Morgane Couchet
email: morgane.couchet@univ-grenoble-alpes.fr

C Breuillard

C Moinard

Citrulline: An important asset in

Keywords:

est destinée au dépôt

Contexte actuel

L'espérance de vie des populations progressant, nos sociétés occidentales doivent faire face à un nouveau défi de taille : préserver les conditions de vie des populations vieillissantes. En effet, le vieillissement s'accompagne de profondes modifications physiopathologiques. L'âge est responsable d'altérations structurales et métaboliques dont l'accumulation va favoriser le développement de maladies chroniques. L'une des caractéristiques majeures du vieillissement est la modification de la composition corporelle. En effet, les personnes âgées perdent de la masse musculaire (dont la forme ultime est la sarcopénie) ce qui les rend plus fragiles.

La sarcopénie peut entrainer une perte de force (dynapénie) et/ou de fonction des muscles, vecteur d'une morbi-mortalité accrue. D'ailleurs, depuis 2016, elle est officiellement reconnue comme une maladie à part entière par l'Organisation mondiale de la santé (OMS, code M62.84). Par ailleurs, cette sarcopénie est difficile à prendre en charge car elle s'accompagne d'une résistance à la renutrition. Ainsi, ces deux éléments sont un véritable enjeu pour la prise en charge nutritionnelle des personnes âgées. Parmi les méthodes étudiées, l'utilisation de nutriments ayant des actions ciblées sur le métabolisme musculaire a été largement explorée. Ainsi, un intérêt majeur s'est porté sur l'utilisation d'acides aminés spécifiques pour La citrulline : un allié de choix dans la prise en charge thérapeutique ? appendiculaire) [START_REF] Bouillanne | Impact of 3-week citrulline supplementation on postprandial protein metabolism in malnourished older patients: The Ciproage randomized controlled trial[END_REF]. Un second travail a étudié l'effet d'une complémentation en CIT (10 g/jour pendant 12 semaines) couplée à de l'exercice physique fractionné chez des sujet âgés et obèses. L'exercice améliore dans tous les cas les paramètres musculaires et fonctionnels, mais ces effets sont potentialisés par la CIT. Ces observations sont à mettre en parallèle avec une légère augmentation de la masse maigre et une diminution de la masse grasse [START_REF] Buckinx | Effect of high-intensity interval training combined with L-citrulline supplementation on functional capacities and muscle function in dynapenicobese older adults[END_REF].

Citrulline et tissu adipeux

En période post-absorptive ou lors d'un jeûne, la lipolyse permet la libération d'acides gras non-estérifiés (AGNE) et de glycérol, source d'énergie pour les tissus périphériques. Cependant, afin de limiter un déstockage massif, l'organisme ré-estérifie une partie des AGNE via la glycéronéogenèse. De plus, avec l'âge, la masse du tissu adipeux blanc (TAB) augmente en parallèle avec l'état pro-inflammatoire, ce qui engendre une altération du statut métabolique et endocrinien, puis, peut entrainer des pathologies comme le diabète de type 2 ou des maladies cardiovasculaires. restaurer l'homéostasie azotée. Parmi les stratégies proposées, la citrulline semble la plus prometteuse, cet acide aminé possédant une capacité d'action directe sur la fonction musculaire.

La citrulline

La citrulline (CIT) est absente de notre alimentation (excepté dans la pastèque où elle y joue un rôle d'antioxydant et de résistance au stress hydrique). Chez l'Homme, la concentration plasmatique en CIT est d'environ 40 μmol/L. Elle est quasi-exclusivement produite par l'intestin grêle, pour être ensuite métabolisée en arginine par le rein (figure 1) [START_REF] Breuillard | Citrulline and nitrogen homeostasis: an overview[END_REF]. De ce fait, la citrulline est reconnue comme l'un des meilleurs marqueurs de la fonctionnalité intestinale [START_REF] Curis | Almost all about citrulline in mammals[END_REF]. En effet, ce concept a pu être validé dans le syndrome de grêle court et dans diverses situations d'insuffisance intestinale, telles que la maladie de Crohn, la réanimation, etc. Cependant, l'intérêt de la CIT repose surtout sur ses propriétés métaboliques qui ont pu être mises en évidence au cours de la dernière décennie.

Tolérance de la citrulline

La CIT est bien absorbée et bien tolérée. Il n'a jamais été rapporté d'effets secondaires liés à sa consommation [START_REF] Moinard | Doseranging effects of citrulline administration on plasma amino acids and hormonal patterns in healthy subjects: the Citrudose pharmacokinetic study[END_REF].

Citrulline et fonction musculaire

Un travail princeps mené chez des rats âgés dénutris, puis complémentés en CIT, a mis en évidence une augmentation de la synthèse protéique musculaire (+90 %) ainsi qu'une accrétion protéique (+20 %) [START_REF] Osowska | Citrulline increases arginine pools and restores nitrogen balance after massive intestinal resection[END_REF]. Ces résultats ont été confirmés dans une étude portant sur l'apport d'un bolus de CIT après un jeûne de 18 h, via une activation de la voie de signalisation intracellulaire mTORC1 impliquée dans l'homéostasie protéino-énergétique. En outre, en utilisant le même modèle expérimental de rats âgés dénutris, la CIT augmentait de pair la masse et la force musculaire avec une augmentation de la motricité.

Une approche in vivo (rats dénutris) et in vitro (myotubes en culture) complémentaires a mis en avant le fait que la CIT a une action directe et ciblée dans le muscle. En effet, elle augmente l'expression des principaux composants des myofibrilles (unités contractiles des fibres musculaires), ainsi que la quantité de protéines impliquées dans le processus de contraction musculaire. Ces données expérimentales ont été confirmées par les résultats d'études cliniques montrant qu'une administration de CIT chez des volontaires sains améliorait le bilan azoté de 57 %. Ces données ont été appuyées par la suite par une étude de Jourdan et al. [START_REF] Jourdan | Citrulline stimulates muscle protein synthesis in the postabsorptive state in healthy people fed a low-protein diet -A pilot study[END_REF] qui a mis en évidence qu'une consommation en aigu de 10 g de CIT augmentait la vitesse de synthèse des protéines musculaires (+25 %). Ces données peuvent être complétées par les résultats de deux études cliniques récentes [START_REF] Bouillanne | Impact of 3-week citrulline supplementation on postprandial protein metabolism in malnourished older patients: The Ciproage randomized controlled trial[END_REF][START_REF] Buckinx | Effect of high-intensity interval training combined with L-citrulline supplementation on functional capacities and muscle function in dynapenicobese older adults[END_REF]. L'étude Ciproâge s'est intéressée à l'impact d'une complémentation en CIT (10 g CIT/jour pendant 3 semaines) au cours d'une renutrition chez le sujet âgé dénutri hospitalisé. Ce travail a permis de mettre en évidence qu'une administration de CIT permettait une augmentation de la masse maigre (+10 % de masse musculaire OAT : ornithine aminotransférase ; OCT : ornithine carbamoytransférase ; ASS : argininosuccinate synthase ; ASL : argininosuccinate lyase.

Dossier thématique

Quelques nouvelles pistes de la biologie métabolique

Une étude réalisée sur des rats âgés sains complémentés en CIT a mis en évidence une diminution de la masse grasse sous-cutanée (-14 %) et intraabdominale (-42 %) parallèlement à une augmentation de la masse maigre (+8 %). Par la suite, une première étude in vitro, en 2012, a permis de montrer que la CIT avait une action directe sur le TAB et augmentait la libération des AGNE par les adipocytes, couplée à une diminution de la glycéronéogenèse. Ces résultats sont retrouvés in vivo, chez des rats jeunes sous régime hyperlipidique et des rats âgés, complémentés en CIT, avec une augmentation de l'expression des enzymes clés de la lipolyse (x 1,5) [START_REF] Joffin | Citrulline reduces glyceroneogenesis and induces fatty acid release in visceral adipose tissue from overweight rats[END_REF]. Ceci confirme l'action lipolytique de la CIT dans le tissu adipeux. Cependant, l'action de la CIT induit une augmentation de la concentration plasmatique d'AGNE qui pourrait s'avérer néfaste en raison de leur implication dans la résistance à l'insuline. La CIT pourrait alors être corrélée au développement de l'obésité. Néanmoins, la dépense énergétique étant plus importante chez les rats en surpoids, l'activité physique couplée à une complémentation en CIT pourrait être une alternative favorable à la perte du tissu adipeux viscéral. L'ensemble de ces données est retrouvé dans des études cliniques [START_REF] Bouillanne | Impact of 3-week citrulline supplementation on postprandial protein metabolism in malnourished older patients: The Ciproage randomized controlled trial[END_REF][START_REF] Buckinx | Effect of high-intensity interval training combined with L-citrulline supplementation on functional capacities and muscle function in dynapenicobese older adults[END_REF].

