Ceramic Thermal Barrier Coatings Research Articles

Abrasive Water Jet Drilling of Ceramic Thermal Barrier Coatings

Ceramic thermal barrier coatings (TBCs) are used to provide thermal insulation for aero-engine components from high temperature gas streams. It is difficult to drill small holes in TBC-coated components using electro-discharge machining (EDM) due to non-conductivity of the TBCs. This paper proposed a novel combined process to drill small holes in TBC-coated components through removing coating layers by abrasive water jet machining (AWJM) followed by drilling metallic substrate using EDM process. The present study investigated the first stage of the proposed process, i.e. AWJM drilling of TBCs with relative low-pressure. The influences of standoff distance, abrasives concentration, jet pressure and drilling time on machining results were evaluated. The relative low pressure (2-3MPa) jet results in a W shape at bottom of the machined hole in case of 90 degrees impact angle. The exposed substrate diameter of the machined hole is smaller than TBC top diameter of that. The results show that the AWJM with relative low-pressure is an efficient and economical approach to remove TBC coatings and expose metal substrate for subsequent EDM drilling.

Fatigue Crack Propagation Simulation in Ceramic Thermal Barrier Coatings subjected to Thermo-Mechanical Loading

Fatigue Crack Propagation Simulation in Ceramic Thermal Barrier Coatings subjected to Thermo-Mechanical Loading

Molten Particulate Impact on Tailored Thermal Barrier Coatings for Gas Turbine Engine

Commercial/military fixed-wing aircraft and rotorcraft engines often have to operate in significantly degraded environments consisting of sand, dust, ash, and other particulates. Marine gas turbine engines are subjected to salt spray, while the coal-burning industrial power generation turbines are subjected to fly ash. The presence of solid particles in the working fluid medium has an adverse effect on the durability of these engines as well as performance. Typical turbine blade damages include blade coating wear, sand glazing, calcia–magnesia–alumina–silicate (CMAS) attack, oxidation, and plugged cooling holes, all of which can cause rapid performance deterioration including loss of aircraft. This research represents the complex thermochemomechanical fluid structure interaction problem of semimolten particulate impingement and infiltration onto ceramic thermal barrier coatings (TBCs) into its canonical forms. The objective of this research work is to understand the underpinning interface science of interspersed graded ceramic/metal and ceramic/ceramic composites at the grain structure level for robust coatings and bulk material components for vehicle propulsion systems. This research enhances our understanding of the fundamental relationship between interface properties and the thermomechanical behavior in dissimilar materials for materials by design systems, and creates the ability to develop and fabricate materials with targeted macroscale properties as a function of their interfacial behavior. This project creates a framework to enable the engineered design of solid–solid and liquid–solid interfaces in dissimilar functionalized materials to establish a paradigm shift toward science from the traditional empiricism in engineering TBCs and high temperature highly loaded bulk materials. An integrated approach of modeling and simulation, characterization, fabrication, and validation to solve the fundamental questions of interface mechanisms which affect the properties of novel materials will be validated to guide component material solutions to visionary 2040+ military vehicle propulsion systems.

Engineered thermal barrier coatings deposited by suspension plasma spray

Yttria stabilized zirconia (YSZ) is susceptible to CMAS (Calcium Magnesium Alumino Silicates) attack at high temperatures (>1200°C) which limits its durability. New ceramic materials which can overcome these high temperature challenges are highly desirable. This work investigates the feasibility of depositing two variations of three ceramic layered thermal barrier coatings. The first variation comprised of yttria as the top ceramic layer with gadolinium zirconate (GZ) as the intermediate layer and YSZ as the base layer. The second variation comprised of Yttrium Aluminum Garnet (YAG) as the top layer with gadolinium zirconate as the intermediate layer and YSZ as the base layer. Microstructural analysis of the as sprayed three layered TBCs were performed by SEM/EDS. Columnar microstructures with a relatively dense top layer were obtained in both the variations. The porosity content of the TBCs was measured by water intrusion and image analysis methods. Phase composition of each layer of the as sprayed TBCs was analyzed using XRD. YAG showed an amorphous phase whereas GZ showed a cubic defect fluorite phase and tetragonal phase was observed in YSZ. In the case of yttria, monoclinic and cubic phases were observed.

