Topographical orientation effects on surface stresses influencing on wear in sliding DLC contacts, Part 2: Modelling and simulations

The effects of surface roughness and topographical orientation on surface stresses influencing wear have been investigated for diamond like carbon (DLC) coated steel surfaces with three levels of surface roughness in the range of 0.004-0.11 µm Ra values, and with topographical groove orientations of 0°, 45° and 90°. A novel multiscale numerical finite element method (FEM) model was developed to integrate the layered and microstructural material features with the grooved topography. Fractal geometry and a surface voxelisation based approach were utilised to derive 3D surface topography. The surface texture representation includes: fractal signatures, which are sets of fractal dimensions calculated at individual scales in different directions, texture aspect ratio describing surface anisotropy, and texture direction signatures calculated by the variance orientation transform (VOT) method. The simulations show details of the main topographical orientation effects on local stresses affecting wear as they appear at a single scratch test with a spherical diamond ball and in a self-mated sliding situation of two rough surfaces. The 45° sliding direction in relation to grooves resulted in a mixed state of surface loading in the scratch test contact. In the complex state of stress-strain within the roughness peaks the overall tensile stress decreased, leading to greater surface resistance to cracking as compared to 0° and 90° directions. Model based calculations showed that the surface structure was about four times more rigid in the direction of grooves as compared to the more flexible behaviour in the perpendicular direction. This behaviour was empirically confirmed. The numerical calculations of rough vs rough sliding surface include real surface topographical features at various scales, material microstructural features down to nano-scale and topographical-microstructural interaction features. This approximation is thus more comprehensive than the classical approach. The real area of contact was 15-30% of the apparent contact area. The macro-topography dominated the tendency for surface cracking and plastic deformation, which is influencing on both wear and friction, while the micro-topographical features contributed to cracking and deformation by less than 40%.