Citrulline et métabolisme cardiovasculaire

Le vieillissement favorise le dysfonctionnement endothélial qui est la cause principale du développement de maladies cardiovasculaires, telle que l'hypertension artérielle, l'athérosclérose, ou encore les lésions d'ischémie-reperfusion. En cas de risque accru, le traitement est souvent chronique, et une complémentation en arginine s'avère peu efficace. La CIT, précurseur de l'arginine, et donc du monoxyde d'azote radicalaire (NO), semble donc être un complément aux effets bénéfiques. En effet, chez l'animal, la CIT augmente la production de NO dans les micro-capillaires. De plus, dans un modèle de rat hypertendu, une complémentation en CIT restaure les concentrations de NO dans le rein. Ces résultats sont confirmés par une étude chez des adultes âgés insuffisants cardiaques. Une ingestion aiguë de CIT (10 g) augmente la synthèse de novo d'arginine et de NO, et donc leur biodisponibilité, favorisant ainsi le maintien du tonus vasculaire. Chez des rats sous régime hyperlipidique et hypercholestérolémique, la CIT permet d'améliorer la structure endothéliale de l'aorte. Ces résultats sont confirmés dans une étude chez le lapin sous régime hypercholestérolémique, montrant une préservation de la fonction endothéliale. Chez des patients pré-hypertendus et hypertendus, des études montrent qu'une complémentation en CIT (6 g/jour pendant 6 semaines) permet de réduire la pression sanguine diastolique et d'améliorer le tonus vasculaire périphérique. Chez des sujets âgés sains et obèses, l'apport de CIT permet de diminuer la rigidité artérielle. Au niveau cardiaque, la CIT améliore la fonction ventriculaire. En effet, une étude chez des patients insuffisants cardiaques montre que la CIT augmente la fraction d'éjection ventriculaire en améliorant l'amplitude maximale artérielle. La CIT parait protéger les altérations de l'endothélium vasculaire par ses effets anti-hypertensifs. Par ailleurs, une complémentation en CIT permet de diminuer la cholestérolémie et d'améliorer le statut inflammatoire, diminuant ainsi l'expansion des plaques d'athéromes, et donc prévenant les altérations cardiovasculaires. Une complémentation en CIT semble être un substitut prometteur pour augmenter la biodisponibilité en arginine afin de produire le NO nécessaire au maintien de l'intégrité vasculaire [START_REF] Allerton | L-Citrulline supplementation: Impact on cardiometabolic health[END_REF].

Citrulline et cerveau

Au vu du rôle de la CIT dans le système vasculaire, couplé au fait qu'elle est un précurseur du NO et qu'elle traverse la barrière hémato-encéphalique, il se pourrait qu'elle puisse moduler des mécanismes du système nerveux central (SNC). Une étude menée chez des rats âgés sains a montré qu'une complémentation en CIT améliorait l'activité locomotrice des animaux et augmentait la quantité striatale d'une enzyme clé impliquée dans la production de dopamine [START_REF] Moinard | Citrulline stimulates locomotor activity in aged rats: Implication of the dopaminergic pathway[END_REF]. Même si le mécanisme d'action de ce système reste à clarifier, il semblerait que la CIT agisse sur la voie dopaminergique, impliquée dans l'initiation des mouvements volontaires. La CIT peut donc générer du NO directement dans le SNC. Dans le cerveau de personnes âgées, la quantité d'enzymes impliquées dans la production de NO est diminuée, pouvant ainsi contribuer à une détérioration des fonctions cognitives. En effet, le NO est impliqué dans la plasticité synaptique, l'apprentissage, et la mémoire. Une étude a montré que la maladie d'Alzheimer impliquait des modifications neuronales liées en partie à une altération de la production de NO. Une étude a exploré l'impact d'une complémentation en CIT sur la structure des radeaux (« rafts ») lipidiques chez le rat âgé. Ces derniers sont des microdomaines membranaires impliqués dans la signalisation cellulaire, et donc dans la dégénérescence cognitive. La CIT diminue les effets délétères du vieillissement en améliorant la structure et les propriétés biochimiques de ces radeaux lipidiques [START_REF] Marquet-De-Rougé | Citrulline diet supplementation improves specific age-related raft changes in wild-type rodent hippocampus[END_REF]. En effet, elle diminue la concentration de peptides amyloïdogéniques et de précurseurs du peptide amyéloïde (APP), permettant la production du peptide amyéloïde dont l'altération du clivage est impliquée dans la diminution de la plasticité neuronale. De plus, des données très préliminaires réalisées dans un modèle murin de maladie d'Alzheimer montrent que la CIT améliore les capacités cognitives de ces animaux.

• De nos jours, il est indispensable d'améliorer les conditions de vie des personnes âgées.

• La citrulline est bien assimilée par l'organisme et bien tolérée.

• La citrulline améliore la fonction musculaire avec des effets sur le tissu adipeux.

Elle pourrait être un acteur dans la prévention des maladies cardiovasculaires et cérébrales.

Les points essentiels

Conclusion

Figure 1 .

 1 Figure 1. Carrefour intestinal arginine-ornithine-citrulline [D'après Ref.1. Breuillard C, Cynober L, Moinard C. Citrulline and nitrogen homeostasis : an overview. Amino Acids 2015 ; 47 : 685-91. Copyright © 2015, Springer-Verlag Wien].

Médecine des maladies Métaboliques -Juin 2019 -Vol. 13 -N°4


I. INTRODUCTION

Electron Energy Loss Spectroscopy (EELS) is now a common method to investigate the spectrum of excitations in a solid. It reflects the response of the electron population of the specimen under the impact of a primary beam

Acknowledgements 171

References .


M.-N Avettand-Fènoël
email: marie-noelle.avettand-fenoel@univ-lille.fr

K Naji

Ph Pouligny

Brazing vs. diffusion welding of graded Fe based matrix composite and yttria stabilized zirconia

Keywords: Brazing, diffusion welding, metal matrix composite, ceramic, microstructure, mechanical properties

Two graded Fe based matrix composite (MMC) -yttria stabilized zirconia (YSZ) joints were developed by brazing with Ticusil TM braze and by diffusion welding. The brazed joint was formed by solidification. During reactive brazing, some few micrometers thick continuous TiO2, Fe2Ti3O9 and FeTiO3 layers were developed at the interfaces of the braze with both YSZ and MMC. On MMC side, TiO2 was formed by reduction of FeO, stemming from MMC, by Ti. The formation of Fe2Ti3O9 and FeTiO3 compounds on YSZ side resulted from the combination of Fe and Ti which diffused through the braze, and O provided by YSZ and dissolved in the braze. The diffusion joint was developed by solid state sintering and YSZ-YSZ, YSZ-FeO and YSZ-Fe discontinuous bonds were created at the MMC-YSZ interface. During 3 points static bending tests, the diffusion joint showed a greater bending resistance compared to the brazed one. Some ways of optimization were finally proposed.

Introduction

The combination of performing functional and mechanical properties to meet specifications of industrial applications generally requires the assembly of dissimilar materials such as ceramics and alloys or metallic matrix composites (MMCs). However ceramics present both ionic and covalent bonds while MMCs essentially present metallic bonds. These materials further present distinct physico-chemical and mechanical properties, which makes their joining a challenge.

Various processes are used to join such distinct classes of materials, namely (i) mechanical joining which does not lead to an atomic bonding between the materials, and (ii) joining processes which proceed a. either at the liquid state such as brazing, melting with eutectic liquid, squeeze casting, field assisted bonding, soldering [START_REF] Suganuma | Handbook of advanced ceramics[END_REF], electron beam welding, (partial) transient liquid phase bonding and anodic bonding/electrostatic bonding/field assisted bonding [START_REF] Zhang | Progress in joining ceramics to metals[END_REF][START_REF] Fernie | Joining of engineering ceramics[END_REF]. In this case, various issues for their joining must be underlined, such as (i) the wettability of the ceramic by the metal which governs the quality of the atomic bonding at the interface, (ii) the eventual formation of thick interfacial layers of brittle phases deleterious for joint mechanical resistance and (iii) the mismatch of coefficients of thermal expansion [3] and of Young moduli [START_REF] Hausner | Brazing of metal and ceramic joints[END_REF] originating some thermal residual stresses upon cooling. b. or at the solid state like friction welding [START_REF] Suganuma | Handbook of advanced ceramics[END_REF], friction stir welding [START_REF] Avettand-Fènoël | Characterization of WC/12Co cermet -steel dissimilar friction stir welds[END_REF][START_REF] Avettand-Fènoël | Effect of a Ni interlayer on microstructure and mechanical properties of WC-12Co cermet / SC45 steel friction stir welds[END_REF],

