MICROSTRUCTURE OF SILICON-NITRIDE CERAMICS AND MORPHOLOGY OF SURFACE DAMAGE-INDUCED BY ROUND INDENTER

Four kinds of silicon nitride ceramics with different grain sizes and different amounts of the boundary phase were prepared, and the polished surface of the samples was indented with a round diamond indenter. The morphology of the surface deformation, cracks and the structure under the indented surface were investigated. After indentation, a round impression was formed on the surface and the swelling was found around the impression. The diameter and the maximum depth of the impression were larger in the samples with a large grain size and a large amount of the boundary phase. Two types of cracks were observed, namely a radial crack and a Hertzian crack. The radial crack propagated from the impression above the indentation load of 147 N, and its length was larger in the samples with a coarse grain size. The Hertzian crack was found inside the impression of the samples with a fine grain size. After etching in a NaOH melt a ring shape area was corroded away from the indented samples. The area of the corroded region depended on the grain size and the amount of the boundary phase.