Tailoring protective coatings for all‐oxide ceramic matrix composites in high temperature‐/high heat flux environments and corrosive media

The application range of porous all‐oxide ceramic matrix composites (CMCs) can be significantly extended through deposition of protective coating systems. Typical applications include protection against erosion, wear and foreign object damage as well as a reduced permeability. Environmental barrier coatings (EBC) are mandatory in order to guarantee sufficient lifetime of the CMC components under high temperature‐, high heat flux conditions and corrosive attack (combustor liners, thermal protection systems for atmospheric reentry). Limited thermal stability of today’s oxide fibers requires additional thermal barrier functionality for EBCs in order to keep the effective CMC bulk temperatures below 1200 °C. Depending on the specific application DLR’s coating concept for all‐oxide CMCs is based on either a single reaction‐bonded aluminium oxide (RBAO) coating or a hybrid coating system consisting of a RBAO bond coat followed by an EB‐PVD YSZ/FSZ top coat and is highlighted for three case studies. Deposition techniques (magnetron sputtering, MOCVD) alternative to EB‐PVD as well as the suitability of fibrous and cellular materials for thick EBC/TBC layers are explored.     

Entwicklung von Oxid – Keramik zur Anwendung als Wärmedämmschichten

Development of Oxide Ceramics for an Application as TBC The standard thermal barrier coating material yttria stabilised zirconia (YSZ) is limited in long term operation to a maximum temperature of about 1200°C. As a result further increase of the gas inlet temperature and hence the efficiency of gas turbines are hardly to achieve with YSZ coatings. In a screening procedure especially perowskite (ABO₃, A = Sr,Ba, B = Zr) and pyrochlore (A₂B₂O₇, e.g. A = La and other rare earth elements, B = e.g. Zr) materials have been identified as possible candidates for thermal barrier coatings. Basic physical properties (e.g. thermal expansion coefficient, thermal diffusivity and conductivity) of several candidates have been determined using sintered, dense samples. The possibility of optimization of the properties by using specific compositions will be discussed. From promising materials powders which are suitable for plasma‐spraying have been produced by spray‐drying. New TBC systems consisting of new materials (BaZrO₃, La₂Zr₂O₇) deposited by atmospheric plasma spraying and vacuum plasma sprayed MCrAlY bondcoats were tested in a gas burner facility. Especially La₂Zr₂O₇ coatings gave promising results. A further improvements could be achieved by the use of layered or graded coatings with a YSZ coating at the bondcoat interface and on top a layer of the new TBC material. First results of thermal cycling tests with 1250 and 1350°C surface temperature will be presented.     

PVD‐Schutzschichten für Werkzeuge des Präzisionsblankpressens

PVD protective coatings for precision molding tools Precision glass molding (PGM) is a replicative hot forming process for the production of complex optical components, such as aspherical lenses for digital and mobile phone cameras or optical elements for laser systems. The efficiency and thus also the profitability of the PGM depend on the unit price per pressed component, which correlates primarily with the service lifetime of the pressing tools. To increase tool lifetime, the tool surfaces are coated with protective coatings based on precious metals or carbon using physical vapour deposition (PVD). The PVD coating technology enables the deposition of thin coatings, which also follow more complex surface geometries and achieve a high surface quality. PVD coatings are also commonly used to protect tools from wear and corrosion. This paper presents two chromium‐based nitride hard coatings produced by an industrial PVD unit and investigated for their applicability for PGM. Two different coating architectures were implemented, on the one hand a single coating chromium aluminium nitride (Cr,Al)N coating and on the other hand a nanolaminar CrN/AlN coating with alternating layers of chromium nitride and aluminium nitride. The latter is a coating consisting of hundreds of nano‐layers, only a few nanometers thick. Both coatings, (Cr,Al)N and CrN/AlN, each have a thickness of s ～ 300 nm in order to follow the tool contour as closely as possible. The properties of the coating systems, which are of particular relevance for PGM, are considered. These include on the one hand the adhesion of glass, the roughness and topography of the surface and the adhesion between the coating and the tool material. In addition, the barrier effect of the coatings against diffusion of oxygen was investigated. In order to reproduce the thermal boundary conditions of the PGM, thermocyclic aging tests are performed and their influence on the different properties is described.     

