Nanodiamonds are known to improve tribological performance when added to lubricants, but their impact on additives that may already be present in the lubricant is poorly documented. Here, we report on a study of their effects on thermal reaction films formed from tricresyl phosphate (TCP) on Fe substrates immersed in a dibasic ester basestock when blended with TCP. Thermal reaction film formation temperatures were recorded in-situ by monitoring the reaction film formation on both Fe and air baked Fe surfaces using a quartz crystal microbalance (QCM). The nanodiamonds were found to raise the thermal reaction film formation temperature by 18 • C, possibly by raising the activation energy for the reaction, but they were not observed to affect the thickness or rate of formation of the films. The nanodiamonds, moreover, were observed to trigger thermal reaction film formation on air baked Fe surfaces that otherwise were highly resistance to reaction film formation. The surface morphology, roughness, and thickness of the thermal reaction films, as measured by atomic force microscopy (AFM), are reported as well as their chemical compositions, as studied with Electron Dispersive X-ray Spectroscopy (EDS). The coefficients of friction measured on the thermal reaction films during dry solid-solid contact are also reported.Lubricants 2018, 6, 56 2 of 14 existing phosphate esters additives, rather than outright replacing them. An appropriate strategy to limit their use levels while also retaining their function is to blend them with a synergistic and environmentally friendly additive. The overarching goal here is to add the synergistic additive without increasing the amount of phosphorus in the system until it is required to do so. This strategy would be completely practical to implement and applicable in various fields, such as aviation, automobiles, manufacturing, defense, etc., that implement a condition-based maintenance schedule [8].Nanomaterials are emerging as environmentally friendly and effective additives for controlling friction and wear in various applications [9]. Several physical mechanisms such as change in friction mode (sliding to rolling), surface polishing/mending, reduction in cohesive forces via layered materials, etc., and chemical mechanisms, including material delivery/transfer for catalysis, tribofilm formation by tribochemical reactions, etc., have been identified as lubrication mechanisms for nanomaterials [10]. However, nanomaterials should also be able to withstand tough environments under EP conditions during boundary lubrication and the high operating temperatures of aircraft engines if they are to be successfully blended with currently used additives. This requires that nanomaterials have, among other properties, a high melting point, high hardness, and favorable chemical reactions to allow rigid bonding onto the surface, together with the anti-wear film produced by the phosphate ester [5].Among the nanomaterials discovered and studied so far, nanodiamonds are the leading candidates from the po...
