Thermal Barrier Coatings Material Research Articles

Analysis of performance and emission characteristics of TBC coated low heat rejection engine

ABSTRACT Present investigation is focused on the effect of thermal barrier coated piston on the performance and emission characteristics of diesel engine using diesel as a fuel. The piston crown top surface were fully coated with three different combinations of thermal barrier coatings (TBC) materials like 7% YSZ, 2% Gd2O3 + YSZ, 5% Gd2O3 + YSZ with thickness of 0.25 mm and in all combinations the bond coat NiCrAlY were used with a thickness of 0.05 mm which is deposited on the target surface using plasma spray technique. Experimental analysis is carried out under different loading conditions in a four-stroke single cylinder diesel engine with diesel as a fuel. Results of this study reveal that the brake thermal efficiency of the coated engine is increased by 12.1% with reduction of brake specific fuel consumption by up to 16.2% compared to baseline engine. Experimental results also revealed that in coated engine Hydro carbon (HC) and carbon monoxide (CO) emissions were reduced by 17.8% and 18.4% respectively and also a significant increment of Nitrogen oxide (NOx) emission by 14.87% compare to baseline engine.

Mechanical properties and thermal cycling behavior of Ta2O5 doped La2Ce2O7 thermal barrier coatings prepared by atmospheric plasma spraying

La2Ce2O7 (LC) is drawing increasing attention to be a new candidate material for thermal barrier coatings (TBCs) because of its higher phase stability, lower thermal conductivity and larger thermal expansion coefficient (TEC) than the traditional yttria partially-stabilized zirconia (YSZ). Unfortunately, the sudden TEC decrease at low temperatures greatly limits its application. In this study, Ta2O5 was added into LC to produce lanthanum-cerium-tantalum-oxide (LCT) solid solution, and LCT coatings were also newly developed by atmospheric plasma spraying (APS) using La2Ce1.7Ta0.3O7.15 powder. LCT coating exhibits excellent thermal stability between room temperature and 1573 K (up to 1000 h). The sudden TEC decrease is effectively suppressed due to the reduced oxygen vacancy concentration resulted from partial Ce4+ substitution by higher valence Ta5+, which is confirmed by Raman spectroscopy. Thermal cycling results showed that LCT TBC has relatively better thermal shock resistance than LC TBC, and spallation failure occurred within the ceramic top coat near the bond coat. It is considered that TEC mismatch stresses, low fracture toughness and coating composition deviation are primarily responsible for the poor thermal cycling performance. These findings provide valuable data for future technical optimization.

Thermophysical Property of Single-phase Strontium Zirconate Co-doped with Double Rare-earth Oxides as a Thermal Barrier Coating Material

Thermophysical Property of Single-phase Strontium Zirconate Co-doped with Double Rare-earth Oxides as a Thermal Barrier Coating Material

Pure waterjet controlled depth machining for stripping ceramic thermal barrier coatings on turbine blades

Gas turbine blades are high-performance composite parts made of nickel-based superalloys coated with a bilayer coating system for thermal protection. This coating system consists of a metallic bond coat and a ceramic thermal barrier coating (TBC), typically made of yttria-stabilized zirconia. During gas turbine operation the coating system undergoes degeneration and needs to be renewed periodically. Conventional overhaul processes remove the TBC and bond coat completely in several machining steps consisting of mechanical grid blasting and chemical stripping processes. Currently selective removal of TBC without influencing the bond coat is not applicable.Pure waterjet (PWJ) controlled depth machining promises to be one of the most flexible non-conventional structuring techniques for TBC materials. Selective TBC stripping promises the flexibility of either complete or locally restricted TBC removal without damaging or contaminating the bond coat. For economic industrial use of this environmental-friendly process a complete Computer-aided (CAx) toolchain comprising Computer-aided drawing (CAD), Computer-aided manufacturing (CAM) and complementary process analysis is needed. Presently no suitable CAx toolchain supports this innovative PWJ process for defined layer machining of composite parts.This paper shows the integration of PWJ controlled depth machining process within CAx toolchain in Siemens NX for selective stripping TBC on complex free-form surfaces. The strong dependence of the machining results on energy input per unit length leads to challenges for synchronization of parametrization and machine dynamics. The evaluation of different machining strategies and machining head configurations resulted in an increasing process performance and economic efficiency of this developed PWJ process.

