Effects of heating temperature and duration on the microstructure and properties of the self-healing coatings

The present work investigates how the heating temperature and duration affect the properties of the self-healing coating on martensitic steels. The coating composed of TiC + mixture (TiC/Al2O3) + Al2O3 is fabricated by means of air plasma spraying. The thermal shock test is performed at 600 °C, 700 °C and 800 °C, respectively, to evaluate the thermal–mechanical stability of the coating. The cross-section morphology of the samples after 1 h, 9 h, 18 h and 30 h of heat treatment shows that the porosity of the coating decreases with the increase of heating duration. The evaluation of electrochemical performance by electrochemical impedance spectroscopy shows that the corrosion resistance of the coating after being heated for 18 h is much better than the other samples due to the process of the inner layer being compacted in the coating. The adhesive tensile strength test between coating and substrate shows that the adhesive strength of the coatings is higher than 9 MPa within 40 h of heat treatment at 600 °C. The residual stress reaches a minimum value after the coating was heated for 9 h at 600 °C, then increases with the heating duration after 9 h. Energy dispersive X-ray analysis at the Vickers indentation indicates that the oxygen content at the crack position increases significantly after being heated for 30 h at 600 °C. These experimental results suggest that this coating can meet the requirement of application under the actual temperature conditions.