Modélisation de l'interdiffusion et du comportement en oxydation cyclique de superalliages monocristallins à base de nickel revêtus d'une sous-couche γ-γ’ riche en platine. Extension aux systèmes barrière thermique

Les systèmes barrière thermique actuels connaissent une importante dispersion de durées de vie liée principalement aux ondulations de surface du revêtement métallique β-(Ni,Pt)Al provoquant l’écaillage du dépôt céramique. Les revêtements γ-γ’ riches en platine sont étudiés en tant qu’alternative au système actuel. Ce travail de thèse s’est intéressé à l’élaboration des revêtements γ-γ’ riches en platine sur un superalliage à base de nickel, l’AM1 à partir de procédés conventionnels : dépôt électrolytique de platine et aluminisation courte. Les mécanismes de dégradation par oxydation cyclique à 1100°C ont été étudiés sur des systèmes revêtement/AM1 et sur des systèmes barrière thermique. Pour comparaison, trois types de revêtement ont été élaborés : γ-γ’ Pt seul, γ-γ’ Pt+Al et β-(Ni,Pt)Al. Ces essais ont mis en évidence une meilleure tenue à l’oxydation cyclique des systèmes revêtus γ-γ’ Pt+Al comparée aux deux autres systèmes revêtus. L’importance de l’ajout d’aluminium dès l’élaboration sur la tenue à l’oxydation cyclique a été soulignée. La modélisation p-kp a mis en avant une augmentation de la proportion d’écaillage au cours du temps du fait de la dégradation de l’interface métal/oxyde et une augmentation du kp du fait de la formation d’un oxyde à croissance plus rapide. Outre l’oxydation, les phénomènes d’interdiffusion lors des tous premiers instants à haute température ont été étudiés à partir de matériaux modèles (Ni13Al et Ni11Al10Cr) et de revêtements de Pt et/ou de Pt-Ir. Ces essais ont mis en avant la rapide formation de la phase α-NiPtAl, les transformations de phases et les chemins de diffusion à 1100°C dans les systèmes Ni-Al-Pt et Ni-Al-Cr-Pt. L’effet du chrome et de l’iridium sur les cinétiques de diffusion a été évalué. La modélisation de l’interdiffusion a mis en évidence les interactions chimiques entre les espèces et une sursaturation en lacunes dans la zone d’interdiffusion prouvant que l’effet Kirkendall est responsable de la formation des pores. ABSTRACT : TBC systems currently used in aircraft engines with a Pt-modified aluminide coating β-(Ni,Pt)Al show an important lifetime dispersion due to the surface undulations of the bond-coating. This phenomenon called rumpling leads to the ceramic scale spallation and is the most common degradation mechanism. Pt-rich γ-γ’ bond-coatings have been extensively studied for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings. The aim of this work was to fabricate Pt-rich γ-γ’ bond-coatings on a first generation Ni-based superalloy, the AM1. Conventional processes were used as a platinum electroplating and a short aluminizing step. The failure mechanisms occurring by cyclic oxidation at 1100°C were studied on coating/superalloy systems and on TBC systems. Three kinds of coatings were fabricated: Pt-only γ-γ’, Pt+Al γ-γ’ and β-(Ni,Pt)Al. These tests highlighted the best oxidation resistance for the Pt+Al γ-γ’/AM1 systems when compared with the two other systems. Al addition during the coating fabrication is necessary to improve the lifetime. The p-kp modeling results pointed out that the oxide scale spalling probability p increases due to the metal/oxide interface degradation with time. If the spallation increases, a breakaway locally occurs with the formation of a fast-growing oxide explaining the kp progression. The interdiffusion phenomena were also investigated during the first times at high temperature from model alloys (Ni13Al and Ni11Al10Cr) and Pt and/or Pt-Ir coatings. These investigations emphasized the rapid formation of the α-NiPtAl phase, the phase transformations and diffusion paths at 1100°C in the ternary Ni-Al-Pt and quaternary Ni-Al-Cr-Pt systems. Chromium and iridium effect was evaluated on the diffusion kinetics. Interdiffusion modeling highlighted the chemical interactions between the species and a vacancy supersaturation in the interdiffusion zone proving that Kirkendall effect is responsible for void formation

Chromium and iridium effects on the short-term interdiffusion behaviour between Pt rich γ-γ′ bond-coatings and a Ni-Al-Cr alloy

