Thermal barrier coating (TBC) is a technology for preventing the rise in surface temperature of the metal substrate by coating a ceramic with low thermal conductivity for the hot section components such as blades, nozzles and combustion chambers. In this paper, for some types of thick TBC specimen which had zirconia based ceramic top coat of 600 micrometer in order to improve the thermal insulation performance, thermal cycle property and heat flux are examined by using thermal simulating facility by gas burner heating. The paper also discusses fracture mechanism of the thick TBC. Paper includes a German-language abstract.

This study investigates a new zirconia-based ceramic for potential use in thermal barrier coatings. In the experiments, Sc 2 O 3 -Gd 2 O 3 -Yb 2 O 3 -ZrO 2 (SGYZ) powder was synthesized by coprecipitation and calcination, then agglomerated and sintered to facilitate spraying. The structure, morphology, and phase stability of the powder and plasma-sprayed SGYZ coatings were analyzed and thermal conductivity was measured. Test results show that the powders and coatings have good phase stability even after 500 h at 1400 °C and do not undergo tetragonal-to-monoclinic phase transition upon cooling. Plasma-sprayed SGYZ also has a lower thermal conductivity than YSZ, which is currently used in gas turbine engines.

Tungsten carbide coatings are often applied to improve surface properties such as wear, high temperature degradation, and corrosion resistance. Zirconia coatings have also been used extensively in various industries due to their excellent tribological and insulation properties combined with high stiffness. It is speculated that adding zirconia to tungsten carbide may result in a coating with combination of excellent thermal and mechanical properties of constituents. In the current study, a powder mixture of 50 wt. % WC-Ni and 50 wt. % ZrO 2 -Y 2 O 3 deposited on a low carbon steel substrate using atmospheric plasma spray technique. The microstructural evolution of deposited sample was investigated. Splat boundaries, micro cracks, pore morphology conversion, and grain growth mechanism were elucidated comprehensively. Results indicated a good adhesion between two different major components. No porosity formed due to mismatch between zirconia and tungsten carbide. This study pays special attention to the dependency of the microstructural characteristics to the phase distribution within the coating.

Single-wavelength pyrometers are widely used as a noncontact temperature measurement tool in material processing, petrochemical and laser-machining industries. In addition, these intensity-based IR sensors are used extensively as a diagnostic and health monitoring in the development and research of advanced high-temperature military and commercial gas turbine engines. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. However, they suffer from some problems, in particular the variation of the material emissivity and perturbations introduced by extraneous radiations. Yttria stabilised Zirconia (YSZ) thermal barriers are known to be more emissive and opaque in the 8-14 µm spectral band, therefore we can take advantage using this spectral band in temperature measurement. Spectral emissivities of an YSZ sample were measured using two commercial pyrometers. And a method for adapting commercial wide-band pyrometers (generally used for low temperature measurements) for high temperature measurements of thermal barrier coatings was tested.

This work focuses on the processing and deposit by suspension plasma spraying (SPS) of ZrO 2 -based ceramic materials for Thermal Barrier Coatings (TBC's) applications. The system of interest is ZrO 2 -16mol%Y 2 O 3 -16mol%Ta 2 O 5 (16YTZ). This ceramic has been reported to keep a non-transformable tetragonal phase (t'-phase), suitable to overcome the thermodynamic limits of the mostly used conventional 7-8wt.% yttria stabilized zirconia (YSZ). The research consists into evaluate the t'-phase stability and performance of the 16YTZ SPS coating. Synthesis of 16YTZ and, the evolution of the resulting microstructure in the dense ceramic and in the coating are a central part of the study. Sintering behavior in dense ceramics prepared from both precursor derived and milled powders is evaluated. Microstructural characterization by XRD, SEM and RAMAN spectroscopy of the as-deposited ceramic coating is presented and discussed.

Abradable seals have been used in jet engines since the late 1960's. Today they are seeing applications in low pressure and high pressure sections of compressors as well as the high pressure turbine module of jet engines. Clearance control systems using abradable coatings are also gaining ever more attention in industrial and steam turbine applications. Thermal spraying is a relatively simple and cost effective means to apply abradable seals. Abradable coatings work by minimizing gaps between rotating and stationary components by allowing the rotating parts to cut into the stationary ones. Typically plasma and combustion spray processes are used for applying abradable coatings. The types of coatings employed in the HP turbine are zirconia based abradable material systems with polymer and, in some cases, solid lubricant additions such as hexagonal boron nitride. The coatings are designed to work at service temperatures of up to 1200°C. Types of matrix materials used in the low and high pressure sections of the compressor are aluminum-silicon, nickel and MCrAlY based systems. These compressor type systems typically also contain fugitive phases of polymer and/or solid lubricants such as hexagonal boron nitride or graphite. Operating temperature, depending on the material of choice, can be up to 750°C. Regardless of the specific application, fugitive phases and porosity are needed for abradable coatings. Polymers are used to create and control porosity in plasma sprayed coatings, a critical design requirement in adjusting abradability and erosion properties of thermal spray coatings. Combustion spray coatings generate porosity through the lower deposition velocities and temperatures compared to plasma and typically do not need polymer phases. Solid lubricants are added to help weaken the structure of thermal spray coatings and reduce frictional heating and material transfer to the blade.

