Microstructures, mechanical and tribological properties of VN films deposited by PLD technique

Consistent stoichiometric FCC structured vanadium nitride (VN) films were fabricated by the pulsed laser deposition (PLD) technique at room temperature and 300°C, for which the microstructures and mechanical and tribological properties were systematically investigated. The results indicated that the films deposited at 300°C displayed a denser structure and possessed higher hardness and elastic modulus values than the films deposited at room temperature that exhibited a columnar structure. The wear behaviors of VN films were investigated at elevated temperature to 900°C against an alumina ball under an ambient atmosphere. Due to the densified structure and excellent mechanical properties, the VN films deposited at 300°C held lower friction coefficients over the investigated temperature range compared to those of the films deposited at room temperature and registered the lowest friction coefficient of about 0.21 at 900°C. According to the XRD and Raman spectroscopy results, the oxidation behaviors of VN films at elevated temperatures formed a series of vanadium oxides, such as V₂O₅, V₃O₇, V₆O₁₁ and V₆O₁₃, which displayed easy crystallographic shear planes with reduced binding strength and influenced the tribological properties significantly. Moreover, liquid self-lubrication existed in the tribology process above 700°C due to the melting of V₂O₅ that registered low melting points of 680°C. Combining the vanadium oxides phase and lubrication phases with the lubricious oxide layer, as well as the liquid lubrication in the contact area, can cause decreased friction coefficients at higher temperatures.