Advances and challenges in direct additive manufacturing of dense ceramic oxides

Ceramic oxides, renowned for their exceptional combination of mechanical, thermal, and chemical properties, are indispensable in numerous crucial applications across diverse engineering fields. However, conventional manufacturing methods frequently grapple with limitations, such as challenges in shaping intricate geometries, extended processing durations, elevated porosity, and substantial shrinkage deformations. Direct additive manufacturing (dAM) technology stands out as a state-of-the-art solution for ceramic oxides production. It facilitates the one-step fabrication of high-performance, intricately designed components characterized by dense structures. Importantly, dAM eliminates the necessity for post-heat treatments, streamlining the manufacturing process and enhancing overall efficiency. This study undertakes a comprehensive review of recent developments in dAM for ceramic oxides, with a specific emphasis on the laser powder bed fusion and laser directed energy deposition techniques. A thorough investigation is conducted into the shaping quality, microstructure, and properties of diverse ceramic oxides produced through dAM. Critical examination is given to key aspects including feedstock preparation, laser-material coupling, formation and control of defects, in-situ monitoring and simulation. This paper concludes by outlining future trends and potential breakthrough directions, taking into account current gaps in this rapidly evolving field.

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Introduction
Ceramic oxides (e.g.Al 2 O 3 , ZrO 2 ) constitute the most extensive group of ceramic materials, and have gained prominence in critical fields such as defense, aerospace, biomedical, energy and environmental sectors.This is attributed to their remarkable properties including high specific strength, excellent wear resistance, thermal stability, chemical stability, and biocompatibility [1][2][3].Notably, the utilization of oxides in vital components like turbine blades, combustion liners, and nozzles within the hot sections of turbine engines significantly enhances thermal efficiency, concurrently reducing fuel consumption and emissions [4,5].Moreover, ceramic oxides play a critical role in biomedical domain, and find extensive applications in medical devices such as artificial joints, dental restoration, acetabular cups and orthopedic implants [6].
The current growing demand for lightweight, integrated, and structural-functional ceramic oxide components has contributed to a rise in their structural complexity.However, conventional manufacturing methods, such as extrusion molding, slip casting, and sol-gel casting, encounter challenges in mold construction, impeding the production of ceramic components with highly complex geometry [7].Also, these methods rely on solid-state sintering to achieve the densification of ceramic green bodies, leading to drawbacks like prolonged manufacturing cycles, high energy consumption, as well as high porosity and significant shrinkage deformations in the final products [8].Moreover, the inherent hardness and brittleness of ceramics can lead to a high cost, defects formation and significant tool wear in post-machining, hindering the achievement of structural complexity via subtractive manufacturing methods [9].
Additive manufacturing (AM) is a revolutionary technology that provides a crucial opportunity to address the abovementioned problems and challenges.AM techniques operate on the discrete-accumulation principle, constructing 3D components from the bottom up layer-by-layer [10,11].Compared to conventional techniques, AM significantly enhances the design freedom and manufacturing flexibility, while eliminating the need for expensive molds or tools [12,13].Consequently, it facilitates the cost-effective production of intricate ceramic components [3].Additional benefits of AM include reduced material waste, on-demand production and customization [14].The AM techniques for ceramic oxides can be broadly categorized as the indirect and direct ones.Common indirect AM (iAM) methods for ceramics include stereolithography, inkjet printing, direct ink writing, laminated object manufacturing, digital light processing, and selective laser sintering.These techniques typically involve layering ceramic powder mixed with organic binders to form green compacts, and subsequent sintering procedures are then employed to density and achieve the final ceramic products [15].While iAM markedly enhances the products' complexity, challenges persist due to the requisite thermal treatments, giving rise to critical issues such as high time expenses, high porosity, and severe shrinkage, similar to those in conventional sintering-based methods.By contrast, the direct additive manufacturing (dAM) methods offer a distinct advantage by eliminating the need for lengthy post-sintering steps.Techniques like laser powder bed fusion (L-PBF) and laser directed energy deposition (L-DED) utilize a high-energy heat source to fully melt the ceramic material, enabling singlestep manufacturing of densely structured ceramic components [16].However, compared to iAM methods, the development of dAM for ceramic oxides is still far from mature.The inherent brittleness, high melting points, and poor thermal shock resistance of ceramics, coupled with the steep thermal gradients inherent in the dAM process, can lead to large thermal stress and massive defects in dAMed ceramic components.Additionally, the challenge of precisely controlling the dimensions of the molten pool during dAM can significantly limit dimensional accuracy and contribute to a rough surface finish in as-printed products.The above-mentioned factors may significantly hinder practical implementation of dAM in ceramics fabrication [3].
Despite these challenges, the past decade has witnessed a surge in interest and advancement in dAM for ceramic oxides.This is evidenced by the rapid growth of scientific publications in the field, as illustrated in figure 1.Researchers have made significant efforts in understanding material design, process optimization, defect control, and shaping mechanisms within the dAM process, all of which are crucial for propelling this field forward.Up to date, several reviews [3,8,[17][18][19][20][21][22][23] have been published on AM of ceramics oxides, with most offering comprehensive overviews of general AM techniques capable of producing ceramic parts.These reviews extensively detail and compare their advantages, disadvantages, and potential applications, whereas limited insights are provided into the specific realm of dAM.Therefore, our review aims to fill this gap by critically scrutinizing the current state of dAM for different ceramic oxides.The focus lies on assessing their shaping quality, microstructure, and mechanical properties, while simultaneously highlighting key technical challenges in this field, as illustrated in figure 2. Additionally, this work outlines promising future trends and potential breakthroughs that could propel dAM techniques towards greater maturity and advance the production of ceramic oxides.

L-PBF
The L-PBF technique was first developed in 1995 at the Fraunhofer Institute ILT in Germany, with the powder bed fusion process employed [42].As illustrated in figure 3(a), the L-PBF process begins with precise lowering of the build platform, followed by the deposition of a thin, uniform powder layer.A focused laser beam then selectively melts the prespread powder surface, forming the first layer.The process iterates by lowering the platform, depositing new powder, and selectively melting, ultimately building the desired 3D component layer by layer.Most of commercial L-PBF devices utilize high-power solid-state Nd:YAG or fiber lasers, with laser wavelengths spanning from visible to near-infrared (NIR) region and spot diameters ranging from tens to hundreds of micrometers [37,38].The L-PBF working chamber typically integrates a preheating platform to mitigate thermal and residual stress in specimens, and a protective gas environment (e.g., N 2 , Ar) is generally required to prevent material oxidation and reduce porosity.To ensure powder flowability, the spherical particles with particle size ranged from 10 µm to 60 µm are commonly chosen as feedstock materials [43], with powder layer thickness controlled between 20 µm and 100 µm [44].
Due to the small laser-melted zone and high energy input in L-PBF, the processed materials experience high cooling rates (10 5 • C•s −1 -10 7 • C•s −1 ) and thermal gradients (10 6 • C•m −1 -10 7 • C•m −1 ) [36].This extreme thermal condition profoundly influences the solidification behavior and phase transformation, leading to distinct microstructures and properties compared to traditional methods.The forming quality of L-PBFed components largely depends on processing parameters like laser power, scanning speed, hatch spacing, and layer thickness, which together determine the laser energy density (LED) in printing.Moreover, beam-material interactions, feedstock attributes, atmospheric environment, and post-processing also play crucial roles in product quality.
Compared to other AM techniques, L-PBF excels in producing components with high dimensional accuracy, low surface roughness, and high density [45,46].The inherent rapid cooling conditions in L-PBF contribute to the microstructure refinement of as-printed products, which effectively enhance their mechanical properties [47].Moreover, the capability of easily printing lattice-like support structures in L-PBF is crucial for supporting overhanging sections, facilitating the production of intricate components with internal passages and porous structures [37].However, limitations exist: the small beam diameter and thin layers result in low manufacturing efficiency, hindering large-scale component production [48,49].Also, high cooling rate and thermal gradient can lead to thermal cracks and porosity, deteriorating part quality [50].

L-DED
The L-DED technique, also known as laser engineered net shaping, was introduced in the mid-1990s by Sandia National Laboratory, building upon the principle of laser cladding/welding [51,52].Similar as L-PBF, L-DED utilizes a high-energy laser beam to melt raw powders under inert atmosphere, and achieves metallurgical bonding between adjacent scan lines or layers.However, L-DED employs a synchronous powder feeding system, in which energy delivery and material deposition are focused on the same region, allowing for a better control of material feed [53,54].The working principle of L-DED is illustrated in figure 3(b): a laser beam is directed onto the substrate surface to locally form a molten pool, and the raw powder is simultaneously transported by inert gas and delivered to the stable melt region through lateral or coaxial powder feeding nozzles.The laser deposition head (or worktable) then moves along a pre-programmed path, completing the material deposition of the first layer.Subsequently, the feeding nozzle is lifted vertically for a certain distance, repeating the above-mentioned steps, until all the layers are built.The L-DED equipment normally utilizes kilowatt-level lasers, with laser beam sizes in the millimeter range [38].To ensure efficient and stable powder feeding, spherical particles with diameters ranging from 45 µm to 150 µm are commonly used as feedstocks [55].The typical cooling rate and thermal gradient for L-DED process are 10   C•s −1 and 10 5 m•s −1 -10 6 m•s −1 , respectively [56].
Compared to L-PBF, L-DED enables the processing of larger part volumes (>1 000 mm 3 [59]) and boasts higher material deposition rate (2.5 kg•h −1 for L-DED versus 0.25 kg•h −1 for L-PBF [59]), rendering it well-suited for the rapid manufacturing of sizable components.By utilizing multiple powder feed lines, L-DED allows for the arbitrary composition of various materials on the same part or the creation of functional graded structures with location-specific properties [39].Its adaptability also enables a wide range of processes like coating, repairing, and part retrofitting [60].However, the laser beam in L-DED is one or two orders of magnitude larger than that in L-PBF, resulting in the formation of a larger molten pool during shaping.This inevitably leads to lower dimensional resolution, poorer surface finish, and a coarser microstructure [37,53].Besides, L-DED also encounters other challenges like low powder efficiency, limitations in part complexity, and the necessity for post-machining [39,59].

dAM of ceramic oxides
Ceramic oxides are defined as inorganic compounds composed of metallic or metalloid elements (e.g.Al, Zr, Ti, Mg, Fe and Si) coupled with oxygen.This family encompasses not only advanced ceramics like alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) but also traditional ceramics featuring silica (SiO 2 ) and silicates in their glassy forms.The term can further extend to include ceramic composites, where multiple oxides are combined.Over the years, researchers have undertaken extensive studies into dAM of the various types of oxides as indicated above.Due to the complete melting and solidification of ceramic feedstock, the dAM techniques have the potential to produce dense structures with intricate geometries.This capability may position dAM favorably over iAM for directly fabricating functional ceramic components with higher loading capacity.In light of their targeted application as structural components, research on dAMed ceramic oxides prioritizes their mechanical performance, which is heavily influenced by the density, composition, microstructure, and defects within the as-printed ceramic components.Given this context, this section aims to provide a comprehensive assessment of dAMfabricated oxides, focusing on their shaping processes, product quality, microstructure, and mechanical properties.