Study on thermal resistance performance of 8YSZ thermal barrier coatings

The application of ceramic thermal barrier coatings (TBCs) can significantly increase gas turbine performance. The microstructure of TBCs fabricated by APS and EB-PVD is remarkably different. The thermal and mechanical properties of TBCs with the same material are determined by its microstructures. In order to establish structure-property relationships of TBCs with complex inhomogeneous geometry, in this work, various 3D-microstructures of TBCs were reconstructed using the geometric reconstruction software developed by our team. A three-dimensional model based on Lattice Bolzmann method (LBM) was developed to investigate the heat transfer behaviors in TBCs. The effect of microstructure parameters such as porosity, pore size and pore shape on the effective thermal conductivity of TBCs was studied in detail. The relationship between microstructures and effective thermal conductivity of TBCs was obtained. The calculated values of the effective thermal conductivity of 3D-TBCs agree well with the experimental values, and all relative errors are less than 4%. It can be found that the layered coatings should have larger size pores, and the columnar coatings should have smaller size pores. This can make the internal temperature distribution of TBCs more uniform. The effect of pores size on the thermal conductivity of two types of TBCs is different, but their thermal conductivity increase with the increase of porosity. The effective thermal conductivity of the columnar TBCs with the same porosity decreases with the decrease of pores size, but that of the layered TBCs increases with the decrease of pores size. The pores shape has a great influence on the effective thermal conductivity of the coating, especially for high porosity columnar coating. The effective thermal conductivity of the columnar coatings logarithmically increases with the length of the elongated pores, and it is increased by 21.6% when the porosity is 20%. However, the effective thermal conductivity of the layered structure coatings logarithmically decreases with the length of the elongated pores. The results will provide us a theoretical guide to design TBCs with a high thermal insulation property.

Role of thermally grown oxide and oxidation resistance of samarium strontium aluminate based air plasma sprayed ceramic thermal barrier coatings

Samarium Strontium Aluminate (SSA) based ceramic oxide was synthesized and coated on a bond coated (NiCrAlY) Inconel 718 superalloy by air plasma spray (APS) process. Thermally grown oxide (TGO) thicknesses were controlled at different pre-oxidation times of 10, 20 and 30h at 1050°C in the argon atmosphere, the highest TGO thickness being found for 30h pre-oxidized samples. The percentage elastic recovery of pre-oxidized samples increased (45, 48, and 50%) with an increasing pre-oxidation time at 1050°C. The isothermal oxidation test was performed on pre-oxidized samples at 1100°C for 15h in air. The parabolic oxidation rate constant decreased from 6.08×10−5 to 3.90×10−5mg2cm−4s−1 (from 10 to 20h) and then increased up to 4.55×10−5mg2cm−4s−1 for 30h pre-oxidized SSA samples at 1100°C. SSA based TBCs showed 65% lower weight gain as compared to conventional YSZ TBCs after oxidation at 1100°C in air. Cross-sectional SEM results revealed that the threshold TGO thickness of SSA TBCs is about 5.3 to 5.8μm. Failure of SSA TBCs was observed at the interface of TGO and ceramic top coat due to the formation of SmAlO3 along with spinel oxides.

Microstructural characterization of PS-PVD ceramic thermal barrier coatings with quasi-columnar structures

Yttria-stabilized zirconia (YSZ) thermal barrier coatings with quasi-columnar structure were produced by plasma spray physical vapor deposition (PS-PVD). To form this typical structured coating, the process-structure relationships response for substrate temperature, carrier gas flow and spray distance were investigated. Furthermore, this study revealed its characterization in coating microstructures. It was found that the substrate temperature played a governor role in determining the formation of quasi-columnar structure partially because it affected the nucleation rate of vapors during deposition. Appropriate carrier gas flow and spray distance were helpful to remove the un-melted particles and droplets from the plasma plume and promote the formation of quasi-columnar microstructure with high-ratio vapors. The quasi-columnar coating is featured with feather-like polycrystal structure with average misorientation angle of ~35.4° and average diameter of ~1.17μm.

