Tribo-Raman-SLIM observation for diamond-like carbon lubricated with fully formulated oils with different wear levels at DLC/steel contacts

Abstract Tribological phenomena of DLC–steel tribo-systems in fully formulated oils were investigated using a Tribo-Raman-spacer layer imaging method (SLIM), which can evaluate the tribological behavior, tribofilm formation process, and tribofilm thickness on the sliding surface. In particular, the effects of the formation process of the tribofilm on the steel surface on the wear phenomena of the DLC were investigated using oil formulations with three different wear levels as test lubricants. The oils were labeled based on the wear results of an engine wear test conducted on the DLC coated-tappet at the DLC coated-tappet/steel-cam contacts. A friction test was conducted at the hydrogenated amorphous carbon DLC (a-C:H DLC) disk/steel ball contacts lubricated with the three formulated oils. The in-situ Raman-SLIM results revealed that thicker phosphate tribofilms with a large number of P–O–P bridges led to lower wear volume of DLC. In other words, the growth of the phosphate tribofilm is a key factor inhibiting the Mo-FMs-induced wear of DLC. Therefore, a booster that can contribute to the growth of the phosphate tribofilms on the steel surface is effective in inhibiting the wear of DLC under the lubrication with fully formulated oils.