窒化ケイ素の研削き裂寸法の破壊力学的評価

抄録

The surfaces of bending specimens of silicon nitride were ground in two different grinding directions with a diamond wheel. One was parallel and the other was perpendicular to the longitudinal direction of specimens. The Weibull distribution of the bending strength of ground specimens was compared to that of lapped specimens. The average fracture strength normal to the grinding direction fell by about 25% in comparison with the average strength of lapped specimens. The residual stress distribution near the ground surface was measured by the cosφ method of X-ray stress measurement. The stress state was biaxial. One of the pricipal directions of stresses was along the grinding direction. Both of the principal residual stresses, σ₁ and σ₂, were compressive and decreased exponentially with increasing the distance from the surface. The residual stress was diminished at about 30μm in depth. The increment of strength due to compressive residual stresses was determined by calculating the stress intensity factor of a semi-elliptical surface crack in the residual stress field. The size of grinding flaws degrading bending strength was estimated from the relation between the fracture strength and the surface crack size. The depth of grinding flaws parallel to the grinding direction ranged approximately from 40 to 70μm; the deepest point of flaws was out of the zone of compressive residual stresses.