Surface Nitriding Process Research Articles

Surface morphology of a laser surface nitrided and etched Ti–6Al–4V alloy

A laser surface nitriding process was applied to a Ti–6Al–4V alloy to obtain a surface layer consisting of TiN dendrites and a titanium alloy matrix. The dendritic volume fraction can be controlled by varying the nitrogen flow rate during the laser gas nitriding. By subsequently applying a selective etching process, the matrix material in the alloyed layer can be selectively removed and a three-dimensional network of TiN dendrites is left on the surface anchoring on the substrate. The 3D dendritic network provides extra surface area and a locking mechanism for the adhesion joint with a ceramic layer. The interface cross-section microstructure of such adhesion jointing surfaces is discussed.

Surface Nitriding of SUS 304 Austenitic Stainless Steel by a Molten Salt Electrochemical Process

Electrochemical surface nitriding of SUS 304 austenitic stainless steel was investigated in a molten system at 773 K, and surface hardening by this molten salt electrochemical process without any special pretreatment was confirmed. From X-ray photoelectron spectroscopy and X-ray diffraction analyses, it was found that the surface nitrided layer contained CrN precipitates in Fe-Ni matrix. The square of the thickness of the surface nitrided layer was proportional to the electrolysis time. The rate constant, increased as the applied potential was more positive. This finding indicated that the chemical potential of adsorbed nitrogen on SUS 304 stainless steel electrodes depended on the applied potential and it affected nitrided layer growth. Furthermore, surface nitriding of a SUS 304 stainless steel tube was achieved. It was confirmed that adhesive coherent nitrided layers formed both on the outside and inside of the tube. These results suggested that this electrochemical surface nitriding process can be applied to specimens of various shapes. © 2004 The Electrochemical Society. All rights reserved.

Surface passivation of GaAs by ultra-thin cubic GaN layer

Attempts were made to passivate the GaAs (001) surface by a pseudomorphic ultra-thin cubic GaN layer formed by a nitrogen radical (N-radical) or nitrogen plasma irradiation technique. Reflection high-energy electron diffraction (RHEED) pattern observations and detailed X-ray photoelectron spectroscopy (XPS) analysis have shown that ultra-thin cubic GaN layer on GaAs (001) surface with desirable surface stoichiometry can be realized with the optimization of surface nitridation process parameters. The passivation effects, characterized by ultra-high vacuum photoluminescence (UHV PL) analysis, revealed strong enhancement in band-edge PL intensity of GaAs after passivation as large as a factor of 10 when compared with the as-grown clean molecular beam epitaxy (MBE) GaAs surface.

DRIFTS Study of the Surface Structure and Nitridation Process of Mixed Galloaluminophosphate Oxynitride (AlGaPON) Catalysts

The spectra of high surface area amorphous oxide AlPO, AlGaPO, and GaPO were compared with those of high surface amorphous oxynitrides AlPON and AlGaPON with similar O/N ratio and with that of the nitride GaN. This allowed us to make a correct assignation of the bands in their IR vibrational spectra, especially those in the new family of solids named galloaluminophosphate oxynitride, AlGaPON. The substitution of aluminum atoms by gallium induces a shift of the framework band vibrations due to a second-neighbor effect. The introduction of nitrogen in the network of the oxide phosphate produces a broadening and poor definition of the structural bands due to a collapse of the structure. NH4+, coordinated NH3, -NH2, and -NH- species are identified on the surface of the AlGaPON solid. A thermal treatment under NH3 allows us to propose the following nitridation sequence: M-O-NH4+ and/or M-NH3 --> M-NH2 --> M-NH-M --> N3-.

Localised corrosion behaviour of Fe-N model alloys

The present investigations were carried out to understand the role played by nitrogen in affecting the passivation and pitting kinetics of Fe-N model alloys. Synthetic alloys of Fe-N, Fe-10%Ni-N and Fe-10%Ni-3%Mo-N were prepared by surface nitriding process with 0.9 wt% nitrogen on the surface. Pit initiation studies were conducted on pure Fe and Fe-N alloy in an acidic sulphate solution (0.01 M H 2SO 4 and 0.99 M Na 2SO 4) containing 0.0007, 0.001, 0.007 and 0.07 M NaCl at three different electrode potentials, namely, +800, +1000 and +1200 mV (NHE). Pit growth kinetics was assessed by studying the dissolution behaviour of these alloys during potentiostatic transient in an acidic sulfate solution containing 0.5, 1, 2 and 4.5 M NaCl. The steady state dissolution current was considered as the parameter reflecting the pit dissolution behaviour of the alloys. Potentiodynamic anodic polarisation of the alloys was also carried out in the acidic sulphate solution at room temperature. The results of the investigation indicated that as the chloride content increased nitrogen addition delayed the pit initiation as well as reduced the dissolution kinetics of the pits. The polarisation study of the model alloys indicated that nitrogen played a greater role in reducing the corrosion behaviour of the alloys by reducing the active peak current density and widening the passive range in which pitting is less probable.

Nitriding mechanisms in ArN 2, ArN 2H 2 and ArNH 3 mixtures in D.C. Glow discharges at low pressures (less than 10 Torr)

The d.c. glow discharge in different ArN 2, ArNH 3 or ArN 2H 2 gas mixtures results in the surface nitriding of titanium metal and its alloy (Ti6Al4V where the composition is in approximate weight per cent). Various gas mixtures were used in order to establish the main active species governing the nitriding process, i.e. N, N 2, NH or NH 2 as excited or ionized particles. The d.c. discharge was sampled and analyzed by quadrupole mass spectrometry and optical emission spectroscopy, and the nitrided samples were analyzed by Auger electron spectroscopy, microhardness measurements and Fourier transform IR reflectance spectroscopy. It was found that the excited and ionized nitrogen and hydrogen atoms were the main species responsible for the surface nitriding process in the d.c. glow discharge.