Thermal conductivity issues of EB‐PVD thermal barrier coatings

The thermal conductivity of electron‐beam physical vapor deposited (EB‐PVD) thermal barrier coatings (TBCs) was investigated by the Laser Flash technique. Sample type and methodology of data analyses as well as atmosphere during the measurement have some influence on the data. A large variation of the thermal conductivity was found by changes in TBC microstructure. Exposure at high temperature caused sintering of the porous microstructure that finally increased thermal conductivity up to 30 %. EB‐PVD TBCs show a distinct thickness dependence of the thermal conductivity due to the anisotropic microstructure in thickness direction. Thin TBCs had a 20 % lower thermal conductivity than thick coatings. New compositions of the ceramic top layer offer the largest potential to lower thermal conductivity. Values down to 0.8W/(mK) have been already demonstrated with virgin coatings of pyrochlore compositions.     

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

Increase of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La₂Zr₂O₇) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.     

Einfluss einer PVD‐Al‐Zwischenschicht auf die Eigenschaften eines thermisch gespritzten Wärmedämmschichtsystems nach Temperaturwechselbelastung

Thermal barrier coatings (TBC) generally consist of a metallic bond coat (BC) and a ceramic top coat (TC). Co–Ni–Cr–Al–Y metallic super alloys and Yttria stabilised zirconia (YSZ) have been widely used as bond coat and top coat for thermal barrier coatings systems, respectively. As a result of long‐term exposure of thermal barrier coatings systems to oxygen‐containing atmospheres at high temperatures, a diffusion of oxygen through the porous ceramic layer occurs and consequently an oxidation zone is formed in the interface between ceramic top coat and metallic bond coat. Alloying components of the BC layer create a so‐called thermally grown oxides layer (TGO). One included oxide type is α‐Al₂O₃. α‐Al₂O₃ lowers oxygen diffusion and thus slows down the oxidation process of the bond coat and consequently affects the service life of the coating system positively. The distribution of the alloying elements in the bond coat layer, however, generally causes the formation of mixed oxide phases. The different oxide phases have different growth rates, which cause local stresses, micro‐cracking and, finally, delamination and failure of the ceramic top coat layer. In the present study, a thin Al inter‐layer was deposited by DC‐Magnetron Sputtering on top of the Co–Ni–Cr–Al–Y metallic bond coat, followed by thermal spraying of yttria‐stabilised zirconia (YSZ) as a top coat layer. The deposited Al inter‐layer is meant to transform under operating conditions into a closed layer with high share of α‐Al₂O₃ that slows down the growth rate of the resulting thermally grown oxides layer. Surface morphology and microstructure characteristics as well as thermal cycling behaviour were investigated to study the effect of the intermediate Al layer on the oxidation of the bond coat compared to standard system. The system with Al inter‐layer shows a smaller thermally grown oxides layer thickness compared to standard system after thermal cycling under same conditions.     

Modeling of thermal barrier coating temperature due to transmissive radiative heating

Thermal barrier coatings are generally designed to possess very low thermal conductivity to reduce the conduction heat transfer from the coating surface to the metal turbine blade beneath the coating. In high-temperature power generation systems, however, a considerable amount of radiative heat is produced during the combustion of fuels. This radiative heat can propagate through the coating and heat up the metal blade, and thereby reduce the effectiveness of the coating in lowering the thermal load on the blade. Therefore, radiative properties are essential parameters to design radiative barrier coatings. This article presents a combined radiation and conduction heat transfer model for the steady-state temperature distribution in semitransparent yttria-stabilized zirconia (YSZ) coatings. The results of the model show a temperature reduction up to 45 K for YSZ of high reflectance (80%) compared to the YSZ of low reflectance (20%). The reflectivities of YSZ and metal blade affect the temperature distribution significantly. Additionally, the absorption and scattering coefficients of YSZ, the thickness of the coating, and the thermal conductivities of YSZ and metal blade affect the temperature distribution.     

