Maßgeschneiderte Innenbeschichtung von Bauteilen mittels Puls Laser Deposition

As the requirements on technical systems are growing in terms of durability and efficiency, coating of the inner walls of highly charged parts becomes more and more important. Well known examples are the protection of combustion chambers or exhaust systems against heat and oxidation as well as wear protection of friction loaded parts like shock‐absorber tubes or casting moulds. The methods available for internal coating are often limited in the inner diameter of the systems to be coated and it has proven to be very difficult to deposit films homogeneously over a wide range. We present a method for coating of complex internal geometries with diameters down to several millimeters. The combination of classic Pulsed Laser Deposition (PLD) and laser evaporation enables synthesis of tailored film systems in terms of microstructure and distribution of single layer thickness. An example for the application of this method is given by means of the preparation of thermal barrier coatings in rocket combustion chambers.     

Architecture of thermal barrier coatings produced by electron beam-physical vapor deposition (EB-PVD)

Extremely high temperatures and severe atmospheric conditions in the hot section of aircraft engines during operation result in degradation and structural failures of turbine components. Replacing these components is very expensive. Thermal barrier coatings (TBC) composed of ZrO₂-8wt%Y₂O₃(8YSZ) applied by Electron Beam-Physical Vapor Deposition (EB-PVD) to turbine components offer excellent properties for thermal protection and resistance against oxidation - induced erosion and corrosion. However, the life of turbine components is still limited due to premature failure of the TBC. It is hypothesized that the life of the coated components can be extended by lowering the thermal conductivity of the TBC by creating multiple non-distinct or distinct interfaces and alloy additions such as Nb-oxide which will result in a reduction in the thermal conductivity and oxygen transport through the coating. This paper presents the microstructural results of standard 8YSZ, layered 8YSZ, Nb-oxide alloyed 8YSZ and functionally graded 8YSZ with Nb-oxide deposited by EB-PVD. TBC samples were examined by various methods including scanning electron microscopy (SEM), high-resolution optical microscopy (OM), X-ray diffraction (XRD), and thermal cycling tests. The preliminary results strongly suggest that multiple interfaced TBC exhibits better oxidation resistant properties as compared to standard and alloyed TBC.     

Erosion of thermal barrier coatings

Abstract

Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines.

Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal.

This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.     

The effects of coating materials in spark ignition engine design

Ceramic coatings provide good thermal barrier properties for designers. In the design of adiabatic engines, reducing in-cylinder heat rejection requires very special thermal barrier coatings on the engine combustion chamber. Partially, thermal barrier coating (TBC) on the top surface of the piston in the annulus form is considered as a solution for unburned HC emission produced by incomplete combustion with respect to crevice volume after SI engines start (the three-way catalytic converter is not yet activated in the period of first 120 s). Because TBC on the top piston surface decreases the thermal conductivity and increases the unburned charge oxidation by increasing the temperature in the flame quenching area near the entrance of the crevice volume between the piston and liner during the compression and the early part of the expansion strokes.In this study, a steady-state thermal analysis was performed to evaluate the temperature gradients in the standard and two different partially stabilized ceramic coated pistons by using Abaqus© finite element (FE) software. A sharp increase in the temperature of the coated area of the piston was observed as a result of FE simulations. It is concluded that the annulus Y-PSZ coating may contribute better, as compared to Mg-PSZ, to decrease the cold start and steady state HC emissions without auto ignition, since the temperature in the area shows a local sharp increase.     

Improved oxidation resistance and diffusion barrier behaviors of gradient oxide dispersed NiCoCrAlY coatings on superalloy

NiCoCrAlY has been used as the bond coat material in thermal barrier coating (TBC) to protect the superalloy substrate from oxidation and hot-corrosion. Inter-diffusion of elements between the coating and substrate could degrade the oxidation resistance of the coating and the mechanical properties of the superalloy. In this work, a gradient oxide dispersed (OD) NiCoCrAlY coating was produced onto DZ125 superalloy using electron beam-physical vapor deposition (EB-PVD). For comparison, conventional NiCoCrAlY (OD free) coated specimens were also produced by EB-PVD. The oxidation and inter-diffusion behaviors of the coated specimens at 1373 K were investigated. As compared to OD free coating, the OD coating exhibits not only a lower oxidation rate but also an improved oxide scale adherence because outward diffusion of the elements such as Ta, W and Hf from the superalloy was effectively blocked by the OD zone. Meanwhile, the presence of minor Hf in the OD coating contributes to the improved oxide scale adherence by reactive element mechanism.     

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.     

