Application Of Thermal Barrier Coatings Research Articles

Structure and Properties of Nonstoichiometric Y1-xNbxO1.5+x for Thermal Barrier Coatings

Understanding the effect of nonstoichiometry on material properties is of critical importance for the Thermal Barrier Coatings (TBCs) application. In this work, the effect of nonstoichiometry was systematically investigated in Y3NbO7, a recently identified promising TBCs material for next-generation gas turbine engines. The results show that the nonstoichiometry effect mediates the microstructure, mechanical properties, thermophysical properties, oxygen ionic conductivity and optical transmittance. The results suggest that the oxygen ionic conductivity is correlated to the mass diffusion in the oxygen-deficient fluorite oxides Y1-xNbxO1.5+x. The stoichiometric composition Y3NbO7, with the lowest thermal conductivity and slowest mass diffusion in Y1-xNbxO1.5+x series, is optimal for the TBCs application, which can be of relevance for TBCs material design and coating fabrication.

Research Progress of Failure Mechanism of Thermal Barrier Coatings at High Temperature via Finite Element Method

In the past decades, the durability of thermal barrier coatings (TBCs) has been extensively studied. The majority of researches emphasized the problem of oxidation, corrosion, and erosion induced by foreign object damage (FOD). TBCs with low thermal conductivity are usually coated on the hot-section components of the aircraft engine. The main composition of the TBCs is top-coat, which is usually regarded as a wear-resistant and heat-insulating layer, and it will significantly improve the working temperature of the hot-section components of the aircraft engine. The application of TBCs are serviced under a complex and rigid environment. The external parts of the TBCs are subjected to high-temperature and high-pressure loading, and the inner parts of the TBCs have a large thermal stress due to the different physical properties between the adjacent layers of the TBCs. To improve the heat efficiency of the hot-section components of aircraft engines, the working temperature of the TBCs should be improved further, which will result in the failure mechanism becoming more and more complicated for TBCs; thus, the current study is focusing on reviewing the failure mechanism of the TBCs when they are serviced under the actual high temperature conditions. Finite element simulation is an important method to study the failure mechanism of the TBCs, especially under some extremely rigid environments, which the experimental method cannot realize. In this paper, the research progress of the failure mechanism of TBCs at high temperature via finite element modeling is systematically reviewed.

Synthesis and characterization of ((La1-xGdx)2Zr2O7; x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) nanoparticles for advanced TBCs

Multicomponent rare-earth oxide-doped zirconia-based thermal barrier coatings (TBCs) are attractive alternatives for yttria-stabilized zirconia (YSZ) in advanced TBC applications. In this study, a zirconia-based nanopowder doped by multiple rare-earth oxides ((La1-xGdx)2Zr2O7; x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) was synthesized using sol-gel and calcination method. The product was then characterized by X-ray diffraction (XRD), simultaneous thermal analysis (STA), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and dilatometric test. According to STA results, the proper calcination temperature of (La1-xGdx)2Zr2O7 nanopowders could be considered > 800 °C. The results indicated that Gd2Zr2O7 exhibits a defect fluorite-type structure, and with the incorporation of larger La3+, it transformed into an ordered pyrochlore-type structure. The microstructure and morphology of the product were studied by field emission scanning electron microscopy (FESEM), revealing the formation of (La1-xGdx)2Zr2O7 nanoparticles with the mean size below 100 nm. Also, the thermal expansion coefficient of (La1-xGdx)2Zr2O7 nanoparticles was determined 8–12.5 × 10−6 K−1 at 1000 °C.

Thermal and oxygen transport properties of complex pyrochlore RE2InTaO7 for thermal barrier coating applications

Pyrochlore-type rare earth zirconate (RE2Zr2O7) has been considered as a new generation thermal barrier coating (TBC) material. We substitute Zr4+ with a pair of In3+ and Ta5+ in RE2Zr2O7 (RE = Pr, Nd, Sm, Eu, Gd), forming a complex pyrochlore material RE2InTaO7. Combined with the structural disordering, the thermal conductivities of RE2InTaO7 were progressively reduced to the amorphous limit. Phonon scattering due to the structural complexity and disordering was confirmed by the reduced phonon mean free path and further demonstrated by the broadened Raman modes. Meanwhile, the oxygen ion conductivities were reduced by two orders of magnitude compared with that of normal pyrochlore structure. This paper demonstrates the approach of structural complexing could simultaneously tune thermal and oxygen transport properties toward high-efficiency and durable TBC applications.

