High-Temperature Resistant Composites in the AlN–SiC–MoSi2 System

Two fully dense composites containing AlN, SiC, and MoSi₂ in different amounts were produced by hot pressing. Young modulus, hardness, electrical conductivity, and thermal expansion coefficient were measured at room temperature. Strength and toughness were evaluated up to 1300°C in air. According to statistical analysis, the temperature has little or no effect on high-temperature fracture behavior of the composites. These electroconductive materials are suitable for thermomechanical applications at high temperature, the strength being about 600 MPa at 1300°C.     

Mechanical Properties of Si2N2O/SiC-Whisker Composites

The mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were studied with emphasis on the effect of matrix composition and of whisker content. The fracture toughness of Si₂N₂O was remarkably improved by 90% with a concomitant 70% strength improvement by addition of SiC whiskers of only 10 vol%. Optimum mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were obtained at an equimolar ratio of Si₃N₄/SiO₂, which is the stoichiometric composition for Si₂N₂O. Additional investigation concerning the Si₂N₂O-matrix/SiC-whisker interface by controlling sintering additives is necessary for further improvement of mechanical and thermal properties of Si₂N₂O/SiC-whisker composites.     

Processing and Mechanical Properties of Dense Si3N4-SiC-Whisker Composites without Sintering Aids

Si₃N₄ with 20 vol% SiC whisker was fabricated without sintering aids by hot isostatic pressing. Density higher than 99.5% was attained after sintering at 2000°C and 170 MPa for 1 h. Careful mixing procedures and the use of an appropriate amount of a dispersant was found to be effective in avoiding whisker segregation and inhomogeneity. Mechanical properties of the composite were investigated by measurements of flexural strength, microhardness, frature toughness, and Young's modulus as a function of temperature. At room temerature, Vickers microhardness and Young's modulus increased from the matrix value about 20% and 5%, respectively. Toughness was about 30% higher, without reduction in flexural strength, up to 1400Deg;C.     

Micromechanical Stresses in SiC-Reinforced Al2O3 Composites

Applying an Eshelby approach, the internal micromechanical stresses within an SiC-inclusion-reinforced (platelet to whisker geometries) polycrystalline alumina matrix composite were calculated. The results are compared to the experimental residual stress measurements of a SiC-whisker-reinforced Al₂O₃ by Predecki, Abuhasan, and Barrett and found to be in excellent agreement. The calculations are then extended to SiC-reinforced composites with polycrystalline mullite, silicon nitride, and cordierite matrices. It is concluded that the internal stresses are significantly influenced by the inclusion geometry as well as the thermoelastic differences between the inclusion and the matrix and also the volume fraction.     

Si3N4/SiC-Whisker Composites without Sintering Aids: III, High-Temperature Behavior

The fracture behavior at high temperature of a Si₃N₄-based SiC-whisker composite fabricated by hot isostatic pressing without sintering aids is compared with that of other highly refractory materials. Particular attention is directed toward evaluating the slow-crack-growth resistance of the composite up to 1440°C and relating this resistance to the microfracture behavior of Si₃N₄ grains, SiC whiskers, and the intergranular, glassy SiO₂ phase. Only thick whiskers operate to bridge the wake of the crack; these whiskers may make a positive contribution to the slow-crack-growth resistance. Impurities detected by EDX microanalysis at the grain boundary, however, apparently degrade the high-temperature properties, a finding supported by internal-friction measurements. Nevertheless, the high potential of the system without sintering aids for high-temperature structural applications has been demonstrated by the time to failure estimated from the measured slow-crack-growth resistance for a fixed flaw size.     

Mechanical Properties of β-Si3N4-Whisker/Si3N4-Matrix Composites

Composites containing 30 vol%β-Si₃N₄ whiskers in a Si₃N₄ matrix were fabricated by hot-pressing. The composites exhibited fracture toughness values between 7.6 and 8.6 MPa · m^1/2, compared to 4.0 MPa · m^1/2 for unreinforced polycrystalline Si₃N₄. The improvements in fracture toughness were attributed to crack wake effects, i.e., whisker bridging and pullout mechanisms.     

