Overview of CMC activities: From high temperature characterization to applications

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Elaboration par voie sol-gel de nouvelles barrières thermiques architecturées présentant des propriétés contre l'infiltration des oxydes CMAS (Ca, Mg, Al, Si) - Etude de la réparabilité de systèmes endommagés

Le développement des systèmes " barrière thermique " (BT) a permis d'accroître la température de fonctionnement des turbomachines, contribuant ainsi à l'augmentation des performances de ces dernières. Néanmoins, ce gain a été contrebalancé par l'apparition de phénomènes de corrosion par des contaminants environnementaux de type CMAS (Calcium, Magnésium, Aluminium et Silicium). L'évaluation de la dégradation des BT et la mise en place de solutions de protection font l'objet du premier axe de recherche présenté dans ce manuscrit. L'interaction entre un CMAS modèle et la zircone yttriée a tout d'abord été étudiée afin de mieux appréhender les phénomènes qui la gouvernent. Par la suite, des matériaux " anti-CMAS " ont été synthétisés par voie sol-gel et leur comportement vis-à-vis du composé silicaté a été évalué. Un effort particulier a été porté sur l'étude de l'oxyde d'yttrium Y2O3. En effet, il a été montré au cours de ces travaux que ce matériau présente une cinétique d'interaction rapide avec le CMAS et conduit à la formation de phases secondaires de type apatite. De plus, il a été mis en évidence que l'élaboration d'une couche sacrificielle à base d'yttrine à la surface d'une BT élaborée par EBPVD, favorise le piégeage des CMAS et permet ainsi d'atténuer la dégradation microstructurale de la barrière thermique sous-jacente. Le second volet de ces travaux de thèse avait quant à lui pour objectif de développer une méthode de réparation locale de barrières thermiques EBPVD endommagées. La technique de dépôt par électrophorèse a été proposée car elle présente l'avantage de pouvoir déposer sélectivement de la matière dans les zones où le revêtement céramique a été dégradé. A l'issue d'une étude paramétrique, les conditions opératoires optimisées ont été appliquées sur des pièces présentant des défauts calibrés. Les résultats obtenus ont mis en évidence la forte potentialité de ce procédé pour répondre à cette problématique. Les dépôts électrophorétiques de zircone yttriée permettent en effet de restaurer la couche céramique dans les zones endommagées sans qu'il n'ait été préalablement nécessaire d'éliminer la barrière thermique EBPVD. Les revêtements mis en forme sont homogènes et uniformes en termes d'épaisseur et de microstructure. A la lumière de ces résultats, il est donc possible d'envisager l'utilisation de l'électrophorèse pour prolonger la durée de vie de pièces partiellement détériorées, mais également pour l'élaboration de barrières thermiques sur pièces complexes.Thermal Barrier Coatings (TBC) have allowed gas-turbine engines to operate at higher gas temperatures leading to a great enhancement in efficiency and performances. However, in the aim to increase the turbine inlet temperature, a new issue has appeared: corrosion by molten Calcium-Magnesium-Alumino Silicates, also known as CMAS. Therefore, the first part of the thesis focus on (1) the degradation of yttria-stabilized zirconia (YSZ)-based TBC systems by CMAS and (2) different materials to protect them. First, the interaction between a synthetic CMAS and yttria-stabilized zirconia (YSZ) has been studied. It has been found that the zirconia in the YSZ dissolved in CMAS and re-precipitate in the YSZ during the corrosion process. Densification of YSZ has also been found. After better understanding of the degradation, different CMAS-resistant materials were synthesized via sol-gel route and their anti-corrosion behaviors have been evaluated. Among different CMAS-resistant materials, yttrium oxide (Y2O3) presents a promising performance since yttria exhibits a fast interaction with CMAS and leads to the formation of secondary apatite phase. In this case, the yttrium oxide coating acts as a sacrificial layer which traps CMAS and helps to mitigate the deterioration of the underneath TBC microstructure. The second part of this research work aims to develop a new repairing method for damaged EBPVD TBC. The electrophoretic deposition (EPD) has been proposed because it has the benefit of selectively deposit material where the ceramic coating has been degraded. Following a parametric study, the optimized operating conditions were applied to pre-damaged reference samples. The results have demonstrated the high potential of this method to tackle this issue. The electrophoretic deposition of yttria-stabilized zirconia allows to restore the ceramic layer in the damaged areas without prior removing the TBC. Coatings shaped by EPD exhibit homogeneous and uniform thickness and microstructure. In light of these results, it is possible to consider the use of electrophoretic deposition to extend lifetime of partially damaged TBC, but also to develop new thermal barrier coatings on complex parts

