Fracture resistance and contact damage of TiN particle reinforced Si3N4 ceramics

The R-curve behavior, spherical Hertzian contact damage and cyclic contact damage of a bearing grade Si3N4 ceramic reinforced with TiN particle dispersion are investigated and compared with the performance of a reference Si3N4 ceramic without TiN but with a similar microstructure. These ceramics were produced by firing powder compacts of Si3N4, Y2O3 and Al2O3, with and without additional AlN and TiO2, at 1800°C for 2 h in 0.9 MPa N2, followed by hot isostatic pressing at 1700°C. The ceramic without TiN has an R-curve with an initial value around 5.5 MPam1/2, which slowly rises to 6.5 MPam1/2 after a crack extension of 300 μm. In contrast, the Si3N4 ceramic with TiN particles has a higher initial KR value but the R-curve remains relatively flat during subsequent crack growth. The latter ceramic exhibits a higher critical stress for permanent deformation and for ring-crack formation on the indented surfaces. It also demonstrates a better retention of the residual strength after single and repeated Hertzian indentation. The superior performance of the TiN-reinforced ceramic is attributed to the strengthening of grain boundary by TiN particles. Specifically, the resistance against contact and fatigue damage of this bearing grade Si3N4 is explained in terms of the effect of high grain boundary strength on the R-curve behavior, grain boundary microcracking, and the balance between crack shielding accumulation and degradation by grain boundary sliding during cyclic loading.