Microstructural investigation of iron nitride layers formed by low-temperature gaseous nitriding

Abstract

Iron nitride layers were formed by a novel low-temperature gaseous nitriding process. Nitriding occurs at a temperature of 325 °C through NH3 decomposition at the surface of Ni (25 nm) coated Fe, followed by N transport through the Ni film into the underlying Fe, where nitride precipitation takes place. The role of Ni is to protect Fe from oxidation by gas impurities and to serve as a catalyst for NH3 decomposition. The precipitation behavior and the development of microstructure were studied by means of elastic recoil detection, cross-sectional transmission electron diffraction (XTEM), and positron annihilation (PA). From PA and XTEM no evidence was found for the occurrence of porosity during nitriding (an effect found at higher temperatures due to the decomposition of the nitrides into Fe and N2). XTEM showed that the original columnar α–Fe grains transform into smaller ′–Fe4N grains which subsequently transform into larger ε–Fe3−xN grains. This microstructural evolution of smaller ′ grains forming in the original columnar α–Fe structure occurs in one of two growth modes of the nitride in the Fe layer, i.e., throughout the entire depth range of the Fe layer, or preferentially at the Ni/Fe interface when an iron oxide layer is present at this interface.

Phase transformations in epitaxial Ni/Fe bilayers upon low-temperature gaseous nitriding

“…This would be in accordance with the findings by Inia et al for polycrystalline Fe samples covered with a Ni layer. 25 From comparison of the two XRD scans it is obvious that the ␥Ј-Fe 4 N layer formed from a single crystal of ␣-Fe is heavily textured with its ͑001͒ planes slightly tilted with respect to the sample surface. To study the OR in greater detail, two-axes XRD scans were measured.…”

Controlled dissolution of colossal quantities of nitrogen in stainless steel

“…A similar method has been applied for gaseous nitriding of pure iron and the formation of pore-free iron nitrides at low temperature. [14] In the present work, the objective is to prevent the development of nitrides and to obtain a controlled dissolution of nitrogen in solid solution.…”

Nitrogen uptake and rate-limiting step in low-temperature nitriding of iron

“…4 It was shown that two growth modes can occur in this low-temperature nitriding method, depending on the interfacial structure and the amount of impurities present at the Ni/Fe interface. 5 In one mode nitrides nucleate throughout the Fe layer and grow by uptake of more N to coalesce and form a continuous nitride layer. In another mode they preferentially nucleate at the Ni/Fe interface after which the layer grows towards larger depths.…”

“…In another mode they preferentially nucleate at the Ni/Fe interface after which the layer grows towards larger depths. 5 In this article, the different steps in the process ͑e.g., dissociation, diffusion, nitride for-mation͒ are discussed in detail. it is concluded that the ratelimiting step in the process occurs in the dissociation reaction of NH 3 at the Ni surface.…”