Step-by-step investigation of degradation mechanisms induced by CMAS attack on YSZ materials for TBC applications

Over the past decades, Thermal Barrier Coatings (TBCs) have become essential parts in gas turbine engines. In working conditions, TBCs are subject to many kinds of degradation (erosion, foreign object damage (F.O.D), oxidation, etc.) which deteriorate integrity and mechanical properties of thewhole system.Moreover, with the aim to increase the turbine inlet temperature, a new type of damage has been highlighted: corrosion by molten Calcium–Magnesium–Alumino Silicates, better known as CMAS. In this paper, interactions between yttriastabilized zirconia (YSZ) materials synthesized via sol–gel process and synthetic CMAS powder were investigated via a step-by-step methodology. The approach was conducted starting from the more severe conditions of interactions and then gradually gets closer to the interactions taking place in service. It was proved that CMAS can induce faster densification of the ceramic leading to a loss of strain tolerance of the protective coating. Besides, a dissolution/re-precipitationmechanismcan also take place between YSZ andCMAS leading to the transformation of the initial tetragonal yttria-stabilized zirconia into globular particles of monoclinic zirconia. CMAS were also found to infiltrate the entire thickness of both EBPVD and sol–gel YSZ coatings at 1250 °C for 1 h. Nevertheless, the original non-oriented microstructure provided by sol–gel route leads to a different way of interaction due to the high reactivity of sol–gel precursors and materials. The behaviors of EBPVD and sol–gel coatings under CMAS exposure are discussed in this paper

Terminating BKZ

Strong lattice reduction is the key element for most attacks against lattice-based cryptosystems. Between the strongest but impractical HKZ reduction and the weak but fast LLL reduction, there have been several attempts to find efficient trade-offs. Among them, the BKZ algorithm introduced by Schnorr and Euchner [FCT\u2791] seems to achieve the best time/quality compromise in practice. However, no reasonable complexity upper bound is known for BKZ, and Gama and Nguyen [Eurocrypt\u2708] observed experimentally that its practical runtime seems to grow exponentially with the lattice dimension. In this work, we show that BKZ can be terminated long before its completion, while still providing bases of excellent quality. More precisely, we show that if given as inputs a basis $(b_i)_{i\leq n} \in Q^{n \times n}$ of a lattice L and a block-size $\beta$, and if terminated after $\Omega\left(\frac{n^3}{\beta^2}(\log n + \log \log \max_i \|\vec{b}_i\|)\right)$ calls to a $\beta$-dimensional HKZ-reduction (or SVP) subroutine, then BKZ returns a basis whose first vector has norm $\leq 2 \gamma_{\beta}^{\frac{n-1}{2(\beta-1)}+\frac{3}{2}} \cdot (\det L)^{\frac{1}{n}}$, where~$\gamma_{\beta} \leq \beta$ is the maximum of Hermite\u27s constants in dimensions $\leq \beta$. To obtain this result, we develop a completely new elementary technique based on discrete-time affine dynamical systems, which could lead to the design of improved lattice reduction algorithms

Advances in the deposition of ceramics by soft chemistry process : example of rare- earth silicate coatings

The dip-coating process consists in immersing a sample to be coated in the liquid medium and then removing it at a controlled speed in order to obtain a film of regular thickness, as shown in Figure 1a). Dip-coating technique is now used in many industrial fields (biomedical, transportation, optics…). It is a very simple, and easy process to implement for the deposition and shaping of different natures of coatings (ceramic, metallic and polymer). In the case of ceramic coatings, after the dip-coating operation, the layers undergo a sintering post-treatment leading to the consolidation and/or the densification of the deposit. The corresponding mechanisms need a rigorous control of many parameters. The parameters involved in the dip-coating process are related to the medium and to the process. Concerning the medium, the dispersion medium nature, the particles concentration, viscosity, and stability are the main ones. The stability of the suspension is a first-order parameter and a preliminary formulation work has been carried out to cope with it. Moreover, parameters relative to the fabrication process such as the number of layers and the thermal profile (intermediary and final temperatures), will also be key factors to be taken into account in the formation of homogeneous and reproducible coatings by dip-coating.This work highlights the influence of these various parameters in the case of rare earth silicates based coatings. The various experiments were carried out in correlation to the coatings quality and microstructure. Homogeneous and conformal ceramic coatings of few tens of micrometers thick, as shown in Figure 1b), were obtained. A multi-layers deposit in a sol loaded at 40% mass generally allows to reach the desired thickness. With these experiments relationship between dip-coating parameters and coatings microstructure and morphology can be established. Please click Additional Files below to see the full abstract

