Analysis of hardness of nanocrystalline coatings of aluminum-rich Ti1 − xAlxN

Titanium aluminum nitride coatings were fabricated by a d.c. magnetron sputtering system from a Ti–Al (60/40 wt%) target. Coatings were deposited on steel substrates, at a substrate temperature of 250 °C and a bias voltage of −80 V. The nitrogen flow was varied from 1·5–6 sccm and the Ar flow was kept constant at 20 sccm. The morphology and microstructure of the coatings were analysed by X-ray diffraction and scanning electron microscopy. The results of X-ray diffraction showed the presence of two cubic crystalline phases, TiN and AlN, which were confirmed by X-ray photoelectron spectroscopy. The Vicker hardness was obtained by the effective model of indentation. It was observed that the hardness of the coatings decreases from 22·8–9·5 GPa with an increased nitrogen content from 1·5–4·5 sccm. Subsequently, the hardness increased to 22·1 GPa by increasing nitrogen to 6 sccm. The behavior of hardness with grain size variation is consistent with the Hall-Peth relationship. The high value in the hardness of the coatings is mainly attributed to small grain sizes and the compressive stress present.