Role of the grain-boundary phase on the elevated-temperature strength, toughness, fatigue and creep resistance of silicon carbide sintered with Al, B and C

doi:10.1016/S1359-6454(00)00246-9

Acta Materialia

Volume 48, Issues 18–19, 1 December 2000, Pages 4599-4608

https://doi.org/10.1016/S1359-6454(00)00246-9 Get rights and content

Abstract

The high-temperature mechanical properties, specifically strength, fracture toughness, cyclic fatigue-crack growth and creep behavior, of an in situ toughened silicon carbide, with Al, B and C sintering additives (ABC-SiC), have been examined at temperatures from ambient to 1500°C with the objective of characterizing the role of the grain-boundary film/phase. It was found that the high strength, cyclic fatigue resistance and particularly the fracture toughness displayed by ABC-SiC at ambient temperatures was not severely compromised at elevated temperatures; indeed, the fatigue-crack growth properties up to 1300°C were essentially identical to those at 25°C, whereas resistance to creep deformation was superior to published results on silicon nitride ceramics. Mechanistically, the damage and shielding mechanisms governing cyclic fatigue-crack advance were essentially unchanged between ∼25°C and 1300°C, involving a mutual competition between intergranular cracking ahead of the crack tip and interlocking grain bridging in the crack wake. Moreover, creep deformation was not apparent below ∼1400°C, and involved grain-boundary sliding accommodated by diffusion along the interfaces between the grain-boundary film and SiC grains, with little evidence of cavitation. Such unusually good high-temperature properties in ABC-SiC are attributed to crystallization of the grain-boundary amorphous phase, which can occur either in situ, due to the prolonged thermal exposure associated with high-temperature fatigue and creep tests, or by prior heat treatment. Moreover, the presence of the crystallized grain-boundary phase did not degrade subsequent ambient-temperature mechanical properties; in fact, the strength, toughness and fatigue properties at 25°C were increased slightly.

Introduction

As a high-temperature structural material, silicon carbide (SiC) ceramics offer many advantages, including high melting temperatures, low density, high elastic modulus and strength, and good resistance to creep, oxidation and wear. This combination of properties makes SiC a promising candidate for use in such applications as gas turbines, piston engines and heat exchangers [1], [2], where load-bearing components are required to be subjected to temperatures up to 1500°C for extended periods of time; however, its application to date has been severely limited by its poor fracture toughness properties.

The low inherent fracture toughness of conventional SiC ceramics (K_c is typically ∼2–3 MPa m^1/2) can be improved by several processing and reinforcement routes. One approach is to produce a composite, typically accomplished by incorporating continuous fibers, whiskers or platelets, or second-phase particles [3]. Critical to composites, besides the high fabrication costs, is tailoring the matrix/second-phase interface to be weak enough to provide pull-out without sacrificing strength of the material; moreover, this interface integrity must be maintained at elevated temperatures. For monolithic ceramics, in situ toughening can also be effective, as has been demonstrated in the silicon nitride (Si₃N₄) system (e.g., [4]).

Using the latter approach to achieve high toughness in these inherently brittle materials, grains are purposely elongated by processing in the presence of a liquid phase. This results in a microstructure consisting of grains covered with a residual glassy film, with pockets of glass at multiple-grain junctions. Toughness values greater than 10 MPa m^1/2 have been achieved for Si₃N₄ processed in this fashion; such high toughnesses are generally attributed to intergranular fracture and resulting elastic bridging and frictional pull-out as the interlocking grains separate [4]. The problem for monolithic ceramics is that although the glassy grain-boundary film is considered to be critical in inducing good low-temperature toughness, its presence typically limits properties at high temperatures, such as oxidation, creep resistance and strength.

