Effect of surface finish and hard coatings on scuffing of steel discs
Dhulipalla, Ajay Krishna.
The principal purpose of the work described in this thesis was to develop an insight into the scuffing performance of steel discs used to simulate gear tooth contacts under severe conditions of load, sliding and high temperature. Different surface conditions of the discs such as ground, superfinished, ground/hard-coated, and superfinished/ hard-coated were investigated in various combinations and at sliding speeds of up to 16 m/s in order to obtain an understanding of the possible beneficial effect of improved surface finish and the application of hard coatings on scuffing performance under the severe conditions experienced in an aerospace transmission such as a helicopter main rotor gearbox. The lubricant used in all tests was an oil commonly used in aerospace transmissions. It was found that the diamond-like coating (DLC) investigated in this work had the effect of markedly improving the scuffing resistance of the hardened steel substrate. In tests using the ground/coated samples, for example, it was not found possible to produce scuffing within the load and temperature limits of the test rig used (2.0 GPa maximum Hertzian contact pressure or 300 C maximum disc bulk temperature) even at the high sliding speed of 16 m/s. In addition to improving scuffing resistance it was found that the effect of the hard coating was to reduce friction and operating temperatures for a given load/speed combination. Superfinishing of the steel discs also led to a reduction in friction, but did not produce a significant improvement in scuffing resistance. It was found that the combination of ground/hard-coated discs running against ground/uncoated discs produced a remarkable behaviour of the discs' bulk temperatures, which suggests that the hard coating acts as a thermal barrier. Micro-elastohydrodynamic lubrication (micro-EHL) analyses were conducted using surface profiles of discs used in the experimental work. These theoretical simulations demonstrated the very severe lubrication conditions which are present in the experiments. Pressure ripples far in excess of the calculated Hertzian pressure are predicted, and the films generated by hydrodynamic action are extremely thin. Under the most severe conditions transitory direct "dry" contact is predicted.