Atomistic study of ternary oxides as high-temperature solid lubricants

Friction and wear are important tribological phenomena tightly associated with the performance of tribological components/systems such as bearings and cutting machines. In the process of contact and sliding, friction and wear lead to energy loss, and high friction and wear typically result in shortened service lifetime. To reduce friction and wear, solid lubricants are generally used under conditions where traditional liquid lubricants cannot be applied. However, it is challenging to maintain the functionality of those materials when the working environment becomes severe. For instance, at elevated temperatures (i.e., above 400 ◦C), most traditional solid lubricants, such as MoS2 and graphite, will easily oxidize or lose lubricity due to irreversible chemical changes. For such conditions, it is necessary to identify materials that can remain thermally stable as well as lubricious over a wide range of temperatures.Among the currently available high-temperature solid lubricants, Ag-based ternary metal oxides have recently drawn attention due to their low friction and ability to resist oxidation. A recent experimental study showed that the Ag-Ta-O ternary exhibited an extremely low coefficient of friction (0.06) at 750 ◦C. To fully uncover the lubricious nature of this material as a high-temperature solid lubricant, a series of tribological investigations were carried out based on one promising candidate – silver tantalate (AgTaO3). The study was then extended to alternative materials, Cu-Ta-O ternaries, to accommodate a variety of application requirements. We aimed to understand, at an atomic level, the effects of physical and chemical properties on the thermal, mechanical and tribological behavior of these materials at high temperatures. Furthermore, we investigated potassium chloride films on a clean iron surface as a representative boundary lubricating system in a nonextreme environment. This investigation complemented the study of Ag/Cu-Ta-O and enhanced the understanding of lubricious mechanisms of solid lubricants in general. Molecular dynamics (MD) simulations was used as the primary tool in this research, complemented by density-functional theoryand experiments from our colleagues.In this research, we first developed empirical potential parameters for AgTaO3 and later Cu- Ta-O ternaries using the modified embedded-atom method (MEAM) formalism. With those parameters, we explored the sliding mechanisms of AgTaO3, CuTaO3 and CuTa2O6 at elevated temperatures. Particularly on AgTaO3, we investigated the effects of applied loads as well as surface terminations on friction and wear as functions of temperature. In addition, to optimize the tribological performance of AgTaO3, film reconstruction mechanisms were investigated on Ta2O5/Ag films with varying amounts of Ag. For the potassium chloride-iron system, we studied the effect of contact pressure on interfacial structure, based on which the origin of the commonly observed pressure-dependent shear strengths was explored. We hope this research will benefit the design and development of solid lubricant materials for a wide range of applications.