M. Caillet

Thermal Oxidation of Metallic Niobium by Water Vapor

The behavior of metallic niobium in pure water vapor (without added oxygen) was studied between 800 and 1000°C. Linear kinetics curves were observed (except for a short initial period) with a homographic influence of H2O pressure. Hydrogen additions in the gas phase largely modified the reaction rates. The oxide formed (NbO2) is different from that predicted by thermodynamics (Nb2O5). Taking into account all these experimental observations, a mechanism is proposed in which the limiting step is oxygen incorporation into the oxide lattice.

Contribution to understanding parabolic oxidation kinetics of dilute alloys. Part II: Oxides with metal deficit or oxygen excess

Concerning the parabolic oxidation of alloys with a low content of a different valence element it is shown that the Wagner-Hauffe valence approach contains some inaccuracies. This paper is devoted to the growth of an oxide MO with metal deficit or oxygen excess. The case is developed where the doping is high enough to modify the defect concentrations over the entire oxide scale. It is shown that lower-valence metal additions decrease the scaling constant and modify the influence of oxygen pressure. For additions of higher-valence metals, as long as electronic conductivity remains over the whole scale, the limiting step might be the diffusion of ionic defects under only an electric field, with an increase of the scaling constant. When the conductivity becomes essentially ionic over the whole scale the limiting step might be the diffusion of electron holes, and the scaling constant will decrease with an increase in the doping level. Globally the parabolic constant might therefore increase and pass through a maximum before decreasing.

Contribution to the understanding of the parabolic oxidation mechanism of dilute alloys. Part I: Oxides with metal excess or oxygen deficit

Concerning the parabolic oxidation of alloys with a low content of a different valence element, we show that the Wagner-Hauffe valence approach contains some inaccuracies. This paper is devoted to the growth of an oxide MO with metal excess or oxygen deficit. The analytical processing consists in solving the differential equation connecting the point defect flux to the oxygen pressure. This equation has been solved analytically in the case where the foreign-element concentration is very high compared to that of the point defects in the pure oxide. On this assumption with a lower-valence dopant, as long as the oxide conductivity remains essentially electronic, the oxidation is limited only by the transport of ionic defects across the scale under the electric-potential gradient. Moreover, an increase of the dopant concentration may lead to an oxide exhibiting partial ionic conductivity. Consequently, the parabolic constant variations as a function of the dopant concentration is not monotonic, contrary to results published before. With a higher-valence dopant, the conductivity always remains electronic, and this result agrees with predictions based on the Wagner Hauffe approach, though the demonstrations are fundamentally different.

An Exploratory Evaluation of the Elaboration and Protection Properties of Chromium- or Aluminum-Containing Laser-Surfaced Coatings on Titanium

In order to protect titanium against high temperature oxidation, coatings were prepared by laser alloying with Cr 3 C 2 or Al 4 C 3 powders predeposited on the metal. These coatings (150 μm thick with Cr 3 C 2 - and 70 μm thick with Al 4 C 3 -incorporation) were two-phase with TiC precipitates in a matrix of a-Ti(Cr) or α-Ti(Al) solid solutions. These surface alloys were shown to be protective against isothermal oxidation only for temperatures near 900°C for which the oxidized scales formed in oxygen in 48 hours did not lead to the breakaway phenomenon. This positive effect could be connected with the growth of an external TiO 2 subscale due to outward cationic diffusion enhanced by the presence of Cr 2 O 3 or Al 2 O 3 mixed with TiO 2 in the inner subscale.

Etude du mécanisme de tantalisation du fer par cémentation activée en caisse

La tantalisation en caisse du fer, activee par CrF2 , conduit a la formation dun revetement compose, dans sa partie externe, de tantale ayant dissous un peu de fer et de chrome et, dans sa partie interne, dun melange TaFe +TaFe2 . La cinetique de tantalisation est parabolique, limitee par la diffusion des especes gazeuses fluorees dans une couche detantalisee du cement. Toutefois, un certain ecart a lequilibre thermodynamique de la reaction interfaciale de tantalisation doit etre pris en compte afin dexpliquer les valeurs de vitesse observees.

Traitements de surface par faisceaux de Haute energie

Resume Nous rappelons tres brievement les principales caracteristiques des traitements de surface par faisceau dions et par faisceau laser. Nous nous efforcons ensuite a laide dexemples choisis pour etre representatifs de faire une synthese des principaux resultats obtenus en utilisant ces techniques de traitements de surface pour lutter contre labrasion et la corrosion par les gaz chauds. Nous proposons ensuite quelques axes de recherche encore tres peu explores et dans lesquels il conviendrait maintenant de faire porter les efforts.