Physical Vapor Deposition Thermal Barrier Research Articles

Multiscale modeling of chemo-thermo-mechanical damage of EB-PVD thermal barrier coatings

The thermal barrier coatings (TBCs) are serviced in elevated thermal conditions characterized by chemo-thermo-mechanical (C-T-M) failure mechanism and multiscale failure modes. In the present work, isothermal, cyclic thermal and mechanical induced spalling tests were carried out. Since the contribution of aluminum migration in TBCs damage evolution and failure was confirmed, the C-T-M life prediction at the micro level, TGO respiratory growth process at the meso level and the C-T-M damage at the macro level have been developed and modeled. Multiscale modeling of C-T-M damage and clarification of EB-PVD TBCs (electron beam physical vapor deposition thermal barrier coatings) failure criterion provides an effective and feasible way to systematically analyze the essential mechanism of TBCs failure and quantitatively evaluate its service performance.

Investigation of TGO stress in thermally cycled plasma-spray physical vapor deposition and electron-beam physical vapor deposition thermal barrier coatings via photoluminescence spectroscopy

Gas turbine engines for aircraft propulsion require thermal barrier coatings (TBCs) to protect metallic components in the hot section from the extreme temperatures of operation, which may exceed 1200 ∘C. One standard method of coating deposition is electron-beam physical vapor deposition (EB-PVD), which produces a high quality and strain tolerant coating with a smooth surface, which does not impair aerodynamical flow. Plasma-spray physical vapor deposition (PS-PVD) is a promising technique that offers several advantages over EB-PVD, including lower cost, shorter coating time, customization of microstructure by varying processing parameters, and the possibility of non-line-of-sight coating. However, the properties and microstructure of PS-PVD coatings and their effect on performance are not fully understood. This work investigates the residual stress in the thermally grown oxide (TGO) layer of samples made by EB-PVD and PS-PVD, thermally cycled at multiple times (uncycled, 300 cycles, and 600 cycles), to evaluate the effects of these two deposition methods on coating lifetime. Area stress maps of the samples were made with photoluminescence piezospectroscopy (PLPS). The PS-PVD samples exhibited stress relaxation with cycling time, suggesting stress relief from damage, and the PS-PVD coatings exhibited greater stress relaxation between 300 and 600 cycles; however, no samples exhibited spallation. SEM imaging revealed different damage modes for the two coating types. These results reveal PS-PVD's potential as a candidate deposition method for coatings in aero-engine applications.

Finite element simulation of residual stress of EB-PVD thermal barrier coatings during thermal cycling

In this study, the residual stress of electron beam - physical vapour deposition thermal barrier coatings (TBCs) with various times of thermal cycling was calculated by the finite element (FE) meth...

Modelling and experimental study of impedance spectra of electron beam physical vapour deposition thermal barrier coatings

A 2-dimensional finite element model has been developed to calculate the impedance spectra of electron beam physical vapour deposition (EB-PVD) thermal barrier coatings (TBCs). The model has been used to examine the effect of thermally grown oxide (TGO) growth and the TGO conductivity change on impedance spectra of TBCs. According to modelling, different spectra were generated due to the TGO growth and TGO conductivity change. Impedance measurements have been carried out on both as-deposited and thermally treated TBCs where the thermal treatments lead to the TGO growth. In addition, the thermal treatment of TBCs at different temperatures produced TGO with different compositions, probably leading to different electrical conductivities of TGO. Measured impedance spectra of TBCs with different TGO thicknesses and TGO compositions agree with modelled spectra of TBCs with different TGO thicknesses and conductivities.

Phase of thermal emission spectroscopy for properties measurements of delaminating thermal barrier coatings

Phase of thermal emission spectroscopy is developed for determining the thermal properties of thermal barrier coating (TBC) in the presence of thermal contact resistance between the coating and the substrate. In this method, a TBC sample is heated using a periodically modulated laser and the thermal emission from the coating is collected using an infrared detector. The phase difference between the heating signal and the emission signal is measured experimentally. A mathematical model is developed to predict the phase difference between the laser and the measured emission, which considers the coating properties and the thermal contact resistance of the interface. An electron-beam physical vapor deposition thermal barrier coating with local regions delaminated by laser shock is characterized using this technique. The measurements are made on two regions of the coating, one where good thermal contact between the coating and substrate exists and the other where the interface has been damaged by laser shock. The results for the thermal properties and thermal contact resistance of the interface are presented and compared.