The purpose of this study was to determine the creep/sintering characteristics of thermally sprayed zirconia coatings and attempt to understand the influence of microstructure on the creep resistance of deposits. The major modification, compared with more typical practice, was employment of powder feedstock with agglomerated sub-micron size particles (Nanox), which is compared to one of the best commercially available powders (HOSP). Thick plasma sprayed coatings were prepared and their physical and mechanical properties were characterized. Creep/sintering experiments were then conducted to investigate the response of the materials when exposed to high temperatures under load. The results showed that it could be possible to correlate the splat thickness to the creep behaviour of the coatings.

Retaining non-melted nano-particles of zirconia in nanostructured coatings has been a challenge in the past. Recently an air plasma spray process was developed to produce coatings which retain up to 30-35% by volume non-melted particles, resulting in a unique structure. The creep/sintering behavior of such thermal barrier coatings deposited from nanostructured feedstock has been measured and compared with deposits produced from hollow sphere powder (HOSPTM). Both feedstocks contain 6-8wt% Y 2 O 3 as stabilizer. Flexure and compression creep testing were conducted under several different loads and temperatures to obtain creep exponents and parameters.

Nanostructured yttria stabilized zirconia abradable coatings were produced via air plasma spray (APS) and compared to conventional abradable coatings based on the composite CoNiCrAlY+BN+polyester also sprayed via APS. The microstructures of the coatings were analyzed using SEM and the hardness determined via Rockwell Y measurements. It was possible to engineer nanostructured abradables with a high degree of plasticity by controlling the amount, morphology and distribution of the nanostructured phase embedded in the coating microstructure. Room temperature rub-rig tests were performed for both types of coatings under different blade tip speeds and seal incursion rates simulating operating conditions of gas turbines. The nanostructured abradables exhibited good performance indicating that abradable coatings engineered in this fashion have potential for industrial application at elevated temperatures.

Thermal barrier coatings (TBCs) are widely used to insulate the combustion chamber of internal combustion engines to improve their performance efficiency, reduce pollution, and protect the metals from high temperature oxidation. In this work, a TBC coating composition of 80% zirconium oxide and 20% gadolinium oxide (GdPSZ) was prepared in the laboratory and plasma spray coated on the combustion chamber of single-cylinder diesel engines. An engine performance test was conducted for both the baseline (uncoated) engines and the coated engines. The coatings in the combustion chamber of the engines were found to be well adhered after 300 hours of rigorous testing. A significant reduction in smoke density was observed, especially at higher loads, for the coated engines. However, the coated engines exhibited a 2-6% reduction in volumetric efficiency and an increase in brake-specific fuel consumption compared to the uncoated engines. The results for other performance parameters are also discussed.

The life and thermal properties of plasma sprayed TBC – widely used in gas turbine engines – are closely related to the microstructure of the ceramic top coating. Especially, the thermal conductivity of this ceramic coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material. This study has been performed to optimize the ceramic coating microstructure. First, a hollow zirconia powder was elaborated in the laboratory and compared with a commercial hollow powder. Then, the Taguchi method was performed on thermal spray operating factors to optimize their beneficial effects on the Thermal Barrier Coating (TBC). A new hollow powder with small grains was elaborated using spray drying process. That spray dried powder based on yttria partially stabilized zirconia was optimized using drying simulation tests. It was shown that the formulation and the state of dispersion of the slurry allow to control the powder morphology (from dense to hollow shape). Moreover, for hollow powder, it is possible to vary the thickness of the shell. According to these results, a hollow powder (size ranging from 36 to 130 µm and an 11 µm shell thickness) was elaborated as an original feedstock to produce plasma sprayed TBC. The agglomerated powder was consolidated in an oven and then was compared to a commercial hollow yttria zirconia powder during thermal spray tests. Coating experiments were based on a limited number of operating factors, which have an influence on the deposit microstructure. The seven selected spraying factors concerned the plasma (primary and secondary gas flow rates, arc current), the cooling and the powder deposition (spraying distance, spraying angle, traverse speed). Experiments based on these factors were carried out to elaborate a Taguchi fractional-factorial L16 design. The resulting as sprayed coatings characteristics were quantified with respect to deposition efficiency, roughness (Ra) and porosity (image analysis technique). Through statistical calculation, the parameters that have relevant influence on the coating properties were identified, and their relative importance and some of their interactions were studied. The final aim is to produce an effective thermal barrier coating with a reduced thermal conductivity.