Structure evolution, thermal properties and sintering resistance of promising thermal barrier coating material La2(Zr0.75Ce0.25)2O7

Rare-earth doped zirconates are promising candidate materials for high-performance thermal barrier coatings (TBCs). The phase and microstructure stability is an important issue for the materials that must be clarified, which is related to the long-term stable work of TBCs at high temperatures. In this work, La2(Zr0.75Ce0.25)2O7 (LCZ) ceramic coatings prepared by atmospheric plasma spraying present a metastable fluorite phase, which can transform into stable pyrochlore under high-temperature annealing. The detailed structure evolution of the ceramic coatings is characterized systematically by SEM, XRD and Raman. The associated thermal properties of LCZ ceramics were also reported. Results show that LCZ ceramic has an ultralow thermal conductivity (0.65 W/m·K, 1200 °C), which is only 1/3 of that of yttria-stabilized zirconia (YSZ). The thermal expansion coefficients of LCZ ceramic increase from 9.68 × 10-6 K-1 to 10.7 × 10-6 K-1 (300 - 1500 °C), which are relatively larger than those of La2Zr2O7. Besides, Long-term sintering demonstrates that LCZ ceramic coating has preferable sintering resistance at 1500 °C, which is desirable for TBC applications.

A novel CMAS‐resistant material based on thermodynamic equilibrium design: Apatite‐type Gd10(SiO4)6O3

AbstractCalcium‐magnesium‐alumino‐silicates (CMAS) melt attack has been a critical issue for the thermal barrier coatings (TBCs) with ever‐increasing engine operating temperature. In this study, a novel CMAS‐resistant material apatite‐type Gd10(SiO4)6O3 is developed for TBCs application based on thermodynamic equilibrium design. The chemical reaction of Gd10(SiO4)6O3 bulk and CMAS melt is investigated at 1300°C. The CMAS corrosion resistance of Gd10(SiO4)6O3 bulk is evaluated and compared with the well‐studied CMAS‐resistant material Gd2Zr2O7 (GZO). It is found that Gd10(SiO4)6O3 shows a significantly enhanced CMAS resistance, including lower intrinsic CMAS infiltration rate (~1.09 μm/h1/2) and smaller infiltration upper limit (50‐62 μm) for a 20 mg/cm2 CMAS deposition. More importantly, for Gd10(SiO4)6O3, the CMAS infiltration only alters the composition but does not change the crystal structure or destroy microstructural integrity. The reaction mechanism is elucidated as following two stages: (a) surface Gd10(SiO4)6O3 quickly transforms into Ca2Gd8(SiO4)6O2 in suit by interdiffusion with CMAS melt and then is thermodynamically stable with CMAS melt, thereby effectively inhibiting the further CMAS infiltration and (b) with the ongoing interdiffusion of Gd/Ca, the CMAS‐infiltrated layer slowly thickens and follows a parabolic law. Meanwhile, the CMAS melt gradually precipitates Ca2Gd8(SiO4)6O2 and CaAl2Si2O8 (anorthite) until the melt is exhausted.

Toward Durable Thermal Barrier Coating with Composite Phases and Low Thermal Conductivity

High-efficiency turbine engines highly rely on the further improvement of the novel technologies related to combustion, cooling and thermal barrier coating (TBC) with the increase in inlet temperatures. Thermal barrier coatings with higher thermal stability and lower thermal conductivity (low-k) than current 8YSZ TBC had attracted a lot of academia and industries’ attentions and activities. The present work aimed to focus on exploring a path toward a durable TBC with better thermal durability and low-k capability by overcoming the technical, practical and economic barriers for current low-k TBC development and applications. The concept of composite phase ceramics was proposed for low-k TBC material design, in an effort to combine the desirable attributes of unique phase constitution, low conductivity k, high fracture toughness and good process economy. Further, thermal spray process was optimized for the topcoats of the low-k ceramics by controlling and measuring the effect of key process parameters on porous coating architecture, deposition rate and process efficiency. To evaluate the performance of the low-k TBCs, both an isothermal oxidation test and a thermal cycling test were conducted. The test results of the composite phase ceramics exhibited promising for a durable low-k TBC measured by several desirable property attributes.