diffusion welding [START_REF] Cam | Progress in joining of advanced materials[END_REF][START_REF] Shirzadi | Joining ceramics to metals using metallic foams[END_REF], microwave welding, ultrasounds welding [2,3], cold spray [START_REF] Ji | Characterization of coldsprayed multimodal WC-12Co coating[END_REF][START_REF] Dosta | Cold spray deposition of a WC-25Co cermet onto Al7075-T6 and carbon steel substrates[END_REF]. The use of solid state joining processes, which overcomes the wetting topic, should limit the excessive growth of brittle compounds at joint interface and may be expected to limit the thermal residual stresses. In the railway sector, railway tracks need insulated joints to be inserted in the rail. This application requires to develop materials presenting at once electrical insulation and high mechanical properties; a novel architectured design of joint between graded Fe based MMCs and a yttria stabilized zirconia (YSZ) thin layer has been proposed [START_REF] Naji | FR 3045078 B1 delivered[END_REF]. First tests enabled to develop the graded MMCs (presenting a graded microstructure with an increasing volume fraction of reinforcements along the material, conferring them an evolution of their mechanical and/or functional properties) as a first step [START_REF] Naji | Design and characterization of an oxides hybrid dispersion strengthened iron based composite with a graded and architectural microstructure[END_REF], and to join them to the YSZ layer as a second step. Two joining processes were considered, namely diffusion welding and brazing with an Ag-Cu based braze. As aforesaid, given the different natures of the atomic bonds in YSZ (covalent and ionic) and in the braze (metallic), the ceramic -metallic braze joining is ticklish. This is the reason why various means were developed in literature to improve the wettability of YSZ by the Ag-Cu based braze, such as (i) the coating of YSZ with a metal [START_REF] Zhang | Progress in joining ceramics to metals[END_REF][START_REF] Fernie | Joining of engineering ceramics[END_REF][START_REF] Hausner | Brazing of metal and ceramic joints[END_REF]20,212,[START_REF] Fernie | Joining of engineering ceramics[END_REF][START_REF] Hausner | Brazing of metal and ceramic joints[END_REF]13,14], (ii) the introduction of TiH2 in the braze and the coating of zirconia with TiH2 leading to TiH2 decomposition during brazing entailing the formation of a Ti layer on zirconia substrate [22,2315,16], (iii) the insertion of an active metal in the braze such as Ti (with a content in-between 2 and 5% [START_REF] Suganuma | Handbook of advanced ceramics[END_REF][START_REF] Zhang | Progress in joining ceramics to metals[END_REF]41,[START_REF] Zhang | Progress in joining ceramics to metals[END_REF][START_REF] Hausner | Brazing of metal and ceramic joints[END_REF][START_REF] Bian | Reactive wetting and interfacial characterization of ZrO2 by SnAgCu-Ti alloy[END_REF]). The active metal modifies the surface chemistry of the ceramics, diminishes its surface energy by forming an intermediate reaction layer and reduces the contact angle of the melted braze on YSZ [START_REF] Zhang | Progress in joining ceramics to metals[END_REF][START_REF] Wei | Microstructural evolution and bonding mechanisms of the brazed Ti/ZrO2 joint using an Ag68.8Cu26.7Ti4.5 interlayer at 900°C[END_REF]21,242,14,[START_REF] Wei | Microstructural evolution and bonding mechanisms of the brazed Ti/ZrO2 joint using an Ag68.8Cu26.7Ti4.5 interlayer at 900°C[END_REF]19]. It is the latter solution, that is the insertion of Ti active metal in the braze, which was chosen during the current study. A satisfactory braze must wet but should also be ductile and bond strongly [2013]. The Ag-Cu braze is a ductile solvent in which Ti has a very high activity coefficient, which is the relevant thermodynamic parameter that governs the interfacial reaction [2520], so that there is no major reservoir of Ti from which too thick brittle reaction product layers may form.

The current paper aims to understand the formation mechanisms of the joints developed by diffusion welding and by brazing, and to compare their microstructural architecture microstructure and their mechanical properties. The pros and the cons of each process in connection with the performances of the joints will finally be discussed.

Experimental procedure

The base materials to assemble are 10 mm diametered and 1 mm thick sintered zirconia stabilized with 3 mol.% yttria (YSZ) layers supplied by Umicore ® (table 1) and graded oxide dispersion strengthened (ODS) Fe based composites (diameter of 10 mm and average thickness of 7 mm). The latter material developed by high energy ball milling, compaction and sintering consists in a stacking of various composite subparts with incremental volume fractions (Vf) of YSZ particle reinforcements (Vf =0 -0.05 -0.1 -0.2 -0.3 -0.4). The subpart that contains the greatest volume fraction of YSZ is the one to be joined to YSZ layer. For further information about the design, the development and the characterization of the graded metal matrix composite (MMC), the reader is encouraged to refer to a previous paper of the authors [START_REF] Naji | Design and characterization of an oxides hybrid dispersion strengthened iron based composite with a graded and architectural microstructure[END_REF]. Two joining processes were tested to assemble the base materials: brazing and diffusion welding.

Concerning brazing, a 51 µm thick ribbon of Ticusil TM braze (table 1) provided by Johnson Matthey ® was used as filler metal between the YSZ layer and the graded MMCs. The braze solidus temperature and the liquidus temperatures of the braze amount toare 778°C and 850°C respectively, according to the supplier. In a first step, the YSZ wettability by the braze was analyzed with the sessile drop method [1621]. The wettability tests were performed under secondary vacuum with roughly or finely polished YSZ substrates surfaces. Two automatic and then reproducible polishing procedures of materials were indeed set up leading to rough and fine surfaces with suspensions containing particles with a size respectively greater and lower than 1 µm so as to investigate the effect of the substrate roughness on the wettability by the braze. The roughness and the Ra parameter (defined as the mean arithmetic roughness of the analyzed surface) of both the YSZ layers and the most charged composite subpart of graded MMC were characterized with a 3 D optical profiler WYKO NT1100 ® . The evolution of the aspect of the braze drop put on the substrate was recorded as a function of temperature and time with a charge coupled device (CCD) camera. The contact angle (wetting angle) between braze and substrate could thus be directly measured on the sideways images of the drop captured by the digital camera (such as those in figure 5) using the ImageJ TM image processing software. In a second step, brazing was carried out in a home-made TZM (Mo based alloy with 0.5% Ti, 0.08% Zr and 0.03% C) device under a uniaxial pressure of 40 cN and secondary vacuum (2.6×10 -7 atm). The brazing cycle is depicted in figure 1a. It was chosen according to literature recommendations [1322]. The 5 min holding time at 750°C enabled to homogenize the temperature along the materials stacking before reaching the brazing temperature of 900°C. The choice of performing no holding time at 900°C was made in order to avoid composite microstructure coarsening during brazing and the formation of too thick interfacial layers prohibitive for joint mechanical resistance. The low cooling rate of 5°C/min was used to limit the thermal residual stresses which may be generated at the joint interfaces because of the difference of thermal expansion coefficients (CTE) between the braze (18.5×10 -6 °C-1 [1423]), YSZ (9.6×10 -6 °C-1 over [20-400°C] range according to the supplier) and the Fe based graded composite (14.6×10 -6 C -1 over [20-800°C] for Fe CTE [1524]).

With regard to diffusion welding, joining occurred simultaneously with graded ODS MMC sintering. Diffusion welding cycle, namely sintering cycle, is displayed in figure 1b. Sintering was performed in a free radial stresses matrix under a uniaxial pressure of 50 kPa and secondary vacuum (2.6×10 -7 atm). The roughness and the Ra parameter (defined as the mean arithmetic roughness of the analyzed surface) of both the YSZ layers and the most charged composite subpart of graded MMC were characterized with a 3 D optical profiler WYKO NT1100 ® . Two automatic and then reproducible polishing procedures of materials were set up leading to rough and fine surfaces with suspensions containing particles with a size respectively greater and lower than 1 µm so as to investigate the effect of the substrate roughness on the wettability by the braze. The YSZ wettability by the braze was analyzed with the sessile drop method [16]. The wettability tests were performed under secondary vacuum with roughly or finely polished YSZ substrates surfaces. The evolution of the aspect of the braze drop put on the substrate was recorded as a function of temperature and time with a charge coupled device (CCD) camera. The contact angle (wetting angle) between braze and substrate could thus be directly measured in the photographies.

Both joints were cut along the longitudinal direction and polished up to 1 µm. Their microstructure was then characterized by scanning electron microscopy (SEM) with a JSM 7800F, JEOL ® microscope equipped with secondary electrons (SE) or back scattering electrons (BSE) mode and X-ray energy dispersive spectroscopy (EDX). The phases crystallography was identified by X-ray diffractometry performed in a Philips X'Pert Pro ® equipment using a Bragg-Brentano configuration and a Co anticathode with a wavelength of 1.78901 Å.

