Electron Beam Physical Vapor Deposition Coatings Research Articles

Performance and failure modes of thermal barrier coatings deposited by EB-PVD on blades under real service conditions in gas turbine

Thermal barrier coatings (TBCs) with a double columnar crystal structure of CoCrAlY/YSZ, prepared using electron beam physical vapor deposition (EB-PVD), were applied to turbine blades and tested in real gas turbine conditions. This study investigates their failure mechanisms under both actual operating conditions and simulated external loads, using three-point bending tests. The performance of full-sized, TBC-coated blades was analyzed through finite element modeling, while the thermomechanical and mechanical properties of the coatings after service were measured to support the simulation and provide insights into the failure processes. A new degradation mode, characterized by penetration cracking, was observed in the EB-PVD coatings. This behavior is linked to the columnar grain structure of YSZ and CoCrAlY, lower operational temperatures, and the reduced thickness of the YSZ layer. Surface cracks in the YSZ coating primarily occurred in regions with large curvature, where stress concentrations are higher. This study offers new insights into the failure modes of TBCs under real gas turbine operating conditions.

Thermally Grown Oxide Stress in PS-PVD and EB-PVD Thermal Barrier Coatings Observed at Various Lifetimes Via Synchrotron X-ray Diffraction

Abstract The current standard application method for thermal barrier coatings (TBCs) on turbine blades for jet engines is electron-beam physical vapor deposition (EB-PVD) due to its high strain tolerance and low thermal conductivity. An emerging deposition method, plasma-spray physical vapor deposition (PS-PVD), presents an opportunity for a tailorable microstructure, and non-line-of-sight deposition that is faster and less expensive. To compare the lifetime behavior of both PS-PVD and EB-PVD coatings, samples subjected to 300 and 600 thermal cycles were measured during a 1 h thermal cycle to determine the strains, which were converted to stress, in the thermally grown oxide (TGO) layer of the TBCs using synchrotron X-ray diffraction (XRD). Room temperature XRD measurements indicated among samples that PS-PVD coatings experienced greater variation in in-plane room temperature strain in the TGO after cycling than the EB-PVD coatings. In-situ XRD measurements indicated similar high-temperature strain and no spallation after 600 thermal cycles for both coatings. Microscopy imaging after cycling showed greater rumpling in PS-PVD coatings that led to different failure modes between the two coatings’ TGO layers. The tailorability of PS-PVD coatings allows for adjustments in the processing parameters to improve their overall performance after aging and bridge the differences between the two deposition methods.

High-energy X-ray phase analysis of CMAS-infiltrated 7YSZ thermal barrier coatings: Effect of time and temperature

Abstract

Wetting, infiltration and interaction behavior of CMAS towards columnar YSZ coatings deposited by plasma spray physical vapor

Thermal barrier coatings (TBCs) produced by electron beam physical vapor deposition (EB-PVD) or plasma spray (PS) usually suffer from molten calcium-magnesium-alumino-silicate (CMAS) attack. In this study, columnar structured YSZ coatings were fabricated by plasma spray physical vapor deposition (PS-PVD). The coatings were CMAS-infiltrated at 1250 °C for short terms (1, 5, 30 min). The wetting and spreading dynamics of CMAS melt on the coating surface was in-situ investigated using a heating microscope. The results indicate that the spreading evolution of CMAS melt can be described in terms of two stages with varied time intervals and spreading velocities. Besides, the PS-PVD columnar coating (∼100 μm thick) was fully penetrated by CMAS melt within 1 min. After the CMAS attack for 30 min, the original feathered-YSZ grains (tetragonal phase) in both PS-PVD and EB-PVD coatings were replaced by globular shaped monoclinic ZrO2 grains in the interaction regions.

Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack

In modern gas turbines, highly loaded components are internally cooled and furthermore covered with thermal barrier coatings (TBCs) to withstand the harsh operating conditions with temperatures exceeding the application limit of such coatings. Under realistic operating conditions, siliceous minerals, of a calcium-magnesium-aluminum-silicate (CMAS) composition, are ingested into the turbine and deposited on the TBCs. Besides erosion, this also leads to degradation by chemical interaction.The plasma spray-physical vapor deposition (PS-PVD) process is an advanced method for manufacturing TBCs, which fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). Due to the unique plasma conditions, coatings with columnar microstructures exhibiting high strain tolerance can be created. However, because of the high amount of open porosity the resistance of such structures to CMAS and erosion attack was expected to be low.In the present work, PS-PVD TBCs were investigated in a burner rig facility under thermal gradient cycling conditions and simultaneous CMAS attack. The interactions of the PS-PVD-deposited YSZ and the CMAS melt were studied by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) and compared to EB-PVD coatings. Additionally, the resistance of PS-PVD TBCs to erosion is compared to APS TBCs by means of room temperature tests according to ASTM G76-13.

Thermal insulation properties of YSZ coatings: Suspension Plasma Spraying (SPS) versus Electron Beam Physical Vapor Deposition (EB-PVD) and Atmospheric Plasma Spraying (APS)

Improving efficiency of hot section components of aero engines such as turbine blades or nozzle guide vanes is critical for the aircraft industry. Over many years, the development of advanced Thermal Barrier Coatings (TBCs) has been a field of active research to achieve this purpose. Electron Beam Physical Vapor Deposition (EB-PVD) and Atmospheric Plasma Spraying (APS) processes are widely used to apply TBCs on metal substrates. High costs and rather high thermal conductivities of EB-PVD coatings, as well as low thermal lifetime of APS ones, are real drawbacks for next generations of turbine engines. In this study, Suspension Plasma Spraying (SPS) was assessed to improve TBC thermal properties. It was shown that the SPS process allows to perform columnar microstructure easily tunable in terms of both compaction of columnar structure and thermal conductivity. Thermal conductivities were in the 0.7–1.25W·m−1·K−1 range for SPS coatings while values of 0.9 and 1.5W·m−1·K−1 were measured for APS and EB-PVD coatings, respectively. The effect of heat conduction paths, which impact thermal diffusivity values, was highlighted for the columnar structure.

