A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contact when lubricated with mineral and biodegradable oils

M. Kalin, J. Vižintin

Wear 261 (2006) 22–31.

Abstract

Diamond-like carbon (DLC) coatings, which can nowadays be applied to many highly loaded mechanical components, sometimes need to operate under lubricated conditions. It is reasonable to expect that in steel/DLC contacts, at least the steel counter body will behave according to conventional lubrication mechanisms and will interact with lubricants and additives in the contact. However, in DLC/DLC contacts, such mechanisms are still unclear. For example, the "inertness" of DLC coatings raises several questions about whether they are able to provide real boundary "lubrication" or whether they are just a "passive" member in these contacts. On the other hand, biodegradable oils, in particular vegetable base oils, possess a good lubricating ability, often much better than mineral or conventional synthetic oils as a result of the large amount of un-saturated and polar components thatcan promote the lubricity of DLC coatings. Accordingly, in this study, we present the results of experiments under severe boundary-lubrication conditions during reciprocating sliding. We look at the effect of the type of mating surfaces - steel/DLC, DLC/DLC and steel/steel - and the type of oil on the tribological performance of DLC coatings. We compare the wear and friction behaviours of two types of DLC coatings, i.e., a 'pure' non-doped a-C:H DLC coating (denoted as a-DLC) and a WC-containing multilayer coating (denoted as W-DLC) tested with a mineral oil and a biodegradable vegetable oil. These oils, which have very different chemical compositions, were used asbase oils and also with mild AW and strong EP additives. Among other things,the results confirm the following (1) coating lubricated contacts can resemble metal-lubrication mechanismsč (2) additives reduce wear in coating/coating contacts by up to 80%; (3) better wear and friction performance are obtained with oils that contain large amounts of polar and un-saturated moleculesč (4) a coating/coating combination generally results inless wear than a steel/coating combination.

Keywords: DLC coating, lubrication, mineral oil, biodegradable oil, additive

URL: http://dx.doi.org/10.1016/j.wear.2005.09.006

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