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

Iron nitride layers were formed by a novel low-temperature gaseous nitridin g process. Nitriding occurs at a temperature of 325 degrees C through NH3 d ecomposition at the surface of Ni (25 nm) coated Fe, followed by N transpor t through the Ni film into the underlying Fe, where nitride precipitation t akes place, The role of Ni is to protect Fe from oxidation by gas impuritie s and to serve as a catalyst for NH3 decomposition. The precipitation behav ior 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 tem peratures due to the decomposition of the nitrides into Fe and N-2). XTEM s howed that the original columnar alpha-Fe grains transform into smaller gam ma'-Fe3-xN grains which subsequently transform into larger epsilon-Fe3-xN g rains. This microstructural evolution of smaller gamma' grains forming in t he original columnar alpha-Fe structure occurs in one of two growth modes o f the nitride in the Fe layer, i.e,, throughout the entire depth range of t he Fe layer, or preferentially at the Ni/Fe interface when an iron oxide la yer is present at this interface.