The adhesion of plasma sprayed polyamide and PMMA coatings to steel depends markedly on the plasma arc power, the spraying distance and the substrate temperature. Each of these process parameters shows an optimum value with respect to adhesion. The underlying reason for this behaviour is the pronounced sensitivity of polymers to temperature. Heat transfer analysis and electron microscopy indicate that a critical amount of heat is required to be transferred from the flame to the feedstock particles in order to provide sufficient splat flow but avoid coating thermal degradation. Inadequate flow leads to interfacial voidage while degradation gives inferior bonding and porosity.

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