The surface spot defects were found on internal tooth and external tooth of gear coupling which is used to joint rotating shaft in nuclear main pump. The spot defects are slightly darker in color and their metallic luster is dim. The spot defects size are from 3 mm to 10 mm. The investigationwascarried on cross-sectional specimen taken from normal and abnormal area, namely spot defect, by scanning electron microscope, electron probe microanalysis, X-ray diffraction and Vickers hardness tester. The compound ...