THE OXIDATION MECHANISM OF CHROMIUM COATING BY A CONTINUOUS CO2-LASERAT CONSTANT-TEMPERATURE

In order to improve the wear resistance of chromium coating, Cr2O3 sca le can be formed by laser oxidation. The objective of this work consis ted of determining the differences in the oxidation mechanism between a classical oxidation and a laser oxidation. The chromium coatings wer e first deposited on a Z32CDV13 steel by an electrochemical technique, but due to their bad protective character, the chromium deposition wa s then carried out by physical vapour deposition. Oxidation treatments were performed for short times between 600 and 800-degrees-C. It was verified that, in all cases, the oxidized layer is thin and consists o f Cr2O3. In the case of classical oxidation, a parabolic law is observ ed, indicating that the oxide growth is controlled by a diffusional pr ocess. The combination of many characterization techniques showed the complexity of the diffusional phenomena; oxygen diffusion in the oxide layer and in the underlying chromium coating, with lattice and interg ranular diffusion combined with interdiffusion between the steel subst rate and the chromium coating. Nevertheless, it was shown that the chr omia scale grows predominantly by anionic diffusion. It appears that l aser oxidation induces the formation of a slightly thicker oxide scale and to a carbon concentration in the scale more important than in cas e of classical oxidation.