An observation on the initiation of micro-pitting damage in as-ground and coated gears during contact fatigue

The effect of surface features on as-ground gears and the influence of BALINIT® C and Nb–S coatings on the initiation of micro-pitting damage have been examined in case-carburised and tempered S156 steel helical gears subjected to contact fatigue using back-to-back gear testing. The initiation of micro-pitting damage has been studied using scanning electron and optical microscopy examination. It has been observed that micro-cracks tend to initiate against the sliding direction preferentially from micro-valleys on the surface of as-ground gears, particularly when the depth of micro-valley is favourably oriented at an angle against the sliding direction. It has been found that the micro-pitting damage is drastically reduced in both coated gears. In BALINIT® C coated gears, the micro-scale surface irregularities are removed by a wear process resulting in a polished surface of the gear flank below the pitch diameter which is attributed to the effect of hard BALINIT® C coating. However, there is region of interface between the polished and unpolished regions of gear flank where there is small scale initiation of micro-pitting due to the absence of BALINIT® C coating at micro-valleys. In Nb–S coated gears, the coating tends to penetrate and fill-up the micro-valleys on as-ground surface modifying the gear flank surface. Hence, the micro-pits tend to initiate only at some micro-valley sites where the Nb–S coating is locally removed during running. This study shows that, both BALINIT® C and Nb–S coated gears show enhanced resistance to micro-pitting damage by removing localised stress concentration at micro-valleys present on as-ground gears.