Fabrication of niobium carbide coating on niobium by interstitial carburization

Ceramic coatings on niobium substrates are fabricated by hot-pressing the niobium with cast iron at 1100 °C. The interstitial carbon atoms diffuse into the niobium, and NbC and Nb2C precipitate in the surface, forming a ceramic coating. The total volume fraction of the ceramic phases in the coating reaches 92%, and the grain size is in the range of 170–1000 nm. Both the volume fraction and the grain size of the ceramic phases decrease with increasing depth with respect to the coating surface, displaying a gradient microstructure. In the coating, the crystallographic orientations of NbC and Nb2C are randomly distributed. In the substrate, some isolated NbC and Nb2C nuclei exhibit the Kurdjumov–Sachs orientation relationship and the Porter orientation relationship with the matrix respectively, namely 11¯1101¯NbC//11¯0111¯Nb and 101¯00001Nb2C//11011¯5Nb. The coating remarkably improves the surface hardness of the niobium from 110 HV to 1600 HV, and it is strongly bonded with the substrate.