Etude et développement de revêtements Gamma-Gamma prime riches en platine élaborés par Spark Plasma Sintering (SPS) - Application au systeme barrière thermique

Le système barrière thermique, permettant la protection des aubes mobiles des turbines aéronautiques, est un système dont l'élaboration est complexe et nécessite de nombreuses étapes. L'utilisation du spark plasma sintering (SPS) a permis de réaliser des systèmes barrière thermique complets en une étape unique. Au-delà des possibilités industrielles de cette méthode, le SPS s'est avéré un outil de recherche précieux pour rapidement tester un vaste champ de compositions et d'ajouts d'éléments réactifs. Les premier essais et la modélisation de la diffusion dans le SPS ont permis de prévoir les phases du revêtement suite à l'étape de SPS. Les travaux se sont ensuite focalisés sur l'optimisation d'une composition de sous couche γ-γ' riche en platine dopée avec des éléments réactifs sur un substrat d'AM1. L'analyse chimique des revêtements SPS a révélé des taux de pollutions en soufre et carbone extrêmement faibles. Au vu de l'influence néfaste de ces éléments sur la tenue en oxydation cyclique ces analyses mettent en valeur la qualité des revêtements élaborés. Les performances des sous couches dopées, avec notamment du hafnium, de l'yttrium et du zirconium ont été évaluées lors d'essais de cyclage thermique à 1100°C sous air. La composition de revêtement γ-γ' la plus prometteuse a ensuite été comparée au système industriel β-(NiPt)Al avec la même barrière thermique de zircone yttriée déposée par EBPVD et le même substrat d'AM1. Les résultats obtenus montre une meilleure durée de vie des systèmes TBC avec sous couches γ-γ'. Par contre la remontée importante des éléments du superalliage dans le revêtement influence la durée de vie du système TBC comme cela a été montré par des dépôts conduits sur d'autres nuances de superalliages à base de nickel. Ces résultats montrent que pour les revêtements γ-γ' la prise en compte du revêtement dans le développement d'un superalliage est essentielle

Proto-TGO formation in TBC systems fabricated by spark plasma sintering

Thermal barrier coatings (TBC) are commonly used in modern gas turbines for aeronautic and energy production applications. The conventional methods to fabricate such TBCs are EB-PVD or plasma spray deposition. Recently, the spark plasma sintering (SPS) technique was used to prepare new multilayered coatings. In this study, complete thermal barrier systems were fabricated on single crystal Ni-based superalloy (AM1®) substrate in a one-step SPS process. The lifetime of TBC systems is highly dependent on its ability to form during service a dense, continuous, slow-growing alumina layer (TGO) between an underlying bond coating and a ceramic top coat. In the present paper, we show that such kind of layer (called proto-TGO in the following) can be in situ formed during the SPS fabrication of TBC systems. This proto-TGO is continuous, dense and its nature has been determined using TEM-EDS-SAD and Raman spectroscopy. This amorphous oxide layer in the as-fabricated samples transforms to α-Al2O3 during thermal treatment under laboratory air at 1100 °C. Oxidation kinetics during annealing are in good agreement with the formation of a protective α-Al2O3 laye

Thermal barrier systems and multi-layered coatings fabricated by spark plasma sintering for the protection of Ni-base superalloys

Aeronautic gas turbine blades, vanes and combustion chambers are protected against high temperature oxidation and corrosion by single or multilayered coatings. These include environmental coatings, generally based on Pt-modified Ni aluminides or MCrAlY overlays (where M = Ni and/or Co), thermal barrier coating (TBC) systems including a ceramic thermally insulating layer, and abradable seals. The present work shows the ability of the Spark Plasma Sintering technique to rapidly develop new coatings compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short production step. Fabrication of MCrAlY overlays with local Pt and/or Al enrichments is shown, as well as fabrication of coatings made of z-PtAl2, e-PtAl, α-AlNiPt2, martensitic and b−(Ni,Pt)Al or Pt-rich g/g’ phases, including their doping with reactive elements. The fabrication of a complete TBC system with a porous and adherent Yttria Stabilized Zirconia (YSZ) layer on a bond-coating is also demonstrated, as well as the fabrication of a CoNiCrAlY-based cermet coating for abradable seal application. Difficulties of fabrication are reviewed, such as Y segregation, risks of carburization, local over-heating, or difficulty to coat complex shaped parts. Solutions are given to overcome these difficulties

Pt-modified Ni aluminides, MCrAlY-base multilayer coatings and TBC systems fabricated by Spark Plasma Sintering for the protection of Ni-base superalloys

Pt-modified Ni aluminides and MCrAlY coatings (where M=Ni and/or Co) are widely used on turbine blades and vanes for protection against oxidation and corrosion and as bond coatings in thermal barrier coating (TBC) systems. The present work shows the ability of a new fabrication technique, the Spark Plasma Sintering, to develop rapidly new coating compositions and microstructures. This technique allows combining powders and metallic foils on a superalloy substrate in order to obtain multilayered coatings in a single short experiment. Fabrication of MCrAlY overlays with local Pt and/or Al enrichment is shown, as well as fabrication of coatings made of ζ-PtAl2, ε-PtAl, α-AlNiPt2, martensitic β- (Ni,Pt)Al or Pt-rich γ/γ′ phases. The realization of a complete TBC system with a porous and adherent Yttria Stabilized Zirconia (YSZ) layer on a γ/γ′ low mass bond coating is also demonstrated. Difficulties of fabrication are reviewed and discussed, such as Y segregation, risks of carburization, local overheating, or difficulty to coat complex shape parts. Finally, some first results of cyclic oxidation are given