PeroxisomeDB: a database for the peroxisomal proteome, functional genomics and disease

Peroxisomes are essential organelles of eukaryotic origin, ubiquitously distributed in cells and organisms, playing key roles in lipid and antioxidant metabolism. Loss or malfunction of peroxisomes causes more than 20 fatal inherited conditions. We have created a peroxisomal database () that includes the complete peroxisomal proteome of Homo sapiens and Saccharomyces cerevisiae, by gathering, updating and integrating the available genetic and functional information on peroxisomal genes. PeroxisomeDB is structured in interrelated sections ‘Genes’, ‘Functions’, ‘Metabolic pathways’ and ‘Diseases’, that include hyperlinks to selected features of NCBI, ENSEMBL and UCSC databases. We have designed graphical depictions of the main peroxisomal metabolic routes and have included updated flow charts for diagnosis. Precomputed BLAST, PSI-BLAST, multiple sequence alignment (MUSCLE) and phylogenetic trees are provided to assist in direct multispecies comparison to study evolutionary conserved functions and pathways. Highlights of the PeroxisomeDB include new tools developed for facilitating (i) identification of novel peroxisomal proteins, by means of identifying proteins carrying peroxisome targeting signal (PTS) motifs, (ii) detection of peroxisomes in silico, particularly useful for screening the deluge of newly sequenced genomes. PeroxisomeDB should contribute to the systematic characterization of the peroxisomal proteome and facilitate system biology approaches on the organelle

Using Argo Floats to Characterize Altimetry Products: A Study of Eddy-Induced Subsurface Oxygen Anomalies in the Black Sea

peer reviewedThe identification of mesoscale eddies from remote sensing altimetry is often used as a first step for downstream analyses of surface or subsurface auxiliary data sets, in a so-called composite analysis framework. This framework aims at characterizing the mean perturbations induced by eddies on oceanic variables, by merging the local anomalies of multiple data instances according to their relative position to eddies. Here, we evaluate different altimetry data sets derived for the Black Sea and compare their adequacy to characterize subsurface oxygen and salinity signatures induced by cyclonic and anticyclonic eddies. In particular, we propose that the theoretical consistency and estimated error of the reconstructed mean anomaly may serve to qualify the accuracy of gridded altimetry products and that BGC-Argo data provide a strong asset in that regard. The most recent of these data sets, prepared with a coastal concern in the frame of the ESA EO4SIBS project, provides statistics of eddy properties that, in comparison with earlier products, are closer to model simulations, in particular for coastal anticyclones. More importantly, the subsurface signature of eddies reconstructed from BGC-Argo floats data is more consistent when the EO4SIBS data set is used to relocate the profiles into an eddy-centric coordinate system. Besides, we reveal intense subsurface oxygen anomalies which stress the importance of mesoscale contribution to Black Sea oxygen dynamics and support the hypothesis that this contribution extends beyond transport and involves net biogeochemical processes

Processing thermal barrier coatings via sol-gel route: crack network control and durability

Thermal barrier coatings (TBC) processed by sol–gel route are deposited onto NiPtAl bond coated superalloy substrates. A crack microstructure, if well controlled, is adequate to get satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. This paper deals with the adjustment of the properties of the microcracked network which is inherent to the process by changing the formulation of the sol and by adding a reinforcement step. The objective is to reduce the size and depth of the surface cracks network. This network controls the release of thermo-mechanical stress in the layers and reduces detrimental propagation of cracks that could result in the spallation of the coatings during engine operation. Several physico-chemical characterizations were performed, associated to image analyses to (i) evaluate the cracks distribution (depth, length and width), in the case of two dispersants, and (ii) to estimate their influence on the performances of TBC systems. Characterizations by cyclic oxidation were carried out using a cyclic oxidation instrumented rig to monitor on a real time basis the crack propagation and spallation. Correlations between the cracked network parameters and the lifetime of the TBC are proposed in this paper