Electron Beam Physical Vapor Deposition Process Research Articles

Influence of Bond Coat Roughness on Adhesion of Thermal Barrier Coatings Deposited by the Electron Beam–Physical Vapour Deposition Process

Thermal barrier coatings (TBCs) are effective protective and insulative coatings on hot section components of turbine engines. The quality and subsequent performance of the TBCs are strongly dependent on the adhesion between the coating and the metal substrate. The adhesion strength of TBCs varies depending on the substrate materials and coating, the coating technique used, the coating application parameters, the substrate surface treatments, and environmental conditions. Therefore, the roughness of the substrate surface has a significant effect on the performance of the TBC system. In this work, the roughness and microstructure of the 7YSZ (7 wt.% yttria-stabilised zirconia) top coat under different bond coat roughness treatments were studied. The purpose of this paper was to investigate the influence of the roughness of the bond coat on the adhesion of 7YSZ TBCs prepared by the electron beam–physical vapour deposition (EB-PVD) process. The VPA (vapour phase aluminium) bond coat was deposited on Inconel 718 nickel superalloy substrate using the above-the-pack technique. The ceramic top coat was applied to the bond coat using the EB-PVD process. The dependence between the TBC coating roughness and the bond coat roughness was determined. Adhesion strength measurements were performed according to the ASTM C 633 standard test method. The highest adhesion value observed in the tensile adhesion tests was 105 MPa. However, it was not determined whether the surface roughness of the bond coat affects the adhesion of the 7YSZ top coat.

Modelling evaporation in electron-beam physical vapour deposition of thermal barrier coatings

This work presents computational models of ingot evaporation for electron-beam physical vapour deposition (EB-PVD) that can be applied to the deposition and development of thermal barrier coatings (TBCs). TBCs are insulating coatings that protect aero-engine components from high temperatures, which can be above the component’s melting point. The development of advanced TBCs is fuelled by the need to improve engine efficiency by increasing the engine operating temperature. Rare-earth zirconates (REZ) have been proposed as the next-generation TBCs due to their low coefficient of thermal conductivity and resistance to molten calcium-magnesium alumina-silicates (CMAS). However, the evaporation of REZ has proven to be challenging, with some coatings displaying compositional segregation across their thickness. The computational models form part of a larger analytical model that spans the whole EB-PVD process. The computational models focus on ingot evaporation, have been implemented in MATLAB and include data from 6 oxides: ZrO2, Y2O3, Gd2O3, Er2O3, La2O3 and Yb2O3. Two models (2D and 3D) successfully evaluate the evaporation rates of constituent oxides from multiple-REZ ingots, which can be used to highlight incompatibilities and preferential evaporation of some of these oxides. A third model (local composition activated, LCA) successfully predicts the evaporation rate of the whole ingot and replicates the cyclic change in composition of the evaporated plume, which is manifested as changes in compositional segregation across the coating’s thickness. The models have been validated with experimental data from Cranfield University’s EB-PVD coaters, published vapour pressure calculations and evaporation rate formulas described in the literature.

Formation of thin foils of high-entropy CrFeCoNiCu alloys by EB-PVD process

Regularities of formation of thin foils (up to 100 μm) of high-entropy alloys (HEAs) of Cr-Fe-Co-Ni-Cu system at electron beam physical vapor deposition (EB-PVD) were studied in the work. Thin foils of CrFeCoNiCu alloy were produced using a single-crucible evaporation with a constant rate of feeding of the alloy into the liquid pool. The influence of deposition temperature on the structure of the produced foils is considered. X-ray methods were used to study the phase composition and the texture of CrFeCoNiCu foils. Foils deposited at temperature below 923 K exhibit a single-FCC structure and low degree of texture. It is shown that increase in deposition temperature to 973 K leads to the transition from the foil microstructure containing almost randomly oriented nanosized crystallites separated by voided boundaries to columnar-like grains microstructure exhibiting a two-component axial 〈111〉 + 〈100〉 texture. Moreover, the increase of deposition temperature to 1023 K results in the transition into dual-FCC phase state. Effect of the structure change on mechanical properties of the deposited foils was analyzed.

