Strength retention of silicon nitride after long-term oil immersion exposure

Two commercial grade silicon nitride ceramics (Honeywell GS44 and Kyocera SN235) were exposed to an oil ash to evaluate the long-term corrosion/oxidation resistance in a simulated diesel engine environment. The exposure condition was at 850 °C for 1000 h in air. Subsequently, the exposed specimens were tested in flexure for strength degradation at room temperature and 850 °C at stressing rates of 30 MPa/s and 0.003 MPa/s in air, respectively. A similar set of specimens not exposed to the oil ash was also tested in flexure for purpose of comparison. Little change in strength was measured after 1000 h exposure in the oil ash environment. Also, the values of Weibull modulus obtained for all of the exposed silicon nitride materials were similar to those with the unexposed specimens whose strength were obtained under the same conditions. However, both exposed and unexposed GS44 specimens exhibited a low fatigue exponent, suggesting a susceptibility to slow crack growth at test temperature. In addition, detailed SEM/EDAX analyses indicated that no oil ash elements (e.g., Zn, Ca, P, Na, and S) were detected beneath a thin layer in the surface; thus, there were no changes in the chemistry of the secondary phase(s) within the bulk. These elements were detected only in a region about 1-3 μm below the exposed surface, but no apparent changes in microstructure observed. Results of mechanical properties and microstructural characterizations indicated that these candidate silicon nitride materials exhibited excellent corrosion/oxidation resistance in the simulated diesel engine environment and, based on their excellent mechanical strengths, would be ideal candidates for diesel engine exhaust valve applications.