Finally, some 3 points static bending tests were carried out according to NF EN ISO 14125 standard [1725] on the 10 mm diametered and 15 mm thick joints (design of the brazed joint: graded MMC-braze-YSZ-braze-graded MMC and design of the diffusion joint: graded MMC-YSZ-graded MMC; for both joints, the greatest reinforcements volume fraction of the graded MMC is close to the YSZ layer (central part) of the joints). During bending tests, the upper mechanical support was a 5 mm diametered cylinder whose speed was 1 mm/min. It has been applied on the YSZ central layer of the joints. The 2 lower static supports were fixed at a distance of 3.25 mm from the centre of the joints. The load applied on the upper mechanical support was in-between 100 N and 20 kN. The fracture surfaces of the joints after bending tests were finally characterized.

Results

3.1.

Base materials 3 mol.% yttria stabilized zirconia sintered commercial sample is characterized by a mean grain size smaller than 0.5 µm. In order to investigate the effect of ceramic roughness on its wettability by the braze, two tetragonal YSZ commercial sintered samples were polished with two different procedures qualified of (i) either rough polishing leading to a mean roughness of 257 nm (figure 2a andb), or (ii) fine polishing entailing a mean roughness of 63 nm (figure 2c andd). The graded Fe based composite is polyphase and is constituted of Fe grains (light grey colored) with an interconnected network of monoclinic zirconia and tetragonal yttria stabilized zirconia (white colored), and of wüstite (FeO) (dark grey colored) (figure 3) [START_REF] Naji | Design and characterization of an oxides hybrid dispersion strengthened iron based composite with a graded and architectural microstructure[END_REF]. The FeO content was shown to not evolve in the various composite subparts of the MMC [START_REF] Naji | Design and characterization of an oxides hybrid dispersion strengthened iron based composite with a graded and architectural microstructure[END_REF]. For further information about the graded composite, the reader may refer to a previous paper of the authors [START_REF] Naji | Design and characterization of an oxides hybrid dispersion strengthened iron based composite with a graded and architectural microstructure[END_REF]. The surface state of the composite seems to be more uneven than that of YSZ (the color scales of figures 2d and f are however different) and its mean roughness amounts to 74 nm (figure 2e andf). The Ticusil TM interlayer presents an average thickness of 54 µm. It is a sandwich shaped film constituted of two layers of Ag-Cu eutectic alloy inserting a Ti layer with an uneven thickness (figure 4); according to EDX analyses, it contains 68% Ag, 27% Cu, 5% Ti (mass%) (54.3%Ag-36.6%Cu-9.1%Ti (at.%)), which is consistent with supplier's data (table 1). It is worth noting that the Ti content is sufficient to render the braze active, according to At its solidus temperature close to 780°C, the braze begins spreading on the substrate. At 900°C, the drop is shiny (figure 5), which means that it is mostly liquid; its angle contact is close to 57.5°. At 950°C, the drop of Ticusil TM spreads more on the YSZ surface and the contact angle reaches 54°.

Figure 6a depicts the effect of the surface roughness of YSZ and of the brazing temperature on YSZ wetting by Ticusil TM braze. It is worth noting that the lower the YSZ surface roughness, the smaller the wetting angle is. In addition, whatever the roughness, the wetting angle is reduced with an increase of the brazing temperature. In order to avoid the coarsening of the microstructure of the graded composite, it was decided to braze at 900°C (figure 1a) the composite and YSZ, both finely polished. Figure 6b shows a reduction of the wetting angle of the braze on the YSZ finely polished up to 20 minutes spent at 900°C. From this time, the wetting angle remains constant. However, in order to avoid (i) a too marked growth of the braze -YSZ and braze -graded composite interfacial layers and (ii) a coarsening of the graded composite microstructure, no soaking at the brazing temperature of 900°C was considered (see figure 1a).

Microstructure of the graded composite -YSZ brazed joint at 900°C.

Compared to the 54 µm primitive braze film thickness (figure 4), the interfacial zone thickness of the brazed joint amounts to only 15 µm at most (figures 7 and 8) because of the pressure applied on the stacked materials and of the solid-liquid transition of the braze during heating step.

The transverse cross section of the brazed joint is displayed in figure 7. On the composite side and from the composite-braze interface, it is worth noting a depletion in iron oxides over a thickness of about 12 µm (figure 7).

The braze -YSZ interface is straight while the braze -composite interface is wavy. The first one is about 1 µm thick while the second one is at most 10 µm thick (figures 7 and8).

The central part [layer 3] of the interfacial zone is composed of the eutectic constituent made of 2 phases, namely a predominant Ag based phase enriched in Fe, Cu and O (point 6, figure 8) and a Ti based one enriched in Fe, Cu and O (point 5, figure 8). From the composite, the composite-braze interface is successively made of (i) a Ti and O bearing layer [layer 1] (point 3, figure 8) with Fe and zirconia rich islands (point 2, figure 8), stemming from the composite and (ii) a Cu and O bearing Ti and Fe based layer [layer 2] with an atomic ratio Fe/Ti equal to 0.5 (point 4, figure 8). This layer is crossed by the eutectic constituent (see yellow arrows in figure 8c andd). The braze-YSZ interface is constituted of a Cu and O bearing Fe and Ti based phase [layer 4] with an atomic ratio Fe/Ti equal to 0.5 (point 7, figure 8) with a similar composition as point 4, figure 8 in layer 2.

These observations indicate that Ti diffused from the centre of the primitive braze (figure 4) towards the composite and penetrates into it over a distance up to 10 µm (figure 8a andf). Fe stemming from the composite is detected up to the YSZ-braze interface in Ti rich phases and in the eutectic constituent. The Fe diffusion distance is about 10 µm (figure 8a andb).

Diffusion weld

Figure 9 depicts the MMC -YSZ interface features. Along the straight interface which is free of porosities, some punctual areas of sintering between YSZ sample and YSZ stemming from the composite are observed, which leads to discontinuous YSZ bonds between both parts of the diffusion joint. Some Fe-YSZ and FeO-YSZ bonds were also formed during solid state sintering at the interface, in the same way that the microstructure was formed in the graded composite from powder particles [1312].

Joints mechanical properties

The global aspects of the laboratory scale joints are displayed in figures 10 a and b. According to the 3 points bending tests, the diffusion joint is twice more performing than the brazed joint in terms of maximum force and deflection. It is also 4 times tougher with regard to the fracture energy, despite its very low value (figure 10c).

Figure 11 depicts pretty smooth fracture surfaces on both MMC and YSZ sides of the brazed joint. XRD analyses prove that the brazed joint fracture surfaces are enriched in Ti3Fe2O9, TiO2 (rutile), TiFeO3 and traces of Ag, Cu, Fe and YSZ (figure 12). The chemical composition of the first two compounds are consistent with EDX analyses on transverse cross section of the brazed joint (see composition of points 4 and 7, figure 8 with a Fe/Ti ratio of 0.5 close to that of 0.66 for Ti3Fe2O9, and composition of point 3, figure 8 very likely consistent with TiO2 phase). The whole results (with EDX analyses of the fractured surfaces (not shown here)) prove that the brazed joint rather fractured in layer 1 and maybe even also in layer 2 at the MMC-braze interface.

Concerning the diffusion joint, its fracture surfaces on MMC and YSZ sides displayed in figure 13 are also rather smooth. The EDX maps (figure 14) suggest that the fracture occurred at the MMC -YSZ interface. the suitable capillarity of the braze (in the case of vertical configuration which is not the case in the configuration used for sessile drop method (figure 5)), the wettability and the capillarity being linked to the braze viscosity, which can evolve with brazing time and temperature in case of dissolution of chemical elements from the base material into the braze, and (iii) the diffusion of braze elements in or towards the base material and the dissolution of the base material or diffusion of its elements in the braze leading to interfacial reactions. As aforesaid, given the different natures of the atomic bonds in YSZ (covalent and ionic) and in the braze (metallic), the ceramic -metallic braze joining is ticklish. This is the reason why various means were developed to improve the wettability of YSZ by the Ag-Cu based braze, such as (iv)

the coating of YSZ with a metal It is the latter solution, that is the insertion of Ti active metal in the braze, which was chosen during the current study. A satisfactory braze must wet but should also be ductile and bond strongly [20]. The Ag-Cu braze is a ductile solvent in which Ti has a very high activity coefficient, which is the relevant thermodynamic parameter that governs the interfacial reaction [25], so that there is no major reservoir of Ti from which too thick brittle reaction product layers may form. With regard to wetting, tThe contact angles obtained (figures 5 and 6) are lower than 90°, which proves a suitable wettability of YSZ by the active braze and the existence of strong chemical interactions, according to [2520]. During sessile drop method, Ti from the braze was supposed to migrate towards the liquid braze / YSZ interface and to react with oxygen coming from YSZ to form a TiO2 layer at the interface according to literature data [2627]; the energy of formation of TiO2 at 900°C is low and amounts to -732 kJ/mol [1524]. The interfacial compounds nature indeed depends on brazing temperature and duration (which promote elements diffusion), on active metal content in braze, on braze thickness [2728] and on the respective nature of the materials of both braze and substrate. Because of the very low surface energy of TiO2 solid (γsv=0.8-0.167×10 -3 ×T (J/m²)) [2627], the Ticusil contact angle decreases and amounts to a value close to 50° for YSZ in the current sessile drop experiment (figures 5 and 6), which is consistent with literature data where a contact angle of 50° under vacuum at 820°C was measured (table of Annex A) [2627].

