Y2O3-stabilized ZrO2 Thermal Barrier Research Articles

Effect of Sc2O3 doping on YSZ TBCs: Morphologies, phase composition, mechanical properties, and high-temperature oxidation resistance

Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology, and then a high-temperature oxidation experiment was carried out at 1100 °C. Whereupon the morphologies, phase composition, and mechanical properties of YSZ ceramic coating and ScYSZ ceramic coating after different high-temperature oxidation time, as well as the high-temperature oxidation resistance of YSZ TBCs and ScYSZ TBCs, were comparatively studied. The results showed that after high-temperature oxidation, ScYSZ ceramic coating presented slighter cracking and exhibited better sintering resistance. After high-temperature oxidation, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was still a single T' phase, and their interplanar spacing became larger; nevertheless, the interplanar spacing of ScYSZ ceramic coating was smaller than that of YSZ ceramic coating. After high-temperature oxidation, ScYSZ ceramic coating had higher hardness, smaller elastic modulus, and smaller fracture toughness. In addition, ScYSZ TBCs exhibited better high-temperature oxidation resistance according to the morphology, equivalent thickness, high-temperature oxidation kinetics, and high-temperature oxidation thermodynamics of TGO.

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc2O3 and Y2O3 co-doping system on the thermal shock resistance of ZrO2 thermal barrier coatings, Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3–Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc2O3 and Y2O3 facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.

Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance

Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) are prone to hot corrosion by molten salts. In this study, the microstructure of atmospheric plasma spraying YSZ TBCs is modified by laser glazing in order to improve the corrosion resistance. By optimizing the laser parameters, a ∼18 µm smooth glazed layer with some vertical cracks was produced on the coating surfaces. The as-sprayed and modified coatings were both exposed to hot corrosion tests at 700 and 1000 °C for 4 h in V2O5 molten salt, and the results revealed that the modified one had improved corrosion resistance. After hot corrosion, the glazed layer kept structural integrity, with little evidence of dissolution. However, the vertical cracks in the glazed layer acted as the paths for molten salt penetration, accelerating the corrosion of the non-modified coating. Further optimization of the glazed layer is needed in the future work.

Effects of laser glazing on CMAS corrosion behavior of Y2O3 stabilized ZrO2 thermal barrier coatings

Calcium-magnesium-alumina-silicate (CMAS) is a serious threat to thermal barrier coatings (TBCs). The goal of this study is to tailor YSZ coating microstructures by laser for alleviating CMAS attack. A smooth laser glazed layer with a columnar microstructure and some open channels is produced on the coating surface. Under molten CMAS conditions, the glazed layer remains phase stability and has little evidence of dissolution. Exposed to thermal shock tests with CMAS deposits, the glazed layer keeps structural integrity. However, the open channels also act as the paths for CMAS penetration, which brings destructive damages to the coating beneath the glazed layer.

Design of friction stir welding tools reducing heat flow into spindle

In the friction stir welding process, a large amount of heat transfers to the spindle from the friction stir tools, resulting in low welding thermal efficiency and adverse impact on spindle bearings. With the help of finite element numerical simulation, ten friction stir welding (FSW) tools with heat insulation features were investigated. It was found that the hollow tool design and smaller tool diameter significantly reduced the temperature of spindle tool holder. Based on the simulation results, five tools were fabricated and three of them were coated with thermal barrier layer. Plasma spraying method was used to spray Y2O3-stabilized ZrO2 thermal barrier coating on the surface of FSW tool shank to reduce heat transfer to the spindle tool holder. It was found that for the tool with all the heat insulation features in the case of tool rotation speed 800 r/min, and welding speed 30 mm/min, the steady temperature of the outer surface of the spindle tool holder decreased more than 50% compared with using the conventional tool.

Hot corrosion evaluation of Gd2O3-Yb2O3 co-doped Y2O3 stabilized ZrO2 thermal barrier oxides exposed to Na2SO4+V2O5 molten salt

1mol% Gd2O3 and 1mol% Yb2O3 co-doped 3.5mol% Y2O3 stabilized ZrO2 (GdYb-YSZ) ceramics in metastable tetragonal (t′) structure were fabricated, and their hot corrosion behaviors were evaluated. The corrosion tests were performed at 700°C, 800°C, 900°C and 1000°C for 4h in the presence of Na2SO4+V2O5 molten salt. After corrosion, t′ phase could still be detected on the corroded surfaces and its content decreased with increasing the corrosion temperature. The corrosion products were composed of doped YVO4 and m-ZrO2, but some amounts of Na4V2O7 were found at low corrosion temperatures (700°C and 800°C). The corrosion resistance characteristic of GdYb-YSZ was compared with that of YSZ. Under an identical corrosion condition, GdYb-YSZ had less monoclinic phase formed, indicative of a better hot corrosion resistance. The hot corrosion mechanism was discussed in detail.

