Abstract
The fuel economy and reduction of harmful elements of lubricants are becoming important issues in the automotive industry. One approach to these requirements is the potential use of low-friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like carbon (DLC) coatings, extensively studied as ultra-low friction films to protect ductile metals surfaces for space applications, are expected to fit the bill. The main purpose of this work is to investigate the friction and wear properties of DLC coatings lubricated with molybdenum dithiocarbamate (MoDTC) and zinc dithiophosphate (ZDDP) under boundary lubrication conditions. The mechanisms by which MoDTC reduces the friction in the centirange were studied using ultra-high vacuum (UHV) analytical tribometer. The UHV friction tests were performed on a tribofilm previously formed on selected DLC material with MoDTC and ZDDP containing oil. Ex-situ characterizations show that the composition of this tribofilm is similar to that of a tribofilm obtained on steel surfaces in the same lubrication conditions with MoS2 single sheets dispersed inside zinc phosphate zones. However, analyses by X-ray photoelectron spectroscopy (XPS) indicate that MoDTC and ZDDP additives seem to be more active on steel surfaces than carbonaceous ones. After UHV friction with the tribofilm formed on selected DLC and steel pin counterpart, the wear scars of both sliding surfaces were characterized by in-situ analytical tools such as Auger electron spectroscopy, scanning Auger microscopy and micro-spot XPS. Low friction is associated with the transfer of a thin MoS2 film to the steel pin counterpart. Copyright © 2006 John Wiley & Sons, Ltd.
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