The origin of the glassy film is associated with the presence of sintering additives. Due to the nature of the bonding, as well as vapor pressures, surface and grain-boundary characteristics make densification in ceramic systems difficult without such additions. These additives promote mass transport either in the solid state or by a liquid-phase formation. While many have been employed in processing SiC, the most common are B and C [5], Al₂O₃ [6], Al₂O₃ and Y₂O₃ [7]. However, in an attempt to avoid the tradeoff between low-temperature toughness and high-temperature strength, recent research has focused on a monolithic SiC which is hot-pressed with additions of Al metal as well as B and C. This material, which has been termed ABC-SiC, has been shown to have an ambient-temperature fracture toughness as high as 9 MPa m^1/2 with strengths of ∼650 MPa [8], mechanical properties that are among the highest reported for SiC. The high toughness, which should be compared with that of a commercial SiC (∼2.5 MPa m^1/2), has been attributed to various crack-bridging processes in the crack wake resulting from the intergranular crack path [9]; specifically, crack-tip shielding from both elastic bridging and frictional pull-out of the grains provides a major contribution, with the frictional pull-out component likely to be the most potent.

In situ toughening in SiC results from the large, elongated, plate-like grains that are brought about by exploiting the β-SiC (cubic) to α-SiC (hexagonal) phase transformation. This is akin to the mechanisms in the Si₃N₄ system where acicular, needle-like, grains are obtained under appropriate processing conditions [10]. The microstructure of ABC-SiC consists of such elongated grains with an aspect ratio of ca. 3 to 7. These plate-like grains exhibit an interlocking network, and are surrounded (in the as-processed state) by a thin (∼1 nm) amorphous grain-boundary film that promotes intergranular fracture; strips of crystalline phases, identified as Al₈B₄C₇, Al₄O₄C and Al₂O₃, are also present at grain-boundary triple points.

The sintering additives function in several capacities in in situ toughened SiC ceramics. In addition to promoting densification, they also accelerate the β (cubic) to α (hexagonal) phase transformation at temperatures below 1800°C. Several other additive systems, including Al₂O₃ [6] and yttrium aluminum garnet (YAG), also lead to an improved resistance to fracture in SiC; however, these have not as yet resulted in a ceramic with a toughness as high as that of ABC-SiC [7], [11], [12].

The sintering additives in SiC and Si₃N₄ are effective densification aids, but the liquid tends to remain as glassy phases at grain boundaries and multiple-grain junctions [13], [14], degrading the high-temperature properties. Strength degradation in Si₃N₄ is generally attributed to softening of the grain-boundary phase [15]. Similarly, the creep rates of ceramics possessing a glassy grain-boundary phase are degraded compared with the inherent creep resistance [16]. The presence of impurities, introduced as sintering additives, often also reduces the oxidation resistance of Si₃N₄ and SiC [17].

Since the deleterious effects of the grain-boundary and secondary phases on the high-temperature properties are well documented, several approaches have been employed to improve creep and strength without compromising the fracture toughness. These include: (1) crystallization of the grain-boundary phase [18], [19], (2) changing the chemistry of the secondary phase [20], and (3) reducing the amount of sintering additives to minimize the volume of glass present in the final microstructure [21]. The most important parameter governing several of the high-temperature properties is the viscosity of the grain-boundary film. By increasing the viscosity, either by crystallization or by altering the additives to produce a more refractory phase, the high-temperature properties, particularly creep and fracture strength, can be significantly enhanced.

In the present work, we examine the first of these strategies, i.e., specifically how the elevated-temperature mechanical properties of ABC-SiC are affected by the nature of the grain-boundary film/phase, and investigate whether its superior room-temperature strength and toughness properties [8], [9] can be retained at high temperatures. Moreover, due to the contradictory nature [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] and paucity of data on creep and especially cyclic fatigue processes in SiC ceramics at such temperatures, not to mention the lack of mechanistic understanding, studies are focused on the characteristics of creep and fatigue-crack propagation behavior from ambient temperatures up to 1500°C.

