Behaviour of TiAl-N coatings for tools applied in the thermoplastic moulding of inorganic glasses

Metal nitride coatings are of particular interest for the improvement of the service behaviour of tools for the thermoplastic moulding of optical components to be formed from inorganic glasses. Major problems in this application concern the wear of the coatings due to interactions with the hot glass and particularly the damage by stresses induced in the coatings by temperature varying from room temperature to the service temperature ranging up to 800 K and even higher. In the present study coatings of the Ti-Al-N system were deposited on steel, fused silica and silicon substrates in reactive sputtering processes by variation of the process parameters such as total pressure, substrate temperature, discharge power and distance between target and substrate. The film composition was systematically varied by applying different argon and nitrogen partial pressures during the deposition. The paper describes the effects of the deposition parameters and film composition on the residual stresses and the service behaviour of coatings. The coatings were investigated by optical and mechanical methods. By appropriate selection of the deposition parameters the film properties and the service behaviour of forming tools could be improved. Results on the service behaviour are discussed on the basis of glass-forming experiments at high temperatures. According to the achieved results the nitrogen content y in Ti-Al-N coatings was increased from y = 0.75 to y = 2.2 and the compressive film stress from 400 to 1500 MPa by increasing the number of collisions of sputtered TiAl species with nitrogen n(N2) between 0.2 and 1.2 in d.c. triode sputtering at 0.05 Pa total gas pressure. In r.f. magnetron sputtering at 1.3 Pa lower stresses between + 200 and - 200 MPa but nitrogen contents in the same ranges as in the d.c process were obtained if the collision number n(N2) was maintained in the same range. Coatings with moderate compressive stresses and high nitrogen content showed the best behavio ur in glass-forming experiments at high temperatures.