High-temperature oxidation resistance of chromium-based coatings deposited by DLI-MOCVD for enhanced protection of the inner surface of long tubes

For nuclear safety issues, there is an international effort to develop innovative “Enhanced Accident Tolerant Fuels” (EATF) materials. EATF cladding tubes are of particular interest because they constitute the first barrier against radioactive fission species dispersal in case of accidental scenario such as LOCA (LOss of Coolant Accident). Actual nuclear fuel claddings are made from Zr-based alloys and to increase safety margins, both mechanical strength and resistance to high-temperature oxidation have to be improved. Several alternatives using high-temperature oxidation resistant coatings for outer-wall protection have been proposed worldwide but there is currently no solution for the inner-wall protection. In order to resist to high temperature steam environment upon LOCA transients, internal Cr-based coatings deposited by DLI-MOCVD (Direct Liquid Injection of MetalOrganic precursors) were investigated. These hard metallurgical coatings could also be used in high-temperature corrosive environments as those encountered in aeronautics and other industries to protect 3D complex components. Thanks to a suitable chemistry of the liquid Cr precursor, bis(ethylbenzene)chromium, different coatings were deposited including: metal Cr, chromium carbides CrxCy and mixed carbides CrxSizCy. The high-temperature behavior of these Cr-based coatings under oxidizing atmospheres has been studied using several techniques and various oxidation tests including pure steam environment followed by water quenching down to room temperature to be representative of LOCA situations. Amorphous CrxCy coatings showed the most promising properties. For instance compared to uncoated substrate, they shift the catastrophic oxidation towards higher temperatures and delay the complete oxidation of the substrate at 1473K of >2h. The results are discussed in terms of oxidation mechanisms and protection of the fuel claddings inner surface deduced from fine characterizations of the samples before and after oxidation tests

Emerging processes for metallurgical coatings and thin films

Innovation in thin-film deposition processes, thermal spraying and cladding technologies mostly rely on evolutions of their previous iteration. Along with other examples, five case studies of emerging elaboration processes for metallurgical coatings are described coupled with their applications. In the frame of the lifetime extension of components exposed to aggressive media or their functionalization, this article depicts all the developments of the detailed processes. Physical vapor deposition (PVD) of coatings with exceptional properties is possible thanks to sources generating highly ionized metallic vapors. The control of the average energy per incident species and particularly metallic ions strongly influences the characteristics of the deposited layer obtained, for example, with HiPIMS (High Power Impulse Magnetron Sputtering). While PVD techniques are mainly directive regarding the growth of the coating, chemical vapor deposition (CVD) processes manage to homogeneously coat complex 3D shapes. The use of specific precursors in DLI–MOCVD (Direct Liquid Injection – MetalOrganic CVD), carefully selected from the whole metalorganic chemistry, allows one to efficiently treat heat-sensitive substrates and broadens their application range. The third detailed example of emerging technology is suspension plasma spraying (SPS). Projection of various solutions containing nanoparticles leads to the growth of unusual morphologies and microstructures and to the generation of porous coatings with multi-scaled porosity. On the other hand, cold-spray uses metallic powders with higher granulometry and does not modify them during the deposition process. As a result, high-purity and dense materials are deposited with properties similar to those of wrought materials. Whereas cold-spray is suitable only for ductile metals, laser cladding can be applied to ceramics, polymers and of course metals. Laser cladding is a key technology for advanced metallurgical engineering and alloy development due to its capability for functionally graded materials production and combinatorial synthesis

Optimization of nanostructured oxide-based powders by surface modification

Ce travail a été consacré dune part à létude de l\u27effet de la dispersion dans une alumine de transition commerciale et dautre part à létude des propriétés mécaniques à basse et haute température des nanocomposites AlO-5 vol. % YAG. 23 Pour étudier leffet de la dispersion dans lalumine de transition, différentes techniques de caractérisation appartenant à la physico-chimie des surfaces et à la science des matériaux ont été utilisées comme lanalyse thermogravimétrique et lanalyse thermique différentielle (ATD-TG), la diffraction des rayons X (DRX), ladsorption dazote (B.E.T.), la microscopie électronique en transmission (MET) et linfrarouge à transformée de Fourier (IR-TF). En particulier, les alumines de transition présentent des phases métastables qui subissent des transformations pendant le frittage et provoquent la formation dune structure vermiculaire avec de larges porosités. La densité finale et la microstructure ont été améliorées grâce à une dispersion efficace de la poudre initialement agglomérée qui permet un réarrangement des particules et facilite la transformation vers la phase alpha. Létude de linfluence de la dispersion sur la cinétique de transformation (Méthode de Kissinger) et la cinétique de frittage (Méthode SID) a été développée. Dans la deuxième partie de la thèse, le travail a été centré sur la production des nanocomposites AlO-5 vol. % YAG à partir de deux alumines commerciales frittées naturellement et par des méthodes non-conventionnelles comme le pressage à chaud (HP) et le spark plasma sintering (SPS). La caractérisation mécanique à température ambiante (dureté, ténacité, module délasticité) a été corrélée à une étude microstructurale (ESEM). Des valeurs intéressantes de dureté et de ténacité ont été mesurées dans les échantillons frittés par SPS et HP, environ 20 GPa et 7 MPa.m, respectivement. 231/2 Pour la caractérisation à haute température, les essais de fluage ont été conduits sous air en flexion 4 points à 1200°C sous une contrainte de 100 MPa. Les résultats montrent que les propriétés mécaniques à haute température dépendent fortement de la répartition de la deuxième phase dans la matrice dalumine. Dans tous les cas, les résultats obtenus sont intéressants

