Ravinder Diwan

Thermo-Mechanical Study of the Role of Gd2Zr2O7 (GZ) in Improving Life of YSZ and GZ Double Layered Thermal Barrier Coatings

Thermo-mechanical properties and thermal cycling behavior of gadolinium zirconate Gd2Zr2O7 (GZ) based thermal barrier coatings (TBCs) was investigated in this study in comparison to conventional yttria-stabilized zirconia (YSZ) coatings. This paper presents results focusing on coefficient of thermal expansion (CTE) measurements, thermal cycling tests, measured elastic properties and porosity of the multilayered GZ/YSZ TBCs deposited by atmospheric plasma spraying (APS) on an Inconel 738 (IN738) superalloy substrate. SEM microstructural images of failed TBC specimens are also presented. Samples of different double layer combinations with one layer being either 100% YSZ or 100% GZ and the second containing varying amounts of the two compounds were prepared to determine optimum combination that maintains good insulating properties while reducing the CTE mismatch between the TBC layers. The temperature range of the tests was 25°C to 1300°C. The samples are processed by APS on (O −12.7 mm, thickness 3 mm) IN738. Using a dilatometer, CTEs of the as sprayed top coat (TC) combinations are measured and compared. The elastic properties are measured using a Hysitron nanoindenter. Results showed that the 10%GZ/ 90%YSZ+ 100YSZ double layered structure was the best among the tested GZ based TBCs and the delamination of GZ/YSZ coating first initiated in the GZ layer close to the interface of GZ and YSZ layers, which was mainly caused by the sintering effect of the GZ layer.Copyright

Effects of Thermal Radiation on Air Plasma Spray (APS) Coated Gas Turbine Blade

Thermal barrier coatings (TBCs) are used to protect hot gas path (HGP) components such as the first two stages of turbine blades and vanes of land-based turbine engines against high temperature environment, corrosion and oxidation. The continuing thrust towards higher thermal efficiencies of gas turbines has resulted in a continuous increase of turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBCs such as air plasma spray (APS) TBCs which are transparent and reflective to radiation. This paper focuses on the combined effects of radiation and conduction heat transfer in the semitransparent yttria-stabilized zirconia (YSZ) coatings used to offer thermal protection to turbine blade. The temperature distribution in the turbine blade depends on the surface convection, reflectivity and refractive index of the grey semitransparent YSZ coatings. The temperature distributions in the metal substrate and the TBC systems are determined by solving the steady state heat diffusion equation and the radiative transport equation simultaneously using ANSYS FLUENT 12.0 CFD commercial package. Preliminary results indicate that substrate metal temperature reduction of about 100K results with the use of the TBC. This temperature drop reduces the thermally activated oxidation rate of the bond coat in the TBC and so delays failure of TBC by oxidation. Furthermore, by taking into account the effect of radiation, the temperature distribution in the metal substrate with TBC exceeds the temperature distribution without radiation by about 40 K, signifying the importance of including radiation in the thermal modeling of TBCs for high temperature applications.Copyright

Thermal Conductivity Prediction for Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve a higher working temperature and thus lead to a better efficiency. Yttria-Stabilized-Zirconia (YSZ), a material with low thermal conductivity, is commonly used as the TBC top coat to provide the thermal barrier effect. In this paper, an analytical model is proposed to estimate the effective thermal conductivity of the TBCs based on the microstructures. This model includes the micro structure details, such as grain size, pore size, volume fraction of pores, and the interfacial resistance. To validate the model, two sets of TBC samples were fabricated and tested for thermal conductivity and associated microstructures. The first set of samples were disk shaped YSZ-Al2 O3 samples fabricated using a pressing machine. The YSZ-Al2 O3 powder mixture was 0, 1, 2, 3, 4 and 5 wt% Al2 O3 /YSZ powder ratio. The second set of samples were fabricated by Atmospheric Plasma Spray process for two different microstructure configurations, standard (STD) and vertically cracked (VC), at two different thicknesses, 400 and 700 urn respectively. A laser flash system was used to measure the thermal conductivity of the coatings. Experiments were performed over the temperature range from 100°C to 800°C. The porosity of the YSZ samples was measured using a mercury porosimetry analyzer, POREMASTER 33 system. A Scanning Electron Microscope (SEM) was used to study the microstructure of the samples. It is observed that the microstructure and the porosity are directly linked with the thermal conductivity values. The relationship of the properties to the real microstructure determines the validity of the proposed model.Copyright