High-rate deposition of high-pure silicon thin films for PV-Absorber layers by crucible-free electron beam physical vapor deposition

In photovoltaics (PV) and microelectronics, there is an ongoing need for fabrication of silicon thin films with excellent material properties and with minimum production cost at the same time. To overcome fabrication gaps we present reached results for depositing silicon thin films by crucible-free electron beam physical vapor deposition (EB-PVD) and for plasma pretreatment of the substrates. The crucible-free EB-PVD process could be improved regarding critical aspects, e.g. occurrence of ingot cracks during heating up of monocrystalline Si evaporation material and spill out of the Si melt on the melting pool edges. Deposition rates above 500 nm/s, corresponding to 30 μm/min, have been reached. Relating heat fluxes will be presented and discussed. By using a double ingot arrangement and superposing vapor from multiple sources the relative deviation of layer thickness could be reduced to < ±5% on a substrate width of 200 mm.

Interfacial phenomena and evolution of modified aluminide bondcoatings in Thermal Barrier Coatings

The paper presents results of microstructural investigations of TBCs on Pt- and PdPt-aluminide bondcoatings with focus on the interfacial phenomena that take place during the pre-oxidation treatment as well as further thermal cycling at 1100 °C. The Pt and Pd coatings were produced using a Physical Vapor Deposition (PVD) method while aluminizing was performed using a high activity Vapor Phase Aluminizing (VPA). The ceramic yttria stabilized zirconia (YSZ) top coating was deposited using Electron Beam Physical Vapor Deposition (EB-PVD) method. Prior to the EB-PVD process the bondcoatings were pre-oxidized at 1140 °C for 2 h in air atmosphere in order to form a stable and adherent α-Al2O3 TGO with minimum transient oxidation. Special effort has been done in order to investigate the microstructure of the Thermally Grown Oxide (TGO) formed during pre-oxidation treatment prior to YSZ deposition and the phenomena occurring at the interface between the YSZ, TGO, and the bondcoatings. Microstructure evolution of the TGOs is described and related to the conditions of the pre-oxidation treatment and the chemistry of the bondcoatings. During thermal cycling at 1100 °C the TGO grows by inward oxygen diffusion forming distinctive columnar grains which grow deeper at triple grain junctions with the bondcoating. Therefore the TGO/bondcoating interface evolves from flat to a convoluted during high temperature oxidation. Several segregation phenomena can be found in the TBCs that include i.a. the Reactive Element Effect involving the presence of Hf and Y on α-alumina grain boundary disconnections in the TGO. It was found that compared to bare René N5 superalloy the Hf segregation is hindered in the presence of modified aluminide bondcoatings.

Application of Multicore Processing and Volume Parallelization in EBPVD Process Simulated Using DSMC

The present work is focused towards exploring the parallelization opportunities to extend the capability of available computational resources for analyzing larger control volumes and complex geometries for an Electron Beam Physical Vapour Deposition (EBPVD) process using Direct Simulation Monte Carlo technique. The work first establishes the efficacy of the model by comparing the simulated results with published experimental results for copper deposition in a vacuum chamber. The model is then extended to incorporate additional geometries and tracks the results of the copper deposition in an extension of the cubic chamber. The copper deposition in the extended geometry is simulated in two ways: initially by considering the entire volume as a single control volume and secondly by creating a virtual partition to divide the entire volume under consideration into two sub-control volumes which are simulated independently. The results obtained show that this independent treatment of the control volume gives comparable results thereby implying that large control volumes can be simulated by dividing them into smaller and more manageable sub-volumes, and simulating these sub-volumes independently. The paper further reports about employing object oriented programming techniques and multicore processing to support development and execution of the program.

Investigation of Isothermal Oxidation Behavior of Thermal Barrier Coatings (TBCs) Consisting of YSZ and Multilayered YSZ/Gd2Zr2O7 Ceramic Layers

Rare-earth zirconates have a big potential as top coat material of TBCs due to their superior high-temperature properties. TBCs are usually produced by thermal spray methods or electron beam physical vapor deposition (EB-PVD) techniques. Cold gas dynamic spray (CGDS) is a promising deposition method for production of dense and non-oxide bond coat compared to other thermal spray processes, while EB-PVD process provides good adhesion and strain tolerance for top coating layers of TBCs. In this study, in order to observe the effect of Gd2Zr2O7 on the oxidation behavior of TBCs, CoNiCrAlY was sprayed on substrate material using the CGDS method, afterward, yttria-stabilized zirconia (YSZ) and YSZ/Gd2Zr2O7 were deposited using EB-PVD technique. After deposition, specimens were isothermally oxidized in a high-temperature furnace at 1100 °C for 8, 24, 50, and 100 h. Microstructures of oxidized samples were examined and thermally grown oxide layer of TBCs were comparatively evaluated.

