IMPROVEMENT OF THE OXIDATION BEHAVIOR OF TI1-XALXN HARD COATINGS BY OPTIMIZATION OF THE TI AL RATIO/

The oxidation behavior of cubic Ti1-xAlxN films was improved by decrea sing the Ti/Al ratio from 50/50 in the direction of the phase transiti on between cubic and hexagonal structure. Metastable, polycrystalline, single-phase Ti1-xAlxN films were deposited on high speed steel (HSS) substrates by reactive magnetron sputtering ion plating (MSIP). The c omposition of the bulk was determined by electron probe microanalysis (EPMA), the crystallographic structure by thin film X-ray diffraction (XRD). A Ti1-xAlxN film with a Ti/Al atomic ratio of 38/62 was deposit ed in cubic NaCl structure, whereas a further decrease of the Ti/Al ra tio down to 27/73 led to a two-phase film with both cubic and hexagona l constituents. The Ti0.38Al0.62N film was oxidized in synthetic air f or 1 h at 800 degrees C. The oxidic overlayer was analyzed by X-ray ph otoelectron spectroscopy (XPS) sputter depth profiling, EPMA crater ed ge linescan analysis, and secondary neutrals mass spectroscopy (SNMS). Scanning electron microscopy (SEM) micrographs of the cross sectional fracture were taken for morphological examination. With higher Ti con tent, the Ti1-xAlxN formed a TiO2-x rich sublayer beneath an Al2O3 ric h toplayer, whereas the oxide layer on the Ti0.38Al0.62N film consiste d of pure Al2O3. The thickness of the oxide layer was determined to 60 -80 nm, about a quarter of the oxide layer thickness detected on Ti0.5 Al0.5N films. The absence of a TiO2-x sublayer was also confirmed by X RD. The results show a distinct improvement of the oxidation resistanc e of cubic Ti1-xAlxN films by increasing the Al content from x = 0.5 t o 0.62, whereas a further increase leads to the hexagonal structure, w hich is less suitable for tribological applications due to its tendenc y to form cracks during oxidation.