セラミック遮熱コーティングにおける損傷・非弾性変形挙動の数値シミュレーション

セラミック遮熱コーティングにおける損傷・非弾性変形挙動の数値シミュレーション

Study on Numerical Analysis Coupled with Stress and Damage in Ceramic Thermal Barrier Coatings under Thermo-mechanical Loading Condition

Study on Numerical Analysis Coupled with Stress and Damage in Ceramic Thermal Barrier Coatings under Thermo-mechanical Loading Condition

Microstructural evaluation of a slurry based Ni/YSZ thermal barrier coating for automotive turbocharger turbine application

In the present study, the method of slurry coating process was used to fabricate functionally graded thermal barrier coating (FG-TBC) for turbocharger application. An automatic film applicator was used for the coating in an attempt to minimize the production cost of ceramic thermal barrier coatings. Yttria stabilized zirconia ceramic and nickel metal powders were mixed in appropriate proportion and produced a functionally graded material mixture. A slurry based coating method was used for the fabrication of the FG-TBC. The coating compositions of 30wt% Yttria Stabilized Zirconia and 70wt% Nickel, 55wt% Yttria Stabilized Zirconia and 45wt% Nickel as well as 75wt% Yttria Stabilized Zirconia and 25wt% Nickel were used in depositing the first, second and third layers of the functionally graded thermal barrier coating on the substrate respectively. Field Emission Scanning Electron Microscopy, Scanning Electron Microscope and X-ray Diffraction were used to study and evaluate the integrity and reliability of the coating produced. The results have indicated the sustainability and suitability of the method adopted in the study with regards to the production of good and quality functionally graded thermal barrier coating with no spallation problem and having good adherence to the substrate.

CaO-MgO-Al2O3-SiO2 (CMAS) Corrosion of Gd2Zr2O7 and Sm2Zr2O7

Ceramic thermal barrier coatings are applied to superalloys used in gas turbine engineering to increase the operating temperature and the energy conversion efficiency. However, dust consisting of CaO-MgO-Al2O3-SiO2 (CMAS) from the air can be injected into the engines and corrode the thermal barrier coatings. Lanthanide zirconates are promising materials in thermal barrier coatings due to their low thermal conductivities, good phase stability and good corrosion resistance. However, the corrosion resistance mechanism of CMAS on lanthanide zirconates is still not clearly understood. In this work, the corrosion mechanism of Gd2Zr2O7 and Sm2Zr2O7 in CMAS is studied. The results show that the CMAS can easily react with lanthanide zirconate thermal barrier coatings to form a dense layer, which can resist further corrosion.

Thermal Barrier Coating Validation Testing for Industrial Gas Turbine Combustion Hardware

As gas turbine (GT) temperatures have increased, thermal barrier coatings (TBCs) have become a critically important element in hot section component durability. Ceramic TBCs permit significantly increased gas temperatures, reduced cooling requirements, and improve engine fuel efficiency and reliability. TBCs are in use throughout the GT hot section with turbine blades, vanes, and combustion hardware, now being designed with TBCs or upgraded with TBCs during component refurbishment (Miller, 1987, “Current Status of Thermal Barrier Coatings,” Surf. Coat. Technol., 30(1), pp. 1–11; Clarke et al., 2012, “Thermal-Barrier Coatings for More Efficient Gas-Turbine Engines,” MRS Bull., 37(10), pp. 891–898). While the industry standard 6–9 wt. % yttria stabilized zirconia (7YSZ) has been the preferred ceramic composition for the past 30+ yr, efforts have been underway to develop improved TBCs (Stecura, 1986, “Optimization of the Ni–Cr–Al–Y/ZrO2–Y2O3 Thermal Barrier System,” Adv. Ceram. Mater., 1(1), pp. 68–76; Stecura, 1986, “Optimization of the Ni–Cr–Al–Y/ZrO2–Y2O3 Thermal Barrier System,” NASA Technical Memorandum No. 86905). The principal development goals have been to lower thermal conductivity, increase the sintering resistance, and have a more stable crystalline phase structure allowing to use above 1200 °C (2192 °F) (Levi, 2004, “Emerging Materials and Processes for Thermal Barrier Systems,” Curr. Opin. Solid State Mater. Sci., 8(1), pp. 77–91; Clarke, 2003, “Materials Selection Guidelines for Low Thermal Conductivity Thermal Barrier Coatings,” Surf. Coat. Technol., 163–164, pp. 67–74). National Aeronautics and Space Administration (NASA) has developed a series of advanced low conductivity, phase stable and sinter resistant TBC coatings utilizing multiple rare earth dopant oxides (Zhu and Miller, 2004, “Low Conductivity and Sintering-Resistant Thermal Barrier Coatings,” U.S. Patent No. 6,812,176 B1). One of the coating systems NASA developed is based on Ytterbia, Gadolinia, and Yttria additions to ZrO2 (YbGd-YSZ). This advanced low conductivity (low k) TBC is designed specifically for combustion hardware applications. In addition to lower thermal conductivity than 7YSZ, it has demonstrated thermal stability and sintering resistance to 1650 °C (3000 °F). The Electric Power Research Institute (EPRI) and cincinnati thermal spray (CTS) have teamed together in a joint program to commercialize the YbGd-YSZ TBC coating system for GT combustion hardware. The program consists of validation of coating properties, establishment of production coating specifications, and demonstration of coating performance through component engine testing of the YbGd-YSZ TBC coating system. Among the critical to quality coating characteristics that have been established are (a) coating microstructure, (b) TBC tensile bond strength, (c) erosion resistance, (d) thermal conductivity and sintering resistance, and (e) thermal cycle performance. This paper will discuss the coating property validation results comparing the YbGd-YSZ TBC to baseline production combustor coatings and the status of coating commercialization efforts currently underway.

