CrN and CrN-based films are widely used for different industrial coating applications. Although, Al-additions to CrN improve the mechanical properties and oxidation resistance their high temperature applications are limited due to the N-loss of CrN-based materials, starting already at temperatures below 1000°C. Due to supersaturated phases, there is potential for several age-hardening processes in this system.

In this work thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS) are used in combination with X-ray diffraction to study thermal stability and phase transformations occurring in Al_xCr_1-xN thin films. Pure CrN decomposes during annealing in inert atmosphere above 950 °C to Cr and N₂ via the formation of Cr₂N. The reaction steps are individually identified with simultaneous use of TGA/DSC/MS measurements at different heating rates.

For as-deposited face-centered cubic (fcc) Al_xCr_1-xN films, the onset temperature of N-loss was found to increase with increasing Al content. However, fcc solid solution with Al contents approaching the solubility limit show a maximum in driving force for precipitation of AlN. This precipitation causes the remaining matrix to become Cr-rich and the onset temperature of N-loss decreases. For Al_0.7Cr_0.3N, the precipitation of AlN starts at ~650°C and the correspondingly Cr-enriched remaining matrix decomposes into Cr and N₂. Exceeding the solubility limit during deposition, results in the growth of hexagonal structured thin films with improved thermal stability, but decreased mechanical properties. The results show that by Al addition the onset temperature for decomposition of CrN into Cr and N₂ can be increased by ~100°C.

Using hard coatings to improve the lifetime of prototyping moulds could allow the production of low and mid series from low strength materials moulds. However, a good compromise between high hardness and low friction coefficient must be reached. Coatings of Ti(Al) with low nitrogen content were deposited by magnetron sputtering on M2 (AISI) steel and aluminium substrates using two facing Ti targets incrusted with Al rods. Three sets of depositions with increasing number of Al rods were carried out. The Al/(Al+Ti)atomic ratio in the films was in the range of 21 to 27 at. %, as measured by Electron Probe Microanalysis (EPMA). For each deposition set increasing N fluxes were used leading to N/(Al+Ti+N) atomic ratios up to 33.2 at %.

X-ray diffraction showed that the h.c.p. Ti phase with a <001> preferential orientation was always obtained for the as-deposited films without N. A decrease in the c lattice parameter was observed as Al was added to these films. On the contrary, N addition increased the c parameter. Collapse of the crystalline h.c.p. Ti lattice was also observed as a result of N incorporation. Amorphization of the films was favored by higher aluminum contents.

A hardness of ±13 GPa was measured for the as-deposited films without nitrogen in spite of their different aluminium content. A continuous increase in hardness was observed with increasing nitrogen content. The highest harness values (up to 27 GPa) were obtained for the amorphous films.

Annealing of the lower Al content films (Al/(Al+Ti) = ± 21 at. %) at 975 K for one hour in a dynamic hydrogenated argon atmosphere did not significantly affect their structure as h.c.p. Ti remained the only phase detected. On the contrary, annealing of the films deposited with higher Al contents (Al/(Al+Ti) = ± 24 and 27 at. %) induced the formation of new phases, including f.c.c Al and Ti₃Al, showing that the thermal stability of the films was degraded by Al incorporation.