A bearing-grade silicon nitride with fine microstructure and a turbine-grade silicon nitride with coarse microstructure were studied with respect to the influence of their microstructures on (a) crack-growth-resistance behavior, (b) strength degradation due to Vickers indentation, and (c) crack initiation in quasi-static indentation with WC spheres. The turbine grade exhibited strong rising crack-growth resistance and less strength degradation due to Vickers indentation as compared to the bearing grade. Partial-ring or C cracks initiated in Hertzian indentation and the critical loads exhibited linear (Auerbach) variation with indenter radius above a critical value. For smaller radius, indentation plasticity preceded C-crack initiation. The bearing grade exhibited higher critical loads for C-crack initiation, but showed greater extension toward a ring crack than the turbine grade. These differences in crack initiation and growth were consistent with the differences in crack initiation and propagation toughness of the two grades. A ball-on-ball impact analysis was used to predict the critical velocities for initiating C cracks in the impact of silicon nitride surfaces with WC spheres.