Evolution of the nano-scale mechanical properties of tribofilms formed from low- and high-SAPS oils and ZDDP on DLC coatings and steel Mitjan Kalin n , Eva Oblak, Somayeh Akbari Laboratory for Tribology and Interface Nanotechnology, Faculty of Mechanical Engineering, University of Ljubljana, Bogišićeva 8, 1000 Ljubljana, Slovenia article info Article history: Received 11 September 2015 Received in revised form 8 December 2015 Accepted 12 December 2015 Available online 18 December 2015 Keywords: DLC AFM Tribofilm Mechanical properties abstract The evolution of the nano-mechanical properties of tribofilms formed in steel/steel, steel/a-C:H and steel/Si-DLC contacts lubricated with two commercial oils containing different amounts of SAPS additives (E6 and E7 grade) and a mineral base oil containing ZDDP additive were examined in this investigation for two very different time periods. An atomic force microscope (AFM) was used in different modes to measure the topography, film thickness and stiffness, while the nano-hardness was measured with a nano-indenter. In addition, FTIR microscope was used on selected samples to explain some of the tri- bofilm's mechanical modifications with chemical changes. The results have shown that the tribofilm's evolution and growth are very much surface and additive dependent, and are different for steel and DLC coatings. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Diamond-like-carbon (DLC) coatings are, due to their low fric- tion, wear resistance, corrosion resistance, high hardness, chemi- cal stability, etc., some of the promising types of hard coatings for various machine components. Their excellent low-friction and anti-wear properties have already been the subject of a number of studies and their properties have been investigated for different non-lubricated conditions [1–5] and lubricated conditions [6–10]. However, the interactions between the DLC surfaces and the lubricant additives have received less attention than for steel surfaces [11]. Nevertheless, several studies have already shown that tribochemical reactions do occur between additives and DLC coatings, and that protective tribofilms do form in the contacts [6,8,12–14]. Moreover, DLC coatings enable very good tribological behavior, even when only lubricated with base oils that do not contain any additives [15,16]. This suggests the possibility that DLC coatings might also have good performance with low-SAPS oils, since the interactions between the oil and the surface will prob- ably be weaker than with conventional, strongly additivated oils, and thus similar to the base oil's behavior. However, the interactions on surfaces and coatings in boundary-lubrication conditions are still not completely under- stood. The chemical reactions between the lubricant molecules and the surfaces are usually accompanied by the formation of a tribofilm [17]. This tribofilm is defined as a thin film generated as a result of a sliding contact, which is adhered to its worn parent surface, but has a different chemical composition, structure, and tribological behavior [18]. This indicates that the generation of tribofilms has a major influence on both the friction and wear. Indeed, it is well known that tribological performance is linked to the tribofilm's properties and, consequently, to the lubricating conditions [19–21]. A large number of studies have been conducted to describe and evaluate the properties of tribofilms, mostly on steel and also some on DLC coatings; however, these relate mostly to the film's chemical properties [16,22–28]. On the other hand, tribofilms' mechanical properties have not been studied in great detail, despite the fact that the mechanical properties are some of the most important parameters for effective films, and so the results are very scarce. Moreover, only a few authors have investigated the changes in the mechanical properties of tribofilms on steel as a function of time. In these studies, usually, only one or very few of the mechanical properties of the tribofilms were investigated, most often the hardness, modulus or morphology [19,21,24,29,30]. It was found that a ZDDP tribofilm's hardness properties evolve as a function of the tribological test's duration, with the early stages characterized by a tribofilm with a relatively low hardness, which increases in hardness as the test proceeds and eventually decreases in hardness as the wear debris removes some of the tribofilm that is present on the surface [24]. On the other hand, some studies [29] report that the indentation modulus and the hardness of the ZDDP tribofilm stay unaffected throughout the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International http://dx.doi.org/10.1016/j.triboint.2015.12.013 0301-679X/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: mitjan.kalin@tint.fs.uni-lj.si (M. Kalin). Tribology International 96 (2016) 43–56