Influence of diamond crystal orientation on their tribological behaviour under various environments

Tribological properties of diamond coatings are very complex; if the wear r ates remain very low, the friction coefficient is in a large range of value s. This behaviour can be explained by: (a) the deposition method: the way t he dangling bonds are saturated is not the same depending on whether diamon d is obtained by chemical vapour deposition (hydrogen saturation) or combus tion flame method (oxygen saturation); (b) by the intrinsic properties of d iamond crystals which also take a predominant part in the tribological beha viour. In particular, a change in the growth direction induces different ph ysico-chemical properties for the coatings, and consequently modifies their friction coefficient and its evolution with time. The aim of this paper is to highlight the influence of two particular crystal orientations, {111} a nd {100}, on the friction of the diamond-coated pin/high-speed steel disc c ouple and on the existence and the importance of the transferred layers. Th e role of two of the experimental parameters was also studied by realising the tribological tests under various environments (vacuum, oxygen and water vapour) and by applying different normal loads (0.2 N and 1.4 N): the wate r vapour exposition under low load induces different results for the {111} and {100} oriented coatings. The sliding surfaces were also observed by SEM and analysed by EDS to study the sight and the composition of the transfer red layers. (C) 1999 Elsevier Science S.A. All rights reserved.