Fatigue behavior of silicon nitride ceramics

This work was aimed to identify the effects of sintering additives on the fatigue life of silicon nitride (Si3N4) ceramics sintered via hot pressing, focusing on the salient toughening mechanisms. The fatigue life and probability of survival of YMH samples using MgO and Y2O3 as sintering additives were observed to be higher than those of YAH samples using Al2O3 and Y2O3 as sintering additives. This was attributed to 1) the higher aspect ratio of the elongated & beta;-Si3N4 grains in the YMH sample which were formed via phase transformation from the initial & alpha;-Si3N4 powders during sintering, promoting a higher extent of crack deflection and crack bridging; 2) the larger compressive residual stresses existing in the YMH sample to suppress fatigue crack initiation and propagation; and 3) the presence of only amorphous (or glassy) phase at & beta;-Si3N4 grain boundaries stemming from the reaction of MgO-Y2O3 additives with the SiO2 layer on the & alpha;-Si3N4 powder surface during sintering, without the formation of an unfavorable crystalline & beta;& PRIME;-SiAlON layer in-between the & beta;-Si3N4 grains and amorphous phase. This further contributed to the increasing crack deflection via more readily debonding, thus enhancing the fatigue life of the YMH sample.