We have seen small black spots or craters on the surface of worn machine parts. They are considered to be caused by discharges of static electricity. However, there is no clear evidence of being caused by discharges of static electricity, as nobody can see the phenomena when discharges are caused there. Using TEM (Transmission Electron Microscope) and SEM (scanning electron microscope), the authors examined the black spots which were found on the lubricated surfaces of machine components and the ones which were artificially caused by spark discharges on a surface of a test material. The results of this study showed that the grains of black spots on the lubricated surface of worn machine parts were almost identical to those which were artificially caused by discharges of static electricity. Moreover, melted wear debris that might have been formed during spark discharge, was found in an oil filter, showing that there was high temperature melting the metal.
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