Deposition of TiN, TiC and Ti1−xAlxN coatings by pulsed d.c. plasma enhanced chemical vapour deposition methods

Unipolar and bipolar pulsed d.c. plasma enhanced chemical vapour deposition techniques have been used under various process conditions to deposit hard crystalline (Ti,Al)N, TiC and TiN coatings on steel and WC/Co hard metal substrates at temperatures between 500 and 700°C. As process gases mixtures of titanium tetrachloride (TiCl4), aluminium trichloride (AlCl3) and nitrogen or methane/hydrogen/argon were applied. The golden, violet grey or dark grey coloured TiN, (TiAl)N, TiC coatings deposited by varying the substrate temperature, plasma power density, excitation mode, and titanium to aluminium ratio in the gas phase were investigated with respect to their composition and structure. At substrate temperatures up to 700°C and gas mixture ratios of Ti/Al=0.3–0.5 using the unipolar pulsed d.c. method cubic (Ti,Al)N coatings with different aluminium and titanium content could be deposited. At constant gas phase ratios and plasma parameters an increased substrate temperature resulted in an increased titanium content. For the bipolar d.c. excitation mode higher aluminium trichloride concentration had to be used in order to obtain the same composition as for the unipolar case. Besides the cubic (Ti,Al)N phase, hexagonal AlN was found in samples prepared at 700°C and 4 mbar by XRD measurements. The crystallite size of the (Ti,Al)N coatings deposited by unipolar and bipolar activation were estimated to be approximately 10 nm. The hardness HV[0.02] was found to be in the range of 25–30 GPa for Ti1−xAlxN, up to 32 GPa for TiN, and up to 40 GPa for TiC. Coating thickness and element composition were determined by glow discharge optical emission spectroscopy depth profile analysis.