On the effect of Ta on improved oxidation resistance of Ti–Al–Ta–N coatings

Abstract

Formation of protective oxide scales is the main reason for the high oxidation resistance of TiAlN based coatings. Here the authors report on further improvement in the oxidation resistance of TiAlN by Ta alloying. An industrial-scale cathodic arc evaporation facility was used to deposit Ti–Al–Ta–N coatings from powder metallurgically produced Ti38Al57Ta5 targets. After oxidation in ambient air, a significantly reduced oxide layer thickness in comparison to unalloyed TiAlN reference material was observed. Energy-dispersive x-ray spectroscopy line scans and secondary ion mass spectroscopy depth profiling showed that the oxide scale consists of an Al-rich top layer without detectable amount of Ta and a Ti–Ta-rich sublayer. Transmission electron microscopy investigations revealed α-Al2O3, rutile-type TiO2, and anatase-type TiO2 as the scale forming oxides. Furthermore, the Ti–Ta-rich sublayer consists of a porous layer at the oxide-nitride interface but appears dense toward the Al-rich top layer. The improved oxidation resistance is explained by doping the TiO2 lattice by replacing Ti4+ with Ta5+ in the rutile lattice, which decreases the oxygen mass transport. This leads to reduced oxidation of Ti under formation of TiO2 at the oxide-nitride interface and is the reason for the excellent oxidation behavior of Ti–Al–Ta–N coatings.

Oxidation resistance and characterization of (AlCrMoTaTi)-Six-N coating deposited via magnetron sputtering

“…Cr induces a similar effect of forming protective Cr 2 O 3 surface layer at high temperatures [22]. The addition of Ta can decrease the number of oxygen vacancies in the surface oxide layer, thereby slowing the diffusion of oxygen [23]. Moreover, alloying Si promotes the formation of nanocomposite structure, thus obstructing oxygen diffusion along the grain boundaries [24].…”

Origin of high temperature oxidation resistance of Ti–Al–Ta–N coatings

“…As mentioned above, the anatase-rutile phase transformation is connected with a contraction of 5-10% [52,53] leading to crack formation and promotion of pores. We suggest, that the suppression of phase transformations within oxide scales is at least as important as reducing the bulk diffusion (as mentioned in earlier studies [24]), to reduce the oxide scale growth rate.…”

Coating Applications for Cutting Tools

“…A pure fcc structure can be reached, even at high Al contents, by applying an enhanced bias voltage to the substrate during the deposition process, which should result in increased hardness and Young's modulus. Beneficial effects on the oxidation resistance and on the wear behavior at high temperatures due to Ta alloying of TiAlN coatings are reported (Pfeiler et al, 2008;Pfeiler, Scheu, et al, 2009). Alloying with high content of Si and B can lead to the formation of a completely different structure.…”