J. P. Larpin

Metal-organic chemical vapor deposition of Cr2O3 and Nd2O3 coatings. Oxide growth kinetics and characterization

Thin oxide films of Cr2O3 and Nd2O3 were prepared, using Metal-Organic Chemical Vapor Deposition (MOCVD) technique, to protect stainless steels against corrosion at high temperature. The conditions of precursor volatilization were studied by thermogravimetry. Deposited film growth kinetics depended on the deposition parameters, particularly substrate temperature, gas flow rate and location of substrate in the coating reactor. The influence of the deposition parameters on the deposition rate and the uniformity of the films is discussed. The oxide films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The aim of this work was to optimize coating parameters in order to prepare mixed Nd2O3–Cr2O3 films, leading to the most important protective effects under high temperature corrosion conditions.

Water vapor effect on high-temperature oxidation behavior of Fe3Al intermetallics

Abstract Fe3Al intermetallics (Fe3Al, Fe3Al-Zr, Fe3Al-Zr,Mo and Fe3Al-Zr, Mo, Nb) were oxidized at 950 °C in dry and humid (11 vol% water) synthetic air. Thermogravimetric measurements showed that the oxidation rates of the tested intermetallics were lower in humid air than in dry air (especially for Fe3Al-Zr, Mo and Fe3Al-Zr, Mo, Nb). The addition of small amounts of Zr, Mo or Nb improved the kinetics compared with that of the undoped Fe3Al. Fe3Al showed massive spallation, whereas Fe3Al-Zr, Fe3Al-Zr, Mo and Fe3Al-Zr, Mo, Nb produced a flat, adherent oxide layer. The rapid transformation of transient alumina into alpha alumina may explain the decrease in the oxidation rate in humid air.

Segregation of Neodymium in Chromia Grain-Boundaries during High-Temperature Oxidation of Neodymium Oxide-Coated Chromia-Forming Alloys

The influence of MOCVD reactive-element-oxide (REO) coatings (Nd2O3) onhigh-temperature, chromia-forming alloy oxidation was investigated. REOcoatings decreased steel oxidation rates and greatly enhanced oxide scaleadherence. Uncoated and coated F17Ti samples were oxidized over thetemperature range 700–1050°C in air at atmospheric pressure. SIMSexperiments were performed on oxidized-coated samples in order to determinethe RE distribution through the oxide scale. Nd was distributed across theoxide layers with a higher concentration in the outer part of thescale. Transmission-electron microscopy (TEM) investigations were performedto more precisely locate the RE through the scale. Transverse crosssections, prepared on oxidized Nd2O3-coatedFe–30Cr (model system), showed that Nd, associated with Cr and O,segregated at chromia grain boundaries. It is thought that this is the maincause of the beneficial effects usually ascribed to the RE inchromia-forming alloys. The effect of chromia grain-boundary segregation onchromia growth mechanism and its influence on the reactive-element effect(REE) are discussed.

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe–30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al2O3 and Cr2O3 films doped with Sm2O3 or Nd2O3 were prepared via the metal-organic chemical vapor deposition technique. Single Cr2O3, Al2O3, Nd2O3 and codeposited Cr2O3–Nd2O3, Al2O3–Nd2O3, Al2O3–Sm2O3 coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11–18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO3 or NdTi21O38 and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.

High-temperature oxidation behavior of the intermetallic compound NbAl3 : Influence of two processing techniques on the oxidation mechanism

The NbAl3 intermetallic compound was prepared two different ways:first, by the classical induction-melting technique; the end product is acoarse-grain massive compound, including cracks and pores. Second, bymechanically activated annealing process (M2AP); the end product is afine-grain, powder of submicron crystallites. The oxidation behavior in airunder atmospheric pressure over the temperature range 500–1350°Cwas studied for each material in order to determine the influence of theNbAl3 microstructure on the oxidation mechanism. In all cases,the massive compound does not form the expected compact alumina, protectivescale. In the lower temperature range, the “pest” phenomenonoccurs. No grain disintegration was evidenced by oxidation of the M2APNbAl3 powder despite the high number of crystallites forming onegrain. This is a good argument with expected behavior for a massive materialproduced from the M2AP precursor by powder metallurgy processing.