Influence of Cryomilling on the Morphology and Composition of the Oxide Scales Formed on HVOF CoNiCrAlY Coatings

Commercially available, gas-atomized CoNiCrAlY powder was cryomilled to produce powder with nanocrystalline grains. The cryomilled powder and conventional gas-atomized powder were thermally sprayed using the HVOF process to prepare two coatings with fine-grain (～15 nm) and coarse-grain (～1 μm) microstructure, respectively. The two coatings were isothermally oxidized in air at 1000° C for up to 330 hr. The morphology and composition of the oxide scales formed on the two coatings were compared with each other. The results indicate that, while a fine-grain microstructure can promote the formation of a pure alumina layer on the coating by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.     

Effects of coating thickness and residual stresses on the bond strength of ASTM C633-79 thermal spray coating test specimens

Wire-arc-sprayed nickel-aluminum is widely used in the aircraft industry for dimensional restoration of worn parts and as a bond coat for thermal barrier coatings and other top coats. Some repair applications require thick coatings, which often result in lower bond strength. A mechanism being investigated to ex-plain this decrease in bond strength is the free edge effect, which includes both coating residual stresses and coating thickness. The layer-removal method was used to determine experimentally the residual stresses in wire-arc-sprayed nickel-aluminum coatings of different thicknesses. Bond strength evalu-ations were performed using an improved ASTM C 633-79 test specimen. Finite-element analysis and fracture mechanics were used to investigate the effects of coating thickness and residual stress state on coating bond strength.     

Sterling,Ringwood, and Greenwood

The growing new research area being developed along the New York-New Jersey border at Sterling Forest recalls metallurgical history that was made in this same region. For it was here that ironmaking was centered from colonial days until after the Civil War. This is an important chapter to be added to historical ironmaking articles featured in earlier issues of Journal of Metals.     

Low-temperature oxidation

Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces dangling bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.     

Thermal Transformation of Transition Aluminas

JOM - This paper records conventional (CTEM) and scanning (STEM) microstructural examination of four transition aluminas (γ, δ, χ and κ) prepared from beohmite and...     

Microstructural Investigation of Protective and Non-Protective Oxides on 11% Chromium Steel

FIB, SEM and STEM/EDX were used to investigate X20 stainless-steel samples exposed to dry O₂, or O₂ containing 40% H₂O, with a flow velocity of 0.5 cm/s or 5 cm/s, for 168 hr or 336 hr at 600°C. Thin protective Cr-rich (Cr,Fe)₂O₃ was maintained on the samples exposed to dry O₂, even after 336 hr, and on the sample exposed to O₂/H₂O mixture with the low-flow velocity (0.5 cm/s) for 168 hr. The oxide scale formed in the latter environment contained less Cr, due to Cr loss through CrO₂(OH)₂ evaporation. Breakaway oxidation occurred on the samples exposed in high-gas-flow velocity for shorter time (168 hr) or in low-gas-flow velocity (0.5 cm/s) for longer time (336 hr). The breakaway scales featured a two-layered structure: an outward-growing oxide “island” consisting of almost pure hematite (α-Fe₂O₃), and an inward-growing oxide “crater” consisting of (Cr,Fe)₃O₄. The transition from a thin protective (Cr,Fe)₂O₃ scale to a non-protective thick scale on this martensitic/ferritic steel originated locally and was followed by rapid oxide growth, resulting in a thick scale that covered the whole sample surface.     

Application of optical transmission interferometry for in-situ structural investigations of titanium dioxide sputter-deposited coatings

Titanium dioxide coatings (from 0.1 to 1.5 μm thick) have been dc sputter-deposited on glass slides from titanium targets in various Ar-O₂ reactive gas mixtures. Deposition rate and optical properties were controlled in-situ by optical transmission interferometry (OTI) with an optical fibre located behind the glass substrate in order to perform a real-time control of transmittance of the growing film. Thus, it is possible to determine in-situ the optical indices (n, k) and the thickness of the as-deposited film by using a simple simulation, developed on Matlab software. The optical properties of the films were investigated in relation to their structure, which depends on the sputtering conditions adopted. In particular, the effects of the sputtering pressure (working pressure and oxygen partial pressure), the discharge power and the substrate location into the reactor are investigated in detail. Films structure is assessed by standard grazing incidence X-ray diffraction (XRD).