Elaboration par Spark Plasma Sintering et caractérisation de composites et multi-couches zircone yttrié/MoSi2(B) pour application barrière thermique auto-cicatrisante

La réparation des revêtements barrières thermiques endommagés par fissuration entraine des coûts de maintenance très élevés. Dans cette étude, qui s’inscrit dans le cadre du projet Européen FP7-SAMBA, il a été proposé d’utiliser des particules de MoSi2(B), revêtues d’une couche d’alumine, comme agent cicatrisant. L’oxydation de celles-ci doit entrainer la formation de silice amorphe qui s’écoule dans la fissure puis réagit avec la barrière thermique en zircone yttriée pour former du zircon. Cette étude traite dans un premier temps de l’élaboration par Spark Plasma Sintering (SPS) de composites modèles composés de zircone yttriée et de particules de MoSi2(B) non revêtues. Les propriétés mécaniques (ténacité, dureté, module d’Young) et thermiques (conductivité thermique, coefficient de dilatation) de ces composites ont été déterminées. Les travaux se sont ensuite orientés vers l’étude du comportement en oxydation cyclique à 1100 °C sous air de ces composites par thermogravimétrie cyclique. La modélisation de l’oxydation de ces composites mais aussi de systèmes multi-couches MoSi2(B)/YPSZ modèles a permis de déterminer les mécanismes et les cinétiques de formation de la silice et du zircon. Une augmentation significative des cinétiques de formation de ces oxydes a été observée lorsque le bore est ajouté dans le MoSi2 ce qui peut être potentiellement très bénéfique pour la cicatrisation des fissures. L'utilisation du procédé SPS a permis de réaliser des systèmes barrières thermiques auto-cicatrisants sur substrats en superalliages à base de nickel revêtus à partir de zircone yttriée et de particules de MoSi2(B) elles-mêmes revêtues d’une couche d’alumine. La pré-oxydation des substrats revêtus favorise la croissance d’une couche d’alumine qui empêche la formation de siliciures par réaction entre les particules et la sous-couche. Ces revêtements présentent une bonne résistance à l’endommagement en cyclage thermique. Les observations post-mortem de ces systèmes mettent en évidence la cicatrisation locale de fissures par formation de silice et de zircon. Bien qu’il ne soit pas possible aujourd’hui de dire si la présence de ces particules augmente ou non la durée de vie de la barrière thermique, par manque de systèmes de référence, ces observations très encourageantes démontrent expérimentalement la validité du concept d’auto-cicatrisation des barrières thermiques proposé dans le cadre de ce projet

Thermal cycling and reactivity of a MoSi2/ZrO2 composite designed for self-healing thermal barrier coatings

Consolidated (relative density of 84%) composite made of molybdenum di-silicide (MoSi2) particles dispersed in a yttria partially stabilized zirconia matrix (8Y2O3–Zr02) was prepared by spark plasma sintering. Cyclic oxidation of the composite at temperature ranging from 1000 °C to 1300 °C was studied. Parabolic rate constants (kp) values of the composite material are in good agreement with those obtained in the literature for the oxidation of bulk MoSi2. Following oxidation exposure, formation of Mo5Si3, SiO2 and ZrSiO4 phases was observed. These observations are compatible with the use of MoSi2 as a self-healing agent in YPSZ thermal barrier coatings

Thermal conductivity of binary ceramic composites made of insulating and conducting materials comprising full composition range – applied to yttria partially stabilized zirconia and molybdenum disilicide

The thermal diffusivity and conductivity of dense and porous binary composites having an insulating and conducting phase were studied across its entire composition range. Experimental evaluation has been performed with MoSi2 particles embedded into yttria partially stabilized zirconia (YPSZ) as prepared by spark plasma sintering (SPS). The thermal diffusivity of the composites was measured with Flash Thermography (FT) and Laser Flash Analysis (LFA) techniques. Subsequently, the thermal conductivity was determined with the measured heat capacity and density of the composites. The actual volume fraction of the conducting phase of the composites was determined with image analysis of X-ray maps recorded with scanning electron microscopy (SEM). The phases present and their density were determined with X-ray diffractometry (XRD) using Rietveld refinement. The thermal diffusivity increases with increasing volume fraction of MoSi2. Porosity reduces the thermal diffusivity, but the effect diminishes with high volume fractions MoSi2. The thermal diffusivity as a function of the MoSi2 volume fraction of the YPSZ composites is captured by modelling, which includes the porosity effect and the high conductivity paths due to the percolation of the conductive phase

