Microstructure and properties of Cr[sbnd]Nb carbide coatings on graphite via powder immersion reaction assisted coating

Abstract

Chromium‑niobium (CrNb) carbide coatings were formed on graphite by powder immersion reaction assisted coating (PIRAC) method. In our experiments, a small amount of iodine was added to the mixture of Cr and Nb powder to lower the annealing temperature to as low as 1173 K. The resultant composite coatings comprised Cr7C3 and NbC phases, of which the Cr7C3 phase was found in the graphite's internal gaps and NbC was distributed on the surface of the coatings. The formation of the multilayer structure might be explained by thermodynamics of metal iodides and carbides. The optimal composite coating's hardness was close to that of CrC compounds coatings, and its toughness was close to that of NbC coatings. Thermal expansion coefficient of the coated sample was similar to the graphite matrix within the temperature range of 313-673 K. No cracks were observed even though stress mismatch was expected due to the large temperature change and large difference in mechanical properties between carbides and graphite. These results showed that the multilayer structure of CrNb composite coating synthesized by PIRAC was promising for improving the mechanical properties of the graphite.