The interdiffusion behaviour of a 5 μm thick layer of Pt deposited by electroplating on a γ-Ni-12Al-10Cr model alloy was studied in order to assess the effect of Cr. Heat treatments were performed for 1 min up to 1 h at 1100 °C under argon. Cr addition increased the uphill diffusion of Al to the surface when compared with Pt/γ-(Ni,Al) systems. Al and Cr had a positive chemical interaction in presence of Pt, as shown by the positive values of the DAlCrNi and DCrAlNi diffusion coefficients determined by modelling. Pt had a negative chemical interaction with Al and with Cr in such a way that Pt decreased their activities. According to the diffusion coefficient values, Pt had a greater influence on the Al activity than on the Cr one. Similarly, 2 μm of Pt and 3 μm of Pt-25Ir were deposited by electroplating on the same model alloy to investigate the effect of Ir. Heat treatments were performed in the same conditions as for Cr. Iridium slowed down the interdiffusion when compared with systems with Pt only. Iridium diffused slower toward the substrate than Pt and a lower Pt + Ir flux toward the substrate was found. As voids formed at the interdiffusion zone/substrate interface due to Kirkendall effect, this lower inward Pt + Ir flux resulted in a lower outward vacancy flux and then Ir reduced Kirkendall voids formation. Moreover, Ir decreased the Pt effect on Al activity by dilution or even gave an opposite contribution to the Pt one. This reduced the uphill diffusion of Al, delaying the α-NiPtAl phase formation. Diffusion paths of each model system were also identified after 15 min at 1100 °C and all highlighted the α-NiPtAl phase formation and its aptitude to be used in TBC systems

Cyclic Oxidation Behavior of TBC Systems with a Pt-Rich γ-Ni+γ′-Ni3Al Bond-Coating Made by SPS

To obtain long-lasting thermal barrier coating (TBC) systems, two types of Pt-rich γ-Ni+γ′-Ni3Al bond-coatings (BC) were fabricated by spark plasma sintering (SPS). The former had the highest possible Pt content (Ni-30Pt-25Al in at.%) while the latter had the highest possible Al level (Ni-28Al-17Pt in at.%). Hf was added as a reactive element. TBCs were fabricated on different superalloys (AM1, René N5 and MCNG) with the aforementioned BCs and with zirconia stabilized with yttria top coats made by SPS or electron beam physical vapor deposition (EBPVD). The cyclic oxidation resistance of these systems was studied at 1,100 °C in air. Most TBCs with a Pt-rich γ–γ′ BC showed better thermal cycling resistance when compared to the reference TBCs (β-(Ni,Pt)Al diffusion BC and EBPVD ceramic top coat), with lifetimes up to 1,745 cycles instead of 700 for the reference, and despite the fabrication defects observed within the SPS BCs. Cu was tested as an addition in the BCs and proved to have a slight negative effect on the system lifetime. Moreover, the fourth generation MCNG substrate led to the best cyclic oxidation behavior

Comparison of the Chondrogenic Potential of Mesenchymal Stem Cells Derived from Bone Marrow and Umbilical Cord Blood Intended for Cartilage Tissue Engineering

International audienceOsteoarthritis (OA) remains incurable in humans or horses and mesenchymal stromal/stem cells (MSCs) represent an attractive solution for producing a neocartilage substitute. However, the best MSC source still needs to be identified. This study compared the chondrogenic potential of equine MSCs derived from bone marrow (BM) and umbilical cord blood (UCB), at their undifferentiated status to check if one cell source is better proned, and after chondrogenic-induced differentiation. Chondrogenesis was induced by culture in collagen scaffold with BMP-2 + TGF-ß1 in hypoxia or normoxia. MSCs chondrogenic potential was evaluated using the mRNA and corresponding protein levels for osteogenic, hypertrophic and chondrogenic markers. MSCs characterization demonstrated that BM- and UCB-MSCs differ in proliferation and tripotencies. At undifferentiated status, they also showed differences in their expression of osteogenic, chondrogenic and hypertrophic markers. Upon chondrogenesis induction, both MSCs sources exhibited increased chondrogenic expression and produce an extracellular matrix (ECM) of better quality in hypoxia, although collagen I remained expressed. UCB-MSCs produced higher amounts of collagen II, particularly its IIB isoform, than BM-MSCs, but also collagen I and Htra1, regardless of the oxygen condition. Finally, immunohistochemistry revealed that the BM-MSCs synthesized an ECM of higher quality, regarding the more homogenous distribution of type IIB collagen, compared to UCB-MSCs. Considering collagen I as the major undesirable component in the neo-synthesis of in vitro cartilage, we recommend using BM-MSCs for horse cartilage engineering

