High pressure and high temperature experiments with a Hall belt apparatus of cathodic arc deposited Ti0.63Al0.37N and Ti0.37Al0.63N on polycrystalline cubic BN substrate reveal a strong influence of pressure on the decomposition rate of TiAlN. The pressure induces enhanced phase stability while keeping the interfaces between coatings and the substrates intact. Hardness measurements of the as-deposited coatings and after high-pressure high-temperature treatment show that the hardness after treatment of Ti0.63Al0.37N at 5.35GPa, 1300°C and after 66min drops from 29.6 to 27.8GPa, because after this treatment this sample contains cubic TiN and hexagonal AlN. This drop is much smaller than if the coatings are just heat treated at 1300°C and suggests that an enhancement of TiAlN-coatings on cubic BN cuttings tools can be achieved by increasing the pressure/stress during cutting. For Ti0.37Al0.63N treated at 5.35GPa, 1300°C and after 66min the hardness drops from 36.1 to 28.6GPa, which means that the coating has first decomposed through spinodal decomposition and one of products of this decomposition has further phase transformed and the final products are cubic TiN and hexagonal AlN.

Nitrides are used as coatings and thin films for a wide range of applications.The study and use of nitrides in the recent decades have shifted towards ternary, quaternary or even higher order (complex) nitrides.There is an interest to use ternary and quaternary nitrides for machining and thermoelectric materials, because it gives the possibility to choose composition and thereby design the materials properties.This thesis presents research results on TiAlN and and TiAlN-based coatings that are used as hard coatings for machining and on ternary scandium nitrides that are of interest for thin films for thermoelectric applications.The high-pressure high-temperature behavior of cubic TiAlN deposited on cubic boron nitride has been experimentally studied.It has been shown that the spinodal decomposition, which means decomposition into cubic domains enriched in TiN and AlN, is delayed as a result of high pressure compared to ambient pressure.No chemical interaction between coating and substrate occurs.TiZrAlN has been theoretically and experimentally studied at high temperature.The results show that the when Zr-content is decreased and the Al-content is increased the decomposition route changes from nucleation and growth to spinodal decomposition.The microstructure evolution with temperature depends on the initial composition.In the case where the decompositon starts with only spinodal decomposition the microstructure at 1100 °C consists of domains that are larger than in the case where the decomposition occurs by nucleation and growth.ScMN 2 (M=V, Nb, Ta) phases have been experimentally demonstrated for M=Nb and Ta in a few studies, but have not been much investigated.In this theseis, their crystal structure, stability, elastic properties, electronic structure and thermoelectric properties have been studied.At 0 K and 0 GPa it has been shown that these three phases are thermodynamically and elastically stable.Additionally, these are narrow-bandgap semiconductors and their thermoelectric properties can be tuned by doping.Pressure has a stabilizing effect on these structures.When pressure increases from 0-150 GPa the elastic constants and moduli increases in the range 53-317 %. v vi

ScMN2-type (M = V, Nb, Ta) phases are layered materials that have been experimentally reported for M = Ta and Nb. They are narrow-bandgap semiconductors with potentially interesting thermoelectric properties. Point defects such as dopants and vacancies largely affect these properties, motivating the need to investigate these effects. In particular, asymmetric peak features in the density of states (DOS) close to the highest occupied state is expected to increase the Seebeck coefficient. Here, we used first principles calculations to study the effects of one vacancy or one C, O, or F dopant on the DOS of the ScMN2 phases. We used density functional theory to calculate formation energy and the density of states when a point defect is introduced in the structures. In the DOS, asymmetric peak features close to the highest occupied state were found as a result of having a vacancy in all three phases. Furthermore, one C dopant in ScTaN2, ScNbN2, and ScVN2 implies a shift of the highest occupied state into the valence band, while one O or F dopant causes a shift of the highest occupied state into the conduction band.

ScMN2-type (M = V, Nb, Ta) phases are layered materials that have been experimentally reported for M = Ta and Nb, but they have up to now not been much studied. However, based on the properties of binary ScN and its alloys, it is reasonable to expect these phases to be of relevance in a range of applications, including thermoelectrics. Here, we have used first-principles calculations to study their thermodynamic stability, elastic, thermoelectric and electronic properties. We have used density functional theory to calculate lattice parameters, the mixing enthalpy of formation and electronic density of states as well as the thermoelectric properties and elastic constants (c(ij)), bulk (B), shear (G) and Young's (E) modulus, which were compared with available experimental data. Our results indicate that the considered systems are thermodynamically and elastically stable and that all are semiconductors with small band gaps. All three materials display anisotropic thermoelectric properties and indicate the possibility to tune these properties by doping. In particular, ScVN2, featuring the largest band gap exhibits a particularly large and strongly doping-sensitive Seebeck coefficient.

ScMN2-type (M = V, Nb, Ta) phases are layered materials that have been experimentally reported for M = Ta and Nb, but their high-pressure properties have not been studied. Here, we have used first-principles calculations to study their thermodynamic stability, elastic and electronic properties at high-pressure. We have used density functional theory to calculate the formation enthalpy relative to the competing binary phases, electronic density of states and elastic constants (c(ij)), bulk (B), shear (G) and Young's (E) modulus as the pressure is varied from 0 to 150 GPa. Our results show that when the pressure increases from 0 to 150 GPa, elastic constants, bulk, shear and elastic moduli increase in the range 53-216% for ScTaN2, 72-286% for ScNbN2, and 61-317% for ScVN2.

Phase, microstructure, and strain evolution during annealing of arc deposited TiZrAlN coatings are studied using in situ x-ray scattering and ex situ transmission electron microscopy. We find that the decomposition route changes from nucleation and growth of wurtzite AlN to spinodal decomposition when the Zr-content is decreased and the Al-content increases. Decomposition of Ti0.31Zr0.24Al0.45N results in homogeneously distributed wurtzite AlN grains in a cubic, dislocation-dense matrix of TiZrN consisting of domains of different chemical composition. The combination of high dislocation density, variation of chemical composition within the cubic grains, and evenly distributed wurtzite AlN grains results in high compressive strains, -1.1%, which are retained after 3 h at 1100 degrees C. In coatings with higher Zr-content, the strains relax during annealing above 900 degrees C due to grain growth and defect annihilation. (C) 2018 Elsevier B.V. All rights reserved.

Through a combination of theoretical and experimental observations we study the high temperature decomposition behavior of c-(TixZryAlzN) alloys. We show that for most concentrations the high formation energy of (ZrAl)N causes a strong tendency for spinodal decomposition between ZrN and AlN while other decompositions tendencies are suppressed. In addition we observe that entropic effects due to configurational disorder favor a formation of a stable Zr-rich (TiZr)N phase with increasing temperature. Our calculations also predict that at high temperatures a Zr rich (TiZrAl)N disordered phase should become more resistant against the spinodal decomposition despite its high and positive formation energy due to the specific topology of the free energy surface at the relevant concentrations. Our experimental observations confirm this prediction by showing strong tendency towards decomposition in a Zr-poor sample while a Zr-rich alloy shows a greatly reduced decomposition rate, which is mostly attributable to binodal decomposition processes. This result highlights the importance of considering the second derivative of the free energy, in addition to its absolute value in predicting decomposition trends of thermodynamically unstable alloys.