Vapor Deposition Thermal Barrier Coatings 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.

Failure and life evaluation of EB-PVD thermal barrier coatings using temperature-process-dependent model parameters

Using experimentally measured temperature-process-dependent model parameters, the failure analysis and life evaluation were conducted for electron beam-physical vapor deposition thermal barrier coatings (EB-PVD TBCs) with Pt-modified β-NiAl bond coats deposited on Ni-base single crystal superalloys. The failure analysis and life model were applied to two failure modes of A and B identified experimentally subjected to thermal cyclic process. The rumpling effect and the associated roughness of the constituent coating layers were shown to play a key role in evaluating the coating’s failure and life. The experimentally determined temperature-dependent thickness of thermally grown oxide (TGO), interfacial roughness, elastic moduli of the constituent coatings and their coefficients of thermal expansion were incorporated into the life prediction model. The maximum average rumpling amplitude of the bond coat/TGO interface associated with bond coat rumpling was used in failure analysis and life evaluation for the failure mode A. The global wavelength related to the interface rumpling and its radius curvature were identified as essential parameters in life evaluation, and the predicted life results for failure mode A were verified by existing furnace cyclic test data. For the failure mode B, the crack growth rate along the TGO/top coat interface was calculated using the crack length dependent fracture toughness.

Research Process in Plasma Spray Physical Vapor Deposited Thermal Barrier Coatings

Research Process in Plasma Spray Physical Vapor Deposited Thermal Barrier Coatings

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.

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.