CoO Layer Research Articles

The oxidation of cobalt-chromium-titanium alloys at 1000�C

Cobalt-based alloys containing 3,5,10,15, and 20% Cr with 1 and 3% Ti were oxidized at 1000°C in slowly flowing oxygen gas. In general, titanium additions decreased the oxidation rate with the most pronounced effect being observed at the 10% Cr level. Titanium accelerated the formation of Cr2O3 layers at the metal-oxide interface. Faceted CrxTiyOz spinel particles were found at the metal-oxide interface which varied in composition according to microprobe results. There was no evidence of spalling on the Co-Cr-Ti alloys studied in contrast to the severe spalling normally encountered in Ni-Cr-Ti alloys. Distinct morphological differences existed on the outer CoO layer of the 1% Ti alloys in comparison to the O and 3% Ti alloys.

Preparation and Coercivity of Cobalt Ultrafine Particles by Reduction of Multilayered CoO–SiO Films

Abstract Films containing Co ultrafine particles were prepared by hydrogen reduction of periodically multilayered CoO–SiO films which were formed by alternating deposition of CoO and SiO. The films containing Co particles maintained the periodicity similar to the original films. Using samples of CoO single layer sandwiched between SiO layers, highly dispersed Co particles in the reduced sample were observed by transmission electron microscopy. The average size of the Co particles was almost as large as the thickness of CoO layer. The coercivities of the multilayered films containing Co particles were measured at room temperature and 77 K; they decreased with decreasing particle size from 20 nm to 3nm. Superparamagnetism was observed for the Co particles with average sizes below 4nm at room temperature. The formation and the magnetic property of the Co ultrafine particles in the Co–SiO system is dicussed.

Formation of artificial superlattice composed of ultrathin layers of CoO and NiO by reactive evaporation

Films composed of alternating ultrathin layers of CoO and NiO have been prepared by reactive evaporation. The structure was investigated by x-ray diffraction and transmission electron microscopy (TEM). The CoO-NiO multilayered films deposited on the (100) plane of NaCl at 250 °C were single crystal, having the NaCl structure with lattice constants intermediate between the ones of CoO and NiO when each layer was ultrathin (<5 nm). The stacking sequence could be directly observed by the cross-sectional TEM method. The artificial periodicity in the films was also confirmed by the low angle Bragg diffraction and the satellite peaks appearing around the (200) peak. It was found that the films had an artificial superlattice consisting of coherently stacked CoO and NiO single-crystal layers. This structure resulted from alternate epitaxial deposition of these oxides. An artificial superlattice formed also on glass substrates although they were polycrystal films.

Anodic oxide film on cobalt in weakly alkaline solution

The anodic oxide film on cobalt was studied in alkaline solutions at pH 10.0–12.5 by means of in situ ellipsometry combined with electrochemical measurements. In the primary passive potential region more negative than 0.86 V (vs. HESS) the anodic film is a single CoO layer about 2 nm thick at pH 10.0, but it becomes a bilayer comprising an inner CoO layer about 2 nm thick and a gelatinous Co(OH)2 overlayer 10–30 nm thick in the pH range more basic than pH 11.0. The anodic film in the secondary passive and oxygen evolution potential regions more positive than 0.86 V is composed of an inner CoO layer and an outer Co3-ΔO4 layer. Depending on the non-stoichiometry, the complex refractive index of the outer layer changes from ¯n=3.2−0.5i for Co3O4 (Δ=0) to ¯n=3.2−0.95i for Co2O3 (Δ=1/3). The outer Co3-ΔO4 layer thickness increases almost linearly with potential until it becomes almost constant in the transpassive potential region.

Anodic oxide film on cobalt in weakly alkaline solution

The anodic oxide film on cobalt was studied in alkaline solutions at pH 10.0–12.5 by means of in situ ellipsometry combined with electrochemical measurements. In the primary passive potential region more negative than 0.86 V (vs. HESS) the anodic film is a single CoO layer about 2 nm thick at pH 10.0, but it becomes a bilayer comprising an inner CoO layer about 2 nm thick and a gelatinous Co(OH) 2 overlayer 10–30 nm thick in the pH range more basic than pH 11.0. The anodic film in the secondary passive and oxygen evolution potential regions more positive than 0.86 V is composed of an inner CoO layer and an outer Co 3-ΔO 4 layer. Depending on the non-stoichiometry, the complex refractive index of the outer layer changes from ¯n=3.2−0.5 i for Co 3O 4 (Δ=0) to ¯n=3.2−0.95 i for Co 2O 3 (Δ=1/3). The outer Co 3-ΔO 4 layer thickness increases almost linearly with potential until it becomes almost constant in the transpassive potential region.