Alumina (Al 2 O 3 )
Al 2 O 3 ceramic exhibits exceptional properties such as high hardness, high strength, low density, superior wear and corrosion resistance, electrical insulation and bio-inertness [61].It thus finds critical applications in a wide range of technical areas, such as automotive engine components, aerospace assemblies, electronic elements, and medical devices [62,63].Owing to its versatility and cost-effectiveness, Al 2 O 3 is by far the most widely studied ceramic material in dAM.The key features of dAMed Al 2 O 3 ceramic components are summarized in table 2.
The pioneer studies have demonstrated the viability of producing Al 2 O 3 ceramic components using both L-PBF and L-DED methods.Deckers et al [64] explored the applicability of L-PBF for Al 2 O 3 ceramics.While their approach involved a preheated powder bed (relative density ∼57%) at 800 • C, the low input laser power (2 W) limited melting and resulted in high residual porosity of as-printed components, yielding only 85% relative density.Seeking to improve the density of L-PBFed parts, Juste et al [29] doped Al 2 O 3 powder with graphite (0.05 vol%-1 vol%) as an absorption additive.This modification facilitated the successful manufacture of intricate ceramic components with a denser structure (figure 4(a)), wherein the incorporation of graphite markedly enhanced the laser absorbance of the feedstock powder, thereby enabling the realization of a relative density surpassing 90%.However, significant thermal shock and combustion reaction of graphite in processing lead to the presence of numerous internal pores and cracks within the Al 2 O 3 specimens (figure 4(b)).For the L-DED technique, the early work by Balla et al [68] demonstrated its potential in achieving dense Al 2 O 3 components (relative density: ⩾94%) in diverse shapes (figure 4(c)).However, inherent thermal gradients and directional heat flow led to a columnar grain structure (figure 4(d)) and thus anisotropic mechanical properties, with higher compressive strength along the building direction (229 MPa) compared to the perpendicular direction (123 MPa).Subsequent heat treatment can improve microhardness and fracture toughness but also coarsen the microstructure, highlighting the inherent trade-offs in process selection.
Although the material performance (e.g.mechanical properties and densification) achieved in the abovementioned works are still far beyond satisfactory when compared with the conventionally sintered one, these pioneered works demonstrate the feasibility of producing advanced ceramic oxides with dAM.Further optimizing the overall performance of dAMed Al 2 O 3 ceramics requires a systematic understanding of how processing parameters influence the final product.Given the intricate relationship between 3D object quality and the microstructure and geometry of each scanned line [74], parametric studies have been commonly initiated through systematical single track experiments, serving as critical facets in both process optimization and material characterization for bulk material preparation.For example, Fan et al [61] focused on the influence of LED in L-PBF, demonstrating its impact on single-track geometry, microstructure, and microhardness.They found that increasing LED enhanced single-track stability by decreasing molten pool viscosity and facilitating substrate melting, thereby preventing the balling effect and enabling the formation of continuous solidified tracks.Additionally, judiciously reducing LED could promote a finer columnar dendritic structure (figure 5(a)), leading to improved microhardness based on the Hall-Petch relation.Similar studies have explored parameter optimization for L-DED.Li et al [63] examined the effects of laser power, scan speed, and powder feeding rate on deposition quality in L-DED.Their findings highlighted the positive impact of high laser power on powder utilization, surface quality, and It was observed that the use of island-scanning strategy yielded the highest relative density of 87.8% in asprinted parts, but resulted in a compromised surface quality.By contrast, employing 45 • rotation scanning strategy ensured both the high density and improved surface morphology.Wu's group [76][77][78][79] conducted a comprehensive study on the influence of critical laser parameters (e.g.power, scan speed, and spot diameter) on the surface morphology and defect distribution of L-DEDed Al 2 O 3 thin-walled structures.Their results revealed that judiciously increasing laser power and spot diameter significantly improved surface quality, while increasing scan speed effectively mitigated crack defects (figure 5(c)).Based on these optimized parameters, they successfully prepared large-sized cylindrical components with a remarkable relative density exceeding 99.5% and mechanical properties (flexural strength of 350 MPa and compressive strength of 618 MPa) comparable to those achieved through conventional sintering [69].Similarly, Mishra et al [80] reported that optimizing laser power, scanning strategy, and substrate material in L-DED led to ceramic specimens with relative densities exceeding 95%.
Adding a small amount of solute constituents in feedstock also represents a viable approach for tailoring the microstructure and mechanical properties of dAMed Al 2 O 3 ceramics.The solute doping strategy shares similarities with metallic alloying in traditional casting processes, wherein the introduced solutes segregate at the solid-liquid interface and are expelled into the remaining liquid upon solidification.This phenomenon engenders the formation of an undercooled region ahead of the growth front, facilitating heterogeneous nucleation events during solidification and ultimately resulting in grain refinement [81,82].Furthermore, the solute phase demonstrates a uniform distribution and accumulates at grain boundaries, thereby establishing reinforcing regions that could effectively impede defects formation [1].The synergistic effects of both fine-grain and grain-boundary strengthening contribute to the mechanical property enhancement in final products.Currently, solute doping is primarily applied in L-DED for Al 2 O 3 ceramics, with ZrO 2 , TiO 2 , and SiO 2 being the most common solutes.Pappas et al [1] and Thakur et al [83] prepared Al 2 O 3 -based ceramics in L-DED with the addition of ZrO 2 (⩽10 wt%).The resulting components (figure 6 [75].Reproduced from [75].CC BY 4.0.(c) defects distribution in L-DEDed Al 2 O 3 thin wall structure prepared using different scanning speeds [77].Adapted from [77], with permission from Springer Nature.composition analysis (figure 6(c)).The microstructure of the Al 2 O 3 ceramic was significantly refined through ZrO 2 doping, reducing the grain size from 100 µm (pure Al 2 O 3 ) to 18 µm (Al 2 O 3 -10 wt% ZrO 2 ).This refinement led to a notable enhancement in flexural strength from 58 MPa to 208 MPa.Studies by Niu et al [84], Wu et al [85,86] and Huang et al [71,87] focused on the impact of TiO 2 addition.The TiO 2 -doped ceramic components (figure 6(d)) predominantly comprised α-Al 2 O 3 and aluminum titanate phases (Al 6 Ti 2 O 13 ), with the latter continuously distributed at the α-Al 2 O 3 cellular/dendritic grain boundaries, as observed through SEM and TEM (figures 6(e)-(g)).The in-situ formed Al 6 Ti 2 O 13 phase, with its low thermal expansion coefficient (TEC), effectively mitigated thermal stress and crack initiation.Optimized TiO 2 content and L-DED parameters led to densities exceeding 99.5%, flexural strengths over 200 MPa, and fracture toughness reaching 3.97 MPa•m 1/2 .Moreover, Zhao et al [72,88] introduced SiO 2 into L-DEDed Al 2 O 3 , achieving ceramic specimens with relative density exceeding 99% (figure 6(h)).The SiO 2 -doped ceramic exhibited a typical cellular structure (figure 6(i)), and consisted mainly of α-Al 2 O 3 and glassy phases (figures 6(j) and (k)).The in-situ formation of a glassy phase at grain boundary facilitated densification and relaxed residual stresses, leading to a maximum flexural strength of 310.1 MPa and fracture toughness of 3.07 MPa•m 1/2 .This could be further improved to 504.4 MPa and 3.54 MPa•m 1/2 via post-heat treatment (1 600 • C, 4 h) [88].
In summary, dAM typically produces Al 2 O 3 ceramics with a columnar grain structure due to epitaxial growth during laser melting.This inherent microstructure leads to predictable anisotropic mechanical behavior.Careful optimization of processing parameters and material composition can yield asprinted components with impressive densities exceeding 99%.The micro-mechanical properties of dAMed Al 2 O 3 ceramics are comparable to those obtained through traditional sintering, but the macro-mechanical performance remains a bottleneck.Despite extensive research on optimizing processing parameters and material compositions for dAMed Al 2 O 3 ceramics, thermal cracking remains a significant challenge during the shaping process, which is the main reason for the poor macromechanical properties.This necessitates a targeted approach to mitigate thermal stresses and address crack formation in dAM processing.[83], with permission from Springer Nature.(c) Adapted from [83], with permission from Springer Nature.(d) Used with permission of EMERALD GROUP PUBLISHING LIMITED, from [84]; permission conveyed through Copyright Clearance Center, Inc. (e), (f), (g1) and (g2) Reprinted from [87], © 2020 Elsevier B.V. All rights reserved.(h) Reprinted from [88], © 2022 Elsevier Inc.All rights reserved.(i1), (i2), (j), (k1) and (k2) Reproduced from [72].CC BY 4.0.