Phase, compositional, structural, and chemical stability of La2Ce2O7 after high temperature heat treatment

High temperature stability of potential materials for thermal barrier coatings becomes increasingly important at elevated temperatures. Therefore, high temperature stability, including phase, compositional, structural, and chemical stability of lanthanum cerate (La2Ce2O7, LC) with yttria stabilized zirconia (YSZ) was investigated after heat treatment at 1300 °C. LC was found to possess excellent phase and compositional stability because of unique fluorite structure. In addition, the mole ration between Ce and La elements was close to 1:1 after prolonged high temperature treatment at 1300 °C. However, LC exhibited poor structural and chemical stability due to poor sintering resistance and severe reaction with YSZ. Rapid grain coarsening and pore-healing were observed. Moreover, interdiffusion reaction between LC and YSZ induced by the large difference in Gibbs free energy values led to the formation of pores with high sintering resistance. Two new materials with pyrochlore and fluorite crystal structures were also present in annealed composite samples.

Effects of doping Yb3+, La3+, Ti4+, Hf4+, Ce4+ cations on the mechanical properties, thermal conductivity, and electronic structures of Gd2Zr2O7

The structural, mechanical, thermal, and electronic properties of a series of Gd-site and Zr-site substituted Gd2Zr2O7 pyrochlores have been investigated by first-principles calculations. Substitution of Gd3+ with smaller and heavier Yb3+ introduces strong phonon scattering effects and results in Gd1.5Yb0.5Zr2O7 exhibiting the lowest thermal conductivity. Substitution of Gd3+ with larger and lighter La3+ does not give as much reduction in the thermal conductivity as in Yb3+-doped Gd2Zr2O7. As for Ti4+, Hf4+ and Ce4+ substitution for Zr4+-site in Gd2Zr2O7, slight changes in thermo-physical properties are observed for Gd2Zr2-yTiyO7 and Gd2Zr2-yHfyO7 pyrochlores, while the substitution of Ce4+ for Zr4+ site results in significantly smaller Young's modulus, better ductility, smaller Debye temperature, and lower thermal conductivity. With the increasing Ce content, the Young's modulus subsequently decreases by 22.2–59.9 GPa. The thermal conductivity of Gd2Zr2O7 is significantly reduced by 21% with complete Ce doping at the Zr-site based on the Clarke's model. This is mainly caused by the fact that larger guest ions incorporation weakens the bonds and enhances the phonon scattering in the host oxides and thus effectively lowers the thermal conductivity. Considering the improved thermo-physical properties, the Gd2Zr2-yCeyO7 may be a more promising candidate of the next-generation topcoat materials for thermal barrier coatings than the pure Gd2Zr2O7. This study provides a theoretical perspective to design and select good thermally insulating materials at high temperatures.

CaO‐MgO‐Al2O3‐SiO2 corrosion behavior of air‐plasma‐sprayed (LaxYb1−x)2Zr2O7

AbstractThe CaO‐MgO‐Al2O3‐SiO2 (CMAS) corrosion of thermal barrier coatings (TBCs) is a crucial problem for the lifetime of blades and vanes of jet engine and gas turbine at high operating temperature. Although many new alternative materials for TBCs have been developed in recent years, their application is limited by the CMAS corrosion. On the other hand, the composition difference of CMAS between regions makes solving this problem very difficult. Therefore, in this study, the yearly composition changes of sand‐dust around Beijing area were investigated. The high‐temperature corrosion behavior of air‐plasma‐sprayed 8YSZ and newly developed (LaxYb1−x)2Zr2O7 TBCs by the representative sand‐dust of Beijing was investigated. In comparison, a universally used CaO‐riched composition of simulated silicate deposit was also adopted for the TBCs corrosion test. It is found that the (LaxYb1−x)2Zr2O7 TBCs performs much better anti‐corrosion behavior than that of 8YSZ, both by Beijing sand‐dust and simulated one. Particularly, Yb2Zr2O7 TBCs appear to be the optimum material against silicate deposits attack. The mechanism of silicate deposits corrosion has also been discussed.

