101.
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
4C
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.
相似文献