Effect of grinding method on bending strength of silicon nitride

The surface of bending specimens of silicon nitride is finished by the surf ace-grinding method or the face-grinding method. A disk-shaped or cup-shape d diamond wheel with the grit size of # 200/230 is used for grinding. The f eed direction of the grinding wheel is either parallel or perpendicular to the longitudinal direction of bending specimens. The grinding flaw size is estimated from the bending strength and the residual stress distribution ne ar the ground surface. In the case of surface-grinding, the depth of grindi ng flaws formed parallel to the feed direction ranges from 40 to 70 mu m. T his grinding flaw extends outside the compressive residual stress zone, so the stress shielding effect is small. The large flaw size reduces the fract ure strength. On the other hand, the depth of flaws of face-ground specimen s ranges from 10 to 30 mu m. The fracture strength after face-grinding is h igher than that of lapped specimens because the flaw lies in the compressio n zone. The face-grinding method is recommended to minimize machining costs because finer finishing is unnecessary after face-grinding.