Zirconia Thermal Barrier Coatings Research Articles

Plasma-sprayed nanostructured YSZ thermal barrier coatings: Thermal insulation capability and adhesion strength

Abstract Adhesion strength and thermal insulation of nanostructured Yttria Stabilized Zirconia (YSZ) thermal barrier coatings (TBC) were investigated and compared with those of conventional YSZ TBCs. A Nickel based superalloy (IN-738LC) was used as the substrate with NiCrAlY bond coat, and nanostructured and conventional YSZ top coats were applied by using air plasma spray (APS). The adhesion strength of coatings was evaluated according to ASTM C633-01, and their thermal insulation capability was evaluated using a specially designed test setup at an electrical furnace. The results revealed the nanostructured YSZ coating to have a bimodal microstructure consisting of nanosized particles and microcolumnar grains. The bimodal microstructure of nanostructured coatings prevented crack propagation by splat boundaries and unmelted particles, thereby improving the bonding strength. Also, due to the presence of nano-zones in the microstructure of nano TBCs, coatings exhibited superior thermal insulation capability.

Microstructure and Properties of Peg-nail Structured YSZ TBCs by Selective Laser Modification after Plasma Spraying

Yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were deposited by an air plasma spraying (APS) method on nickel-based superalloy with initially sprayed NiCoCrAlYTa bond coat. After that, the surface of the plasma sprayed TBCs was subjected to selective laser modification using a Nd:YAG pulsed laser. The microstructure and performance of the plasma sprayed and peg-nail structured YSZ coatings by selective laser modification were investigated. The results reveal that selective laser modification helps to reduce the surface roughness considerably, eliminate surface porosities and produce a network of continuous cracks perpendicular to the surface. The microstructure of peg-nail structured units consists of columnar grains in the cross-section and equiaxed grains on the surface. XRD patterns show that both as-sprayed and the peg-nail structured coatings by selective laser modification consist of nonequilibrium tetragonal (T') phase; however, monoclinic (M) phase disappeared and intensity of T' phase slightly increases in the range of 72°∼76°(2 θ ) after selective laser modification. It has been found that the average bonding strength YSZ coatings is enhanced from 7.3 MPa to 13.3 MPa after selective laser modification. Thermal insulation capability of the peg-nail structured YSZ coating by selective laser modification, as compared to the as-sprayed YSZ coating, is decreased due to microstructure change in the ceramic top coat.

Thermal Shock Failure Mechanism of Nanostructured Zirconia Coating by Atmospheric Plasma Spraying

Nanostructured zirconia coatings has been prepared based on reasonable spraying technical parameters and the corresponding thermal shock property of the as-sprayed coating was examined at 1100 °C. The structure and the surface/interface morphology of the coatings have been analyzed using X-ray diffraction (XRD), metallographic microscope, scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and transmission electron microscopy (TEM). Based on the detailed analysis of the structure and phase composition, a rational mechanism has been proposed for thermal shock failure of the coating. Compared with other nanoparticles, those particles close to pores and pre-existing microcracks would preferentially grow up during the thermal shock process due to a better growth space. The growth of these nanoparticles is conducive to the formation of new microcracks which would lead to the growth of the other nanoparticles. With the growth of most or all of the nanoparticles, the nanostructured zirconia coating correspondingly changes into the quasi-microstructured coating. The thermal shock failure mode of the as-sprayed coating is similar to that of traditional zirconia thermal barrier coatings (TBCs).

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.

Effect of CMAS Composition on Hot Corrosion Behavior of Gadolinium Zirconate Thermal Barrier Coating Materials

The resistance of synthesized pyrochlore-type Gd2Zr2O7 bulk specimens to four calcium-magnesium aluminosilicate (CMAS) compositions at different temperatures was investigated. The reaction products were identified by X-ray diffraction and penetration depths were examined using scanning electron microscopy. A dense reaction layer is comprised mainly of Ca2Gd8(SiO4)6O2 and a cubic fluorite phase formed during the CMAS attack, and some unreacted CMAS was found in a transition layer below the reaction layer. The overall infiltration depth changed slightly with temperature, however, the thickness of the reaction layer and the morphology of the transition layer varied distinctly with temperature. The sintered sample underwent the most severe degradation by the CaO-lean CMAS, whereas the effect of CaSO4 and CaCO3 was not significant. Moreover, the Gd content of the ZrO2-based cubic fluorite phase depends on the temperature and the molar ratio of Ca:Si in the CMAS.