Optimization of nanostructured oxide-based powders by surface modification

This thesis is divided into two parts. The first part deals with the study of surface modification of a transition alumina nanopowder and the second part is devoted to the production and mechanical characterization at room- and high-temperature of Alumina 5 vol. % YAG nanocomposites. In order to study, the effect of the dispersion on a transition alumina several physico-chemical characterization techniques were employed such as the DTA-TG analysis, XRD, BET Specific Surface Area, HR-TEM and the FT-IR spectroscopy. In particular, the transition aluminas present metastable phases which suffer transformations during sintering and induce the formation of a vermicular microstructure, consisting of a network of large pores. As a consequence, the final density and the microstructure have been improved thanks to the dispersion which allowed to achieved higher densities and promotes the transformation into alpha-phase. Moreover , the influence of the dispersion on the transformation kinetics (Kissinger Method), as well as, on the sintering kinetics (SID Method) has been evaluated. In the second part, it is presented the development of Alumina/YAG nanocomposites from two commercial nanopowders naturally sintered and sintered by non-conventional methods, such as HP and SPS. The mechanical characterization at room temperature (Hardness, Toughness, Elastic modulus) has been correlated to a microstructural study (ESEM). Interesting values regarding hardness and toughness have been measured in samples sintering by SPS and HP, around 20GPA and 7 MPA.m1/2, respectively. For the characterization at the high temperature, creep tests were carried in a 4-point bending fixture at 1200C and an applied stress of 100 Mpa. The results show that mechenical properties depend on the second phase distribution into the alumina matrix. In all cases, the obtains results were interestingVILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceItalyFRI

Procédés d'insertion de dopants dans poudre industrielle INCO718 = ODS (Oxide Dispersion Strengthened,) pour fabrication LPBF (SLM) pour des applications nucléaires

International audienceLes alliages à base de Ni sont des matériaux prometteurs pour le nucléaire en raison de leur excellente résistance mécanique sous haute température et de leur bon comportement dans des environnements corrosifs ou oxydants. Cependant, en raison de la complexité de ses composants, les techniques de traitements conventionnelles sont limitées. De nouvelles technologies, élaboration à base de poudre métalliques, permettent d'élargir les possibilités de conception, de s'affranchir des problèmes de soudabilité, en particulier la fusion laser en lit de poudre (LPBF). Les performances mécaniques remarquables de l'Inconel 718 sont obtenues par le renforcement de la phase de matrice γ en solution solide, la précipitation γ' et γ''. Cependant l'ajout d'oxyde peuvent permettre d'augmenter les performances des matériaux. Dans cette étude nous nous sommes intéressés à l'influence de l'ajout d'Y2O3 sur la microstructure de l'Inconel 718. L'intégration de particules d'oxyde nanométriques dans une matrice relativement ductile n'est pas facile à réaliser. Afin de résoudre ce problème, divers procédés d'insertion ont été essayés, notamment le procédé MA (mécanosynthèse), le mélange par turbula et le dépôt d'oxyde par PVD. De plus, l'ajout d'oxyde nécessite une adaptation des paramètres du procédé LPBF. Les principaux paramètres tels que la vitesse du processus, la puissance du laser, la distance de hachage et les stratégies de balayage sont pris en compte pour chaque processus d'insertion d'oxyde. Enfin, les macro et microstructures sont analysées et des conclusions sont données sur les potentialités et les limites de chaque condition de fabrication

Creep behaviour of alumina/YAG composites prepared by different sintering routes

Al2O3-5 vol.% Y3Al5O 12 (YAG) composite powders have been prepared by surface doping of α-alumina powders by an yttrium chloride aqueous solution. Two commercial, one submicron-sized, the other ultra-fine, alumina powders were compared as matrix materials. YAG phase was yielded by an in situ reaction promoted by the subsequent thermal treatment of the doped powders. In particular, a flash soaking into a tubular furnace kept at a fixed temperature in the range 1050-1150 °C was employed, for inducing the crystallization of yttrium-aluminates on the alumina particles surface, but avoiding a relevant crystallites growth. After that, aqueous suspensions of the calcined powders were dispersed by ball-milling and cast into porous moulds or simply dried in a oven. Slip cast green bodies were densified by pressure-less sintering, while powdered samples were consolidated by hot pressing or spark plasma sintering. The low- and high-temperature mechanical performances of the sintered materials were investigated and related to monolithic aluminas behaviour as well as to the composites microstructures. It is shown that the hot-pressed and spark plasma sintered composites present a significantly lower creep rate as compared to reference, monolithic alumina sample