Non-steady state heating of substrate and coating during thermal-spray deposition

The temperature of the substrate and coating during thermal spraying may have a significant effect on coating quality, especially for low-temperature materials such as polymers. The temperature must be high enough to thermally activate the necessary interfacial bonding mechanisms but low enough to avoid degradation of the coating material or of the substrate surface. A computer model is developed in this paper on the basis of fluid mechanics and heat transfer theory to predict the temperature distributions in the coating and substrate during scanning with a thermal-spray gun. The three-dimensional temperature profile of the gas jet was measured experimentally and the data used in the computational model. The model shows how the important practical parameters used in the operation of thermal spraying affect the temperature distributions of the coating and the substrate throughout deposition. For example, coatings on small substrates heat up considerably during repeated scanning, whereas there is little temperature build-up in large substrates. Increasing the scanning speed reduces the temperature fluctuations during a given cycle but does not change the overall temperature rise in the coating. The model can contribute to the understanding and control of thermal spraying by simulating the many possible combinations of operating parameters and predicting the shifting temperature distributions across and within the coating and substrate.