The purpose of this study is to confirm the validity for evaluation of fracture toughness on the surface layer of ceramics by the sphere indentation test. The sphere indentation tests using HIP sintered Si_3N_4 specimens were carried out to obtain the ring crack initiation load for various indentation diameters 2R. On the assumption that a ring crack is induced from microcracks on the surface layer, the crack length c_i of microcracks were estimated from the experimental results in terms of a stress intensity factor K_I for a surface crack under the sphere-plate contact loading condition. The ring crack initiation condition was assumed as K_I.>K_<th> where K_<th> is the threshold fracture toughness and was assumed as 5.3 MPa・m^<1/2>. The mean value of c_i ws 8-9 μm and was almost independent of 2R in the case of large 2R.