Section snippets

Material processing

ABC-SiC was processed with submicrometer β-SiC starting powders, which were mixed with additions of 3 wt% Al (as powder with a nominal particle diameter of ∼5 μm), 0.6 wt% B (as powder) and 2 wt% C (as Apiezon wax) additions. The calculated value of 2% C was doubled since the carbon yield upon pyrolysis was determined to be ∼50%. This carbon source also served as a binder. The Apiezon wax was dissolved in toluene, and the other powders were added; the resulting suspension was agitated

Microstructure prior to high-temperature testing

The microstructure of as-processed ABC-SiC consisted of a network of interlocking plate-like grains of 5 vol% β-phase (cubic polytype 3C) and 95 vol% α-phase (49 vol% 4H and 46 vol% 6H hexagonal polytypes), with a maximum grain aspect ratio of ca. 4 to 5. This grain morphology is quite distinct from that of toughened silicon nitrides, where the grains typically possess high aspect ratios and are acicular, or needle-like, in shape [10]. Between the grains, an amorphous grain-boundary film,

Concluding remarks

Structural ceramics have often been regarded as exhibiting a conflict between toughness, strength and fatigue resistance. Indeed, this conflict is not unlike the competition between brittleness and strength in metallic systems. A summary of many results on creep and toughness at 1300°C for SiC and Si₃N₄, extracted from the literature, is shown in Fig. 10 to illustrate this paradigm, and to show that ABC-SiC can be processed to negate it. Essential in the success of retaining simultaneously

Summary and conclusions

The high-temperature mechanical properties, including strength, fracture toughness, creep and cyclic fatigue properties, of an in situ toughened silicon carbide sintered with Al, B and C (ABC-SiC) have been studied, and related to the corresponding microstructural and mechanistic characteristics. Based on this work, the following conclusions can be made.

1.
High-temperature annealing at 1100 to 1500°C was found to lead to a remarkable improvement in mechanical properties. For example, although the

Acknowledgements

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy under Contract No. DE-AC03-76SF00098. Particular thanks are due to Drs J. M. McNaney and C. J. Gilbert for helpful discussions and experimental assistance.

References (65)

C.J Gilbert et al.
Acta mater.
(1996)
S.M Wiederhorn et al.
Mater. Sci. Eng. A
(1994)
Y.H Zhang et al.
Mater. Sci. Eng. A
(1998)
S.-Y Liu et al.
Acta mater.
(1996)
J.L Chermant et al.
Mater. Sci. Eng.
(1985)
D Chen et al.
Acta mater.
(2000)
M Li et al.
Acta metall. mater.
(1995)
R.M.L Foote et al.
J. Mech. Phys. Solids
(1986)
R.H Dauskardt
Acta metall. mater.
(1993)
H.E Helms et al.

N.S Jacobson

J. Am. Ceram. Soc.

(1993)

A.G Evans

J. Am. Ceram. Soc.

(1990)

P.F Becher

J. Am. Ceram. Soc.

(1991)

S Prochazka et al.

J. Am. Ceram. Soc.

(1975)

M.A Mulla et al.

Ceram. Bull.

(1991)

D.-H Kim et al.

J. Am. Ceram. Soc.

(1990)

J.J Cao et al.

J. Am. Ceram. Soc.

(1996)

M.N Menon et al.

J. Am. Ceram. Soc.

(1994)

S.K Lee et al.

J. Am. Ceram. Soc.

(1994)

N.P Padture

J. Am. Ceram. Soc.

(1994)

D.R Clarke et al.

J. Am. Ceram. Soc.

(1977)

M.K Ferber et al.

J. Am. Ceram. Soc.

(1992)

D.E Lloyd

R.F Davis et al.

S.C Singhal et al.

J. Am. Ceram. Soc.

(1975)

A Tsuge et al.

J. Am. Ceram. Soc.

(1975)

D.R Clarke et al.

J. Am. Ceram. Soc.

(1982)

H Park et al.

J. Am. Ceram. Soc.

(1997)

T Hansson et al.

C.-K.J Lin et al.

J. Am. Ceram. Soc.

(1991)

U Ramamurty et al.

J. Am. Ceram. Soc.

(1994)

L Ewart et al.

