Relationship between Microstructure and Fracture Toughness of Toughened Silicon Carbide Ceramics

Different microstructures in SiC ceramics containing Al₂O₃, Y₂O₃, and CaO as sintering additives were prepared by hot-pressing and subsequent annealing. The microstructures obtained were analyzed by image analysis. Crack deflection was frequently observed as the toughening mechanism in samples having elongated α-SiC grains with aspect ratio >4, length >2 μm, and grain thickness ( t ) <3 μm (defined as key grains 1). Crack bridging was the dominant toughening mechanism observed in samples having grains with thickness of 1 μm < t < 3 μm and length >2 μm (key grains 2). The values of fracture toughness varied from 5.4 to 8.7 MPa·m^1/2 with respect to microstructural characteristics, characterized by mean grain thickness, mean aspect ratio, and total volume fraction of key grains. The difference in fracture toughness was mainly attributed to the amount of key grains participating in the toughening processes.     

Microstructural Control for Strengthening of Silicon Carbide Ceramics

Fine-grained (<1 μm) silicon carbide ceramics with high strength were obtained by using ultrafine (∼90 nm) β-SiC starting powders and a seeding technique for microstructural control. The microstructures of the as-hot-pressed and annealed ceramics without α-SiC seeds consisted of fine, uniform, and equiaxed grains. In contrast, the annealed material with seeds had a uniform, anisotropic microstructure consisting of elongated grains, owing to the overgrowth of β-phase on α-seeds. The strength, the Weibull modulus, and the fracture toughness of fine-grained SiC ceramics increased with increasing grain size up to ∼1 μm. Such results suggested that a small amount of grain growth in the fine grained region (<1 μm) was beneficial for mechanical properties. The flexural strength and the fracture toughness of the annealed seeded materials were 835 MPa and 4.3 MPa·m^1/2, respectively.     

Effects of α-Sic versus β-Sic Starting Powders on Microstructure and Fracture Toughness of Sic Sintered with Al2O3-Y2O3 Additives

Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al₂O₃-Y₂O₃ additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α-SiC powder was composed of equiaxed grains, whereas SiC obtained from α-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α-SiC powder increased slightly from 4.4 to 5.7 MPa.m^1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m^1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.     

Microstructures and mechanical properties of high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C ceramics with the addition of SiC secondary phase

The relationships between microstructures and mechanical properties especially strength and toughness of high-entropy carbide based ceramics are reported in this article. Dense (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C (HEC) and its composite containing 20 vol.% SiC (HEC-20SiC) were prepared by spark plasma sintering. The addition of SiC phase enhanced the densification process, resulting in the promotion of the formation of the single-phase high-entropy carbide during sintering. The high-entropy carbide phase demonstrated a fast grain coarsening but SiC particles remarkably inhibited this phenomena. Dense HEC and HEC-20SiC ceramics sintered at 1900 °C exhibits four-point bending strength of 332 ± 24 MPa and 554 ± 73 MPa, and fracture toughness of 4.51 ± 0.61 MPa·m^1/2 and 5.24 ± 0.41 MPa·m^1/2, respectively. The main toughening mechanism is considered to be crack deflection by the SiC particles.     

Fine grained Al2O3/SiC composite ceramic tool material prepared by two-step microwave sintering

Fine-grained Al₂O₃/SiC composite ceramic tool materials were synthesized by two-step microwave sintering. The effects of first-step sintering temperature (T1), content and particle size of SiC on the microstructure and mechanical properties were studied. It was found that the sample with higher content of SiC was achieved with finer grains, and the incorporation of SiC particles could bridge, branch and deflect the cracks, thus improving the fracture toughness. Higher T1 was required for the densification of the samples with higher content of SiC (>5?wt%). The sample containing 3?wt% SiC particles with the mean particle size of 100?nm, which was sintered at 1600?°C (T1) and 1100?°C (T2) for 5?min had the fine microstructure and optimal properties. Its relative density, grain size, Vickers hardness and fracture toughness obtained were 98.37%, 0.78?±?0.31?μm, 18.40?±?0.24?GPa and 4.97?±?0.30?MPa?m^1/2, respectively. Compared to the sample prepared by single-step microwave sintering, although near full densification can be achieved in both two methods, the grain size was reduced by 36% and the fracture toughness was improved by 28% in two-step microwave sintering.     

Pressureless Sintering of Hafnium Carbide–Silicon Carbide Ceramics

HfC‐30 vol%SiC ceramics with a relative density of 99.7% was obtained by pressureless sintering at 2300°C for 0.5 h. The resultant ceramics showed fine microstructure with HfC grain size around 1 μm. The hardness (20.5 ± 0.2 GPa), bending strength (396 ± 56 MPa), and fracture toughness (2.81 ± 0.18 MPa·m^1/2) of HfC‐30 vol%SiC ceramics were at least 20% higher than those of monolithic HfC ceramics. The influences of SiC particle size, volume fraction, and the oxide impurity on the microstructure evolution of HfC‐based ceramics were examined. The results indicate that SiC addition and the oxygen impurity introduced by ball milling play opposite roles in the HfC grain growth during sintering. The oxide impurity introduced by ball milling caused the HfC grain coarsening, whereas SiC particles inhibited the grain growth of HfC significantly.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

Technique for the R-Curve Determination of Y-TZP Using Indentation-Produced Flaws

A technique was developed to estimate the R -curve behavior of Y-TZP using indentation-produced flaws. This technique allows crack-growth resistance to be determined for short crack extensions (micrometers). For Y-TZP samples sintered at 1600°C for 1 h (mean grain size, 0.7 μm), the crackgrowth resistance increases rapidly over the first 10 μm. Peak toughness values determined from this technique correlate with chevron-notch fracture toughness if the ellipticity of the indentation-produced flaw is considered.     