一种热障涂层的形貌和相结构特征研究

研究了一种热障涂层的形貌和相结构组成。溅射NiCrAlY粘结层由γ-Ni相和β-NiAl相组成，是柱状结构，其晶粒度小，属于微晶，有利于形成α-Al2O3膜。溅射NiCrAlY粘结层经真空处理后，发生γ和γ’的转变，以及β-NiAl和脱溶。EB-PVD TBC 具有开放性的柱状结构，并且存在大量的t'-ZrO2相，使涂层性能大大优化。     

Study of failure of EB-PVD thermal barrier coating upon near-α titanium alloy

The cracking failure of a conventional thermal barrier coating (TBC), consisting of a near-α titanium substrate, a NiCoCrAlY bond coat (BC), and a 8 wt.% yttria-stabilized zirconia ceramic layer deposited by electron beam-physical vapor deposition (EB-PVD) method, was studied by cyclic furnace testing and isothermal exposure. The scanning electron microscope, electron probe microanalysis, and microhardness indentation were used to probe the failure mechanism. It is found that due to the mismatch of the coefficient of thermal expansion, the as-deposited BC is suffered the long-term tensile creeping at room temperature. During the high-temperature exposure, the TBC locally rumples, bringing in-plane tensile stress at the shoulders, and out-of-plane tensile stress at the peak of the rumpled BC, where primal cracks are originated. During the cooling period, the ridges of substrate pulled by the local rumpling of the BC blocks the contracting of the BC, originating new cracks in planar BC, and aggravating the original cracks. Furthermore, the oxidation products pushed into the BC and the 8YSZ enlarges the TBC and cracks the substrate along the weakest diffused grain boundaries. The cracking failure related to the diffusion of the BC to the substrate is also discussed.     

Thermal conductivity investigation of zirconia co-doped with yttria and niobia EB-PVD TBCs

A technique used to improve the life cycle and/or the working temperature of the turbine blades uses ceramic coatings over metallic material applied by electron beam-physical vapor deposition (EB-PVD). The most usual material for this application is yttria doped zirconia. Addition of niobia, as a co-dopant in the Y₂O₃–ZrO₂ system, can reduce thermal conductivity. The purpose of this work is to evaluate the influence of the addition of niobia on the microstructure and thermal properties of the ceramic coatings. This new formulation will, in the future, be able to become an alternative to the composition currently used by the aerospace field in EB-PVD thermal barrier coatings (TBC). A significant reduction of the thermal conductivity, measured by laser flash technique, in the zirconia ceramic coatings co-doped with yttria and niobia when compared with zirconia–yttria coatings was observed.     

Metal‐Glass Based Composites for Novel TBC‐Systems

A new concept of thermal barrier coating (TBC) system is presented, based on a metal‐glass composite (MGC). Coatings of metal‐glass composite can be deposited by vacuum plasma spraying and slip casting with a subsequent sinter step. In this TBC system the thermal expansion coefficient depends on the metal‐glass ratio. It is chosen in such a way that the thermal expansion coefficient of the composite is close to the one of the substrate. This leads to reduced thermal stresses and hence improved thermal cycling life times. Because of the low thermal mismatch, coatings of more than 600 μm thickness can be realized. Another advantage of the gas tight composite coatings is their ability to protect the bondcoat from severe oxidation. Correspondingly, long life times have been found for these TBCs in oxidation tests. Also good results were found during thermal cycling tests. Furthermore some aspects of the microstructure evolution of the composite during heat treatment are described.     

Tests of NASA ceramic thermal barrier coating for gas turbine engines

A two-layer thermal barrier coating system with a bond coating of nickel-chromium-aluminum-yttrium (Ni-16Cr-6Al-0.6Y, in wt.%) and a ceramic coating of yttria-stabilized zirconia (ZrO₂12Y₂O₃, in wt.%) was tested for corrosion protection, thermal protection and durability. Full-scale gas turbine engine tests demonstrated that this coating eliminated burning, melting and warping of uncoated parts. During cyclic corrosion resistance tests made in marine diesel fuel products of combustion in a burner rig, the ceramic cracked on some specimens. However, metallographic examination showed no base metal deterioration.     

Multidisciplinary analysis of the operational temperature increase of turbine blades in combustion engines by application of the ceramic thermal barrier coatings (TBC)

The improvement of the temperature resistance of the aircraft engine elements can be obtained by application of a single ceramic thermal barrier coating (TBC) (e.g. Noda [1]) or several composite layers (e.g. Sadowski [2]). Engine elements protected by TBC can work safely in elevated temperature range above 1000 °C. Continuous endeavour to increase thermal resistance of engine the elements requires, apart from laboratory investigations, also numerical study of the different aero-engine parts. The most important are turbine blades, where high temperatures and stress concentrations during thermal shocks or thermal fatigue can be observed during engine exploitation. The high temperatures and stress concentrations can act as the local sources of damage initiation and defects propagation in the form of cracks.The present paper deals with the solution of the transient temperature transfer problem in bare and thermal barrier coated alloy Inconel 713 for the temperature range up to 1000 °C. The computational fluid dynamics (CFD) part of analysis was performed by application of ANSYS Fluent code receiving the temperature field of combustion gas, whereas computational structural mechanics (CMS) part concerning the temperature distribution inside the turbine blade was done by ABAQUS. Finally, the efficiency of the TBC layer (0.5 mm thickness) protecting and cooling channels was discussed in order to explore the operational temperature increase in the aero-engines.     

Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications

Failure of turbine blades generally results from high-temperature oxidation, corrosion, erosion, or combinations of these procedures at the tip, and the leading and trailing edges of a turbine blade. To overcome these limitations, functionally gradient ceramic/metallic coatings have been produced by high-energy beams for high-temperature applications in the aerospace and turbine industries to increase the life of turbine components. Thermal spray processes have long been used to apply high-temperature thermal barrier coatings to improve the life of turbine components. However, these processes have not met the increased demand by the aerospace and turbine industries to obtain higher engine temperatures and increased life enhancement as a result of the inhomogeneous microstructure, unmelted particles, voids, and poor bonding with the substrate. High-energy beams, i.e. electron beam-physical vapour deposition (EB-PVD), laser glazing, laser surface alloying, and laser surface cladding, have been explored to enhance the life of turbine components and overcome the limitations of the thermal spray processes. EB-PVD has overcome some of the disadvantages of the thermal spray processes and has increased the life of turbine components by a factor of two as a result of the columnar microstructure in the thermal barrier coating (TBC). Laser glazing has been used to produce metastable phases, amorphous material, and a fine-grained microstructure, resulting in improved surface properties such as fatigue, wear, and corrosion resistance at elevated temperatures without changing the composition of the surface material. Laser surface alloying and laser surface cladding have shown promising results in improving the chemical, physical, and mechanical properties of the substrate's surface. Metal-matrix composite coatings have also been produced by a laser technique which resulted in increased wear and oxidation-resistant properties. The advantages and disadvantages of thermal spray processes, EB-PVD, laser glazing, laser surface alloying, and laser surface cladding will be discussed. Microstructural evolution of thermal barrier coatings, recent advancements in functionally gradient coatings, laser grooving, and multilayered textured coatings will also be discussed.     

APPLICATION OF HIPING-THICK CERAMIC COATINGS ONTO METALS

The demand for high performance in the combustion equipment used in the automobile and aerospace industries is creating renewed interest in the use of ceramic protective coatings on metal surfaces. Sometimes, thick coating layers are required as thermal barriers or for wear resistance and hardness. Although plasma spraying is one of the promising processes available for depositing thick ceramic coatings onto metal surfaces, the presence of porosity in the coating coupled with lack of corrosion resistance of the coated materials, and the generally low strengths of both the coating layer and the coating-matrix interface may limit the use of the process. HIP treatment of ceramic coatings allows one to obtain dense coatings and also to increase the interfacial bond strength. The present paper reviews the recent advances in the post-HIPing of ceramic coatings as well as the use of HIP for sinter-coating by which a ceramic powder compact is sintered and bonded simultaneously to a metal surface.     

Hot Isostatic Pressing of Plasma Sprayed Thermal Barrier Coating Systems

Thermal barrier coatings (TBC) are important to aerospace and high performance gas turbine engines because they help to keep the temperature experienced by the base metal low; thus, prolonging the life span of the material. Plasma spraying is a technique commonly used to deposit the ceramic-based TBC. An intermediate layer is applied to enhance the bond between the substrate and the ceramic top coat. However, the oxidation of the bond coat due to the infiltration of gas through the porous ceramic layer is a major problem encountered in TBC. This in turn leads to spalling and eventual destruction of the whole coating system. Hot isostatic pressing (HIP) was performed on a number of plasma sprayed thermal barrier coating systems to investigate the effects the process has on micro structure and other physical properties. Due to the fact that the majority of TBC is exposed to thermal cycling and thermal fatigue, it is hoped that the changes brought about by HIP in the porosity and microstructure will improve the life span and performance of TBC. HIP was performed in the temperature range 750-1300° C and pressures of 50-200 MPa. The bond coats that were studied include Ni-5% Al, Ni-20 percnt; Al, NiCrAl and NiCrAlY, while the ceramic coat was Zr0₂-5 wt percnt; CaO. Characterization of the coatings was carried out using scanning electron microscopy (SEM) and image analyser. The results showed the porosity of the coatings to be dramatically reduced to near zero levels. In addition, the other physical properties like hardness and Young's modulus increased over a wide temperature range.     