The braze contact angle decreases with brazing temperature (figures 5 and 6a) and with holding time at brazing temperature (figure 6b), which is consistent with results reported in literature [25,2820,29]. Figure 6 indicates a contact angle of 49° after 30 min at 900°C while a stabilized contact angle of 22° is reached in literature after the same duration at 950°C [2930]. The distinct results can be explained at least by the difference of brazing temperature and of vacuum quality (table of Annex A). Some authors actually claim the dependence of contact angle with brazing atmosphere [2728].

Besides, a decrease of the YSZ substrate roughness is shown to reduce the contact angle (figure 6a) at least of 10° for the three tested brazing temperatures. This tendency agrees with data reported in literature [3031]. A good wetting requires a minimum number of porosities, cracks and asperities on the ceramics surface [START_REF] Hausner | Brazing of metal and ceramic joints[END_REF]. By the way, it is suggested in literature that ceramic should not present open porosities otherwise they could be filled by the braze which would behave as stresses raiser, harmful for the joint mechanical resistance [2419]. The dependence of contact angle on substrate roughness is however controversial in literature [4] and three theories have been proposed to explain the effect of roughness on contact angle [3031]:

(i) a thermodynamic approach of Wenzel for which an extra area of surface generated by the substrate roughness leads to an increase of surface energy by assuming that the substrate surface features are low compared to drop dimensions and that their geometry affects only the surface area. This would entail an increase of contact angle for wetting liquids.

(ii) according to Shuttleworth and Bailey, the asperities on rough surfaces could create some barriers which hinder liquid flow attempting to take up the minimum energy contact angle predicted by Wenzel. With this theory, the contact angle would increase with both wetting and unwetting liquids.

(iii) according to Dettré and Johnson, Huh and Mason, Eick and Good, the asperities are some energy barriers which have to be overcome when the liquid front goes ahead and spreads on the surface. With this theory, the contact angle will decrease if the vibrational energy of the liquid (linked to its enthalpy) is greater than the energy barriers to overcome. Some authors finally assert that, with regard to contact angle, for reactive systems, the relevant roughness to consider in brazing is that of the reaction product which is in direct contact with liquid during braze drop spreading, rather than that of substrate [2728].

Concerning the substrate surface nature in the case of YSZ ODS Fe based MMC, the presence of chemical heterogeneities inducing also various surface states may also modifiy the contact angles and lead to different local contact angles according to the phase nature on the susbtrate, as suggested in literature [2520].

Finally and although it has not been investigated in the current study, the contact angle can also depend on crystallographic texture of the substrate and on the polycrystalline or monocystalline feature of the substrate [2829].

Spreading kinetics

Given the typical diffusion coefficient of chemical elements in molten alloy of 5×10 -9 m²/s, the distance covered by Ti in the liquid braze is at most of 1.8 mm (this height corresponds to the Ticusil TM drop height in figure 5, and figure 4 further shows that Ti layer is intercalated in-between Ag-Cu alloy) which implies that the experimental time required to initiate spreading is lower than 650 s at 900°C. The Ti concentration into the braze however evolves with brazing time.

According to the Reaction Product Control model, the contact angle in a brazeceramic reactive system with the formation of a new compound at the braze-substrate interface evolves between the initial contact angle θ0 (defined as the contact angle of the substrate which has not reacted) and the final contact angle θf (which is the contact angle on the reaction product) [2520]. In the current case, θ0 amounts to 57.5° and θf to 49°, θf being linked to the formation of interfacial TiO2 at 900°C (figures 5 and 6b). In addition, 1200 s are required to obtain a stationary contact angle (figure 6b), which is in agreement with the usual spreading duration limited by the interfacial reaction in braze-ceramic reactive system over the [10-10 4

Work of adhesion

The strength of the chemical interfacial bonding is governed by the existence of crystal orientation relationships and by a low lattice misfit between the compound formed at the interface and the substrate [2829]. The quality of the adhesion between the materials to join can be estimated by the wettability [3633] and also evaluated by the work of adhesion [3633]. The work of adhesion Wadh is defined according to Young-Dupré relationship (Equation 1). Wadh = γLV × (1 + cosθ) Equation 1 with θ contact angle and γ LV surface tension of liquid-vapor interfaces.

According to literature, the addition of Ti in the braze improves the work of adhesion Wadh between the ceramic and the braze [2728]. At 900°C for a brazing duration of 20 min, the contact angle amounts to 49° (figure 6b). By assuming an insignificant effect of Ti on liquid alloy surface tension [2728] and by extrapolating the Ti surface tension of 1650 mJ/m² at its melting point [1621] to 1.74 J/m² at 900°C [2728], which is greater than that of eutectic Ag-Cu of 1 J/m² [2728], the work of adhesion of Ticusil TM -YSZ would amount to 2.88 J/m², which is greater than those calculated at higher temperatures for other dissimilar brazed joints reported in literature [34].

Reactive brazing

Brazing mechanism

The brazed joint very likely forms following the sequential stages displayed in figure 16.

Once the solidus temperature of the braze is reached in the MMC-braze-YSZ stacking (figure 16a), the Ag-Cu alloy begins melting at the MMC-YSZ interface. Because of the presence of a miscibility gap in the Ag-Cu-Ti system at 900°C given the liquidus projection [3735] and the chemical composition of Ticusil TM braze, there is very likely a separation between a liquid enriched in Ti and a liquid depleted in Ti.

Concomitantly, Ti diffuses through the liquid phase towards both YSZ and the MMC and wets the solid surfaces (figure 16b). As already said, the presence of 5 mass.% Ti in the interlayer renders the braze reactive and Ti is well-known as an active metal which improves the wettability of YSZ by Ag-Cu braze [1,2,41,2,4]. In addition, figure 8b proves that Fe stemming from the MMC diffuses through braze (it is noting that Fe diffusion through the liquid must modify the wettability results obtained with the sessile drop method performed with Ticusil TM braze on YSZ). Subsequent interdiffusion and chemical reactions occur at interfaces.

On MMC side, Ti, after having diffused through the liquid, diffuses at the solid state into the MMC over a distance of 10 µm and reduces FeO contained in the MMC, leading to Fe and to the formation of a continuous layer of titanium oxide TiO2 (layer 1). Figure 7 actually displayed a 12 µm large FeO depleted zone in the MMC next to the joint interfacial area, which suggests that O 2-from FeO was attracted by Ti and carried through this area to combine with Ti. The formation of TiO2 on MMC side can be explained by the high chemical affinity between Ti and O and by its low energy of formation of -739.5 to -652.7 kJ/mol over [727-1227°C] [1524] (table 2). TiO2 is further more stable than Fe0.95O whose energy of formation amounts to -197.2 kJ/mol to -165.8 kJ/mol over [727-1227°C] [1524], which justifies the FeO reduction by Ti. At a less extent, TiO2 in layer 1 may also further result from the combination of Ti and O 2-stemming from YSZ reinforcements of the MMC.

Concerning the formation of layer 2, figure 8c and d depicts the presence of eutectic constituent between layer 1 and layer 2. Two assumptions may thus be formulated to explain the formation of layer 2 at TiO2-braze interface: (i) Layer 2 was formed in the liquid: some Fe, either resulting from FeO reduction or initially contained in the MMC, was dissolved in the liquid phase and reacted with the decomposition products of TiO2 at high temperature to form, under a low partial pressure of oxygen, Ti3Fe2O9 and TiFeO3; (ii) Layer 2 was formed at solid state by combination of both TiO2 (layer 1) and Fe (resulting from FeO reduction and present in the MMC) or FeO to form Ti3Fe2O9 and TiFeO3. It is worth noting that both pseudo-rutile Ti3Fe2O9 [38,3936,37] and ilmenite TiFeO3 [4038] have already been identified in mineralogy field as Fe2O3.3TiO2 and FeO.TiO2 respectively. TiO2 is also well-known to be able to accept a high amount of FeO (~ 48 mass.%) in solution [4139]. The pressure applied during brazing on this thick brittle layer 2 may also have initiated some cracks leading to the eutectic penetration at layer 1 -layer 2 interface.

Because of the very low solubility of Cu into Ag [1524] and of Fe and Ti diffusion in liquid phase, some oxidized Ti, Fe, Cu bearing particles are formed in the braze (layer 3). During solidification, a eutectic constituent with 91Ag6Cu2FexO and 87.5Ti8.3Fe4.2CuyO phases is formed at the joint interface (layer 3) (figure 16b).

