Plasma-enhanced CVD of (Ti,Al)N films from chloridic precursors in a DC glow discharge

Metastable Ti1−xAlxN films have been deposited from gaseous mixtures of TiCl4–AlCl3–N2–H2–Ar in a pulsed DC glow discharge at 510°C. When the discharge voltage was kept constant, the Al content x of the films increased linearly with the AlCl3/TiCl4 ratio in the feed gas. Increasing the discharge voltage also increased the Al content. Up to compositions of Ti0.09Al0.91N the layers remained single-phase cubic with a strong {100} texture. Films with a higher Al content consisted of two phases and their cubic phase showed a weak {111} texture. The lattice parameter of the homogeneous cubic films decreased with increasing Al content in accordance with Vegardʹs law. Films with a low Al content exhibited a columnar morphology, while the films with high Al contents had a fine-grained structure. Increasing the discharge voltage also caused the grain size to decrease. The microhardness of the single-phase coatings increased with increasing Al content up to 3947 HV 0.05 for x=0.83, while the two-phase layers showed hardness values of approximately 5000 HV 0.05. The metastable films began to decompose at temperatures between 750 and 800°C, depending on the Al content. The decomposition of the films with an Al/Ti ratio below 1 caused the lattice parameter of the cubic phase to increase and the microhardness to decrease. Films with high Al contents did not show any increase in the lattice parameter after annealing and their microhardness strongly increased. Investigation of the oxide layer formed on a Ti0.21Al0.79N film after annealing in air at 800°C showed that an amorphous alumina layer with a thickness of approximately 100 nm was formed on the surface, preventing further oxidation. The films with high Al content exhibited advantageous tribological properties with friction coefficients of 0.5. Thus, they seem to be especially well suited for an application on cutting and metal working tools.