C.A. Figueroa

Effect of hydrogen and oxygen on stainless steel nitriding

The influence of hydrogen and oxygen on stainless steel implanted by nitrogen low-energy ions is systematically studied. It is shown that hydrogen intervenes moderately in the process only when the oxygen partial pressure in the deposition chamber is relatively high. For very low-oxygen partial pressures, the energetic nitrogen molecules impinging on the substrate sputter the thin oxide layer formed on the substrate. This allows the growing of a rich nitrogen layer beneath the surface, improving the diffusing of the implanted atom deeper in the bulk material. For higher-oxygen partial pressures, the sputtering is ineffective, and an oxide layer partially covers the surface even in the presence of hydrogen. The maximum depth penetration of nitrogen depends on the degree of oxygen coverage, which is fairly well described by a Langmuir absorption isothermal. Hardness depth profiling is consistent with the existence of a diffusion barrier formed by the oxygen absorbed on the surface. In order to understand th...

Magnetic and structural properties of ion nitrided stainless steel

The magnetic properties and crystalline structure of expanded austenite obtained by ion beam nitriding of AISI 316 steel are investigated. Magnetic force microscopy reveals that the nitrogen expanded austenite has two different layers, an outermost ferromagnetic layer and a paramagnetic layer beneath it. Superimposing the nitrogen concentration profile determined by secondary neutral mass spectrometry and the magnetic force microscopy image, one can see that the paramagnetic-ferromagnetic transition takes place at the inflection point of the nitrogen concentration profile at about 14±2 N at. %. Conventional and glancing angle x-ray diffraction suggests that nitrogen could occupy first tetrahedral interstitial positions (nitrogen-poor paramagnetic phase) and then, after saturation of Cr traps, octahedral interstitial positions (nitrogen-rich ferromagnetic phase). The ferromagnetic-paramagnetic transition is seen to be governed by Cr (traps)–N interactions.

Identification of the mechanism-limiting nitrogen diffusion in metallic alloys by in situ photoemission electron spectroscopy

In situ photoemission electron spectroscopy is used to identify the mechanism limiting the thermally activated nitrogen diffusion in metallic alloys. The samples were prepared by bombarding stainless steel with a broad ion source in a high-vacuum chamber. The photoemission spectra evolution on increasing controlled oxygen partial pressure is correlated with bulk material properties. The presence of oxygen inhibits the formation of iron nitrides and γN-phase (austenitic face-centered-cubic lattice containing nitrogen), which proved to be fundamental for efficient nitrogen penetration in the alloy.

Influence of the ion mean free path and the role of oxygen in nitriding processes

In this article we report the mechanism involved in the nitriding process of stainless steel by ion implantation. The importance of the nitrogen ion mean-free path on the stainless steel nitrated layer obtained by using a broad ion source is established. The energy distribution of the nitrogen ions arriving at the substrate is basically determined by the inelastic scattering suffered by the ions on the way to the material surface, i.e., the ion mean-free-path λ. Besides this effect, the ion current density arriving at the sample surface is modified by the dispersion introduced by the collisions of the nitrogen ions with the chamber background molecules. This multiple scattering process is modeled assuming a stochastic phenomenon and its conclusions used to explain experimental results of hardness, diffusion profile, and nitrated layer thickness. A controlled oxygen-background partial pressure is also introduced and its role on the nitrated layer reported. At relatively low ion energies and oxygen partial ...

A comprehensive nitriding study by low energy ion beam implantation on stainless steel

In this paper we report nitriding studies of stainless steel 316 using a broad ion beam source. Experiments performed by changing the ion energy (0.2-1.5 KeV), ion current density (1.4-5.7 mAycm ) and implantation times ( 1a nd 8h ) at a 2 temperature around 3808C are reported. The microstructure and morphology are studied by glancing angle X-ray diffraction and scanning electron microscopy. For constant ion energy, higher nitrogen ion flux increases the hardness. At higher ion energies the sputtering process prevents the formation of a thick-nitrated layer, even for longer implantation times. The results are examined in the light of recent studies on physical models for ion implantation. 2001 Elsevier Science B.V. All rights reserved. 2q

