G. Bonnet

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.

Comparison of the High-Temperature Oxidation of Uncoated and CVD-FBR Aluminized AISI-304 Stainless Steel

Aluminide coatings were obtained by means of the CVD–FBR technique at 525°C for 1.5 hr under a reactive-gas mixture composed of 10 vol.% H2+1 vol.% HCl, the rest being Ar as the fluidizing inert gas. Subsequent heat treatment at temperatures to 900°C was conducted to enhance interdiffusion of the components. As a result, substoichiometric Ni–Al phases were found to form. Uncoated and coated plus heat-treated specimens were then subjected to 950°C oxidation for up to about 200 hr under isothermal conditions. The coated plus heat-treated specimens had much lower oxidation rates than the uncoated ones because of the formation of protective alumina scales. Loss of protective behavior occurred only by spalling of the alumina scales upon cooling from the oxidation temperature. The higher oxidation rates of uncoated specimens have been attributed to nodule formation and minor subsequent spalling of the oxide scale.

pH-distribution of cerium species in aqueous systems

Abstract Cerium-based oxide coatings can be obtained through either chemical or electrochemical processes on various conductor and semiconductor substrates. In both cases the films develop through a precipitation mechanism, which strongly depends on the solution chemistry. In the particular case of the electrolytic approach, the elaboration parameters play a key role on the interfacial pH modification thereby leading to an indirect precipitation mechanism. Indeed, the nucleation and growth mechanisms of crystallites and the composition of the resulting layers have been shown to be also strongly affected by the deposition conditions as well as by the substrate composition, which could in turn modify the protectiveness provided by such coatings. Therefore a better fundamental understanding of the system is required, in particular of the distribution of cerium-containing species in aqueous solution. To this end, the present work intended to develop a diagram showing the distribution as well as the relative amount of Ce(III)/Ce(IV) species in aqueous media as a function of the pH range. The resulting pH-distribution diagram turned out to be a useful tool to predict the relevant precipitation mechanisms and species involved during the growth of cerium-containing films and to draw correlations with the characteristics of the as-deposited films.

Growth of oxide scales upon isothermal oxidation of CVD-FBR aluminide coated stainless steel

Abstract The performance of different alloys exposed at high temperature environments depends upon their mechanical resistance as well as their corrosion/oxidation properties. When the mechanical requirements are not critical, austenitic stainless steels may play a role in substituting the more expensive Ni and Co base alloys. However, at temperatures close to 950°C, the chromia scale usually grown to protect the alloy may be further oxidised into CrO 3 , which is a volatile oxide and thus, the naked material may undergo a catastrophic oxidation. Fe–Cr–Al alloys have been shown to be oxidation resistant at high temperatures. This relies on the formation of alumina scales to protect the alloy, having a chromium reservoir so as to reduce the aluminium amount needed to maintain the oxide scale. In this work, aluminide coatings were deposited by means of the CVD-FBR technique on AISI 304 substrates, at 525°C for 1.5 h. A subsequent heat treatment up to 900°C was applied to the coated specimens to enhance interdiffusion of the species, which led to substoichiometric NiAl phases. Uncoated as well as coated plus heat-treated specimens have been oxidised at 950°C, up to 200 h, under continuous and discontinuous isothermal conditions. The results indicate that aluminide coatings provide a much higher beneficial effect under continuous oxidation than in discontinuous tests, the latter undergoing breakaway oxidation. Growth morphologies and compositions will show an iron enrichment during the latter oxidation stages, which will be responsible for oxidation resistance failure in the discontinuous tests.

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.

Effects of chromia coatings on the high-temperature behavior of F17Ti stainless steel in air: Analytical studies of the effect of rare-earth-element oxides

The MOCVD deposition of neodymium oxide and/orchromium oxide provided beneficial effects both onisothermal- and cyclic high-temperature behavior ofcommercial F17Ti stainless steel. Fracture crosssections provided information about the morphology ofthe oxide scales formed on bare steel and coatedspecimens. The chromia scales developed small equiaxedgrains on the Nd2O3-coated samplesand columnar grains on the uncoated ones. Neo dymium segregatedwithin a surface layer composed ofMn1.5Cr1.5O4 spineloxide. A complex phase (close to the structure ofCeTi21O38) was identified in thiszone. It could act as a source of neodymium ions, which couldsegregate to the grain boundaries of the chromia scale.Polished cross sections associated with X-ray mappingstudies confirmed the scale structure and the location of the rare-earth element in the outer part ofthe oxide layer.

