Self-organized nanostructures in the Ti–Al–N system

Abstract

The phenomenon of age hardening could be evidenced in thin film applications. A model system, ${\mathrm{Ti}}_{\text{1−x}}{\mathrm{Al}}_x\mathrm{N}$ was chosen as such coatings are known for their excellent wear resistance enabling improved machining processes like high-speed and dry cutting. Here, we show unambiguously that metastable ${\mathrm{Ti}}_{\text{1−x}}{\mathrm{Al}}_x\mathrm{N}$ coatings initially undergo spinodal decomposition into coherent cubic-phase nanometer-size domains, causing an increase in hardness at elevated temperatures. These intermediate metastable domains transform into their stable phases TiN and AlN during further thermal treatment. Activation energies for the processes indicate defect-assisted segregation of Ti and Al. The findings are corroborated by ab initio calculations. A long-standing discussion on the thermal stability of this important class of ceramics is thus resolved.