Welding of dissimilar materials in particular ceramics to metals is a technical challenge with attractive economical consequences. In this study a ceramic coating was processed by plasma transferred arc (PTA). ZrO 2 –7wt.%Y 2 O 3 powders were deposited on low carbon steel plates and on Ni based alloys layers previously welded on a steel plate. Coatings were evaluated regarding the soundness and features of the metal/ceramic bond. Results showed that the pair ZrO 2 –7wt.%Y 2 O 3 /metallic alloy played a major role on the quality of the processed surfaces determining the effectiveness of the bonding. The presence of Al in the Ni based intermediate layer was detrimental to the adhesion of the ceramic coating. Deposition of ZrO 2 –7wt.%Y 2 O 3 on NiCrFe intermediate layers allowed for a metal/ceramic bond resulting on 3,0mm thickness coatings. Ceramic deposits exhibited cracks, whose features were altered after a stress relief treatment of the substrate (AISI 1020+NiCrFe layer) prior to the deposition of ZrO 2 –7wt.%Y 2 O 3 . Transverse section analysis revealed the presence of second phase particles in the ceramic coating and the diffusion of elements from the intermediate Ni based layer into the ceramic deposit.

Die-casting moulds are subjected to severe conditions of cyclical thermal and mechanical loads, as well as chemical and mechanical wear. Dies mostly fail due to a combination of wear mechanisms. Heat checking, erosion, melt corrosion and soldering often lead to complete die failure. Ceramic plasma-sprayed coatings have a great potential for die protection, since they are chemically inert, and show high form stability at elevated temperatures. Furthermore, they show good resistance against thermal shock due to the porous state of the coated layer and low Young’s modulus. In the present study, the influence of hot isostatic pressing on thermal fatigue resistance of plasma spraying coatings is investigated. Thermal and thermomechanical tests were applied to characterise their ability to protect dies. The coatings were evaluated by residual stress measurements, metallography, hardness test and X-ray diffraction.

The dense refractory is applied for the high-temperature glass melting equipment. Furthermore, platinum or its alloy-clad refractory bricks are utilized to melt high quality glass in fields of optical and display glasses. From the viewpoint of the resource saving of rare metal, the decreasing of Pt-consumption is very serious problem for glass manufacturing. The platinum thermal spray coating is effective alternative technology to solve this problem for platinum cladding. A Pt-spray coated ceramics is difficult to get reliability due to the large difference of thermal expansion between the ceramic and the metal. In this study, we have investigated the bond strength of the platinum coat on the dense refractories which was textured by the mechanical process. As a result, the bond strength between the ceramic and the metal is 3-5 MPa. It is almost the same as the thermal spray ceramics coating on the metal. Furthermore, in case of the ceramics substrate that contains the glass phase, the bond strength between the ceramic and the metal increased to 14-17 MPa after 1773 K heat treatment.

Plasma-sprayed thermal barrier coating (TBC) systems are widely used in gas turbines blades in order to increase turbine entry temperature (TET) with better efficiency. Yttria stabilized zirconia (YSZ) has been usually chosen as the top thermal barrier coating material because of its low thermal conductivity, high thermal expansion coefficient and good corrosion resistance. However as a new candidate commercial TBC material, ceria stabilized zirconia (CSZ) currently looks to be promising. Ceria and ceria based ceramics show an outstanding potential for use at temperatures exceeding 1200 °C. CSZ coatings do not only have high temperature stability, good corrosion resistance and high fracture toughness but also lower thermal conductivity and higher thermal expansion coefficient than YSZ coatings. The sintering and phase transformation characteristics of both ceramic thermal barrier coatings under high temperature conditions are complex phenomena. In this paper, microstructural differences, sintering behaviours (1200 oC, 10h, 25h and 50h) and phase transformations of the plasma sprayed ceria stabilized zirconia (CSZ: ZrO 2 –2.5 wt.%Y 2 O 3 – 25 wt.%CeO 2 ) and conventional yttria stabilized zirconia (YSZ; ZrO 2 –8 wt.%Y 2 O 3 ) coatings and their powder materials have been investigated and compared using thermal analysis techniques, XRD and scanning electron microscope.

