Daniel Monceau

Determination of Parabolic Rate Constants from a Local Analysis of Mass-Gain Curves

A method is proposed to allow a more accurateevaluation of thermogravimetric data to identifydiffusion or partial diffusion control of scalingkinetics. This method is based on the fitting ofmass-gain data to a parabola over a short time interval.The translation of the time interval over the entiretest time period provides an actual instantaneousparabolic rate constant independently of any transient stage or simultaneous reaction steps. Theusefulness and limitations of this procedure areillustrated from oxidation tests performed on severalmetallic materials (pure nickel, single-crystalsuperalloys, and NbTi-Al alloy).

Correlations between growth kinetics and microstructure for scales formed by high-temperature oxidation of pure nickel. II. Growth kinetics

The oxidation kinetics of high-purity nickel were studied between 500 and 1200°C, in pure oxygen at atmospheric pressure, for average oxide-scale thicknesses of 1, 5, 10, and 30 μm. In the overall temperature range studied, a decrease in the parabolic rate constant kp with increasing scale thickness was observed. Depending on temperature and oxide-scale thickness, growth kinetics can be interpreted as a mixture of parabolic- and cubic-growth kinetics. Possible correlations between growth kinetics and microstructures of the oxide scales were investigated. From this set of experimental data, oxidation-kinetics models were tested. In particular, the effect of grain-boundary diffusion on NiO-growth kinetics was discussed. The correlations between growth kinetics and oxide microstructures appear to be more complex than usually reported.

Correlations Between Growth Kinetics and Microstructure for Scales Formed by High-Temperature Oxidation of Pure Nickel. I. Morphologies and Microstructures

The morphology and microstructure of NiO scales grown during oxidation of high-purity nickel were investigated. The oxidation tests performed between 450 and 1200°C in pure oxygen at atmospheric pressure were stopped at given scale thicknesses ranging from 1 to 70 μm. The specific specimen preparation and test procedure used in this work led to very good test reproducibility. The morphological and microstructural evolution of NiO scales are described by temperature-thickness maps. The scale morphology can be faceted or cellular. In addition, the growth of NiO platelets was observed in a specific temperature-thickness range. The microstructure of oxide scales can be simplex compact, simplex porous, or duplex. An important effect of substrate orientation was also observed, independent of the test temperature and scale thickness.

Application of a simple statistical spalling model for the analysis of high-temperature, cyclic-oxidation kinetics data

A statistical cyclic-oxidation model is presented. This model gives analytical formulas to assess stabilized metal consumption in cyclic-oxidation experiments. The model is first detailed, then applied to se eral Ni-base superalloys, which form an α-alumina scale during oxidation above 1000C. A new map is introduced in order to compare the cyclic-oxidation beha ior of these alloys.

Investigations on the Diffusion of Oxygen in Nickel at 1000 ° C by SIMS Analysis

High-purity polycrystalline nickel foils have been oxidized at 1000°C in laboratory air before being analyzed in secondary ion mass spectrometry to locally measure the oxygen content in solid solution. The values obtained in metallic grains are surprisingly the same before and after the oxidation treatments (between 5 and 10 atom ppm) and they are much lower than the ones predicted from the literature solubility and diffusion coefficient data at 1000°C. It is shown that this discrepancy could have its origins in the purity level of the samples but also in the exclusive oxygen diffusion in nickel grain boundaries. This last assumption is supported by the occurrence of nickel oxide particles on the walls of voids located in grain boundaries.

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).

Continuous thermogravimetry under cyclic conditions

Thermogravimetry during cyclic oxidation of metallic alloys is described. A methodology is given in order to determine the Net Mass Gain, the Gross Mass Gain, the total mass of spalled oxide, the rate of metal consumption and the average oxide scale thickness as a function of the number of cycles. The fraction of oxide scale which spalls at each cycle can be also calculated, and the parabolic constant can be estimated at each cycle. Two examples are given: the cyclic oxidation of a NiAl single crystal in flowing oxygen at 1150°C, and the cyclic oxidation of alloy P91 at 800°C in laboratory air. Advantages and disadvantages of this technique are discussed in regards to classical interrupted tests in crucibles. Thermogravimetry during cyclic oxidation appears to be a powerful tool in order to model and quantify the cyclic oxidation test which is of great interest in order to qualify the resistance of materials to oxidation in conditions close to their actual use, but a specific aspparatus need to be developed in order to obtain data in an efficient and economical manner. A new apparatus designed for this purpose is described briefly.

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.

Preliminary Results of the Isothermal Oxidation Study of Pt-Al-NiCoCrAlYTa Multi-Layered Coatings Prepared by Sparks Plasma Sintering (SPS)

MCrAlY coatings (where M = Co, Ni or Co/Ni) are widely used on turbine blades and vanes as oxidation and corrosion resistant overlays or as a bond-coating in thermal barrier coatings systems. MCrAlY are usually fabricated by Plasma Spraying, Physical Vapour Deposition, High Velocity Oxy-Fuel spraying or electrolytic techniques. The use of emergent Spark Plasma Sintering technique as a preparation method for NiCoCrAlYTa coatings has been presented previously [1]. SPS technique allows fast development of new coatings with a one-step fabrication of multilayered coatings. This work presents first results of the long term isothermal oxidation behaviour of Pt-Ni aluminide/NiCoCrAlYTa multi layered coatings. The obtained coating is dense and homogeneous. Isothermal oxidation up to 500 h at 1100°C leads to the formation of an adherent alumina scale with Y-rich precipitates and deep intergranular oxidation.