On YSZ side, some O from YSZ migrates towards the YSZ-braze interface (the long range transport of oxygen ions occurs by hopping between anion sites in YSZ via the vacancies • Ti, Y and Zr bearing phase, Ag, Cu, Ti, Y and Zr bearing phase [34], and whose available energy of formation of most of them is tabulated in table 2.

The Ti3Fe2O9 and TiFeO3 ternary compounds of the braze / YSZ interfacial layer differ from the TiO2 usually detected during such an experiment with Ticusil TM braze [2627] because of the diffusion of Fe stemming from the MMC through the braze. The energy of formation of FeTiO3 is -26.4 to -24.8 kJ/mol over [727-1227°C] [1524], but no data about that of Ti3Fe2O9 could be found in literature. Since the energy of formation of FeTiO3 is greater than that of TiO2 (table 2), the kinetics of formation of Ti3Fe2O9 and TiFeO3, and the local chemical enrichment can very likely explain the formation of these phases on both sides of the braze.

Thickness and morphology of interfacial zones

The Ti bearing global layer (layer 1 + layer 2) at the MMC-braze interface is thicker than layer 4 at braze-YSZ interface, while layer 2 and layer 4 present the same thickness and nature (figures 7 and 8). There was actually a competition between YSZ and MMC to attract Ti. As there was two O available sources on MMC, namely FeO and YSZ, the Ti bearing global layer (layer 1 + layer 2) at the MMC-braze interface is thicker. In addition, YSZ already presents some oxygen vacancies. The formation of layer 4 at the braze -YSZ interface consumes some oxygen from YSZ, which introduces more oxygen vacancies in YSZ and may entail its destabilization.

With regard to the interfacial layers morphology, the braze -YSZ interface is rather straight while the braze -MMC interface is wavy (figures 7 and 8). According to roughness maps in figure 1d andf, the mean roughness parameter of YSZ is slightly smaller than that of MMC. Moreover, the roughness amplitude of the braze -MMC interface which amounts 8 µm (figures 7 and 8a) is greater than the 1.3 µm roughness amplitude of the MMC surface (figure 1f). The existence of a wavy and thick interfacial zone on MMC side suggests its formation partly at the liquid state, the growth of the layers being important at liquid state and slower during solidification [47].

Diffusion welding

Diffusion welding is recommended in literature to join MMCs provided the thermal cycle is mastered in order to preserve the fineness of the MMCs microstructure [START_REF] Cam | Progress in joining of advanced materials[END_REF]. In the current study, it is worth noting that the thermal cycle presented in figure 1 enabled the sintering of the graded composite and the concomitant diffusion welding of the ceramic to the graded composites. The finely polished YSZ and MMC present a Ra of 63 and 74 nm, respectively (figure 2) which is lower than the maximal 0.4 µm surface roughness recommended in literature for diffusion welding [START_REF] Zhang | Progress in joining ceramics to metals[END_REF][START_REF] Fernie | Joining of engineering ceramics[END_REF][START_REF] Cam | Progress in joining of advanced materials[END_REF]. In addition, the melting temperatures of Fe and ZrO2 are respectively 1538°C and 2690°C while the diffusion welding temperature amounts to 1250°C. The homologous temperatures (HT defined as the ratio between the processing absolute temperature and the absolute melting temperature of the material) for Fe and YSZ are thus respectively 0.84 and 0.51. These values agree well with literature which recommends proceeding either a few hours at a HT of 0.5-0.6 or few minutes at a HT of 0.9 The applied low pressure of 50 kPa (compared to that recommended between 1 and 200 MPa [3]) enabled to deform the Fe based composite compacted particles (which constitute the MMC) and to suppress the cavities at the MMC particles -bulk YSZ interface. An intimate contact was then generated between the MMC composite compacted particles and the bulk YSZ. An atomic bonding was subsequently established via diffusion and the joint interface was formed via solid state sintering of YSZ-YSZ, YSZ-Fe and YSZ-FeO (figure 17).

Brazing vs. diffusion welding

Concerning the metal-oxide adhesion of MMC and YSZ, two distinct architectural interfaces were developed according to the joining process. As aforementioned, brazing leads to a few micrometers thick oxide layers at the braze-MMC and braze-YSZ interfaces separated by a eutectic phase, while diffusion welding led to discontinuous chemical bonds resulting from YSZ sintering, YSZ stemming from both MMC and bulk YSZ. The mechanical results show that the bending strength and toughness of the diffusion joint is far greater than those of the brazed joint, even if they are rather low (it is worth noting that the test configuration was the most critical since the support was located on the YSZ and its interfaces and did not mimic the real case). The mechanical properties are directly governed by the joints microstructure. It is important to notice that for both joints in the current study, the ceramic YSZ, which presents a small number of slip systems, a low toughness, a low ability to plastic deformation and spontaneous crack propagation [START_REF] Hausner | Brazing of metal and ceramic joints[END_REF], is not the most critical defect (figures 11 to 14). Both joints are characterized by a brittle interfacial and non-cohesive fracture. Concerning the brazed joint, it fractured in the 5 µm (TiO2) and 1 µm (Fe2Ti3O9 and FeTiO3) thick interfacial layers rather than in the ceramic and the eutectic constituent (figures 12, 13). TiO2 toughness (K1c) amounts to 2. 4-3.3 MPa.m 0.5 [48] compared to YSZ toughness (K1c) of 8 MPa.m 0.5 [1524] while no data were found for Fe2Ti3O9 and FeTiO3 compounds. Indeed, according to [START_REF] Frear | Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys[END_REF], under shearing solicitations, the brazed joints fracture first through the braze unless (i) the oxides or intermetallic compounds formed at the interfaces occupy a significant fraction of the joint greater than 10% or (ii) the strain rate is high (greater than 0.1 mm/min). Thick and brittle interfacial layers are indeed well-known to be deleterious for the joints mechanical resistance [3633]; a 2 µm thick titanium oxide layer is for instance known to be deleterious for joint mechanical resistance [4442]. With regard to the diffusion joint, fracture occurs at the MMC-YSZ interface (figure 14) but the YSZ-YSZ discontinuous bonds improve the mechanical resistance of the joint, compared to the brazed joint.

Despite the difference of cooling rates during brazing (5°C/min) and diffusion welding (10°C/min) (figure 1), the 3 points bending strength of the diffusion joint is greater than that of the brazed one (figure 10). Even if the temperature during diffusion welding reached 1250°C, joining proceeded at the solid state and the greater cooling rate seems to have not increased as much the residual stresses into the joint. During brazing, the Young moduli mismatches (E(ZrO2) = 220 GPa °C-1 [14,34,4323,34,41]) leading to braze shrinkage were expected to generate an elastic mismatch, that is tensile stresses in YSZ and compressive stresses in the braze [START_REF] Fernie | Joining of engineering ceramics[END_REF]. Indeed, a thermal strain equal to ∆α×∆T, with ∆α: the difference of CTE of YSZ (and Fe) and the braze, and ∆T: the temperature difference between the braze temperature (900°C) and the solidus temperature (which may be, in first approximation, close to Ag-Cu eutectic temperature (figure 8), that is 777°C [1524]), can occur during cooling stage. With the previous data (although they are not rigorous at 900°C), the thermal strains are close to 1.09×10 -3 at YSZ-braze interface and 4.79×10 -4 at Fe-braze interface. These values suggest that strain accommodation may occur via interfacial plastic flow due to a suitable ductility of Ag-Cu based braze [34].

Compared to brazing, which is an additional step to the elaboration stage of the MMC, diffusion welding occurs concomitantly with MMC sintering, which is cost-effective even without taking into account the braze price. Diffusion welding further presents the advantage of avoiding an additional coarsening of the MMC microstructure. Besides, the use of a Cu bearing braze to join the MMC and YSZ is not particularly suitable since it (i) can diffuse into Fe favoring its embrittlement and (ii) may entail corrosion, harmful for the aimed application.

Ways of optimization

Brazing -The applied load on the stacked materials during brazing could be optimized by taking into account the wetting properties and the braze viscosity in order to reduce the interfacial layers thicknesses [3633] which constitute the weak mechanical areas in the current study. Besides, thin reaction zones at the brazed joint interfaces may also be obtained by induction brazing which should require a shorter thermal joining cycle [START_REF] Hausner | Brazing of metal and ceramic joints[END_REF].

The enhancement of the mechanical resistance of the braze may be solved (i) by the improvement of the interfaces quality, (ii) by the development of an architectural braze such as a metallic foam (ex: 316L stainless steel) filled by the braze [8] or (iii) by the addition of a controlled volume fraction of ceramic particulate reinforcements [START_REF] Fernie | Joining of engineering ceramics[END_REF][START_REF] Hausner | Brazing of metal and ceramic joints[END_REF]24,503,[START_REF] Hausner | Brazing of metal and ceramic joints[END_REF]19,[START_REF] Lin | Characterization of yttria-stabilized-zirconia / stainless steel joint interfaces with gold-based interlayers for solid oxide fuel cell applications[END_REF] in the braze.