Failure Behavior of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings Under Three-Point Bending Test via Acoustic Emission Technique

In this paper, the failure behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings fabricated by atmospheric plasma spraying (APS-TBCs) under three-point bending (3PB) test has been characterized via acoustic emission (AE) technique. Linear positioning method has been adopted to monitor dynamic failure process of the APS-TBCs under 3PB test. The investigation results indicate that the variation of AE parameters (AE event counts, amplitudes and AE energy) corresponds well with the change of stress–strain curve of the loading processes. The failure mechanism was analyzed based on the characteristics of AE parameters. The distribution of frequency of crack propagation has been obtained. The AE signals came from two aspects: i.e., plastic deformation of substrates, initiation and propagation of the cracks in the coatings. The AE analysis combined with cross-sectional observation has indicated that many critical cracks initiate at the surface of the top-coat. And some main cracks tend to propagate toward the substrate/bond-coat interface. The actual failure mechanism of the APS-TBCs under 3PB test is attributed to the debonding of metallic coating from the substrates and the propagation of the horizontal crack along the substrate/bond-coat interface under the action of flexural moment.

Thermal Insulation and Thermal Shock Behavior of Conventional and Nanostructured Plasma-Sprayed TBCs

This study investigated the thermal insulation and thermal shock resistance behavior of nanostructured versus conventional yttria-stabilized zirconia (YSZ) thermal barrier coatings. To evaluate their coating performance in service conditions, samples were furnace sintered at 1150 °C for 100 h in ambient atmosphere. The results show that the nanostructured coatings exhibited better heat transfer resistance and thermal shock resistance compared with the conventional coating. In addition, the larger size of the initial agglomerates in the nanostructured coatings increased the percentage area of nanozones and decreased the heat transfer resistance. The thermal insulation behavior of the conventional coating was improved after heat treatment because of horizontal cracking. Disappearance of cracks, bridging between grains, and their growth by connecting with each other were observed in the conventional coating. However, in the nanostructured coatings, the nanoareas and their related properties disappeared. Microstructural and phase investigations were carried out by optical microscopy, field-emission scanning electron microscopy (FE-SEM), and x-ray diffraction (XRD) analysis. The thermal behavior was investigated by thermal insulation capability testing.

The role of the surface morphology and segmented cracks on the damage forms of laser re-melted thermal barrier coatings in presence of a molten salt (Na2SO4 + V2O5)

Hot corrosion behaviour of yttria-stabilized zirconia thermal barrier coatings (TBCs) with various segmented cracks and surface morphologies, obtained using ultrasonic-assisted laser re-melting, was investigated during exposure to Na2SO4+V2O5 salt at 1100°C. It is demonstrated, that the damage types of TBCs were significantly affected by segmented cracks and surface morphology. Four main damage mechanisms: (i) chemical reaction, (ii) Coefficient thermal mismatch stress, (iii) oxidation of bond coating and (iv) intergranular corrosion, based on the observed transport behaviour of oxygen and molten salt, stress-tolerance and self-healing of various segmented cracks, were discussed to account for the failure behaviour.