Experimental and numerical investigation on gas turbine blade with the application of thermal barrier coatings

The engine parts material used in gas turbines (GTs) should be resistant to high-temperature variations. Thermal barrier coatings (TBCs) for gas turbine blades are found to have a significant effect on prolonging the life cycle of turbine blades by providing additional heat resistance. This work is to study the performance of TBCs on the high-temperature environment of the turbine blades. It is understood that this coating will increase the lifecycles of blade parts and decrease maintenance and repair costs. Experiments were performed on the gas turbine blade to see the effect of TBCs in different combinations of materials through the air plasma method. Three-layered coatings using materials INCONEL 718 as base coating, NiCoCrAIY as middle coating, and La2Ce2O7 as the top coating was applied. Finite element analysis was performed using a two-dimensional method to optimize the suitable formulation of coatings on the blade. Temperature distributions for different combinations of coatings layers with different materials and thickness were studied. Additionally, three-dimensional thermal stress analysis was performed on the blade with a commercial code. Results on the effect of TBCs shows a significant improvement in thermal resistance compared to the uncoated gas turbine blade.

Heat treatment for TGO growth on NiCrAlY for TBC application

The TGO (thermally grown oxide) is a layer of TBC (Thermal Barrier Coating), which develops and grows at the interface between at the bond coat (BC) and ceramic top coat (TC) in high temperature, and is composed mainly of aluminum oxides. The TGO layer improves the effective adhesion between BC/TC and effectively protects against oxidation and hot corrosion. However, this layer is the main failure of TBC and, thus, required further research. This work studied the initial growth of the TGO when exposed to NiCrAlY (BC) at high temperatures. The NiCrAlY samples obtained from its powder by isostatic pressing, sintered by hot uniaxial pressing at 1150 °C for one hour (10 MPa), and the TGO was grown by heat treatment in the air and vacuum at 800, 1100, and 1150 °C for four hours. The results indicated that with increased temperature, surface roughness increases and the oxide layers in the vacuum process are larger than in the air. SEM found that at 1100 °C, samples obtained the most appropriate oxide layer thickness. In vacuum treatments, x-ray diffraction analyses showed the formation of greater quantities of aluminum oxides.

Evaluation of thermal barrier coatings and surface roughness in a single-cylinder light-duty diesel engine

The effect of two thermal barrier coatings and their surface roughness on heat transfer, combustion, and emissions has been investigated in a single-cylinder light-duty diesel engine. The evaluated thermal barrier coating materials were plasma-sprayed yttria-stabilized zirconia and hard anodized aluminum, which were applied on the piston top surface. The main tool for the investigation was cylinder pressure analysis of the high-pressure cycle, from which the apparent rate of heat release, indicated efficiency, and heat losses were derived. For verification of the calculated wall heat transfer, the heat flow to the piston cooling oil was measured as well. Application of thermal barrier coatings can influence engine operating conditions like charge temperature and ignition delay. Therefore, extra attention was paid to choosing stable and repeatable engine operating points. The experimental data were modeled using multiple linear regression to isolate the effects of the coatings and of the surface roughness. The results from this study show that high surface roughness leads to increased wall heat losses and a delayed combustion. However, these effects are less pronounced at lower engine loads and in the presence of soot deposits. Both thermal barrier coatings show a reduction of cycle-averaged wall heat losses, but no improvement in indicated efficiency. The surface roughness and thermal barrier coatings had a significant impact on the hydrocarbon emissions, especially for low-load engine operation, while their effect on the other exhaust emissions was relatively small.

The formation of pyrochlores during plasma spraying of REO and zirconia oxides powder mixture

The pyrochlores are the promising low-thermal conductive materials for Thermal Barrier Coatings applications. In present work the concept of pyrochlore formation during APS spraying of ZrO2 and ReO was analyzed. The specially prepared agglomerated mixtures of ZrO2 with Nd2O3, Yb2O3, Er2O3 and Gd2O3 oxides were plasma sprayed using A60 plasma torch on NiCoCrAlY-type bond coat. The influence of plasma gasses composition on coatings’ microstructure was investigated. The results of XRD phase analysis proved formation of pyrochlores from Gd2O3 +ZrO2 and Nd2O3+ZrO2 mixtures. The formation of Er4O12Zr3 from Er2O3+ ZrO2 mixture as well as Zr3Yb4 from Yb2O3+ ZrO2 powder was detected. The presence of pure rare earth oxides (REO) and zirconia oxides was observed in all types of sprayed coatings. The microstructural assessment showed differences in porosity and thickness of obtained ceramic coatings depending of type of REO oxides. The analysis of results showed that it is possible to obtain pyrochlore ceramic coatings from pure oxides mixture. The plasma energy was not efficient for full formation of pyrochlores, therefore the presence of pure oxides was observed.

