Marcus Hans

Surface chemistry of TiAlN and TiAlNO coatings deposited by means of high power pulsed magnetron sputtering

It is shown that the investigated TiAlN and TiAlNO protective coatings show an enrichment of oxygen in the surface near region with concurrently higher oxidation states of titanium. We suggest that surface oxidation is caused by reaction with atmospheric oxygen at room temperature and/or with residual gas immediately after deposition during cooling in the chamber. As this coating system is used during cutting and forming operations, the here-established difference between the coatings composition and the composition of the surface near region of the coating is of great relevance for understanding the interactions thereof with the materials to be cut or formed.

Effect of oxygen incorporation on the structure and elasticity of Ti-Al-O-N coatings synthesized by cathodic arc and high power pulsed magnetron sputtering

Ti-Al-O-N coatings were synthesized by cathodic arc and high power pulsed magnetron sputtering. The chemical composition of the coatings was determined by means of elastic recoil detection analysis and energy dispersive X-ray spectroscopy. The effect of oxygen incorporation on the stress-free lattice parameters and Youngs moduli of Ti-Al-O-N coatings was investigated by X-ray diffraction and nanoindentation, respectively. As nitrogen is substituted by oxygen, implications for the charge balance may be expected. A reduction in equilibrium volume with increasing O concentration is identified by X-ray diffraction and density functional theory calculations of Ti-Al-O-N supercells reveal the concomitant formation of metal vacancies. Hence, the oxygen incorporation-induced formation of metal vacancies enables charge balancing. Furthermore, nanoindentation experiments reveal a decrease in elastic modulus with increasing O concentration. Based on ab initio data, two causes can be identified for this: First, the metal vacancy-induced reduction in elasticity; and second, the formation of, compared to the corresponding metal nitride bonds, relatively weak Ti-O and Al-O bonds.

Unprecedented Al supersaturation in single-phase rock salt structure VAlN films by Al+ subplantation

Modern applications of refractory ceramic thin films, predominantly as wear-protective coatings on cutting tools and on components utilized in automotive engines, require a combination of excellent mechanical properties, thermal stability, and oxidation resistance. Conventional design approaches for transition metal nitride coatings with improved thermal and chemical stability are based on alloying with Al. It is well known that the solubility of Al in NaCl-structure transition metal nitrides is limited. Hence, the great challenge is to increase the Al concentration substantially while avoiding precipitation of the thermodynamically favored wurtzite-AlN phase, which is detrimental to mechanical properties. Here, we use VAlN as a model system to illustrate a new concept for the synthesis of metastable single-phase NaCl-structure thin films with the Al content far beyond solubility limits obtained with conventional plasma processes. This supersaturation is achieved by separating the film-forming species in ...

Unprecedented thermal stability of inherently metastable titanium aluminum nitride by point defect engineering

ABSTRACT Extreme cooling rates during physical vapor deposition (PVD) allow growth of metastable phases. However, we propose that reactive PVD processes can be described by a gas–solid paraequilibrium defining chemical composition and thus point defect concentration. We show that this notion allows for point defect engineering by controlling deposition conditions. As example we demonstrate that thermal stability of metastable (Ti,Al)Nx, the industrial benchmark coating for wear protection, can be increased from 800°C to unprecedented 1200°C by minimizing the vacancy concentration. The thermodynamic approach formulated here opens a pathway for thermal stability engineering by point defects in reactively deposited thin films. GRAPHICAL ABSTRACT IMPACT STATEMENT A novel thermodynamic methodology to predict stoichiometry of coatings is utilized to increase thermal stability of today’s industrial benchmark hard coating TiAlN from 800°C to 1200°C by point defect engineering.

High-throughput exploration of thermoelectric and mechanical properties of amorphous NbO2 with transition metal additions

To increase the thermoelectric efficiency and reduce the thermal fatigue upon cyclic heat loading, alloying of amorphous NbO2 with all 3d and 5d transition metals has systematically been investigated using density functional theory. It was found that Ta fulfills the key design criteria, namely, enhancement of the Seebeck coefficient and positive Cauchy pressure (ductility gauge). These quantum mechanical predictions were validated by assessing the thermoelectric and elastic properties on combinatorial thin films, which is a high-throughput approach. The maximum power factor is 2813 μW m−1 K−2 for the Ta/Nb ratio of 0.25, which is a hundredfold increment compared to pure NbO2 and exceeds many oxide thermoelectrics. Based on the elasticity measurements, the consistency between theory and experiment for the Cauchy pressure was attained within 2%. On the basis of the electronic structure analysis, these configurations can be perceived as metallic, which is consistent with low electrical resistivity and ductil...

