A Review on Effects of Lubricant Formulations on Tribological Performance and Boundary Lubrication Mechanisms of Non-Doped DLC/DLC Contacts

Tribological efficiency of industrial applications involving boundary lubrication regime can be improved to an appreciable extent by the deposition of hard coatings on interacting surfaces. Among such coatings, diamond-like carbon (DLC) coatings are considered to be one of the most suitable ones for the said role. DLC coatings possess a unique combination of physical, chemical, and material properties due to which they can help in minimizing friction-induced energy and material losses even under starved lubrication conditions. Since commercial lubricants are optimized for steel surfaces, therefore, a lot of experimental investigations were carried out to analyze the tribological compatibility of these lubricants with various DLC coatings. However, there is still a lack of understanding about how DLC coatings interact with conventional lubricant additives. Some researchers reported tribologically beneficial interactions between DLC coatings and formulated lubricants while others observed no such behavior. To address these inconsistencies, there is a need to rearrange the published data in a more apprehensible and organized manner with a special emphasis on the mechanisms responsible for a particular tribological behavior. In this way, it can be determined whether synergistic or antagonistic correlation exists between a particular DLC-lubricant combination and research on DLC coatings can be continued in a logical way. In this article, most widely investigated non-doped DLC coatings (ta-C, a-C:H, a-C, and ta-C:H) are tribologically analyzed. Average values of friction and wear coefficients are calculated for various DLC-lubricant combinations using already published data and compared to quantify the effectiveness of a particular lubricant additive in enhancing tribological characteristics of symmetrical non-doped DLC contacts. Moreover, tribological performance parameters of non-doped DLC coatings are compared with those of doped-DLC coatings to understand differences in their tribological behavior in combination with additives.     

XANES characteristics of tribofilms generated by dithiophosphate additive in water at different test temperatures and durations

Water‐based lubricants and metal‐working fluids are becoming more and more popular in modern manufacturing due to environmental concerns. Owing to the poor lubricating properties offered by pure water, additives are generally needed to improve the antiwear and extreme pressure properties. In the present work, the tribological behaviors of synthesized additives in water‐based lubricants are studied for different durations and temperatures by a four‐ball machine, and the worn surfaces were analyzed by scanning electron microscopy (SEM) and X‐ray absorption near‐edge structure (XANES) spectroscopy. It was found that the tribological properties changed with the duration and temperature, and the tribofilms mainly consisted of an adsorbed layer and a tribochemical layer. The compositions of tribofilms also changed with duration and temperature, and the different compositions of tribofilms would result in different tribological properties accordingly. Copyright © 2009 John Wiley & Sons, Ltd.