W.M. Seidl, M. Bartosik, S. Kolozsvári, H. Bolvardi, P.H. Mayrhofer, Improved mechanical properties, thermal stabilities, and oxidation resistance, of arc evaporated Ti-Al-N coatings through alloying with Ta, Surf. Coat. Technol. (2018) in press DOI10.1016/j.surfcoat.2018.03.014
Improved mechanical properties, thermal stabilities, and oxidation resistance, of arc evaporated Ti-Al-N coatings through alloying with Ta W.M. Seidl1, M. Bartosik1, 2*, S. Kolozsvári3, H. Bolvardi4, and P.H. Mayrhofer1, 2 1
Christian Doppler Laboratory for Application Oriented Coating Development, TU Wien, Austria 2 Institute of Materials Science and Technology, TU Wien, Austria 3 Plansee Composite Materials GmbH, Germany 4 Oerlikon Balzers, Oerlikon Surface Solutions AG, Liechtenstein
Abstract Ti-Al-N is widely used as protective coating in various industrial applications. Here, we investigate the influence of Ta content – ranging from 0 to 28 at% on the metal sublattice – on the mechanical properties, thermal stability, and high temperature oxidation resistance of arc evaporated Ti-Al-N coatings. We found an increase in hardness from 30.0±1.0 GPa for Ti0.54Al0.46N to 35±0.7 GPa for Ti0.44Al0.41Ta0.15N. Furthermore, age hardening due to spinodal decomposition of the supersaturated solid solution into cubic AlN-, TiN-, and TaN-rich domains lead to maximum hardness values of 39.5±1.0 GPa upon annealing to 1100 °C for Ti0.38Al0.34Ta0.28N. This behaviour is directly linked with the extremely retarded formation of wurtzite structured (w) AlN. Even after annealing Ti0.38Al0.34Ta0.28N at the maximum temperature of 1100 °C, no crystalline w-AlN phase could be detected. Additionally, the incorporation of Ta to Ti-Al-N leads to a significantly higher oxidation resistance. While Ti0.54Al0.46N is already fully oxidised during exposure to ambient air at 900 °C, following a linear like oxide scale growth kinetic with 2.96±0.48 µm/h, the Ti0.49Al0.44Ta0.07N and Ti0.44Al0.41Ta0.15N coatings provide the lowest parabolic-like oxide growth rates of 7.1±0.4·10-2 µm²/h. The even higher-Ta-containing Ti0.38Al0.34Ta0.28N, exhibits already a much higher oxide scale growth rate of 23.1±1.7·10-2 µm²/h. Consequently, the Ta content for an optimised oxidation resistance needs to be balanced to the Ti content of the Ti-Al-N coatings. *corresponding author:
[email protected] Keywords: Mechanical properties; Oxidation resistance; TiAlTaN; Ta alloying; PVD. 1. Introduction Ti-Al-N is one of the most versatile and therefore industrially relevant protective coating with applications in a broad variety of industry branches. It exhibits excellent mechanical properties and – depending on the Ti/Al ratio – oftentimes a distinct age hardening effect based on spinodal decomposition [1,2,3]. However, compared to other established coatings, the high temperature oxidation resistance of Ti-Al-N is in need of improvement. Previous studies and density functional theory (DFT) calculations predict a beneficial effect for the addition of a transition metal with higher valence electron concentration (VEC), such as Tantalum, with respect to toughness and cohesive strength [4,5]. In previous studies, specific Ti1-x-yAlxTayN compositions have been analysed regarding their mechanical properties and oxidation resistance [3,6,7,8]. For example, Rachbauer et al. [6] found a significant increase in thermal stability for magnetron sputtered Ti-Al-N films by alloying up to 10 at% Ta (the maximum content investigated). The increasing Ta content leads to a hardness increase from ~30 GPa to about 40 GPa. Furthermore, the formation of wurtzite structured AlN (w-AlN) is retarded by 300 °C to ~1200 °C, with an accompanying increase in maximum hardness from 38 GPa to 42 GPa. This is in agreement with the results obtained for arc evaporated coatings [3], where