Kinetics and mechanism of high-temperature oxidation of dilute cobalt-chromium alloys

Kinetics of oxidation of Co-Cr alloys containing 0.4%–15% Cr was studied as a function of temperature (1273–1573 K) and oxygen pressure (4 × 102–105Pa). The oxidation process was found to be approximately parabolic and faster than that for pure cobalt. The scales are double-layered and consist of a compact outer CoO layer and a porous inner layer containing CoO slightly doped by chromium and spinel CoCr2O4. The oxidation mechanism was investigated by means of platinum markers and the18O isotope. The scale on the alloys containing less than 1% Cr grows exclusively by outward diffusion of cobalt, while that on the alloys containing more chromium—with a significant contribution of inward oxygen transport from atmosphere. This transport is not a lattice diffusion, but proceeds presumably through microfissures resulting from the secondary process of perforating dissociation of the outer scale layer.

Spray-pyrolysed cobalt black as a high temperature selective absorber

Cobalt oxide coatings with an integrated solar absorptance of 0.93 and a hemispherical emittance at 100°C of 0.14 were prepared by spray pyrolysis on stainless steel substrates kept at 300°C. The coatings are strongly adherent and stable up to temperatures of about 600°C. Auger electron spectroscopy, X-ray photoelectron spectroscopy and X-ray and electron diffraction investigations revealed the coatings to be composed of an upper layer of Co 3O 4 and subsequent layers of CoO down to the substrate. The absorption mechanism is primarily intrinsic. Lower substrate temperatures (about 150°C) can be used for preparing coatings from equimolar aqueous solutions of cobaltous acetate and thiourea. Such coatings, containing both cobalt oxide and cobalt sulphide, have a comparable absorptance but a higher emittance and are stable only up to about 300°C.

The isothermal oxidation of Co-Cr alloys in 760 Torr oxygen at 1000�C

The isothermal oxidation of Co-Cr alloys containing 0–30% Cr in 760 Torr oxygen at 1000° C has been studied kinetically and by appropriate physical techniques. Chromium additions to cobalt increase the parabolic oxidation rate to an almost constant level from 2 to 15% Cr, while further additions to 20–30% Cr decrease the rate. All the alloys produce a virtually pure CoO layer outside a layer containing Co-Cr spinel particles in a Cr3+ -doped CoO matrix. The variation of oxidation rate with alloy chromium content is explained in terms of the complex interplay of doping, blocking of cation transport by voids and spinel particles and short circuiting by transport of dissociative oxygen across these voids and other processes, internal oxidation making a negligible direct contribution to weight gain. Complete spinel layers are never quite developed under the conditions studied, although formation of spinel does slow the oxidation rate. The improved protection eventually obtained at higher chromium levels is produced by the tendency to form a Cr2O3 healing layer.

Evidence of mechanical Film Breakdown and ESCA Analysis of a Film on Thermally Aged Cobalt Hardened Gold

Electron spectroscopy for chemical analysis (ESCA) has been used to determine the chemical formulation of thermally grown films on cobalt hardened-gold electroplate (150°C/8000 h). K2S04 (or SO3) ~ 60-Å, thickness has been identified overlaying a 50-Å-thick CoO layer on cobalt hardened gold. It was found that the dielectric breakdown voltage of this complex film depends on contact force. At Iow contact forces (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> N), electronic (as opposed to thermal) dielectric breakdown events are observed at a critical voltage of ~ 3.5 V. I-V characteristics are typically nonohmic. Mechanical film breakdown is evidenced by ohmic I-V characteristics. A statistically important number of mechanical breakthrough events are observed at 0.2-N contact force.

The effect of small amounts of silicon on the oxidation of high-purity Co-25 wt. % Cr at elevated temperatures

Some investigators have reported that Co-25 wt.% Cr oxidizes slowly at temperatures in the range 1000–1200°C forming a protective Cr2O3 scale; and this is the normal behavior of cobalt-base superalloys. Others have reported very rapid oxidation, forming a two-layer scale: an outer CoO layer and an inner mixture of Cr2O3 and CoCr2O4 particles in a CoO matrix. This investigation shows that the principal reason for this behavior is the purity of the material; it appears that the rapid mode of oxidation is the intrinsic behavior for high purity material. The most probable impurity to produce the slower mode is silicon, and it is shown that as little as 0.05 wt. % Si is sufficient to change the mode of oxidation provided sufficient oxygen is also present in the alloy: it seems probable therefore that a fine dispersion of SiO2 is responsible.

THE USE OF VACUUM-EVAPORATED GOLD FILMS TO INVESTIGATE THE OXIDATION MECHANISM OF METALS

Vacuum-evaporated gold films were used as inert markers to study the oxidation mechanism of cobalt at 850°C. Gold-marked and unmarked specimens were simultaneously oxidized for 138 hr. With the exception of the gold marker appearing as a dispersed row of spheriods within the CoO layer closest to the metal-oxide interface, and the random distribution of voids in the specimens, the microstructures of the gold-marked and unmarked specimens were identical, and are in agreement with microstructures reported in the literature.