Advanced Materials & Technology Research Laboratories, Nippon Steel Corporation, Kawasaki
The relationship between fracture toughness and bending strength in Si3N4 ceramics was studied. Three-point bending tests were carried out. The fracture toughness was estimated by the single edge precracked beam method. Grain morphology was observed using transmission microscopy.
The fracture toughness increased with grain growth. The strength increased with fracture toughness up to KIC=7 MPam0.5 and decreased in spite of increasing fracture toughness over 7 MPam0.5.
The linear elastic fracture mechanics is not applicable to small cracks in ceramics. Because of this, the effective crack size for fracture is not equal to the original equivalent crack length. In order to elucidate the fracture toughness-strength relation, the effective crack size must be estimated. In the previous work, the tendency in the crack size-strength relation was explained by adding a material-dependent constant to the original equivalent crack length. But this constant was not related to the physical process of fracture and was calculated according to the experimental results. In this study, the effective crack size of fracture origin was estimated experimentally and the fracture toughness-strength relation was investigated using the effective crack size.
The increase in strength up to 7 MPam0.5 resulted from a constant effective crack size. The decrease in strength over 7 MPam0.5 was due to both the increase in fracture toughness by the R-curve behavior and the increase in effective crack size.
(Received September 29, 1994)
ceramics, silicon nitride, fracture toughness, strength, fracture mechanics, grain size, equivalent crack length
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