Vacuum plasma spray deposition of YSZ–NiO–Ni coatings at different Ar and H2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ–NiO–Ni) ceramic composite coatings deposited on Al₂O₃ ceramic and stainless steel substrates produced at different Ar and H₂ gas flow rates. The Ar and H₂ gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H₂ gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ–NiO–Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.     

Fabrication of TBC-armored rocket combustion chambers by EB-PVD methods and TLP assembling

A thermal barrier coating (TBC) system for rocket chambers made of Cu-based high strength alloys has been developed in a pilot project in line with EB-PVD (electron-beam physical vapor deposition) technology aiming at TBC application on Cu-based walls of real rocket combustion chambers. The TBC system consists of a metallic bond coating compatible with Cu-based material and an yttria partially stabilized zirconia TBC. The TBC overlayer is a distinctive ceramic structure designed for an exceptionally low Young’s modulus to withstand the extreme mismatch stresses between the internally LN-cooled high thermal expansion Cu metal base and the low thermal expansion hot ceramic shell. The TBC system has been qualified under close-to-service conditions on cylindrical LH₂-cooled combustion chamber segments, where they have performed superior.

As EB-PVD technology is a line-of-sight process that is rather able to coat internal cavities, a transient liquid phase (TLP) joining technique for fully coated parts has been developed, that allows to assemble complete components out of vapor-accessible fully coated parts. It is capable, e.g. to incorporate sinuous cooling passages in the throat areas of combustion chambers, and/or to assemble oversized parts out of smaller components by maintaining parent metal properties. A manufacturing process is outlined for making internal TBC armored combustion chambers.     

Review Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB-PVD)

The objective of this paper is to demonstrate the versatility of electron beam-physical vapor deposition (EB-PVD) technology in engineering new materials with controlled microstructure and microchemistry in the form of coatings. EB-PVD technology is being explored in forming net-shaped components for many applications including space, turbine, optical, biomedical and auto industry. Coatings are often applied on components to extend their performance and life under severe environmental conditions including thermal, corrosion, wear, and oxidation. In addition, coatings have been used in designing and developing sensors. Performance and properties of the coatings depend upon its composition, microstructure and deposition condition. This paper presents recent results of various materials including ceramic, metallic, and functionally graded coatings produced by EB-PVD. Simultaneous co-evaporation of multiple ingots of different compositions in the high energy EB-PVD chamber has brought considerable interest in the architecture of functional graded coatings, nano-laminated coatings and designing of new structural materials that could not be produced economically by conventional methods. In addition, high evaporation and condensate rate allowed fabricating precision net-shaped components with nanograined microstructure for various applications. This paper will also present the results of various metallic and ceramic coatings including chromium, titanium carbide (TiC), hafnium carbide (HfC), tantalum carbide (TaC), hafnium nitride (HfN), titanium-boron-carbonitride (TiBCN), and partially yttria stabilized zirconia (YSZ), and HfO₂-based TBC coatings deposited by EB-PVD for various applications.     

Microstructure behaviour and influence on thermally grown oxide formation of double‐ceramic‐layer EB‐PVD thermal barrier coatings annealed at 1,300 °C under ambient isothermal conditions

To resist high thermal loads in turbines effectively, turbine blades are protected by thermal barrier coatings in combination with additional air cooling. State‐of‐the‐art yttria stabilised zirconia top coats do not operate at temperatures higher than 1,200 °C. Promising candidates for alternative top coats are pyrochlores, lanthanum zirconate and gadolinium zirconate. But lifetime of pyrochlores is short because of spallation. However, combinations of yttria stabilised zirconia and lanthanum zirconate or gadolinium zirconate as multilayer systems are promising top layers operating at higher temperatures than yttria stabilised zirconia. Such thermal barrier coatings top coats as double‐ceramic‐layer systems consisting of 7 wt.% yttria stabilised zirconia and lanthanum zirconate or gadolinium zirconate were deposited by Electron Beam‐Physical Vapour Deposition. The focus of the work was set on the influence of the coating design and the microstructure variation generated at different rotating speeds on the adhesion and thermally grown oxide behaviour after isothermal oxidation at 1,300 °C. Phase formation of the thermal barrier coatings top coats was obtained using X‐ray diffraction. After isothermal oxidation tests for 50 h at 1,300 °C, both, microstructure change and the formation of the thermally grown oxide were investigated. While the pyrochlore single‐ceramic‐layer are completely spalled off, microstructure of the double‐ceramic‐layer reveals only crack initiation. The thermally grown oxide thickness was determined by means of scanning electron microscopy. A high aluminum and oxygen content in the thermally grown oxide is found using X‐ray spectroscopy. Existence of α‐phase in Al₂O₃ was proved by X‐ray diffraction. After isothermal testing, no phase transformation can be detected regarding the double‐ceramic‐layer coatings.     