Effect of Carbon on the Compound Layer Properties of AISI H13 Tool Steel in Pulsed Plasma Nitrocarburizing

Due to the mechanical and inertness properties of the e-Fe2-3N phase, its formation as a compact monolayer is most wanted in plasma surface treatments of steels. This phase can be obtained by the inclusion of carbon species in the plasma. In this work, we present a systematic study of the carbon influence on the compound layer in an AISI H13 tool steel by pulsed plasma nitrocarburizing process with different gaseous ratios (0% ≤ [CH4]/[N2 + CH4 + H2] ≤ 4%). The plasma treatment was carried out for 5 h at 575 °C. The microstructure and phase composition of the modified layers were studied by scanning electron microscopy and X-ray diffraction, respectively. X-Ray photoelectron spectroscopy was used to measure the relative concentration of carbon and nitrogen on the surface. The hardening profile induced by the nitrocarburized process is also reported.

Electrostatically Confined Plasma in Segmented Hollow Cathode Geometries for Surface Engineering

A segmented hollow cathode (SHC) geometry was used for electrostatic confinement of plasma, and surface engineering treatments were conducted in this arrangement. The assessed processes included plasma nitriding, reactive deposition of sputtered material, and deposition of carbonaceous films by plasma-enhanced chemical vapor deposition with a bipolar pulsed-dc power supply on steel substrates. The treated specimens exhibited uniform surface morphology and deposition layers. Characterization techniques included optical microscopy, scanning electron microscopy with energy dispersive X-ray capability, and X-ray diffraction. The advantages and potential applications of the SHC arrangement are discussed in view of these results.

Enhanced nitrogen diffusion induced by atomic attrition

The nitrogen diffusion in steel is enhanced by previous atomic attrition with low energy xenon ions. The noble gas bombardment generates nanoscale texture surfaces and stress in the material. The atomic attrition increases nitrogen diffusion at lower temperatures than the ones normally used in standard processes. The stress causes binding energy shifts of the Xe 3d5∕2 electron core level. The heavy ion bombardment control of the texture and stress of the material surfaces may be applied to several plasma processes where diffusing species are involved.

Physicochemical, structural, and mechanical properties of Si3N4 films annealed in O2

The physicochemical, structural, and mechanical properties of silicon nitride films deposited by radio frequency reactive magnetron sputtering were investigated before and after thermal annealing in O182. As-deposited films were essentially amorphous, stoichiometric, and free from contaminants for a wide range of deposition parameters, with hardness figures ranging from 16.5–22 GPa, depending mainly on the deposition temperature. After O182 annealing at 1000 °C, films hardness converged to 21 GPa, independently of the deposition temperature, which is explained based on the crystallization of the films at this annealing temperature. Moreover, oxygen is incorporated only in 7.5 nm of the Si3N4, forming silicon oxynitride at the top surface of the film, indicating a good oxidation resistance at high temperature. Finally, the elastic strain to failure (H3/E2), which mimics the wear resistance of the film, doubles after the 1000 °C annealing. These observations show the great potential of silicon nitride as a ...

Hydrogen etching mechanism in nitrogen implanted iron alloys studied with in situ photoemission electron spectroscopy

In situ photoemission electron spectroscopy (XPS) is used to elucidate the hydrogen etching mechanism in nitrogen implanted iron alloys. The samples were prepared by bombarding stainless steel with a broad nitrogen ion source in a high vacuum chamber. The photoemission spectra evolution on increasing hydrogen ion current is correlated with the nitrided surface properties. The presence of hydrogen is associated with oxygen removal, augmenting the surface nitrogen concentration. The total active sites at the surface are constant, i.e., oxygen competes with nitrogen sites on the surface. The absorbed oxygen is etched following a linear law on hydrogen ion flux. Simultaneously, the formation of metallic nitrides is enhanced. At the working temperature, the efficiency of the process is determined by a characteristic time that depends on hydrogen retention time, water formation and desorption time.