According to figure 1a, the 5°C/min cooling rate during the brazing cycle was chosen so as to limit the thermal residual stresses into the joint. Nevertheless and as mentioned with regard to brazing, the mismatch of both Young moduli and CTE between YSZ, the braze and the MMC generate some thermal residual stresses in particular in YSZ where the stresses are the greatest near the joint interface, which can lead to joint fracture in this area. These stresses depend, inter alia, on the braze thickness and on the materials size [START_REF] Hausner | Brazing of metal and ceramic joints[END_REF]244,19]. It was shown that the amplitude of residual stresses increases with the ceramic thickness [START_REF] Suganuma | Effect of Thickness on Direct Bonding of Silicon Nitride to Steel[END_REF]. The 50 µm thickness of the braze used in the current study is usual and the YSZ thickness is already rather low. In order to tempt to tackle the problem of residual stresses, various solutions can be proposed provided they do not diminish the wetting effect of Ti: (i) the addition of a buffer interlayer to the braze Diffusion welding -One or several interlayers may be added at the ceramic-MMC interface to assist joining and to adapt the stresses repartition after welding [START_REF] Fernie | Joining of engineering ceramics[END_REF][START_REF] Cam | Progress in joining of advanced materials[END_REF]. The metal-to-ceramic adhesion bonds may finally be enhanced by performing sintering under microwave fields characterized by a short sintering duration. These adhesion bonds were shown to be increased of about 200-300% compared to those obtained by conventional sintering [52].

Conclusions

To summarize, two graded Fe based composite (MMC) -yttria stabilized zirconia (YSZ) joints were developed by brazing, with Ticusil TM braze, and by diffusion welding. The various following points were evidenced:

-Ticusil TM braze presented a better wettability on smooth YSZ substrate. Wetting further increased with brazing temperature. -The brazed joint was formed by solidification while the diffusion joint was developed by solid state sintering. -During reactive brazing, some few micrometers thick continuous TiO2, Fe2Ti3O9 and FeTiO3 layers were developed at both the braze-YSZ and braze-MMC interfaces. On the MMC side, TiO2 was formed by reduction of FeO (contained in the MMC) by Ti. The formation of Fe2Ti3O9 and FeTiO3 compounds on the YSZ side resulted from the combination of Fe and Ti which diffused through the braze, and O provided by YSZ and dissolved in the braze. -During diffusion welding, some YSZ-YSZ, YSZ-FeO and YSZ-Fe discontinuous bonds were created by sintering at the MMC-YSZ interface. -Both joints fractured at interfaces. The diffusion joint showed the best bending resistance.

Aknowledgments

The authors are very thankful to Ch. Vermander, Ecole Centrale de Lille, France for the brazing TZM device, dies and pistons machining, to P.

Figure 1 :

 1 Figure 1: Brazing (a) and diffusion welding (b) cycles.

Figure 2 :

 2 Figure 2: Surfaces of YSZ after rough (a) and fine (c) polishing (light microscopy) and their corresponding roughness maps (b and d, respectively). Surface of Fe-40vol.% YSZ composite after fine polishing (light microscopy) (e) and its corresponding roughness map (f). The color scales indicate the roughness level.

Figure 3 :

 3 Figure 3: Details of the polished microstructure of the Fe-40 vol.% YSZ subpart of the graded metallic matrix composite (BSE/SEM).

  literature data [1,2,4]. The latter Ti content in the braze is also known to improve the 4 points bending strength of YSZ-YSZ brazed joints [18].

Figure 4 :

 4 Figure 4: Transverse cross section of Ticusil TM braze film (a) (SE/SEM) and associated Cu Kα (b), Ag Lα (c) and Ti Kα (d) X-ray maps (EDX).

1 .

 1 Contact angle and wetting kinetics.

Figure 5

 5 Figure 5 displays an example of the evolution of YSZ wettability by Ticusil TM braze with temperature according to the sessile drop method. It shows the progressive spreading of the braze on the substrate. At its solidus temperature close to 780°C, the braze begins spreading on the substrate. At 900°C, the drop is shiny (figure5), which means that it is mostly liquid; its angle contact is close to 57.5°. At 950°C, the drop of Ticusil TM spreads more on the YSZ surface and the contact angle reaches 54°.Figure6adepicts the effect of the surface roughness of YSZ and of the brazing temperature on YSZ wetting by Ticusil TM braze. It is worth noting that the lower the YSZ surface roughness, the smaller the wetting angle is. In addition, whatever the roughness, the wetting angle is reduced with an increase of the brazing temperature. In order to avoid the coarsening of the microstructure of the graded composite, it was decided to braze at 900°C (figure1a) the composite and YSZ, both finely polished. Figure6bshows a reduction of the wetting angle of the braze on the YSZ finely polished up to 20 minutes spent at 900°C. From this time, the wetting angle remains constant. However, in order to avoid (i) a too marked growth of the braze -YSZ and braze -graded composite interfacial layers and (ii) a coarsening of the graded composite microstructure, no soaking at the brazing temperature of 900°C was considered (see figure1a).

Figure 5 :

 5 Figure 5: Effect of temperature on YSZ (finely polished) wettability byTicusil TM braze spreading on YSZ (finely polished) (sessile drop method) [the thickness of YSZ sample is about 1.5 mm].

Figure 6 :

 6 Figure 6: a) Effect of different roughness values and of temperature on contact angle between Ticusil TM braze and YSZ substrate. A constant mass of 0.14 g of braze was used for the experiments. b) Effect of time at a brazing temperature of 900°C on wetting angle of Ticusil TM braze (mass of 0.158 g) on YSZ surface presenting a mean roughness of 63 nm. The contact angle values were measured on the sideways images of the drop captured by the digital camera (such as those in figure 5) using the ImageJ TM image processing software.

Figure 7 :

 7 Figure 7: Global view of the transverse cross section of the interface of the brazed [Fe -40 vol.% YSZ composite / YSZ] joint (BSE/SEM).

Figure 8 :

 8 Figure 8: Transverse cross section of the interface of the brazed [Fe -40 vol.% YSZ composite / YSZ] joint (BSE/SEM) (a) and Fe Kα (b), Cu Lα (c), Ag Lα (d), O Kα (e), Ti Kα (f) and Zr Lα (g) X-ray maps (EDX/SEM). Magnified zone of micrograph a (h) and atomic compositions of the various phases reported in the table (the calculations are made without taking into account O content) (EDX/SEM).

Figure 9 :

 9 Figure 9: a) Global view of the transverse cross section of the [Fe -40 vol.% YSZ composite / YSZ] diffusion joint interface and b) magnification of the black encircled area in figure (a) (BSE/SEM) with Fe Kα (c), Zr Lα (d), Y Lα (e) and O Kα (f) X-ray maps (EDX/SEM).

Figure 10 :

 10 Figure 10: Brazed (a) and diffusion (b) joint features. Maximum force, largest deflection and fracture energy of both joints during 3 points bending tests (c).

Figure 11 :

 11 Figure 11: Fracture patterns of brazed joint after 3 points bending tests (SE/SEM).

Figure 12 :

 12 Figure 12: Magnified X-ray diffraction patterns of fracture surfaces of the brazed joint on composite (a) and YSZ (b) sides. M and T are the respective abbreviations of monoclinic and tetragonal crystal lattices.

Figure 13 :

 13 Figure 13: Fracture patterns of diffusion joint after 3 points bending tests (SE/SEM).

Figure 14 :

 14 Figure 14: Fracture patterns of diffusion joint after 3 points bending tests: YSZ side micrograph (SE/SEM) (a) and Fe Kα (b), O Kα (c), Zr Lα (d) and Y Lα (e) X-ray maps; composite side micrograph (SE/SEM) (f) and Fe Kα (g), O Kα (h), Zr Lα (i) and Y Lα (j) Xray maps (EDX/SEM).

  TiH2 in the braze and the coating of zirconia with TiH2 leading to TiH2 decomposition during brazing entailing the formation of a Ti layer on zirconia substrate [22,23], (vi) the insertion of an active metal in the braze such as Ti (with a content in-between 2 and 5% [1,2,4]). The active metal modifies the surface chemistry of the ceramics, diminishes its surface energy by forming an intermediate reaction layer and reduces the contact angle of the melted braze on YSZ [2,18,21,24].

  s] range [2520]. According to literature [3532], the logarithmic plots of the ratio (cosθf -cosθ) / (cosθf -cosθ0) (with θ, instantaneous angle contact angle) vs. brazing holding time at 900°C gives a kinetic constant with opposite sign of -0.0052 s -1 (figure 15), which is in agreement with literature data for reaction-limited spreading [29,3530,32].

