Polished diamond coatings for dry tapering of aluminum

Abstract

For economic and environmental reasons, dry forming is of increasing interest due to the shortening of process chains, cost savings and reduction of environmental pollution. The aim of these investigations is to examine to what extent CVD diamond coatings are suitable for dry forming of aluminum and to identify the surface topology requirements for a low friction coefficient and low wear. It was discovered that the inner contour of a 5 mm high forming die including undercuts could be coated with a CVD diamond coating of a homogeneous layer thickness by turning the forming die upside down after half the coating time, so that the plasma flame enters through the other opening of the die. Based on oscillating ball on plate tribometer tests and tapering tests it could be shown that for the realization of dry piece goods processes with aluminum material, diamond coatings offer a great potential. It was possible to determine a roughness parameter which is equally capable of predicting the coefficient of friction and the wear rate of the aluminum counter body in the dry oscillating ball on plate tribometer tests for unpolished and mechanically polished CVD diamond coatings. This parameter is the peak material volume (Vmp) and is a measure of the volume of the top 10% of the surface. For dry piece goods processes of aluminum workpieces, the deposition of microcrystalline CVD diamond layers with subsequent post-treatment of the surface is recommended in order to achieve a peak material volume of less than 0.04 ml/m² in order to minimize wear of the aluminum counter body. The lower the Vmp value, the lower the coefficient of friction for the dry contact of diamond with aluminum. Nanocrystalline CVD diamond coatings offer a potential for dry forming of aluminum without post-treatment, but they are more suitable for processes such as wire drawing as they do not withstand repeated static friction. For post-treatment of CVD diamond coatings laser polishing with a picosecond-laser was investigated. It could be shown that the surface roughness could be significantly reduced by 72.5%. As soon as the CVD diamond coating is accessible by laser irradiation, the laser polishing process has a lower risk of delamination of the CVD diamond layer due to lower mechanical stress than applied by mechanical polishing.