Zirconia (ZrO 2 )
Zirconia (ZrO 2 ), often referred to as 'ceramic steel', earns this moniker due to its exceptional mechanical properties such as high strength, wear resistance, and fracture toughness [89].Its impressive biocompatibility has led to widespread use in biomedical applications, encompassing dental restorations, joint prosthetics, bone repair plates, and orthopedic hip replacements [90,91].Additionally, the low thermal conductivity and advantageous ion conductivity render ZrO 2 ceramic a suitable material for applications in gas turbines within the aerospace sector [5,92], as well as in solid oxide fuel cells and oxygen sensors for various electrochemical applications [93].It is noteworthy, however, to note that pure ZrO 2 ceramics commonly exhibit numerous microcracks, impacting their mechanical performance.This phenomenon is attributed to their martensitic phase transformation (t-ZrO 2 ↔m-ZrO 2 ) during heating or cooling, which results in an increase or decrease of volume (∼4%) [94].To improve the stability of hightemperature phases (e.g.t-ZrO 2 , c-ZrO 2 ) in ceramic components, it is a standard practice to incorporate stabilizers such as Y 2 O 3 , MgO, and CaO.Despite its considerable engineering applications, there has been a relative paucity of research efforts devoted to dAMed ZrO 2 ceramics in comparison to Al 2 O 3 .This is probably due to their higher melting temperatures and markedly lower thermal conductivity, rendering this material more challenging to shape with dAM.The reported structure and mechanical performance of dAMed ZrO 2 ceramics are presented in table 3.
Bertrand et al [95] pioneered the dAM research for Y 2 O 3stabilized ZrO 2 (YSZ) ceramic production, achieving successful L-PBF printing of near-net-shaped YSZ objects with precise geometry (figure 7(a)).However, the relative density of asfabricated YSZ components was constrained to a mere 56%.This was caused by the use of a low laser power input (⩽50 W) in this work, leading to incomplete melting of the raw ceramic powders (figure 7(b)).Notably, post-furnace sintering did not significantly improve the densification of L-PBFed YSZ parts.
The later work either incorporated a high energy input or employed absorbing agents for obtaining highly-dense YSZ ceramics in dAM [2,89,93,96,98,99].Liu et al [96] used an 1 µm fiber laser in L-PBF to fully melt YSZ powder with laser power of 60 W and scanning speeds of 0.01 m•s −1 -0.2 m•s −1 .The resulting components (figure 7(c)) achieved 88% density and 12.6 GPa average microhardness, but exhibited substantial orderly cracks (figure 7(d)) due to uneven laser distribution.An inferior macro-mechanical performance of this YSZ sample can be expected due to the high fraction of cracks.Likewise, Fan et al [89] prepared YSZ thin-wall structures (figure 7(e)) using L-DED with laser power of 250-350 W and 0.006 m•s −1 scan speed.The microstructure showed lenticular t-ZrO 2 embedded in c-ZrO 2 (figure 7(f)).Optimized L-DED parameters yielded highly dense parts (up to 98.7% density) with maximum nano-hardness of 19.8 GPa and elastic modulus of 236.1 GPa.Notably, Liu and Bai [98] used a femtosecond laser system to produce YSZ ceramics, achieving a remarkable relative density exceeding 99%.However, only thin layer samples could be produced with significant surface cracks.
In instances involving the absorbers addition, Ferrage et al [93] and Urruth et al [99] successfully enhanced the densification of L-PBFed YSZ parts (figure 7(g)) by doping trace amounts of carbon-related materials, such as graphite, TiC, and SiC.The resulting YSZ ceramics featured columnar grains growing along the build direction (figure 7(h)) and exhibited relative densities exceeding 96% after laser parameter optimization.Unfortunately, these samples are still featured with high fractions of crack that are primarily distributed at both columnar grains and interlayer boundaries (figures 7(h) and (i)).Verga et al [2] employed organic binder pyrolysis to achieve a uniform distribution of carbon absorber in ceramic feedstocks and successfully densified ZrO 2 -based ceramics (20 wt% Al 2 O 3 ) in L-PBF.Highly complexed ceramic structures (figure 7(j)) were prepared, showing relatively high densities surpassing 90%.Microstructural observations revealed a dense appearance (figure 7(k)), but closer inspection at higher magnification revealed numerous small cracks (thickness <1 µm) (figure 7(l)).This may explain the modest fracture strength ((31 ± 11) MPa) observed in these parts.
Defect control and mechanical enhancement in dAMed ZrO 2 ceramic have proven to be exceptionally challenging as indicated in a number of previous studies [2,89,93,99], which stems from the substantial thermal stresses generated when fusing this material at extremely high temperatures.In response to this predicament, Liu et al [97] introduced a secondary Nd:YAG laser for preheating the YSZ ceramic powder bed at 1 500 • C-2 500 • C and investigated its influence on microstructure and density of the as-printed YSZ parts.They observed effective mitigation of extensive orderly cracks when the preheating temperature exceeded 2 000 • C, resulting in a significant increase in parts density from 84% to 91%.Despite this, small disordered cracks persisted in the as-printed YSZ samples.
In summary, most of the studies in dAMed ZrO 2 ceramics focus on maximizing part density through increased energy input or energy absorbers.While these optimizations have yielded part densities exceeding 99%, the persistence of substantial internal cracks remains a critical impediment, significantly limiting their mechanical performance.In fact, the thermal cracking issue in dAMed ZrO 2 is more pronounced compared to Al 2 O 3 ceramic, primarily attributed to its higher melting point and extremely low thermal conductivity.

Silica (SiO 2 ) and silicate
SiO 2 ceramic ranks among the most abundant compounds in the Earth's crust.It exists in both crystalline and amorphous forms, and shows notable features such as high optical transparency, low thermal expansion, chemical inertness, thermal insulation, electrical insulation and piezoelectric capabilities [100,101].These versatile characteristics render it wellsuited for a broad range of applications such as electronic devices, precision optical components, thermal insulation boards, laboratory vessels, molding tools and capacitors.Current research of dAMed SiO 2 predominantly focuses on its amorphous form (i.e.SiO 2 glass), which stands out as a promising candidate for the production of defect-free ceramic parts.This distinction is attributed to its exceptionally low TEC (∼0.5 × 10 −6 • C −1 [102]), efficiently mitigating the thermal stress generated in dAM process.However, the high brittleness and low strength of amorphous structure [103] also impose limitations on the use of dAMed SiO 2 components for structural applications, particularly under high-temperature environments.
Initial attempts like Tang et al [104] and Wang et al   [32]; (k) HRTEM image showing the phase constituents at grain boundary regions [32].(l) the L-PBFed soda lime glass components with intricate structure [112,113]; (m) The dAMed silicate components made by bioactive glass [114] and spodumene glass [115].g1) and (g2) Reprinted from [110], © 2018 Elsevier Ltd.All rights reserved.(h) and (i) Reproduced from [111], with permission from Springer Nature.(j) and (k) Reprinted from [32], © 2021 Elsevier B.V. All rights reserved.(l1)Reproduced from [113].CC BY 4.0.(l2) and (l3) [112] John Wiley & Sons.Used with permission of BLACKWELL PUBLISHING, from [112]; permission conveyed through Copyright Clearance Center, Inc. (m1) Reprinted from [114], Copyright © 2011 Acta Materialia Inc. Published by Elsevier Ltd.All rights reserved.(m2) Reprinted from [115], © 2018 Elsevier Ltd and Techna Group S.r.l.All rights reserved.and low mechanical performance (compressive strength: 6-16 MPa) in as-printed parts.Increasing laser power or decreasing scanning speed slightly improved mechanical properties, though at the cost of increased surface roughness.Seeking highly dense and defect-free products, Khmyrov et al [106,107] conducted systematic L-PBF experiments on SiO 2 glass, investigating the influence of processing parameters on single tracks/layers (figure 8(c)).The narrow processing window for SiO 2 glass stemmed from its high viscosity.While cracking was not observed, pores were present near the substrate (figure 8(d)).Finer feedstock and thinner deposited layers demonstrably reduced pores and improved adhesion to the substrate.Protasov et al [108] further proposed an analytical model to calculate the thermal conductivity of the melted SiO 2 powder bed and numerically simulated the structure of single tracks.Combined with the experimental results, they concluded that decreasing layer thickness or preheating the substrate could improve L-PBFed SiO 2 glass quality.Additionally, sufficient laser power to reach the boiling point at the powder bed surface was crucial for complete consolidation.
Compared with L-PBF, L-DED offers superior optical performance for SiO 2 , allowing for the fabrication of transparent SiO 2 glass with intricate structures [116].To ensure highly dense and transparent parts, L-DED often favors SiO 2 glass wires or filaments over powders as the preferred feedstock (figure 8(e)).Luo et al [117] conducted fundamental studies on single and multi-tracks in L-DED, exploring the influence of laser power, scanning speed, and layer thickness on the morphology of printed parts.They systematically mapped the operational processing window for SiO 2 with the aid of a numerical thermal model.Witzendorff et al [109] employed a filament-based L-DED method with precise heat input control to achieve continuous material deposition and prevent excessive evaporation.Their successfully printed transparent cylinder component (figure 8(f)) showcases a dense structure free of cracks and pores.Likewise, Peters et al [110] implemented a unique molten pool temperature controller to prevent bubble formation and proposed a path planning method to ensure the dimensional accuracy of deposited SiO 2 tracks.Consequently, they successfully manufactured transparent thin-walled star patterns and a spring-like structure (figure 8(g)) without structural support.
Beyond pure SiO 2 , silicate ceramics have also gained significant traction in the dAM field.For example, production of mullite-based ceramic was reported by Gahler et al and Heinrich et al [118,119] using L-PBF, where ceramic slurries with different Al 2 O 3 /SiO 2 ratios were prepared as feedstocks.The as-printed samples comprised of blocky α-Al 2 O 3 , amorphous SiO 2 , and acicular mullite phase, achieving a relative density of 92%.The post-heat treatment (1 600 • C, 2 h) in conventional sintering furnace could further increase the density to 96%, while promoting the transformation of the amorphous phase into cristobalite.Likewise, Wu et al [32,111] synthesized mullite ceramics in L-DED process (figure 8(h)), utilizing pre-mixed α-Al 2 O 3 and SiO 2 powders as feedstocks with a weight ratio of 71.8: 28.2.Their microstructural characterizations (figures 8(j) and (k)) revealed that the L-DEDed mullite primarily composed of cellular mullite grains growing along the [001] direction, and the Si-rich glass phase was recognized at grain boundaries.Through process optimization, the obtained mullite ceramics exhibited a density of over 97%, with an average flexural strength and microhardness of 108.6 MPa and 14 GPa, respectively.The studies in [112,113,120,121] have substantiated the efficacy of using L-PBF to produce soda lime glass parts with complex shapes figure 8(l), and systematically probed the impact of process parameters and feedstocks on the parts qualities.Fateri et al [112,120] noted that high energy density could induce glass deformation, while low energy density might result in poor surface quality.Through meticulous control of L-PBF parameters, the resulting glass samples demonstrated a relative density of 99% and a surface roughness of 0.88 µm.Datsiou et al [113,121] found that the optimal process window was independent of particle size of feedstocks.Nevertheless, the use of finer powders could improve the geometrical accuracy of the samples.After parameter optimization, the L-PBFed samples would exhibit a porosity of approximately 10% and a flexural strength of 6.2-6.9MPa.Recently, a parametric study was also conducted in L-DED process for producing soda lime glass [122], and optimal parameters were determined to produce consistent single-tracks on glass substrates without cracking.Moreover, researchers also engaged in comprehensive experimental investigations on dAM for some other silicate glass materials, such as spodumene glass [115,123], bioactive glass [114,124], and borosilicate glass [125].Examples of these printed components are presented in figure 8(m).
In sum, the past decade has witnessed a surge of interest in utilizing dAM for processing silica and various silicate materials.This stems from the advantageous low TEC of SiO 2 , which facilitates the fabrication of complex, defect-free SiO 2based ceramic components via dAM.However, it is important to note that these materials predominantly consist of an amorphous structure, resulting in significantly lower mechanical performance compared to their crystalline counterparts.Consequently, dAM-processed silica and silicates might not be readily suitable for engineering structural applications.

