Bernard Pieraggi

Transition in high-temperature oxidation kinetics of Pd-modified aluminide coatings: Role of oxygen partial pressure, heating rate, and surface treatment

The isothermal oxidation of Pd-modified Ni aluminide coatings was studied as a function of P o 2 and temperature (900–1200 °C). A kinetic transition was observed between 900 and 1000 °C. Grazing incident x-ray diffraction, thermogravimetric analysis, x-ray photoelectron spectroscopy, scanning electron microscopy/energy dispersive spectroscopy, and secondary ion mass spectrometry analyses are consistent with the growth of δ-alumina or α-alumina below or above this transition temperature. Moreover, because P o 2 was established before specimen heating, an effect of heating rate was observed and analyzed. More importantly, no kinetic transition was observed for sand-blasted specimens oxidized at low P o 2 . Thus conditions for the direct growth of an α-alumina scale could be determined from the reported results.

Effects of Bond-Coat Preoxidation and Surface Finish on Isothermal and Cyclic Oxidation, High Temperature Corrosion and Thermal Shock Resistance of TBC Systems

A systematic study of the effects of bond-coats surface modifications prior to ceramic deposition is presented. TBC systems have been made and tested with most combinations of the following parameters: 1/substrate: IN 100 or AM3 (single crystal) Ni-based superalloys; 2/ bond-coat: VPS NiCoCrAlYTa or CoNiCrAlY, (Ni,Pd)Al, (Ni,Pt)Al; 3/ bond-coat surface finish: as deposited, machined, grit blasted, preoxidized; 4/ top coat: APS or EB-PVD Y 2 O 3 stabilized ZrO 2 . The preoxidation treatments of bond-coats were determined from the results of a study (TGA-GIXRD-SEM-SIMS) of their short-term (6 hours) isothermal oxidation behavior as a function of temperature (900 to 1100°C), oxygen partial pressure and heating rate. Under these conditions all bond-coats are alumina formers, but depending on the oxidation conditions, transition aluminas may be formed. The characterization of complete TBC systems was also performed, including isothermal oxidation (TGA), cyclic oxidation at 1100°C, cyclic corrosion at 900°C and thermal shock (fast cooling from 1200°C).

Chemical vapor deposition of ruthenium on NiCoCrAlYTa powders followed by thermal oxidation of the sintered coupons

Abstract This paper introduces spouted-bed metal–organic chemical vapor deposition, an original technique for doping by platinum group metals of commercial powders that are used as raw materials in the processing of bond coats in thermal barriers. In this context, Praxair NI-482 NiCoCrAlYTa powders were doped by ruthenium, starting from ruthenocene. The doping level, purity, microstructure and ruthenium distribution of the powders were established. Doped and undoped (reference) powders were sintered at 1473 K by uniaxial hot pressing. Isothermal oxidation tests on the coupons obtained were carried out in a thermogravimetry apparatus, at temperatures ranging between 1173 and 1423 K, in order to determine the parabolic rate constants. Microscopic and spectrometric characterization of the oxidized samples provided information on the morphology of the scale, its nature and the distribution of its components over the surface. The addition of 0.8 wt.% of ruthenium in NiCoCrAlYTa powders is not detrimental to their oxidation resistance in isothermal conditions. The present study reveals that spouted-bed metal–organic chemical vapor deposition is a convenient method for the systematic screening of raw materials used in thermal barriers doped by different elements.

Effect of Cycle Frequency on High Temperature Oxidation Behavior of Alumina-forming Coatings Used for Industrial Gas Turbine Blades

Oxidation kinetics of platinum modified aluminide and overlay coatings on nickel base superalloys were investigated. Isothermal oxidation tests were carried out at 1050°C in synthetic air. Cyclic oxidation tests were performed with 2 cycle frequencies : n- Short term cycles : 1h dwells at 1050°C in synthetic air ×1800 cycles n- Long term cycles : 300h dwells at 1050°C in laboratory air × 6 cycles (experiment planned to totalize at least 10 000 hours at high temperature) nThe mass gain curves point out a large effect of the cycle frequency at 1050°C for overlay NiCoCrAlYTa coating whereas the effect is less significant for Pt-modified nickel aluminide coating. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy was used to evaluate the effect of cycle frequency on microstructural evolution. A simple statistical spalling model [1,2], assuming that the parabolic rate constant kp and the spalling probability p are constant, is tentatively applied and discussed in view of the microstructural evolution complexity.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

Abstract The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

In situ ESEM investigations of the oxide growth on hot work tools steel: effect of the water vapour