Effects of heat treatment on adhesion strength of thermal barrier coating systems

Thermal barrier coatings (TBCs) are widely used as protective and insulative coatings on hot section components of gas turbines and their applications, like blades and combustion chambers. The quality and performance properties of TBCs are of great importance in terms of their resistance to service conditions. In a TBC system, there is a close relationship between the adhesion properties of coating layers. The adhesion strength of TBCs varies depending on the coating technique used and the surface treatments. In this study, CoNiCrAlY and YSZ (ZrO₂ + Y₂O₃) powders were deposited on stainless steel substrate. High Velocity Oxy-Fuel (HVOF) and Atmospheric Plasma Spraying (APS) techniques were used to produce the bond coats. The ceramic top layers on CoNiCrAlY bond coats were produced by the APS technique. The TBC specimens were subjected to heat-treatment tests. Adhesion strength for top coat/bond coat interface of as-sprayed and heat-treated samples was investigated. The results showed that the heat treatment of the coatings in different temperatures led to an increase in the adhesion strength of TBCs.     

In-depth and In-plane Thermal Diffusivity Measurements of Thermal Barrier Coatings by IR Camera: Evaluation of Ageing

Ceramic thermal barrier coatings (TBCs) are widely used for protecting hot path components from combustion gases in gas turbines for both aero- and land-based applications. TBCs undergo degradation and eventually detach from the substrate. Forecasting of the detachment of TBCs for timely maintenance is an open problem in gas turbine technology. It is known that sintering happens in the TBCs when exposed to high temperature. Sintering affects the mechanical properties of TBCs and mainly their strain compliance for which degradation causes the detachment. As sintering strongly affects the thermal diffusivity of TBCs also, the idea is to measure the latter parameter to account for the former. Pulsed thermography is the technique selected to monitor the thermal diffusivity variation due to TBC ageing. In perspective, it should be applied to monitor the gas turbine during the normal stop for maintenance. This article reports preliminary laboratory tests carried out on a set of metal samples coated with TBCs. The samples were aged during cyclic oxidation tests at various percentages of their estimated life, the end of life being the time of the TBC detachment from the substrate. The identification of the thermal diffusivity in the coating layer is carried out for the general case of anisotropic conductivity.     

Development of Catalytic Converters Using Detonation Spraying

One of the trends in hydrogen power engineering is the development of devices for the preparation of synthesis gas by the catalytic reforming of a hydrocarbon feedstock. Studies show the advantages of catalytic converters based on a modular catalyst support with a honeycomb-type structure produced from a metal foil, both sides of which are coated with highly porous oxide ceramics. The drawback of this design is a poor ability of the coating to withstand high-temperature operating conditions. The coating may detach from the substrate because of the difference in thermal expansion coefficients between the metal foil and the ceramic coating. Besides, a corrosion of metal foil takes place. The result of the present study is the development and application of a two-step coating method, which allows significantly increasing the service life of the catalyst supports. At the first step, a low-porosity ceramic layer is deposited on a metal foil by detonation spraying. At the second step, a high-porosity layer of a ceramic catalyst is deposited from suspension. In this work, the peculiarities of the detonation spraying of the ceramic coating on a metal foil and the design of the obtained catalytic converter have been addressed.     

Adhesion of thermal barrier coatings – the development of metastable alumina

Abstract

The adhesion of thermal barrier coatings (TBCs) is dependent upon the characteristics of the thermally grown oxide (TGO) that forms between the TBC and the corrosion resistant bond coat. Work has been carried out to investigate the properties of the TGO as a function of ageing treatments using piezospectroscopy. Residual stress maps were generated for an electron beam physical vapour deposited (EB-PVD) TBC which showed a large variation in residual stress over the surface of a coated sample. The two peaks generally associated with a alumina (R1 and R2) frequently appear as doublets with a high and low stress component. In addition, the presence of a metastable θ alumina was detected in aged samples. It is believed that these observations can be related to incipient spallation of the TBC. The development of residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC.     

La2(Zr0.7Ce0.3)2O7——新型高温热障涂层

采用电子束物理气相沉积技术(EB-PVD)制备了新型La2(Zr0.7Ce0.3)2O7 (LZ7C3)热障涂层.研究了涂层的组分、显微结构、表面和横截面形貌以及恒温氧化行为.结果表明:涂层中La2O3/ZrO2/CeO2的相对含量偏离了化学计量比,但X 射线衍射(XRD)相结构与靶材非常相似.通过CeO2 掺杂后,LZ7C3体材料的热膨胀系数比La2Zr2O7 (LZ)大;在1100℃恒温氧化890h的条件下,LZ7C3涂层的抗氧化增重性能明显优于传统的Y2O3部分稳定化的ZrO2(8YSZ)涂层.此外,热膨胀不匹配、黏结层氧化和陶瓷涂层内部微观裂纹的出现可能是导致LZ7C3涂层恒温氧化失效的主要原因.