PVD‐Beschichtungen für trockenlaufende Hybridwälzlager

PVD coatings for dry running hybrid roller bearings PVD coatings are able to improve the characteristics of dry running hybrid roller bearings. Wear and friction moment can be reduced by coatings. Therefore PVD coatings were adjusted for the appearing loads. Examinations with the help of the standardised FE8 test‐rig were used to verify the potential of PVD coatings in dry running applications. The results of the tests were taken into account for further coating development.     

Untersuchung des Sputter‐Ätzens auf die Eigenschaften von PVD‐CrN Hartstoffschichten auf Magnesium AZ91hp

Investigating the Influence of the Sputter Etching Process on the Properties of PVD‐CrN Coatings on Magnesium Die Cast Alloy AZ91hp A common method prior to the PVD deposition is the sputter etching process of the substrate itself to clean the surface from adhesion products and to improve the coating adhesion. This report deals with the sputter etching of magnesium die cast alloy AZ91hp to investigate the influences on the coating‐substrate interface, the surface properties and the mechanical properties of PVD‐CrN hard coatings. The coating‐substrate interface of the Cr‐AZ91 coating systems was investigated with XPS and SIMS. Surface studies were carried out by high resolution electron microscopy and AFM. The characterization of the mechanical properties of the CrN‐AZ91 compound systems includes thickness, coating hardness and hardness depth profiles, coating adhesion, structure and residual stresses. Some properties show a strong dependency of the etching time, especially the mechanical properties and the coating roughness. Increasing etching times lead to an improvement of the coating quality.     

Understanding the dry sliding wear behaviour of atmospheric plasma-sprayed rare earth oxide coatings

In this paper, the friction and wear behaviours of Yttria Stabilized Zirconia (YSZ) coatings; Lanthanum Zirconate (LZ) coatings and Inconel 738 base material (BM) sliding against a sintered tungsten carbide surface were studied and compared under unlubricated conditions. Sliding experiments were performed in a pin-on-disc configuration. Further, relationships among porosity, normal load and disc speeds with respect to wear loss were derived by using the response surface methodology, and the wear mechanisms were discussed adequately. It was found that the wear resistance of the ceramic coatings gets deteriorated with the increase in the percentage volume of porosity.     

Plasmagestützte Flascheninnenbeschichtung

Application of substrate biasing for plasma enhanced inner coating of plastic bottles to improve barrier properties The article summarizes the results of a research project aiming at the improvement of the barrier performance of plastic beverage bottles by means of plasma enhanced barrier coatings. A pilot plant for the biased barrier coating of PET‐bottles was designed and successfully commissioned within the scope of the investigations. In the process, two excitation types are examined: the so‐called in‐plasma process and a single‐sided plasmaline process for the deposition of barrier coatings. Advantages as well as draw backs of both processes are investigated with respect to an industrial implementation. As a result, improvement of the barrier performance by adjustment of the substrate bias is estimated by a factor of 2‐3. At the same time, substantial challenges for the realization of the processes as well as possible approaches for their solution are identified with the support of gas flow simulations.     

Nitridische und oxinitridische HPPMS‐Beschichtungen für den Einsatz in der Kunststoffverarbeitung (Teil 2)

Nitride and oxy‐nitride HPPMS coatings for the application in the plastics processing (Part 2) In the previous issue three oxy‐nitridic hard coatings on CrAl‐basis were investigated. These coatings were deposited by physical vapour deposition (PVD) as protective coatings against adhesive and abrasive wear in polymer extrusion. The coatings were developed using a variation of the oxygen content to investigate the influence of the chemical composition on the coating properties as well as composite properties between the coating and the coated tool. Following up on these findings this article will focus on the application oriented system properties of the three investigated coating systems towards the polycarbonate melt.     

