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In order to control wear and friction in industry, Diamond like carbon (DLC) hard coatings are widely applied in recent years due to their well appreciated mechanical and tribological properties. However, those properties are not yet well understood, as well as behavior of DLC hard coatings when rubbed against other materials. Other researchers reported that DLC hard coatings film show high wear when rubbed against steel counterpart and their durability decrease with the increase of the temperature. In this research, we investigated on friction and wear properties of DLC coatings and the results reveal that carbon diffusion has a key role in the wear and friction behavior of DLC coatings when rubbed against steel.

Diamond-like carbon (DLC) coatings are amorphous hard carbon films which offer excellent mechanical and tribological properties. Recently, these coatings have been applied to engine and powertrain components in passenger cars, including those under boundary lubricated conditions. Tribological performance of DLC coatings in oil lubricated conditions and their interaction with various lubricant additives are very complex due to the variety of DLC coatings and lubricants. In this study, the effects of oil temperature and additive concentration on the wear and friction performance of non-hydrogenated tetrahedral amorphous carbon (ta-C DLC) coating have been investigated. Tribological tests were conducted using a pin-on-disc tribotester with ta-C DLC coated steel pins sliding against non-coated steel discs. Tribological tests were also carried out between DLC coated pin against germanium disc in base oil for clarification of wear mechanism. The ta-C coating gave very low boundary friction with base oil at 50 °C but the coating experienced limited durability with increased wear rate at higher temperature against steel disc. Organic friction modifier glycerol-mono-oleate was able to preserve low friction behavior of the coating at higher temperature with enhanced durability. The anti-wear additive Zinc Dialkyldithiophosphate (ZnDTP) provided excellent wear protection for the ta-C coating due to the formation of thick pad-like tribofilm on steel counterpart at all temperature ranges but rubbing in ZnDTP additivated oil lead to higher friction coefficient. The results obtained at high temperature show the significant and beneficial influence of oil additives on the wear performance of the coating. Based on the obtained results and analysis, the wear performance of non-hydrogenated DLC was found to have a clear dependence on the concentration of the lubricant additives, test temperature and counter material.

Wear tests were conducted on hydrogen-free DLC coatings lubricated with engine oil containing carbon black (CB) in order to clarify their wear resistance. The influence of CB on accelerating wear was noticeable in the region where the minimum oil film thickness was less than the averaged secondary particle diameter of CB aggregates. According to a Raman spectroscopic analysis, the amount of DLC coating wear when lubricated in oil without CB tended to increase with increasing oil temperature and with an increasing Id/Ig ratio in the coating surface layer. In contrast, the Id/Ig ratio of the DLC coatings tested in oil containing CB tended to decrease, though the amount of DLC coating wear increased, which indicated a different wear mechanism.

Diamond-like carbon (DLC) coatings have increased demand as protective hard coating for mechanical components due to their exceptional mechanical and tribological performance. They provide ultra-low friction, excellent anti-wear properties and adhesive protection. In recent years, researches focus on the use of DLC coatings under boundary lubricated contacts to increase fuel efficiency of automotive by controlling the friction and to reduce the emission of harmful elements. Commercially available fully formulated oils are designed for ferrous surfaces and contain various additives to enhance the base oils, but interaction between DLC surfaces and those formulated oils is not yet fully understood. In this study we investigated frictional properties of ta-C DLC coating when lubricated with a base oil poly alpha-olefin (PAO) and PAO containing friction modifier glycerol mono-oleate (GMO), anti-wear additives ZnDTP and mixture of GMO+ZnDTP to understand the mechanism of ultra-low friction. The results exhibit that ta-C give ultra-low friction in pure PAO for DLC/steel and DLC/DLC tribopair. GMO additivated PAO provide smooth run-in period for transition to ultra-low friction regime and also enhance the durability of coating. ZnDTP behave differently depending on the presence of ferrous surfaces on the contact. It forms pad-like wear protective tribofilm both on ta-C and steel surfaces for DLC/steel contact, while it form thin white layer on ta-C surfaces for DLC/DLC contact. Using GMO and ZnDTP together does not show any synergistic correlation for steel/steel, DLC/steel and DLC/DLC combinations.

Hydrogen-free diamond-like carbon (DLC) coatings display markedly lower friction coefficients under a lubricated condition owing to the adsorption of oiliness agents and their decomposed components. This study examined the effect on friction properties of the sp2/sp3 bonding ratio of DLC coatings and the addition of nitrogen for improving surface wettability. Evaluations of friction properties under a lubricated condition showed that DLC coatings with a lower ratio of sp3 bonds and nitrogenated DLC coatings displayed higher friction coefficients. Results obtained by electron spin resonance revealed that adding nitrogen decreased the number of dangling bonds of DLC coatings in proportion to the sp3 bond ratio. Assuming that such dangling bonds are oiliness agent adsorption sites, it is concluded that adding nitrogen is not conducive to reducing friction coefficients.