Microstructural, mechanical and oxidation features of NiCoCrAlY coating produced by plasma activated EB-PVD

NiCoCrAlY coatings produced by electron beam-physical vapor deposition (EB-PVD) have been extensively used as the oxidation resistance coatings or suitable bond coats in thermal barrier coating (TBC) system. However, the inherent imperfections caused by EB-PVD process degrade the oxidation resistance of the coatings. In the present work, NiCoCrAlY coatings were creatively produced by plasma activated electron beam-physical vapor deposition (PA EB-PVD). The novel coatings showed a terraced substructure on the surface of each grain due to the increased energy of metal ions and enhanced mobility of adatoms. Also a strong (111) crystallographic texture of γ/γ′ grains was observed. The toughness of the coatings got remarkably improved compared with the coatings deposited by conventional EB-PVD and the oxidation behavior at 1373K showed that the novel coatings had excellent oxidation resistance. The possible mechanism was finally discussed.

Corrosion properties and contact resistance of TiN, TiAlN and CrN coatings in simulated proton exchange membrane fuel cell environments

In this study, the contact resistance (CR) and electrochemical properties of TiN, CrN and TiAlN electron beam physical vapor deposition (EBPVD) coatings and their stainless steel 316L (SS316L) substrate were investigated in a simulated proton exchange membrane (PEM) fuel cell environment. The potentiodynamic polarization corrosion tests were conducted at 70 °C in 1 M H 2SO 4 purged with either O 2 or H 2, and the potentiostatic corrosion tests were performed under both simulated cathodic (+0.6 V vs. Ag/AgCl reference electrode purged with O 2) and anodic conditions (−0.1 V vs. Ag/AgCl reference electrode purged with H 2) for a long period (4 h). SEM was used to observe the surface morphologies of the samples after corrosion testing. All the TiN-, TiAlN- and CrN-coated SS316L showed a lower CR than the uncoated SS316L. While the corrosion performance of the coatings was dependent on the cathodic and anodic conditions, the CrN coating exhibited a higher (in the anodic environment) or similar (in the cathodic environment) corrosion resistance to the uncoated SS316L. Thus, the CrN-coated SS316L could potentially be used as a bipolar plate material in the PEM fuel cell environment. Although the EBPVD process greatly reduced number of pinholes in the coatings compared to other plasma enhanced reactive evaporations, future research efforts should be directed to eliminate the pinholes in the coatings for long-term durability in fuel cell applications.

Simulation model for an EB-PVD coating structure using the level set method

A comprehensive modeling approach to link machine dynamics, deposition, and substrate kinematics in an electron beam physical vapor deposition (EB-PVD) is presented in this paper. The machine dynamics in EB-PVD process are captured by finite element models, resulting in the prediction of evaporation rate and vapor distribution. The deposition process is modeled using the level set method, which is one of the computational techniques for tracking topographic evolution. The proposed simulation model is implemented in Matlab and is compared with experimental results published by other researchers. Results indicate that the proposed simulation model can be used to predict microstructure features such as zigzag and helical columnar shapes. The pitch of a zigzag microstructure can be predicted within 20% at the 0.3 to 6 μm level for Yttira-stabilized Zirconia (YSZ) coating.

Residual Stresses and Structure of EB-PVD Thermal Barrier Coatings Exposed at High Temperature

The substrate material was nickel based superalloy (In738LC), CoNiCrAlY was pressureless plasma-sprayed on the substrate as the bond coating. As the top coating, zirconia with 4 mol% yittria was made by a electron beam-physical vapor deposition (EB-PVD) with a rotation. The thickness of the bond coating was about 0.18mm, and the thickness of the top coating was about 0.12 mm. The rotation speeds of specimens in the EB-PVD process were 5 rpm, 10rpm and 20rpm. The specimens were exposed at 1273K for 200h. The in-plane residual stress was measured by a conventional X-ray method, and the out-of-plane residual strain was measured by a strain scanning method with hard synchrotron X-rays. For the specimens with 5rpm and 10rpm, the in-plane compressive residual stresses were released by the high temperature exposure. For the specimen with 20 rpm, the in-plane residual stress did not change by the high temperature exposure, because it was very small before the exposure. The in-plane residual stresses were large compression near the interface. For the specimens with 5 rpm and 10rpm, the out-of plane stresses were small from the coating surface but became a compression near the interface. Both in-plane and out-of-plane residual stresses of the specimen with 20rpm were very small. In the effect of decreasing the residual stress, the rotation speed of 20rpm was the excellent condition. According to the observation with a scanning electron microscope, the feather-like columnar structure decreased after the high temperature exposure. This volume decrease caused the release of the in-plane residual stresses.

