Temperature distribution on thermally sprayed heating conductor coatings

Cavities in injection moulding can be coated to increase the corrosion and wear resistance or to adapt the heat flux through the cavity wall. The latter is to be achieved by thermal spraying of TiO₂/Cr₂O₃ on the surface and its electrical heating. A thermally sprayed Al₂O₃ coating serves as an electrical insulator between the steel substrate and the TiO₂/Cr₂O₃ coating. In this study, the feasibility of a homogeneous surface heating is investigated. The heating behaviour was analysed using a thermographic camera. Depending on the process parameters during the coating process and used electrical current, inhomogeneity in the temperature distribution was detected. The observed inhomogeneity was distributed in linear patterns in form of "hot lines" perpendicular to the electrical current. To identify the root cause of the observed inhomogeneity, numerical modelling was utilized. The results of the simulations were validated with the experimental measurements. It was found that the cracks in the coating microstructure were the prime cause of the temperature increase that was distributed in linear patterns. Moreover, even though the crack distribution does not have a directional preference, the pronounced heating of the cracks aligned in the perpendicular direction to the electrical current created the temperature pattern.