Influence of embedded MoSi2 particles on the high temperature thermal conductivity of SPS produced yttria-stabilised zirconia model thermal barrier coatings

To prolong the lifetime of thermal barrier coatings (TBCs) recently a new method of microcrack healing has been developed, which relies on damage initiated thermal decomposition of embedded molybdenum disilicide (MoSi2) particles within the TBC matrix. While these MoSi2 particles have a beneficial effect on the structural stability of the TBC, the high thermal conductivity of MoSi2 may have an unfavourable but as yet unquantified impact on the thermal conductivity of the TBCs. In this work the thermal conductivity of spark plasma sintering (SPS) produced yttria-stabilised zirconia (YSZ) model thermal barrier coatings containing 10 or 20 vol.% of MoSi2 healing particles was investigated using the laser flash method. Measurements were performed on free-standing composite material over a temperature range from room temperature up to 1000 °C. Microstructural analysis was carried out by SEM combined with image analysis to determine the size, distribution and area fraction of healing particles. The measurements were compared with the results from microstructure-based multi-physics finite element (FE) models and analytical models (the asymmetric Bruggeman model and the Nielsen model) in order to study the effects of the addition of MoSi2 particles as well as the presence of micro-pores on the apparent thermal conductivity. The results show a strongly non-linear increase in the thermal conductivity of the composite material with the MoSi2 volume fraction and a dependence on the aspect ratio of MoSi2 particles. Interparticle connectivity is shown to play a big role too

Elaboration by Spark Plasma Sintering and characterization of yttria partially stabilized zirconia/MoSi2(B) composites and multi-layer systems for self-healing thermal barrier coatings

La réparation des revêtements barrières thermiques endommagés par fissuration entraine des coûts de maintenance très élevés. Dans cette étude, qui s’inscrit dans le cadre du projet Européen FP7-SAMBA, il a été proposé d’utiliser des particules de MoSi2(B), revêtues d’une couche d’alumine, comme agent cicatrisant. L’oxydation de celles-ci doit entrainer la formation de silice amorphe qui s’écoule dans la fissure puis réagit avec la barrière thermique en zircone yttriée pour former du zircon. Cette étude traite dans un premier temps de l’élaboration par Spark Plasma Sintering (SPS) de composites modèles composés de zircone yttriée et de particules de MoSi2(B) non revêtues. Les propriétés mécaniques (ténacité, dureté, module d’Young) et thermiques (conductivité thermique, coefficient de dilatation) de ces composites ont été déterminées. Les travaux se sont ensuite orientés vers l’étude du comportement en oxydation cyclique à 1100 °C sous air de ces composites par thermogravimétrie cyclique. La modélisation de l’oxydation de ces composites mais aussi de systèmes multi-couches MoSi2(B)/YPSZ modèles a permis de déterminer les mécanismes et les cinétiques de formation de la silice et du zircon. Une augmentation significative des cinétiques de formation de ces oxydes a été observée lorsque le bore est ajouté dans le MoSi2 ce qui peut être potentiellement très bénéfique pour la cicatrisation des fissures. L'utilisation du procédé SPS a permis de réaliser des systèmes barrières thermiques auto-cicatrisants sur substrats en superalliages à base de nickel revêtus à partir de zircone yttriée et de particules de MoSi2(B) elles-mêmes revêtues d’une couche d’alumine. La pré-oxydation des substrats revêtus favorise la croissance d’une couche d’alumine qui empêche la formation de siliciures par réaction entre les particules et la sous-couche. Ces revêtements présentent une bonne résistance à l’endommagement en cyclage thermique. Les observations post-mortem de ces systèmes mettent en évidence la cicatrisation locale de fissures par formation de silice et de zircon. Bien qu’il ne soit pas possible aujourd’hui de dire si la présence de ces particules augmente ou non la durée de vie de la barrière thermique, par manque de systèmes de référence, ces observations très encourageantes démontrent expérimentalement la validité du concept d’auto-cicatrisation des barrières thermiques proposé dans le cadre de ce projet.Repair of thermal barrier coatings (TBC) systems damaged by cracking leads to significant maintenance costs. In this project (FP7-SAMBA), it was proposed to use MoSi2(B) particles, coated with an alumina shell, as healing agent for TBCs. Healing particles intercepted by cracks will oxidize preferentially, leading to the formation of amorphous SiO2, which flows into cracks and subsequently reacts with the TBC leading to the formation of a load bearing ZrSiO4 phase. In this study model composite materials were prepared from mixtures of yttria partially stabilized zirconia (YPSZ) and uncoated MoSi2(B) particles by using Spark Plasma Sintering (SPS) technique. Mechanical (toughness, hardness, Young modulus) and thermal (conductivity, coefficient of thermal expansion) properties of these materials were determined. Then, cyclic thermogravimetry analysis (CTGA) was used to study the oxidation behavior of these materials at 1100 °C in air. Kinetics of silica and zircon formations were determined through modelling of the oxidation of composite materials but also the oxidation of multi-layer YPSZ/MoSi2(B) materials. Boron addition was shown to significantly increase silica and zircon formation rates which could be very beneficial for the healing of the cracks. Then, SPS technique was used to sinter self-healing thermal barrier coatings on bond coated Ni-based superalloys from mixtures of YPSZ and Al2O3-coated MoSi2(B) particles. The pre-oxidation of coated substrates was shown to prevent the detrimental formation of silicides by the reaction of MoSi2(B) particles and the bond coat. Good results were obtained upon thermal cycling and post-mortem observations highlight local healing of cracks. At this time, it is too early to quantify the potential effect of the particles on the TBC lifetime due to a lack of reference systems and statistics. However, these observations demonstrate, experimentally, the validity of the self-healing mechanism proposed in the framework of this project

