Tantalum-modified Stellite 6 thick coatings: microstructure and mechanical performance

Thick Co-based coatings with different contents of tantalum were prepared by simultaneous powder feeding laser cladding technique on 304 stainless steel substrate, with the Ta wt% being 0, 2, 7 and 12. Laser processing was carried out with a continuous 3.3 kW Yt:YAG fiber laser. Microstructural observations were executed using scanning electron microscopy, energy dispersive X-ray spectroscopy analysis, and transmission electron microscopy. Observations indicated that, with an increase in the Ta contents, the Ta-rich MC-type carbides were formed in interdendritic regions. Also, hexagonal M₇C₃-type carbides were formed instead of orthorhombic M₇C₃-type carbides. The orientation relationships between different phases and the matrix were determined by electron diffraction. Mechanical properties were determined using microhardness measurement at room temperature and wear resistance measurement at room and elevated (500 °C) temperatures. The research demonstrated that alloying any amount of tantalum, in spite of increasing the microhardness, could be detrimental for increasing the wear resistance of Stellite 6, both at room and elevated temperatures. The relationship between microstructure and mechanical properties is explained.