Structure, hardness and thermal stability of Ti(Al,N) coatings

In the past two decades, coatings of the Ti-Al-N ternary system have attracted considerable research and industrial interest. Nevertheless, the Ti-Al-N system still offers new interesting possibilities for coating developments such as the addition of low N contents to Ti-Al films in order to reach a good compromise between high hardness and a low friction coefficient.Ti-Al-N coatings with low nitrogen content were deposited by closed field unbalanced magnetron sputtering using two facing Ti targets inserted with Al rods. The Al/(Al + Ti) and N/(Al + Ti + N) atomic ratios were varied from 21 to 28 at.% and 0 to 33 at.%, respectively.Hexagonal close-packed (hcp) α-Ti with a preferential <001> orientation was the only phase detected by X-ray diffraction in the as-deposited films. A decrease in the α-Ti c lattice parameter was observed as aluminium was added to the films. Nitrogen addition increased the c lattice parameter and led to a progressive loss of crystallinity until quasi-amorphous films were obtained. A hardness of ≈ 13 GPa was obtained for the as-deposited films without nitrogen. A continuous increase in hardness was observed with increasing nitrogen content. The highest hardness values (up to 27 GPa) were obtained for the quasi-amorphous films. Annealing of the films with low aluminium content (Al/Al + Ti ≈ 21 at.%) did not significantly affect their structure as hcp Ti remains the only phase detected. On the contrary, annealing of the films deposited with higher aluminium contents (Al/(Al + Ti) ≈ 24 and ≈ 28 at.%) resulted in the formation of face centered cubic (fcc) Al or Ti₃Al, showing that the thermal stability of the films decreased with aluminium incorporation.