Effects of surface roughness on scratch resistance and stress-strain fields during scratch tests

A three-dimensional scratch model is proposed to numerically investigate the effects of surface roughness on the scratch resistance, stress concentration, residual stress and plastic deformation during scratch tests, with the help of the finite element method. Without loss of generality, the surface roughness of a coating is modelled by a sinusoidal function. The stress and plastic strain fields with various geometry parameters of roughness are obtained and discussed. In comparison with a smooth coating, the stress concentration, the residual stress, and the scratch resistance are significantly intensified and the plastic strain is increased by a factor of five in a corrugated coating, which may tremendously reduce the material performance. Consequently, the effects of surface roughness should not be ignored in both experiments and simulations. The geometry of roughness is determined by both the wave amplitude and wavelength, and their influences on the tribological behaviors can be significant. While existing experiments are only focused on the roughness in vertical direction (corresponding to the wave amplitude), the effects of wave amplitude and wavelength can be qualitatively different in many aspects and should be considered separately. The scratch depth and the area of contact region between the indenter and coating are not very sensitive to a moderate change in the wave amplitude, while they are reduced obviously with a rising wavelength. While the stress concentration increases monotonously with a rising amplitude, it becomes more complex and the decrease of wavelength can reduce the stress concentration in some cases. A growth in roughness by a large wave amplitude or a small wavelength leads to an enhancement on scratch friction coefficient and friction resistance, which is qualitatively consistent with experiments. The findings here contribute to a new way for quantitative evaluations of the effect of surface roughness and should be beneficial to modelling and simulation in scratch tests and also to the optimum designs on the surface coating for experiments.