Adhesion analysis of polycrystalline diamond films on molybdenum by means of scratch, indentation and sand abrasion testing

Diamond films have been grown by hot-filament chemical vapour deposition (CVD) on molybdenum substrates under different growth conditions. The films grown with increasing substrate temperatures show a higher interconnection of diamond grains, whereas increasing methane concentrations in the 0.5-4.0% range lead to a transition from micro- towards nanocrystalline films. X-ray diffraction analysis shows Mo2C interlayer formation. Indentation, scratch and sand erosion tests are used to evaluate the adhesion strength of the diamond films. Using steel ball indenters (circle divide 750 mum), indentation and scratch adhesion tests are performed up to final loads of 200 N. Upon indentation, the load values at which diamond film failure such as flaking and detachment is first observed, increase for increasing temperatures in the deposition temperature range of 450-850 degreesC. The scratch adhesion tests show critical load values in the range of 16-40 N normal load for films grown for 4 h. In contrast, diamond films grown for 24 h at a methane concentration of 0.5% do not show any failure at all upon scratching up to 75 N. Film failure upon indenting and scratching is also found to decrease for increasing methane concentration in the CVD gas mixture. The sand abrasion tests show significant differences in coating failure for films grown at varying CH4/H-2 ratios. In contrast to the other tests, here best coating performance is observed for the films deposited with a methane concentration of 4%. (C) 2004 Elsevier B.V. All rights reserved.