Effect of Cr3C2 Addition on the Sintering of SiC-TiC Composite

The densification behavior and mechanical properties of SiC-30TiC (in volume percent) composites prepared with Cr₃C₂ additive were investigated. By hot-pressing a SiC-30TiC-lCr₃C₂ specimen at 1950°C, 98.5% of theoretical density was achieved and the specimen exhibited a fracture strength of 750 MPa. For the SiC-30TiC-10Cr₃C₂ specimen, (β-α transformation of SiC was observed to occur during hot-pressing and in situ growth of elongated α-SiC grains resulted in an increase of fracture toughness. Micro-structural observations using high-resolution TEM indi-cated that no liquid phase was present at the interfaces.     

Fabrication and Mechanical Properties of Silicon Carbide-Silicon Nitride Composites with Oxynitride Glass

A microstructure that consisted of uniformly distributed, elongated β-Si₃N₄ grains, equiaxed β-SiC grains, and an amorphous grain-boundary phase was developed by using β-SiC and alpha-Si₃N₄ powders. By hot pressing, elongated β-Si₃N₄ grains were grown via alpha right arrow β phase transformation and equiaxed β-SiC grains were formed because of inhibited grain growth. The strength and fracture toughness of SiC have been improved by adding Si₃N₄ particles, because of the reduced defect size and the enhanced bridging and crack deflection by the elongated β-Si₃N₄ grains. Typical flexural-strength and fracture-toughness values of SiC-35-wt%-Si₃N₄ composites were 1020 MPa and 5.1 MPam^1/2, respectively.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.     

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.     

Effect of Grain Growth of β-Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

β-Si₃N₄ powder containing 1 mol% of equimolar Y₂O₃–Nd₂O₃ was gas-pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of " in-situ composites" as a result of exaggerated grain growth of some β Si₃N₄ grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa-m^1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of β-Si₃N₄ powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.     

Effect of the Amount of Additives and Post-Heat Treatment on the Microstructure and Mechanical Properties of Yttrium–α-Sialon Ceramics

The yttrium–sialon ceramics with the composition of Y_0.333Si₁₀Al₂ON₁₅ and an excess addition of Y₂O₃ (2 or 5 wt%) were fabricated by hot isostatic press (HIP) sintering at 1800°C for 1 h. The resulting materials were subsequently heat-treated in the temperature range 1300–1900°C to investigate its effect on the α→β-sialon phase transformation, the morphology of α-sialon grains, and mechanical properties. The results show that α-sialons stabilized by yttrium have high thermal stability. An adjustment of the α-sialon phase composition is the dominating reaction in the investigated Y–α-sialon ceramics during low-temperature annealing. Incorporation of excess Y₂O₃ could effectively promote the formation of elongated α-sialon grains during post-heat-treating at relatively higher temperature (1700° and 1900°C) and hence resulted in a high fracture toughness ( K _IC= 6.3 MPa·m^1/2) via grain debonding and pullout effects. Although the addition of 5 wt% Y₂O₃ could promote the growth of elongated α grains with a higher aspect ratio, the higher liquid-phase content increased the interfacial bonding strength and therefore hindered interface debonding and crack deflection. The heat treatment at 1500°C significantly changed the morphology of α-sialon grains from elongated to equiaxed and hence decreased its toughness.     

In Situ-Toughened Silicon Carbide

A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (i) uniformly distributed elongate-shaped α-SiC grains and (ii) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of β-SiC powder doped with α-SiC, Al₂O₃, and Y₂O₃. The Al₂O₃ and Y₂O₃ aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the α-SiC "seeds" assist in controlling in situ growth of the elongated α-SiC grains. The fracture pattern in the in situ -toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si₃N₄ ceramics. The elongate nature of the α-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ -toughened SiC relative to a commercial SiC.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

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.     

Microstructure Characterization of Gas-Pressure-Sintered β-Silicon Nitride Containing Large β-Silicon Nitride Seeds

Fine β-Si₃N₄ powders with or without the addition of 5 wt% of large β-Si₃N₄ particles (seeds) were gas-pressure sintered at 1900°C for 4 h using Y₂O₃ and Al₂O₃ as sintering aids. The microstructures were examined on polished and plasmaetched surfaces. These materials had a microstructure of in situ composites with similar small matrix grains and different elongated grains. The elongated grains in the materials with seeds had a larger diameter and a smaller aspect ratio than those in the materials without seeds. A core/rim structure was observed in the elongated grains; the core was pure β-Si₃N₄ and the rim was β-SiAION. These results show that the large β-Si₃N₄ particles acted as seeds for abnormal grain growth and the rim was formed by precipitation from the liquid containing aluminum.     

Microstructural Development of Silicon Carbide Containing Large Seed Grains

Fine (}0.1μm) β-SiC powders, with 3.3 wt% large (}0.44μm) α-SiC or β-SiC particles (seeds) added, were hot-pressed at 1750°C and then annealed at 1850°C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into β-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable β→α phase transformation occurred. The growth of matrix grains in materials with β-SiC seeds was slower than that in materials with α-SiC seeds. The material with β-SiC seeds, which was annealed at 1850°C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with α-SiC seeds, also annealed at 1850°C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with α-SiC and β-SiC seeds were 5.5 and 5.4 MPa·^1/2, respectively. Such results suggested that further optimization of microstructure should be possible with β-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.     