Evolution of mechanical properties of thermal barrier coatings subjected to thermal exposure by instrumented indentation testing

The evolution of mechanical properties, namely Young's modulus, hardness, fracture toughness and residual stresses of air plasma-sprayed 8wt% Y2O3-stabilized ZrO2 thermal barrier coatings (TBCs) subjected to thermal exposure at 1100°C, was evaluated based on an instrumented indentation testing system. It is found that the variation of Young's modulus and hardness with the increase of thermal exposure time can be divided into two regimes: a rapid initial increase, followed by a more gradual increase. Conversely, fracture toughness decreased from the initial value of 1.48–1.05MPam1/2 after 350h, indicating the degradation of TBCs. And the corresponding residual stress in top coat varied from the deposition stress of −29.9MPa to a maximum of −56.0MPa within 100h, and then decreased to −15.0MPa after 350h. The evolution of mechanical properties was correlated with microstructural changes in the top coat. Microstructure observations revealed that thermally grown oxide (TGO) formed at the interface between ceramic coat and bond coat, and was predominantly comprised of Al2O3 layer, along with some oxide clusters of (Cr, Al)2O3, (Co, Ni)(Cr, Al)2O4 and NiO. The growth of TGO followed a parabolic law with a rate constant of 0.13236μm2/h. After 350h, porosity in 8YSZ coatings was reduced from 8.39% to 5.79% as a result of activated diffusion-controlled sintering.

Integrated Approach for the Development of Advanced, Coated Gas Turbine Blades

AbstractThis paper describes a through‐process modelling on a microstructural level of the production of a coated turbine blade, including its in‐service properties and degradation, accompanied by the actual production and testing of a CMSX‐4 single crystal turbine blade dummy. The following steps are dealt with by modelling and experiment: solidification of the blade alloy during casting, microstructural changes during homogenization and aging heat treatments, chemical vapour deposition of an Al2O3 diffusion barrier coating, physical vapour deposition (sputtering) of a (Ni,Co)CrAlY bond coat, atmospheric plasma spraying of an Y2O3 stabilized ZrO2 thermal barrier coating and microstructural changes and development of critical stresses at in‐service conditions. This work forms a part of the Collaborative Research Centre 370 (SFB 370) “Integrative materials modelling”.

Detection of Micro-Damage Evolution of Air Plasma-Sprayed Y<SUB>2</SUB>O<SUB>3</SUB>-ZrO<SUB>2</SUB> Thermal Barrier Coating through TGO Stress Measurement

The micro damage evolution behavior in air plasma-sprayed 8 mass% Y2O3 stabilized ZrO2 thermal barrier coating (APS-TBC) after isothermal heat exposure at 1150 � C up to 200 h was observed. Residual stress of thermally-grown-oxide (TGO) layer was measured through the TBC layer using Cr 3þ luminescence spectroscopy and the measured stress levels were compared with micro damage evolution behavior stored in the TBC system. With heat exposure time of longer than 10 h, microfracture behaviors were noted in the TBC layer. In-plane residual stress of the TGO layer increased for up to 50 h of heat exposure time, and thereafter, diminished with further increase. These behaviors are strongly correlated with the thickness and the residual stress of the TGO layer. This decrease in the TGO stress well contrasts with the evolution of the micro fracture behavior, which is also related to the thickness and undulation of the formed TGO layer. The change of TGO stress, � tgo, depending on the average thickness of the TGO layer, �

A high-temperature displacement-sensitive indenter for studying mechanical properties of thermal barrier coatings

Electron beam physical-vapor-deposited Y2O3-stabilized ZrO2 thermal barrier coating (TBC) samples were indented from room temperature to 900 °C using an instrumented high-temperature vacuum displacement-sensitive indenter. Hardness and elastic modulus were determined from the load–displacement curves recorded during indentation. Both the hardness and the elastic modulus of the TBCs were much lower than those of dense ceramics of a similar composition; this is attributed to the increased compliance that results from the porous columnar microstructure of the TBCs. In addition, the TBCs showed an unusual absence of elastic recovery at the residual indents compared to the dense ceramics.

Properties of Plasma Sprayed CeO<sub>2</sub>-ZrO<sub>2</sub>-Y<sub>2</sub>O<sub>3</sub> Thermal Barrier Coating

The plasma sprayed CeO2-ZrO2-Y2O3 thermal barrier coating was developed, and thermal conductivity, thermal expansion, bending strength and hot corrosion resistance were evaluated. The results indicated that CeO2-ZrO2-Y2O3 compositions are useful for the plasma sprayed thermal barrier coatings. The thermal conductivity of the sprayed CeO2-ZrO2-Y2O3 coating was 1-2W/m·K, lower than that of sintered materials of the same composition and one of the lowest thermal conductivities of ceramics. Thus, the CeO2-ZrO2-Y2O3 coating is useful for the thermal barrier, since the sprayed layer is expected to insulate heat flow well. The thermal expansion coefficient of the sprayed CeO2-ZrO2-Y2O3 coating was 12×10-6/K, comparable to that of the metal substrate 16×10-6/K. The bending strength was 2-3kgf/mm2, favorable for the excellent thermal fatigue property. The hot corrosion resistance was almost the same as that of Y2O3-stabilized ZrO2 thermal barrier coatings in a low vanadium environment.