Ti0.45Al0.36Ta0.19N exhibits a ~300 °C higher thermal stability than Ti0.54Al0.46N, during vacuum annealing. While the difference in hardness after annealing at 700 °C is small (28 GPa for Ti0.54Al0.46N and 31 GPa for Ti0.45Al0.36Ta0.19N), the difference is significant after annealing at 1000 °C (21 GPa for Ti0.54Al0.46N and 35.5 GPa for Ti0.45Al0.36Ta0.19N) [3]. A more recent study on arc evaporated coatings (on cemented carbide substrates) [8], shows again an increase in mechanical strength and thermal stability (during vacuum annealing) of Ti-Al-N when alloyed with Ta. Due to the high number of possible (metastable) Ta-N phases, and especially the huge impact of point defects such as vacancies on the various phase stability regions (of Ta-N [9] and also Ta-Al-N [10]), the addition of Ta to TiAl-N while remaining with their single-phase face centred cubic (fcc) structure, is not straightforward. Forming a multilayer arrangement with fcc-Ti-Al-N can be very beneficial in this respective [11]. But Ta also helps Ti-Al-N to form protective oxide scales. The significantly improved oxidation resistance of Ti-Al-N when alloyed with Ta [12] is based on several effects: 1) Tantalum has a higher valency than Ti, thus the oxygen vacancy content of TiO2 (which typically forms when TiN or Ti-Al-N oxidizes) decreases when Ta substitutes for Ti in these oxides [13]. Consequently, the diffusion through such solid solution Ti1-xTaxO2 oxides will be more difficult than through TiO2. 2)
1
W.M. Seidl, M. Bartosik, S. Kolozsvári, H. Bolvardi, P.H. Mayrhofer, Improved mechanical properties, thermal stabilities, and oxidation resistance, of arc evaporated Ti-Al-N coatings through alloying with Ta, Surf. Coat. Technol. (2018) in press DOI10.1016/j.surfcoat.2018.03.014
Tantalum assists Ti-Al-N in the formation of a stable and protective corundum type α-Al2O3 and promotes the direct formation of the stable rutile TiO2 phase [7]. Thereby, phase-transformation induced volume changes (which often lead to crack formation within the oxide scale) can be minimised. These mechanisms result in the formation of dense, well adherent, and protective oxide scales. Hollerweger et al. [7], by studying the thermal stability and oxidation resistance of arc evaporated Ti-Al-Ta-N coatings, showed that the growth rate of their oxide scales is not just determined by the Ta content, but especially by the Ti-to-Ta ratio. For low-Ti-containing Ti1-xAlxN coatings (with Al/(Al+Ti) ratios of ~0.66) lower Ta contents lead to best results. For example, Ti0.32Al0.60Ta0.08N shows oxide scale growth constants of 1.23·10-4 and 1.40·10-4 mg²/(mg²·s) at 850 and 950 °C, respectively. These growth rates are higher [1.60·10-4 and 2.40·10-4 mg²/(mg²·s) at 850 and 950 °C, respectively] for the higher Ta-containing Ti0.30Al0.54Ta0.16N. However, the oxide growth rates decrease with increasing Ta content (up to the maximum of 15 at% tested) for higher-Ti-containing coatings (with an Al/(Al+Ti) ratio of ~0.5). While the Tafree Ti0.51Al0.49N coating exhibits an oxide scale growth rate of 1.83·10-3 mg²/(mg²·s) at 850 °C, which becomes already linear at 950 °C, these rates decrease with increasing Ta content. Ti0.47Al0.45Ta0.08N shows oxide scale growth rates of 7.20·10-4 and 1.13·10-3 mg²/(mg²·s) at 850 and 950 °C, which further decrease to 5.80·10-4 and 4.50·10-4 mg²/(mg²·s) at 850 and 950 °C, respectively, for the even higher Ta containing Ti0.43Al0.42Ta0.15N [7]. However, these detailed studies of the oxide-scale growth rates of arc evaporated Ti-Al-Ta-N coatings have been conducted for powdered coating materials using a combined differential scanning calorimetry and thermo gravimetric analyser. The evaluation of the oxidation kinetics (in addition to investigating the mechanical properties and phase stabilities) of arc evaporated Ta-containing Ti-Al-N coatings (with Al/(Al+Ti) ratios of ~0.5) on flat substrates is the aim of this study. Furthermore, we want to find the optimum Ta content also for the higher-Ti-containing Ti-Al-N coatings. As mentioned above, Hollerweger et al. [7] showed that for Ti-Al-N coatings with Al/(Al+Ti) ratios of ~0.66, the optimised Ta content (for best oxidation resistance) is between 8 and 16 at% of the metal fraction. But for Ti-Al-N with Al/(Al+Ti) ratios of ~0.50, the optimised Ta content was not identified, as the oxidation resistance increased up to the maximum Ta content of 15 at% investigated. Therefore, we studied the mechanical properties, thermal stability, and in particular oxidation resistance of ~2-µm-thick Ta containing Ti-Al-N coatings with Al/(Al+Ti) ratios of ~0.50 and Ta contents up to 28 at% (at the metal sublattice).
