High temperature behavior of arc evaporated ZrAlN and TiAlN thin films

Hard coatings can extend the life time of a tool substantially and enable higher cutting speeds which increase the productivity in the cutting application. The aim with this thesis is to extend the understanding on how the microstructure and mechanical properties are affected by high temperatures similar to what a cutting tool can reach during operation. Thin films of ZrAlN and TiAlN have been deposited using cathodic arc-evaporation. The microstructure of as-deposited and annealed films has been studied using electron microscopy and x-ray scattering. The thermal stability has been characterized by calorimetry and thermogravity and the mechanical properties have been investigated by nanoindentation. The microstructure of Zr1−xAlxN thin films was studied as a function of composition, deposition conditions, and annealing temperature. The structure was found to depend on the Al content where a low (x < 0.38) Al-content results in cubic-structured ZrAlN while for x > 0.70 the structure is hexagonal. For intermediate Al contents (0.38 < x < 0.70), a nanocomposite structure with a mixture of cubic, hexagonal and amorphous phases is obtained. The cubic ZrAlN phase transforms by nucleation and growth of hexagonal AlN when annealed above 900 ◦C. Annealing of hexagonal ZrAlN thin films (x > 0.70) above 900 ◦C causes formation of AlN and ZrN rich domains within the hexagonal lattice. Annealing of nanocomposite ZrAlN thin films results in formation of cubic ZrN and hexagonal AlN. The transformation is initiated by nucleation and growth of cubic ZrN at temperatures of 1100 ◦C while the AlN-rich domains are still amorphous or nanocrystalline. Growth of hexagonal AlN is suppressed by the high nitrogen content of the films and takes place at annealing temperatures of 1400 ◦C. In the more well known TiAlN system, the initial stage of decomposition is spinodal with formation of cubic structured domains enriched in TiN and AlN. By a combination of in-situ x-ray scattering techniques during annealing and phase field simulations, both the microstructure that evolves during decomposition and the decomposition rate are found to depend on the composition. The results further show that early formation of hexagonal AlN domains during decomposition can cause formation of strains in the cubic TiAlN phase.