Influence of sol–gel derived ZrO2 and ZrC additions on microstructure and properties of ZrB2 composites
Introduction
Ultra High Temperature Ceramics (UHTCs) are materials with melting points exceeding 3000 °C, due to strong covalent bonds that yield excellent hardness and thermo-mechanical strength. ZrB2 materials are candidates for extreme environments such as solar receivers, nuclear energy, Sharp Hot Structures (SHS) or Wing Leading Edges (WLEs) for hypersonic flight (>2000 °C).1, 2, 3 A major processing challenge is attaining high-density without incorporating low melting point sintering additives, or metals that depreciate thermal performance.4, 5 A common example of a highly refractory composition ZrB2–SiC. Addition of SiC improves oxidation resistance up to 1800 °C, but composites may still have limited service temperatures due to a eutectic at 2270 °C.6, 7 These composites are also viable because SiC-precursor technology is mature in fibre, whisker, solution/polymer form, whereas UHTC carbides – and particularly diborides – are still emerging.8, 9 There have been several powder UHTC compositions with very high thermal stability such as NbB2–NbC, ZrB2–TiC and ZrB2–ZrC, which have been densified by B and C diffusion without the addition of metals or silicides.10, 11, 12 In developing high temperature oxidation resistance, there is potential for ZrC oxidation to provide protection at higher temperatures not achievable by SiC.13, 14, 15 However, the composition and microstructure must be carefully controlled; the synergy is focused on improving the intrinsic oxidation resistance of diboride materials. A small vol% UHTC phase dispersed in a powder could provide both improvement in densification and sacrificial oxidation. Previous work on ZrB2 composites reported successful sintering of a ZrB2–ZrC composite, without the milling process that introduces contaminants that can adversely affect other properties. Instead, the powder had a 5 mol% (4.6 wt%) nano-ZrC coating onto the surface of ZrB2 derived from a sol–gel precursor.16 Until pre-polymer UHTC precursors become commercially available, sol–gel is a useful alternative. Control of the decomposition of the carbon precursor – polyfurfuryl alcohol – in sol–gel was not only crucial in the formation of ZrC, but was also used to remove the surface oxide films on ZrB2.16 The differences in properties caused by changing the carbon content were not explored in detail.
Non-oxide ceramics can use a small amount of carbon as a prudent sintering additive if refractory performance is important. Carbon can also improve toughness in boride ceramics through microcracking.17 It can be introduced through other polymer precursors, such as phenolic resin, sucrose and chitosan, which can homogenously coat ceramic powders.18, 19, 20 The decomposition (also called pyrolysis or carbonisation) to carbon must be controlled to ensure subsequent densification is not adversely affected. Control at the precursor stage requires understanding of chemical reactions, polymer pyrolysis and empirical knowledge of carbon yield. After carbon is formed, the interaction of carbon with particles, phase boundaries, and the graphitization process must be considered.21, 22 The processing of carbon and nano-oxides on the surface of ZrB2 powders is the major focus of this paper. The variable carbon yield in this system produces two contrasting phases and morphologies: ZrB2 with 5 mol% ZrO2 or ZrB2 with 5 mol% ZrC + C. This work examines in detail the microstructure of both compositions and briefly explores their properties.
Section snippets
Powder preparation
The powder-gel process is shown schematically in Fig. 1. A block co-polymer surfactant P123 (Sigma–Aldrich) was dissolved in 99.7% ethanol. Zirconium n-propoxide (ZNP) (Sigma–Aldrich) 70 wt% in 1-propanol was chelated with acetylacetone (AcAc). The composition ratio of P123:EtOH:AcAc:ZNP in the sol was 0.05:40:1:1, yielding a slightly yellow sol. Furfuryl alcohol (C5H6O2) (FA) (Sigma–Aldrich) was added at an FA:Zr molar ratio of at least 4:1 turning the sol an intense yellow. This theoretically
Sol–gel coated powders
We previously reported a sol–gel process to produce nano-ZrC particles using ZNP and FA as the respective oxide and carbon precursors.15 Using a surfactant, the gel precursor was fired at 550 °C. This decomposed PFA to amorphous carbon and crystallized 4 nm ZrO2. Heat treatment at 1450 °C in Ar yielded ∼50 nm ZrC nanoparticles, which typically formed micron size agglomerates, particularly if residual carbon was present.34, 35 These needed to be dispersed in a composite, so the sol–gel process was
Conclusions
A sol–gel derived nanoparticle coating can be applied on ZrB2 powders to form <200 nm ZrC on the surface of ZrB2. These can be formed into dense composites by SPS. Carbon is crucial in the evolution of the reinforcing phase. Excess carbon guarantees formation of very fine ZrC nanoparticles due to carbon preventing their coalescence. If carbon is insufficient the carbothermal reduction cannot complete and ZrB2 is coated with ∼300 nm nano-ZrO2 particles. Densification is still assisted by
Acknowledgements
The authors would like to thank Sonya Slater (DSTO), Daniel Curtis (Monash) and Don Rodrigo (Monash) for technical assistance and discussion. The authors acknowledge the use of facilities within the Monash Centre for Electron Microscopy (MCEM), DSTO Aerospace Division and CSIRO Process Science and Engineering. This research used equipment funded by the following Australian Research Council LIEF grants LE0882821 and RIEFP99, and the CoE for Design in Light Metals.
References (45)
- et al.
