R-CURVE AND FATIGUE-CRACK GROWTH-BEHAVIOR IN SILICON-NITRIDE CERAMICS

The R-curve and fatigue behavior were investigated for two kinds of silicon nitride with different microstructures, In these materials the toughening exponents determined by the indentation-strength-in-bending method are almost zero, Moreover, the values of fracture toughness obtained by the indentation crack size measurement and the single-edge-precracked-beam method are the same. Therefore, it is concluded that these materials show hat R-curves. On the other hand, it is found that a considerable cyclic-fatigue effect, distinct from static fatigue, exists in these materials, Cyclic-fatigue crack growth rates are higher in small grained silicon nitride than in large grained one, When the crack growth behavior is represented by a power law relationship including two terms of the stress intensity range Delta K and the maximum stress intensity K-max i,e, da/dN = C(K-max)(p) (Delta K)(q), significant differences in the values of the exponent p and q between these materials are not recognized, On the basis of the above results and other data available, the factors affecting the cyclic-fatigue behavior and cyclic fatigue mechanisms are discussed.