Fatigue mechanism of a coating film under rolling contact loading: Study of methods for evaluation of rolling contact fatigue strength of coating films (1st Report)

Surface coatings are widely used to enhance the properties of machine components such as paper machine rolls. These surface-coated rolls are at present empirically designed because no method for evaluation of coating film strength has been developed.This paper describes fatigue tests of chrome-plated rolls under rolling contact loading conditions together with a detailed examination of the fracture mode to develop a method for evaluation of coating film strength.The results show that the coating film first fractures in the axial direction, a number of cracks are then initiated, and the surface film finally exfoliates. The number of stress cycles to exfoliation (flaking) is found to depend on the coating film thickness, whereas the number of stress cycles for crack initiation does not.A finite element stress analysis was performed to evaluate the coating film thickness dependence of crack initiation. The tangential stress responsible for the cracking showed little change when the coating film thickness was changed.The results suggest that cracking is caused by fatigue due to compressive tangential stress.