Microstructure and Properties of Self-Reinforced Silicon Nitride

Problems associated with manufacturing Si₃N₄/SiC-whisker composites have been overcome by developing selfreinforced Si₃N₄ with elongated β-Si₃N₄ grains formed in situ from oxynitride glass. This Si₃N₄–Y₂O₃–MgO–SiO₂–CaO-based material has a flexure strength >1000 MPa and fracture toughness >8 MPa·m^½. The optimum combination of mechanical properties has been obtained with Y₂O₃:MgO ratios ranging from 3:1 to 1:2, CaO contents ranging from 0.1 to 0.5 wt%, and Si₃N₄ contents between 90 and 96 wt%.     

Microstructural Evolution in Liquid-Phase-Sintered SiC: Part I, Effect of Starting Powder

The effect of starting SiC powder (β-SiC or α-SiC), with simultaneous additions of Al₂O₃ and Y₂O₃, on the microstructural evolution of liquid-phase-sintered (LPS) SiC has been studied. When using α-SiC starting powder, the resulting microstructures contain hexagonal platelike α-SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in α-SiC. When using β-SiC starting powder, the β→α phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of β→α phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high-aspect-ratio SiC grains is proposed.     

Nd2O3-Doped Sialons with ZrO2/ZrN Additions Formed by Sintering and Hot Isostatic Pressing

β-sialon and Nd₂O₃-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO₂ (3 mol% Nd₂O₃) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO₂(Nd₂O₃) or ZrN were seen in both the HIPed β-sialon/ZrO₂(Nd₂O₃) composites and the HIPed Nd₂O₃-stabiIized α-sialons with ZrN additions.     

Microstructure and Mechanical Properties of the in situβ-Si3N4/α'-SiAlON Composite

The in situ β-Si₃N₄/α'-SiAlON composite was studied along the Si₃N₄–Y₂O₃: 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si₃N₄/α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si₃N₄ during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si₃N₄ fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m^1/2. α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions.     

High Thermal Conductivity in Silicon Nitride with Anisotropie Microstructure

Silicon nitride was fabricated by tape casting of α-Si₃N₄ powder with 5 wt% Y₂O₃ and 5 vol% rodlike β-Si₃N₄ seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1850°C for 2-66 h under a nitrogen pressure of 0.9 MPa. Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the casting direction. Thermal conductivities parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m.K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.     

Flaw-Tolerance and R-Curve Behavior of Liquid-Phase-Sintered Silicon Carbides with Different Microstructures

β-SiC powder containing 6 wt% A1₂O₃ and 4 wt% Y₂O₃ as sintering additives was pressureless sintered at 2000°C for 1 h (AYE-SiC) and 3 h (AYP-SiC). AYE-SiC consisted of an equiaxed grain structure and AYP-SiC exhibited a micro-structure with platelike grains as a result of grain growth related to β→α phase transformation during sintering, R -curve behavior and flaw tolerance for these silicon carbides were evaluated by the indentation-strength technique. For comparison, the R -curve behavior of conventional sintered, boron- and carbon-doped SiC (SS-SiC) was evaluated. AYE-SiC and AYP-SiC exhibited rising R -curve behavior with toughening exponents of m = 0.042 and m = 0.135, respectively. AYP-SiC exhibited better flaw tolerance and more sharply rising R -curve behavior than AYE-SiC. The more sharply rising R -curve behavior and the better flaw tolerance of AYP-SiC were attributed mainly to grain bridging of crack faces by platelike grains. Because of the high degree of transgranular fracture, SS-SiC exhibited a flat R -curve despite a microstructural feature with platelike grains.     

Effect of α to β (β') Phase Transition on the Sintering of Silicon Nitride Ceramics

By using α-Si₃N₄ and β-Si₃N₄ starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si₃N₄·9Al₂O₃·9Y₂O₃ (wt%) and 80Si₃N₄·13Al₂O₃·5AIN·5AIN·2Y₂O₃. When α-Si₃N₄ powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification.     

Phase Assemblages of Pr α-sialon Derived from SHS-ed Powders and TEM Study on the Nucleation of Pr α-sialon

The Pr α-sialon powders prepared by self-propagating high-temperature synthesis (SHS), consisting of 55 wt% Pr α-sialon and 45 wt% of β-sialon (abbreviated as α' and β'), were hot-pressed at 1800°C for 1 h. The results showed that Pr α' phase would transfer to β' with the appearance of JEM phase (Pr(Si_{6− z} Al _z )(N_{10− z} O _z )) after sintering, thus resulting in the increase of β' phase to 86 wt%. The addition of Y₂O₃ into SHS-ed Pr α' powders as the starting materials restrains the transformation of α' to β' and prevents the formation of JEM phase as well. The nucleation mechanism of Pr α' grain during hot-pressing was investigated in terms of transmission electron microscope and energy-dispersive spectrometer analysis. Two nucleation modes of Pr α' grains were found, i.e., nucleating on the undissolved Pr α' grains and on the nuclei of (Pr, Y) α' grains precipitated from liquid phase.     

Pressureless Sintering of Alumina-Titanium Carbide Composites

The densification of Al₂O₃-TiC composites is detrimentally affected by chemical reactions between Al₂O₃ and TiC. These reactions must be suppressed in order to promote sintering. In this study, the specific reactions occurring in Al₂O₃-TiC composites were modeled, using thermodynamic calculations, and verified by experiments. The reaction between Al₂O₃ and TiC was suppressed by the use of specially prepared embedding powders allowing pressureless sintering to closed porosity. The Al₂O₃-TiC composites were subsequently hot isostatically pressed to > 99% of theoretical density without encapsulation. Typical flexural strength and fracture toughness of Al₂O₃-30 wt% TiC composites were 690 MPa and 4.3 MPa · m^1/2, respectively.