A Comparative Experimental Study on Radiative Properties of EB-PVD and Air Plasma Sprayed Thermal Barrier Coatings

In this paper, the radiative properties of electron beam physical vapor deposition (EB-PVD) and air plasma sprayed (APS) partially yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been comparatively studied for the first time by measuring the spectral diffuse reflectance and transmittance in a broad spectral band ranging from 250 nm to 15 μm. The radiation transfer mechanisms inside the coatings are explored based on the experimental data and theoretical model. The results indicate that the distinctive micronanostructures of APS and EB-PVD coatings have an important effect on the radiative heat transfer. In particular, the larger grain boundary and the total porosity strongly affect the volume scattering properties of the coatings, and the scattering coefficient is closely related to the arrangement of grain boundary as well as the pore architecture (i.e., its size, morphology, and its distribution). Compared to the laminar microstructure of APS TBCs, the columnar microstructure of EB-PVD freestanding coatings exhibits a higher transmittance, a lower reflectance, and a larger absorption in the spectral region from 400 nm to 10 μm, which leads to an increase of the total heat flux. By modifying the microstructure of TBCs properly, the radiative heat flux can be reduced and thereby providing a better thermal protection for the metallic substrate.

Thermal Conductivity Analysis and Lifetime Testing of Suspension Plasma-Sprayed Thermal Barrier Coatings

Suspension plasma spraying (SPS) has become an interesting method for the production of thermal barrier coatings for gas turbine components. The development of the SPS process has led to structures with segmented vertical cracks or column-like structures that can imitate strain-tolerant air plasma spraying (APS) or electron beam physical vapor deposition (EB-PVD) coatings. Additionally, SPS coatings can have lower thermal conductivity than EB-PVD coatings, while also being easier to produce. The combination of similar or improved properties with a potential for lower production costs makes SPS of great interest to the gas turbine industry. This study compares a number of SPS thermal barrier coatings (TBCs) with vertical cracks or column-like structures with the reference of segmented APS coatings. The primary focus has been on lifetime testing of these new coating systems. Samples were tested in thermo-cyclic fatigue at temperatures of 1100 °C for 1 h cycles. Additional testing was performed to assess thermal shock performance and erosion resistance. Thermal conductivity was also assessed for samples in their as-sprayed state, and the microstructures were investigated using SEM.

The Fatigue Life Prediction for EB-PVD Thermal Barrier Coating

The electron beam-physical vapor deposition (EB-PVD) coatings were selected to analyze the fatigue life. Firstly the failure mechanisms of oxidation and thermal fatigue of TBCs were considered together. The weight of thermally grown oxide (TGO) were measured during oxidation experiments and modeled. Thermal fatigue experiments were induced by thermal mismatched stress and thermal fatigue with holding time. The failure mechanism was obtained by the finite element analyses. The life prediction model was modified by the shear and normal strain range close to the interface of the top coat and TGO. Based on the experiment data, the life prediction model of thermal fatigue of TBCs was set up for EB-PVD coatings. Finally, using the life prediction model, the life of TBCs was predicted under the designed load case. The life prediction model was verified and all experiment data fall into the factor of 3 scatter band.

Thermal Fatigue Life of Thermal Barrier Coatings

Two thermal barrier coatings (TBCs), the plasma sprayed (PS) and electron beam-physical vapor deposition (EB-PVD) coatings were selected to analyze the fatigue life. Firstly, the oxidation and thermal fatigue experiments were carried out, which were domestic spraying technology. The failure mechanisms of oxidation and thermal fatigue of TBCs were considered together. The weight of thermally grown oxide (TGO) was measured during oxidation experiments and modeling. Thermal fatigue experiments were induced by thermal mismatched stress and thermal fatigue with holding time. Secondly, based on the experiment data, the life prediction model of thermal fatigue of TBCs was set up for PS and EB-PVD coatings. Thirdly, the method of obtaining the parameters of the life prediction model was given. Finally, using the life prediction model, the life of TBCs was predicted under the designed load case. The life prediction model was verified.

Thermophysical properties of EB-PVD coatings and sintered ceramics of 4 mol% Y 2O 3-stabilized zirconia

The specific heat, thermal diffusivities and thermal conductivities of electron beam physical vapor deposition (EB-PVD) coatings and sintered ceramics of 4 mol% Y 2O 3-stabilized zirconia (YSZ) have been determined in temperature range between room temperature and 1000 °C. EB-PVD coatings reveal porous columnar microstructure. The thermal conductivities and thermal diffusivities of both EB-PVD coatings and sintered ceramics decrease with increasing temperature from room temperature to 1000 °C. The specific heat increases gradually as the temperature increase up to about 600 °C and thereafter does not vary much. The thermal diffusivities and thermal conductivities of EB-PVD YSZ coatings were lower than those of sintered YSZ ceramics at given temperature.

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.