Abstract
NiMoAl–Cr3C2–Ag composite coatings with different mass fraction of silver were deposited by high velocity oxy-fuel (HVOF) spraying, and their tribological properties from 20 to 800 °C under unlubricated conditions were evaluated by CSM high-temperature tribometer. Scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. Meanwhile, thermal stability of as-sprayed composite coatings were investigated by heat treatment at 800 °C for 200 h. The HVOF-sprayed composite coating showed an extremely dense structure, high bonding strength, excellent friction reduction and wear resistance property. The addition of silver effectively decreased friction coefficient of the coating and reduced wear rate below 400 °C. Characterization of NiMoAl–Cr3C2–Ag coating revealed that silver provided lubrication below 400 °C and Ag2MoO4 that was formed through tribo-chemistry reactions acted a as a high-temperature lubricant. However, when the content of silver increased to 15 %, more evident enrichment of silver on the coating’s surface at 800 °C was observed, which ultimately increased the oxidation degree of the composite coating.
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