Effects of antiwear additives in the base oil on the tribological performance of hydrogen-free DLC coating. Issue 6 (19th August 2014) - Ng, Edward
Abstract : Purpose: For many years, zinc dialkyl dithiophophates (ZDDP) have traditionally been used in engine oils as antiwear (AW) and extreme pressure (EP) additives. However, additives containing sulfated ash, phosphorus and sulfur (SAPS) have a detrimental effect on the exhaust after-treatment device found on modern vehicles. Besides the automotive industry, Diamond like carbon (DLC) is also used in hydraulic applications where zinc-free and ashless hydraulic fluids have gradually gained popularity in recent years. The objective of this study is to investigate the effects of ZDDP and ashless triphenyl phosphorothionate (TPPT) on hydrogen-free DLC coatings. Design/methodology/approach: The tribological tests were performed using a disk-on-cylinder tribometer, where the stationary hydrogen-free DLC coated steel disk formed a line contact with an uncoated rotating steel shaft under lubricated conditions. Findings: It was found that TPPT and ZDDP separately at a concentration of 1.0 wt% increased the amount of friction of the base oil by approximately four times. TPPT appeared more effective than ZDDP in minimizing wear on the DLC coated surface. Also, primary ZDDP seemed to have a more detrimental effect on the DLC coated surface compared to a mixture of primary and secondary ZDDP. With regard to surface roughness of the hydrogen-free DLC-coated surface, the values corresponding to a lubricant containing TPPT were lower than those obtained for a lubricant with ZDDP and a lubricant without any additive. Originality/value: This is the first report on the effects of ZDDP and ashless triphenyl phosphorothionate (TPPT) on the tribology of hydrogen-free DLC coatings.