A study on the oxidation resistance of electrodeposited and nanostructured YSZ thermal barrier ceramic coatings

A method is developed to electrodeposit nanostructured yttria-stabilized zirconia (YSZ) as thermal barrier ceramic coating on In.738LC Ni base superalloy substrate. The microstructure of the deposit and its oxidation resistance were investigated in detail in the present study. The first step was to applying an incipient conversion coating from an acidic solution in order to improve the coating adhesion. The deposition was performed in an aqueous solution containing yttrium and zirconium soluble salts as well as potassium chloride. Oxidation resistance of the specimens was tested at 1000°C through a cyclic voltammetry method. The surface morphology and cross section of specimens were examined by a scanning electron microscope. According to the obtained results, it was concluded that nanostructured YSZ coating can obtained by electrochemical synthesis with better oxidation resistance compared to the conventional coatings. This was attributed to the formation of the ultrafine and compact structures. Meanwhile, homogenous distribution of micro-pores which was observed within the coating might have also affected the oxidation phenomenon.

High‐Temperature Aging of Plasma Sprayed Quasi‐Eutectoid LaYbZr2O7–Part II: Microstructure & Thermal Conductivity

LaYbZr2O7 ceramic thermal barrier coatings (TBC) of meta‐stable structure were prepared by an air plasma spraying process. Their microstructure and associated thermal transport properties evolution during high‐temperature annealing at 1300°C were characterized. The as‐sprayed LaYbZr2O7 TBCs underwent a fast crystallization and a quasi‐eutectoid transformation during annealing, resulting in a biphase composite consisting of La‐rich pyrochlore phase and Yb2Zr2O7 fluorite phase with coherent phase boundaries. Due to the diffusion barriers between the two phases as well as the low interface energy of the coherent boundaries, sintering and grain growth of materials was significantly refrained. Therefore, a final thermal dynamically stable microstructure with a grain size of ~300 nm and a total porosity about 5% could be maintained even after long‐term aging at a high temperature of 1300°C. Resulting from this stable microstructure, an ultralow thermal conductivity of 1.3 W·(m·K)−1 could be obtained even after 216 h high‐temperature aging, which is much lower than that of the state‐of‐art 7 wt% yttria‐stabilized zirconia TBCs. Both the high phase and microstructure stability and the extremely low thermal conductivities could be particularly beneficial for TBC material in gas turbine applications.

Novel thermal barrier coatings based on rare-earth zirconates/YSZ double-ceramic-layer system deposited by plasma spraying

Sm2Zr2O7 and (Sm2/3Yb1/3)2Zr2O7 feedstocks prepared by spray drying have been used to produce ceramic thermal barrier coatings. Double-ceramic-layer thermal barrier coatings with an under-layer of 8 wt.% yttria-stabilized zirconia (YSZ) and a top-layer of Sm2Zr2O7 or (Sm2/3Yb1/3)2Zr2O7 were deposited by plasma spraying. Plasma sprayed Sm2Zr2O7 coatings are composed of a pyrochlore-type phase and a t-ZrO2 phase, while (Sm2/3Yb1/3)2Zr2O7 coatings exhibit a defect fluorite-type structure. The number of thermal shock failure at 1250 °C is 52 and 33 for Sm2Zr2O7/YSZ and (Sm2/3Yb1/3)2Zr2O7/YSZ coatings, respectively. The thermal shock failure of zirconates/YSZ double-ceramic-layer coatings mainly occurs inside ceramic layers, and is not caused by the thermal grown oxide layer from the bond coat.

