Variation in the Oxidation Mechanism of Cr3C2-NiCr Coatings as a Function of Extended High Temperature Exposure

Cr₃C₂-NiCr thermal spray coatings are extensively applied to combat erosion at high temperatures where the oxidation resistance of the WC based systems is inadequate.  Under these conditions oxide layers take on an ever more significant role in determining the composite response.  The response of blended powder based carbide coatings for erosion applications has formed the basis for application of cermet based coatings at elevated temperature.  The assumed oxidation response of these coatings is largely based on the oxidation mechanism of the individual metallic and carbide components.  However, little work has been carried out to determine the interrelationship of these phases and the effect on the oxidation response.  Even less is known about what changes in oxidation mechanism occur as the coating phases interact with long-term high temperature exposure.  In this work the oxidation mechanism of as-sprayed Cr₃C₂-NiCr blended powder based coatings is characterised.  The variation in oxidation as a function of microstructure development at high temperature is discussed.  Inter-diffusion between the coating phases with long term exposure increased the Cr content of the matrix phase.  This had a significant effect on the oxidation mechanism.  The implications of the change in oxidation mechanism and oxide morphology on the coating response to high temperature erosion are discussed.