J. Mater. Sci.

(1992)

Cited by (108)

Effects of boron source on properties of pressureless solid-state sintered silicon carbide ceramics
2024, Journal of the European Ceramic Society
The effects of the boron source (B, BN, and B₄C) on the thermal, electrical, and mechanical properties of pressureless solid-state sintered SiC ceramics were investigated. A high relative density of 99.9 % was successfully achieved for all ceramic specimens using an appropriate additive composition and ideal sintering conditions. SiC ceramics sintered with B and B₄C additives displayed higher electrical and thermal conductivities and higher flexural strength than those sintered with the BN additive at the same relative density. The presence of intrinsically-weak, electrically-insulating BN grains at the junction and grain boundaries increased both the fracture toughness and electrical resistivity, while the flexural strength and thermal conductivity of the ceramics decreased. The electrical resistivity, thermal conductivity, and flexural strength of the SiC ceramics were able to be adjusted with values ranging from 3.2 × 10⁴–1.6 × 10⁶ Ω‧cm, 72.4–147.5 W‧m⁻¹‧K⁻¹, and 386–545 MPa, respectively, depending on the boron source used.
Synthesis and characterization of functionally graded SiC-mullite thermal material
2024, Journal of Solid State Chemistry
SiC-mullite is considered for high temperature applications due to its high mechanical strength and thermal shock-resistance. However, maintaining these properties is challenged by the high processing temperature requirement for the formation and densification of the mullite phase which leads to abundant surface oxidation of SiC into cristobalite. Cristobalite has relatively poor mechanical strength and thermal properties compared to SiC. Herein, we report on using coal fly ash as a source of alumina (Al₂O₃) that reacts in situ with the silica (SiO₂), oxidation product of SiC. The instantaneous mullite formation on the surface of SiC facilitated due to the presence of minor concentrations of metal oxides in coal fly ash, resulted in a strong bonding zone between the two phases at relatively low temperature. In this work, SiC was mixed with coal fly ash at weight ratios of 90SiC/10ash, 85SiC/15ash, 80SiC/20ash, and 75SiC/25ash and sintered at 1400 °C. Measurements of mechanical properties showed that the 85SiC/15ash composition had the highest mechanical strength among samples. XRD analysis showed the phase composition of thermally treated 85SiC/15ash to be 81.8 wt% SiC, 11.4 wt% mullite, and 6.8 wt% cristobalite. SEM-EDX revealed a concentration gradient of Al in the cristobalite which enhanced formation of functionally graded bonding zones between phases and resulted in SiC-mullite composite with high thermomechanical properties. The compressive strength, nanoindentation elastic modulus, and Vickers hardness were 434 ± 20 MPa, 370.9 ± 22.6 GPa, and 11.5 ± 1.2 GPa respectively. The thermal shock resistance test showed high dimensional and mechanical stabilities after quenching in liquid nitrogen (−196 °C) from 1400 °C. The SiC-mullite composite showed low thermal expansion co-efficient from 3.17 × 10⁻⁷/K to 5.615 × 10⁻⁶/K when the sample was heated from 182 K to 354 K. The specific heat capacity, thermal diffusivity, and thermal conductivity were 7.83 ± 0.0014 J/g.K, 1.04 ± 0.013 mm²/s, and 17 W/m.K at 100 °C, respectively. The SiC-mullite composite exhibited moderate electrical conductivity of 3.48 × 10⁻² S/m at 1000 °C.
Microstructure, mechanical properties and strengthening mechanism of high strength hot-rolled ferrite-martensite dual phase steel
2024, Materials Characterization
This study is focused on investigating the microstructure and mechanical properties of high strength ferrite-martensite (F-M) dual phase steels manufactured through a simple hot rolling and step cooling. Additionally, it seeks to elucidate the strengthening mechanism of the ‘unexpected microstructures’, namely lower bainite and auto-tempered martensite, within F-M dual phase steels. During the step cooling process, the morphology of ferrite is determined by the air-cooling temperature, while the ferrite content is affected by the air-cooling temperature and time. Since the morphology of continuously cooled bainite is determined by the cooling rate, granular bainite is formed in the air-cooling stage and lower bainite is formed in the laminar water-cooling stage. Combined with the reasonable microstructure construction and the strengthening effect of ‘unexpected microstructures’, the sample air-cooled at 700 °C for 10s achieves the best comprehensive mechanical properties (yield strength: 1155 MPa, tensile strength: 