Owing to the high hardness and low thermal conductivity, the plasma electrolytic oxidation (PEO) coatings, also known as micro-arc oxidation (MAO) coatings, have been developed to improve the wear resistance and thermal shock resistance of valve alloys. The thick PEO coatings are deposited on the aluminium and magnesium substrates by Curran et al. and their thermal conductivity (about 1 W m−1 K−1) is one order of magnitude lower than that of the bare substrates. A kind of PEO coating on Ti6Al4V alloy is prepared by Yao et al., the phase component and adhesion of which are kept stable when subjected to thermal shock under 20–500°C for 50 cycles. Therefore, the PEO coatings are expected to reduce the excessive surface oxidation and material deterioration under excessively high temperature environment.In recent years, a variety of traditional solid lubricants (e.g. graphite, MoS2, PTFE) have been applied on the PEO coatings to decrease their friction coefficients. Tekin et al. impregnates molybdenum disulfide friction modifier onto the surface of PEO coatings, resulting in the remarkable reduction of friction coefficient (<0.15). To prolong the lubricating life of MoS2 films, the laser surface textured arrays combined with PEO coatings are also fabricated on Ti6Al4V alloy, which can provide lubricant reservoirs and hard support, respectively. However, the effective lubricating conditions of these solid lubricants are restricted to room temperature and medium temperature. At high temperature (MoS2∼ 350°C, graphite ∼ 550°C), the solid lubricants rapidly get oxidized and lose the lubricating effect due to the loss of their easily sheared layer structure. Differently, silver molybdate with layer structure has been conformed as an effective high temperature solid lubricant which exhibits low friction coefficient over medium temperature to high temperature range. Two aspects play an important role in preventing the wear failure of LST/PEO coating on Ti6Al4V substrate, namely a compacted microstructure resulted from the seal of electroplating Ag and the heat consumption during the melting and migration of Ag. The friction heat between rubbing surface and thefresh Ag migrating from the micro-pore reservoirs
can effectively promote the formation of high temperature lubricant Ag2MoO4.The LST/PEO/Ag/MoS2 coating shows low frictioncoefficients over a wide temperature range attributed to the lubrication of MoS2 and Ag at 25–350°C and the formation of low friction contact interface at 600°C.
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