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.