Intermetallic target materials in PVD based deposition techniques: : A case study for Ti-Al-N coatings

Abstract

Hard protective coatings are an essential surface modification for state of the art cutting and milling inserts, enabling higher performance with respect to cutting speed, thermal stability, or durability. Within this context, titanium aluminium nitride (TiAlN) has evolved to an application proven material system, offering high hardness, combined with excellent wear-, and oxidation resistance. As the performance of the coatings not only relies on the physical vapor deposition (PVD) technology, but also on the quality of the source material – setting highest requirements with regard to purity and density – targets are a cost-intensive part of the thin film synthesis process. Industrially sized Ti-Al targets with an aluminium content > 20 at. % are typically produced via forging, as a uniform elemental distribution and a fine-grained microstructure are of significance to achieve excellent target performance. Nevertheless, the once elaborately manufactured PVD-targets become inoperable after ~30 to 70 % of the material has been eroded, leaving most of the material unused. Therefore, an attempt has been made to recycle the remaining high-quality target materials, leading to a novel intermetallic, multiphased TixAly target design. Within this study, an in-depth comparison of recycled intermetallic TiAl (50/50 at. %) targets (IM) and conventional powder metallurgically ones (PM) of identical composition is presented from the aspect of different PVD techniques, and respective film properties. A nitrogen variation is conducted during direct current magnetron sputtering (DCMS), high-power impulse magnetron sputtering (HiPIMS), as well as cathodic arc evaporation (CAE). Structural analysis of the virgin IM target reveals the presence of several intermetallic phases (e.g., γ-TiAl, α2-Ti3Al, or TiAl2), whereas a pure two-phased structure (i.e. hexagonal-Ti and cubic-Al) is identified for the PM target. While no phase transition is observed on the target surface after DCMS and HiPIMS, significant changes occur during CAE. For both targets identical intermetallic as well as nitride-based phases – such as face centered cubic (fcc) TiAlN – have been recognized after arc evaporation. In addition, DCMS discharges of the two target types obtained strongly varying poisoning behaviour. IM targets show a smooth and quasi hysteresis-free transition from the metallic to the compound dominated mode, whereas an abrupt poisoning is observed for the PM target accompanied by a pronounced hysteresis effect – transition zone from 0.22 to 0.34 Pa nitrogen partial pressure. Consequently, this unequal poisoning behaviour is reflected in the deposition rates and the required nitrogen partial pressure during DCMS to stabilise fcc-structured Ti0.43Al0.57N. On the other hand, reactive HiPIMS deposition from both target types, performed under significantly low nitrogen partial pressure (0.08 to 0.12 Pa) allows for a steady transition from wurtzite to purely fcc-structured coatings without revealing a distinct decrease in the deposition rate due to target poisoning. During CAE the usage of the two different target type reveals only minor changes with respect to structural evolution growth characteristics. Despite the absence of low-melting, elemental aluminium within the IM target, top view analysis of the arc evaporated coatings reveals the formation of more, yet smaller macroparticles compared to thin films deposited from PM targets. Indentation hardness and modulus of all synthesised fcc-phased Ti1-xAlxN are empirical superior with the highest hardness obtained during HiPIMS (H ~ 35 GPa, E ~ 400 GPa) followed by CAE (H ~ 33 GPa, E ~ 460 GPa) as well as DCMS (H ~ 31 GPa, E ~ 450 GPa).This study highlights the importance of a well-described target constitution, based on either purely metallic but also intermetallic phases, leading to interesting effects with respect to target poisoning and phase stabilization. The utilization of IM TiAl targets allows for depositions at high nitrogen partial pressures without pronounced target poisoning and related process instabilities.