Fluidized Bed CrN Coating Formation on Prenitrocarburized Plain Carbon Steel Peter C. King, Ray W. Reynoldson, Allan Brownrigg, and John M. Long (Submitted February 10, 2004) CrN coatings were formed on plain carbon steel by prenitrocarburizing, followed by thermoreactive deposition and diffusion (TRD) in a fluidized bed furnace at 570 °C. During TRD, Cr was transferred from Cr powder in the fluidized bed to the nitrocarburized substrates by gas-phase reactions initiated by reaction of HCl gas with the Cr. The microstructural processes occurring in the white layer, caused by N diffusion toward the surface during this stage were studied. This study compares TRD atmospheres employing inert gas and HCl or inert gas, H 2 , and HCl. Surface characterization was performed by scanning electron microscopy (SEM), x-ray diffraction (XRD), and glow-discharge optical-emission spec- troscopy (GDOES). Keywords fluidized bed, glow-discharge optical-emission spec- troscopy, nitrocarburizing, plain carbon steel, thermo- reactive deposition and diffusion 1. Introduction Thermoreactive deposition and diffusion (TRD) is a hard- coating process whereby a metallic element [i.e., a transition metal carbide- or nitride-former-chromium (Cr), vanadium (V), titanium (Ti), or aluminum (Al)] is thermochemically depos- ited on the surface of a substrate, and that metallic element reacts with an interstitial element [i.e., carbon (C), nitrogen (N), or boron (B)] that diffuses to the surface from the sub- strate. The driving force for diffusion of the interstitial element is the thermodynamic stability of the surface compound(s) formed. The Toyota Diffusion (TD) Process, developed by the Toyota Central Research Institute, is one example of TRD. [1] In the TD process, a V carbide layer is formed at 800-1050 °C by outward diffusion of C from a steel substrate. The thickness of the layer formed under identical processing conditions is lim- ited by the C content of the substrate being treated. [2] It has been shown that V carbide/carbonitride/nitride coat- ings grow at a faster rate by pretreating with carburizing, ni- trocarburizing, or nitriding. [3] Pretreatment raises the concen- tration of interstitials in the surface of the substrate prior to TD treatment. Chicco et al. [3] carburized H13 tool steel, followed by TD vanadizing, to yield a V carbide coating of thickness 4.6 m. Even thicker coatings were achieved with prenitrocarbur- izing and prenitriding, because V has a higher affinity for N than for C. However, superior hardnesses were achieved when C was present in the coating. Similar results can be expected with other transition metals, because they generally form harder carbides than nitrides. [4] There is some limited experimental evidence that TRD coat- ing may be performed at lower temperatures due to the effect of interstitial surface enrichment. Vanadium carbonitride and Cr carbonitride coatings have been formed on a variety of substrate materials by nitriding, followed by TD salt-bath treat- ment at 530-700 °C. [5,6] A number of patents have been filed for low-temperature TRD processes performed in a fluidized bed. [7-9] The equip- ment described was a fluidized bed of inert refractory powder, mixed with 10-20 wt.% of powdered Ti, V, or Cr. Ferrous substrates were suspended in the bed during treatment. The powder was activated by sublimated ammonium-halide pellets. Reaction at the surface of the metallic powder formed volatile metal halides, which subsequently transferred the coating metal to the substrate surface. In this way, the authors of Ref 9 claim to have achieved a 1 m layer of CrN after 6 h of treatment at 570 °C and an 8 m layer after 50 h. Thin Cr(N,C) coatings have also been applied to H13 tool steel by nitrocarburizing, followed by TRD in a fluidized bed containing Cr metal powder. Both steps were performed at 570 °C, just below the normal tempering temperature of this steel. The Cr powder was activated by gaseous HCl in this investigation. [10] No matter what coating method was used, the nature of the surface formed during pretreatment must have an important influence on the final coating properties. It is also vital to understand the processes whereby interstitial atoms diffuse to the surface during TRD. For example, Chicco et al. [3] reported a modification of the diffusion-zone nitride distribution after TRD treatment at 1000 °C. This finding indicates that diffusion of interstitials had occurred over large distances (several hun- dred microns), whereas research at lower temperatures [5,6,10] shows diffusion of a more local nature. The structural aspects of diffusion have been unexplored in the limited amount of work published so far. Nitrided and nitrocarburized compound layers on plain carbon steel consist of one or both of the Fe carbonitrides: -Fe 2-3 (N,C) and '-Fe 4 (N,C). Occasionally cementite Fe 3 C is This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15-17, 2003, in Indianapolis, Indiana, and appears on pp. 467-75 of the Proceedings. Peter C. King, Allan Brownrigg, and John M. Long, School of Engineering and Technology, Deakin University, 221 Burwood High- way, Burwood, Vic. 3125, Australia. Ray W. Reynoldson, Reynold- son Technologies, 160 Wickhams Road, Launching Place Vic 3139, Australia. Contact e-mail: pckin@yahoo.com. JMEPEG (2004) 13:431-438 ©ASM International DOI: 10.1361/10599490419982 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 13(4) August 2004—431