Porosity and Thermal Cycling Behavior of Plasma Sprayed and EBPVD Thermal Barrier Coatings

Traditionally, thermal barrier coatings (TBCs) are used in gas turbine engines to create an insulation layer between the metallic components and the gases in the hot section. Atmospheric plasma spray (APS) is a common method used to produce TBCs. The goal of this study is to study the porosity and thermal cycling behavior of standard (STD) and vertically cracked (VC) thermal barrier coatings (TBCs) fabricated by Atmospheric Plasma Spray (APS) for two different thicknesses, 300 and 600 μm respectively. Electron Beam Physical Vapor Deposition (EBPVD) coatings with 300 micron thickness prepared under tumbled and non-tumbled conditions were studied. For this study, mercury porosimeter equipment (POREMASTER 33) by Quantachrome Instruments was used to measure porosity, and pore size distribution. Scanning Electron Microscopy (SEM) images were obtained for all the samples. The images showed clear microstructural difference between the APS and EBPVD coatings. All the coatings were thermal cycled to 1200°C and the conventional APS-STD (300μm) performed the best followed by APS-VC coatings and EBPVD coatings which performed similarly.Copyright

Thermal Property Measurements of YSZ-Al2O3 Ceramic Composites

Thermal barrier coatings (TBCs) are used in gas turbine engines to achieve a higher working temperature, and thus lead to a better efficiency. Yttria-stabilized-zirconia (YSZ), a material with low thermal conductivity, is commonly used as the top coat layer to provide the thermal barrier effect. Recent studies demonstrated that YSZ-Al2 O3 composite layer could reduce the oxygen diffusion through the TBC, thus YSZ-Al2 O3 composite layer potentially could be used to mitigate the spalling induced failure of a TBC coating. The goal of this study is to investigate the effect of the addition of Al2 O3 on the thermal properties of YSZ based TBCs. In this study, a stainless steel die was used to make disk shaped samples with 0, 1, 2, 3, 4 and 5 wt% Al2 O3 /YSZ powder ratios under uniaxial pressure. A laser flash system was used to measure the thermal diffusivity for all samples and the porosity of the samples is measured using mercury porosimetry. It is found that adding Al2 O3 to YSZ decreases the thermal conductivity and increases the porosity of the ceramic composites.Copyright

Interfacial Kinetics of High Temperature Oxidation in YSZ Thermal Barrier Coatings With Bond Coatings of NiCoCrAlY With 0.25% Hf

This paper presents the results of an experimental study of the high-temperature isothermal oxidation behavior and micro-structural evolution in plasma sprayed thermal barrier coatings (TBCs) at temperatures between 900 and 1200 °C. Two types of specimens were produced for testing. These include a standard and vertically cracked (VC) APS. High temperature oxidation has been carried out at 900, 1000, 1100 and 1200 °C. The experiments have been performed in air under isothermal conditions. At each temperature, the specimens are exposed for 25, 50, 75 and 100 hours. The corresponding microstructures and microchemistries of the TBC layers are then examined using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy EDS. Changes in the dimensions of the thermally grown oxide (TGO) layer are determined as functions of time and temperature. The evolution of bond coat microstructures/interdiffusion zones and thermally grown oxide (TGO) layers are compared in TBCs with standard (STD) and vertically cracked (VC) microstructures.Copyright

TITLE: A STUDY OF ADVANCED MATERIALS FOR GAS TURBINE COATINGS AT ELEVATED TEMPERATURES USING SELECTED MICROSTRUCTURES AND CHARACTERISTIC ENVIRONMENTS FOR SYNGAS COMBUSTION

