Single-point scratch tests, using a hemispherical pin specimen made of diamond on a flat specimen, were carried out in a scanning electron microscope. Five kinds of heat-treated bearing steels (SUJ 2) were used for the flat specimen. One was quenched at 860 °C (750 HV), and the others were quenched at 860 °C and then tempered at 200 °C, 350 °C, 500 °C and 750 °C (650 HV, 560 HV, 380 HV and 250 HV) respectively. In situ observations of the wear processes in the scanning electron microscope show that the abrasive wear mechanism changes from the ploughing mode to the wedge-forming mode and then to the cutting mode as the degree of penetration increases. Based on the observations, an abrasive wear diagram was obtained as a function of the degree of penetration and hardness of the flat specimen. The critical degree of penetration for the transition between the ploughing mode and the wedge-forming mode was independent of the hardness. However, the critical degree of penetration for the transition between the wedge-forming mode and the cutting mode decreased with increasing hardness. The degree of wear for the cutting mode increased with increasing hardness.
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