Eutectic oxides
Eutectic oxides represent a class of naturally occurring in-situ composites, comprising two or more constituent oxide phases that undergo direct melting and solidification at the eutectic composition [126].Their solidification process is characterized by the congruent crystallization of multiple solids from a single liquid, fostering the formation of a fine and homogeneous eutectic structure [127,128].Examples of typical eutectic oxide systems include Al and Al 2 O 3 -GAP-ZrO 2 .Due to their distinctive microstructure, strong interphase bonding and the absence of grain boundaries, eutectic oxides demonstrate exceptional thermal stability and mechanical performance.For example, Waku et al [129] reported a superior flexural strength of 695 MPa for Al 2 O 3 -GAP eutectic at 1 600 • C, and the eutectic displays plastic deformation rather than brittle fracture at elevated temperatures.Oliete et al [130] produced the Al 2 O 3 -YAG-ZrO 2 components with the best achieved flexural strength of 4.6 GPa at the room temperature.Therefore, these materials hold significant value for critical components for the aerospace uses, such as nozzles, blades, and combustion chamber linings in turbine engines [4].The adoption of dAM techniques for eutectic oxides has gained significant attention recently, attributing not only to the inherent technical superiorities of dAM but also to the unique multiphase structure in dAMed eutectics, contributing to effective defects alleviation.The microstructure and mechanical performance of previously reported dAMed eutectic oxides are demonstrated in table 4.
Microstructure refinement is a common phenomenon observed in dAMed eutectic oxides, stemming from the high cooling rates inherent to the dAM process.For example, Chen et al [90] and Zhang et al [91] reported the production of highly-dense Al 2 O 3 -YAG and Al 2 O 3 -ZrO 2 binary eutectic components in L-DED, achieving a minimum eutectic spacing close to 100 nm.The as-solidified Al 2 O 3 -YAG eutectic displayed a distinctive irregular eutectic structure ('Chinesescript'), featuring entangled Al 2 O 3 and YAG phases in a threedimensional and continuous network (figure 9(a)).The formation of irregular structure is attributed to the high fusion entropies of both constituent phases, inducing a faceted/faceted growth mode during solidification.In contrast, the dAMed Al 2 O 3 -ZrO 2 eutectic exhibited a complex regular structure due to the non-faceted growth behavior of ZrO 2 , and ordered ZrO 2 lamellas or rods were discerned to be embedded in Al 2 O 3 matrix within the colony (figure 9(b)).Likewise, the preparation of finely structured ternary eutectic oxides in dAM  [144,145] has also been well documented, encompassing compositions such as Al 2 O 3 -YAG-ZrO 2 [58] and Al 2 O 3 -GAP-ZrO 2 [144,146] ceramics.The ternary eutectic components predominantly feature a refined irregular structure with interpenetrated constituent phases (see figures 9(c) and (d)).The introduction of the ZrO 2 phase in Al 2 O 3 -YAG or Al 2 O 3 -GAP binary systems is believed to impart additional grain refining and toughening effects to the eutectic components [147,148].
It is noteworthy to note that the microstructural size and morphology of dAMed eutectics can be significantly influenced by the imposed solidification conditions during processing.Numerous studies [58,132,133,136,142,146, 149] have observed a strong correlation between eutectic spacing and solidification rate, which can be effectively modeled by the Jackson-Hunt equation [151]: where λ is the eutectic spacing, V s is the solidification rate and C is a constant.This equation clearly demonstrates that a finer eutectic structure can be achieved by increasing the laser scanning rate in dAM.However, it is crucial to recognize that the C values vary significantly among different eutectic systems.For instance, Liu et al [142] reported a relationship of λ 2 V s = 1 µm 3 •s −1 for dAMed Al 2 O 3 -ZrO 2 eutectic, while the values of 10 µm 3 •s −1 s and 23.2 µm 3 •s −1 were observed for Al 2 O 3 -YAG and Al 2 O 3 -GAP-ZrO 2 systems, respectively [133,136].Moreover, the solidification parameters also exert a substantial influence on the eutectic morphology in dAM.
The transition from irregular to a complex regular eutectic structure was commonly observed in Al 2 O 3 -YAG (ZrO 2 ) and Al 2 O 3 -GAP (ZrO 2 ) eutectic systems under high solidification rates [58,[132][133][134][135]150].This phenomenon is attributed to the change in growth mode of the constituent phases, transitioning from faceted growth at low undercooling to non-faceted growth at high undercooling [150].
One of the major challenges in achieving uniform microstructure in dAMed eutectic oxides is the emergence of a periodic banded coarsening structure at the bottom of each deposited layer, primarily due to the layer-wise building strategy employed.Notably, the precise formation mechanism of the banded coarsening structure remains a topic of ongoing controversy.Ma et al [152] and Yan et al [149] investigated the microstructural feature of the banded structure in L-DEDed Al 2 O 3 -ZrO 2 eutectic (see in figure 9(e)).The banded structure consisted mainly of discrete block of Al 2 O 3 /ZrO 2 granules and a small amount of coarsening eutectics, and their formation was explained by the thermal annealing effect from the successive layer deposition.Further EBSD characterization of this banded structure (see in figure 9(f)) was reported in [56], revealing that the discrete blocks of Al 2 O 3 phase exhibited random orientation, while the ZrO 2 phase grew epitaxially, forming a halo structure enveloping the Al 2 O 3 facets.By contract, Fan et al [150] researched the banded structure in Al 2 O 3 -YAG system (figure 9(g)), and found its formation to be closely linked to the local solidification conditions of the molten pool.Within each deposited layer, the eutectic spacing exhibited a rapid decrease from the bottom to the top of the molten pool, attributed to an increased solidification rate along the building direction.The EBSD analysis disclosed that the YAG phase in the banded structure displayed random orientation, while the Al 2 O 3 phase grew consistently along [1010] direction (figure 9(h)).Moreover, Liu et al [144] conducted a partial remelting experiment to elucidate the banded structure formation mechanism in Al 2 O 3 -GAP-ZrO 2 eutectic.Their results indicated that the appearance of the banded structure was related to the drastic abnormal coarsening of the fine eutectic structure adjacent to the molten pool.Recent studies have proposed various strategies to mitigate or eliminate this undesired feature.Yan et al [149] successfully controlled the thickness of banded structure in Al 2 O 3 -ZrO 2 eutectic to 10 µm through optimization of laser parameters.In a subsequent investigation [33], they further refined this thickness to 2 µm with the assistance of an ultrasonic device.Liu et al [153] achieved a complete disappearance of the banded structure in Al 2 O 3 -GAP-ZrO 2 eutectic via post-heat treatment (1 500 • C, 300 h).However, this approach may face limitations due to potential thermal coarsening in the overall microstructure.
Mechanical testing of dAMed eutectic oxides primarily focuses on their micro-mechanical properties.Specifically, microhardness measurements fall within the range of 15-19 GPa, and fracture toughness ranges from 3-9 MPa•m 1/2 , demonstrating a fundamental comparability to conventional directional solidification methods.Through the implementation of high-temperature laser preheating, Wilkes et al [16] successfully eliminated internal defects in large-sized L-PBFed Al 2 O 3 -ZrO 2 eutectic samples, determining their macroscopic strength to exceed 500 MPa.However, the surface quality is compromised due to the formation of excessively large melt pools during laser preheating.Further mechanical enhancement of dAMed eutectics can be achieved by introducing ultrasonic vibration or incorporating hard reinforcing particles.For example, Yan et al [138,139] utilized ultrasonic vibration-assisted L-DED technology to produce Al 2 O 3 -ZrO 2 eutectics.Their results indicated that ultrasonic vibration significantly refined the eutectic size to 60-70 nm, leading to an enhanced fracture toughness of 7.67 MPa•m 1/2 .Their subsequent work [143] combined ultrasonic vibration and carbon fiber doping in L-DED, effectively reducing the eutectic spacing to 50 nm, with an enhanced fracture toughness of 8.7 MPa•m 1/2 .Wu et al [136] reported the mechanical enhancement of Al 2 O 3 -YAG eutectic through the use of a water-cooled substrates in L-DED processing.The increased cooling rate led to a drastic reduction in eutectic spacing by 78.1%, resulting in a 10.6% increase of microhardness and an 8.5% improvement in fracture toughness.
Compared to single-phase ceramics, eutectic ceramics are more suitable for dAM.Their intricate multi-phase structure inherently provides an additional strengthening effect, leading to improved mechanical properties.Optimizing dAM parameters and incorporating external fields further enable the fabrication of fully dense eutectic specimens with refined and uniform microstructures, effectively reducing the occurrence of defects in as-printed samples.Notably, the micromechanical properties of dAMed eutectic ceramics often match or surpass those of traditionally directionally solidified counterparts, attributable to their finer eutectic structure.Nevertheless, achieving comparable macroscopic properties remains a significant challenge.

Other ceramic oxides
Researchers have also conducted systematic dAM experiments on other ceramic oxides.For instance, Bernard et al [154] fabricated lead zirconate titanate (PZT, Pb(Zr x Ti 1−x )O 3 ) components using L-DED.The relative density of the produced components was measured at 90%, and the microstructure predominantly consisted of fine perovskite and pyrochlore grains, contributing to a high hardness of 3.51 GPa.The PZT samples exhibited favorable dielectric properties, indicating the potential of L-DED in preparing PZT-based sensors or transducers on structural components.Pappas et al [155][156][157] explored the feasibility of shaping transparent magnesium aluminate (MgO•Al 2 O 3 ) ceramic using L-DED.They successfully produced high-density parts with a transmittance of 82% at wavelength of 632.8 nm.The microhardness and fracture toughness of the samples fell within the range of 12.7-14.2GPa and 2-3 MPa•m 1/2 , respectively.These values demonstrated comparability to those achieved through traditional sintering techniques.

Key issues in dAM of ceramic oxides
The viability of employing dAM to produce various ceramic oxides has been proven in previous studies, and highlydense components could be fabricated with a certain level of complexity.Nevertheless, the intrinsic properties of ceramic oxides, including their brittleness, high melting points, limited thermal shock resistance, and poor NIR laser absorptance, present substantial difficulty in dAM of these materials.The high thermal gradient, rapid cooling, and cyclic thermal loading inherently in dAM inevitably lead to the defect formation, which subsequently contribute to degraded mechanical performance of as-printed components.Several researchers [22,79] have noticed that the processing window for ceramic oxides in dAM is considerably narrower compared to metallic materials, posing a challenge in optimizing product quality through processing parameters adjustment.Furthermore, current dAM techniques struggle to meet the demands of largescale and intricate ceramic structures.These limitations significantly restrict the practical application of dAMed ceramics in critical engineering fields, hindering the full potential of the dAM technologies.To address these limitations and promote the continued advancement of dAM for ceramic oxide production, researchers are actively focusing on several key areas: (1) developing optimized feedstocks, (2) enhancing lasermaterial coupling, (3) understanding and controlling defect formation, and (4) implementing advanced in-situ monitoring and simulation tools.