A direct comparative study of the corrosion behaviour of Si-free and Si-rich slurry aluminide coatings in molten carbonate melts

ABSTRACT: This study is focused on the evaluation of the hot corrosion behaviour of two low-cost slurry aluminide coatings with and without Si addition deposited on ferritic-martensitic steels, comparing them with uncoated steel, after 1000-h of exposure to Li, K, Na molten carbonates at 650ºC, under static and dynamic conditions with a high linear velocity (1.3 m/s). Both coatings evidenced a high performance increase in comparison with uncoated substrates after exposure in both conditions. Both coatings behaved in a similar way according to the gravimetric results. However, after dynamic exposure, the Si-free aluminide maintained its morphology and composition after 1000 h with α-LiAlO2 as the only corrosion product while the Si-rich coating showed a higher attack extent in the outer part and developed a large quantity of voids at the coating/substrate interface, leading to substrate corrosion.info:eu-repo/semantics/publishedVersio

High-temperature cyclic oxidation behaviour of Pt-rich γ-γ’ coatings. Part I: Oxidation kinetics of coated AM1 systems after very long-term exposure at 1100 °C

The cyclic oxidation behaviour of several compositions of Pt-rich γ-γ’ bond-coatings on AM1 superalloy was studied at 1100 °C and was compared to the β-(Ni,Pt)Al coated and uncoated superalloy. AM1 superalloy exhibited an outstanding performance due to an optimized Hf doping and a low sulfur content. The Pt-rich γ-γ’ bond-coatings showed a better cyclic oxidation resistance than the reference system with a β-(Ni,Pt)Al coating. Aluminium addition during fabrication was found to be beneficial to improve the oxidation behaviour of Pt-rich γ-γ’ bond-coatings. Their breakaway resulted from an insufficient aluminium content below the TGO whereas the reference system suffered from rumpling

Diagnostic imaging of equine thoracolumbar disorders

Equine thoracolumbar conditions represent a major cause of poor performance and locomotor disorders in sports and race horses. Advances in diagnostic imaging during the last 15 years allow today to diagnose most equine back lesions in the standing sedated horse. Radiography is the first choice imaging modality due to its high diagnostic performance. Ultrasonography is a complementary modality to further investigate back soft tissues and the caudal lumbar area not evaluable radiographically in the standing horse. Bone scintigraphy is a more complex and expensive technique and is usually dedicated to complicated clinical cases. The most frequent injuries diagnosed using these 3 modalities are kissing spines, osteo-arthrosis of the synovial intervetebral joints, vertebral spondylosis and muscle injuries. Clinical significance of these lesions should be interpreted with care considering the clinical signs exhibited by the horse but also its sports or racing useLes affections vertébrales thoracolombaires représentent une cause fréquente de baisse de performances et/ou de troubles locomoteurs chez les chevaux de sport et de courses. Le développement des techniques d’imagerie ces 15 dernières années permet aujourd’hui au vétérinaire équin de diagnostiquer un grand nombre de lésions affectant la colonne thoracolombaire sur le cheval debout tranquillisé. La radiographie du dos du cheval constitue la modalité d’imagerie de premier choix par sa capacité diagnostique élevée. L’échographie complète la radiographie dans l’exploration des tissus mous et de la région lombaire caudale, cette dernière n’étant pas accessible par radiographie sur le cheval debout. La scintigraphie, technique plus onéreuse et plus contraignante, est envisagée pour des cas cliniques complexes. Les lésions les plus fréquemment diagnostiquées avec ces techniques d’imagerie sont représentées par les conflits et fractures des processus épineux, les arthroses synoviales intervertébrales, les spondyloses vertébrales et les lésions musculaires. La signification clinique de ces lésions doit être évaluée en considérant les signes cliniques du patient mais également sa discipline sportiv

Axial speed of sound for the monitoring of injured equine tendons: a preliminary study.