Influence of microstructure on the erosion behaviour of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are applied on the surface of hot parts of gas turbine engines to increase the turbine efficiency by providing thermal insulation and to protect the engine parts from the harsh environment. Typical degradation of TBCs can be attributed to bond coat oxidation, thermal stress etc. In addition to this, erosion can also lead to partial or complete removal of the TBCs especially when the engine operates under erosive environment such as flying over desert area, near active volcanic or offshore ocean environment. Suspension Plasma Spraying (SPS) is a promising technique for TBC applications by virtue of its ability to produce a strain-tolerant porous-columnar microstructure that combines the benefits of both electron beam physical vapor deposited (EB-PVD) as well as atmospheric plasma sprayed (APS) coatings. This work investigates the influence of various coating microstructures produced by SPS on their erosion behavior. Six different coatings with varied microstructures produced using different suspensions with distinct characteristics were studied and their erosion resistance was compared. Results showed significant influence of SPS TBCs microstructures on the erosion resistance. Furthermore, the erosion resistance of SPS TBCs showed a close correlation between fracture toughness and the erosion rate, higher fracture toughness favours superior erosion resistance.

Discrete crystallization of fluorite and subsequent pyrochlore phase transitions in Dy2Zr2O7 facilitated by Ti4+ additions

The impact of Ti4+ substitutions in Dy2Zr2O7 and the resultant structural, mechanical, morphological behaviour and hot corrosion characteristics were evaluated for potential applications in thermal barrier coatings. The results ensured the ability of Dy2Zr2O7 to hold 20 mol. % of Ti4+ to retain fluorite structure of Dy2Zr2O7 while the attempt to substitute an excess Ti4+ led to the partial phase transitions from fluorite to pyrochlore. Quantitative analysis revealed the significant contraction of Dy2Zr2O7 unit cell due to the replacement of relatively larger Zi4+ by the lower sized Ti4+. Morphological analysis ensured well-defined microstructures with definite grain size and boundaries and this inference displayed a direct impact on the resultant mechanical properties of the system. The results from the hot corrosion studies also indicated a better resistance displayed by the Dy2Zr2O7 fluorite due to Ti4+ substitutions.

Titanium substitution in Gd2Zr2O7 for thermal barrier coating applications

Abstract The study underlines the impact of Ti4+ substitution in Gd2Zr2O7 for applications in thermal barrier coatings (TBC). Depending on the Ti4+ content, two different crystal structures of Gd2Zr2O7 namely pyrochlore and fluorite were determined. Ti4+ substitutions in the increasing order induced a gradual contraction of Gd2Zr2O7 unit cell; however, with the accomplishment of concentration dependent crystal structures of either single phase pyrochlore or mixtures of pyrochlore and fluorite. Absorption measurements enunciated the enhanced infra-red reflectance behaviour of Gd2Zr2O7 due to Ti4+ substitutions. A gradual increment in the concentration of Ti4+ substitutions in Gd2Zr2O7 envisaged a simultaneous porous to dense morphological features, which reflected in the resultant mechanical data. Hot corrosion studies ensure the critical role of Ti4+ to retain the crystal structure of Gd2Zr2O7.

Mg2SiO4 as a novel thermal barrier coating material for gas turbine applications

Forsterite-type Mg2SiO4 was investigated systematically for thermal barrier coating (TBC) applications. Results showed that Mg2SiO4 synthesized by solid-state reaction possessed the good phase stability up to 1573 K. The thermal conductivity of Mg2SiO4 at 1273 K was lower ˜20% than that of yttria stabilized zirconia (8YSZ). Mg2SiO4 also presented moderate thermal expansion coefficients, which increased from 8.6 × 10−6 K−1 to 11.3 × 10−6 K−1 (473˜1623 K). Mechanical properties including hardness, fracture toughness, and Young’s modulus of Mg2SiO4 were comparable to those of 8YSZ. The sintering results indicated a promising low-sintering activity of Mg2SiO4. Mg2SiO4 samples were subjected to water quenching test at 1573 K and showed a superior thermal shock resistance compared to 8YSZ. Mg2SiO4 coating with stoichiometric composition was produced by atmospheric plasma spraying. The thermal cycling test result showed that Mg2SiO4 coating had a lifetime more than 830 cycles at 1273 K, which is desirable for TBC applications.

