Static and dynamic aspects of coupling between creep behavior and oxidation on MC2 single crystal superalloy at 1150 °C

Creep tests were performed on thin wall specimens made of MC2 single crystal superalloy at 1150 °C and under controlled atmosphere. The results highlight the deleterious oxidation effect on creep properties. The assumption that oxidation leads to a non-load-bearing affected zone is insufficient to explain the difference in creep rate that was noticed between tests performed under synthetic air and under hydrogenated argon, and cannot explain the decrease of the strain rate during the tests that were carried out with a change of atmosphere from synthetic air to hydrogenated argon. On the other hand, these experimental results are consistent with vacancy injection due to partial cationic oxidation, which accelerates the creep rate by promoting creep mechanisms controlled by diffusion. The anionic protective alumina scale formed under hydrogenated argon prevents this vacancy flux

Advanced burner-rig test for oxidation-corrosion resistance evaluation of MCrAlY/superalloys systems

Protective coatings are used on gas turbine components to enable them to survive in engine-operating conditions. This study presents a recently developed cyclic burner-rig test that is used to simulate helicopter engine conditions and to assess the oxidation and hot corrosion behaviour of MCrAlY coatings on nickel-base superalloys. A diluted sea-salt solution is atomised into the burner-rig to simulate hot-corrosion. Each cycle lasts 1 h with temperatures varying in the range of 900 °C to 1000 °C followed by 15 min cooling to room temperature. Specimens are tested up to 1000 such cycles. Three different NiCoCrAlYTa coating thicknesses are used to determine the influence of the Al reservoir on the lifetime of the coated MC2 superalloy. The evolving microstructural features are identified using high resolution scanning electron microscopy and energy dispersive spectroscopy and compared with isothermal testing in pure oxidising conditions. The NiCoCrAlYTa microstructure obtained after the burner-rig test has typical features of a Type 1 hot corrosion degradation, with internal oxidation and nitruration and a front of chromium and yttrium-rich sulphides. This type of advanced burner-rig test cycle is successful in reproducing the accelerated combined hot-corrosion/oxidation damage

The effect of thermal cycling on the high-temperature creep behaviour of a single crystal nickel-based superalloy

Isothermal and thermal cycling creep behaviours of a single crystal nickel-based superalloy have been studied by means of tensile tests at 1150 °C and 80 MPa. We have demonstrated that thermal cycling creep rates are faster than isothermal creep rates and that lifetimes at high temperatures are shorter for creep tests under thermal cycling conditions. Furthermore, it is shown that thermal cycling creep lifetime increases as the thermal cycle frequency decreases

Influence of Environment on Creep Properties of MC2 Single Crystal Superalloy at 1150°C

In order to reveal the effect of oxidation on thin blade walls, a new machine allowing tests up to 1250°C under controlled atmosphere has been designed. Creep tests were performed on MC2 single crystal superalloy at 1150°C, under hydrogenated argon and dry air, but also with a switch from one atmosphere to the other after reaching the steady state creep stage. The results point out the decrease of the minimum creep rate in case of tests performed or at least started under hydrogenated argon, compared with the value obtained under synthetic dry air. This effect of oxidation was attributed to the protective oxide scale formed under hydrogenated argon. The low growth rate of alumina layer leads to a thinner zone affected by metal consumption, which is assumed to be non bearing, and prevents from vacancy flux toward the alloy. The second point results in slowing down creep mechanisms controlled by diffusion and therefore dislocation motion and microstructure evolution. Thermogravimetric tests confirm the difference of oxidation kinetics regards to environment (hydrogenated argon and dry air). However, oxide scales have different microstructures on thermogravimetric and creep samples when tested under air

Isothermal Oxidation Behaviour of NiCoCrAlYTa Coatings Produced by HVOF Spraying and Tribomet™ Process

Protective NiCoCrAlYTa coatings are used on gas turbine single crystal superalloy blades to provide environmental resistance. They can be deposited by several processes. In this study, isothermal oxidation behaviour of NiCoCrAlYTa coatings produced by HVOF spraying and Tribomet TM process and deposited on single crystal nickel-based superalloy CMSX-4 were compared between 950 and 1,150°C for several exposure durations. Microstructure and chemical composition of both coatings were examined before and after oxidation testing and quite similar observations were made for both coating processes. The combination of phase and chemical analyses allowed the establishment of an occurrence diagram of phases for both coating processes, according to temperature and duration of exposure. The obtained diagrams seemed similar for both processes. Finally both processes appeared to be equivalent for the protection of CMSX-4 superalloy in isothermal oxidation conditions

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

Tensile properties of a non-line-of-sight processed β-γ-γ′ MCrAlY coating at high temperature

