Friction and wear of coated surfaces — scales, modelling and simulation of tribomechanisms Kenneth Holmberg a, ⁎ , Helena Ronkainen a , Anssi Laukkanen a , Kim Wallin b a VTT Technical Research Centre of Finland, Finland b Academy of Finland, Finland Available online 21 August 2007 Abstract Coating a surface with a thin layer changes the surface material properties and is an important tool for controlling friction and wear. The tribological mechanisms, scale effects and parameters influencing the friction and wear of coated surfaces are discussed. The basic friction and wear mechanisms can be reduced to: friction by adhesion, ploughing and hysteresis and wear by adhesion, abrasion and fatigue combined with material fracture. The tribochemical and surface physical effects and surface fatigue taking place before material fracture are treated here as pure surface material modification mechanisms. Scale effects in a tribological contact are illustrated by explaining typical surface roughness related tribological mechanisms for diamond and DLC coated surfaces. For diamond coatings asperity interlocking effects are important for rough surfaces, graphitisation is a dominating mechanism for smooth engineering surfaces and hydrogenising of dangling bonds may be crucial for physically smooth surfaces. For DLC coated surfaces, surface graphitisation is important with rougher surfaces; building up transfer layers and graphitisation is crucial for smooth engineering surfaces and hydrogenising of dangling bonds can explain superlubricity for physically smooth surfaces. An analysis of dominating surface parameters such as elastic, plastic and fracture behaviour of the top surface, the coating, the coating/ substrate interface and the substrate in addition to the coating thickness forms the basis for surface modelling. A stress intensity factor analysis of crack growth shows the importance of considering both modes I, II and III loading, crack spacing and location of crack, while crack orientation, location in crack field as well as load biaxiality have minor influences. It is shown how surface 3D FEM modelling generates stress and strain values at the nano level, within bond layers at coating/substrate interfaces and around cracks and forms the basis for better understanding the origin of wear. © 2007 Elsevier B.V. All rights reserved. Keywords: Tribology; Coatings; Modelling; Scale effects; Diamond; Diamond-like carbon (DLC) 1. Introduction Energy saving, environmental, economic and safety aspects in our society all emphasise the importance of controlling friction and wear in machinery and devices. Lubrication with oil is the most common way to control friction and wear. However, the use of liquid lubricants is often not so desirable for environmental reasons, problems with keeping it in the contact zone, ageing, circulating, storing, contamination etc. Surface engineering, where the surface properties of the moving contacts are changed in a favourable way by deposition or surface treatment now offers another efficient way of controlling friction and wear. The development of the vacuum deposition techniques, chemical vapour deposition (CVD) and physical vapour deposition (PVD), has been of major impact, since they make it possible to deposit a thin layer of only a few micrometers (or down to nanometer thickness) on the surfaces of most engineering materials. The geometrical change is minimal and the surface layer may have properties covering an extremely wide range, from hard diamond and ceramic coatings to very soft polymeric or lamella-structured films [1]. In the 1980s hard ceramic TiN, TiC and Al 2 0 3 coatings were commercially introduced as surface layers on tools in the production industry, and wear rates were decreased by one to two orders of magnitude or more. In the 1990s very low friction Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2007) 1034 – 1049 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. VTT Technical Research Centre of Finland, P.O. Box 1000, 02044 VTT, Finland. Tel.: +358 20 7225370; fax: +358 20 7227077. E-mail address: Kenneth.holmberg@vtt.fi (K. Holmberg). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.105