Microstructural, mechanical and thermal characteristics of zirconia-based thermal barrier coatings deposited by plasma spraying

Ceramic thermal barrier coatings (TBCs) are potential tools to increase the durability of metal parts operating in turbine engines. In this work, partially yttria stabilized zirconia (YSZ) and lanthanum zirconate (LZ) coatings were deposited by plasma spraying on stainless steel substrates. X-Ray Diffraction (XRD) analysis revealed that plasma spraying promoted the formation of metastable phases in both the cases. Both YSZ and LZ coatings exhibited porous microstructure with a network of embedded pores and microcracks, that resulted in similar porosity values. The average microhardness of LZ coating was about 92% of that of YSZ one (5.4 against 5.9GPa), whereas the thermal expansion coefficient (CTE) of LZ coating was about 86% of that of YSZ one. Based on the results herein discussed the development of multilayered microstructures represents a promising solution in order to develop TBCs with enhanced performance.

Thermo-mechanical Experiments of Y-PSZ Thermal Barrier Ceramic Coating with Bond Coat of Alumina

In the present study, aluminum alloy (AlSi) substrates coated by Yttria stabilized zirconia used as thermal barrier have been investigated. The ceramic coating on the substrate is applied by plasma-spraying technique. Alumina (Al2O3) is used as a bond coat material. Four types of ceramic coating thickness (150, 250, 350 and 450 µm) were applied to AlSi materials which were cut out of diesel engine pistons with 100 µm thickness of bonding coat of Al2O3. The thermal torch and the thermal shock tests were conducted to coated samples which were made according to the international standards. Constant shock intensities were tested for 70 cycles on different thicknesses of the ceramic material and special attention was paid to the influence of thickness of the coating on the crazing phenomenon. Thermal resistance of thermal torch tested samples has been checked by drilling a hole in the middle of the samples and deformation time has been noted. Thermal shock tested samples were analyzed by X-ray diffraction machine and scanning electron microscopy machine. The diffractograms and micrographs have been obtained respectively for the samples. The diffractograms and micrographs of as sprayed samples and shock tested samples for different thickness have been compared, even the effect of thermal shock experiment on the different coating thickness of the ceramic have been analyzed. At the end of the tests, the most suitable ceramic coating thickness was determined.

高温曝露されたセラミック遮熱コーティングの引張負荷下での損傷過程について

高温曝露されたセラミック遮熱コーティングの引張負荷下での損傷過程について

Experimental determination of thermal-fatigue life of turbine blades with various coatings made of an advanced heat resistant nickel alloy

An original CIAM know-how for the experimental determination of thermal-fatigue life of parts of the gas-turbine engine hot section, including parts with heat-resistant and ceramic thermal barrier coatings, in the conditions of superficial uneven heating by high-frequency currents with continuous cooling of the parts at the preset air consumption is outlined in the paper. The possibility of using high-frequency heating of the parts with coatings (including thermal protection coatings) for modeling superficial heating of full-scale parts of gas-turbine engines is shown on the basis of the tests of model samples with coatings controlling the temperature of the surface by means of a thermal imager. The method used makes it possible to simulate the thermal state of the part in the operating conditions. Temperature differences observed both over the surface and the thickness of a part create thermal stresses in the material, similar to those arising during the operation of the engine. If necessary, mechanical loads synchronized with changes in temperature, to simulate for example, the centrifugal force may be applied to the part. The article presents the results of experimental studies of thermal-fatigue life of rotor blades of the 1st stage of the turbine made of an advanced monocrystal alloy VZhM-5 without coating and with various protective coatings, conducted according in the regime Tmin↔Tmax = 350↔1050˚C.

OS1204-136 その場観察に基づくセラミック遮熱コーティングの高温疲労き裂進展挙動

OS1204-136 その場観察に基づくセラミック遮熱コーティングの高温疲労き裂進展挙動