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.     

Deformation of Alumina/Titanium Carbide Composite at Elevated Temperatures

The deformation behavior of an Al₂O₃/30 wt% TIC composite in uniaxial tension was evaluated under vacuum over the temperature range of 1300° to 1550°C. The Al₂0₃/TiC composite exhibited the maximum elongation of 66% at an initial strain rate of 1.19 X l0^-4 s^-1 at 1550°C. The stress exponent calculated from peak stresses of true stress-true strain curves at 1500^OC was 3.8, which was in good agreement with that obtained by changing the crosshead speed during the tension test. The apparent activation energy at 20 MPa was 853 kJ/mol. In addition the deformation of the Al₂O³/TiC composite in uniaxial tension at elevated temperature was accompanied by cavitation.     

Experimental Observations of High Fracture Resistance in Silicon Carbide/Alumina Laminated Composites

Model laminated composites were fabricated with porous-Al₂O₃ interfaces between SiC bars. The porous Al₂O₃ was deposited using an aerosol spray deposition technique, and the sandwich specimen was fabricated by hot pressing. Residual thermal stresses were present in the interface because of the difference in the coefficients of thermal expansion of SiC and Al₂O₃. Crack deflection was observed with measured interfacial fracture resistances that were considerably higher than the deflection threshold predicted by the He–Hutchinson criterion. Examination of the fracture surface revealed a tortuous crack path and significant crack–flaw interaction.     

Preparation of Titanium Nitride/Alumina Laminate Composites

A preparation route for TiN/Al₂O₃ laminate composites has been described. A water-based process using Al₂O₃ and TiN slurries with solids contents of 40 and 35 vol%, respectively, was used to make TiN and Al₂O₃ tapes. The removal of the binder was monitored by weight-loss measurements in a thermogravimetry unit. Bodies composed of Al₂O₃ and TiN tapes were densified at temperatures of 1400° and 1500°C using the Spark Plasma Sintering^® (SPS) technique. Densities of >98% of the theoretical densities were approached. Crack-free and almost fully densified TiN/Al₂O₃ compacts were prepared by heating the burned-out green bodies to the final sintering temperature (1500°C) at a rate of 100°C/min, and with a holding time of 5–10 min, under a pressure of 75 MPa. The microstructures of the obtained compacts were studied using scanning electron microscopy. Grain sizes in the sintered Al₂O₃ and TiN compacts were similar to those of the precursor powders. Hardness and indentation fracture toughness were measured at room temperature, and the monolithic compacts as well as the laminate composites exhibited anisotropic mechanical behavior; i.e., the cracks propagated much more easily in a direction parallel to the laminas than perpendicular to them.     

Fracture Resistance Behavior of Silicon Carbide Whisker-Reinforced Alumina Composites with Different Porosities

Fracture resistance behavior was characterized for SiC-whisker-reinforced alumina composites with porosities ranging from 0.6% to 11.5% The composites were hot-pressed from an Al₂O₃ powder with 25 wt% SiC whiskers. Strengths of individual specimens were measured in four-point flexure either for natural flaws or for Vickers-indentation flaws as a function of radial crack size. Indentation crack sizes were controlled with indentation loads which varied between 2 and 200 N. A novel method of analysis of these measurements indicates that the fracture resistance of these composites increases as a function of crack extension, a rising R curve. This behavior is interpreted in terms of tractions from both crack-bridging whiskers and interlocking grains, which develop in the wake of the crack tip as it extends. A decrease in porosity raises the level of fracture resistance, but has a negligible effect on the relative steepness of the R curve. The sizes of natural flaws which causes failure in flexure testing were also estimated from analysis of the data.     

Enhanced Mechanical Properties of Alumina by Dispersed Titanium Diboride Particulate Inclusions

The mechanical properties of composite ceramics composed of 0 to 20 vol% of titanium diboride particles dispersed in an α-alumina matrix were investigated. The alumina–titanium diboride composite powder was hot-pressed at 1470°C for 20 min to achieve over 98.8% of the theoretical composite density. The strength and fracture toughness of the twophase, hot-pressed composite were both significantly improved compared to the single-phase alumina. Results from different methods of measuring the stress intensity factor, ( K _{I c} ) are compared and discussed.     

Silicon Carbide Whisker/Alumina Matrix Composites: Effect of Whisker Surface Treatment on Fracture Toughness

The fracture toughness of a 30 vol% SiC whisker/Al₂O₃ matrix composite was evaluated as a function of whisker surface chemistry. Two types of SiC whiskers (Silar-SC-9 and Tateho-SCW-1-S) were investigated. Modification of the whisker surface chemistry was achieved by subjecting the whiskers to thermal treatments under controlled atmospheres. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy, was correlated to the fracture toughness of the composites.     

