Hiroyuki Umehara

Chrome-free surface treatments for magnesium alloy

Conversion coatings for magnesium have traditionally been based on immersion treatment in a solution containing hexavalent chromium compounds. However, the need for a replacement surface treatment has been strongly emphasized by the present environmental drive to eliminate hexavalent chromium. The development of permanganate bath chemical conversion coatings for magnesium alloys was studied, using mass gain measurements, measurements of corrosion potential, X-ray diffraction, X-ray photoelectron spectroscopy and microscopic examination. The chemical conversion coating obtained from a permanganate bath consists of a film, composed mainly of manganese oxides and magnesium oxide or hydroxide. Such films were found to be amorphous composite coatings. The corrosion resistance of permanganate chemical conversion coatings was comparable with that of chromating.

Effect of deposition parameters on composition, structures, density and topography of CrN films deposited by r.f. magnetron sputtering

Abstract CrN thin films were fabricated on single crystal silicon substrates by using the reactive radio frequency magnetron sputtering. The effects of deposition parameters on the deposition rate, composition, microstructures, density, and surface roughness of the CrN films have been investigated by AES, XRD, XRR, and AFM. It is found that the cubic-CrN phase can be observed in films with a relatively wide range of nitrogen gas flows. The composition of CrN films was controlled by the gas flows of N 2 and the gas flow ratio (Ar/N 2 ). The density of CrN films decreases with the increase of gas flows at room temperature. It is considered that the observed density decrease of CrN films is due to an increasing number of collision between CrN particles with the increasing gas flows of Ar and N 2 . It is beneficial to produce dense films at high temperature of substrate. The effects of gas flows on the deposition rate showed that the deposition rate decreases with the increase of nitrogen gas flows. The surface roughness of the films increases with the increase of Ar and N 2 gas flows, and Ar gas flows play a main role in the surface roughening. It is suggested that the ion and particles bombardment at low gas pressures cause a smoother surface.

Thermal stress hardening of a-Si3N4/nc-TiN nanostructured multilayers

Amorphous/nanopolycrystalline Si3N4/TiN nanostructured multilayer films have been fabricated by radio-frequency reactive magnetron sputtering. The effects of deposition temperature, modulation period, and the layer thickness ratio on the hardness have been studied, in order to elucidate the hardening mechanisms in these multilayers. The hardness of the Si3N4/TiN multilayers is affected not only by the modulation periods, but also by the layer thickness ratio and deposition temperature. The hardness value is about 40% higher than the value calculated from the rule of mixtures at a deposition temperature of 500u200a°C and a modulation ratio (lSi3N4/lTiN) of 3/1. Based on the experimental results, it is suggested that the alternating stress field caused by thermal mismatch between Si3N4 and TiN is one of the main reasons for the superhardness effect in Si3N4/TiN nanostructured multilayers.

The preparation of iron complex oxide nanoparticles by pulsed-laser ablation

Abstract Nanoparticles of a calcium–iron complex oxide were prepared by pulsed laser ablation on silicon wafer substrates placed at off-axial positions against a target. An ArF excimer laser was used to irradiate a CaFe 2 O 4 target in atmospheres of Ar and O 2 at room temperature. The effects of ambient pressure and laser pulse energy on size and composition of nanoparticles were investigated using atomic force microscopy and X-ray photoelectron spectroscopy. The nanoparticles obtained were almost spherical and 2 to 26 nm in diameter. The size distributions of the nanoparticles were very narrow and agreed with the log-normal distribution function. The nanoparticle size increased with ambient pressure and pulse energy. The Ca/Fe atomic ratios in the nanoparticles, however, decreased with increasing ambient pressure and were independent of pulse energy. By this technique, the size and composition of nanoparticles can be easily controlled by laser fluence and pressure during the laser ablation.

One-step growth of silica nanotubes and simultaneous filling with indium sulfide nanorods

A high temperature approach has been utilized for the mass production of ultra-long, straight silica nanotubes and indium sulfide filled cable-like structures by heating of an Si wafer and In2S3 powder under a stream of hydrogen/argon gases at 950 °C. Transmission electron microscopy, scanning electron microscopy and X-ray diffraction analysis demonstrate the formation of hollow cylindrical amorphous silica tubes with lengths up to hundreds of micrometers and outer diameters in the range of 70 to 300 nm, silica tubes encapsulating indium sulfide nanorods with a cubic structure and S-filled silica tubes with high sensitivity to a focused electron beam. A modified vapor–liquid–solid growth was proposed for the formation of amorphous silica nanotubes under the coexistence of low melting point In and S. The capillary induced filling promoted simultaneous formation of indium sulfide nanorods upon further crystallization.

Structure and Corrosion Behavior of Conversion Coatings on Magnesium Alloys

We studied various types of surface treatments for magnesium alloys that involved the application of chromium or manganese by measuring their corrosion potential using X-ray diffraction, X-ray photoelectron spectroscopy, auger electron spectroscopy, and microscopic examination. The chromium-conversion coating is composed primarily of amorphous trivalent chromium oxide (or hydroxide) and a fluoride of magnesium and aluminum. The permanganate-conversion coatings, we obtained by adding HF to the immersion bath, consisted of a film that was composed primarily of manganese oxides and magnesium fluoride, We found that these films formed an amorphous composite coating In addition, we examined the corrosion resistance of AZ91D magnesium alloy die-cast panels with various surface treatments. The exposure tests, we performed in Miyakojima, indicated that the anodic c oxidation coating and the MX1-type surface treatment containing chromate, which result in relatively thick coatings, are superior in corrosion resistance. The corrosion resistance of permanganate chemical conversion coatings is comparable with that of chromium conversion coatings.

Corrosion Resistance of Surface Treated AZ91D Magnesium Alloys Outdoor Exposure after 10 Years

The high-purity magnesium alloy specimens prepared by applying die-cast [1] paint and various types of surface treatments, and then the corrosion resistance of practical coating materials after ten-year outdoor exposure tests was evaluated. These tests were performed at the Choshi (long. 140°45 E, lat. 35°43 N), the Asahikawa (long. 142°18 E, lat. 43°47 N), and the Miyakojima (long. 125°20 E, lat. 24°44 N) Exposure Laboratories of the Japan Weathering Test Center. The environmental factors generally believed to be associated with corrosion mechanisms (temperature, humidity, precipitation, wetness time, solar and UV radiation, and concentration of chloride and sulfur oxide) were measured. Evaluation of the adhesion of paint and corrosion resistance, chromate and anodic oxide films were superior as pretreatment paint films on AZ91D magnesium alloys; these films were comparable to paint-coated ADC12 aluminum alloys.