Papers by Author: G. Bonnet

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Authors: S.Y. Brou, G. Bonnet, Jean Luc Grosseau-Poussard
Abstract: TiAl, TA6V and titanium coupons were treated by a phosphoric acid solution by dipping. They were then submitted to a heat treatment and, in the case of TiAl and TA6V, to high temperature discontinuous oxidation under laboratory air. The H3PO4 treatment allowed to decrease the mass gains for TiAl and TA6V, in particular during the first 100 hours. XRD analyses demonstrated for the three substrates the formation of a pyrophosphate layer during the heating period. This pyrophosphate evolved towards TiO2 with oxidation time increase, quicker for smaller aluminium content (or higher titanium content) in the metallic substrate. The decrease of mass gains was attributed to a diffusion barrier effect of the pyrophosphate layer as long as it was present.
Authors: Jean Michel Brossard, Anne Marie Huntz, J. Balmain, G. Bonnet
Authors: F. Pedraza, Baptiste Bouchaud, J. Balmain, G. Bonnet, Justine Menuey
Abstract: 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.
Authors: F. Pedraza, Baptiste Bouchaud, J. Balmain, G. Bonnet, Justine Menuey
Abstract: Cathodic electrodeposition was used to generate a rare earth (RE)-containing deposit on a single crystal Ni-based superalloy. The deposition parameters were optimised in order to get a RE oxy-hydroxide coating with a “well-fitted” dry-mud like morphology, i.e. presenting a multi-cracks network. A further thermal treatment was applied to dehydrate the deposit to obtain a well crystallised oxide coating (RExOy). The uncoated and RExOy-coated substrates were then submitted to cyclic oxidation tests at 1100°C in laboratory air. They demonstrated the efficiency of the coating as uncoated samples severely spalled after a few cycles whereas the coated ones did not lose their protective oxide layer even after 2000 cycles. This result was attributed to the formation of a duplex oxide scale very similar to that obtained on g/g’ coatings, to the presence of nanograins at the RExOy/scale interface and to the Hf-rich oxide pegs at the scale/substrate interface.
Authors: F. Pedraza, Jean Luc Grosseau-Poussard, J.F. Dinhut, J. Balmain, G. Bonnet
Abstract: Nitriding by low energy high flux processing has been carried out at about 400°C in fcc metal substrates (pure Ni, Ni-20Cr model alloy and a conventional AISI 304L stainless steel). The gradual ingress of this element into the structures will be shown to depend markedly on the chemical composition of the substrate. The associated expansion of the fcc lattices and surface roughness will be discussed in this work with the support of X-ray diffraction, atomic force, scanning and transmission electron microscopy techniques. In light of the resulting composition, microstructures and thickness of nitrided layers, some preliminary results of their behaviour under isothermal oxidation conditions at high temperatures will be discussed. The high temperatures will provoke decomposition of the expanded austenite into a conventional gamma phase and some chromium nitrides. Trapping of chromium therefore shall explain a reduction of the high temperature oxidation resistance against the untreated substrates.
Authors: Mbaihoudou Kemdehoundja, Jean Luc Grosseau-Poussard, J.F. Dinhut, G. Bonnet
Abstract: Using Raman microprobe spectroscopy made it possible to study the buckling phenomenon in chromia films grown at 900°C in air from a Ni30at%Cr alloy. Blisters have been optically observed to be circular and, from the top view, the mean radius has been measured with an accuracy of about 1%m. An autofocus device allows the characterisation of the profile of each blister and the shift of the A1g Raman peak of chromia gave the local stress far from the blister and all along the buckled zone. From these observations, the induced spalling has been related to the blister morphology.
Authors: Marcia Silva, Sébastien Chevalier, G. Bonnet, J.C. Colson
Authors: S.Y. Brou, G. Bonnet, Jean Luc Grosseau-Poussard
Abstract: Gamma-TiAl samples were treated by phosphoric acid solutions at different concentrations. With 15 mol/dm3, a viscous deposit was left on the surface leading, after drying and high temperature oxidation, to a very heterogeneous surface aspect. Concentrations below 0.5 mol/dm3 were then used and allowed to greatly ameliorate the homogeneity of the superficial layer. After heating from ambient to oxidation temperature, however, cracks were always observed, due to the departure of gaseous molecules. Isothermal oxidation tests come out at 800°C and 900°C under reconstituted air showed that weight gains were strongly reduced when TiAl had been treated. A compound containing titanium, oxygen and phosphorus was detected at the end of the heating period, identified as titanium pyrophosphate, TiP2O7. This compound remained the only one detected for 100 h when oxidation was come out at 800°C, but evolved towards TiO2 when oxidation time was increased. In the case of oxidation at 900°C, the evolution from TiP2O7 to TiO2 also happened but appeared to go faster.
Authors: G. Bonnet, M. Mollard, B. Rannou, J. Balmain, F. Pedraza, X. Montero, M. Galetz, Michael Schütze
Abstract: Novel, unconventional type of high temperature coating systems can be elaborated by depositing Al micro-particles on nickel base substrates, using an appropriate binder, and converting them into a thermal barrier type coating by a two-step heat treatment under argon. Final result is a coating structure consisting of a quasi-foam top coat, constituted by spherical hollow alumina particles, surmounting a β-NiAl diffusion layer able to form during high-temperature oxidation a protective alumina scale. In this work, pure nickel was employed as a model material to evaluate the effects of moderate temperatures (550-700°C), dwelling times and Al particle size on the final characteristics of the coatings. Almost no diffusion occurred below 600°C. In contrast, a Ni2Al3 layer very quickly formed at 650 or 700°C. The rapidity of coating formation was attributed to the appearance of a liquid phase at the coating/substrate interface. The increase of dwelling time did not provide any significant thickness increase as the Al particles got practically emptied after 2h. In addition, the use of different micro-sized particles resulted in similar Al diffusion coatings under the investigated conditions.
Authors: Sébastien Chevalier, G. Bonnet, Günter Borchardt, J.C. Colson, Jean Pierre Larpin
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