Evaluation of microstructural evolution and corrosion types in ultrasonic assisted laser re-melted thermal barrier coatings under exposure to molten salts

In this study, yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) with improved microstructure and properties were re-melted using the novel ultrasonic assisted laser re-melting technique and its hot corrosion behavior was investigated in 50wt% Na2SO4+50wt%V2O5 molten salts at 1100°C. The results indicated that the microstructure and corrosion types of the laser re-melted TBCs were significantly affected by the ultrasonic vibration power output (UVPO). The increased convection effect of low UVPO (20%) caused the enhanced nucleation of equiaxed grain and improvement in crack distribution and size. High strain tolerance led to the enhancement in the self-healing (eliminate cracks spacing) performance of cracks when exposed to the thermal expansion and phase transformation volume expansion, and seal the macroscopic diffusion channels of particles, thus promoting the propagation of only intergranular corrosion (internal corrosion). However, intensive ultrasonic nonlinear effect of high UVPO (50%) led to the generation of the uneven surface with coarse irregularly distributed cracks that are not entirely self-healing when exposed to volume expansion. Thus, the massive coatings particles could migrate preferentially outward through the cracks and voids to react with molten salts on the surface (external corrosion), leading to formation of the corrosion products.

Atmospheric Plasma Spraying of Single Phase Lanthanum Zirconate Thermal Barrier Coatings with Optimized Porosity

The shortcomings at elevated operation temperatures of the standard material yttria-stabilized zirconia (YSZ) for thermal barrier coatings (TBCs) have initiated many research activities seeking alternatives. One candidate is the pyrochlore lanthanum zirconate La2Zr2O7 (LZ), which is phase-stable to its melting point. At the same time, it shows a lower thermal conductivity and a lower sintering tendency when compared to YSZ. Because of its low thermal expansion coefficient and poor toughness, it is applied in combination with YSZ in double layer TBC systems. It is the current state of knowledge that LZ is prone to lanthanum depletion if processed by plasma spraying. The process conditions have to be selected carefully to avoid this. Furthermore, the amount and morphology of the coating porosity is essential for a good thermo-mechanical performance. In this work, the development and testing of LZ/YSZ double layer TBC systems is described. Initially, suitable basic parameters (torch, plasma gas composition, and power) were tested with respect to coating stoichiometry. Then, microstructures were optimized by adjusting feed rate, spray distance, and by selecting a more appropriate feedstock. Powder particles and coatings were characterized by digital image analysis.

Advanced analysis on growth mechanisms of thermally grown oxide at elevated temperature for thermal barrier coatings

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by plasma spraying. The as prepared YSZ TBCs were characterized by scanning electron microscopy (SEM), Electron backscatter diffraction (EBSD) and focused ion beam (FIB) systems before and after heat treatment. A thermally grown oxide (TGO) layer was formed between YSZ topcoat and bond coat after heat treatment. EBSD results revealed that the TGO layer was composed of α-Al2O3 and cubic Al2NiO4 layers. Mechanism controlling the growth of the TGO layer were analyzed in detail. It was found that the AlNi phase gradually disappeared from bond coat during heat treatment, which provided Al and Ni sources for the formation of TGO. Meanwhile, EBSD results confirmed that the growth of the TGO layer was dominated by the diffusion of Al and Ni rather than O penetration. Furthermore, direct evidence on the diffusion pathways of Al and Ni was also found, indicating that Al and Ni proceeded along the pores, cracks and YSZ grain boundaries to promote the further growth of TGO layer with increased heat treatment time.

Thickness and microstructure characterization of TGO in thermal barrier coatings by 3D reconstruction

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are prepared by plasma spraying. Thermally grown oxide (TGO) would be formed between YSZ topcoat and bond coat after 50h thermal service for YSZ TBCs. The electron back scattered diffraction (EBSD) results reveal that the TGO layer is composed of α-Al2O3 and cubic Al2NiO4 layers. Measured values of TGO thickness from the 2D-SEM image are greater than or equal to its real thickness due to the fact that the TGO layer is much rolling so that up and down surfaces of the TGO can't be completely perpendicular to the cross-section direction confirmed by 3D-SEM. Furthermore, 3D-SEM results reveal that the real thickness of TGO layer is 3.10μm instead of 7.1μm. In addition, 3D-EBSD confirmed that α-Al2O3 layer in TGO is composed of single layer of grains and Al2NiO4 layer consist of multilayer of grains while α-Al2O3 layer is mixed with single layer and multilayer of α-Al2O3 grains from observation of the 2D-EBSD image. It provides a new method to characterize real thickness and microstructure of TGO, which is also applied to other film materials.

