No description available.

Les carburants BtL (Biomass to Liquid) font partie des carburants alternatifs au pétrole dits de seconde génération car synthétisés à partir de biomasse solide (contenant de la lignocellulose). Le procédé de fabrication de tels carburants par voie thermochimique repose sur deux étapes successives : la gazéification de la biomasse suivie par un procédé de Fisher-Tropsch. Certains éléments contenus dans la biomasse comme les sels alcalins peuvent se retrouver après l’étape de gazéification et être à l’origine d’attaques sous forme de sels fondus et ainsi endommager les infrastructures. Le travail réalisé dans ce manuscrit consiste à simuler la corrosion de parois d’échangeur thermique en présence de sulfate et chlorure de sodium sous atmosphère de gazéification (CO/H2/CO2) très faiblement oxydante (~ 10−18 bar). Afin de comparer les résultats à ceux issus de la bibliographie, les essais ont également été conduits sous atmosphère fortement oxydante (Ar/O2). Les essais réalisés sur alliage chrominoformeur, HR-120 (38Ni-34Fe-25Cr) à une température de 900°C en présence de sulfate de sodium en milieu faiblement oxydant ont mise en évidence une corrosion de type catastrophique localisée et réversible de l’alliage. Le comportement de l’alliage aluminoformeur, 214 (76Ni-16Cr-4Al) apparaît plus protecteur dans des conditions similaires. En présence de chlorure de sodium, les deux alliages se comportent d’une manière totalement identique : corrosion catastrophique en milieu fortement oxydant et impact du sel négligeable sous atmosphère faiblement oxydante. Un chapitre remède prometteur a été développé en fin de manuscrit. / BTL (biomass to liquid) is an innovative process to synthesize second generation bio-gasoline from wood and farm residues. This process includes a gasification step in order to generate the synthetic gas (syngas) which is subsequently transformed into gas oil by the Fisher Tropsch process. Alkaline salts (mainly potassium and sodium sulphates and chlorides) are present in the biomass and can induce detrimental high temperature corrosion of the refractory alloys where they can condensate as a liquid phase. In this work, we simulate high temperature corrosion of heat exchanger tubes in presence of sulphate and sodium chloride under gasification environment (CO/H2/CO2), consider as weakly oxidizing (~ 10−18 bar). To compare the results with those from the literature, tests were also conducted under highly oxidizing atmosphere (Ar/O2). Results with the chromia-forming alloy HR-120 (38Ni-34Fe-25Cr) at 900°C with sodium sulfate under low oxygen partial pressure shows reversible localized catastrophic oxidation. The behavior of the alumina-forming alloy 214 (76Ni-16Cr-4Al) appears more protective under similar conditions. In presence of sodium chloride, the behavior of the two alloys is identical. Indeed, with oxidizing atmosphere corrosion became totally catastrophic while the impact of sodium chloride was insignificant under gasification atmosphere.

No description available.

Thermal barrier coatings (TBC) are used in the hot sections of gas turbine engine in order to insulate the substrate at high temperature. Molten salt infiltration retards the durability of TBCs. The current standard material, i.e. 8YSZ is susceptible to molten salt infiltration. Therefore, alternate TBC materials are desirable. In addition to material composition, the TBC microstructure plays an important role in mitigating molten salt infiltration. Therefore, in this work, three different TBC variations were investigated. The first variation was a columnar microstructured 48YSZ TBC processed by SPS (48YSZ-SPS). The second variation was a columnar microstructured 8YSZ TBC processed by SPS (8YSZ-SPS), and the third variation was a lamellar microstructured 8YSZ TBC deposited by APS (8YSZ-APS). The as-sprayed TBC specimens were characterized by SEM/EDS, porosity analysis and XRD measurements. Later, the TBC specimens were exposed to hot corrosion test and their interaction with the molten salts were investigated using SEM (EDS and XRD). It was shown that an increase in stabilizer content (yttria content) in zirconia (in the case of 48YSZ) leads to an improved hot corrosion resistance due to the adequate amount of yttria content, which restricts the molten salt infiltration by forming needle like YVO4 phase. In terms of microstructure comparison, the infiltration behavior was similar for columnar microstructured 8YSZ and lamellar microstructured 8YSZ-APS as the molten salts infiltrated the coatings completely compared to the 48YSZ TBC. Furthermore, it seems that the molten salt infiltrates the TBC through globular pores, delamination cracks and splat boundaries in the case of APS-TBCs whereas the column gaps favor easier infiltration of molten salts in the case of columnar microstructured SPS processed TBCs.

