Wear Resistance of Niobium Carbide Layers Produced on Gray Cast Iron by Thermoreactive Treatments Fa´ bio Edson Mariani, Galtiere Correˆa Reˆ go, Pedro Gabriel Bonella, Amadeu Lombardi Neto, George Edward Totten, and Luiz Carlos Casteletti (Submitted October 28, 2019; in revised form January 21, 2020) 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, NH 4 Cl, Al 2 O 3 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. Keywords gray cast iron, micro-abrasive wear test, micro-adhesive wear test, niobium carbide layers, thermoreactive treatments 1. Introduction Gray cast irons are widely used in the manufacture of mechanical parts and components, due to its characteristics, such as lower melting temperatures compared to steels, high fluidity of the molten metal, low solidification contraction, good machinability and high vibration damping capacity. Parts such as engine blocks, clutches, disks brakes and agricultural machines are some of the main applications for this material (Ref 1-3). The use of wear and/or corrosion-resistant coatings can enhance the application potential of gray cast irons such as the thermoreactive deposition/diffusion (TRD) process. What makes this process attractive is its low cost since it is performed at atmospheric pressure and does not require expensive equipment such as those required in PVD and CVD coating techniques (Ref 4). The TRD process consists of diffusing the carbon or nitrogen present in the substrate to the surface and combining it with carbide-forming elements (CFE) or nitride- forming (NFE) such as niobium, chromium, vanadium, molyb- denum and tungsten, resulting in the formation of high- hardness layers at the surface of the material (Ref 5-8). In TRD treatment using niobium, layers of niobium carbide (NbC) with thicknesses between 1 and 15 lm are produced depending on the alloying elements present in the substrate and time and temperature of the treatment. Usually, treatment times ranging from 1 to 8 h at 800 to 1250 °C are used (Ref 7-13). These layers exhibit high hardness and wear resistance, good toughness, high modulus of elasticity, excellent chemical stability, high melting temperature (about 3873 °C) and good adhesion to the substrate (Ref 6-17). There is an extensive body of work regarding the deposition of niobium carbide layers via liquid medium, where the substrate material is immersed in a molten salt bath composed of borax and a Fe-Nb alloy (Ref 9-14). Solid diffusion treatment has already been successfully applied for the same goal, thus enabling formation of layers in plain carbon steels (Ref 15, 16) and tool steels (Ref 17). However, it is not found in the literature a comparison of both techniques applied to the same substrate. Therefore, it is important to evaluate the characteristics of the formed layers in each case and their respective mechanical and wear performances. In this work, diffusion thermoreactive treatments utilizing solid powder and a molten salt bath were used for the production of niobium carbide layers on a gray cast iron substrate with the objective of improving its tribological properties. This article is an invited submission to JMEP selected from presentations at the 30th Heat Treating Society Conference and Exposition held October 15-17, 2019, in Detroit, Michigan, and has been expanded from the original presentation. Fa´bio Edson Mariani, Production Engineering Department, Sa˜o Carlos School of Engineering, University of Sa˜o Paulo, Sao Carlos, Sao Paulo, Brazil; Galtiere Correˆa Reˆgo, Pedro Gabriel Bonella, and Luiz Carlos Casteletti, Materials Engineering Department, Sa˜o Carlos School of Engineering, University of Sa˜o Paulo, Sao Carlos, Sao Paulo, Brazil; Amadeu Lombardi Neto, Materials Engineering Department, Federal University of Technology – Parana´, Londrina, Parana, Brazil; and George Edward Totten, Materials Engineering Department, University of Portland, Portland, OR. Contact e-mails: mariani.fabioe@gmail.com and mariani.fabio@usp.br. JMEPEG ÓASM International https://doi.org/10.1007/s11665-020-04645-9 1059-9495/$19.00 Journal of Materials Engineering and Performance