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.

${\mathrm{TiB}}_2$ thin films are well known for their high hardness which makes them useful for wear-resistant applications. Overstoichiometric ${\mathrm{TiB}}_2$ deposited at 300 °C by nonreactive sputtering has been shown to exhibit superhardness $(H⩾40\mathrm{GPa})$ , while the hardness of their bulk stoichiometric counterparts is $\sim 25\mathrm{GPa}$ . We show, using high-resolution transmission electron microscopy, that overstoichiometric ${\mathrm{TiB}}_{2.4}$ layers have a complex self-organized columnar nanostructure. The $\sim 20\mathrm{nm}$ wide columns, encapsulated in excess B and oriented along 0001, consist of a bundle of $\sim 5\mathrm{nm}$ diameter ${\mathrm{TiB}}_2$ subcolumns separated by an ultrathin B-rich tissue phase. The nanocolumnar structure, which is thermally stable to postannealing temperatures up to 700 °C, inhibits nucleation and glide of dislocations during hardness indentation measurements, while the high cohesive strength of the B-rich tissue phase prevents grain-boundary sliding. The combination of these effects results in the observed superhardness of $\sim 60\mathrm{GPa}$ .