Densification and Mechanical Properties of B4C with Al2O3 as a Sintering Aid

The densification behavior and mechanical properties of B₄C hot-pressed at 2000°C for 1 h with additions of Al₂O₃ up to 10 vol% were investigated. Sinterability was greatly improved by the addition of a small amount of Al₂O₃. The improvement was attributed to the enhanced mobility of elements through the Al₂O₃ near the melting temperature or a reaction product formed at the grain boundaries. As a result of this improvement in the density, mechanical properties, such as hardness, elastic modulus, strength, and fracture toughness, increased remarkably. However, when the amount of Al₂O₃ exceeded 5 vol%, the level of improvement in the mechanical properties, except for fracture toughness, was reduced presumably because of the high thermal mismatch between B₄C and Al₂O₃.     

Oxidation Effects on the Mechanical Properties of a SiC-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

The room-temperature mechanical properties of a SiC-fiberreinforced reaction-bonded silicon nitride composite were measured after 100 h treatment in nitrogen and oxygen environments to 1400°C. The composite heat-treated in nitrogen to 1400°C showed no appreciable loss in properties. In contrast, composites heat-treated in oxygen from 600° to 1000°C retained ∼65% and 35% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites, and those heat-treated from 1200° to 1400°C retained greater than 90% and 65% of the matrix fracture and ultimate strength, respectively, of the as-fabricated composites. For all nitrogen and oxygen treatments, the composite displayed strain capability beyond the matrix fracture strength. Oxidation of the fiber surface coating, which caused degradation of bond between the fiber and matrix and reduction in fiber strength, appears to be the dominant mechanism for property degradation of the composites oxidized from 600° to 1000°C. Formation of a protective silica coating at external surfaces of the composites at and above 1200°C reduced oxidation of the fiber coating and hence degrading effects of oxidation on their properties.     

Processing and Mechanical Properties of Al2O3/Ni3Al Composites with Interpenetrating Network Microstructure

Composites with microstructures of interpenetrating networks were manufactured by gas pressure infiltration of Ni₃Al into porous preforms of aluminum oxide. Composites with Ni₃Al contents of between 15% and 30% by volume were made and evaluated mechanically at temperatures between room temperature and 1000°C. The fracture strength, the fracture toughness, Young's modulus, and the thermal expansion coefficient were measured for each composite and test condition and were correlated with the microstructures of the composites. Composites with low Ni₃Al contents had strengths below 400 MPa, presumably due to microcracking along the interface between the Ni₃Al and the Al₂O₃. The composite with the highest content of Ni₃Al, 30 vol%, had a mean fracture strength of 675 ± 16 MPa, a Weibull modulus of 23.9, and a room-temperature toughness of 9.2 ± 0.5 MPa·m^1/2.     

Mechanical Properties of Alumina/Silicon Carbide Whisker Composites

The improvement of mechanical properties of Al₂O₃/SiC whisker composites has been studied with emphasis on the effects of the whisker content and of the hot-pressing temperature. Mechanical properties such as fracture toughness and fracture strength increased with increasing whisker content up to 40 wt%. In the case of the high SiC whisker content of 40 wt%, fracture toughness of the sample hot-pressed at 1900° decreased significantly, in spite of densification, compared with one hot-pressed at 1850°. Fracture toughness strongly depended on the microstructure, especially the distribution of SiC whiskers rather than the grain size of the Al₂O₃ matrix.     

Microstructure and Mechanical Properties of Mullite–Silicon Carbide Composites

Mullite-SiC-whisker composites were prepared by powder processing using two commercial SiC whiskers. These composites were prepared by sintering rather than hot-pressing. A mulliteSlC-powder composite and a base line mallite material were also prepared for comparison with the two whisker composite materials. Fracture toughness measurements showed significant enhancement in only one of the whisker composite materials. The microstructure of the four materials was examined by scanning electron microscopy and transmission electron microscopy to assist in the explanation of the mechanical behavior of these composites. The examinations suggested that most of the toughening results from second-phase particles, with only limited toughening from effects associated with whiskers per se. In one case, higher toughness was partially associated with the formation of sialon phase by reaction with the whiskers and the furnace environment.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Microstructure and Mechanical Properties of Sinter–Post-HIPed Si3N4–SiC Composites

The microstructural evolution and mechanical properties of Si₃N₄–SiC composites obtained by the sinter–post-HIP process were investigated. SiC addition prohibited β-Si₃N₄ grain growth; however, the grain growth followed the empirical growth law, with exponents of 3 and 5 for the c - and the a -axis directions, respectively. Mechanical properties were strongly influenced by SiC addition and sintering conditions. Short-crack propagation behavior was measured and analyzed by the indentation-strength in-bending (ISB) method. The present composites had high short-crack toughness, compared with the values for monolithic Si₃N₄. The enhanced short-crack toughness was attributed to crack-tip bridging by the SiC particles.     

