Deposition of chromium oxide-chromium carbide coatings via high velocity suspension flame spraying (HVSFS)

A chromium carbide containing suspension was thermally sprayed through high velocity suspension flame spraying (HVSFS). Therefore, an aqueous suspension with 20 wt% of pure Cr3C2 and an organic stabilizing additive with a pH of 9.6 was prepared. Due to oxidation of the carbide particles in the flame the deposited lamellar coating is composed of both chromium oxide and carbide, namely Cr2O3 phase and Cr7C3 phase as identified via XRD measurements. The HVSFS coatings are analysed regarding their microstructure, chemical composition, microhardness and surface roughness. The gas parameters, specifically the oxygen-to-ethene ratio and the total gas amount, determine the flame temperature and gas velocity and therefore control the microstructure and chemical composition. Substoichiometric gas compositions lead to higher deposition efficiencies. They create a reducing atmosphere that hinders the oxidation, whereas oxygen excess favours the oxidation of the chromium carbide particles. The higher the chromium oxide content, the higher is the coating's micro hardness. When the surface temperature during the deposition process gets high enough, post-oxidation of the as-sprayed surface takes place and finally results in increased surface roughness and a layered microstructure with alternating chromium carbide and chromium oxide layers.