Tribological properties of duplex coatings of chromium-vanadium carbide produced by thermo-reactive diffusion (TRD)

Abstract

Carbide coatings are very important in molding industries due to their good anti-wear properties increasing the life of molds of hot and cold forging, extrusion and powder metallurgy exposing to abrasive forces. Diverse processes are applied to produce carbide coatings. One of them is the thermo-reactive diffusion method (TRD) using a molten salt bath. This process has many advantages including cost-effectiveness over other similar surface coating methods. The aim of this study is the formation of carbide-composite coatings using molten salt baths containing oxides of carbide forming elements individually and in a mixed form on SKD-11 cold work tool steel at 1000 °C. For this purpose, two sets of experiments were considered in this study. In experiment A, a chromium oxide bath and then a mixed chromium oxide and vanadium oxide bath with a molar ratio of Cr to V equal to 0.66 were used. In experiment B, at first a mixed chromium oxide and vanadium oxide bath (Cr/V ratio = 0.66) and next a single bath of vanadium oxide were utilized. To evaluate and compare the produced coatings, FE-SEM, EPMA (point, line and map), XRD analysis, nano-indentation and wear tests were performed. The results showed that the coatings include chromium carbides (Cr3C2, Cr7C3 and Cr23C6) and vanadium carbides (V2C, V6C5 and V8C7) as well as a carbon-chromium-vanadium triple phase with the composition of Cr2C2V. Moreover, the best hardness and abrasion resistance were gained for the coating produced in the molten bath containing chromium oxide and vanadium oxide after experiencing a chromium oxide bath. For the sample, the hardness was between 17.2 and 19.6 GPa and the lowest amount of COF was 0.32.