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 NH3decomposition 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 NH3decomposition. 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.