Vacuum plasma spray deposition of YSZ-NiO-Ni coatings at different Ar and H2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ-NiO-Ni) ceramic composite coatings deposited on Al₂O₃ ceramic and stainless steel substrates produced at different Ar and H₂ gas flow rates. The Ar and H₂ gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H₂ gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ-NiO-Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.     

Adhesive strength of new thermal barrier coatings of rare earth zirconates

La₂Zr₂O₇ (LZ) and La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) as novel candidate materials for thermal barrier coatings (TBCs) were prepared by electron beam-physical vapor deposition (EB-PVD). The adhesive strength of the as-deposited LZ and LZ7C3 coatings were evaluated by transverse scratch test. Meanwhile, the factors affecting the critical load value were also investigated. The critical load value of LZ7C3 coating is larger than that of LZ coating, whereas both values of these two coatings are lower than that of the traditional coating material, i.e. 8 wt% yttria stabilized zirconia (8YSZ). The micro-cracks formed in the scratch channel can partially release the stress in the coating and then enhance the adhesive strength of the coating. The width of the scratch channel and the surface spallation after transverse scratch test are effective factors to evaluate the adhesive strength of LZ and LZ7C3 coatings.     

Optical and structural study of EB-PVD ZrO2 thin films

The behaviour of the crystalline properties of ZrO₂ films prepared by electron beam physical vapour deposition (EB-PVD) is investigated as a function of their deposition rates. In this paper, the conditions for the preparation of tetragonal zirconia from yttria stabilized zirconia and from monoclinic powder as starting materials are reported. The variation of the crystallite size as a function of the deposition rate is studied and, additionally, the optical characterization that permits to determine the refractive index is presented. The obtained values are in agreement with the bulk ones showing that EB-PVD prepared samples have good performance for optical and protective coatings.     

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.     

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.     

Thermal barrier coatings produced by laser cladding

A 2kW C0₂ laser has been used to clad a mild steel substrate with two different ceramic coatings, namely yttria partially stabilized zirconia (8 wt% YPSZ) or a mixture of YPSZ and pure alumina powder. A range of laser processing parameters has been investigated. Results have been obtained showing the possibility of using the laser beam for producing a clad layer of thermal barrier coating with different topography depending on the processing conditions.On leave from Scientific Research Council, Baghdad, Iraq.     

Phases in Plasma Sprayed Thermal Barrier Coatings from Yttria Partially Stabilized Zirconia

The application of thermal barrier coatings (TBC) is increasing in aeroengines. Surface temperatures up to 1450°C require the application of ceramic TBCs because the temperature capability of metallic substrate materials is not high enough. The service life of turbine components could be improved by the use of yttria partially stabilized zirconia top coatings. The most successfull TBCs are made from 7–9 wt‐% yttria partially stabilized zirconia. One of the most discussed reasons of damages of such TBCs is the transformation between monoclinic and tetragonal phase in zirconia in connection with a dramatic change in volume. Thus in this work resulting phases of plasma sprayed zirconia coatings were investigated. It was found that no monoclinic phase could be detected after heat treatments at 1300, 1400 and 1466°C with cooling rates > 2°/min. Only with cooling rates < 2°/min monoclinic phases occured. It can be concluded that the metastable tetragonal high temperature configuration of yttria partially stabilized zirconia is “very stable”. The conditions in aeroengines with cooling rates > 2°C prevent the formation of the monoclinic phase in zirconia.     

Coatings on graphite crucibles used in melting uranium

Standard coatings for graphite crucibles used for melting uranium have generally been zirconia based and have been applied as a paint or by flame spraying. These coatings do not provide adequate protection at the temperatures normally required for melting uranium alloys. Yttria provides superior protection above 1300°C but becomes less satisfactory above 1450°C when applied directly on graphite. The utilization of a protective niobium/zirconia bilayer coating between the yttria and the graphite results in improved performance at 1500°C. Yttria has been satisfactorily applied both by plasma spraying and by brush applying a stable suspension. When the protective niobium layer is used, coating adherence after melting is excellent and multiple use of coatings is practical. The coatings adhere to graphite with a high coefficient of thermal expansion (CTE) (≈7 μm m^-1 °C^-1) much better than to standard crucible grade graphite (≈4μm m^-1 °C^-1). A single Nb/Y₂O₃-coated high CTE graphite crucible has been used for seven melts at 1450°C without repair or increased carbon contamination.The yttria paint coating is cost effective when compared with flame- or plasma-sprayed zirconia.     

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.     

