Effect of N2 partial pressure on the microstructure and mechanical properties of reactively sputtered (Ti,Al)N coatings

A series of (Ti,Al)N coatings were deposited by using Ti–Al mosaic target in the mixture gas of Ar and N2 with different N2 pressures. The deposition rate, composition and microstructure of coatings were characterized with energy dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and a nanoindentor was employed to measure their mechanical properties. The results reveal that N2 partial pressure has an important effect on the (Ti,Al)N coatings. (Ti,Al)N coatings with stoichiometric ratio can be easily synthesized at a suitable range of N2 partial pressure and the coatings present fcc structure single phase with (111) texture preferential orientation. The coatings are strengthened and reach maximum hardness of 34.4 GPa and elastic modulus of 392 GPa, respectively. (Ti,Al)N coating prepared at lower N2 partial pressure shows low N and thus has low hardness. On the other hand, at overhigh N2 partial pressure, the deposition rate of the coatings decreases greatly because N2 reacts with Ti–Al target and forms a nitride layer on its surface, while the composition of the coatings has almost no change and the coatings show a nanocrystalline or amorphous structure and lower hardness.