Thermal conductivity plays a critical role in the thermal transport of thermal sprayed coatings. In this paper, a combined image analysis and finite element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy (SEM) images of the coating microstructure. Images are analyzed with a collection of image processing algorithms to reveal the microscopic coating morphology. The processed digital image is used to generate a two-dimensional finite element meshing in which pores, cracks and the bulk coating material are identified. The effective thermal conductivity is then simulated using a commercial finite element code. Results are presented for three coating material systems: yttria stabilized zirconia (YSZ), molybdenum and NiAl, and results are found to be in good agreement with experimental values, obtained using the laser flash method. YSZ coatings are also annealed and the analysis procedure repeated to determine if the technique could accurately assess changes in coating morphology.

Ceramic layers, such as yttria-stabilized zirconia or scandia-stabilized zirconia, used for functional layers of solid oxide fuel cells, i.e. the gas tight oxygen ion conductive electrolyte or as ceramic component in the porous cermet anode, were obtained by the Solution Precursor Plasma Spray (SPPS) process. The influence of different solvent types on microstructure was analyzed by comparison of coatings sprayed with water-based solution to ethanol-based one. Use of solvent with low surface tension and low boiling point enhances splat formation, coating microstructure and crystalline structure. Parameter adjustment to receive coatings from nitrate solutions with ethanol as solvent was carried out. Results of Raman spectroscopy indicate that an intermediate of both nitrates (zirconyl and scandium nitrate hydrate) was deposited.

Thermal barrier coatings have got considerable importance for the improvement of gas turbine efficiency. These materials are applied on the surface of gas turbine blades and vanes and are based on a layer of low-oxidation material (mainly MCrAlY alloys, where M stay of Co, Ni or a combination of both) and a ceramic top layer that acts as proper thermal barrier (normally Yttria Partially Stabilized Zirconia). Coating removal is an important aspect in the production of these blades and vanes. “Decoating” or “stripping” is needed during the production of new components as well as for the reconditioning of existing ones. The present paper is dedicated to a new removal method of the ceramic Zirconia layer, based on dry ice blasting. This method will not impact on the roughness and morphology of the bond coat surface, making it suitable for re-coating with TBC, without any further operation before TBC recoating. This possibility has an important impact on the stripping costs and time, avoiding all the operations related to the bond coat. The paper presents the process tests to get the process set up and the characterization of the surfaces comparing the stripped ones with the “original ones” coated by LPPS on new components, ready to be TBC coated. Optical and SEM microscopy, 3D profilometry have been used for characterization. Finally a Thermal Cycling Fatigue test has been carried out in order to validate the procedure of stripping and re-coating.

Abradable seals have been used in jet engines since the late 1960's. Today abradable seals are seeing applications not only in low pressure and high pressure sections of the compressor but also in the high pressure turbine module of jet engines and are gaining ever more attention in industrial gas turbines. Thermal spraying is a relative simple and cost effective means to apply abradable seals. These work by minimizing gaps between rotating and stationary components by allowing the rotating parts to cut into the stationary ones. The types of coatings employed are zirconia based abradable material systems with polymer and, in some cases, hexagonal boron nitride additions. The coatings are designed to work at service temperatures of up to 1200 °C. The objective of this paper is to review various types of commercial and experimental ceramic abradable systems and to assess their performance profiles. The paper will review yttria stabilized zirconia based systems with modified polymer additions and with variable particle sizes of the ceramic phase. Alternative stabilizers and their influence on key coating properties such as thermal shock resistance and abradability will be studied. The paper will also review the influence of plasma spray parameters on coating properties and study the general influence of coating porosity on coating erosion properties and abradability.

A transpiration cooling system for gas turbine applications has significant benefit for reducing the amount of cooling air and increasing cooling efficiency. In this paper, the porous ceramic coating using a plasma-spraying process, which can infiltrate a cooling gas and control the loss of coolant gas pressure, and the properties of the porous coating material fabricated in this study, such as permeability of the cooling gas, thermal conductivity and adhesion strength, are shown. The mixture of 8wt% yttria stabilized zirconia and polyester powders was employed as the coating, in order to deposit the porous ceramic coating onto the Ni-based super alloy substrate. The ceramic coating deposited with such mixed powder showed superior permeability for cooling gas and thereby to lead to considerable reduction of the substrate temperature based on the transpiration cooling mechanism. The adhesion strength of the porous ceramic coating was almost the same level as that of the ceramic coating utilized in current gas turbine components. The high-temperature exposure test revealed that the permeability of cooling gas did not change though a rigorous experience for a long period of up to 1000hours. It was also confirmed that the coating peeling damage did not occur after the thermal cycling test. It is concluded consequently that the transpiration cooling system proposed here for gas turbine could be achieved using the porous ceramic coating.