From quantum to continuum mechanics: studying the fracture toughness of transition metal nitrides and oxynitrides

ABSTRACT We show here, based on VAlN, TiAlN and the related oxynitrides, that the (brittle) fracture and elastic properties may be consistently modelled from quantum- to continuum mechanics using micromechanical testing to link both scales. The measured elastic moduli match closely with those predicted by density functional theory calculations. Good agreement was also observed between the micro-cantilever bending experiments and cohesive-zone-finite element modelling. These scale-bridging data serve as a baseline for future improvements of the fracture toughness of these coating systems based on microstructure and coating architecture optimization. GRAPHICAL ABSTRACT IMPACT STATEMENT We link ab initio calculations with finite element modelling using micro-mechanical testing to consistently model the elastic and fracture behaviour of transition metal nitrides and oxynitrides.

Extended metastable Al solubility in cubic VAlN by metal-ion bombardment during pulsed magnetron sputtering: film stress vs subplantation

Dynamic ion-recoil mixing of near-film-surface atomic layers is commonly used to increase the metastable solubility limit xmax in otherwise immiscible thin film systems during physical vapor deposition. Recently, Al subplantation achieved by irradiating the film growth surface with Al+ metal-ion flux was shown to result in an unprecedented xmax for VAlN, far above values obtained with gas ion irradiation. However, it is reasonable to assume that ion irradiation necessary for subplantation also leads to a compressive stress σ buildup. In order to separate the effects of Al+ bombardment on σ and xmax, and realize low-stress high-xmax nitride alloys, we grow metastable cubic V1-xAlxN (0.17 ≤ x ≤ 0.74) films using reactive magnetron sputtering under different ion irradiation conditions. Al and V targets are operated in Ar/N2 discharges employing (i) conventional DC (Ar+, N2+), (ii) hybrid High-power pulsed magnetron sputtering (HIPIMS)/DC processing with one type of metal ion present (Al+ or V+/V2+), and (iii...

Crystallite size-dependent metastable phase formation of TiAlN coatings

It is well known that surface energy differences thermodynamically stabilize nanocrystalline γ-Al2O3 over α-Al2O3. Here, through correlative ab initio calculations and advanced material characterization at the nanometer scale, we demonstrate that the metastable phase formation of nanocrystalline TiAlN, an industrial benchmark coating material, is crystallite size-dependent. By relating calculated surface and volume energy contributions to the total energy, we predict the chemical composition-dependent phase boundary between the two metastable solid solution phases of cubic and wurzite Ti1−xAlxN. This phase boundary is characterized by the critical crystallite size dcritical. Crystallite size-dependent phase stability predictions are in very good agreement with experimental phase formation data where x was varied by utilizing combinatorial vapor phase condensation. The wide range of critical Al solubilities for metastable cubic Ti1−xAlxN from xmax = 0.4 to 0.9 reported in literature and the sobering disagreement thereof with DFT predictions can at least in part be rationalized based on the here identified crystallite size-dependent metastable phase formation. Furthermore, it is evident that predictions of critical Al solubilities in metastable cubic TiAlN are flawed, if the previously overlooked surface energy contribution to the total energy is not considered.

Structural, mechanical, and magnetic properties of GaFe3N thin films

Using the density-functional theory, the structural, mechanical, and magnetic properties were investigated for different GaFe3N configurations: ferromagnetic, ferrimagnetic, paramagnetic, and nonmagnetic. Ferrimagnetic and high-spin ferromagnetic states exhibit the lowest energy and are the competing ground states as the total energy difference is 0.3 meV/atom only. All theoretically predicted values could be fully confirmed by experiments. For this, the authors synthesized phase pure, homogeneous, and continuous GaFe3N films by combinatorial reactive direct current magnetron sputtering. Despite the low melting point of gallium, the authors succeeded in the growth of GaFe3N films at a temperature of 500 °C. Those thin films exhibit a lattice parameter of 3.794 A and an elastic modulus of 226 ± 20 GPa. Magnetic susceptibility measurements evidence a magnetic phase transitions at 8.0 ± 0.1 K. The nearly saturated magnetic moment at ±5 T is about 1.6 μB/Fe and is close to the theoretically determined magneti...

Metastable phase formation of Pt-X (X = Ir, Au) thin films

The dependence of phase formation and mechanical properties on the chemical composition has been investigated for Pt-Ir and Pt-Au combinatorial thin films. The formation of a single, metastable Pt-Ir solid solution has been observed for all experimental compositions and temperatures. Upon Ir addition to Pt the experimentally determined changes in lattice parameter and Young’s modulus display rule of mixture behavior which is in good agreement with our ab initio data. Whereas, in the Pt-Au system, the single metastable solid solution decomposes into two phases as the growth temperature is raised to ≥600 °C. The lattice parameters in the dual phase region are independent of chemical composition. The substrate temperature and chemical composition dependent phase formation in Pt-Ir and Pt-Au thin films can be rationalized based on CALPHAD (CALculation of PHAse Diagrams) results combined with estimations of the activation energy required for surface diffusion: The metastable phase formation during film growth is caused by kinetic limitations, where Ir atoms (in Pt-Ir) need to overcome an up to factor 6 higher activation energy barrier than Au (in Pt-Au) to enable surface diffusion.