Microstructures of Cr3C2-NiCr sprayed coatings modified by chromium diffusion treatment

In oder to improve density and bond strength of atmospheric plasma sprayed Cr3C2-NiCr coatings, chromium diffusion treatments have been carried out in the range of 1073 to 1373 K. In this work, the change of structure and har dness were examined by SEM, EDX, XRD and micro vickers hardness tester. The results obtained are as follows. (1) Thin Cr23C6 carbide layer which slightly contains Cr2C has been formed on the sprayed coating surface by the chromium diffusion treatment. This ca rbide layer is extremely high desity and high micro vickers hardness, HV150 0. (2) The thickness of this carbide layer became large (about 20 mu m at 1373 K for 57.6 ks), as the temperature and period for chromium diffusion treat ment incereased. It is found that at higher temperature than 1203 It this c arbide layer covered the sprayed coating surface with nearly unform thickne ss. The growth rate of this carbide layer goes with a parabolic rule and th e activity energy for the growth is 113.9 kJ/mol.