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Stress-corrosion cracking in a commercially available, hot isostatically pressed (HIPed), yttria-fluxed, silicon nitride was the prevalent mode of failure in specimens creepruptured at 1370°C. High-temperature diffusional processes associated with oxygen were responsible for the creation of an advancing stress-corrosion front that had formed at the specimen surface and advanced radially inward. The volume of material in the wake of the stresscorrosion front possessed a high concentration of lenticular cavities at two-grain boundaries, a high concentration of multigrain junction cavities, and large amorphous "pockets" in other multigrain junctions that were abnormally rich in oxygen and yttrium. The combination of tensile stress and the high concentration of cavities in the nearsurface volume of the material resulted in microcrack coalescence or the formation of a planar, stress-corrosion crack. The concurrent growth of the stress-corrosion front and crack during the tensile creep-rupture tests ultimately led to stress-induced failure.

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Quasi‐static Weibull strength‐size scaling of hot‐pressed silicon carbide is described. Two surface conditions (uniaxial ground and uniaxial ground followed by grit blasting) were explored. Strength test coupons sampled effective areas from the very small (4 × 10−3 mm2) to the very large (4 × 104 mm2). Equibiaxial flexure and Hertzian ring crack initiation were used for the strength tests, and characteristic strengths for several different specimen geometries were analyzed as a function of effective area. Characteristic strength was found to substantially increase with decreased effective area for both surface conditions. Weibull moduli of 9.4‐ and 11.7 well‐represented strength‐size scaling for the two ground conditions between an effective area range of 10−1 and 4 × 104 mm2. Machining damage was observed to be the dominant flaw type over this range. However, for effective areas <10−1 mm2, the characteristic strength increased rapidly for both ground surface conditions as the effective area decreased, and one or more of the inherent assumptions behind the classical Weibull strength‐size scaling were in violation in this range. The selections of a ceramic strength to account for ballistically induced tile deflection and expanding cavity modeling are considered in context with the measured strength‐size scaling. The observed size‐scaling is briefly discussed with reference to dynamic strength.

Executive SummaryThe inert strength and fatigue performance of a diesel engine exhaust valve made from silicon nitride (Si~Nq) ceramic were assessed. The Si~Na characterized in this study was manufactured by Saint Gobain / Norton Industrial Ceramics and was designated as NT551. The evaluation was performed utilizing a probabilistic life prediction algorithm that combined censored test specimen strength data with a Weibull distribution function and the stress field of the ceramic valve obtained from finite element analysis. The major assumptions of the life prediction algorithm are that the bulk ceramic material is isotropic and homogeneous and that the strength-limiting flaws are uniformly distributed.The results from mechanical testing indicated that NT551 was not a homogeneous ceramic and that its strength was a function of temperature, loading rate, and machining orientation.Fractographic analysis identified four different failure modes; 2 were identified as inhomogeneities that were located throughout the bulk ofNT551 and were due to processing operations. The fractographic analysis concluded that the strength degradation of NT551observed from the temperature and loading rate test parameters was due to a change of state that occurred in its secondary phase.. Pristine and engine-tested valves made from NT551 were loaded to failure and the inert strengths were obtained. Fractographic analysis of the valves identified the same four failure mechanisms as found with the test specimens.The fatigue performance and the inert strength of the Si~NAvalves were assessed from censored and uncensored test specimen strength dat~respectively. The inert strength failure probability predictions were compared to the inert strength of the Si~Nqvalves.. . . The inert strength failure probability predictions were more conservative than the strength of the valves. The lack of correlation between predicted and actual valve strength was due to the . nonuniform distribution of inhomogeneities present in NT551. For the same reasons, the predicted and actual fatigue performance did not correlate well.

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