Anisotropy of Surface Roughness on Aluminium Sheet Studied by Atomic Force Microscopy

Atomic force microscope images of aluminium sheet are used to calculate the mean roughness Ra and the root mean square roughness Rq for different directions (theta) across the samples. The angle determined values of Ra(theta) and Rq(theta) are significantly lower in the rolling direction than in any other direction. The results provide a clear demonstration of the vector nature of surface roughness. Local features in an angle determined Ra(theta) profile are also compared with the positions of peaks and valleys in the matching topographical cross-section of the surface running at right angles to theta. It is further found that the distribution of z heights for an imaged sample is usually not Gaussian in shape unless the z heights are measured about mean lines constructed across the surface parallel with the rolling direction. The results are relevant for controlling lubrication performance in the aluminium rolling process.