Effects of EB-PVD Process TGO Formation and Growth within Thermal Barrier Coatings

It has been found that under oxygen partial pressure of ~2×10-6 kPa, the high-temperature oxidation of thermal barrier coatings (TBCs) occurred during an electron beam physical vapor deposition (EB-PVD) process for producing the TBCs top ceramic coating. In the present investigation, two modified bond coats (BCs) of NiCrAlY with Si addition, and NiCrAlY with Co and Hf additions, were developed by Arc Ion-plating technique to study the effects of the EB-PVD process on thermally grown oxide (TGO) formation and growth. The isothermal and cyclic oxidation tests were conducted and the cross-sectional morphologies of the specimens were examined to compare the high-temperature oxidation behaviors of the two TBCs. It was found that a mixed oxide layer have been developed in the as-deposited TBCs with a NiCrAlYSi BC. The mixed oxide layer mainly included Cr2O3, NiO, Al2O3 and their spinel. With the mixed oxide layer, TBCs with the NiCrAlYSi BC showed a superior high-temperature resistance on later high-temperature exposure to TBCs with NiCoCrAlYHf BC, where no mixed oxide layer was observed. The pre-formed mixed oxide layer apparently shortened the time to fully develop a protective α-Al2O3 layer and therefore restrained the TGO growth in TBCs.

Residual Stress in Zirconia Coating by EB-PVD Method

The Ni-based superalloy IN738LC was used as the substrate material, and CoNiCrAlY powder was pressureless plasma-sprayed on the substrate as the bond coating. Zirconia was coated as the top coating by the electron beam-physical vapor deposition (EB-PVD) method. In the EB-PVD process, the specimens were kept at 1223K and rotated with 5rpm, 10rpm and 20rpm. According to the microscopic observation and the result of the pole figures, the top coatings had a columnar structure, which was made by the piling up of (111) planes. The cross section of the column had a diamond shape, and its diagonal was parallel to the rotation axis. The residual stress on the surface of the top coatings was evaluated by the X-ray diffraction method. Each diffraction profile was separated into the 133 and the 331peak, and the residual stress was measured by the sin2ψ method. The measured residual stresses were –76.7MPa for 5rpm, –63.0MPa for 10rpm and –25.1MPa for 20rpm.

Indentation on YSZ thermal barrier coating layers deposited by electron beam PVD

We have deposited an yttria-stabilized zirconia (YSZ) layer consisting of microsized grains on an alumina substrate using electron beam physical vapour deposition (EBPVD). The effects of Y2O3 concentration and the coating microstructures of YSZ on indentation damage are investigated. Different coating microstructures are prepared by varying the substrate temperature during deposition. Hertzian and nanoindentation are used to characterize the damage responses of the YSZ thermal barrier-coated alumina layered systems. The damage and failure behaviours have been investigated in relation to the effects of microstructures formed at various substrate temperatures as well as indentation loads from indentation tests. Elastic modulus, hardness, and stress–strain behaviours are characterized by results of the indentation tests. With increasing substrate temperature during the EBPVD process, the overall grain sizes became coarser with a smaller fraction of weak interfaces between the columns, and became more faceted. This change in microstructure ultimately influences the indentation behaviour. The results according to the use of smaller and larger scaled indenters during Hertzian/nanoindentations were also analyzed.

