Microstructure and wide temperature range self-lubricating properties of laser cladding NiCrAlY/Ag2O/Ta2O5 composite coating

Abstract

In this work, laser cladding has been employed for the preparation of nickel-based self-lubricating coating NiCrAlY/Ag2O/Ta2O5 on Inconel 625 alloy substrate. In order to study the evolution of Ag2O and Ta2O5 in the coating and its effect on the microstructure, three coatings of NiCrAlY/Ag2O, NiCrAlY/Ta2O5 and NiCrAlY were prepared for comparison. The composition and microstructure of the powders and the corresponding composite coatings were studied by SEM-EDS and XRD. The average hardness of Coating (A, B, C, D) and substrate is 369.5 HV, 300.7 HV, 306.5 HV, 332.1 HV, and 246.8 HV, respectively. The friction and wear properties of the self-lubricating coating were studied by MFT-5000 multifunctional friction and wear tester at RT (room temperature) ~750 °C. In order to compare with the self-lubricating coating, the same friction and wear test was done on the substrate under the same condition. It can be seen that the friction coefficient of the self-lubricating coating from RT to 750 °C is 0.64, 0.23, 0.62 and 1.5 times lower than that of the substrate, and its wear rate is 2.03, 2.11, 2.82 and 1.04 times lower than that of the substrate, respectively. SEM-EDS and XRD were used to analyze the morphology and composition of the worn surface. It can be seen from the results that Ag is the main lubrication phase at room temperature, and NiO and Ag work together to lubricate at 350 °C. When the temperature reaches 550 °C, Ag loses its effective lubrication, while the wear-resistant glaze layer composed of NiO and CrO2 provides the main lubrication. At 750 °C, AgTaO3 and Ag2Ta4O11 formed by in-situ reaction will act as a synergistic lubrication with the wear resistant glaze layer composed of oxide. The synergistic lubrication of Ag, NiO, CrO2, AgTaO3 and Ag2Ta4O11 enables self-lubrication of the composite coating in a wide temperature range.