Friction Properties of Tribofilm Formed From Engine Oil Additives (Part 1): The Effect of DLC-Coated Material on Forming Tribofilms

摘要

Friction tests simulating a cam follower sliding condition were conducted by using a pin-on-disc test rig. In the case of 5W-30 engine oil, the friction coefficient of steel pins sliding on a steel disc increased from 0.11 to 0.12. On the other hand, that of steel pins sliding on DLC-coated disc decreased from 0.12 to 0.10. For 5W-20 oil containing MoDTC, the friction coefficient of steel pins sliding on the steel disc decreased markedly from 0.12 to 0.04. In contrast, that of steel pins sliding on the DLC disc decreased moderately from 0.11 to 0.08. In both cases, Zn, P, S and Mo elements derived from ZDDP and MoDTC were not detected on the DLC disc by EPMA and XPS analysis. The lateral force of the film formed on the steel disc was then measured in an LFM test. The lateral force of the film resulting from 5W-30 oil was much higher than that of the film formed from 5W-20 with MoDTC. Judging from these results, it is thought that the high friction coefficient observed for the steel pins on the steel disc in 5W-30 was caused by the higher shear strength of the film formed from ZDDP. On the other hand, the very low friction coefficient observed for the steel phi-steel disc combination in 5W-20 oil was presumably caused by the formation of the solid lubricant MoS{sub]2 derived from the MoDTC additive.