Relationship between flexural strength and surface roughness for hot-pressed Si3N4 self-reinforced ceramics

The hot-pressed Si3N4 self-reinforced ceramics with Y2O3 and Al2O3 addition were first ground using the diamond wheel with the diamond grit of 64 μm size, and then polished using the diamond abrasives of 15, 6 and 1 μm size. The surface morphologies were examined by scanning electron microscopy (SEM) and atomic force microscope (AFM). The flexural strength at room temperature showed a net relationship with surface roughnesses (Ra, Rz, R3z, Rt, Rq) the increase of which followed the surface finished and defect disappearance. The flexural strength and the negative square root of surface defect size, maximum peak-to-valley height of surface, fit the following equation: σf=ARt−BRt+C, where surface compressive residual stress and defect size interact competitively on strength. Sharp peaks and deep grooves were undoubtedly the fracture origins after the first grinding by the diamond wheel. The obvious surface defects had been eliminated by the following polishing, where fracture may be initiated from subsurface defects; however, there was still some local damage observed by AFM due to diamond abrasion. The mirror finished surfaces should be stress free.