Microstructural evolution of the nickel platinum-aluminide bond coat on electron-beam physical-vapor deposition thermal-barrier coatings during high-temperature service

The microstructural evolution of a (Ni,Pt)-aluminide bond coat underneath the ZrO2-based thermal-barrier coating (TBC) topcoat system on a Rene N5 Ni-based superalloy turbine blade during prolonged high-temperature service has been characterized using transmission electron microscopy (TEM). The as-deposited bond coat has a spatially varying microstructure, which consists of an outer layer of single-phase β-(Ni,Pt)Al, a middle layer of a β-(Ni,Pt)Al matrix containing a high number density of μ-phase precipitates, and an inner layer containing a γ/γ′ matrix and numerous μ- and σ-phase precipitates. During service, microstructural changes in the hotter sections of the blade are more extensive than those in the cooler parts, as expected. As a result of interdiffusion, the inner layer grows into the γ/γ′ substrate, with the formation of some M23C6 precipitates, and the β matrix in the middle layer is transformed into a two-phase mixture of β and γ′. Corresponding changes occur in the morphologies and volume fractions of the various precipitate phases present in the bond coat. The single-phase β material in the outer layer retains its basic structure, except that the compositional changes due to diffusion between the bond coat and turbine blade cause a martensitic transformation to occur in the hottest sections during the final cooling of the blade. The distribution of various elements in the different layers has also been analyzed, as has growth of the thermally grown oxide (TGO) at the bond coat/TBC interface.

Oxidation induced stress fields in an EB-PVD thermal barrier coating

An elastoviscoplastic analysis based on the finite element method is proposed for the calculation of stress fields responsible for the spallation of an electron beam physical vapor deposition thermal barrier coating. Computations of the mechanical behavior are coupled to a diffusion model to reproduce the growth of the oxide scale. A periodic unit cell, characterizing the local microstructure, is used to compute stress fields at the bond-coat/oxide interface as a function of the interfacial roughness, oxidation time and growth stress. Results show a good agreement between experimental observations of crack locations and calculated spallation stress at the oxide/bond-coat interface.

A comparison between the erosion behaviour of thermal spray and electron beam physical vapour deposition thermal barrier coatings

Thermal barrier coating (TBC) technology is used in the hot sections of gas turbines to extend component life. To maximise these benefits, the TBC has to remain intact through the life of the turbine. High velocity ballistic damage can lead to total thermal barrier removal, while erosion may lead to progressive loss of thickness during operation. This paper compares the erosion behaviour of thermally sprayed and electron beam physical vapour deposited (EB-PVD) TBCs. A unique high velocity gas gun has been developed with the capability of impacting TBCs at particle velocities up to 300 m/s at test temperature up to 920°C. Using this facility it was found, both at room temperature and 910°C, that EB-PVD ZrO 2–8 wt.% Y 2O 3 thermal barriers are significantly more erosion-resistant than the equivalent plasma spray coating, when impacted with either alumina or silica in the particle size range 40–100 μm. Examination of tested hardware reveals that cracking occurs within the near surface region of the columns for EB-PVD ceramic and that erosion occurs by removal of these small blocks of material. In stark contrast, removal of material for plasma sprayed ceramic occurs through poorly bonded splat boundaries. This difference in material removal mechanism accounts for the seven- to 10-fold improvement, in erosion resistance observed for EB-PVD TBCs over those produced by plasma spraying. At both room temperature and 910°C, erosion rates are linear with velocity for both types of coating. When impacted with hard particles (alumina at room temperature and 910°C, silica at room temperature) peak erosion rates occur for normal impact, with impact angle dependence scaling similarly for the air plasma sprayed (APS) and EB-PVD coatings. At high temperatures, erosion rates are increased over those measured at room temperature, consistent with the higher test velocities achieved at 910°C.

Relationships between residual stress, microstructure and mechanical properties of electron beam–physical vapor deposition thermal barrier coatings

Abstract Residual stresses develop in coatings during deposition and can have a large impact on coating mechanical properties and durability. In this study, in-plane residual stresses in Electron beam–physical vapor deposited (EB–PVD) thermal barrier coatings (TBCs) were characterized by the change in substrate curvature upon coating removal, and in-plane elastic moduli were measured from the resonant frequency of the coating–substrate system. Variations in deposition conditions were observed to produce in PVD TBCs a wide range of stress levels, between −70 and 20 MPa. The residual stress was observed to be correlated strongly with the in-plane elastic modulus. A significant difference in the in-plane elastic modulus was measured along different directions of PVD TBC specimens fabricated by rotating the specimens over the evaporation source. The elastic modulus in the direction perpendicular to the axis of rotation was always significantly lower than the modulus measured along the axis of rotation. The elastic modulus measured perpendicular to the axis of rotation was associated with compliant microstructural features produced by the rotation of the substrate over the melt pool. Strain tolerance was measured directly by a new mechanical test that measured the strain at delamination of a coating from an edge-initiated crack from a substrate that was loaded in compression. The strain tolerance of the coating decreased with increasing residual stress.