All the moving components in an internal combustion engine require a lubricant that allows smooth sliding and/or rolling of interacting surfaces. Lubricant not only minimizes the friction and wear but also dissipates the heat generated due to friction and removes debris from the area of contact. Environmental concerns, decreasing mineral oil reserves and difficult disposal of nonbiodegradable conventional lubricants have urged the researchers to shift towards environmental-friendly lubricants. Number of tribological studies carried out in the past have proved that ionic liquid-based bio-lubricants are sustainable and biodegradable alternative to mineral oils. This paper presents a brief review of properties of ionic liquids and their ability to reduce friction and wear between the interacting surfaces. Tribological performance and compatibility of ionic liquids with various base-oils have been compared under boundary lubrication. The results reveal that phosphonium-based ionic liquids namely tetra-decyl tri-hexyl phosphonium bis(2,4,4-trimethylpentyl) phosphinate (P66614)i(C8)2PO2 and tri-hexyl tetra-decyl phosphonium bis(2-ethylhexyl) phosphate (P-DEHP) are more suitable for tribological applications. Since, ionic liquids can be tailored according to the application and millions of combinations are possible therefore, there is a need to summarize the published data in a more systematic and logical way.

In past few years DLC coatings and bio based lubricants have gained significant attraction due to their excellent tribological properties. Biolubricants showed synergetic behavior with contact surfaces, when used as a lubricant. Reason behind the attraction towards biolubricants is that they are renewable and biodegradable source of energy. The dominant properties of biolubricants are high flash point, less coefficient of friction, good wear resistance, high viscosity index, lower toxicity and high biodegradability. On the other hand, diamond like carbon (DLC) coatings have also gained attraction due to their excellent tribological properties which enables them to reduce the COF and wear of contact surfaces. In literature, many experimental studies have been carried out by researchers on DLC coating and biolubricants to analyze their interaction. Although biolubricants are not much applying in practical tribology fields but their properties are significant as compared to conventional synthetic lubricants. In this review paper, data from past few years published papers have been arranged in an organized manner to study the interaction of DLC coatings with biolubricants. Most widely used DLC coatings (W-DLC, a-C:H DLC, ta-C DLC) and biolubricants (palm oil, coconut oil, canola oil, sunflower oil, jatropha oil and rapeseed oil) were considered for this study. Tribological performance of symmetric (DLC) contacts and asymmetric (DLC and steel) contacts with biolubricants have been analyzed by comparing the average values of coefficient of friction and coefficient of wear. Synergetic behavior was obtained when biolubricants were used with symmetric DLC coated contact while tribological results were not much effective in case of asymmetrical contact of DLC coatings and steel.