Microstructure and mechanical properties of Ti-6Al-4V alloy diffused with molybdenum and nickel by double glow plasma surface alloying technique

Ti-6Al-4V alloy substrates are diffused with molybdenum (Mo) and nickel (Ni), respectively, using the double glow (DG) plasma surface alloying technique. The crystal structures and surface morphologies of the diffused substrates are investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, the elemental compositions of the Mo- and Ni-diffusion modified surface layers are examined using an Auger electron spectrometer (AES) and an electron probe X-ray microanalyzer (EPMA). Finally, the hardness and wear resistance properties of the two surface layers are evaluated by means of Knoop hardness tests and pin-on-disk sliding tests, respectively. It is shown that the Mo-diffusion modified layer has a mixed Mo-Ti structure. By contrast, the Ni-diffusion modified layer consists of NiTi phase and Ni3Ti and NiTi2 intermetallic phases. Both surface alloying treatments increase the hardness of the Ti-6Al-4V substrate and improve the wear resistance as a result. Notably, the Ni-diffusion modified substrate has a higher friction coefficient than the Mo-diffusion modified substrate, but exhibits a lower wear rate. The improved wear resistance can be attributed to the presence of the NiTi, Ni3Ti and NiTi2 phases, which increase the yield strength of the contact surface and therefore reduce the wear mass.