Thermally induced metallurgical processes in Cr3C2-NiCr thermal spray coatings as a function of carbide dissolution

Cr3C2-NiCr thermal spray coatings are widely used to mitigate wear and corrosion at high temperatures. The aim of this work was to determine what minimum treatment temperature is required to transform the non-equilibrium as-sprayed coating composition back to an equilibrium composition, with the aim to improve the wear resistance. Cr3C2-NiCr coatings were sprayed using two HVOF techniques and a shrouded plasma spraying technique to produce samples with a broad spectrum of carbide dissolution and peritectic decomposition of Cr3C2. Shrouded plasma spraying was found to be highly effective in decreasing the carbon loss and the oxygen uptake. Differential scanning calorimetry (DSC) was used to characterize the exothermic solid-state phase transformations within the coatings. For the HVOF coatings, one main exothermic peak was observed, which was attributed to the crystallization of the metastable Ni binder material. In the plasma spray coating an additional higher temperature peak was also observed. This was attributed to the transformation of (Cr,Ni)7C3 and a high Cr content Ni phase, into the equilibrium phases Cr3C2 and a low Cr content Ni binder.