Microstructure and stress influence on phase transformations

Hard coatings are successfully used for various tribological applications in particularly to increase the life-time of machining tools since the 1970`s. In recent years, binary and ternary transition metal nitride materials are playing a crucial role and are used in various engineering applications due to their remarkable physical and mechanical properties including high hardness, high melting point, chemical inertness, and good thermodynamic stability. Due to the processing by vapor deposition techniques, these materials often contain metastable phases. Consequently, the stability and transition of these phases are of great importance as thereby the overall coating properties (like hardness, thermal and oxidation resistance) are influenced. Titanium Aluminum Nitride (Ti1-x Alx N) was developed in the late 1980`s as a promising alternative to TiN coatings for cutting and forming tools with a higher oxidation resistance at elevated temperatures and improved performance in machining operations. Previous studies on Ti 1-x Alx N mainly concentrate on the effect of varying Al contents on the occurring phase transitions, whereas only recently also the effect of various sputtering parameters on microstructural and phase transitions have been studied by the applicant. Here, especially the stresses in the coatings during deposition and phase transition are of great interest, as there is only limited information on the correlation between phase transition, compressive stresses and microstructural features such as grain size for this class of coatings. Consequently, the main focus of the proposed project is to investigate the influence of sputtering parameters on phase transition and structural, mechanical and thermal properties of Ti 1-x Alx N coatings. The thermal stability will be studied as a function of the annealing temperature to substantiate the structural changes of the coating. The stresses developed in the coating during deposition and phase transition is essential to investigate as it plays an important role (as cubic AlN is the stress-dependent metastable form of stable hexagonal AlN). The experimental studies will be supported by computational studies on mechanical properties and phase transition of the coatings using finite element analysis and density function theory. Multilayered or superlattice Ti 1-x Alx N/AlN coatings will be used to study the influence of coherency strains on the phase transformation of the individual cubic (c) stabilized Ti 1-x Alx N and AlN layers. Varying the chemical composition of the Ti1-xAlxN layers allow to vary the generated coherency strain as thereby the lattice parameter mismatch between the layers (c-Ti 1-x Alx N and c-AlN) changes. The studies on these model systems will contribute towards the understanding and hence controlling of phase transformations in Ti 1-x Alx N based hard coatings. This enables knowledge-based tailoring of materials to meet future industrial demands.