THE EFFECT OF SUBSTRATE PROPERTIES ON TRIBOLOGICAL BEHAVIOR OF COMPOSITE DLC COATINGS

The unique features of DLC coatings in providing low friction and low wear and, at the same time, causing low wear to the counterface make t hem very attractive in industrial applications, in improving tribologi cal performance of mechanical components on various substrates. In thi s study, composite DLC coatings have been deposited on sintered ferrou s alloy, M42 tool steel, 2618 aluminium alloy, and 6063 aluminium extr usion substrates using the combined CFUBMS-PACVD technique. The effect of mechanical properties of substrate materials on tribological behav iour of the composite DLC coatings has been investigated at various lo ads on a ball-on-disk wear machine in dry air. A transition load was u sually observed for coatings on the various substrates except for the aluminium extrusion; above the transition load the coating was complet ely destroyed via some spallation/fragmentation process after 2 h slid ing, and the wear rate increased dramatically with further increase in load. The coating system on sintered ferrous alloy substrate exhibite d the highest transition load among the four types of substrates studi ed. This is considered to have resulted from the combined effects of t he lower elastic modulus of the porous sintered ferrous alloy substrat e, which decreases the stress concentrations in the contact region, an d the surface roughness and porosity, which enhance the bonding streng th between the coating and the substrate under multi-contact condition s. The high elastic modulus of the tool steel substrate leads to tensi le stress conditions in the sliding contact region and therefore makes coatings deposited on such a substrate more prone to breakdown/fragme ntation, resulting in a transition load close to that for coatings on the soft 2618 aluminium alloy substrate. For coatings on the 6063 alum inium extrusion substrate, significant plastic deformation occurred in the substrate at loads above 1.5 N. However, despite the heavy deform ation in the substrate, coatings on this substrate were not scraped of f, as were coatings on the 2618 aluminium alloy substrate, even at a l oad as high as 20 N. The specific wear rate increased continuously wit h load, no apparent transition load being explicitly identifiable. Thi s study shows that hard DLC coatings can be applied on both hard and s oft substrates for improvement of the tribological behaviour of mechan ical components. (C) 1997 Elsevier Science Ltd.