PVD multilayer VN–VN/Ag composite coating with adaptive lubricious behavior from 25 to 700 °C

Abstract

The doping of Ag nanoparticles in a VN matrix was developed on Inconel 718 and Si (100) substrates using a multi-arc ion plating technique. The microtopographies and microstructures of the as-deposited multilayer VN–VN/Ag and VN coatings were studied using a scanning electron microscope, an X-ray diffractometer, and a transmission electron microscope. The mechanical properties were investigated using a scratch test and a nanoindentation test. The results show that the incorporation of Ag-dopant into a VN coating can promote the decrease of average grain size, and the formation of a diffuse structure in the VN/Ag layer. The lamellar structure of the VN–VN/Ag coating can achieve an enhanced adhesion strength in comparison to the monolayer VN coating, but it has a lower hardness and elastic modulus because of the formation of a high content of soft metallic Ag. Furthermore, tribological testing was performed on a UMT-3 ball-on-disk tribometer, with Al₂O₃ balls as the counterpart, at room temperature (RT), 300, 500, or 700 °C in ambient air. The VN–VN/Ag coating had an excellent self-adaptive lubricating behavior with a decrease of friction coefficient as the temperature increased, but its wear rate showed a drastic increase especially at the high temperature region because of oxidization and a tribochemical reaction. The phase composition of the worn area was investigated using a micro-Raman spectrometer, which indicated a smooth Ag, Magnéli phase (V₂O₅) and bimetallic oxides (Ag₃VO₄ and AgVO₃) were responsible for the decrease of the coefficient of friction in (i) room to mid-range temperatures and (ii) mid-range to high temperatures.