Abstract Tempered martensitic steel modified AISI H11 is used in forging processes where tool failure can result from the combination of thermo-mechanical and chemical damage. A better knowledge of the oxidation mechanisms in this material could be useful for a better appreciation of its service behaviour and lifetime. The low chromium content of this Fe–Cr type steel allows the development of mainly Fe2–xCrxO3 oxides with corundum structure and leads to enhanced oxidation in the presence of water vapour. In situ FEG–ESEM images show the scale microstructural modifications during high temperature exposure, as well as the lateral growth of oxide particles. Together with GIXRD, SEM/EDS and SIMS analysis, FEG–ESEM also allows assessment of the H2O effect on oxidation behaviour during high temperature exposures (600 and 700°C). Water vapour induces either pores or crystallites size increase, favours faceted oxides particles with enhanced density at the highest partial pressure. At this microscopic scale, anisotropic growth of crystallites is observed, and size expansion rates are found to be linear and characteristic of each individual particle. Temperature acts principally on oxide film microstructure. Whatever the environment, homogeneous scale growth is observed at 600°C whereas the steel surface is heterogeneously covered by oxides at 700°C.

Direct Examinations of Oxide Scales upon Cooling: A New Way to Analyse Oxide Scale Spallation

The spallation behaviour of thick alumina scale grown at 1300 degreesC on PM2000 alloy was investigated by recording the events occurring upon cooling from 1300 degreesC down to ambient temperature with CCD cameras. The evolutions of the spalled surface fraction with time and temperature as well as the trajectories of spalled particles were observed and analysed. These observations and analysis were used to determine the critical temperatures drops and incubation times for the onset of spallation as well as to calculate the kinetic energies of spalled oxide particles which is a part of the energy balance describing the interfacial decohesion, the fracture and finally the spallation of oxide scales. In addition to the usual spallation phenomenon resulting in the ejection of small but measurable oxide spalls, localized and sudden outbursts of very fine oxide powders were also observed, showing that oxide scale spallation is likely a phenomenon more complex than usually considered.

Isothermal Oxidation Behaviour of a Hot-Work Tool Steel

Isothermal oxidation behaviour of a hot-work tool steel (X38CrMoV5) was investigated at 600degreesC and 700degreesC in dry and wet air. Growth kinetics were determined by using TGA and oxide scales were characterised by means of SEM (EDS, X-ray mapping) and XRD examinations. Moreover, as the microstructural properties of the studied hot-work steel strongly depend on the carbides precipitates formed during its heat treatment, these carbides were extracted from the X38CrMoV5 matrix and their oxidation behaviour in dry and wet air was also studied. Oxidation behaviour of X38CrMoV5 is very sensitive to the presence of water vapour : a large increase of the scale growth kinetics was observed as soon as the water vapour partial pressure exceeds a value of 9 mbar. Microstructural characterisations showed that scales grown in wet air are porous and sometimes cracked and deformed. They are composed of an external iron-rich oxides scale (hematite alpha-Fe2O3), an internal oxide scale enriched in Cr (spinel oxides (Fe,Cr)(3)O-4) and a narrow zone of internal oxidation. Whereas no significant influence of the water vapour partial pressure has been observed on the oxide scale microstructure (composition, morphology), texture of the superficial hematite scales becomes more pronounced when p(H2O) increases from 9 to 310 mbar. Preferential orientation of alpha-Fe2O3 scales is also favoured by increasing thickness of oxides. On the other hand, the oxidation behaviour of carbide precipitates is rather complex and strongly affected by the presence of water vapour in air.

Short-Term High Temperature Oxidation of Lamellar Cast Iron

The present study is focused on the oxidation behavior, at temperatures above 900 °C, of lamellar cast irons containing various amounts of species commonly present in these alloys (mainly P, S and Si) and less common metallic impurities (Cu and Cr). Oxidation kinetics were investigated from TGA tests performed in flowing air for four hours. The morphology, microstructure and composition of oxide scales were investigated by means of SEM and XRD. It was found that the oxidation kinetics and the microstructure of oxide scales strongly depend on composition and temperature. Slower oxidation kinetics is associated with the growth of oxide scales showing a two-layered microstructure consisting in an outer scale of hematite and magnetite and an inner scale of wustite and fayalite. Faster oxidation kinetics, usually linear, relates to the growth of a single stratified scale primarily composed of hematite, with a small amount of magnetite and a dispersion of silicon-rich precipitates, most likely amorphous silica.

In Situ and Ex Situ Investigation of the Spallation of Thermally Grown Oxides

A review of various experimental techniques, simply implemented or specifically developed to investigate the spallation of thermally grown oxides subject to thermal shocks and cycles, is proposed. This includes thermogravimetric analysis, acoustic emission, interfacial indentation and a newly developed technique based on the real time in situ monitoring of spallation events using CCD cameras. Benefits and drawbacks of each method are discussed. It is shown that the CCD monitoring technique shows enhanced mass resolution and, in addition to the global information derived from the spallation kinetics, reveals straightforwardly the local correlation between spall and substrate micro structures.