La sélection des matériaux utilisés dans les moteurs aéronautiques est un enjeu majeur pour assurer la sécurité des passagers, optimiser les performances de l’avion et maîtriser les coûts. Dans les parties les plus chaudes des moteurs (i.e. chambre de combustion et turbine), les pièces sont généralement constituées de superalliages à base nickel en raison de leurs excellentes propriétés mécaniques à haute température. Vulnérables aux phénomènes de corrosion et d’oxydation à haute température, les superalliages doivent la plupart du temps être revêtus afin de prolonger leur durée de vie (ingénierie de surface). La composition chimique et l’architecture des revêtements sont alors adaptées en fonction du régime de température et des phénomènes de dégradation rencontrés (i.e. corrosion à chaud, oxydation et/ou érosion). En vue de répondre aux nouvelles réglementations environnementales, de nouvelles voies de synthèse et de fonctionnalisation sont à l’étude comme alternatives aux procédés industriels actuels. Dans le cadre du projet Européen « PARTICOAT », le LaSIE a démontré la faisabilité d’élaborer des systèmes barrières thermiques complets (couche de diffusion + barrière thermique) en une seule étape à partir de barbotines (« slurries ») à base aqueuse contenant des microparticules d’Al. Dans cette étude, l’ajout de Cr comme dopant a été étudié. L’addition de Cr a permis d’abaisser l’activité de l’Al lors de l’étape d’aluminisation et de limiter les réactions exothermiques entre Al et substrat à base de nickel. L’optimisation des ratios entre Al et Cr a permis d’obtenir différentes microstructures de revêtement. Diverses architectures de dépôts ont également pu être testées grâce à la souplesse d’élaboration des revêtements par barbotines. L’influence de l’atmosphère (Ar, air) et celle des conditions de traitement thermique ont également été étudiées. Enfin, la durabilité des revêtements développés au cours de la thèse a été évaluée au cours d’essais de corrosion à chaud et d’oxydation. / The selection of materials is of utmost importance in gas turbine engines to ensure the security of the passengers, optimize the performances of the aircraft and be cost efficient. In the hottest region of the engines (i.e. combustion chamber and turbine), the components are usually made of nickel-based superalloys. These materials can resist to high mechanical loads at high temperature but are vulnerable to aggressive environments. Therefore, nickel-based superalloys are usually coated to increase their durability in the engine (surface engineering). The chemical composition and the coating architecture are carefully adjusted depending on the temperature regime and the mechanisms of degradation encountered (hot corrosion, oxidation and/or erosion). New synthesis routes and functionalization are currently developed as alternative solutions to industrial processes. As a promising alternative approach, different studies were carried out in the LaSIE laboratory under the European project “PARTICOAT” and confirmed the possibility to elaborate complete thermal barrier systems (diffusion coating + thermal barrier coating) from Al-containing water-based slurries. In this work, the role of Cr as a doping agent was investigated. The addition of Cr decreased the thermodynamic activity of Al upon aluminizing and limited the exothermic reactions usually reported between Al and nickel-based materials. Different architectures of coatings were obtained thanks to the flexibility and the adaptability of the slurry coating process. The gas composition (Ar, air) and the heat treatment conditions were also investigated. Finally, the high temperature resistance of the slurry coatings developed during this work was evaluated under hot corrosion and oxidation conditions.

