Micro-grit blasting to enhance adhesion of diamond coating on Ti6Al4V

Abstract

Diamond coatings are of industrial importance due to their tribological properties like wear resistance and low friction. These properties can be exploited only when the coating has good adhesion to the substrate. Substrates such as titanium alloys exhibits poor adhesion due to differences in the coefficient of thermal expansion (CTE) between the diamond coating and the substrate. This study investigates the enhancement of diamond coating adhesion to substrate by micro-grit blasting. The effect of micro-grit blasting pressure and angle of blasting are systematically studied. The micro-grit blasted surface is characterised by the roughness parameters such as skewness and kurtosis. Diamond coating is carried out on the grit-blasted substrates using the hot filament chemical vapour deposition (HFCVD) method and then characterised using electron microscopy, Raman spectroscopy, X-ray diffraction, and scratch testing. The skewness was seen to be negative, and did not vary much with grit-blasting conditions. A high value of residual stress has been observed in all the samples. Pressure was inversely correlated with kurtosis and is one of the dominant factors influencing kurtosis variation. Surfaces with lower kurtosis value were found to have higher adhesion strength. The effect of angle on adhesion depends on the relative value of skewness at the given angle. Adhesion strength is proportional to the skewness in the case of angle variation. The crystal size of the diamond grown for both parameters (angle & pressure) initially increases and then decreases depending on the surface free energy. Higher the surface free energy lower will be the average crystal size.