Fatigue Crack Growth Behavior of Silicon Nitride: Roles of Grain Aspect Ratio and Intergranular Film Composition

摘要

The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO-RE_2O_3 and results were compared with a commercial Al_2O_3-Y_2O_3-doped material. Decreasing stress intensity range (AK) fatigue tests were conducted using compact-tension specimens to measure steady-state fatigue crack growth rates. Specimens doped with MgO-RE_2O_3 additives showed a significantly higher resistance to crack growth than those with Al_2O_3-Y_2O_3 additives and this difference was attributed to the much higher grain aspect ratio for the MgO-RE_2O_3-doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R-curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO-RE_2O_3-doped silicon nitrides displayed similar steady-state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.