Meccanica (2008) 43: 623–637 DOI 10.1007/s11012-008-9149-z Review of boundary lubrication mechanisms of DLC coatings used in mechanical applications M. Kalin · I. Velkavrh · J. Vižintin · L. Ožbolt Received: 19 November 2007 / Accepted: 16 May 2008 / Published online: 4 June 2008 © Springer Science+Business Media B.V. 2008 Abstract In the last ten years significant progress has been made on development and understanding of diamond-like carbon coatings (DLC), which enabled them to become one of the most promising types of protective coatings for various mechanical applica- tions. Their main advantages are low friction, good anti-wear properties, and adhesive protection. How- ever, due to their low surface energy their reactivity with conventional oils and additives is limited, which makes achieving effective boundary lubrication a com- plex task. For a qualitative step-change that would im- prove the performance and allow effective optimizing and tailoring of these boundary-lubricated tribologi- cal systems, it is necessary to understand the mecha- nisms of why, how, under which conditions, and with which materials and lubricants, the actual boundary lu- brication is possible—if at all. The current results on boundary lubrication of DLC coatings are not many, and are due to the different types of coatings, lubri- cants and additives used in these studies, often dif- ficult to compare and sometimes even contradictive. However, a recent great demand in different industries to apply the DLC coatings to lubricated systems re- quires a better understanding of these phenomena and M. Kalin ( ) · I. Velkavrh · J. Vižintin · L. Ožbolt Centre for Tribology and Technical Diagnostics, University of Ljubljana, Bogisiceva 8, 1000 Ljubljana, Slovenia e-mail: mitjan.kalin@ctd.uni-lj.si overall performance. Therefore, if we wish to see a more effective continuation of the research and a bet- ter understanding of the scattered results, an overview of today’s state-of-the-art of lubricated DLC contacts is needed. In this paper we analyse the behaviors and suggested mechanisms from already-published studies and we summarize the present understanding of the boundary lubrication of DLC coatings. We focus on the DLC-lubricant interaction, thus we analyse only self-mated DLC/DLC contacts in order to avoid the inevitable effects from interactions with other counter materials such as steel. Keywords Boundary lubrication · DLC · Oil · Additive · Tribofilm · Nanolayers 1 Introduction Modern advanced mechanical systems need to op- erate under demanding working conditions, such as high loads, high speeds, high temperatures and aggres- sive environment. Under lubricated conditions, which are far the biggest portion of all mechanical sys- tems, this suggests that lubrication frequently (per- manently or temporarily) corresponds to mixed or boundary regime, where direct contacts between the surfaces are predominant. Namely, in boundary lubri- cation, the oil film thickness is less than the compos- ite surface roughness and the surface asperities come