Phase stability of ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3 compound at 1100 °C and 1300 °C for advanced TBC applications

Phase instability of conventional zirconia compounds (YSZ: Yttria Stabilized Zirconia) at temperatures above 1200 °C, is one of the important reasons of thermal barrier coatings (TBCs) destruction in new generation of turbines. In this study, ZGYbY: ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3 powder, as a new material for TBC applications, was synthesized through a chemical co-precipitation method to enhance the phase stability of the YSZ at higher temperatures. A cyclic method with 5 h cycles at 1100 °C and 1300 °C for 50 h was employed to investigate the phase stability of YSZ and ZGYbY compounds. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis confirmed the phase stability of both of powders during thermal cycling at 1100 °C for 50 h. The results of thermal cycling at 1300 °C for 50 h indicated that ZGYbY powder exhibits excellent phase stability due to full retention of t-prime zirconia phase and restriction of tetragonal to monoclinic phase transition upon cooling. However, in the case of YSZ powder, after thermal cycling at similar condition, it decomposed to two new phases including cubic and monoclinic zirconia with 62 wt% and 38 wt%, respectively.

Plasma Sprayed WC-12%Co-Coatings for TBC Applications on Diesel Engine Piston

In the present study, plasma sprayed WC-12%Co coatings with 100µm NiCrAlY bond coat on a substrate of A336 cast aluminum alloy have been investigated for a thermal barrier coating (TBC) application. The coatings deposited with varying topcoat thickness up to 500µm were deposited on the piston top surface of an Indian hatchback diesel car to act as a thermal barrier and enhance the thermal efficiency of the engine. Although all the specimens with distinct coating overlays survived 350 thermal cycles, the one with 200µm thickness exhibited the best thermal shock behavior as they exuded the most cycles to surface cracks initiation. Moreover, SEM analysis also suggested 200 µm thick coating to be optimal for thermal shock behavior in diesel engine components. The coating phase analysis by XRD and the lattice strain analysis performed by a Williamson-Hall (W-H) analysis did not reveal any structural changes after the thermal shock experiment.

Synthesis and Thermal Properties of Europium-Cerium Oxide as a New Fluorite-Type Ceramic Material for Thermal Barrier Coatings Applications

Synthesis and Thermal Properties of Europium-Cerium Oxide as a New Fluorite-Type Ceramic Material for Thermal Barrier Coatings Applications

Modification of NiCoCrAlY with Pt: Part II. Application in TBC with pure metastable tetragonal (t′) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4 mol.% yttria stabilized zirconia (4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4YSZ thermal barrier coatings (TBCs) with and without Pt modification was evaluated in ambient air at 1100 °C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide (TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top (Ni,Co)(Cr,Al)2O4 spinel and underlying α-Al2O3, was formed at the NiCoCrAlY/4YSZ interface with thickness of 8.4 μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al2O3 scale with thickness of 5.6 μm was formed at the interface of Pt-modified NiCoCrAlY and 4YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al2O3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.

Prolong the durability of La2Zr2O7/YSZ TBCs by decreasing the cracking driving force in ceramic coatings

Service lifetime and thermal insulation performance are both crucial for the application of thermal barrier coatings (TBCs). In this study, layered structure design under equivalent thermal insulation conception is introduced to lower the cracking driving force in TBCs, and with the goal of prolonging TBCs lifetime. Three groups of layered LZO/YSZ TBCs were designed with same thermal insulation of 500 μm YSZ, the LZO layers were deliberately designed with different initial elastic modulus. Virtual crack closure technique (VCCT) calculation result showed that the energy release rates at the crack tips are 28.2, 22, and 18.8 N/m corresponding to the initial elastic modulus of 70, 60, and 50 GPa. After gradient thermal cyclic tests with surface temperature of 1300 °C, TBCs with lowest initial elastic modulus showed the longest lifetime, and more than double of pure YSZ TBCs. This study provides a new option for the improvement of TBCs lifetime.