Zrc ceramics incorporated with different-sized SiC particles

ABSTRACT

Three different SiC powders with average particle sizes of 0.45, 3.5 and 10?µm were used to prepare ZrC-20vol.-% SiC ceramics by hot pressing. The effects of SiC particle size on the densification, microstructure, mechanical properties and thermal properties of ZrC–SiC ceramics were studied. Ceramics prepared from SiC with finer particle sizes exert higher bending strength, hardness and lower thermal conductivity. The ZrC–SiC ceramics with a starting SiC particle size of 3.5?µm has relative high fracture toughness than others. Analysis indicates that SiC grain size and the grain boundaries control the thermal conductivity ZrC–SiC ceramics. Ceramics prepared from SiC with the particle size of 10?µm exhibits the highest thermal conductivity due to the larger grains and less grain boundaries.     

Toughening of Silicon Nitride Matrix Composites by the Addition of Both Silicon Carbide Whiskers and Silicon Carbide Particles

Si₃N₄ matrix composites reinforced by SiC whiskers, SiC particles, or both were fabricated using the hot-pressing technique. The mechanical properties of the composites containing various amounts of these SiC reinforcing materials and different sizes of SiC particles were investigated. Fracture toughness of the composites was significantly improved by introducing SiC whiskers and particles together, compared with that obtained by adding SiC whiskers or SiC particles alone. On increasing the size of the added SiC particles, the fracture toughness of the composites reinforced by both whiskers and particles was increased. Their fracture toughness also showed a strong dependence on the amount of SiC particles (average size 40 μm) and was a maximum at the particle content of 10 vol%. The maximum fracture toughness of these composites was 10.5 MPa·m^1/2 and the flexural strength was 550 MPa after addition of 20 vol% of SiC whiskers and 10 vol% of SiC particles having an average particle size of 40 μm. These mechanical properties were almost constant from room temperature to temperatures around 1000°C. Fracture surface observations revealed that the reinforcing mechanisms acting in these composites were crack deflection and crack branching by SiC particles and pullout of SiC whiskers.     

Effect of Iron and Boron Carbide on the Densification and Mechanical Properties of Titanium Diboride Ceramics

The effect of Fe and B₄C on the sintering behavior and mechanical properties of TiB₂ ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition-milled TiB₂ powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B₄C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B₄C grains did not act as a grain growth inhibitor, and B₄C grains were frequently trapped in large TiB₂ grains. The best mechanical properties were obtained for the TiB₂–10 wt% B₄C–0.5 wt% Fe ceramics, which exhibited a three-point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m^1/2.     

无压烧结Al2O3／SiC纳米复相陶瓷的研究

将粒径为30～35nm的β-SiC粉,加入亚微米尺寸的α-Al     

Dependence of Fracture Toughness of Alumina on Grain Size and Test Technique

The fracture toughness ( K_Ic ) of sintered alumina was measured using notched beam (NB) and indentation/strength-in-bending (ISB) techniques. K_Ic (NB) decreases with increasing grain size. For fine-grained materials (<5 μ m ), NB results overestimate K_Ic , and exhibit a substantial notch-radius sensitivity. A stress-intensity-derived model is used to explain this notch sensitivity. The ISB results are very similar to those obtained using the double-cantilever-beam (DCB) method and show an increasing fracture toughness with increasing grain size. The differences between the NB and ISB (DCB) results for coarser-grained materials are thought to be related to R -curve behavior.     

The Effect of Conformation Method and Sintering Technique on the Densification and Grain Growth of Nanocrystalline 8 mol% Yttria-Stabilized Zirconia

Uniaxial dry pressing (DP) and slip casting (SC) were used to form green bodies of nanocrystalline 8 mol% yttria-stabilized zirconia powder processed via the glycine-nitrate combustion method. The SC method was shown to be a more efficient, yielding more homogenous green bodies with higher green density (60% theoretical density) which contained smaller pores with narrower distribution. Improved green properties resulted in lowering the sintering temperature of SC bodies by about 200°C compared with DP compacts. Consequently, the grain growth in sintered bodies formed by SC was relatively abated. By taking the benefits of the wet conformation method, the final grain size of nearly full dense (>97% TD) structures was reduced by 39% (i.e. from 2.15 to 1.3 μm). To reveal the effect of sintering technique, DP bodies were sintered via both microwave and two-step sintering methods. While the grain size of two-step sintered samples was <300 nm, sintering via microwave radiation yielded a nearly full dense structure with a mean grain size of 0.9 μm. The results show that conventionally sintered SC bodies posses higher indentation fracture toughness (FT) (∼3 MPa·m^1/2) compared with DP samples (1.6 MPa·m^1/2). Interestingly, it was shown that, without applying any modified sintering technique, the hardness and FT of SC bodies with coarser structures are completely close to those of samples sintered via microwave heating.     