Thermal barrier coatings (TBCs) that can be suitable for use in industrial gas turbine engines have been processed and compared with electron beam physical vapor deposition (EBPVD) microstructures for applications in advanced gas turbines that use coal-derived synthesis gas. Thermo-physical properties have been evaluated of the processed air plasma sprayed TBCs with standard APS-STD and vertically cracked APS-VC coatings samples up to 1300 C. Porosity of these selected coatings with related microstructural effects have been analyzed in this study. Wet and dry thermal cycling studies at 1125 C and spalling resistance thermal cycling studies to 1200 C have also been carried out. Type I and Type II hot corrosion tests were carried out to investigate the effects of microstructure variations and additions of alumina in YSZ top coats in multi-layered TBC structures. The thermal modeling of turbine blade has also been carried out that gives the capability to predict in-service performance temperature gradients. In addition to isothermal high temperature oxidation kinetics analysis in YSZ thermal barrier coatings of NiCoCrAlY bond coats with 0.25% Hf. This can affect the failure behavior depending on the control of the thermally grown oxide (TGO) growth at the interface. The TGO growth kinetics is seen to be parabolic and the activation energies correspond to interfacial growth kinetics that is controlled by the diffusion of O{sub 2} in Al{sub 2}O{sub 3}. The difference between oxidation behavior of the VC and STD structures are attributed to the effects of microstructure morphology and porosity on oxygen ingression into the zirconia and TGO layers. The isothermal oxidation resistance of the STD and VC microstructures is similar at temperatures up to 1200 C. However, the generally thicker TGO layer thicknesses and the slightly faster oxidation rates in the VC microstructures are attributed to the increased ingression of oxygen through the grain boundaries of the vertically cracked microstructures. The plasma sprayed TBC microstructure (VC and STD) with NiCoCrAlY-Hf bond coat are stable up to 1100 C. However, as with other TBC structures, a considerable amount of interdiffusion was observed in the different layers, although the TBC growth was self-limiting and parabolic. The addition of Hf to the VC microstructure appears to have some potential for the future development of robust TBCs with improved isothermal and service temperatures in advanced gas turbines.

Performance of Vertical Cracked Air Plasma Spray Processed Thermal Barrier Coatings: Thermo-Physical Properties Measurements, and Effects of Thermal Stress on Thermal Cycling Failure

In this study two variations of Air plasma spray (APS) coated Inconel738 (IN738) materials are evaluated for their thermal cycling and spalling behavior as well as thermo-physical properties variations as a function of temperature. The emphasis in this study was to investigate two different processing regimes that yield a standard APS and vertically oriented columnar microstructures in terms of their thermomechanical performance. The thermal cycling studies involved ramping the samples from 25°C to a temperature of about 1125°C in duration of 30 minutes, and followed by maintaining the samples at that temperature for 5 hours, followed by cooling down from 1125°C to 25°C for 30 minutes. The thermal properties measurements involved using Laser Flash method to measure thermal diffusivity and thermal conductivity from 100°C to 900°C. A thermal stress analytical study was also conducted in order to draw inference in the above studies as to the plausible causes of failure in the samples. Preliminary results of this thermal cycling and thermo-physical properties study are presented in this paper. The results are being correlated with the stress analysis results.Copyright

Two-Dimensional Thermal Performance Analysis of a Semi-Transparent Spectral Zirconia Thermal Barrier Coating

The performance of an aero engine can be increased in two ways: one by reducing the air requirement for the cooling of the turbine blades and secondly by increasing the turbine inlet temperature (TIT) that is operating temperature of the turbine blades. Taking into account the latter approach the blade material must withstand high temperatures of above 1350°C. For this enhancing purpose, protective coatings called the thermal barrier coatings (TBC) are being employed. The thermal barrier coating mainly consists of two layers; one is the metallic coating MCrAlY, which is the premiere layer over the substrate Ni based super alloy. The other is the ceramic layer made of Yttria Stabilized Zirconia (YSZ). Apart from these two layers, an intermediate layer of Al2 O3 is formed by the oxidation of the aluminum in MCrAlY called the diffusion layer which also enhances the adhesion between the two layers. M stands for Nickel or Cobalt. The present study is an investigation on the in-situ thermal performance of TBCs by considering the ceramic layer as a semi-transparent media and varying its thickness and simultaneously increasing the operating temperature on its other boundary surface. The above thermal boundary value problem is modeled in 2-dimensions and solved numerically using the discrete ordinate model for radiative heat transfer in a commercial computational fluid dynamics and heat transfer software. Two samples of Ni based super alloy substrate with dimensions 40 × 100 × 3mm are considered; one sample with a thickness of 0.25 mm ceramic layer and the other sample with 1 mm coating thickness for transient thermal analysis. Simulated transient temperature histories are presented for use in a thermo-mechanical analysis in order to predict the failure modes in the TBC. The temperature distribution in TBC coating mainly depends on the radiative effects combined with heat conduction and convection and radiation at the material boundaries.Copyright