AUTO-ORGANIZED NANOSTRUCTURES IN THE Ti-Al-N THIN FILM SYSTEM

摘要

Age hardening by spinodal decomposition in ceramic thin film systems is reviewed. This is a new concept for advanced surface engineering with applications for wear-resistant coatings in machining processes like high-speed and dry cutting. The reactive arc deposition method with relatively low substrate temperatures is employed to produce supersaturated solid solutions of the material by ion-bombardment-induced mixing of atoms and kinetic limitation to reduce thermodynamically-driven segregation during synthesis. It is shown using electron microscopy, X-ray diffraction, and nanoindentation techniques that Ti_(1-x)Al_xN (0 <= x <= 1) coatings with compositions in the miscibility gap undergo spinodal decomposition during annealing at temperatures between approx 900 deg C and 1100 deg C. As a result, coherent cubic-phase nanometer-size domains form that cause an increase in hardness. These intermediate metastable domains transform into their stable phases TiN and hexagonal wurtzite-structure AlN during further thermal treatment. The findings are corroborated by Ab initio calculations of phase stability and molar volume for competing phases. Activation energies for the processes indicate defect-assisted segregation of Ti and Al. It is inferred that the success of Ti_(1-x)Al_xN coatings is not only based on its superior oxidation resistance, but also on its ability for self-adaptation to the thermal load applied during cutting by age hardening. The findings and experimental approach have implications also for other ternary and multinary ceramic systems including the group-Ill nitride alloys.