Effect of long-term oil immersion test on mechanical reliability of candidate silicon nitride ceramics for diesel engine applications

Two lots of commercially available silicon nitride ceramics (i.e., Honeywell GS44 and Kyocera SN235) were exposed to an oil ash environment, as so-called oil immersion test, to evaluate the long-term corrosion/oxidation resistance in a simulated diesel engine environment. The exposure condition was at 850°C for 1000h in air. Subsequently, the exposed bend bars were tested for strength degradation at room temperature and 850°C at stressing rates of 30 MPa/s and 0.003 MPa/s in air, respectively. Little change in strength was measured after 1000h exposure to the mentioned oil ash environment. Also, the values of Weibull modulus obtained for all of the exposed silicon nitride materials were similar to those with as-machined surfaces tested under the same conditions. In addition, the detailed SEM/EDAX analyses indicated that no oil ash elements were detected in the bulk of materials, especially along the grain boundaries, resulting in no changes in chemistry of secondary phase(s). The oil ash elements were only present in a region about 1-3 μm below the exposed surface with 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 to the diesel engine environments, and would be excellent candidates for diesel engine exhaust valve applications.