Understanding the composition and low friction tribofilm formation/removal in boundary lubrication

The thickness of a tribofilm formed from additives in a lubricant is determined by the nature of a dynamic process which involves film formation and film removal. In this work, the removal rate of tribofilms is studied using a distinctive testing procedure. Tribofilms are produced using a pin-on-reciprocating plate tribometer with lubricants comprising a combination of base oil (PAO6), zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyldithiocarbamate (MoDTC) additives. In some tests the same lubricant is used throughout the test to assess the formation of the tribofilm. However, an important aspect of the study is the measurement of the tenacity of the tribofilm and to do this the lubricant was changed after a tribofilm had formed, to a lubricant free from additives. Tribofilms (fully developed and partially removed) have been chemically characterised using energy dispersive X-ray analysis (EDX). Qualitative and quantitative information regarding the tenacity of the low friction tribofilms is obtained. Surface analyses have shown that there is a distinct link between chemical composition of the tribofilms formed and their tenacity. The low friction tribofilm formed from the lubricant with ZDDP and MoDTC was shown to be more durable than the one formed from the lubricant with only MoDTC. The mechanism of how the ZDDP tribofilm provides this durability is discussed.