RESISTANCE TO WEAR OF NITRIDING ON TITANIUM-ALLOY USING THE LASER-IRRADIATION TECHNIQUE

The introduction of nitrogen to titanium alloy (Ti6Al4V) by laser irradiation under atmospheric pressure is characterized by : (i) the presence of a thin layer of TiN; (ii) the possible presence of an equally thin layer of Ti2N; and (iii) enrichment of the nitrogen-rich alpha phase film to depths of tenths of mm. The use of a laser leads preferentially to the substrate surface melting due to energy absorption, which is necessary for gas diffusion in the base metal. When nitrogen is present, nitrides are produced and are distributed throughout the melted area. The layer thus obtained, which is in fact a mixture of TiN, Ti2N and Ti dendrites, is generally thicker that obtained by conventional methods (ionic nitriding). The superficial hardness is around 900 HV100g but is considerably lower where the melted and non-melted areas meet. Turbine blades in nitrided titanium alloy were tested for resistance to wear in a series of trials aiming to determine an efficient method for protection against the effects of erosion by droplets of water. Extremely rigourous comparative tests with other techniques (laser-deposited metallic powders, sintered and soldered plates) showed that nitriding by laser irradiation displayed good resistance to wear in the incubation/initiation phase. In the weight-loss over time phase, it seems clearly that the melted and irradiated area should be around 1 mm thick.