1511 MPa, total elongation: 11.5%, impact toughness in −20 °C: 16.6 J). The strengthening effect of ‘unexpected microstructure’ is mainly to improve strain strengthening and impact toughness. The presence of lower bainite and auto-tempered martensite both promotes the incompatible strain during the deformation, resulting in higher strain hardening. This effect is further enhanced by the segmentation of the prior austenite grains by lower bainite and the plastic constraints imposed by surrounding martensite during deformation. Auto-tempering contributes to an increase in impact toughness by reducing areas with high dislocation density. The improvement of impact toughness by lower bainite is mainly attributed to the increase of high angle grain boundaries when partitioning the prior austenite.
Improving specific stiffness of silicon carbide ceramics by adding boron carbide
2022, Journal of the European Ceramic Society
Citation Excerpt :
Silicon carbide (SiC) ceramics are among the most important structural materials because of their useful engineering properties, such as excellent mechanical properties [1–5], excellent strength retention at high temperatures [6–9], good oxidation resistance [10], high thermal conductivity [11], good wear resistance [12], excellent corrosion resistance [13], and adjustable electrical conductivity [14].
A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²‧s⁻² to ~153 × 10⁶ m²‧s⁻² when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 W‧m⁻¹‧K⁻¹ when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 25 wt% B₄C were ~690 MPa and ~95 W‧m⁻¹‧K⁻¹, respectively.
Artificial neural network molecular mechanics of iron grain boundaries
2022, Scripta Materialia
This study reports grain boundary (GB) energy calculations for 46 symmetric-tilt GBs in $α$ -iron using molecular mechanics based on an artificial neural network (ANN) potential and compares the results with calculations based on the density functional theory (DFT), the embedded atom method (EAM), and the modified EAM (MEAM). The results by the ANN potential are in excellent agreement with those of the DFT (5% on average), while the EAM and MEAM significantly differ from the DFT results (about 27% on average). In a uniaxial tensile calculation of $\sum 3 (1 \bar{1} 2)$ GB, the ANN potential reproduced the brittle fracture tendency of the GB observed in the DFT while the EAM and MEAM mistakenly showed ductile behaviors. These results demonstrate the effectiveness of the ANN potential in calculating grain boundaries of iron, which is in high demand in modern industry.
Electromagnetic wave absorbing performance of multiphase (SiC/HfC/C)/SiO<inf>2</inf> nanocomposites with an unique microstructure
2021, Journal of the European Ceramic Society
Dielectric properties and electromagnetic (EM) wave absorbing performance of monolithic (SiC/HfC/C)/SiO₂ nanocomposites (denoted as SHCOs) have been investigated in the X-band (8.2–12.4 GHz). The multiphase SHCOs are composed of insulating SiO₂ and SiC/HfC/C nanocomposite fillers (SHC), which fillers composed of semiconducting β-SiC, conductive HfC-Carbon core-shell nanoparticles, and interconnected carbon nanoribbons. Dielectric response indicates that the increased SHC content results in an enhanced imaginary part of the permittivity and dielectric loss, leading to an improved EM absorbing performance. The unique microstructure with an EM wave-transparent SiO₂ matrix is favorable for impedance matching and effective EM wave propagation. The enhanced interface polarization and conduction loss are considered as the key mechanisms for EM wave attenuation. The minimum reflection loss of the SHCOs achieves – 60.7 dB containing 20 vol% of SHC (at 9.98 GHz) with the sample thickness of 3.33 mm, and the effective absorbing bandwidth (EAB) covers ca. 72 % of the X-band. The monolithic (SiC/HfC/C)/SiO₂ nanocomposites with outstanding EM wave absorbing performance are promising candidates for EM application at high temperatures.

View all citing articles on Scopus

View full text

Role of the grain-boundary phase on the elevated-temperature strength, toughness, fatigue and creep resistance of silicon carbide sintered with Al, B and C

Abstract

Introduction

Section snippets

Material processing

Microstructure prior to high-temperature testing

Concluding remarks

Summary and conclusions

Acknowledgements

Acta mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Acta mater.

Mater. Sci. Eng.

Acta mater.

Acta metall. mater.

J. Mech. Phys. Solids

Acta metall. mater.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

Ceram. Bull.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

J. Mater. Sci.