Micro-Focused X-Ray Diffraction Imaging Analysis on the Fe Nitrides and Carbides in Compound Layers of Nitrocarburized and Gas Nitrided Coatings on Steels

Abstract

Steel loading components frequently require hardened surfaces to enhance performance in diverse mechanical applications. Nitrocarburizing and gas nitriding are established thermal treatments that introduce nitrogen and carbon atoms into the lattice structure, forming complex surface coatings composed of tunable metal nitrides and carbides with superior mechanical and chemical properties. Optimizing these coatings demands advanced characterization techniques, traditionally relying on optical and electron microscopy for detailed structural analysis. Recent advancements in X-ray characterization, particularly at synchrotron radiation facilities, have enabled us to re-examine the compositional characteristics of hardened steel coatings produced through nitrocarburizing and gas nitriding. Three different steel grades were treated to produce different phase compositions and distribution. The samples were prepared into specimens via a focused ion beam for X-ray characterization. The greater penetration power of X-ray provides bulk sensitivity, and the detailed line profile analyses allows straightforward mapping of the elastic strain distribution. By utilizing a micro-focused X-ray beam in a fine-step mesh grid, we achieved spatially resolved phase and strain distribution data. Our findings indicate that ε phase typically exhibits isotropic deformation with a depth-dependent transition from expansion to compression, whereas ϒ’ phase undergoes increasing anisotropic deformation, expanding perpendicular to the surface and compressing parallel to it as depth increases.

Keywords: Synchrotron radiation, XRD, coatings, steel, thermal treatment