Experimental Simulation of Tribochemical Reactions Between Borates Esters and Steel Surface

In this study, tribochemical reactions of borate ester additives on steel surfaces were simulated experimentally by gas phase lubrication. Trimethylborate (TMB) was employed as a model molecule of the borate chemical function. TMB was introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests were conducted at room temperature. In order to identify the nature of the generated tribofilm, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were carried out inside and outside the track of the tested samples. The results showed a reduction of the friction coefficient down to 0.2 by increasing the TMB gas pressure. This reduction in friction was induced by the formation of a tribofilm of iron oxide partially digested by a borate glass network. The reduction in friction was attributed to the formation of the borate glass. The digestion of abrasive iron oxides by the borate glass was purported to be responsible for the reduction in the wear. The tribochemical reaction of TMB on steel surfaces is discussed in relation to the hard and soft acid base (HSAB) principle.