Kinetic Study of Niobium Carbide Coating Formation on AISI L2 Steel using Thermo-Reactive Deposition Technique

In the present study, Thermo-Reactive Deposition method (TRD) in a molten bath was used to form niobium carbide coating on AISI L2 steel. The coating was formed in five different composition of borax (Na2B4O7), boric acid (B2O3), and ferro-niobium. For all of the five compositions of molten bath, niobium carbide coating treatment was carried out at 1173 K, 1273 K, and 1373 K for 2, 4 and 8 hours. The thickness of coating ranged between 3.6 ± 0.1 µm to 33.6 ± 0.5 µm depending on the bath composition, treatment time, and temperature. Kinetic study of the formation of NbC coating demonstrated that for the molten baths containing 10 wt.% ferro-niobium or more, growth of the coating is under the controled of by diffusion. For such molten baths, the activation energy of the process was estimated to be 122.5 kJ/mol. A practical formula to estimate the coating thickness in these molten baths has been suggested.