Functional phase grading of ζ‐Ta 4 C 3‐x : Kinetics and properties

Phase‐graded carbides that exhibit high hardness and corrosion resistance on the surface and high fracture toughness in the bulk are useful in severe wear applications. A method for producing hard γ‐TaC y layer on the surface of a tough ζ‐Ta 4 C 3‐x substrate by carburization is described in this study. The γ‐TaC y layer exhibited parabolic growth kinetics in the temperature and gas‐flow regimes investigated in this study. Analyses of the rate constants using a model of mass transport through two phases in equilibrium at the interface indicated diffusion of carbon through the product layer was the rate‐controlling step with diffusivities (1‐5 × 10 −15 m 2 /s in the 1623‐1773K range) and enthalpy (213‐217 kJ /mol) matching those for carbon diffusion in γ‐TaC y . Nanohardness of the transformed γ‐TaC y layer increased as a function of distance from the carburized surface, with a maximum hardness occurring near the γ/ζ interface. Measurements of lattice parameters at different depths in the γ‐TaC y layer indicated that the hardness increase with depth was related to the increasing carbon deficiency of the γ‐TaC y phase through the thickness.