Feedstock preparation
The powder supply process in dAM is crucial in determining the final quality of ceramic products.Compared to metallic materials, ceramic oxides have lower density, resulting in poor powder flowability [158], making it challenging to achieve uniform spreading in L-PBF or continuous transport in L-DED.In addition, the features of ceramic feedstock such as morphology, density, chemical composition, humidity, particle size and distributions, also exert a substantial influence on the dynamics of powder supply.Researchers have observed that the size of feedstock particles influences the accuracy and quality of dAMed components.Yap et al [158] and Sing et al [17] found that large particles can compromise dimensional accuracy, while smaller ones, prone to van der Waals aggregation, can lead to uneven deposition or blockages.Wilkes et al [16] demonstrated that spherical powders, compared to irregular shapes, exhibit better flowability and reduce porosity in the final product.Several studies have explored the potential of bimodal particle size distributions to improve dAMed ceramic quality [94,97,157].This improvement is credited to the smaller particles filling the gaps between larger particles, ultimately enhancing the overall density of the raw powder.For example, Liu et al [97] successfully increased the density of L-PBFed YSZ components by utilizing a bimodal feedstock with coarse (22.5-45 µm) and fine (9-22.5 µm) powders.Notably, their research also revealed a decreased flowability when the coarse fraction fell below 80 wt%.Similarly, Pappas et al [157] achieved pore elimination in L-DEDed MgAl 2 O 4 components by incorporating nano-particles into micron-sized ceramic powder.
The two primary feedstock preparation techniques for dAM are gas atomization (figure 10(a)) and spray drying (figure 10(b)).Gas atomization utilizes high-speed gas flow to break the molten ceramic stream into fine droplets, converting the kinetic energy of the gas into surface energy for the molten ceramic.The resulting fine droplets are rapidly cooled and solidified in air, forming spherical-shaped powder particles.The ceramic powders prepared by gas atomization offer notable advantages, including fine particle size, good sphericity, good chemical purity, and high production efficiency [159,160].However, a notable drawback of gas atomization is the associated high production costs.This stems from the extreme temperatures required to melt ceramics due to their high melting points.Spray drying, in contrast, bypasses the energyintensive melting process.Instead, it atomizes a ceramic slurry into droplets, which are then dried with hot air to form solid powder.While significantly reducing operating costs, this method compromises certain powder properties.The resulting ceramic powders often exhibit lower strength and density due to incomplete densification during drying, leading to poor feedstock flowability and increased porosity in printed components [17].
dispersants, using a scraper to achieve uniform coating on heated substrates (figure 10(c)).Likewise, Zhang et al [162] achieved a well-flowing ceramic slurry by mixing ceramic powder and deionized water in a 1:1 ratio.This high-load slurry was then uniformly spread as feedstocks using a plastic scraper.Waetjen et al [161] also prepared water-based ceramic slurries containing high solids content (>70 wt%), and proposed a layer-wise slurry deposition method based on airbrush spraying technique (figure 10(d)).While this method offers uniform layers, subsequent drying can lead to warping and partial detachment from the substrate.Overall, ceramic slurries exhibit superior flowability over the solid powders, and the use of finer particles in slurry contributes to the enhanced densification of powder layer after heating and drying [165].Thus, ceramic slurries have the potential to improve the density and surface quality of dAMed ceramic components.Practical challenges including complex preparation processes, prolonged drying times, and potential impurity introduction still remain [22,166].
For dAM feedstocks containing multiple oxides, it is crucial to ensure different constituents are sufficiently mixed and uniformly distributed in feedstocks before melting.A common approach involves mechanical mixing via ball milling or stirring, offering advantages like low cost, short processing times, and operational ease [167].However, this method can negatively impact feedstock flowability by damaging spherical particles.To address this concern, Su's group [132,144] introduced a powder preparation process that integrates ball milling spray The process commenced with ball to homogenize the small and irregularly shaped ceramic powders (1-5 µm), and centrifugal spray drying technology was subsequently employed to spray the well-mixed precursor, ultimately yielding the spherical powders with particle sizes ranged from 10 µm to 50 µm.Verga et al [2] and Pfeiffer et al [67] adopted a similar strategy, utilizing ball milling to blend micron/sub-micron primary phase powders with nanosized dopants.Spray drying was then employed to produce composite powders with a particle size of 30-40 µm.This preparation process not only ensures homogeneous mixing of different ceramic constituents, but also assures excellent flowability of multi-component powders.Moreover, Yu et al [168] developed a combustion synthesis-spray cooling process for powder preparation.This innovative method produced composite ceramic melt via a combustion reaction, followed by the injection of the melt into the air under high pressure.Highlydense spherical composite powders can be prepared with the particle size of 3-12 µm, and exhibit a uniform two-phase structure.

Laser-material coupling
The limited laser-material interaction is another critical challenge in dAM when dealing with ceramic oxides.Most commercial L-PBF/L-DED devices are tailored for processing metallic materials, typically employing NIR lasers with wavelengths (λ) between 1 030 nm and 1 070 nm [169].Unfortunately, at this λ range, the energy absorption rate of ceramic oxides is remarkably low with high temperaturesensitivity.Tolochko et al [170] and Ferrage et al [93] measured the laser absorption rates of various oxide powders (e.g.Al 2 O 3 , SiO 2 , ZrO 2 ) using an integrating sphere apparatus.Their results revealed a strong wavelength dependence of energy absorbance, with most oxides exhibiting absorption rates below 0.05 for Nd:YAG lasers (λ = 1.06 µm).This poor coupling between oxides and NIR lasers was also observed in [29], and the relationship between absorption and laser wavelength in Al 2 O 3 ceramics is presented in figure 11(a).[171] reported the laser absorption rate of bulk Al 2 O 3 ceramic was less than 0.12 for Nd:YAG laser at room temperature but significantly increased to 0.95 at 2 500 • C. Qian and Shen [172] further demonstrated the nonlinear energy absorption behavior in ceramic oxides interacting with Nd:YAG lasers, characterized by an avalanche-like temperature rise.The inherent optical features present issues for dAM processing, including high energy losses, low production efficiency, insufficient melting, and increased defects formation [17,22,35].

Zhang and Modest
Adding high-absorption materials into raw powder is effective for enhancing the optical properties of ceramic oxides.Researchers have conducted a number of experiments to explore various types of additives that can stabilize and enhance the laser absorbance of ceramic oxide powders.The previously reported dopant material and the corresponding absorption rate for doped ceramics are summarized in table 5.
Most of these studies have demonstrated an enhanced laser absorbance of oxide particles through the incorporation of trace amounts of carbon-based materials, such as graphite, carbon black, TiC, and SiC [29,93,99,173].The absorbance evolution of oxides for increasing amounts of carbonbased dopant can be well illustrated in figure 11(b).The observed absorption enhancement arises from a better matching of the optical bandgap in carbon-based materials with the excitation energy of NIR photons, which facilitates the laser radiation absorption of ceramic powders.Moreover, materials like graphite or carbon black can be easily oxidized and volatilized in laser processing, effectively mitigating concerns regarding impurity introduction [29].The enhanced absorption rate of the feedstock resulted in a more stable molten pool and a denser structure in dAMed ceramics.Recently, Leung et al [169] demonstrated a significant advancement in laser absorptance by utilizing reduced graphene oxide (rGO) as the absorbing material.Compared to conventional carbon additives, rGO achieved a remarkable threefold improvement in absorptance.This enhancement was ascribed to a smaller optical bandgap of rGO (0.553 eV) in contrast to carbon (0.656 eV).Consequently, high relative density up to 99.6% can be achieved for r-GO doped ceramics at lower laser energy input, as compared to the carbon-doped ones.Other than carbon-based materials, Pfeiffer et al [65,66] and Florio et al [26,30] reported an absorption enhancement by incorporating colored oxide nanoparticles into raw Al 2 O 3 powders, such as Fe 2 O 3 and MnO 2 .They achieved an absorption rate of 90%, leading to a relative density exceeding 98% for the as-printed components.Verga et al [2] introduced 0.1 wt% organic binder into ZrO 2 -based ceramic powders, and the subsequent calcination procedure led to the organic pyrolysis in forming the uniformly distributed carbon residues within the feedstock, thus effectively increasing the NIR laser absorption.
Another potential solution to the low absorptivity challenge lies in exploring alternative heating sources.The use of mid-infrared lasers, in contrast to the NIR ones, demonstrates a significant enhancement in energy absorption for ceramic oxides.For instance, the energy absorption rate of Al 2 O 3 powder under CO 2 laser irradiation (λ = 10.6 µm) can reach up to 96% [170].However, CO 2 lasers suffer from a large beam size and pronounced light scattering, compromising the processing accuracy and surface quality of dAMed ceramic components [29].Exner's team [176][177][178] employed Q-switched pulse laser in dAM for ceramic components preparation.Their findings suggested that the use of pulse laser not only enhanced the energy absorption rate but also effectively mitigated overheating issues associated with the avalanche effect.This is because strong laser pulses facilitated the multiphoton excitation of electrons in the valence band of oxides, enabling the powder to efficiently absorb energy at the initial stages of laser-material interaction [176].

Defects formation and control
The formation of defects, including cracks and pores, acts as critical roadblocks impeding the widespread adoption of dAM for ceramic oxides.The inherent mechanical and physical properties of these materials contribute to the challenges associated with fabricating defect-free and high-performance components using dAM techniques, as highlighted in numerous studies.Therefore, attaining a comprehensive understanding of defects formation mechanisms and developing effective defects mitigation strategies become paramount for unlocking the full potential of dAM techniques.

Cracks formation mechanism.
Cracking stands out as the predominant and detrimental defect in dAMed ceramic oxides.It significantly constrains the mechanical performance of as-printed components, rendering them unreliable and susceptible to premature failure [179].The ceramic oxides are particularly vulnerable to cracking in laser processing, which arises from the interplay of inherent material properties, characterized by high melting point, low thermal shock resistance and the brittleness, and the dAM procedure itself, which features rapid cooling, steep thermal gradients, and complex thermal cycling.As a result, excessive thermal stresses are accumulated in the deposited ceramics during dAM processing, and microcracks are easily nucleated and propagated when the local thermal stress is larger than the fracture strength of the deposited material.The critical threshold for crack propagation in dAM can be expressed mathematically as follows [77]: where f (x, L) is a function of analysis position and specimen length, α is the TEC of the deposited material, E is the elastic modulus, ∆T is the temperature difference, c is the half-length of initial microcrack, and γ s is the fracture surface energy.Notably, the left-hand side of the equation quantifies the magnitude of thermal stresses induced by dAM processing, while the right-hand side reflects the theoretical fracture strength of the deposited material.Previous research has systematically explored the cracking behavior of dAMed ceramics, spanning various scales from single deposited tracks to bulk components.Liu et al [34,162] investigated the crack formation and extension in L-PBFed Al 2 O 3 ceramics.They identified two types of cracks within single deposited tracks and layers, namely transverse and longitudinal cracks (figures 12(a) and (b)).The transverse cracks were perpendicular to the laser scanning direction, and were closely related to the high thermal gradient and the accumulated internal stresses in L-PBF.In contrast, the longitudinal cracks, aligned with the laser scanning direction, arose primarily from solidification shrinkage of the molten pool and, to a lesser extent, from deflection of transverse cracks.Notably, crack propagation preferentially followed the path of the least resistance along the columnar grain boundaries, although transgranular fracture could also occur under high internal stress conditions.Similar work was also conducted by Qiu et al [180], who observed that intergranular transverse cracks dominated surface defects in single deposited Al 2 O 3 layers, and the distribution of cracks was significantly influenced by hatch spacing and laser scanning speed.Liu et al [96,97] noted the formation of vertical cracks at the central region of single deposited tracks of ZrO 2 (figure 12(c)), attributed to the uneven Gaussian energy distribution of the laser beam.Short vertical cracks in single tracks could readily extend into continuous large cracks during subsequent layer depositions, as schematically illustrated in figure 12(d).This finding directly elucidates the mechanism behind the formation of the massive orderly cracks observed in bulk ceramic components.Niu et al [77,84,181] conducted research on crack formations in L-DEDed thin wall structures (figure 12(e)).Their work revealed a predominant tendency for cracks to propagate along the deposition direction, and this was attributed to the combined effects of high thermal tensile stress and the inherent columnar growth nature of the dAM process.Importantly, they identified the formation of intergranular liquid films, arising from impurity segregation in solidification, as the fundamental cause of crack initiation (see the schematic view in figure 12(f)), and further crack propagation could be activated if sufficient interlayer tensile stress is achieved.In summary, the occurrence of cracking in dAMed ceramic oxides manifests a notable level of complexity, determined by diverse factors such as local thermal conditions, microstructure, initial crack sources, and material properties.

