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

Chromium‑niobium (Cr Nb) 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 Cr 7 C 3 and NbC phases, of which the Cr 7 C 3 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 Cr C 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 Cr Nb composite coating synthesized by PIRAC was promising for improving the mechanical properties of the graphite. • Graphite has been widely used for making various devices. Its soft and fragile nature is, however, a shortcoming that shortens service life of graphite-based components. • We test the viability of an inexpensive approach, namely powder immersion reaction assisted coating (PIRAC) method. • We prepared composite Cr-Nb carbide coatings on graphite using the PIRAC method with the aid of iodine, and investigated the effect of processing conditions on the material microstructure and mechanical properties. • We observed a layering structure in which the Cr C compound in the lower layer and NbC in the upper layer. • The optimal composite coating has high hardness close to that of chromium carbides, and high elastic modulus and toughness close to that of niobium carbide. [ABSTRACT FROM AUTHOR]