Surface molybdenizing on titanium by halide-activated pack cementation

Abstract

To improve the surface hardness of titanium, molybdenized layer was fabricated on titanium surface by halide-activated pack cementation process. Coupons were analyzed using optical microscopy (OM), scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). Microhardness values were obtained by Vickers hardness test. It was found that molybdenized layer consists of diffusion layer and deposition layer formed above 883 °C. Attributed to the difference of Mo content, the phase transformation of Mo → β → α″ → α′ occurred from outside to inside in the diffusion layer, which led to the gradual decrease of microhardness values from the deposition layer to the substrate for the different hardness levels of β, α″ and α′ phases. Molybdenized layer could obviously improve the surface microhardness of the titanium substrate. The highest microhardness value of deposition layer and diffusion layer is about 1400 HV and 1200 HV respectively, which is approximately four times higher than that of the titanium substrate. Based on Fick's second law, the relation between the thickness of diffusion layer, process temperature and time is discussed.