Figure 15 :

 15 Figure 15: Spreading kinetics of the Ticusil braze drop on YSZ substrate at 900°C (sessile drop method).

  [2829]) and is subsequently very likely dissolved in the braze (Ag presents a very low chemical affinity for O but can dissolve a big amount of O [2520]). It is worth noting that there is no complete reduction of YSZ in Zr by Ti since ZrO2 is more thermally stable than TiO2 according to Ellingham diagram [4240]. Since the addition of Y 3+ cations in ZrO2 lattice introduces oxygen vacancies [2829], the O depletion of YSZ during brazing generates a greater density of oxygen vacancies in YSZ lattice. It has already been suggested that Ti can partially deplete zirconia in oxygen [4,29,43,44,4,30,41,42] via the likely formation of a Ti-O complex oxide [4341]; then the freed-up oxygen atoms combine with titanium atoms to form titanium oxides which present a low Gibbs energy [4341]. This oxide layer, whose formation is presumably fast, is expected to hinder the dissolution of YSZ in the braze [18] contrary to observations reported in literature [4341]. The layer 4 thus results from the diffusion of Fe stemming from the composite through the liquid phase over a distance of 10 µm towards the braze-YSZ interface and its combination with Ti and O to form Ti3Fe2O9 and TiFeO3 according to the mechanisms previously proposed to explain formation of layer 2.

Figure 16 :

 16 Figure 16: Schematic stages of the formation mechanism of the brazed joint. a) Stacking of MMC, braze and YSZ. b) During heating, diffusion of elements and ions as well as chemical reaction; the arrows indicate diffusion directions. c) At room temperature, architecture of the brazed joint.

  [2,3,7]. Young moduli of Fe and ZrO2 are respectively 221 and 220 GPa [1524] while the coefficient of thermal expansion (CTE) of YSZ and Fe are respectively 8.9-10.6×10 -6 °C-1 [34,4334,41] and 12.1×10 -6 °C-1 [1524]. Since the values of these parameters are rather close for both materials, it suggests once again that these materials were suitable to be joined by diffusion welding [2829].

Figure 17 :

 17 Figure 17: Schematic stages of the formation mechanism of the diffusion joint. a) MMC and YSZ workpieces. b) Beginning of sintering cycle under applied pressure which enables to bring the workpieces into contact. c) During heating, creation of YSZ-YSZ, YSZ-Fe, YSZ-FeO bonds at the joint interface via solid state sintering and formation of the interface.

  [1524], E(Fe) = 221 GPa [1524], E(Ticusil TM ) = 85 GPa [49]) and the CTE mismatches (CTE (YSZ) = 9.6×10 -6 °C-1 over [20-400°C] according to the supplier, CTE(Fe) = 14.6×10 -6 °C-1 over [20-800°C] [1524] and CTE(Ticusil TM ) = 18.5×10 -6

[ 2 ]

 2 , this interlayer being ductile (for instance Ag, Cu, Pd, Ni) [4,324,45], (ii) the addition, to the braze, of interlayers with graded CTE [1,4,8,361,4,8,33], (iii) the use of filler metals or alloys, with CTE similar to those of ceramics [2114] and (iv) the insertion of a 'braze joint design' which maintains the ceramic in a compressive state [2114].

Table 1 :

 1 Chemical composition (mass. %).

	Material	ZrO2 Y2O3 Fe2O3 Na2O	Ag	Cu	Ti	Al	Bi	Cd	P	Pb	Si
	YSZ	94.8	5.2	<0.01 <0.01	-	-	-	-	-	-	-	-	-
	Ticusil TM	-	-	-	-	68.8	26.7	4.5 <0.001 <0.03 <0.01 <0.008 <0.025 <0.05

4. Discussion 4.1. Brazing 4.1.1. Effect of Ticusil TM -YSZ brazing conditions on wettability, braze spreading kinetics and work of adhesion

	4.1.1.1. Wettability
		Four fundamental principles are required for suitable brazing [13,19,2013,22,26], namely
	(i)	the wetting of the substrate by the braze,
	(ii)

Table 2 :

 2 Energy of formation of compounds (kJ/mol). Phases nature The Ti3Fe2O9 and TiFeO3 phases observed along the brazed joint differ from the phases usually observed in literature during steel or Ti -YSZ brazing with Ag-Cu-Ti interlayer (table of Annex A), namely

	Compound	Fe2Ti	TiO	Ti2O3	TiO2	CuTi	CuTi2	Cu4Ti	Cu4Ti3	Cu2O Ti3Cu3O
	727°C	-	-447.1	-245.7	-739.5					-95.5
	900°C	-67.9	-	-	-	-14.7	-6.6	-3.2	-7.1		-520.5
	1227°C	-	-402.7	-116.5	-652.7					-59.5
	Reference	4543		1524		18		4644		1524	18
	4.1.2.2.

• Fe2Ti [32,3345,46], • TixOy such as Ti2O3 [32,3345,46], TiO, Ti2O [18],

  Denoirjean, SPCTS, Limoges University, France for the wetting analyses, to A.-L. Cristol, LaMcube, Ecole Centrale de Lille, France for her assistance in roughness measurement and to M. Bouhallier, Agence d'Essais Ferroviaires (AEF), France for his help in 3 points static bending tests. They also thank the Microscopy platform of Lille University, France for the use of electron microscopes. K. Naji is finally grateful to ANRT (French National Association of Research and Technology) for her financial support under CIFRE convention n° 2014/1316. Table:Brazing conditions, interfacial phases and mechanical strength of joints.

	Annex A Material 1 YSZ YSZ (Ra = 0.02 µm) ZrO2 YSZ YSZ YSZ coated with TiH2 YSZ YSZ	Material 2 ---Ferritic stainless steel Ferritic stainless steel (17-19%Cr) Stainless steel coated with Ni Ti Steel	Braze (mass.%) (layer thickness) Ag63Cu35.25 Ti1.75 Ag60at-Cu40at-Ti5at (melting T = 788°C) Ag-Cu-Ti Ag58-Cu32-Pd10 or Ag65-Cu20-Pd15 Ag68.8-Cu26.7-Ti4.5 (0.1-0.12 mm) (T liquidus = 900°C) Ag72-Cu28 + 25 µm and 2.7 µm sized TiH2 particles Ticusil TM Ticusil TM	Brazing cycle and conditions Vacuum (2.5×10 -6 atm) Vacuum (< 9.8× 10 -11 atm) or pure Ar, Brazing at 950°C, Cooling rate = 4°C/min Vacuum (1.3×10 -9 atm), Heating up to 915°C, holding time of 5 min at this temperature, Slow cooling up to room temperature, Pressure = 0.3N No pressure Vacuum 0.1, 1 et 6 h and 3°C/min cooling rate, Applied pressure = 0.1 MPa, 910°C for 5 min -at 900°C of Interfacial phases Strength + fracture location TiO2 -4 µm layer with Ti and O with various Ti/Cu/O atomic ratios: 54/0/46, 40/30/30 and 3-5µm thick layer of Ti54-Cu27-O19 (at%) -TixO at interface and formation of an out-of stoichiometric zirconia Bending strength of 300 MPa Cu rich phase containing Zr in solid solution as well as Fe and Cr, Cu3Pd layer at steel side with 58Ag-32Cu-10Pd and 65Ag-20Cu-15Pd particles and Fe(Cr) needle-like phase, SiO2 and Ti3O5 at YSZ side Fe2Ti at steel-braze interface and Ti2O3 and Fe2TiO4 at braze -YSZ interface. Tensile strength of 16.7 MPa Shear strength of 40.2 MPa Fracture in braze From YSZ to stainless steel: CuTi3, Ti3Cu3O, Cu4Ti3, NiTi2, Ni3Ti, Ti Shear strength of 90 MPa at Ticusil-braze Along the braze: Ag + TiCu4 for 0.1h, Ag + Cu2O for 1h and Ag+Cu2O+(Ag+Ti3Cu3 O) for 6h at Ti-braze interface: Ti2Cu, TiCu, Ti3Cu4, Fe, Cr, Zr and Y diffusion in braze; Ag diffusion in YSZ. Ag and Zr bearing layer at YSZ-Ticusil TM interface; Ti enrichment at YSZ surface; adjacent Ti, Cu, Ag, Zr, Y bearing diffuse layer; Ag-Cu eutectic in braze; Fe, Cr, Ti bearing phases in eutectic constituent on steel side; Zr,Y, Ti bearing phases in eutectic constituent on YSZ side. TiCu4, α-Ti+Ti2Cu Brazing at Ti diffusion in steel; -	Reference 2627 2930 3 3153 32,3345,46 22,2315,16 34
			(68.8Ag-26.7Cu-4.5Ti) (0.05 mm)	(9.8×10 -10 atm) and Ar purge, Heating at 5°C/min up to 900°C,	interface: 0.1 h brazing: TiCu and TiO 1 and 6 h brazing: Ti3Cu3O, Ti2O, TiO
				holding time