The tensile properties of a NiCoCrAlYTa coating manufactured by the Tribomet® process were examined in a wide range of temperatures representative to turbine in-service conditions. Thin and bulky freestanding coating specimens (FSCS) were tested and compared in order to evaluate the mechanical strength and ductility of this material. Comparable results were found for both the specimen geometries, i.e. a brittle-to-ductile transition temperature (BDTT) between 650 and 700°C, a significant increase of the ductility (up to 5.5% at 800°C) and a drop of the tensile strength above the BDTT. Thin FSCS demonstrated slightly lower yield strength and ultimate tensile stress compared to bulky FSCS. Fractographic examinations aimed to suggest that this difference in mechanical properties was partly attributed to a lesser cohesion and brittle behavior of a β-rich layer found in the extreme surface of the overlay coating due to the deposition technique. This poor local mechanical strength is believed to be detrimental for the integrity of thermal barrier coatings (TBC) systems and attentions in the design of such functionally graded materials have to be paid to prevent early in-service damages

Mechanical and thermo-physical properties of plasma-sprayed thermal barrier coatings: a literature survey

Atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) have been studied from an extensive review of the dedicated literature. A large number of data have been collected and compared, versus deposition parameters and/or measurement methods, and a comparison was made between two different microstructures: standard APS coatings and segmented coatings. Discussion is focused on the large scattering of results reported in the literature even for a given fabrication procedure. This scattering strongly depends on the methods of measurement as expected, but also—for a given method—on the specific conditions implemented for the considered experimental investigation. Despite the important scattering, general trends for the correlation of properties to microstructure and process parameters can be derived. The failure modes of TBC systems were approached through the evolution of cracking and spalling at various life fractions

Characterization of the mechanical properties of thermal barrier coatings by 3 points bending tests and modified small punch tests

Issu de : ICMCTF 2017 - 44th International Conference on Metallurgical Coatings and Thin Films, San Diego, UNITED STATES, April 24-28, 2017International audienceThe Atmospheric Plasma Sprayed Thermal Barrier Coatings (APS TBCs) is commonly used to insulate hot sections in gas turbines. Cyclic oxidation failure usually results from the spallation of the ceramic top coat. In order to predict such spalling phenomena, understanding the mechanisms for cracks initiation and propagation in thermal barriers is a major issue for engine-makers. Failure of the TBC is strongly related to the thermal and mechanical properties of each component of the multi-materials system (substrate, bond coat and ceramic) but also to the response of the TBC as a whole. The purpose of the present work is to assess the mechanical behavior of a complete TBC using comparative experimental (uniaxial and biaxial loading) and Digital Images Correlation (DIC) analysis approaches for classical TBC microstructures obtained through APS coatings.The experimental characterization of the mechanical behavior of the TBC systems was studied on as deposited specimens. Three Points Bending (3PB) tests were performed at room temperature, with the ceramic coating either under tension or compression. Additionally, in situ observations during 3PB tests by a camera, associated to a DIC analysis, allow determining the evolution of the strain field of surface sample correlated with the damage evolution. Location of crack initiation and crack propagation paths up to macroscopic failure were investigated. These tests highlighted the strong differences in mechanical behavior and fracture mode depending on the tension or compression stress state in ceramic coating. Small Punch Tests (SPT) were also performed at room temperature using both geometries (tension/compression). This allows pointing out the similarities of failure modes between uniaxial solicitation and biaxial flexure. Tests performed at 850 °C in the SPT ring show that when temperature varies, different mechanical properties can be observed

Advanced burner-rig test for oxidation-corrosion resistance evaluation of MCrAlY/superalloys systems

International audienceProtective coatings are used on gas turbine components to enable them to survive in engine-operating conditions. This study presents a recently developed cyclic burner-rig test that is used to simulate helicopter engine conditions and to assess the oxidation and hot corrosion behaviour of MCrAlY coatings on nickel-base superalloys. A diluted sea-salt solution is atomised into the burner-rig to simulate hot-corrosion. Each cycle lasts 1 h with temperatures varying in the range of 900 °C to 1000 °C followed by 15 min cooling to room temperature. Specimens are tested up to 1000 such cycles. Three different NiCoCrAlYTa coating thicknesses are used to determine the influence of the Al reservoir on the lifetime of the coated MC2 superalloy. The evolving microstructural features are identified using high resolution scanning electron microscopy and energy dispersive spectroscopy and compared with isothermal testing in pure oxidising conditions. The NiCoCrAlYTa microstructure obtained after the burner-rig test has typical features of a Type 1 hot corrosion degradation, with internal oxidation and nitruration and a front of chromium and yttrium-rich sulphides. This type of advanced burner-rig test cycle is successful in reproducing the accelerated combined hot-corrosion/oxidation damage