Electrosynthesis of Rare Earth Oxide Coatings for High Temperature Applications

Rare earth oxides are commonly employed as dopants or coatings to improve the development and adherence of alumina scales. However, for practical applications, doping is difficult to control and the use of coatings is preferred. Nevertheless the thickness of such coatings is relatively limited for long term exposures at high temperatures and thicker coatings are hence required. With this in mind, the cathodic electrodeposition technique has been investigated in this work. The results show that deposits of about 20 µm RExOOHy coatings can be obtained on a Ni superalloy in 20 min. The applied current density and time significantly influence the microstructure, thickness, crystallite size and number of oxygen vacancies of the coatings. Their needle-like microstructure is indicative of non negligible amounts of rare earth hydroxides. However, the hydroxide peaks overlap with the oxide peaks in the X-ray diffraction (XRD) patterns. XRD also suggests that the coatings are either amorphous or of nanocrystalline nature, as supported by Raman spectroscopy. Their multicracked morphology is related to the shear stresses between the coating and the substrate, hydrogen bubbling and mostly by drying of the coatings in air. The number of cracks is increased after a heat treatment which also allows full crystallization of the RExOy coating and pre-oxidation (α-Al2O3) of the superalloy. The combined effect of both oxides results in an improved oxidation resistance of the Ni-base superalloy at 1100°C in air.

On the Development of a Protective Oxide System in Rare Earth Oxide Coated Nickel Superalloy under Isothermal Oxidation Conditions

Isothermal oxidation experiments at 1100°C in air were carried out to evaluate the protective capability of a new rare earth oxide coating realized by electrodeposition onto a Ni-base single crystal superalloy. A subsequent heat treatment of the RExOy coating already allowed the establishment of a very thin and discontinuous inwardly grown alumina scale. Under isothermal conditions at 1100°C in air a fully parabolic regime installed from 25h leading to parabolic rate constants of 2.5 10-7 mg2.cm-4.s-1 after 200h, similar to those of conventional β-NiAl coatings. The initial, transition and parabolic regimes were ascribed to the major development of NiAl2O4/Al2O3 mixed oxides by in situ high temperature X-ray diffraction (HT-XRD). No major transient alumina was observed. The α-Al2O3 scale intensity increased with increasing oxidation time, in particular with respect the rare earth oxide coating signal. The scanning electron microscopy (SEM) images showed an oxide system consisting on a top NiAl2O4 oxide and a bottom α-Al2O3 scale underneath the RExOy coating. Alumina grew within the substrate surface. After 500 and 1000h of oxidation, very scarce nodules grew between the alumina and the rare earth oxide deposit. Despite the thermodynamic calculations suggested a REAlOy perovskite at the alumina-RExOy interface, this was not observed experimentally either by XRD or scanning electron microscopy (SEM).

High-temperature oxidation behavior of low-energy high-flux nitrided Ni and Ni–20% Cr substrates

Abstract Nitridation at low-energy, high-flux implantation-diffusion has been performed on pure Ni and Ni–20at.% Cr substrates in order to study their high-temperature oxidation behavior at 700 and 800xa0°C in synthetic air. The nitridation treatment leads to significant sputtering on pure Ni, but no implanted nitrogen has been detected. However, the Ni–20at.% Cr substrates are able to incorporate nitrogen, with a very different surface state. Porosity is found on both substrates after the nitridation treatment. No particular difference is found in the oxidation kinetics of Ni specimens, but in their scales morphology. In contrast, in Ni–20at.% Cr specimens, oxidation is enhanced mainly upon the first exposure times owing to trapping of chromium by the implanted nitrogen. Furthermore, the expanded austenite γ N layer formed on the nitrided Ni–20at.% Cr samples is stable up to 700xa0°C for 24 h, where after nitride precipitation sets in.