Wear Resistance of Niobium Carbide Layers Produced on Gray Cast Iron by Thermoreactive Treatments

Abstract

Gray cast iron is commonly used due to its high damping capacity, machinability and low cost, 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 gray cast iron wear resistance and expand its usage range. The high hardness of niobium carbide indicates 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 thermoreactive niobizing treatments. The first process (a pack treatment) utilized a powder mixture composed of iron niobium, NH4Cl, Al2O3 at 900 °C during 2 h. The second treatment, a thermoreactive deposition process (TRD), utilized a liquid molten bath of sodium borate and iron niobium performed at 900 °C for 2 h. X-ray diffraction (XRD), Vickers hardness, 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.