科学研究費補助金 研究種目:基盤研究(C)(2) 課題番号:08650815 研究代表者:村田 純教 研究期間:1996-1997 年度

The development of thermal barrier coatings (TBCs) has been undoubtedly the most critical advancement in materials technology for modern gas turbine engines. TBCs are widely used in gas turbine engines for both power-generation and propulsion applications. Metallic oxidation-resistant coatings (ORCs) are also widely employed as a stand-alone protective coating or bond coat for TBCs in many high-temperature applications. Among the widely studied durability issues in these high-temperature protective coatings, one critical challenge that received greater attention in recent years is their resistance to high-temperature degradation due to corrosive deposits arising from fuel impurities and CMAS (calcium-magnesium-alumino-silicate) sand deposits from air ingestion. The presence of vanadium, sulfur, phosphorus, sodium and calcium impurities in alternative fuels warrants a clear understanding of high-temperature materials degradation for the development of fuel-flexible gas turbine engines. Degradation due to CMAS is a critical problem for gas turbine components operating in a dust-laden environment. In this study, high-temperature degradation due to aggressive deposits such as V2O5, P2O5, Na2SO4, NaVO3, CaSO4 and a laboratory-synthesized CMAS sand for free-standing air plasma sprayed (APS) yttria stabilized zirconia (YSZ), the topcoat of the TBC system, and APS CoNiCrAlY, the bond coat of the TBC system or a stand-alone ORC, is examined. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. This study demonstrated that the V2O5 melt degrades the APS YSZ through the formation of ZrV2O7 and YVO4 at temperatures below 747°C and above 747°C, respectively. Formation of YVO4 leads to the depletion of the Y2O3 stabilizer and the deleterious transformation of the YSZ to the monoclinic ZrO2 phase. The investigation on the YSZ degradation by Na2SO4 and a Na2SO4 + V2O5 mixture (50-50 mol. %) demonstrated that Na2SO4 itself did not degrade the YSZ, however, in the presence of V2O5, Na2SO4 formed vanadates such as NaVO3 that degraded the YSZ through YVO4 formation at temperature as low as 700°C. The APS YSZ was found to react with the P2O5 melt by forming ZrP2O7 at all temperatures. This interaction led to the depletion of ZrO2 in the YSZ (i.e., enrichment of Y2O3 in t' -YSZ) and promoted the formation of the fluorite-cubic ZrO2 phase. Above 1250°C, CMAS deposits were observed to readily infiltrate and significantly dissolve the YSZ coating via thermochemical interactions. Upon cooling, zirconia reprecipitated with a spherical morphology and a composition that depended on the local melt chemistry. The molten CMAS attack destabilized the YSZ through the detrimental phase transformation (t - > t - > f + m). Free standing APS CoNiCrAlY was also prone to degradation by corrosive molten deposits. The V2O5 melt degraded the APS CoNiCrAlY through various reactions involving acidic dissolution of the protective oxide scale, which yielded substitutional-solid solution vanadates such as (Co,Ni)3(VO4)2 and (Cr,Al)VO4. The molten P2O5, on the other hand, was found to consume the bond coat constituents significantly via reactions that formed both Ni/Co rich phosphates and Cr/Al rich phosphates. Sulfate deposits such as Na2SO4, when tested in encapsulation, damaged the CoNiCrAlY by Type I acidic fluxing hot corrosion mechanisms at 1000°C that resulted in accelerated oxidation and sulfidation. The formation of a protective continuous Al2O3 oxide scale by preoxidation treatment significantly delayed the hot corrosion of CoNiCrAlY by sulfates. However, CoNiCrAlY in both as-sprayed and preoxidized condition suffered a significant damage by CaSO4 deposits via a basic fluxing mechanism that yielded CaCrO4 and CaAl2O4. The CMAS melt also dissolved the protective Al2O3 oxide scale developed on CoNiCrAlY by forming anorthite platelets and spinel oxides. Based on the detailed investigation on degradation of the APS YSZ and CoNiCrAlY by various corrosive deposits, an experimental attempt was carried out to mitigate the melt-induced deposit attack. Experimental results from this study demonstrate, for the first time, that an oxide overlay produced by electrophoretic deposition (EPD) can effectively perform as an environmental barrier overlay for APS TBCs. The EPD protective overlay has a uniform and easily-controllable thickness, uniformly distributed closed pores and tailored chemistry. The EPD Al2O3 and MgO overlays were successful in protecting the APS YSZ TBCs against CMAS attack and hot corrosion attack (e.g., sulfate and vanadate), respectively. Furnace thermal cyclic oxidation testing of overlay-modified TBCs on bond-coated superalloy also demonstrated the good adhesive durability of the EPD Al2O3 overlay.