High Temperature Thermal Properties of Columnar Yttria Stabilized Zirconia Thermal Barrier Coating Performed by Suspension Plasma Spraying

Performance enhancement of gas turbines is a main issue for the aircraft industry. Over many years, a large part of the effort has been focused on the development of more insulating Thermal Barrier Coatings (TBCs). In this study, Yttria Stabilized Zirconia (YSZ) columnar structures are processed by Suspension Plasma Spraying (SPS). These structures have already demonstrated abilities to get improved thermal lifetime, similarly to standard YSZ TBCs performed by EB-PVD. Thermal diffusivity measurements coupled with differential scanning calorimetry analysis are performed from room temperature up to 1100 °C, first, on HastelloyX substrates and then, on bilayers including a SPS YSZ coating. Results show an effective thermal conductivity for YSZ performed by SPS lower than 1 W.m-1K-1 whereas EB- PVD YSZ coatings exhibit a value of 1.5 W.m-1K-1.

Thermal Insulation Property of Nanostructured Alumina Coated Zirconia Thermal Barrier Coating

nanostructured Al2O3 coated ZrO2-Y2O3 thermal barrier coatings were prepared by plasma spraying method using nanostructured alumina coated 8mol%Y2O3 stabilized ZrO2 powder as starting material and the thermal insulation property was investigated as a function of the thickness of the coating. The results indicate that the structure and property of thermal barrier coating using nanoAl2O3 coated ZrO2-Y2O3 powder was superior to that of using single zirconia powder. The thermal insulation property of the thermal barrier coating increased with thickness of the coating increasing, and the advantage is more obvious with temperature increasing.

Thermal and Metrological Studies on YTTRIA Stabilized Zirconia Thermal Barrier Coatings and a Simulated Model to Co-Relate the Findings

Thermal Barrier Coatings (TBCs), routinely prepared from Ceramic based compositions (typically 8%Y 2O3-ZrO 2or 8YSZ) are being engineered to protect the metallic components from degradation in applications like gas turbines, jet and automotive engines. With a goal of finding improved TBC materials a wide variety of ceramics are being researched worldwide. Before physically preparing the TBCs of uncommon compositions in the laboratory, their suitability to perform can be predicted. Limited accessibility to detailed and realistic information on the influence of newer compositions (other than 8YSZ) on TBCs warrants methods to obtain this information. In this paper, 8YSZ TBCs coated onto aluminium substratesare studied for thermal fatigue, thermal barrier and materials characteristics to determine the reliability of the coating configuration to withstand the harshness of test conditions under the framework of experiments. Thereafter, the results have been used to corroboratethe developed simulation model. Results obtained via thermal tests confirm the suitability of the model and we can predict the thermal barrier effects of TBCs when prepared from materials other than YSZ.

Remote temperature sensing on and beneath atmospheric plasma sprayed thermal barrier coatings using thermographic phosphors

Investigations on remote temperature sensing of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) at the surface and at the bond-coat/top-coat interface were carried out. Using Y2O3:Eu thermographic phosphor as an embedded temperature sensing layer, sub-surface temperature probing through 300 μm of atmospheric plasma sprayed YSZ is demonstrated. The Y2O3:Eu thermographic phosphor displays a temperature sensitivity ranging between 400 °C up to a maximum of 900 °C when utilizing the luminescence originating from the 611 nm emission band. Dysprosium stabilized zirconia (10 wt.% DySZ), a TBC material, is also investigated and established as a temperature sensor from 400 °C up to a temperature of 1000 °C using both the intensity decay time and emission intensity ratio methods. In addition, the luminescence of presumed optically inactive YSZ materials was spectroscopically investigated in terms of optical interferences caused by impurities. A validation temperature probing measurement through 300 μm of YSZ top-coat was successfully performed in a SGT-800 Siemens burner running at six different operating conditions in an atmospheric combustion rig.

