Influence of film thickness on the structural transition cubic/hexagonal within Ti0.38Al0.62N films

Metastable Ti1-xAlxN films have the more outstanding functional properties in the range of composition around x = 0.6 for their use as protective coatings. They exhibit complex texture because of the phase transition between TiN-like rocksalt and AlN-like wurtzite structures. The competitive growth between cubic and wurtzite domains is studied in metastable Ti0.38Al0.62N thin film deposited by reactive magnetron sputtering for thicknesses between 1 and 3.7 mu m. Long-range order is analysed within the film by X-ray diffraction and cross-sectional TEM imaging. The study is complemented by using Ti K-edge diffraction anomalous spectroscopy to obtain differentiated information on short-range order around Ti atoms incorporated in cubic and wurtzite domains. The growth along the [111]c direction has the effect of promoting the appearance of crystallites with wurtzite symmetry oriented along [001]h. In the middle of the layer, cubic and wurtzite domains become coherently oriented. Lattice distortions, which affect long- and short-range order, are revealed by the anisotropy of broadening of the diffraction lines and by spectroscopic data collected on the main diffraction lines. The fine preedge structure indicates a strong distortion of the tetrahedral sites occupied by Ti atoms along the c axis, which minimizes the in-plane lattice mismatch. Near the film surface, a dramatic change in growth directions is observed. A better understanding of the structural transition mechanism in the (Ti,Al)N system could help improve the mechanical performances of these coatings in the future.