EFFECT OF GROUNG SURAFACE LAYER ON BENDING STRENGTH OF STRUCTURAL CERAMICS

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Effect of ground surface layr on bending strength of structural ceramics (Al_2O_3, SiC,Si_3N_4, and ZrO_2) was investigated. The main results were as follows.(1) Plastic deformation causing the compressive residual stress was mainly generated by the burnishing action of abrasive grains. The effect of burnishing diminished with an increase in the magnitude of fracture during the material removal ; the compressive residual stress was larger in the ceramics with higher bending strength but smaller in those with higher hardness under conventional grinding conditions.(2) The depth of the compressive residual stress can be estimated from the plastic deformation zone obtained by the sphere indentation which gives an analogous effect to the the burnishing action of abrasive grains.(3) The compressive residual stress of ceramics measured by X-ray is estimated to be considerably smaller than the peak stress realy existing in the surface layr even if the characteristic X-ray of shallower penetrat … More ion depth is used. This is because the effective penetration depth of the X-ray is not sufficiently shallow in comparison with the depth where compressive stress exists.(4) Deterioration of bending strength of ground ceramics varied depending upon fracture toughness and phase transformability : the deterioration was higher in Si_3N_4 but lower in ZrO_2.(5) The bending strength deteriorated was considerably recovered by annealing in Si_3N_4. This implied that crack healing effect overcame the disadvantage of elimination of compressive residual stress. On the other hand, the annealing of ZrO_2 rather deteriorated strength of ground specimen.(6) The finite element method analysis of elasto-plastic stress field in a spherical indentation and the fracture analysis of pre-existing crack in its stress field showed that plastic deformation could be caused without any crack extension. This implied that grinding could be carried out without strength deterioration.(7) Grain depth of cut which was statistically evaluated by newly developed method could be applied to estimate crack extension length and residual stress caused by grinding. Less