2. Experimental All coatings were deposited using arc evaporation (Oerlikon Balzers INNOVA deposition plant) of four powder metallurgical produced cathodes (either Ti0.50Al0.50, Ti0.475Al0.475Ta0.05, Ti0.45Al0.45Ta0.10, or Ti0.40Al0.40Ta0.20). To ensure a maximum of comparability between the different coatings, all processes used exact the same parameters and differed only in the cathode materials. Prior to the deposition process, a vacuum below 5·10-6 mbar was ensured before heating the chamber to 480 °C using radiators and plasma heating for up to 40 min. Afterwards, we ion etched the carousel, samples, and chamber for 30 min in pure Ar-atmosphere with the built-in central beam etching technology. During the deposition, all arc sources were operated with a constant arc current of 200 A, while the substrate bias voltage was kept constant at -40 V. The deposition temperature was kept constant at 480 °C. The necessary nitrogen for the formation of stoichiometric nitrides was provided by a pressure-controlled gas flow of 3.5 Pa (3.5·10-2 mbar) N2 in the chamber. To achieve a constant film thickness of 2 µm, we used a deposition time of 40 min. All coatings were synthesised on multiple substrate types, Si (100) (20 x 7 x 0.38 mm³), polished austenitic stainless steel (20 x 7 x 0.8 mm³), polycrystalline Al2O3 (20 x 7 x 0.6 mm³), and monocrystalline Al2O3 (1-102) (10 x 10 x 0.4 mm³). Before mounting the substrates on the two-fold rotating carousel, they were cleaned in an ultrasonic bath, first in acetone and then in ethanol for three minutes each. The deposition chamber featured a minimum cathode-to-substrate-distance of 25 cm. The coatings were analysed regarding their chemical composition as well as the thickness of the oxide scales using Energy-Dispersive X-ray Spectroscopy (EDXS) attached to a FEGSEM Quanta F200 Scanning Electron Microscope (SEM). For the chemical analyses, we used an acceleration voltage of 20 keV. All measured metal/nitrogen ratios were normalised to 1 from their original slightly overstoichiometric values (the N/Me ratio increases from ~1.05 to ~1.15 with increasing Ta content [14]). The coatings deposited from cathodes with 0, 5, 10, and 20 at% Ta exhibited a metal fraction of Ta of 0, 7, 15, and 28 at%, respectively. The same FEGSEM was used to obtain fracture cross section images of all deposited coatings. The film structure was analysed by X-ray diffraction using a PANalytical X’Pert Pro MPD diffractometer equipped with a CuKα radiation source. There, the films were investigated in Bragg-Brentano as well as Gracing Incidence geometry regarding their structural evolution with annealing temperature. Vacuum annealing was done in a Centorr LF22-2000 vacuum furnace (base pressure 5h = 8.8 * 10-2 µm²/h
3.5 3.0
[µm²]
kl= 2.96 µm/h
2.5
4.0
2.0
2.5 2.0
d2Ox
dOx [µm]
3.0
7 at% Ta 15 at% Ta 28 at% Ta
1.5
kp
