Wear mechanism of Ag as solid lubricant for wide range temperature application in micro-beam plasma cladded Ni60 coatings

Abstract

The effect of Ag content on the microstructure, compositions, and wear resistance of micro-beam plasma cladded Ni60 coating was studied. The friction layer formed on the wear surface at different temperatures and its influence on tribological properties were discussed, and wear mechanism of Ag added Ni60 coating were clarified. The results show that adding Ag can effectively improve the wear resistance of cladding layer. The friction coefficient of the cladding layer decreases from 0.40, 0.10 and 0.22–0.20, 0.08 and 0.18 at 25 ℃, 300 ℃ and 600 ℃, respectively. At room temperature, the wear rate of nickel based coating is 14.13 * 10−7 mm/Nm, and reduces to 10.55 * 10−7 mm/Nm with 5%Ag addition. The excellent tribological performance at room temperatures is attributed to the existence of Ag layer formed in cladding process. The addition of silver significantly reduced the degree of abrasive wear. However, at higher temperature of 300 ℃, Ag precipitates from the coating and begins to generate Ag2O oxide, which avoids the direct contact of metal and effectively reduces the friction coefficient. The wear mechanism is oxidation wear and abrasive wear. At 600 ℃, high temperature softening results in obvious adhesive wear of the coating, and Ag particles accumulate on the surface of the cladding layer in the form of "sweating", and results in the formation of self-lubricants with NiO and AgCrO2. Therefore, oxide lubrication layer formation such as Ag2O, NiO and AgCrO2 at high temperatures is benefit for low friction coefficient at elevated temperature.