Korrosionsuntersuchungen an mittels PVD mit Zn und Zn/Mn beschichteten Stählen

Corrosion Studies of Steels Coated by means of PVD with Zn and Zn/Mn Alternative methods for hot dip‐ or electrogalvanic deposition of zinc coatings on steel are gas phase depositions (PVD). They posess high flexibility with respect to alloy composition, and are environmentally harmless. However, a PVD‐coated steel must have at least the same corrosion resistance than steels with “classical” surface finishing. Therefore, the corrosion behaviour of Zn‐coatings and Zn/Mn/system‐coatings deposited by electron beam evaporation without and with ion beam assistance (IBAD) on low alloy steel, was determined by means of salt spray test and electrochemical potential/time measurements. At first the influence of chemical and irradiation pre‐treatment and ion bombardment during deposition on the corrosion resistance of the coatings was investigated. Than the effect of the Zn‐layer thickness was determined in comparison with an 8μm thick electrogalvanized reference coating. Finally Zn/Mn‐alloys, Zn/Mn‐multilayers and Zn‐coatings with Mn‐ or Zn/Mn‐surface layers (top layers) were investigated. By means of optimised pre‐treatment and ion bombardment conditions one obtains, considering the layer thickness, PVD‐Zn coatings with corrosion resistance comparable with the reference layer. The best Mn‐containing coatings are Zn‐coatings with Mn‐toplayer. They surpass the corrosion resistance of the reference layer considerably. Additionally it could be shown that in tendency the potential/time measurements agree very well with the results of the salt spray test.     

热障涂层热冲击试验研究

随着先进发动机的研制和生产,对高温涂层的要求也不断提高。热障涂层（TBCs) 作为一种新型隔热涂层,在发动机涡轮叶片上的应用得到更多的重视。对利用电子束物理气相沉积（EB－PVD）制备的TBCs进行了高温热冲击试验,并对试验前后的TBCs进行了形貌、SEM和XRD分析。     

TUBA 800 — die Realisierung eines Inline-Coaters für Thermo-Barriere-Beschichtungen mittels EB-PVD

TUBA 800 — an In-line Plant for Thermal Barrier Coating by EB-PVD TUBA 800 represents a highly productive multichamber plant for thermal barrier coating (TBC) of gas turbine blades, furnished with VON ARDENNE high power EB guns for PVD. Basis of the plant design is a new type of carrier (trolley) which is handling the blades during travelling from air to air through the plant. Each trolley is equipped with a docking unit for connecting it to cooling and electric supplies and for transmission of measured data to external PC. The realized concept is compared with well established sting and sting-trolley plants.     

PVD – Eine Erfolgsgeschichte mit Zukunft

PVD – A success story with a future PVD coatings in the range of a few nanometers up to some microns have become state of the art in engineering technology. PVD coatings can be found anywhere in our everday lives. They are used in data storage mediums such as CDs or DVDs. Car or architectural glasses are improved by thermal insulation coatings. A diffusion barrier is achieved via PVD coatings at food packaging. For decorative aspects sham jewelery and accessoires are coated as well as fittings. In the last three decades PVD coatings have been established in a variety of technical applications acquiring wear protection and/or friction reduction. First, coatings for tools have been developed, later on for components as well. So, in the past lots of experiences have been made not only in coating development, but likewise in methodical product design. By contrast, the surface has not yet been regarded as construction element. Here the knowledge is just at the beginning. The achieved performance of coated components can be improved drastically if the tribological system consisting of coating, substrate and intermediate material is designed for one single application with regard to the macro‐ and micro geometry. An exemplary application derived from the collaborative research center (SFB 442) “Environmentally friendly tribosystems” at the RWTH Aachen university is discussed. Results of fundamental research and their way into industrial applications are presented. The research development is reflected with regard to the development of the industrial PVD market. Regarding a process chain for the exemplary application the development method of surface technology is explained beginning with the production up to field testing of a new product.     

Damage mechanisms of coated systems under thermomechanical fatigue

Abstract

The damage mechanisms of several kinds of coatings on a single crystal nickel base superalloy under thermomechanical fatigue (TMF) are described. The systems investigated were diffusion platinum aluminide coatings, Co–Ni–Cr–Al–Y overlay coatings, and thermal barrier coatings (TBCs). The TMF experiments were carried out on hollow specimens over a temperature range from 300 to 1050°C, at strain ranges Δ? = 0·5 and 0·7%, and at a strain ratio R = -∞. No coating cracking was found for the platinum aluminide coating. Instead, specimens failed owing to oxidation induced crack initiation from the uncoated inner surface of the hollow testpieces, although coating surface roughening caused by non-homogeneous oxidation was observed. For the overlay coating, roughening in terms of coating rumpling and coating cracking occurred, resulting in reduced TMF life. For TBC specimens with a thin ceramic coating processed by electron beam–physical vapour deposition (EB–PVD), TMF life was comparable with that of specimens with the overlay coating. Failure once again occurred owing to Co–Ni–Cr–Al–Y bond coat cracking and propagation into the substrate. In this system, some bond coat cracks penetrated through the top ceramic coat although others did not. In contrast with specimens coated with the overlay alone, no significant rumpling on the bond coat surface was observed and the crack density was low.     

