Properties and Tribological Performance of Vanadium Carbide Coatings on AISI 52100 Steel Deposited by Thermoreactive Diffusion B.L. STRAHIN, 1,3 D.D. SHREERAM, 2 and G.L. DOLL 1,2 1.—Department of Mechanical Engineering, The University of Akron, Akron, OH 44325, USA. 2.—Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA. 3.—e-mail: bls103@zips.uakron.edu Vanadium carbide coatings were formed on AISI 52100 steel specimens by thermoreactive diffusion and characterized using nanoindentation, x-ray diffraction, and chemical analysis. The deposition process formed a 4-lm coating of vanadium carbide (V 4 C 3 ) with an average grain size of 33 nm and a [200] crystallographic texture. The hardness and elastic modulus of the coatings were determined to be 35 ± 7.5 GPa and 334 ± 67 GPa, respectively. Friction and wear of the coatings were examined in reciprocating sliding contact against tungsten carbide (WC) balls in dry and in an abrasive envi- ronment. It was determined that in the abrasive environment, the V 4 C 3 coating provided wear protection comparable to WC. INTRODUCTION Hard coatings have been used to provide wear resistance to mechanical components for many years. Three common treatment processes are chemical vapor deposition (CVD), physical vapor deposition (PVD), and thermochemical conversion treatments. Thermoreactive diffusion (TRD) is a thermochemical treatment that can produce dense ceramic coatings on the surface of a substrate. 1 TRD consists of three primary phases. First, metal diffuses into the substrate as carbon diffuses out. This diffusion of metal into the substrate provides the foundation for the excellent adhesion of TRD coatings to the substrate. Next, metal and carbon begin to react at the surface and nucleation of metal carbide crystals initiates. Finally, the coating thick- ens following a parabolic growth law for the remainder of the treatment time. 2–4 Fan et al. 4 and Kong and Zhou 3 provide excellent descriptions of the stages of TRD coating growth. TRD is typically performed between 850°C and 1100°C, although temperatures as low as 570°C have been reported with pre-treatment. 1,5,6 Without pre-treat- ment, carbon diffusion is insufficient to form an adequate coating below 770°C. 3 Because of the higher temperatures, distortion and dimensional changes, as well as microstructural changes of the substrate can be issues during treatment. Despite these limitations, TRD has many advantages over CVD and PVD processes. TRD produces coatings that are thicker with comparable hardness as their PVD and CVD counterparts. 7,8 For example, the typical hardness of a TRD vanadium carbide layer is 24–35 GPa. 3,9–16 TRD can also be performed on less expensive equipment than vacuum-based processes, which makes it a more economical process in many instances. TRD can be carried out in a molten salt bath, fluidized bed, or in powder compaction. The most common method for applying TRD coatings is the use of a borax-based molten salt bath. Four factors affect the rate of coating growth: time, temperature, interstitial element potential (e.g., nitrogen, carbon, or boron), and the concentration of the carbide/ nitride forming element (e.g., vanadium, chromium, or titanium) in the treatment media. 1,7 Since com- position affects the thermodynamic activity of the material and the interstitial element potential, proper substrate selection is very important. 8 Although many studies have been reported on the diffusion kinetics for TRD coatings, fewer studies have evaluated the wear or corrosion resistance of coatings produced by TRD. Most of those studies were performed in dry contact or by using non- standardized methods. 7,9,15,17,18 The goal of this JOM DOI: 10.1007/s11837-017-2370-2 Ó 2017 The Minerals, Metals & Materials Society