PublishedJournal ArticleResearch Support, Non-U.S. Gov'tEquine superficial digital flexor tendons (SDFT) are often injured, and they represent an excellent model for human sport tendinopathies. While lesions can be precisely diagnosed by clinical evaluation and ultrasonography, a prognosis is often difficult to establish; the knowledge of the injured tendon's mechanical properties would help in anticipating the outcome. The objectives of the present study were to compare the axial speed of sound (SOS) measured in vivo in normal and injured tendons and to investigate their relationship with the tendons' mechanical parameters, in order to assess the potential of quantitative axial ultrasound to monitor the healing of the injured tendons. SOS was measured in vivo in the right fore SDFTs of 12 horses during walk, before and 3.5 months after the surgical induction of a bilateral core lesion. The 12 horses were then euthanized, their SDFTs isolated and tested in tension to measure their elastic modulus and maximal load (and corresponding stress). SOS significantly decreased from 2179.4 ± 31.4 m/s in normal tendons to 2065.8 ± 67.1 m/s 3.5 months after the surgical induction, and the tendons' elastic modulus (0.90 ± 0.17 GPa) was found lower than what has been reported in normal tendons. While SOS was not correlated to tendon maximal load and corresponding stress, the SOS normalized on its value in normal tendons was correlated to the tendons' elastic modulus. These preliminary results confirm the potential of axial SOS in helping the functional assessment of injured tendon.Direction Générale de l’Enseignement et de la Recherche (French Ministry of Agriculture)Région Basse-NormandieInstitut National de la Recherche AgronomiqueAgence Nationale de la Recherch

Interest of magnetic resonance imaging in the diagnosis of equine locomotor disorders

Magnetic Resonance Imaging (MRI) is the current gold standard in human medicine for locomotor apparatus imaging. This paper presents the interest of MRI in the diagnosis of equine locomotor disorders, based on patients examined at the CIRALE centre with a C-shape low-field system (0.2 T). The protocols used and indications of this technique are described. The diagnostic interest of MRI is then presented by anatomical entities. MRI is a very reliable technique to identify bone lesions which are radiologically invisible. Compared to ultrasonography, MRI provides images of all soft tissues in the foot, with greater tissue discrimination within the lesions. The downside, however, is that a general anesthesia is necessary to obtain diagnostic images of high quality. Moreover, current MRI systems are able to visualise only the distal segments of the limbs (including carpus and tarsus), the head, and the cervico-cranial hinge. The development of MRI systems designed to examine a standing horse is an interesting avenue of research, which would overcome these limitations.L'Imagerie par Résonance Magnétique (IRM) est aujourd'hui en médecine humaine la technique d'imagerie de référence pour l'appareil locomoteur. L'objectif de cet article est de présenter les apports de l'IRM dans le diagnostic des affections locomotrices du cheval à partir des patients examinés au CIRALE à l'aide d'une machine bas champ (0,2 T) avec une configuration en C. Les protocoles utilisés et les indications de cet examen sont décrits. Les apports de l'IRM au plan diagnostic sont ensuite présentés par type de formation anatomique. L'IRM permet notamment d'identifier, avec une grande fiabilité, des lésions osseuses qui ne sont pas visibles en radiographie. Elle présente, par rapport à l'échographie, la capacité d'imager l'ensemble des tissus mous du pied et permet une meilleure discrimination tissulaire au sein des lésions. En revanche, l'IRM requiert une anesthésie générale pour obtenir des images de grande qualité diagnostique. D'autre part, seules les régions distales des membres (jusqu'au carpe et tarse inclus), la tête et la région cervicale crâniale peuvent être examinées sur les machines actuelles. Pour pallier ces limites, le développement de machines IRM permettant l'imagerie sur cheval debout représente une voie de recherche intéressante

High-temperature cyclic oxidation of Pt-rich γ-γ’ bond-coatings. Part II: Effect of Pt and Al on TBC system lifetime

Three kinds of Pt-rich γ-γ’ bond-coating were processed with different contents in Pt and Al. The cyclic oxidation tests performed at 1100 °C on TBC systems showed the superiority of the Pt-rich γ-γ’ coatings when compared with the β-(Ni,Pt)Al reference system. TBCs with a Pt-only bond-coating provided the highest performance. Whatever the bond-coating, the failure occurred at the TGO/bond-coating interface which appeared to be the weak point of these γ-γ’ bond-coating based systems. Al addition during bond-coating fabrication did not improve the durability. A decrease of 2 μm of electroplated Pt thickness led to a higher performance than the reference systems