Thermal cycling behavior of EBPVD TBC systems deposited on doped Pt-rich γ–γ′ bond coatings made by Spark Plasma Sintering (SPS)

In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire thermal barrier coating systems (TBC). In the present study, this technique was used to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr, Si and metallic additions of Ag. These samples were then coated by electron beam physical vapour deposition (EBPVD) with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with SPS Pt rich γ–γ′ bond coatings were compared to conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Thermal cycling tests were performed during 1000-1 h cycles at 1100 °C under laboratory air. Spalling areas were monitored during this oxidation test. Most of the Pt rich γ–γ′ samples exhibited a better adherence of the ceramic layer than the β-samples. After the whole cyclic oxidation test, cross sections were prepared to characterize the thickness and the composition of the oxide scales by using scanning-electron microscopy. In particular, the influence of the doping elements on the oxide scale formation, the metal/oxide roughness, the TBC adherence and the remaining Al and Pt under the oxide scale were monitored. It was shown that RE-doping did not improve the oxidation kinetics of the studied Pt rich γ–γ′ bond coatings, nevertheless most of the compositions were superior to “classic” β-(Ni,Pt)Al bond coatings in terms of ceramic top coat adherence, due to lower rumpling kinetics and better oxide scale adherence of the γ–γ′-based 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

Relationship between mechanical properties and microstructure of yttria stabilized zirconia ceramics densified by spark plasma sintering

Porous ceramics are widely used for many applications such as filters, insulators, electrodes for SOFC, membranes or bone scaffolds, with porosity in the typically range of 20–50%vol. The functionality of those materials comes at the expense of the degradation of their mechanical properties which are highly impacted by the rate, distribution, shape and size of the porosity. Among them tetragonal stabilized zirconia (TSZ) is one of the most industrially used; it is sometime called: “the ceramic steel” since in its dense state it exhibits the highest toughness for ceramics. It is known that the porosity has a huge impact on the thermo-mechanical properties of refractory ceramics as Yittria Stabilized Zirconia (YSZ). This study aims to capitalize the mechanical properties as a function of porosity to provide future applications and ensure the behavior in service of thermal barrier coating. In the present paper, the correlation between the microstructure and the mechanical properties such as Young modulus, hardness and strength of YSZ ceramics obtained by Spark Plasma Sintering (SPS) was investigated. Two types of YSZ powder, a nanometric one from Tosoh and a micrometric one obtained by sol-gel route were studied to prepare homogeneous mesoporous or oriented macroporous microstructure by partial sintering. SPS parameters have been determined and optimized to manage the porosity rate. Furthermore, a bimodal microstructure can be obtained, by mixing both powders, allowing the formation of linking bridges between the microporous zone and the nanopowder during sintering. The macroporous ceramics have lower Young modulus, hardness and strength than mesoporous ones. These characteristics are discussed in the paper taking into account the differences between microstructure and contacts between particles with various form factors. Thus, it is clearly evidenced that the morphology of raw powders and the level of porosity are key parameters to optimize the mechanical properties of such porous material

High temperature tensile properties of β-γ-γ\u27-MCrAlY and β-Ni(Al,Pt) bond-coatings and interdiffusion zone with Ni-based single crystal superalloys

MCrAlY overlay coatings and Pt-modified aluminide diffusion coatings are commonly used in thermal barrier coating (TBC) systems for turbine blade and vane applications. Purposely designed for oxidation and corrosion protection, MCrAlY and aluminide coatings have a ductile-to-brittle transition temperature (DBTT) of about 600 to 800°C, i.e. in the temperature range of service conditions. Therefore, these coatings can be a source of premature crack initiation under thermomechanical loading at low/intermediate temperature. They also creep at high temperature. This drastic change in local mechanical properties significantly impair the structural integrity of such multi-layered materials. Current damage-tolerant design of TBC systems preferentially deals with DBTT than with the effective temperature- and time-dependent mechanical properties of the individual layers constituting the TBC systems. Data on high temperature properties of the bond-coatings and the interdiffusion zone with the substrate are lacking. Indeed, these local properties are particularly difficult to assess up to 1100°C, both using freestanding-layer[1-4] or multi-layer specimen approaches[5]. Improvements in the prediction of the mechanical behavior and the lifetime of TBC systems require the understanding and the quantification of such local mechanical properties. Please click Additional Files below to see the full abstract

High temperature micromechanical behavior of a Pt-modified nickel aluminide bond-coating and of its interdiffusion zone with the superalloy substrate

The micromechanical properties of a b-(Ni,Pt)Al bondcoating was investigated between 700°C and 1000°C using ultrathin freestanding bond-coating specimens. Its brittle-to-ductile transition temperature was close to 750°C, with a significant ductility above 800°C (up to 23 pct at 1000°C). The tensile strength decreased from 450 MPa at 750°C down to 50 MPa at 1000°C. Fractographic observations evidenced the material brittleness at intermediate temperature with large cleaved grains and its ductility above 750°C with important necking of individual grains

Oxidation behaviour of a CoNiCrAlY/h-BN based abradable coating

The oxidation resistance of a thermally sprayed CoNiCrAlY/h-BN abradable coating was studied at 750 and 900 °C. First, the high porosity of the abradable coating was carefully characterised in order to estimate the specific surface area of the coating, required to evaluate the oxidation kinetics. Then the formed oxides were investigated by XRD, Raman spectroscopy and SEM. At 900 °C, the mass variations exhibited a deviation from parabolic behaviour due to rapidly growing oxides. Meanwhile, at 750 °C, after a transient state, the oxidation rate reaches a steady state, indicating that a protective alumina scale was maintained at this temperature