Crack suppression methods.
Minimizing crack formation in dAMed ceramic oxides is primarily tackled through two fundamental strategies: (1) reducing thermal stresses in laser processing and (2) engineering the microstructure/composition of the ceramic components to enhance their cracking resistance.To achieve these, researchers have developed a range of crack suppression methods, including processing parameters optimization, field-assisted devices implementation, and secondary phase doping.
Processing parameter optimization serves as a convenient and fundamental approach to control cracks via modifying both local thermal conditions and microstructures [182].Also, it lays the groundwork for further implementing other strategies to suppress defects, such as field-assisted dAM techniques and secondary-phase toughening [77].The impact of laser energy on the cracking behavior of dAMed ceramics has been thoroughly investigated in [34,70,71,89,174] for both L-PBF and L-DED techniques.These studies demonstrated that judiciously increasing laser power is advantageous for crack suppression (figures 13(a) and (b)).This is attributed to the diminished thermal tensile stress during processing, and a reduction of lack-of-fusion (LOF) defects that could potentially serve as crack nucleation sites.However, exceeding a critical energy threshold led to an undesirably reduced cooling rate during solidification.This, in turn, promotes grain coarsening within the ceramic specimen, compromising its fracture strength and enhancing its susceptibility to crack formation.Likewise, Niu et al [77,78] explored the role of laser scanning speed and layer thickness in crack mitigation.Their work demonstrated that within specific ranges, increased scanning speed and interlayer spacing reduced crack density.This effect stems from faster solidification rates, shorter deposition times, and consequently, grain refinement and diminished impurity segregation in dAMed ceramics.Additionally, reducing energy consumption through these adjustments further minimizes thermal stress.Recently, Su et al [183] highlighted the crucial role of scanning vector length in managing macroscopic cracking in L-DEDed ceramics.By shortening the vector length below 5 mm, they successfully fabricated corner-shaped components without macrocracks (figure 13(c)).In conclusion, these studies confirm the effectiveness of optimizing processing parameters in reducing cracks to a certain extent.However, it is also well acknowledged that relying solely on parameter adjustment is insufficient to achieve defect-free dAMed ceramics.Despite the mitigation of macroscopic cracking, considerable microcracks persist within the printed products.
Fields-assisted AM refers to the introduction of external energy fields such as thermal, magnetic, and acoustic fields in dAM to enhance the product quality.Currently, two main methods are employed in dAMed ceramics: high-temperature preheating and ultrasonic vibration-based methods.The hightemperature preheating method lies in its ability to manipulate thermal gradients within the deposited material.By increasing the ambient temperature and controlling the cooling process, this approach effectively reduces the thermal stress in dAM processing, thereby inhibiting crack initiation and propagation in printed ceramics.Wilkes et al [16] incorporated a secondary CO 2 laser in L-PBF to preheat the ceramic powder bed, as depicted in figure 13(d).Their study revealed that preheating above 1 600 • C could eliminate crack defects in Al 2 O 3 -ZrO 2 ceramics (figure 13(e)), and the resulting defectfree components had a volume of 400 mm 3 .The alternative heating sources were also explored in dAM, such as induction heating [134,142] and microwave heating [186], all of which exhibit effective suppression of cracks.However, the application of high-temperature preheating encounters challenges in practical use.The constrained preheating area results in relatively small ceramic components, potentially falling short of meeting the industrial requirement.Additionally, high preheating temperature leads to excessively large and uncontrollable melt pools, thereby impacting the surface quality of specimens.In terms of ultrasonic-assisted dAM method, the periodic oscillation of ultrasonic waves induces nonlinear effects such as acoustic streaming and transient cavitation in the melt pool.These effects play a crucial role in accelerating melt flow, improving temperature uniformity, and promoting heterogeneous nucleation.Therefore, the incorporation of ultrasonic vibration in dAM contributes to the reduction of thermal stress and the refinement of grain size, effectively suppressing the formation of cracks.Hu et al [70] and Yan et al [33,138] utilized an ultrasonic-assisted L-DED device (figure 13(f)) to produce thin-wall and cubic structures of Al 2 O 3 -ZrO 2 composites.They observed a significant decrease of crack density with the increase of ultrasound power, attributing this improvement to the notable microstructure refinement in the as-printed components after introducing ultrasonic vibration (figure 13(g)).However, the ultrasonic vibration system is typically situated beneath the forming area, and its effectiveness diminishes rapidly as the material deposits layer by layer.Consequently, applying this technology in manufacturing of large-sized ceramic components remains challenging.
In addition to adjusting the external environment in dAM (e.g.laser parameters, external fields), modifying the ceramic composition can also achieve crack suppression effects.This strategy involves introducing foreign materials, categorized as solutes or reinforcing hard particles, into the ceramic matrix.The solute material is completely melted with the primary phase during laser heating, and segregated at solid/liquid front in solidification, ultimately enriching at grain boundaries.The solute segregation leads to large undercooling ahead of growth front, which promotes the grain refinement of the printed ceramics.Also, the solutes enriched at grain boundaries form locally reinforced regions, which effectively hinders the crack propagation.For example, Pappas et al [1] reported that the internal cracks in L-DEDed Al 2 O 3 ceramic were almost eliminated after adding small amount of ZrO 2 solutes (figure 13(h)), and the fracture toughness was increased from 2.6 MPa•m 1/2 for pure Al 2 O 3 to 3.4 MPa•m 1/2 for 10 wt% ZrO 2 -Al 2 O 3 ceramic.Additionally, doping specific solutes could optimize the thermal physical properties of the asprinted products.An illustrative case involves doping TiO 2 in dAMed Al 2 O 3 ceramics, where the reaction between TiO 2 solute and Al 2 O 3 induced the formation of Al 6 Ti 2 O 13 phase at grain boundaries.The Al 6 Ti 2 O 13 phase exhibited a remarkably low TEC of 0.5 × 10 −6 • C −1 .Consequently, the overall TEC of dAMed components decreased from 8 × 10 −6 • C −1 for pure Al 2 O 3 to 2.5 × 10 −6 • C −1 for the TiO 2 doped counterpart.This significantly reduced the thermal stress in solidification, thereby reducing the crack propagation energy.Similarly, Pfeiffer et al [187] introduced the ZrW 2 O 8 phase, a phase with a negative TEC of-4.9 × 10 −6 • C −1 , into L-PBFed Al 2 O 3 ceramics.This led to an extremely low CTE of 5.2 × 10 −8 • C −1 for the modified ceramic component, resulting in a reduced crack density.In contrast to solutes, reinforcing particles typically involve the incorporation of highmelting-point and hard materials.For instance, Wu's group [184,188] introduced SiC and TiC particles into L-DEDed Al 2 O 3 -ZrO 2 ceramics.These hard particles did not undergo melting during laser heating and disperse uniformly in the as-printed components, as shown in figure 13(i).The mismatch in TEC and elastic modulus between the reinforcing phase and the matrix material created a residual stress field around the particles.The interaction of this residual stress with the crack tip induced crack pinning, deflection, and bifurcation (figure 13(j)), thereby enhancing the toughness of dAMed ceramics.While second-phase doping demonstrates significant potential for crack suppression, modifying ceramic compositions can also come with trade-offs, potentially affecting mechanical performance, thermal stability, and chemical stability of the printed products.
Moving beyond crack suppression, Verga et al [185] recently introduced a novel crack self-healing approach for dAMed ceramics.Their strategy involved fabricating 10 vol% TiC-Al 2 O 3 composites using L-PBF followed by postprocessing in air at 900 • C.This exposure triggered the oxidation of TiC, generating TiO 2 as a by-product.Notably, this oxidation product spontaneously migrated and filled fissures within existing cracks (figure 13(k)), effectively repairing the defects.Interestingly, their research revealed exceptional efficacy in healing cracks with widths under 2 µm and lengths reaching several hundred micrometers.A similar phenomenon was also observed by Zhao et al [88,189] during the thermal treatment of Al 2 O 3 -based ceramics, where the capillary action of the high-temperature liquid glass phase facilitated crack filling.[33], © 2017 Elsevier Ltd and Techna Group S.r.l.All rights reserved.(c) Reprinted from [193], © 2023 Elsevier Ltd and Techna Group S.r.l.All rights reserved.(d) Reproduced from [184], with permission from Springer Nature.

Pores formation mechanism.
While less critical than cracks, pores remain another prevalent defect type in dAMed ceramics.Their presence not only compromises the mechanical properties of printed components but also acts as stress concentrators, potentially triggering crack initiation and propagation [164].The pore defects can be broadly categorized as three distinct types for dAMed ceramics: LOF pores, gas pores, and shrinkage pores, each arising from unique formation mechanisms.
The LOF pores display an elongated and irregular morphology (figure 14(a)), and primarily arise from inadequate melting of feedstock powders in dAM due to low energy input [14].This energy deficiency impedes the coalescence of adjacent deposited tracks or layers during solidification, inducing the formation of interstitial LOF voids.Compared to other pore defects, LOF pores are less frequently reported in dAMed ceramics, likely attributed to their ease of avoidance through laser parameter tuning.For example, the studies on Al 2 O 3 and SiO 2 ceramics [107,190] reported cases of LOF defects between the deposited layer and substrate (figure 14(a)), which can be eliminated via either increasing laser power or reducing layer thickness.In contrast to the LOF defect, gas pores are originated from entrapped gas in the ceramic melt, and display a spherical or ellipsoidal shape with a smooth inner wall as shown in figure 14(b).This defect arises due to the insuf-ficient time for the gas bubble to escape from the melt pool before solidification.Notably, the viscosity of ceramic melt is considerably higher than that of metal melt (e.g.Ti ∼ 2.2-5.2mPa•s vs. Al 2 O 3 ∼ 30-135 mPa•s [17,191]).Since viscosity is inversely proportional to bubble floating speed [140], the elevated viscosity in ceramic melt substantially prolongs the time required for bubbles to escape from the liquid.Thus, mitigating gas pore defects seem to be more challenging in dAMed ceramics than metals.Previous research has identified three primary sources of gas pores in dAMed ceramics [39,140,188]: (i) impurity evaporation at high temperature, (ii) residual gas in the feedstock, encompassing both air within the hollow powder and air in the inter-particle gaps, and (iii) entrapment of shielding inert gas in the molten pool.Furthermore, the shrinkage pore defects primarily arise from the excessive volume shrinkage of the solid phase at the final stage of solidification, leaving insufficient time for the liquid to fill the resulting vacancies.These irregular pores are predominantly concentrated in the rapid quenching region at top of dAMed ceramic specimens [140,157,192,193], as shown in figure 14(c).Also, this defect was frequently observed at grain boundary due to inadequate liquid filling (figure 14(d)), and such occurrences can easily lead to the initiation and propagation of intergranular cracks under thermal tensile stress, as documented in previous works [33,142,152].

