Microstructural investigation of iron nitride layers formed by low-temperature gaseous nitriding
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Microstructural investigation of iron nitride layers formed by low-temperature gaseous nitriding D. K. Inia and A. M. Vredenberga) Section Interface Physics, Debye Institute, Utrecht University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands
D. O. Boerma Section Interface Physics, Debye Institute, Utrecht University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands and Department of Nuclear Solid State Physics, Materials Science Center, Groningen University, Nijenborgh 4, 9747 AG Groningen, The Netherlands
F. D. Tichelaar National Center for HREM, Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands
H. Schut and A. van Veen Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands (Received 17 March 1998; accepted 17 February 1999)
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 a –Fe grains transform into smaller g 0 –Fe4 N grains which subsequently transform into larger e–Fe32x N grains. This microstructural evolution of smaller g 0 grains forming in the original columnar a –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 NiyFe interface when an iron oxide layer is present at this interface.
I. INTRODUCTION
Improvement of material properties of iron, such as enhancement of corrosion resistance and hardness, can be obtained by applying an iron nitride layer at the surface. Conventionally, such nitride layers are formed thermochemically in an NH3 and H2 containing atmosphere. Mostly, compound layers of e–Fe32x N and g 0 –Fe4 N are formed, the exact composition of which depends on the temperature and the nitriding potential, 3/2 which is given by pNH3 ypH2 . Temperatures usually range from 500 to 600 ±C.1 These relatively high process temperatures have some disadvantages. The most important disadvantage is the possible development of porosity in the nitride layer. Due to the thermodynamic instability of g 0 and e nitrides with respect to Fe and N2 gas, the nitrides partially decompose during the nitriding a)
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