Influence of Ta on the fracture toughness of arc evaporated Ti-Al-N

Adaptive hard and tough mechanical response in single-crystal B1 VNx ceramics via control of anion vacancies

The addition of Ta to Ti1−xAlxN hard coatings has been proven to be a promising strategy to enhance their properties, consequently leading to superior performance of coated cutting tools. Within this work, tribological properties and oxidation behavior of arc evaporated Ti1−x−yAlxTayN hard coatings were systematically investigated for Ta contents varying between 0 ≤ y ≤ 0.23. The combination of ball-on-disk tests at elevated temperatures and differential scanning calorimetry with subsequent X-ray diffraction analysis provided profound insight into the microstructural changes occurring at elevated temperatures. Additionally, the investigation of the oxide layers, formed within the wear track and on the coating surface outside the wear track, allowed to establish correlations between the beneficial effect of Ta on the oxidation process, the oxide scale formation and the tribological properties. The addition of Ta to the metastable Ti1−xAlxN solid solution increases the oxidation onset temperature by >100 °C, promotes the formation of the stabile rutile phase within the oxide scale, and leads to a significant reduction of the friction coefficient.

High-temperature tribology and oxidation of Ti1−x−yAlxTayN hard coatings

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.

Improved mechanical properties, thermal stabilities, and oxidation resistance of arc evaporated Ti-Al-N coatings through alloying with Ta

We carried out density-functional ab initio molecular dynamics (AIMD) simulations of Ti adatom ($\mathrm{T}{\mathrm{i}}_{\mathrm{ad}}$) migration on, and descent from, square $\mathrm{TiN}\ensuremath{\langle}100\ensuremath{\rangle}$ epitaxial islands on TiN(001) at temperatures ($T$) ranging from 1200 to 2400 K. Adatom-descent energy barriers determined via ab initio nudged-elastic-band calculations at 0 Kelvin suggest that Ti interlayer transport on TiN(001) occurs essentially exclusively via direct hopping onto a lower layer. However, AIMD simulations reveal comparable rates for $\mathrm{T}{\mathrm{i}}_{\mathrm{ad}}$ descent via direct hopping vs push-out/exchange with a Ti island-edge atom for $T\ensuremath{\ge}1500\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We demonstrate that this effect is due to surface vibrations, which yield considerably lower activation energies at finite temperatures by significantly modifying the adatom push-out/exchange reaction pathway.

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Effects of surface vibrations on interlayer mass transport: Ab initio molecular dynamics investigation of Ti adatom descent pathways and rates from TiN/TiN(001) islands

Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/propagation leading to brittle failure during application, is a primary industrial requirement. In this work, experimental methods supported by ab initio density functional theory (DFT) calculations and electronic structure analyses are used to investigate the mechanical behavior of magnetron sputtered Ti-Al-Ta-N hard coatings. The as-deposited Ti1-x-yAlxTayN (y = 0–0.60) films exhibit a single phase cubic sodium chloride (B1) structure identified as TiAl(Ta)N solid solutions. While the hardness H of Ti0.46Al0.54N (32.5 ± 2 GPa) is not significantly affected by alloying with TaN (H of the quaternary nitrides varies between 26 ± 2 and 35 ± 4 GPa), the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents, which indicates improved toughness in TiAlTaN. Consistent with the experimental findings, the DFT results show that Ta substitutions in TiAlN reduce the shear resistance due...

Experimental and computational studies on toughness enhancement in Ti-Al-Ta-N quaternaries

Superconductor/semiconductor hybrid devices have attracted increasing interest in the past years. Superconducting electronics aims to complement semiconductor technology, while hybrid architectures are at the forefront of new ideas such as topological superconductivity and protected qubits. In this work, we engineer the induced superconductivity in two-dimensional germanium hole gas by varying the distance between the quantum well and the aluminum. We demonstrate a hard superconducting gap and realize an electrically and flux tunable superconducting diode using a superconducting quantum interference device (SQUID). This allows to tune the current phase relation (CPR), to a regime where single Cooper pair tunneling is suppressed, creating a $ \sin \left( 2 \varphi \right)$ CPR. Shapiro experiments complement this interpretation and the microwave drive allows to create a diode with $ \approx 100 \%$ efficiency. The reported results open up the path towards monolithic integration of spin qubit devices, microwave resonators and (protected) superconducting qubits on a silicon technology compatible platform.

Marián Janík

Papers

Experimental and computational studies on toughness enhancement in Ti-Al-Ta-N quaternaries

Radio frequency driven superconducting diode and parity conserving Cooper pair transport in a two-dimensional germanium hole gas