A New Low Friction Concept for High Temperatures: Lubricious Oxide Formation on Sputtered VN Coatings

Abstract

The binary system of vanadium and nitrogen offers promising phases in order to enhance the tribological properties of common hard coatings, owing to their ability to form lubricious oxides, often also referred to as Magne´li phases, at elevated temperatures. The aim of this work is to characterize VN coatings prepared by reactive unbalanced magnetron sputtering, and to verify the new concept of solid/liquid oxide lubrication. Oxidation of the coatings and possible melting of the oxides were investigated by dynamic Thermo-Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) up to 900°C in air. On heating the coating samples, exothermal reactions appeared during the DSC measurements indicating oxidation in the temperature range of 500-650°C. Subsequently, endothermal melting reactions were observed between 650 and 850°C. To investigate the effect of oxide formation and melting, dry sliding experiments against alumina and austenitic stainless steel balls were performed using a ball-on-disc tribometer in the temperature range between 25°C and 700°C. For all coatings evaluated, a significant decrease of the friction coefficient at temperatures above 500°C was observed compared to room temperature. Additionally, oxide phases were identified by X-ray Diffraction (XRD) after DSC and tribometer testing. Finally, it can be concluded that the lubricious oxide concept should strongly contribute to the reduction of friction for future machining applications.

Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures

“…This is consistent with the static oxidation studies that showed the V2O5 dominant outer surface on top of AlVO4 and TiO2 particles [10]. The present observations are also consistent with the findings of Gassner et al [7] and Fateh et al [8], who observed VO2 and possibly V6O13 as well as V2O5. In any event, it is clear that, as expected, the low friction is promoted by the outer V2O5 layer, which provides some mechanical support as a result of the compacted nanocrystalline nature (often referred to as a glaze layer).…”

“…Similarly, wear tests of Ti-Al-V-N monolithic films (2.40 at.% ≤ V ≤ 11.10 at.%) against an Al2O3 ball exhibited friction coefficients in the range of 0.6-0.85 at 25 0 C, which increased to 1.1 at 500 0 C and dropped to the range of 0.8-0.2 at 700 0 C (friction coefficient value depended on the concentration of V) [6]. Wear studies of VN coatings by Gassner et al [7] and Fateh et al [8] confirmed the reduced friction coefficient associated with the formation of lubricious vanadium oxides, i.e. Magne´li phases at elevated temperatures.…”

TEM-EELS study of low-friction superlattice TiAlN/VN coating: the wear mechanisms

“…Due to the lower shear strength of V2O5 oxide than other oxides, the V-O containing tribofilm exhibited lower friction coefficient. Similar low friction has been found in binary VN coatings [37] where TiAlN, another constituent subphase in the TiAlN/VN, generally exhibits high friction [13,14]. The extremely low wear coefficient of TiAlN/VN can be explained in three aspects.…”

Oxidation and diffusion processes during annealing of TiSi(V)N films

“…V 2 O 5 , Ag 2 Mo 2 O 7 ) [6][7][8][9]. Among these elements, particular attention has been given to the vanadium-containing coatings (Magnéli phases V n O 2n−1 ), which showed interesting tribological properties in the temperature range 500-700•C [2,[10][11][12][13][14][15]. Dissimilar series of V-based hard coatings have been developed, such as ternary…”

“…Note that Ti 0.80 Si 0.15 N film was tested at higher temperature than V rich coating since no signals of oxidation were detected at lower temperature. On the other hand, the oxidation resistance of the V rich coating could not be studied at 900 ºC due to melting of V 2 O 5 (~685 ºC) [7,13,24] and consequent degradation of the film [26]. respectively.…”