Moritz to Baben

Experimental and computational study on the phase stability of Al-containing cubic transition metal nitrides

The phase stability of Al-containing cubic transition metal (TM) nitrides, where Al substitutes for TM (i.e. TM1−xAlxN), is studied as a function of the TM valence electron concentration (VEC). X-ray diffraction and thermal analyses data of magnetron sputtered Ti1−xAlxN, V1−xAlxN and Cr1−xAlxN films indicate increasing phase stability of cubic TM1−xAlxN at larger Al contents and higher temperatures with increasing TM VEC. These experimental findings can be understood based on first principle investigations of ternary cubic TM1−xAlxN with TM = Sc, Ti, V, Cr, Y, Zr and Nb where the TM VEC and the lattice strain are systematically varied.However, our experimental data indicate that, in addition to the decomposition energetics (cubic TM1−xAlxN → cubic TMN + hexagonal AlN), future stability models have to include nitrogen release as one of the mechanisms that critically determine the overall phase stability of TM1−xAlxN.

Origin of the nitrogen over- and understoichiometry in Ti0.5Al0.5N thin films

To identify the origin of the experimentally observed nitrogen over- and understoichiometry in TiAlN thin films, various point defect configurations were studied by ab initio calculations in terms of formation energies, equilibrium volume and elastic moduli. From formation energies and comparison to existing experimental equilibrium volume and elasticity data, it is shown that nitrogen vacancies and metal vacancies are responsible for nitrogen understoichiometry and overstoichiometry, respectively. Irrespective of the type of vacancies, the bulk modulus is decreased by approximately 7% as the nitrogen concentration is increased or decreased by 3 at.%.

(Nbx, Zr1–x)4AlC3 MAX Phase Solid Solutions: Processing, Mechanical Properties, and Density Functional Theory Calculations

The solubility of zirconium (Zr) in the Nb4AlC3 host lattice was investigated by combining the experimental synthesis of (Nbx, Zr1-x)4AlC3 solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH0.89, ZrH2, Al, and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum of 18.5% of the Nb content in the host lattice) in Nb4AlC3 observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb0.85, Zr0.15)4AlC3 was mapped using atom probe tomography. The hardness, Youngs modulus, and fracture toughness at room temperature as well as the high-temperature flexural strength and E-modulus of (Nb0.85, Zr0.15)4AlC3 were investigated and compared to those of pure Nb4AlC3. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 ± 0.1 MPa/m(1/2) for pure Nb4AlC3 to 10.1 ± 0.3 MPa/m(1/2) for the (Nb0.85, Zr0.15)4AlC3 solid solution.

Phase stability predictions of Cr1?x, Mx)2(Al1?y, Ay)(C1?z, Xz) (M?=?Ti, Hf, Zr; A?=?Si, X?=?B)

The phase stability of (Cr1?x, Mx)2(Al1?y, Ay)(C1?z, Xz) (M?=?Ti, Hf, Zr; A?=?Si, X?=?B, space group P63/mmc, prototype Cr2AlC) was studied using ab initio calculations. Based on the energy of mixing data as well as the density of states (DOS) analysis, (Cr1?x, Zrx)2AlC and (Cr1?x, Hfx)2AlC are predicted to be unstable, whereas (Cr1?x, Tix)2AlC, Cr2(Al1?y, Siy)C and Cr2Al(C1?z, Bz) are predicted to be stable or metastable. The density of states analysis reveals that small differences in the position of the Fermi level alters the phase stability: (Cr1?x, Zrx)2AlC and (Cr1?x, Hfx)2AlC are predicted to be unstable or metastable as the Fermi level lies at a peak position. While the Cr dominated DOS for (Cr1?x, Tix)2AlC plateaus at the Fermi level indicating stability. Implications of these results for the vapour phase condensation of self-healing Cr2AlC based materials are discussed.

Modeling of metastable phase formation diagrams for sputtered thin films

Abstract A method to model the metastable phase formation in the Cu–W system based on the critical surface diffusion distance has been developed. The driver for the formation of a second phase is the critical diffusion distance which is dependent on the solubility of W in Cu and on the solubility of Cu in W. Based on comparative theoretical and experimental data, we can describe the relationship between the solubilities and the critical diffusion distances in order to model the metastable phase formation. Metastable phase formation diagrams for Cu–W and Cu–V thin films are predicted and validated by combinatorial magnetron sputtering experiments. The correlative experimental and theoretical research strategy adopted here enables us to efficiently describe the relationship between the solubilities and the critical diffusion distances in order to model the metastable phase formation during magnetron sputtering.

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.

Theoretical study of elastic properties and phase stability of M0.5Al0.5N1−xOx (M = Sc, Ti, V, Cr)

The influence of oxygen content and transition metal valence electron concentration on the phase stability and elastic properties of cubic M0.5Al0.5N1−xOx (M = Sc, Ti, V, Cr; x = 0 – 0.5) was studied using ab initio calculations. The negative value of enthalpy of mixing was observed for all phases indicating full miscibility of M0.5Al0.5N with the hypothetical M0.5Al0.5O. Bulk moduli are decreased as x in M0.5Al0.5N1−xOx is increased. This can be understood based on the electronic structure. As N is substituted by O, there are no noticeable changes in the chemical bonding nature. However, O is more electronegative than N, giving rise to an increase in the ionic character of the overall bonding. In spite of that, the M – O bond in M0.5Al0.5N1−xOx is longer than the corresponding M–N bond, which implies that this bond becomes weaker. Hence, we propose that the decrease of bulk moduli upon O incorporation into M0.5Al0.5N1−xOx is caused by weaker M–O bonds.

Correlative theoretical and experimental investigation of the formation of AlYB14 and competing phases

The phase formation in the boron-rich section of the Al-Y-B system has been explored by a correlative theoretical and experimental research approach. The structure of coatings deposited via high power pulsed magnetron sputtering from a compound target was studied using elastic recoil detection analysis, electron energy loss spectroscopy spectrum imaging, as well as X-ray and electron diffraction data. The formation of AlYB14 together with the (Y,Al)B6 impurity phase, containing 1.8 at. % less B than AlYB14, was observed at a growth temperature of 800 °C and hence 600 °C below the bulk synthesis temperature. Based on quantum mechanical calculations, we infer that minute compositional variations within the film may be responsible for the formation of both icosahedrally bonded AlYB14 and cubic (Y,Al)B6 phases. These findings are relevant for synthesis attempts of all boron rich icosahedrally bonded compounds with the space group: Imma that form ternary phases at similar compositions.