Effect of laser power on the microstructure and mechanical properties of TiN/Ti3Al composite coatings on Ti6Al4V

Laser nitriding is one of the effective techniques to improve the surface properties of titanium alloys and has potential application in the life extension of last-stage steam turbine blades. However, cracking of surface coating is a common problem due to heat concentration in laser nitriding process. Conventionally, the cracks can be avoided through heat treatment, which may have an important influence on the mechanical properties of coating. Crack-free TiN/Ti3Al IMC coatings on Ti6Al4V are prepared by plasma spraying and laser nitriding. The microstructures, phase constitutes and compositions of the coating are observed and analyzed with scanning electron microscopy(SEM), X-ray diffraction(XRD) and X-ray energy-dispersive spectroscopy(EDS). Microhardness, elastic modulus, fracture toughness of the coating are measured. The results show that the crack- and pore-free IMC coatings can be made through the proposed method; with increasing laser power, the amount and density of TiN phase in the coating first increased and then decreased, leading to the similar trend of microhardness and elastic modulus and the reverse trend of fracture toughness of the coating. Both the average microhardness and elastic modulus of the coating increase three times higher than those of the substrate. The volume fraction of the TiN reinforced phase in composite can be controlled by varying the laser power and the cracking problem in laser nitriding process is successfully solved.