Pores suppression methods.
To address the various types of pore defects as mentioned above, researchers have explored strategies to effectively mitigate these undesirable features, focusing on controlling process parameters, incorporating refractory particles, and employing ultrasonic vibration assistance.All these approaches have demonstrated effectiveness in reducing porosity in dAMed components.Optimizing dAM parameters plays a crucial role, and increasing LED judiciously is a well-established method for minimizing porosity.For example, Zhang et al [162] reported that increasing laser power could facilitate the complete melting of deposited Al 2 O 3 feedstocks in L-PBF, thereby eliminating LOF pores and boosting overall density.Similarly, Liu et al [142] observed that adjusting LED by increasing laser power or decreasing scanning speed suppressed intergranular shrinkage pores in L-DEDed Al 2 O 3 -ZrO 2 ceramics.Higher LED improved the flowability and filling capacity of the ceramic melt, leading to pore suppression.They identified the optimal parameters for shrinkage pore elimination as laser power exceeding 400 W and scanning speed below 100 µm•s −1 .Wu's group [32,71,87] systematically investigated the relationship between various dAM parameters and porosity (figures 15(a)-(c)).Their findings confirmed that increasing laser power, decreasing scanning speed, and reducing layer thickness effectively suppressed gas pore formation.These adjustments prolonged the molten pool lifetime and decreased its viscosity due to elevated liquid temperature, facilitating gas bubble escape and porosity reduction.However, exceeding critical LED can be counterproductive, leading to an unstable molten pool or excessive material evaporation [190,194].In addition to LED, other parameters such as scanning strategy, feedstock powder size, and substrate material also significantly impact porosity as demonstrated in [16,80,157].Meticulous selection and control of these parameters are also crucial for achieving high-dense dAMed components.
Other than the parameter control, adding refractory particles, such as SiC and TiC, to ceramic feedstocks can also suppress the pore formation in dAM.These particles act as stirring agents within the molten pool, facilitating the escape of gas bubbles and minimizing their entrapment within the printed component.Wu et al [184] demonstrated the remarkable effectiveness of this strategy, eliminating large spherical and ellipsoidal gas pores in dAMed Al 2 O 3 -ZrO 2 ceramics through the addition of SiC particles (figure 15(d)).Notably, incorporating 20 wt% SiC particles reduced porosity from 11.7% to a mere 0.2% (figure 15(e)).Similarly, their another work with TiC particles [188] also achieved a significant decrease in porosity, from 6.37% to 0.29%.However, it is crucial to note that exceeding the optimal amount of carbide additives can be counterproductive.As Ur Rehman et al [163,195] observed, excessive carbide additions would intensify their chemical reactions with ceramic oxides, generating gaseous byproducts like CO and SiO.This paradoxically increased porosity instead of reducing it.
Furthermore, ultrasonic vibration assistance emerges as another effective method for mitigating pore defects.This approach capitalizes on the dual benefits of ultrasonic vibration: (i) inducing stirring and crushing effects within the melt, which facilitate gas expulsion, and (ii) reducing melt viscosity, therefore enhancing the gas bubble escape velocity.Building upon these suppression mechanisms, Yan et al [33,140] successfully fabricated Al 2 O 3 -ZrO 2 ceramic specimens with an exceptionally low porosity of 0.1% using an ultrasonic-assisted dAM device.The highly dense structure of the ceramic specimen is evident in figure 15(f).

In-situ monitoring and simulation
Compared to metallic materials, ceramic oxides present unique challenges for dAM due to their high melting points and low thermal conductivity.These intrinsic properties necessitate higher processing temperatures and lead to more intensive heat accumulation.This can cause problems like melt pool instability, excessive evaporation, significant thermal stress, and cracking during the process.These risks emphasize the critical importance of rigorous process control when dAMing ceramic oxides.Two key methods for achieving efficient and reliable process control in dAM are in-situ monitoring and simulation.In-situ monitoring captures real-time data on crucial process features like temperature, cooling rate, melt pool morphology, defects, and elemental composition.This real-time feedback loop allows for timely detection and response to potential quality issues, preventing them from jeopardizing the final product.Simulation modeling, on the other hand, utilizes advanced numerical models to virtually replicate the entire dAM process, including melt pool dynamics, material solidification, temperature distribution, and stress-strain development.By analyzing the simulation data, researchers can optimize dAM parameters to minimize defect formation and improve forming accuracy.In essence, both in-situ monitoring and simulation play an integral role in ensuring the controllability and stability of ceramic components, ultimately leading to enhanced forming efficiency and product quality.

4.4.1.
In-situ monitoring in dAM process.In-situ monitoring has achieved maturity in dAM for metals, but its application to ceramic oxides remains in its infancy.Several challenges hinder this advancement, the high melting points of ceramics pose thermal constraints for certain sensors, impeding their reliable operation due to extreme environments.Additionally, the optical properties of ceramic oxides, including transparency and high reflectivity, present obstacles for accurate monitoring data acquisition.Current monitoring efforts for ceramic oxides primarily focus on real-time temperature, molten pool state and defects formation, employing predominantly non-contact sensors like photodiodes, dualcolor pyrometers, high-speed cameras, and infrared thermography.It is noteworthy that the application of in-situ monitoring technology in this field is currently more pronounced in L-PBF compared to L-DED.This discrepancy may arise from the simultaneous deposition and melting process in L-DED, posing challenges in capturing real-time, layer-wise information.Temperature stands as the most crucial variable for process monitoring and control in dAMed ceramics.Hagedorn et al [196,197] exemplified the potential of in-situ monitoring in closed-loop temperature control in L-PBFed Al 2 O 3 -ZrO 2 ceramics.By integrating infrared thermography and dual-color pyrometry, they implemented real-time control of the preheating temperature on powder bed surface, and the heat map of ceramic processing region was captured as shown in figure 16(a).Likewise, Liu et al [97] employed infrared thermography to capture the surface temperature of ZrO 2 powder bed under high-temperature preheating (figure 16(b)).Their observations suggested that employing a preheating temperature over 2 000 • C could effectively reduce the laser energy input, requiring only a ∼100 • C increase for ceramic powder melting.In addition to directly observing temperature, the fluctuation patterns in temperature signal can also serve as indicators of the molten pool state.Qian et al [198] investigated effect of laser energy on surface temperature using a high-speed pyrometer and spectrometer.Their findings revealed the surface temperature fluctuation decreased with the energy input, indicating a more stable melting process.However, an opposite trend was reported from the work in [199] with the use of a novel multi-photodetector method.This method overcame the measurement deviations introduced from single-photodiode setup, providing an accurate depiction of the thermal state.The fluctuation range of collected photodiode signal increased with the laser power (figure 16(c)), indicating a less stable molten pool at high laser power, and this aligns well with their SEM observations.High-speed cameras have been frequently utilized in dAM for recording the molten pool/solidification dynamics during laser scanning.For instance, the in-situ observations conducted by Wilkes et al [16] using a high-speed camera revealed the morphology and flow patterns of Al 2 O 3 -ZrO 2 ceramic melt pools under laser preheating.The results showed that high preheating temperatures caused significant deviations of the molten pool from its intended motion trajectory due to its increased volume (figure 16(d)).Notably, these deviations were identified as a primary cause of the poor surface quality in final components.Qian et al [198] utilized off-axial high-speed cameras to monitor the time-evolution of molten pool under different energy inputs, demonstrating its power in tracking defects formation in dAMed components figure 16(e).Florio et al [26] conducted in-situ observations of the melt dynamic behavior in absorber-doped Al 2 O 3 , employing an external laser illumination for image data acquisition.They identified the appearance of denudation zone around the molten pool (figure 16(f)) and found its formation is due to convective flow in the melt, causing nearby powder particles to be attracted into the molten pool.This explains the formation of geometrical defects in consolidated part.Moreover, Moniz et al [173] investigated the powder splattering and shrinkage evolution in L-PBF under varying laser inputs.Significant powder ejection was observed at lower energy density, with the melt pool gradually stabilizing as laser energy increased.The collective findings above demonstrate that high-speed cameras have the potential in achieving control over forming defects and dimensional accuracy of dAMed ceramics.
Recently, the application of high-energy synchrotron x-ray imaging has garnered significant attention for tracking the melting process for dAMed ceramic oxides.This technology provides enhanced penetration power and spatiotemporal resolution, allowing for detailed examination of the internal structures and dynamics of molten pool.For instance, Makowska et al [190] achieved a stunning 3D visualization of the Al 2 O 3 melt through operando x-ray computed tomography, as depicted in figure 16(g).They observed, in real-time, the influence of laser power on balling, powder denudation, and pore formation.Their findings suggested that high energy input could eliminate balling, thereby improving the surface quality of specimens.However, they noted that powder denudation became more severe at the same time.Additionally, high energy input was found to effectively suppress LOF defects but simultaneously increased the occurrence of gas pores.Similarly, Leung et al [169] utilized synchrotron x-ray imaging device to illuminate the transient dynamic behavior of SiO 2 melt doped with absorption additives (figure 16(h)).Their in-situ observations revealed a sequence of physical phenomena in melted tracks, including cavity formation, plume generation, particle splattering, and the formation and collapse of bubbles.Notably, their study provided compelling evidence for the inhibitory effect of second-phase doping on pore formation and warping deformation in SiO 2 ceramics.In essence, high-energy synchrotron x-ray imaging offers a powerful tool for elucidating the intricate melt flow behavior and defect formation mechanisms in dAM of ceramics.This detailed in-situ visualization provides valuable insights for guiding the development of strategies to optimize process parameters and ultimately enhance product quality.

