Wear resistance of niobium carbide layers produced on gray cast iron by thermo-reactive treatments

Gray cast iron has frequent industrial use due to its high damping capacity, machinability and low cost. These outstanding properties are due to the presence of free graphite and the high fluidity of the molten metal, which facilitates the casting of complex parts with thin walls. Suitable coatings can increase their wear resistance and expand their usability range. The high hardness of niobium carbide suggests that it may be a good candidate for this purpose. In this work, samples of gray cast iron with composition 3.47%C-2.39%Si-0.55%Mn-0.15%Ni-0.65%Cu - balance Fe were subjected to two thermo-reactive niobizing treatments. The first niobizing process (a pack treatment) utilized a powder mixture composed of iron-niobium, NH4Cl, Al2O3 at 900 °C during 2 hours. The second treatment, a thermo-reactive deposition/diffusion process (TRD), utilized a liquid molten bath of sodium borate and iron-niobium performed at 900 °C for 2 hours. Vickers hardness and micro-adhesive (fixed-ball) and micro-abrasive (free-ball) wear tests were used to characterize the treated samples. Hardness layers of 2000 HV, typical for niobium carbides, were obtained. Wear tests demonstrated a substantial increase (2 to 12 times higher than that the substrate) in wear resistance obtained with niobizing treatments.