Lab Scale Preparation and Evaluation of Yttria Stabilized Zirconia Thermal Barrier Coatings and its Influence on the Diesel Engine Performance

Lab Scale Preparation and Evaluation of Yttria Stabilized Zirconia Thermal Barrier Coatings and its Influence on the Diesel Engine Performance

Microstructure and thermal properties of nanostructured gadolinia doped yttria-stabilized zirconia thermal barrier coatings produced by air plasma spraying

In this article, the nanostructured 2mol% Gd2O3-4.5mol% Y2O3-ZrO2(2GdYSZ) coating was developed by the atmospheric plasma spraying technique. And the microstructure and thermal properties of plasma-sprayed 2GdYSZ coating were investigated. The result from the investigation indicates that the as-sprayed coating is characterized by typical microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks. The 2GdYSZ coating shows a lower resistance to destabilization of the metastable tetragonal (t′) phase compared to the yttria stabilized zirconia(YSZ). The thermal diffusivity and thermal conductivity of the nano-2GdYSZ coating at room temperature are 0.431mm2s−1 and 1.042W/m K, respectively. Addition of gadolinia to the nano-YSZ can significantly reduce the thermal conductivity compared to the nano-YSZ and the conventional YSZ. The reduction is mainly attributed to the synergetic effect of gadolinia doping along with nanostructure.

An experimental investigation on transpiration cooling with phase change under supersonic condition

The extremely high heat loads on hypersonic vehicles make great challenges for the research and development of more effective Thermal Protection Technique (TPT). Transpiration cooling using liquid coolant has been confirmed as a promising TPT, because the phase change of liquid can release a large amount of latent heat. An experimental investigation on transpiration cooling using water as coolant was conducted in the arc-heated wind tunnel of China Academy of Aerospace Aerodynamics (CAAA), with a free stream specific enthalpy, mass flow rate and Mach number of 2700kJ/kg, 645g/s and 4.2, respectively. The specimen used in this experiment, a nose cone, consists of two types of materials. One is sintered porous material in the leading edge region as transpiration cooling matrix, and the other is impermeable alloy with zirconia Thermal Barrier Coating (TBC). The thermodynamic and aerodynamic parameters in coolant chamber are controlled by a quantitative injector of coolant mass flow rate to prevent the phenomena of overcooling and ablation. Surface temperature distributions are captured by an infrared thermal imaging system (ITIS). This paper exhibits and analyzes some interesting experimental phenomena, including ice formation in high enthalpy environment, hysteresis of thermal balance due to TBC, and variations of surface cooling effectiveness. A numerical approach to estimate the water mass flow rate is introduced and the relative error referenced experimental results is analyzed.

Modeling the thermal radiation properties of thermal barrier coatings based on a random generation algorithm

Thermal barrier coatings (TBCs) are porous media in which many different pores and cracks are induced by different manufacturing procedures. Many studies have been conducted to investigate the impact of microstructures of TBCs on thermal conductivity; nevertheless, the influence of microstructures on the radiative properties of TBCs has not drawn significant attention. In addition, the working condition of thermal barrier coatings is at high temperatures at which the contribution of radiative heat transfer plays a very crucial role. Therefore, it is necessary to study the radiative properties of TBCs to characterize their insulation performance. In this work, the microstructures of air-plasma- sprayed (APS) 8wt% yttria stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) are constructed by the quartet structure generation set (QSGS) algorithm. A finite-difference-time-domain (FDTD) method is carried out to simulate radiative heat transfer through TBCs. Three parameters—average pore size, directional growth probability Di (especially horizontal growth probability D13) and porosity—have been investigated to study the microstructural effect on the radiative properties of TBCs. The reflectance of freestanding 50-μm-thick thermal barrier coatings is studied using Lumerical FDTD Solutions in the wavelength range from 1 to 6μm at normal incidence. The absorption and scattering coefficient as a function of wavelength are extracted using the four-flux model. The results will help us to characterize the radiative heat transfer process across the TBCs and provide us with a theoretical guide to design TBCs with a high thermal insulation property.