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Erosion rates of SiC-Al2OC ceramics, with Al2OC content varying from 5 to 75wt%, were assessed using 240-grit alumina abrasive particles accelerated to a velocity estimated at 120msec-1 and impacting the target at normal incidence. The target ceramics varied in hardness from 27.1 GPa for SiC-5wt% Al2OC to 10.8 GPa for SiC-75wt% Al2OC, but the fracture toughness was essentially independent of composition (Klc ã 3.5 MPa m1/2). The erosion weight loss varied linearly with the test duration for all the ceramics and the erosion rate decreased systematically with increasing target hardness; the hardness dependence of the erosion rate was, however, much greater than the predictions of the currently available erosion models.

Critical components in coal-liquefaction system, like letdown value trim and valve seats, are subject to severe wear from the erosive coal slurries. The slurry-erosion behavior of candidate valve trim and seat materials, like WC-Co cermets and hard ceramics (SiC, B4C), was studied by measuring the wear rates in a slurry-jet impingement test and by examining the wear craters in a scanning electron microscope. The erosion rates of the WC-Co cermets decreased monotonically with decreasing cobalt volume fraction in the range 36.9 to 5.1 Percent. The mechanism of erosion changed systematically from one involving preferential removal of binder and release of undamaged WC grains in high-binder alloys to a uniform transgranular wear of WC grains in the low-binder alloys. A sintered grade of SiC and a hot-pressed grade of B4C showed wear rates less than those of the cermets.

Transformation yield and plasticity, transformation zone sizes at crack tips and rising crack-growth-resistance (R-curve) behaviours were studied in a commercial-grade ceria partially stabilized zirconia polycrystalline material (CeO2-TZP). The yield stresses measured in three-point bending decreased from 390 to 176 MPa when the sintering temperature was varied from 1425 to 1525° C. The corresponding total plastic strain to fracture increased with decreasing yield stress. Crack-tip transformation zones in precracked, annealed and loaded single-edge-notch-bend specimens decreased significantly in size with increasing transformation yield stress; however, the R-curves were relatively insensitive to the yield stresses or the transformation zone sizes. The measured zone sizes and R-curves were examined in terms of both crack shielding and plastic yield strip zone models.

Increasing use of ceramics in structural applications has led to the development of a probabilistic design methodology that combines three elements: linear elastic fracture mechanics theory that relates strengths of ceramics to size, shape, and orientation of critical flaws, a characteristic flaw size distribution function that accounts for the size effect on strength via the weakest-link concept, and a time-dependent strength caused by subcritical crack growth or other mechanisms. This paper reviews recent research that has been focused on the first of the above three elements, the investigation of fracture criteria for arbitrarily oriented flaws in ceramics, i.e., the mixed-mode fracture problem in linear elastic fracture mechanics theory. Experimental results obtained with two-dimensional through cracks and three-dimensional surface (indentation) cracks are summarized and compared to mixed-mode fracture criteria. The effects of material microstructure and the stress state on mixed-mode fractures are discussed. The application of mixed-mode fracture criteria in reliability analysis is illustrated for several simple stress states in the absence of time-dependent strength degradation.

Fracture toughness of polycrystalline alumina and ceria partially stabilized tetragonal zirconia (CeO2-TZP) ceramics were assessed in combined mode I and mode II loading using precracked disk specimens in diametral compression. Stress states ranging from pure mode I, combined mode I and mode II, and pure mode II were obtained by aligning the center crack at specific angles relative to the loading diameter. The resulting mixed-mode fracture toughness envelope showed significant deviation to higher fracture toughness in mode II relative to the predictions of the linear elastic fracture mechanics theory. Critical comparison with corresponding results on soda-lime glass and fracture surface observations showed that crack surface resistance arising from grain interlocking and abrasion were the main sources of the increased fracture toughness in mode II loading of the polycrystalline ceramics. The normalized fracture toughness for pure mode II loading, (KII/KIc), increased with increasing grain size for the CeO2-TZP ceramics. Quantitative fractography confirmed an increased percentage of transgranular fracture of the grains in mode II loading.

Chevron-notched bend specimens in three-point loading are increasingly used for evaluation of fracture toughness of structural ceramics. Its advantages include stable precracking of the specimen during loading, a simple specimen geometry, and a compressive mode of loading that is convenient for testing ceramics at elevated temperatures. Subcritical crack growth caused by environmental interactions or mass transport phenomena at elevated temperatures, however, can affect the apparent fracture toughness measured. A numerical computational study was carried out to assess the effects of subcritical crack growth on crack stability in the chevron-notched three-point bend specimens. A power-law relationship between the sub-critical crack velocity (v) and the applied stress intensity (KI) was used along with compliance and stress-intensity relationships for the chevron-notched bend specimen to calculate the load response under fixed deflection rate and a machine compliance. The results indicate that the maximum load during the test occurs at the same crack length for all the deflection rates; the maximum load, however, is dependent on the deflection rate for rates below a critical rate. The resulting dependence of the apparent fracture toughness on the deflection rate is compared to experimental results on soda-lime glass and polycrystalline alumina.