Strain Tolerance and Microstructure of Thermal Barrier Coatings Produced by Electron Beam Physical Vapor Deposition Process

Several kinds of thermal barrier coatings (TBCs) deposited by electron beam physical vapor deposition (EB-PVD) were produced as a function of electron beam power in order to evaluate their strain tolerance. The deposition temperatures were changed from 1210 K to 1303 K depending on EB power. In order to evaluate strain tolerances of the EB-PVD/TBCs, a uniaxial compressive spallation test was newly proposed in this study. In addition, the microstructures of the layers were observed with SEM and Young’s moduli were measured by a nanoindentation test. The strain tolerance in as-deposited samples decreased with an increase in deposition temperature. In the sample deposited at 1210 and 1268 K, high-temperature aging treatment at 1273 K for 10 h remarkably promoted the reduction of the strain tolerance. The growth of thermally grown oxide (TGO) layer generated at the interface between topcoat and bondcoat layers was the principal reason for this strain tolerance reduction. We observed TGO-layer growth even in the as-deposited sample. Although the thickness of the initial TGO layer in the sample deposited at high temperature was thicker, the growth rate during aging treatment was smaller than those of the other specimens. This result suggests that we can improve the oxidation resistance of TBC systems by controlling the processing parameters in the EB-PVD process.

Effect of morphology on thermal conductivity of EB-PVD PYSZ TBCs

Partially yttria stabilized zirconia (PYSZ) based thermal barrier coatings (TBC) manufactured by electron beam-physical vapour deposition (EB-PVD) protect turbine blades, working under severe service conditions in aero engines and stationary turbines. These coatings show a high strain tolerance relying on their unique morphology which is comprised of weakly bonded, preferred-oriented columns, voids between feather-like sub-columns and, finally, of intra-columnar closed pores.The results obtained in this work demonstrate that variation of the EB-PVD process parameters alters the resulting columnar morphology and porosity of the coatings. The physical properties and, most importantly, thermal conductivity, are greatly affected by these morphological alterations. This study investigates three morphologically different EB-PVD PYSZ TBC top coats in terms of the spatial and geometrical characteristics of their porosity and correlates those with the thermal conductivity values measured in as-coated state and after heat treatment at 1100 °C for 1 h and 100 h. Changes in the open and closed porosity caused by heat-treatment are characterized by small-angle neutron scattering (SANS), Brunauer–Emmett–Teller Method (BET) and scanning electron microscope (SEM). Correlation of shape and surface-area changes in all porosity types of the analysed coatings revealed that the thermal conductivity of these coatings is influenced primarily by size and shape distribution of the pores and secondarily by the pore surface-area available at the cross section perpendicular to the heat flux.

Peculiarities of the structure and properties of quasicrystalline Al–Cu–Fe coatings produced by EBPVD process

The effect of both composition and substrate temperature on the structure and properties of quasicrystalline Al–Cu–Fe coatings obtained by high-rate (˜100 nm s−1) electron beam physical vapour deposition (EBPVD) was studied. An icosahedral phase has been observed in thick coatings in a much wider concentration range compared with in cast alloys. A lowering of substrate temperature results in a decrease of grain size of the quasicrystalline coating (down to nanosized scale) and an increase of the concentration of vacancy-type defects in the quasicrystalline structure. The features of the quasicrystalline structure of the coatings are shown to have an influence on their mechanical behaviour at both static and alternate loadings.

Intelligent automation of electron beam physical vapour deposition

The present paper presents an intelligent automation of the electron beam physical vapour deposition (EBPVD) process to achieve high quality and cost efficient coatings for low volume part production, using realtime feedback control. A computational model of EBPVD for predicting coating thickness is used with an optimisation heuristic for reducing coating thickness variance and feedback control approaches for substrate temperature control and melt pool control. The computational model can be readily generated using a standard computer aided design (CAD) model of the workpiece, which makes the method applicable to workpieces with complex three-dimensional geometry. Based on this model, an optimisation heuristic for the EBPVD process is developed to control workpiece motion systematically with the objective of reducing coating thickness variance, i.e. providing a uniform coating. These computational developments are illustrated using a simulation of a turbine blade coating in which the coating thickness variance is reduced significantly. Process level intelligence is incorporated using realtime feedback control for substrate temperature and melt pool control using an open architecture control system. Results using thermocouple based temperature control and realtime vision for melt pool control are presented. Video images of the melt pool are analysed on a block by block basis, using a technique to identify critical regions of the melt pool. Simulation results demonstrate the feasibility of automating the electron gun beam steering sequence. The proposed methods offer the prospect of eliminating dedicated tooling/fixtures and improving the cost effectiveness of the process, especially for low volume production.

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y2O3–ZrO2 (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y2O3–HfO2 (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ.©2003 Elsevier Science Ltd. All rights reserved.