Elevated temperature oxidation of laser surface engineered composite boride coating on steel

The effects of long duration exposure of laser surface engineered composite boride coating on plain carbon steel in air at high temperatures were investigated in this study. Exposures at 600 degrees C, 800 degrees C, and 1000 degrees C for 10, 30, and 50 hours of composite-TiB2 coated samples were conducted to study oxide scale growth and morphology. Kinetics of oxidation of the coating during elevated temperature exposures were separately studied using the thermogravimetric analysis (TGA) technique. The oxidation rate for all samples was parabolic in nature and the oxidation kinetic rate constant, K, increased with increasing temperature of exposure. Activation energy, Q for composite TiB2 coating was found to be 205 kJ/mol. A thick (>35 mu m) oxide layer formed for all duration of exposure at temperatures greater than or equal to 800 degrees C. In case of 1000 degrees C exposure, a very thick (>150 mu m) oxide layer was formed, which was separated from the substrate. X-ray diffractometry analysis revealed the complex nonstoichiometric nature of the oxides of type TiaOb, FemOn, and FexTiyOz. Profilometric measurements indicated an increase in the surface roughness of the oxide layer with an increase in temperature of exposure. These physical observations indicated that the nature and morphology of the oxides formed at various temperatures and duration of exposure are complex.