The effect of (Ti+Al):V ratio on the structure and oxidation behaviour of TiAlN/VN nano-scale multilayer coatings

Abstract

Nano-scaled multilayered TiAlN/VN coatings have been grown on stainless steel and M2 high speed steel substrates at U B=−85 V in an industrial, four target, Hauzer HTC 1000 coater using combined cathodic steered arc etching/unbalanced magnetron sputtering. X-ray diffraction (XRD) has been used to investigate the effects of process parameters (Target Power) on texture evolution (using texture parameter T*), development of residual stress (sin 2 ψ method) and nano-scale multilayer period. The composition of the coating was determined using energy dispersive X-ray analysis. The thermal behaviour of the coatings in air was studied using thermo-gravimetric analysis, XRD and scanning electron microscopy. The bi-layer period varied between 2.8 and 3.1 nm and in all cases a {1 1 0} texture developed with a maximum value T*=4.9. The residual stress varied between −5.2 and −7.4 GPa. The onset of rapid oxidation occurred between 628 and 645 °C depending on the (Ti+Al):V ratio. After oxidation in air at 550 °C AlVO 4, TiO 2 and V 2O 5 phases were identified by XRD with the AlVO 4, TiO 2 being the major phases. The formation of AlVO 4 appears to disrupt the formation of Al 2O 3 which imparts oxidation resistance to TiAlN based coatings. Increasing the temperature to 600 and 640 °C led to a dramatic increase in the formation of V 2O 5 which was highly oriented (0 0 1) with a plate-like morphology. At 640 °C there was no evidence of the coating on XRD. Increasing the temperature to 670 °C led to further formation of AlVO 4 and a dramatic reduction in V 2O 5.