Reduced complexity, high performance digital delta-sigma modulator for fractional-N frequency synthesis

This paper presents the design consideration of high order digital ΔΣ modulators used as modulus controller for fractional-N frequency synthesizer. A third-order MASH structure (MASH 1-2) is designed and implemented which allows for the input to operate over 75% of the input adder capacity. The number of the output levels is reduced to two bits. The circuit was verified through simulation, ASIC implementation and exhibits high potential for a gigahertz range, low-power monolithic CMOS frequency synthesizer

Self-healing thermal barrier coating systems fabricated by spark plasma sintering

The present paper focuses on the Spark Plasma Sintering (SPS) manufacturing of a new type of self-healing thermal barrier coating (TBC) and a study of its thermal cycling behaviour. The ceramic coating consists on an Yttria Partially Stabilized Zirconia (YPSZ) matrix into which healing agents made of MoSi2-Al2O3 core-shell particles are dispersed prior to sintering. The protocol used to sinter self-healing TBCs on MCrAlY (M: Ni or NiCo) pre-coated Ni-based superalloys is described and the reaction between the particles and the MCrAlY bond coating as well as the preventive solutions are determined. Thermal cycling experiments are performed on this complete multilayer system to study the crack healing behaviour. Post-mortem observations highlighted local healing of cracks due to the formation of silica and the subsequent conversion to zircon at the rims of the cracks.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-1Novel Aerospace Material

Thermo – mechanical properties of SPS produced self-healing thermal barrier coatings containing pure and alloyed MoSi₂ particles

Yttria – partially stabilised zirconia (YPSZ) MoSi2 composites have been designed to prolong the lifetime of the matrix by self – healing cracks during thermal cycling. The healing reaction at high temperatures is based on the decomposition of MoSi2, leading to a volumetrically expanding reaction product, which seals the crack. In this work, coefficient of thermal expansion (CTE) and the fracture toughness of composites containing MoSi2 particles, produced by spark plasma sintering (SPS) have been compared to conventional YPSZ. The CTE mismatch between YPSZ and MoSi2 was found to be small, implying that thermally induced mismatch stresses will be small and the composites have a similar CTE to conventional YPSZ. Fracture toughness was found not to be affected by the particles and showed similar values to unreinforced YPSZ. Cracks introduced by indentation have been shown neither to prefer, or avoid, the particles suggesting that such a composite system is capable of autonomously activating the self – healing reaction.</p