Si3N4/SiC-Whisker Composites without Sintering Aids: II, Fracture Behavior

The fracture behavior of an Si₃N₄/SiC-whisker composite fabricated without sintering aids is investigated using a double approach based on the examination of R -curve behavior and a statistical analysis of crack propagation. In the composite with 20 vol% whisker, a 30% increase in toughness over the matrix value can be attributed to crack-tip phenomena. Strong interfacial bonding prevents any contribution to toughening by mechanisms operating in the wake region of the crack. Based on experimental observations of microfracture in both SiC whiskers and Si₃N₄ grains, toughening caused by crack-tip phenomena is quantitatively discussed in terms of fracture energy and whisker-distribution parameters.     

Mechanical Properties of Interpenetrating Microstructures: The Al2O3/c-ZrO2 System

Composites of Al₂O₃ (A) and cubic ZrO₂ (Z) (8 mol% Y₂O₃) (with c -ZrO₂ volume fractions ranging from O to 1) were fabricated by pressureless sintering of mechanically mixed powders. The microstructures of the AZ composites were duplex, with the grains of both phases exhibiting similar size. Room-temperature mechanical properties including Young's modulus (determined from elastic wave velocity measurements), strength and toughness (by indentation-strength-in-bending), and Vickers microhardness were evaluated for the full range of compositions. All properties exhibited a linear decrease with increasing c -ZrO₂ content, and no R -curve behavior was seen in any of the composite compositions. Fracture morphology, observed from cracks emanating from microhardness indentations, changed from essentially intergranular for the tougher Al₂O₃ to transgranular for c -ZrO₂, with AZ composites exhibiting mixed morphology.     

High-Temperature Mechanical Properties of a Uniaxially Reinforced Zircon-Silicon Carbide Composite

Mechanical properties of a monolithic zircon ceramic and zircon-matrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide monofilaments were measured in flexure between 25° and 1477°C. Monolithic zircon ceramics were weak and exhibited a brittle failure up to about 1300°C. An increasing amount of the plastic deformation was observed before failure above about 1300°C. In contrast, composites reinforced with either uncoated or BN-coated SiC filaments were stronger and tougher than the monolithic zircon at all test temperatures between 25° and 1477°C. The ultimate strength and work-of-fracture of composite samples decreased with increasing temperature. A transgranular matrix fracture was shown by the monolithic and composite samples tested up to about 1200°C, whereas an increasing amount of the intergranular matrix fracture was displayed above 1200°C.     

Fabrication and Mechanical Properties of Si3N4/Carbon Fiber Composites with Aligned Microstructure Produced by a Seeding and Extrusion Method

Si₃N₄/carbon fiber composites have been produced with and without seeding by an extrusion and sintering process. In both cases the carbon fibers were aligned along the direction of extrusion, but the Si₃N₄ grains were only aligned in the seeded material. The mechanical properties of the specimens showed anisotropy with respect to the grain alignment, with both strength and toughness being highest in the direction parallel to the extruding direction. In this direction the seeded specimen, where both the Si₃N₄ grains and the carbon fibers were aligned, showed both higher fracture toughness and higher fracture strength than the nonseeded specimen where only the fibers were aligned.     

Mechanical Properties and Oxidation Behavior of Al2O3–AlN–TiN Composites

The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al₂O₃–42 wt% TiN, differing in the AIN/Al₂O₃ ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al₂O₃ and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al₂O₃-rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters.     

Elastic Properties of Al2O3 and Si3N4 Matrix Composites with SiC Whisker Reinforcement

A study of the elastic moduli of Al₂O₃ and Si₃N₄ ceramics reinforced with 0 to 25 wt% SiC whiskers has been performed. The Young's moduli, shear moduli, and longitudinal modulus are compared with calculated predictions for aligned fiber composites by Hill and Hashin and Rosen, and for fibers randomly oriented in three dimensions by Christensen and Waal. The measured values are in excellent quantitative agreement with those derived for the random orientation of the SiC whiskers.