High temperature oxidation of ZrC–20%MoSi2 in air for future solar receivers
Sol Energy Mater Sol Cells
(2011) The addition of SiC particles into a MoSi2-doped ZrB2 matrix: effects on densification, microstructure and thermo-physical properties
Mater Chem Phys
(2009)- et al.
ZrB2-ceramic toughened by refractory metal Nb prepared by hot-pressing
Mater Des
(2010) - et al.
Current issues in recrystallization: a review
Mater Sci Eng A
(1997) - et al.
Oxidation behavior of zirconium diboride-silicon carbide at 1800 °C
Scr Mater
(2007) - et al.
Fabrication of SPS compacts from NbC-NbB2 powder mixtures synthesized by the MA-SHS in air process
J Alloys Compd
(2006) - et al.
Effect of carbon and titanium carbide on sintering behaviour of zirconium diboride
J Alloys Compd
(2008) - et al.
Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C
Ceram Int
(2013) - et al.
Microstructure and ablation properties of zirconium carbide doped carbon/carbon composites
Carbon
(2010) - et al.
Oxidation of ZrC–30 vol% SiC composite in air from low to ultrahigh temperature
J Eur Ceram Soc
(2012)
Modification of ZrB2 powders by a sol–gel ZrC precursor – a new approach for ultra high temperature ceramic composites
J Asian Ceram Soc
Pressureless sintering of carbon-coated zirconium diboride powders
Mater Sci Eng A
Role of some technological parameters during carburizing titanium dioxide
J Eur Ceram Soc
Overcoming the barrier to graphitization in a polymer-derived nanoporous carbon
Carbon
Microstructural characterization of spark plasma sintered boron carbide ceramics
Ceram Int
Ultra-high temperature HfB2–SiC ceramics consolidated by hot-pressing and spark plasma sintering
J Alloys Compd
In situ formation of ZrB2–ZrO2 ultra-high-temperature ceramic composites from high-energy ball-milled ZrB2 powders
J Alloys Compd
Synthesis of zirconium carbide powders using chitosan as carbon source
Ceram Int
Solid state reaction of zirconia with carbon
Solid State Ionics
In situ synthesis and sintering of ZrB2 porous ceramics by the spark plasma sintering–reactive synthesis (SPS–RS) method
Int J Refract Met Hard Mater
Effect of the addition of silicon nitride on sintering behaviour and microstructure of zirconium diboride
Scr Mater
Advances in microstructure and mechanical properties of zirconium diboride based ceramics
Mater Sci Eng A
Cited by (18)
Mechanical and thermal properties of densified ZrC<inf>x</inf> (x = 0.5, 0.7 and 1.0) ceramics
2024, Journal of the European Ceramic SocietyProcessing and properties of reactively densified TiB<inf>2</inf>-AlN-hBN conductive ceramics with tunable compositions
2023, Journal of the European Ceramic SocietyEffect of residual carbon on the phase transformation and microstructure evolution of alumina-mullite fibers prepared by sol-gel method
2023, Journal of the European Ceramic SocietyMultiscale simulation of elastic response and residual stress for ceramic particle reinforced composites
2022, Ceramics InternationalCitation Excerpt :ZrB2 is widely used as typical ultra-high temperature ceramic materials (UHTCs) in extreme environments [1]. The effective improvement of the sinterability [2] and high temperature oxidation [3] of such ceramic materials has received a significant research attention owing to their high temperature (above 3000°C) [4] with additives such as SiC [5–7], and ZrC [8,9] to obtain the multiphase ceramic composites [10–12]. In particular, ZrB2 matrix composites reinforced with the dispersed SiC particles have attracted widespread attention due to their excellent comprehensive properties [13–15].
Influence of dispersion method of LaF<inf>3</inf> in ZrB<inf>2</inf>-based ceramics on high-temperature oxidation resistance
2021, Ceramics InternationalCitation Excerpt :The introduction of additives by coating method can optimize the composite structures and improve their performances [19,20,22,23]. For instance, several researchers have employed coating technologies, e.g., co-precipitation method and sol–gel method, to prepare coated composites to improve the oxidation resistance of monolithic ZrB2-based ceramics [18,19,24]. However, the mechanism of improved oxidation resistance of ZrB2-based ceramics, prepared by coating powders, has rarely been studied [18,25].
A CrSi<inf>2</inf>-HfB<inf>2</inf>-SiC coating providing oxidation and ablation protection over 1973 K for SiC coated C/C composites
2020, Corrosion ScienceCitation Excerpt :The elements which can produce high melting point oxides after oxidation are good choices to be introduced into SiC ceramic coatings and modify them, so that the viscosity of the glass layer can be increased, thermal stability of coatings can be improved, and oxidation protective time of coatings can be prolonged in ultra-high temperature environment. Ultra-high temperature ceramics (UHTCs) [10–12] are a kind of thermal protection material which can maintain good chemical and physical stability in the ultra-high temperature environment above 2273 K. And UHTCs have been widely used in supersonic flight (air temperature above 1673 K), atmospheric reentry (in oxygen and nitrogen environment, above 2273 K), rocket engine (in chemical reaction atmosphere, above 3273 K) and other extreme environments [13,14] because of their high melting point (>3273 K), high specific strength, high thermal conductivity, high conductivity and corrosion resistance. UHTCs mainly include carbides and borides of transition metals Zr, Ta and Hf.