Intermediate temperature inert strength and dynamic fatigue of candidate silicon nitrides for diesel exhaust valves

The flexure strength distributions of AS800, GS44, KYON3000, KYON3500, N7202, NT154, and NT551 silicon nitrides were determined at 850°C in ambient air at 30 and 0.003 MPa/s with ASTM C1161-B bend bars. Their inert strength distributions at 850°C were compared to those obtained at 20°C, and it was found that all grades with the exception of NT154 exhibited a loss in inert characteristic strength between 20 and 850°C. Additionally, all exhibited further decreases in strength to varying degrees when tested at 0.003 MPa/s at 850°C indicating varied susceptibilities to fatigue. Lastly, the inert strength was dependent on machining orientation at 20 and 850°C; however, the strength dependence at 850°C and 0.003 MPa/s was relatively small indicating a change in material state and strength-limiting flaw mechanism at this slow test rate. It was found that at a temperature as low as 850°C, that the secondary phase in silicon nitride can still suffer problems associated with oxidation that can lower its strength and fatigue resistance. Therefore, it is clearly advantageous to use a silicon nitride for diesel exhaust valves that have a completely crystallized grain boundaries that are stable to oxidation up to 850°C or a glassy phase that has a softening temperature above the intended service temperature.