Achieving and maintaining low friction at high temperatures (i.e., 300–1000℃) have been very difficult in the past and still are the toughest problems encountered in the field of tribology. In the past, tribologists have explored the feasibility of lubricating hot surfaces with vapor, liquid, and solid lubricants. When liquid- or vapor-phase lubricants are used, most often oxidation and/or chemical breakdown occurred and thus rendered these lubricants useless. As an alternative approach, lubrication by catalytic cracking of carbonaceous gases at high temperatures was also tried but the very specific and difficult lubricantdelivery systems that were needed for its success limited the use of this approach. Solid lubricants are certainly more appealing for use at high temperatures. However, it has been shown that conventional solid lubricants (such as MoS2, graphite, hexagonal boron nitride [h-BN], etc.) become ineffective mainly because of chemical and/or structural degradations at elevated temperatures. In the past, soft metals, such as silver and a series of plasma spray coatings that consisted of silver and alkaline halides (i.e., CaF2, BaF2 ), as the self-lubricating entities and chrome carbide and/or oxide as wear resisting entities were also developed and used for high-temperature lubrication . It was demonstrated that very low friction and wear coefficients could be achieved under dry and lubricated sliding conditions by depositing adherent films of various solid lubricants on ceramic surfaces. However, at very high temperatures (i.e., above 500℃), solid lubricant coatings either delaminated from the substrates or were removed quickly from the surfaces mainly because of oxidation and/or corrosive wear. Because of their low friction at elevated temperatures, oxides of certain metals and metalloids (i.e., Re, Ti, Mo, Zn, V, W, B, etc.) have also been used as lubricants. Oxide-based self-lubricating materials can be prepared as alloys or by designing appropriate coatings or composite structures. The lubricious layers that form by oxidation of metallic surfaces or alloys would be very desirable and exceptionally advantageous when compared with the solid lubricant coatings with finite lifetimes. At high temperatures, as the oxide layer is depleted from the surface by wear, the most useful alloying ingredients diffuse toward the surface where the oxygen potential is higher, and oxidize again to replenish the consumed lubricious layer which has low shear strength and/or surface energy to decrease friction. In a series of fundamental studies, Gardos demonstrated that at a very narrow range of anion vacancies and at high temperatures, crystalline TiO2 (rutile) and rutile-forming surfaces can provide low friction coefficients to sliding tribological interfaces. Further work by Woydt et al. demonstrated the formation of Magn´eli phases on sliding surfaces containing titanium-based alloys and compounds. Their findings suggested that Magn´eli phases are principally the result of tribo-oxidation and that once formed, they can dominate the tribological behavior of sliding ceramic interfaces mainly because of their unique shear properties.Briefly, a significant amount of research has been carried out in the past to develop lubricious oxides for use at high temperatures. However, in many cases, it was found that some of the oxides that formed at sliding interfaces were abrasive or hard to shear; thus, they led to severe abrasion and plowing. Oxides with low shear strength could be formed and maintained only in a very narrow temperature range. This paper introduces a crystal-chemical model that can be used to determine the kind(s) of lubricious oxides that are needed on sliding surfaces at high temperatures. It will demonstrate that the crystal chemistry of certain oxides forming on sliding surfaces is related strongly to shear rheology and hence lubricity of the oxides at high temperatures. This model can allow prediction of not only the kind(s) of lubricious oxides that are needed on a sliding surface but also of their shear rheology, and hence lubricity, at elevated temperatures. Therefore, the purpose of this paper is to describe in detail the crystal chemistry of certain oxides with good lubricating characteristics, rank their lubricating capacities based on their crystal chemistry, and combine this knowledge with lubrication performance to elucidate their solid-lubricating mechanisms.
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