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Effect of Al content on the hardness and thermal stability study of AlTiN and AlTiBN coatings deposited by HiPIMS

Abstract:

The microstructure, mechanical properties and thermal stability of Al<inf>x</inf>Ti<inf>1−x</inf>N and Al<inf>1</inf>Ti<inf>1-</inf><inf>x</inf>BN coatings grown by reactive high-power impulse magnetron sputtering (HiPIMS) have been analyzed as a function of Al/(Al + Ti) ratio (x) between 0.5 and 0.8. The coatings were predominantly formed by a face-centered cubic Ti(Al)N crystalline phase, both with and without B, even for x ratios as high as 0.6, which is higher than the ratio typically encountered for Al<inf>x</inf>Ti<inf>1−x</inf>N coatings deposited by reactive magnetron sputtering. B doping, in combination with the highly energetic deposition conditions offered by HiPIMS, results in the suppression of the columnar grain morphology typically encountered in Al<inf>x</inf>Ti<inf>1−x</inf>N coatings. On the contrary, the Al<inf>x</inf>Ti<inf>1−x</inf>BN coatings grown by HiPIMS present a dense nanocomposite type microstructure, formed by nanocrystalline Ti(Al)N domains and amorphous regions composed of Ti(Al)B<inf>2</inf> and BN. As a result, high-Al content (x ≈ 0.6) Al<inf>x</inf>Ti<inf>1−x</inf>BN coatings grown by HiPIMS offer higher hardness, elasticity and fracture toughness than Al<inf>x</inf>Ti<inf>1−x</inf>N coatings. Moreover, the thermal stability and the hot hardness are substantially enhanced, delaying the onset of formation of the detrimental hexagonal AlN phase from 850 °C in the case of Al<inf>0.6</inf>Ti<inf>0.4</inf>N, to 1000 °C in the case of Al<inf>0.6</inf>Ti<inf>0.4</inf>BN.