R-Curve Behavior in a Silicon Carbide Whisker/Alumina Matrix Composite

R -curve measurements were performed on a SiC whisker/Al₂O₃ matrix composite. A controlled flaw/strength technique was utilized to determine fracture resistance as a function of crack extension. Rising R -curve behavior with increasing crack extension was observed, confirming the operation of wake toughening effects on the crack growth resistance. Observations of crack/microstructure interactions revealed that bridging by intact whiskers in the crack wake was the mechanism responsible for the rising R -curve behavior.     

Processing and Properties of Sol–Gel-Derived Alumina/Silicon Carbide Nanocomposites

Dense alumina/5 vol% SiC nanocomposites were prepared by sol–gel processing using nanosized (180 nm) precoated SiC powders and a commercial boehmite sol. The SiC powder was precoated with boehmite by a controlled heterogeneous precipitation from an aluminum nitrate solution. The coated SiC powder was then dispersed in a boehmite sol, gelled, calcined, and densified by gas pressure sintering under argon atmosphere at 7–8 MPa pressure. The dependence of the calcination conditions on densification, the effect of seeding on the microstructural development, as well as the mechanical behavior of the sintered specimens, are presented and discussed.     

Thermal Diffusivity/Conductivity of Alumina—Silicon Carbide Composites

Thermal diffusivity and conductivity values for several Al₂O₃-SiC whisker composites were determined. The thermal diffusivity values spanned the range from 373 to 1473 K, and thermal conductivity data wre obtained between 305 and 365 K. The thermal diffusivity decreased with increasing temperature and increased with SiC-whisker content. An estimate of the thermal conductivity of the whiskers was obtained from the direct thermal conductivity measurements, but attempts to derive whisker conductivity values from the thermal diffusivity data were not successful because the laser flash method lacks the required accuracy and precision. Specimens were subjected to two different thermal quench experiments to investigate the effect of thermal history on diffusivity. In the most severe case, multiple 1073- to 373-K quenches, radial cracks were observed in the test specimens; however, there was no change in diffusivity. The lack of sensitivity to thermal cycling appears to be related to the sample size.     

Effect of Silicon Carbide Whisker and Titanium Carbide Particulate Additions on the Friction and Wear Behavior of Silicon Nitride

A Si₃N₄/TiC composite was previously demonstrated to exhibit improved wear resistance compared to a monolithic Si₃N₄ because of the formation of a lubricious oxide film containing Ti and Si at 900°C. Further improvements of the composite have been made in this study through additions of SiC whiskers and improved processing. Four materials—Si₃N₄, Si₃N₄/TiC, Si₃N₄/SiC_wh, and Si₃N₄/TiC/SiC_wh— were processed to further optimize the wear resistance of Si₃N₄ through improvements in strength, hardness, fracture toughness, and the coefficient of friction. Oscillatory pin on flat wear tests showed a decrease in the coefficient of friction from ∼0.7 (Si₃N₄) to ∼0.4 with the addition of TiC at temperatures reaching 900°C. Wear track profiles illustrated the absence of appreciable wear on the TiC-containing composites at temperatures above 700°C. Microscopic (SEM) and chemical (AES) characterization of the wear tracks is also included to deduce respective wear and lubricating mechanisms.     

Oxidation Resistance of Alumina–Titanium Nitride and Alumina–Titanium Carbide Composites

The presence of TiC or TiN paritcles in an Al₂O₃ matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed at T < 900°C and at 900°C ≤ T ≤ 1100°C. At low temperatures ( T < 900°C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO₂ rutile crystals that subsequently grow and coalesce. When a continuous TiO₂ rutile layer is formed ( T ≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O₂ through a thick TiO₂ layer is proposed as the governing step.     

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.     

Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites

Ceramic-matrix composites are being developed to operate at elevated temperatures and in oxidizing environments. Considerable improvements have been made in the creep resistance of SiC fibers and, hence, in the high-temperature properties of SiC fiber/SiC (SiC_f/SiC) composites; however, more must be known about the stability of these materials in oxidizing environments before they are widely accepted. Experimental weight change and crack growth data support the conclusion that the oxygen-enhanced crack growth of SiC_f/SiC occurs by more than one mechanism, depending on the experimental conditions. These data suggest an oxidation embrittlement mechanism (OEM) at temperatures <1373 K and high oxygen pressures and an interphase removal mechanism (IRM) at temperatures of ≳700 K and low oxygen pressures. The OEM results from the reaction of oxygen with SiC to form a glass layer on the fiber or within the fiber–matrix interphase region. The fracture stress of the fiber is decreased if this layer is thicker than a critical value ( d > d _c) and the temperature below a critical value ( T < T _g), such that a sharp crack can be sustained in the layer. The IRM results from the oxidation of the interfacial layer and the resulting decrease of stress that is carried by the bridging fibers. Interphase removal contributes to subcritical crack growth by decreasing the fiber-bridging stresses and, hence, increasing the crack-tip stress. The IRM occurs over a wide range of temperatures for d < d _c and may occur at T > T _g for d > d _c. This paper summarizes the evidence for the existence of these two mechanisms and attempts to define the conditions for their operation.     

Silicon Carbide Platelet/Alumina Composites: II, Mechanical Properties

The mechanical properties, i.e., Young's modulus, fracture toughness, and flexural strength, of SiC-platelet/Al₂O₃ composites with two different platelet sizes were studied. Both Young's modulus and the fracture toughness of composites using small platelets (12 μm) increased with increasing SiC volume fraction. Maximum values for toughness and Young's modulus of 7.1 MPa·m^1/2 and 421 GPa were obtained for composites containing 30 vol% platelets. Composites fabricated using larger platelets (24 μm), however, showed spontaneous microcracking at SiC volume fractions of ≤0.15. The presence of microcracks decreased Young's modulus and the fracture toughness substantially. Two types of radial microcracks were identified by optical microscopy and found to be consistent with a residual stress analysis. Anisotropy in fracture toughness was identified with a crack length indentation technique. Cracks propagating in a plane parallel to platelet faces experienced the least resistance, which was the the lowest toughness plane in platelet composites with preferred orientation. Enhanced fracture toughness was found in the plane parallel to the hot-pressing direction, but no anisotropy in toughness was observed in this plane. The flexural strength of alumina showed a decrease from 610 to 480 MPa for a 30 vol% composite and was attributed to the presence of the platelets.     

Effect of Silicon Carbide Reinforcement on the Properties of Combustion-Synthesized Titanium Silicide

The formation of various phases in self-propagating high-temperature synthesis of titanium silicide with 15 wt% SiC was studied. It was observed that the addition of SiC as a reinforcing agent significantly influences the thermodynamics and kinetics of the combustion process when compared with the combustion synthesis of titanium silicide alone. Hardness and fracture toughness of this composite were directly obtained from the porous compact by using micro-indentation technique and the fracture toughness of this composite was found to be significantly enhanced with the addition of SiC.     

Silicon Carbide/Silicon and Silicon Carbide/Silicon Carbide Composites Produced by Chemical Vapor Infiltration

Composites of SiC/Si and SiC/SiC were prepared from single yarns of SiC. The use of carbon coatings on SiC yarn prevented the degradation normally observed when chemically vapor deposited Si is applied to SiC yarn. The strength, however, was not retained when the composite was heated at elevated temperatures in air. In contrast, the strength of a SiC/C/SiC composite was not reduced after this composite was heated at elevated temperatures, even when the fiber ends were exposed.     

Elevated-Temperature-Delayed Failure of Alumina Reinforced with 20 vol% Silicon Carbide Whiskers

Alumina composites reinforced with 20 vol% SiC whiskers were exposed to applied stresses in four-point flexure at temperatures of 1000°, 1100°, and 1200°C in air for periods of up to 14 weeks. At 1000° and 1100°C, an "apparent" fatigue limit was established at stresses of ∼ 75% of the fast fracture strength. However, after long-term (>6 weeks) tests at 1100°C, some evidence of crack generation as a result of creep cavitation was detected. At 1200°C applied stresses as low as 38% of the 1200°C fracture strength were sufficient to promote creep deformation and accompanying cavitation and crack generation and growth resulting in failures in times of <250 h.     

Microstructure and Composition of Alumina/Aluminum Composites Made by Directed Oxidation of Aluminum

Two Al₂O₃/Al composites, grown by the directed oxidation of molten Al alloys at 1400 and 1600 K, were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and wet chemical analysis. The materials were found to contain a continuous network of Al₂O₃, which was predominantly free of grain-boundary phases and was made up of nanometer- to micrometer-sized crystallites, a continuous network of Al alloy, and isolated inclusions of Al alloy. No crystallographic orientation was observed in the metallic phase, whereas the Al₂O₃ was oriented with its c axis parallel to the growth direction. The higher process temperature yielded a lower metal content and less connectivity of the metallic consituent.     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.