Simulation in dAM process.
While real-time monitoring offers valuable insights into dAM process, computational simulation provides a powerful tool for unraveling the intricate interplay of physical phenomena involved, such as laser absorption, heat and mass transfer, fluid flow, phase change, non-equilibrium solidification, and cyclic heating/cooling.It also circumvents limitations of real-time monitoring, such as instability, low resolution, and restricted access, providing valuable support for optimizing processing parameters and designing dAM structures.However, compared to metals, the simulation of a dAM process for ceramic oxides is still in its infancy.The major challenge is the lack of comprehensive datasets for ceramics, including their optical and thermophysical properties in both the solid and molten states, as well as their local variations with temperature and chemical composition.This scarcity of data significantly hinders the ability of simulation models to accurately capture the intricate interplay of laser-material interaction, heat transfer and molten pool dynamics in dAM, ultimately compromising the simulation accuracy and complicating the experimental validation of these models.Despite the challenge, the past decade has yielded encouraging advancements in dAM process simulation for ceramic oxides, and researchers have primarily directed their efforts towards developing two key types of models: macroscale and mesoscale.
Developing macroscale model enables a systematic examination of temperature distribution and stress-strain states in ceramic components during dAM processing.Abdelmoula et al [75] and Zhang et al [200] developed a 3D FEM model using ANSYS software for demonstrating the temperature field in L-PBFed single layer of Al 2 O 3 ceramics (figure 17(a)).Their models treated the Al 2 O 3 powder bed as a uniform continuum, accounting for temperature-dependent material properties.Through analyzing the influence of laser power, scan speed, and scanning strategy on simulated temperature distributions, the optimal laser parameters for single-layers shaping can be determined.Huang et al [174,182] further advanced the understanding of dAM processes by developing a thermal-mechanical coupled FEM model to analyze the dynamic temperature evolution and thermal stress distribution in L-DEDed Al 2 O 3 -TiO 2 thin-walled structures (figures 17(b) and (c)).This enabled them to elucidate the crack formation mechanism in dAMed specimens, revealing a strong correlation between crack patterns, local thermal stress, and microstructure.Notably, their study suggested that increasing laser scanning speed significantly reduced thermal stress, thereby suppressing crack formation.Similarly, Shen et al [27,132,201] also conducted the thermal-mechanical simulation for dAMed eutectic oxides.After analyzing the thermal stress distribution at different deposition layers (figure 17(d)), they unraveled the crack initiation and propagation mechanisms in ceramic shaping.Specifically, they found that the longitudinal cracks propagated along the material deposition direction were closely linked to the abrupt change in maximum principal stress at initial deposition stage.Further, the gradually increasing maximum principal stress promoted the formation of transverse cracks and their extension along the melt pool boundaries.Also, their simulation work determined the optimal preheating strategy for L-DEDed ceramics for reducing the cracks after comparing the maximum principal stress level at different preheating conditions.These studies demonstrate the effectiveness of macroscale models in elucidating temperature and stress-strain field evolution in dAMed ceramics.It not only reveals the intricate mechanisms governing thermal crack formation, but also provides guidance for the selection of processing parameters and defect control strategies.
In contrast to macroscale models, mesoscale numerical simulations focused more on the intricate dynamics of individual molten pools in dAM processing, aiming at characterizing their transient temperature distribution, morphology, liquid flow behavior, and solidification conditions.For example, Fan et al [61] developed a thermal FEM model to capture the molten pool features in L-PBFed Al 2 O 3 ceramics (figure 17(e)).Their results showed that higher LED led to an increased temperature and geometrical size of the molten pool, while decreasing the temperature gradient, solidification rate, and cooling rate at the solid/liquid interface.In their follow-up study [58], a similar model was used to elucidate the microstructure evolution mechanism in dAMed eutectic oxide.They found that the evolution of ceramic microstructure along the building direction was mainly controlled by the local temperature gradient and solidification rate at the molten pool interface.Chen et al [35,202] developed a thermo-fluid FEM model for further characterizing the dynamic process of Al 2 O 3 melt in L-PBF (figure 17(f)).Their model used the level set method to track the gas/liquid interface of the molten pool, and simulated the powder melting and shrinkage process based on the compressible Newtonian law.Also, the model assumed that the laser energy distribution and penetration followed the Beer-Lamberts law.The simulation results demonstrated that the material absorption coefficient and Marangoni convection had a significant impact on the temperature distribution and size of the molten pool, while the morphology of the solidified track was closely related to the viscosity and surface tension of the ceramic melt.Moreover, Moniz et al [194] used the same model to analyze the molten pool states of Al 2 O 3 ceramics at different L-PBF parameters.They observed that the molten pool tended to exhibit a discontinuous morphology at high scan speeds (figure 17(g)), and this was attributed to insufficient melting of the powder bed due to lower energy input, thus leading to the spheroidization (or balling) of the melt.In sum, the above-mentioned studies demonstrate that mesoscale models have the potential to guide the control over melt pool morphology and microstructure, contributing to an improved dimensional accuracy, forming quality, and mechanical performance of dAMed ceramics.

Conclusion and outlook
dAM has emerged as a promising technology for the onestep fabrication of complex ceramic oxide components, characterized by dense melt-grown structures.The dAM techniques also offer significant advantages such as near-net shaping, high material utilization, and mold-free production.Owing to above-mentioned features in dAM, and the excellent properties of ceramic oxides themselves, such as low specific material density, high strength, high temperature stability, and biocompatibility, the dAMed ceramic parts can find applications across various industries, such as aerospace, biomedical, and automotive.This is particularly advantageous for functional components that necessitate the ability to withstand certain mechanical loads.Over the past decade, researchers have undertaken systematic investigations into various dAMed ceramic oxides.These studies have validated the feasibility of dAM in producing high-density ceramic components with geometric complexity.However, a significant gap remains between dAMed ceramic oxides and their translation into practical engineering applications.This discrepancy arises from the interplay between the extreme processing conditions in dAM and the intrinsic properties of ceramics themselves.In this context, the present work delves into the latest advancements in dAM processing of ceramic oxides, focusing on two prominent technologies: L-PBF and L-DED.We systematically examine prior research on the forming quality, microstructure, and mechanical properties of various ceramic oxides, while highlighting key breakthroughs in critical areas like feedstock preparation, laser-material coupling, defect formation and control, and in-situ monitoring and simulation.The main conclusions of this review are drawn as follows: (1) In the past decade, researchers have systematically explored dAM feasibility for producing various ceramic oxides, including the advanced ceramics like Al 2 O 3 and ZrO 2 , and the traditional ceramics such as SiO 2 -based amorphous materials.Notably, there is a rising focus on ceramic composites like Al 2 O 3 -based eutectic oxides in current dAM research.(2) In contrast to its mature use in metals, dAM for ceramic oxides is still in early stages.While dAM enables the fabrication of intricate and dense ceramic components, the large thermal stresses produced in process limit the maximum product size and contribute to defect formation.Also, despite some studies suggesting dAM-processed ceramics match traditional sintered counterparts in macroscopic mechanical performance, most investigations highlight a persistent performance gap.(3) The microstructure of dAMed ceramic oxides notably differs from those produced by traditional sintering.Unlike the equiaxial grain structure formed in the sintering, dAMed ceramic oxides experience complete melting, leading to the epitaxial growth of large columnar grains along the deposition direction in most cases.This microstructural feature imparts anisotropic properties to the ceramic specimens, and makes them more prone to defects formation.(4) Compared to single-phase ceramics, the utilization of dAM in fabricating multiphase ceramics appears more appealing and promising.The introduction of a second phase effectively refines the microstructure and reinforces grain boundaries, leading to significantly improved fracture resistance in dAMed components.(5) Crack formation, intrinsic to ceramics and amplified by harsh thermal environment in dAM, remains the major obstacle to the use of dAM for manufacturing ceramics.While promising techniques like parameter tuning, second-phase doping and introduction of external fields have demonstrated some success in crack mitigation, their limitations necessitate further research in suppression strategies for wider adoption of dAM in ceramic oxides.(6) The critical issue of low energy absorption in preparing ceramic oxide using commercial dAM equipment has been well addressed in the recent decade.This was achieved by the addition of absorption aids and the adjustment of laser types, which greatly improved energy absorption and promoted the densification of ceramic materials during the forming process.(7) Notable advancements have been achieved in recent years within the realms of feedstocks preparation, realtime monitoring, and simulation of the dAM process.These advances have played a key role in controlling defects, optimizing dimensional accuracy, and enhancing the overall properties of dAMed ceramic components.
Nevertheless, there are plenty of opportunities for ongoing development in the future.
Drawing upon the findings, we propose the following perspectives to propel the continued advancement and real-world implementation of dAM techniques for ceramic oxides: (1) The dAM equipment tailored for ceramic oxides: current commercial L-PBF/L-DED equipment, optimized for metals, often falls short in producing high-quality ceramic oxide components.This necessitates the development or adaptation of the dAM equipment.This involves the optimization of laser sources, where careful design considerations for wavelength, spot size, and energy distributions are made to maximize energy absorption and minimize thermal stress based on specific oxides properties.Also, addressing solidification defects is imperative, and this may be achieved through updated preheating and ultrasonic vibration assistance, as well as the incorporation of multi-field assisted technologies.Another perspective for further development is the integration of dAM with subtractive manufacturing technologies, such as employing milling and grinding to address surface roughness and other finishing concerns.The implementation of advanced online monitoring and feedback systems is also crucial, involving the integration of real-time process monitoring and adaptive control to optimize the quality and consistency of dAM-produced components.(2) Material design in dAMed ceramic oxides: recent research in dAM of ceramic oxides has yielded effective material design approaches like solute doping, reinforcing particle incorporation, and eutectic ratio manipulation.These strategies refine microstructures, enhance crack resistance, and strengthen the material.However, compared to metal alloy design, compositional research in dAMed oxides remains in its infancy and often relies on empirical exploration.Moreover, existing strategies offer limited quality improvement, especially in mitigating cracking for large components.Therefore, future efforts should prioritize the development of dAM-specific material design theories for ceramics, considering both their intrinsic properties and solidification behavior.Beyond material knowledge-based approaches, incorporating cutting-edge machine learning techniques may offer exciting potential for accelerating material design with precisely targeted properties.Furthermore, the intrinsic layer-by-layer nature of dAM provides a unique opportunity to manipulate the composition and structure of the material locally.This manipulation offers exciting avenues for designing and developing novel oxides-based functional gradient materials (FGMs).Illustrative instances of FGM design schemes can be found in the recent groundbreaking work conducted by Wu's research team [203][204][205], exemplified by Al 2 O 3 /ZrO 2 and Al 2 O 3 /TiC systems.(3) Solidification mechanism in dAMed ceramic oxides: limited insight into grain formation mechanisms during dAM of ceramic oxides hinders optimal control of their microstructure and performance.Unlike traditional sintering processes, dAM involves complete melting and solidification, complicating the application of existing theories.Therefore, it is necessary to enhance the understanding of solidification mechanism by integrating classical solidification models with experimental validation, and this will elucidate the intricate relationships between processing parameters, solidification conditions, and the resulting microstructure of dAMed oxides.Besides, microscale FEM models should be established for demonstrating the nucleation and epitaxial growth behaviors of oxides in dAM, aiming at understanding their microstructure formation mechanism under non-equilibrium solidification conditions.Moreover, systematic thermophysical property testing of different oxide ceramics is required to improve the prediction accuracy of microscale models.

Figure 1 .
Figure 1.Research trends in dAM of ceramic oxides reflected by the publication numbers over time.
(a)) exhibited a typical columnar cell/dendrite structure (figure 6(b)), with ZrO 2 solutes accumulating at Al 2 O 3 grain boundaries, as confirmed by EDS

Figure 5 .
Figure 5.The influences of processing parameters on parts quality and microstructure of dAMed Al 2 O 3 ceramics: (a) microstructure of L-PBFed Al 2 O 3 single-tracks produced under different LEDs [61].Reprinted from [61], © 2018 Elsevier Ltd and Techna Group S.r.l.All rights reserved.(b) Geometrical morphology of L-PBFed Al 2 O 3 cubic samples using different scanning strategies [75].Reproduced from [75].CC BY 4.0.(c) defects distribution in L-DEDed Al 2 O 3 thin wall structure prepared using different scanning speeds [77].Adapted from [77], with permission from Springer Nature.
[105] employed L-PBF with a 200 W CO 2 laser and powder bed preheating up to 400 • C to fabricate SiO 2 casting molds (figure8(a)).These parameters resulted in partial melting of the SiO 2 feedstock (figure 8(b)), leading to high porosity

Table 2 .
Microstructure and mechanical performance of dAMed Al 2 O 3 -based ceramics.

Table 3 .
Microstructure and mechanical performance of dAMed ZrO 2 ceramics.

Table 4 .
Microstructure and mechanical performance of dAMed eutectic oxides.

Table 5 .
The dopant compositions and the corresponding laser absorption rates of doped ceramics in dAM.