Pressureless Sintering of Carbon Nanofibre/SiC Composites and Their Properties

To enhance the fracture toughness of silicon carbide (SiC) ceramics and prevent the generation of cracks and chippings in the SiC ceramics during machining process, carbon nanofibres (CNFs) were compounded with SiC. The densification and microstructure development of the CNFs/SiC composites pressureless sintered in Ar atmosphere were investigated. The fracture toughness of SiC ceramics was enhanced by the addition of 1–3 wt% CNFs, which resulted from the pullout and/or bridging effect of CNFs bonded much more closely with SiC. The addition of 3 wt% CNFs prevented the chippings from generating in the composite during precision machining process.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Erosion behavior of SiC–WC composites

Dense silicon carbide (SiC) ceramics were prepared with 0, 10, 30 or 50 wt% WC particles by hot pressing powder mixtures of SiC, WC and oxide additives at 1800 °C for 1 h under a pressure of 40 MPa in an Ar atmosphere. Effects of alumina or SiC erodent particles and the WC content on the erosion performance of sintered SiC–WC composites were assessed. Microstructures of the sintered composites consisted of WC particles distributed in the equi-axed grain structure of SiC. Fracture surfaces showed a mixed mode of fracture, with a large extent of transgranular fracture observed in SiC ceramics prepared with 30 wt% WC. Crack bridging by WC enhanced toughening of the SiC ceramics. A maximum fracture toughness of 6.7 MPa*m^1/2 was observed for the SiC ceramics with 50 wt% WC, whereas a high hardness of 26 GPa was obtained for the SiC ceramics with 30 wt% WC. When eroded at normal incidence, two orders of magnitude less erosion occurred when SiC–WC composites were eroded by alumina particles than that eroded by SiC particles. The erosion rate of the composites increased with increasing angle of SiC particle impingement from 30° to 90°, and decreased with WC reinforcement up to 30 wt%. A minimum erosion wear rate of 6.6 mm³/kg was obtained for SiC–30 wt% WC composites. Effects of mechanical properties and microstructure on erosion of the sintered SiC–WC composites are discussed, and the dominant wear mechanisms are also elucidated.     

Preparation and study of the mechanical and optical properties of infrared transparent Y2O3–MgO composite ceramics

In this study, fine Y₂O₃–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y₂O₃–MgO composite ceramics were fabricated by pre-sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre-sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre-sintering temperature. A maximum fracture toughness of 1.42 MPa·m^1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C.     

Preparation,mechanical properties,and toughening mechanisms of SiCw/SiCp-reinforced zirconia-toughened alumina ceramics

Zirconia-toughened alumina (ZTA) ceramics with high mechanical properties were sintered by hot-pressing method using SiC particles (SiCp) and SiC whiskers (SiCw) as the reinforcing agents simultaneously. The influences of sintering temperature, SiCp, and SiCw contents on the microstructure and mechanical properties of ZTA ceramics were investigated. It was found that both SiCp and SiCw could contribute to grain refinement significantly and promote the mechanical properties of the ceramics. However, the excess addition of SiCp or SiCw led to the formation of pores with large sizes and degraded the mechanical properties instead. When 13 wt% SiCp was introduced, the maximum flexural strength of 1180.0 MPa and fracture toughness of 15.9 MPa·m^1/2 were obtained, whereas the maximum flexural strength of 1314.0 MPa and fracture toughness of 14.7 MPa·m^1/2 were achieved at 20 wt% SiCw. Interestingly, the simultaneous addition of SiCp and SiCw could further improve the mechanical properties, and the highest flexural strength of 1334.0 MPa and fracture toughness of 16.0 MPa·m^1/2 were achieved at a SiCw/SiCp ratio of 16/4. The reinforcement mechanisms in the ceramics mainly included the phase transformation toughening of ZrO₂, the crack deflection and bridging of SiCp and SiCw, and the pull-out of SiCw.     

Lower-Temperature Hot-Pressed Dy-α-Sialon Ceramics with an LiF Additive

Hot-pressed Dy-α-sialon ceramics, using LiF as a sintering additive, were fabricated at lower temperatures (≤1650°C). Some of the densified samples possessed higher hardness and fracture toughness up to 19.00–20.00 GPa and 4.00–6.00 MPa·m^1/2, respectively. The amount of LiF had a strong effect on the densification behavior in sialon preparation. As one of the experimental results, the sample with 0.1 wt% of LiF additive sintered at 1600°C produced an optical translucence of about 50% in the range of 1.5–5.0 μm wavelengths. The maximum infrared transmission reached ∼60% at a wavelength of 2.4 μm. It is inferred that these more easily sintered materials would be practical for optical applications in certain fields.