SiC纤维/YSZ复合热障厚涂层韧性及热震性能研究

采用大气等离子喷涂(APS)技术, 以ZrO₂-8wt%Y₂O₃(8YSZ)和团聚的P7216(8YSZ和珍珠岩粉)粉末为原料, 在基体上制备了厚度大于4 mm的SiC纤维/YSZ(SFY)复合厚热障涂层, 通过扫描电子显微镜(SEM)分析了涂层的显微结构, 发现SFY涂层具有钢筋混凝土结构, 这种结构能够防止因为涂层厚度增加而引起的失效。此外, 基于计算机的断层成像技术分析热障涂层孔隙率的变化, 考察了SFY涂层和YSZ 热障涂层的抗热震性能、断裂韧性以及热导率性能, 并探讨了纤维的增韧机制。研究结果表明, SFY涂层具有更高的断裂韧性值和更好的抗热震性能, 25℃时SFY涂层的热导率为0.632 W/(m·K), 大约是传统YSZ热障涂层热导率的一半。SiC纤维对涂层内部裂纹的偏转和截止作用, 防止了裂纹扩散长大, 形成网状微裂纹结构, 有效提高了涂层的抗热震性能和断裂韧性。     

Silver diffusion and high-temperature lubrication mechanisms of YSZ–Ag–Mo based nanocomposite coatings

Yttria-stabilized zirconia (YSZ) nanocomposite coatings consisting of silver and molybdenum were produced by a hybrid process of filtered vacuum arc, magnetron sputtering and pulsed laser depositions for tribological investigations at different temperatures. The coatings with 24 at.% Ag and 10 at.% Mo contents showed a friction coefficient of 0.4 or less for all temperatures from 25 to 700 °C. The wear scar surfaces and coating cross-sections were studied using scanning electron, transmission electron, scanning transmission electron and focused ion beam microscopes, which also provided the information on chemical composition distributions of silver and molybdenum along with microstructure features. It was demonstrated that silver diffusion and coalescence on surfaces played an important part in the high-temperature lubrication mechanism of the YSZ–Ag–Mo coatings. Silver was found to be an effective lubricant at temperatures below 500 °C and its coalescence on the surface isolated molybdenum inside coatings from ambient oxygen. Lubricious oxides of molybdenum were formed and lubricated at temperatures above 500 °C when the silver was worn off the contact surface. For silver containment inside the coating at high temperatures, a multilayer architecture was built by inserting a TiN diffusion barrier layer in the composite coatings. Microscopic observations showed that this barrier layer prevented silver exit to the coating surface. At the same time, this enabled a subsequent lateral lubricant supply toward a wear scar location where the diffusion barrier layer was worn through and/or for a next thermal cycle. The multilayer coating maintained a friction coefficient of 0.4 or less for more than 25,000 cycles, while the monolithic coating lasted less than 5000 cycles. In addition, a TiN surface barrier layer with pinholes was deposited on the YSZ–Ag–Mo composite surface to control vertical silver diffusion. With this coating design, the coating wear lifetime was significantly increased beyond 50,000 cycles.     

Composite ceramics thermal barrier coatings of yttria stabilized zirconia for aero-engines

Composite ceramics thermal barrier coatings(TBCs) are widely used in the aero-engines field due to their excellent thermal insulation, which improves the service life and durability of the inherent hot components. The most typical, successful and widely used TBCs material is yttria stabilized zirconia(YSZ). In this paper, fabrication methods, coating structures, materials, failure mechanism and major challenges of YSZ TBCs are introduced and reviewed. The research tendency is put forward as well. This review provides a good understanding of the YSZ TBCs and inspires researchers to discover versatile ideas to improve the TBCs systems.