Numerical study on modification of ceramic coatings by high-intensity pulsed ion beam

ZrO₂ ceramic coatings, which often call thermal barrier coatings (TBCs), fabricated by electron beam physical vapor deposition (EB-PVD), are widely used in high-temperature environment of aircraft and industry gas-turbine engines, because of the excellent strain tolerance imparted by the columnar structure. However, channels separating the columnar grains in EB-PVD TBCs provide paths for oxygen or other aggressive species from ambient atmosphere into the bond coat, resulting in the premature spallation-failure during high-temperature service. In our previous study, high-intensity pulsed ion beam (HIPIB) technique has been proposed to modify the EB-PVD TBCs, where a melted, densified top layer can be produced as a result of extremely thermal effect induced by the HIPIB irradiation. In this paper, HIPIB melting process is investigated numerically using a physical model based on experimental data, taking into account the surface morphology of HIPIB-melted TBCs to explore the mechanism of interaction between HIPIB and the coatings. Deposition process of the beam energy in TBCs was simulated by Monte Carlo method, and the non-linear equations describing the thermal conducting process were solved numerically based on the deposited energy to obtain the evolution of the temperature field of TBCs. The calculated melting depth of irradiated EB-PVD TBCs is consistent with results obtained in the HIPIB irradiation experiments.     

Solar Process for Surface Glazing of Zirconia-Based Plasma Sprayed Thermal Barrier Coatings

A solar process for thermal barrier coatings surface glazing is presented. Results obtained on an yttria partially-stabilized zirconia plasma sprayed coating are described. The optical behavior of the ceramic during the thermal treatment is also discussed.     

Thermal barrier coatings from sol-gel-derived spray-grade Y2O3-ZrO2 microspheres

For the development of ceramic thermal barrier coatings, spray-grade yttria-stabilized zirconia microspheres were prepared by the sol-gel technique. Oxide microspheres were obtained by calcination of the corresponding gel spheres at 1000 °C. Scanning electron microscopic and optical microscopic observations revealed the material thus obtained to have a predominantly spherical morphology and the requisite size distribution (5–50 m). The dense, calcined microspheres showed good flowability. X-ray diffraction studies indicated the presence of the tetragonal polymorph of ZrO₂ as the major phase, in addition to about 14% monoclinic ZrO₂. The plasma-sprayed YSZ coatings made from the sol-gel-derived microspheres showed a further decrease in the monoclinic ZrO₂ content (6%). The coatings survived 40–50 thermal cycles (30 min at 1200 °C followed by a water quench), indicating good thermal shock resistance.     

Characterization of Thermal Barrier Coatings Using Thermal Methods

Although low thermal conductivity was one of the major reasons for the development of thermal barrier coatings (TBCs) based on yttria stablilized zirconia (YSZ), its mechanical properties are often considered crucial in failure analysis and assessment of coatings. Thermal properties usually are treated as just basic material parameters and have not been utilized widely in TBC characterization and failure analysis. In fact, thermal transport properties such as thermal conductivity and thermal diffusivity are closely related to the microstructural changes of the TBC. In this paper, a brief overview of characterization of TBCs using thermal thermophysical properties and temperature mapping is presented.     

Thermally grown oxide scales on γ-TiAl coated with thermal protection systems

Abstract

Thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on gamma TiAl samples using electron-beam physical vapour deposition. The specimens were coated with intermetallic Ti –Al – Cr layers and CrAlYN/CrN nanoscale multilayer coatings. The lifetime of the TBC systems was determined performing cyclic oxidation tests in air at temperatures between 850 and 950–C. The TBC systems with Ti –Al – Cr and CrAlYN/CrN layers did not fail at 850 and 900–C during the maximum exposure time period of 1000 cycles of 1 h dwell time at high temperature. No spallation of the thermal barrier coatings was observed. As revealed by post-oxidation microstructural analysis, the protective coatings were severely degraded when exposed at 900–C, resulting in growth of mixed oxides on the substrate. Underneath the thermal barrier coating an outer oxide scale with a columnar structure was observed, consisting of rutile and α-Al₂O₃. Energy-dispersive X-ray spectroscopy analysis revealed zirconia and chromia being dissolved in the outer oxide scale. The columnar structure and the presence of zirconia indicated an effect of the TBC on the morphology of the outer oxide scale. The zirconia top coat exhibited an excellent adherence to this oxide scale formed on the protective layers when degraded, and at defects like cracks. When thermally cycled at 950–C, the TBC system on specimens coated with Ti –Al – Cr failed by spallation of the thermally grown mixed oxides, whereas the thermal barrier coating was well adherent to the outer oxide scale at this temperature, too.     

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.     

La2(Zr0.7Ce0.3)2O7—A new oxide ceramic material with high sintering-resistance

Oxide ceramics with high sintering-resistance above 1473 K have very important applications in thermal barrier coatings (TBCs), catalytic combustion and high-temperature structural materials. Lanthanum zirconate (La₂Zr₂O₇, LZ) is an attractive TBC material which has higher sintering-resistance than yttria stabilized zirconia (YSZ), and this property could be further improved by the proper addition of ceria. The composite La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) has the highest sintering-resistance, and it is observed that fine particles of the second phase in the composite concentrate at the grain boundary to prevent the grain growth of the main phase. The formation of hollow fibers is helpful to the further improvement of sintering-resistance of LZ7C3, and this technique may be widely applied to protect the catalyst support against thermal sintering.