Long-term compositional/microstructural development of Cr3C2-NiCr coatings at 500 °C, 700 °C and 900 °C
Cr3C2-NiCr composite thermal spray coatings are widely used to mitigate erosive, abrasive and sliding wear at temperatures between 500-900°C. During deposition a wide variety of compositional changes occur in the coating material due to carbide dissolution, carbide decomposition, carbon loss, oxidation, and the formation of metastable phases upon rapid solidification. With exposure at elevated temperature the metastable as-sprayed structure begins to transform back to an equilibrium composition. The rate of this transformation and the steady state composition achieved is time and temperature dependent. The aim of this work was to extend the current understanding of these compositional and microstructural transformations by determining the effect of heat treatment temperature on the mechanisms and rate of coating development at 500-900°C for up to 30 days. The coating compositional development was characterised by X-ray diffraction. Image analysis was used to study the mechanisms of microstructure development and quantify the rate of carbide grain growth. Rapid transformation of the metastable as-sprayed coating phases to an equilibrium composition of Cr3C2 and a Ni alloy occurred within 1-5 days at 500°C and within 1 day at higher temperatures. The steady state composition matched that of the starting powder. The effect of the compositional and microstructural development on the coating microhardness is discussed.