Synthesis and thermal properties of zirconate, hafnate and cerate of samarium

The basic direction of the development of thermal barrier coatings (TBC) systems is connected with new materials solutions dedicated for bond-coats and ceramic insulating layers. The present article shows the synthesis and thermal properties of different types of insulating ceramic materials based on zirconia, hafnia and ceria. The analyzed materials were synthesized via solid state reaction (SSR) of the mixtures of nanocrystalline feedstock powders of zirconia, hafnia, ceria and samaria. The synthesized materials were analyzed in view of chemical and phase composition. The morphological characterization of the utilized feedstock powders was showed as well. The basic range of the investigation was related to the analysis of thermal parameters such as a thermal diffusivity and coefficient of thermal expansion in temperature range 25 ÷ 1100 °C. The P-type samarium zirconate and hafnate were synthesized as well as F-type non-stoichiometric samarium cerate. The chemical homogeneity of obtained solid materials was satisfactory. The dilatometric measurements showed that Sm2Zr2O7 and Sm2Hf2O7 may be beneficial materials for TBC insulating layer due to the thermal expansion coefficient close to metallic substrate. These materials were also beneficial taking into account the results of thermal diffusivity measurements. In the temperature range 25–1100 °C, samarium hafnate and zirconate were characterized by substantially lower thermal diffusivity compared to that of 8YSZ. In the case of cerate of samarium with fluorite type of structure, the more beneficial thermal properties compared to that of YSZ were observed, however, were considerably less promising related to pyrochlore ceramic materials based on zirconia and hafnia.

Thermo-Physical Properties Measurement of Advanced TBC Materials with Pyrochlore and Perovskite Structures

Thermal barrier coatings (TBCs) serve as thermal insulator in the hot region of an aircraft engine. Besides this, it also protects the underlying metal surface from the harsh corrosive and eroding environment. The associated lower thermal conductivity of TBC ceramic materials plays an important role in the improvement of thermal efficiency of the engine in term of increased combustion temperature and power. The thermal conductivity of the conventional yttria stabilized zirconia (YSZ) and three advanced ceramic materials with perovskite (CaZrO3) and pyrochlore structure (La0.75Nd0.25)2Zr2O7& Nd2Ce2O7) have been determined using differential scanning calorimetry (DSC). With thin metallic disk on the ceramic samples of different heights were heated / scanned using a standard DSC apparatus. The results were evaluated for the thermal conductivity measurement using well established procedure /calculations. The analyzed results were compared with that of other techniques given by other researchers and found to be in good agreement with an error of 10-15%. The result of coefficient of thermal expansion (CTE) that was measured using a dilatometer up to 1273°K has also given.

A theoretical study on the effects of thermal barrier coating on diesel engine combustion and emission characteristics

In recent years, thermal barrier coating (TBC) has been used as an effective way to reduce the heat transfer losses and to improve the thermal efficiency of internal combustion (IC) engine. In this study, a mathematic model has been proposed by taking the TBC material parameters into account, which is applied to explore the effects of TBC on engine combustion and emissions. Details of the combustion process is analyzed for the coated and uncoated engines under both low and high load conditions. The result shows that TBC has the ability of reducing wall heat transfer losses, thus improving the indicated thermal efficiency. A redistribution of different temperature regions is found with TBC and the coating shows better performance at rich mixture region due to higher temperature increase rate and at squish region due to higher surface/volume ratio. Both the soot oxidation process and NOx formation process are accelerated with TBC. However, TBC also enhances the overlap region between the high soot region and high temperature region, which accelerates the soot oxidation rate and greatly improves the soot emission.

Evaluation of LaAlO3 as top coat material for thermal barrier coatings

Perovskite is a versatile group of oxide materials allowing their properties to be tailored by composition towards specific requirements. LaAlO3 was prepared to study and report its properties in the context of its potential in thermal barrier coatings (TBCs) technology. A citric acid method was used for synthesis and the perovskite structure was confirmed using XRD and FT-IR. Viscosity of the solution precursor was checked as well as the particle size by laser particle size analysis. Densification behavior of the material was followed by conventional sintering and by spark plasma sintering. Apparent porosity by the Archimedes method, thermal conductivity and thermal expansion coefficient were studied. Mechanical and fracture properties were measured at elevated temperatures up to 1300 °C. For samples sintered at 1200−1400 °C, coefficient of thermal expansion ranged from 5.5×10−6 to 6.5×10−6 K−1 and thermal conductivity ranged between 2.2 and 3.4 W/(m·K). Elastic modulus and ultimate stress were measured at 1000−1300 °C, while by micro-indentation, fracture toughness was found to be 3 MPa·m1/2. As the sintering temperature increased from 1200 to 1500 °C, significant densification from 3.21 to 5.81 g/cm3 was found, indicating that material annealing should be made at least at 1400 °C. Under this condition, negligible dimensional change in phase transition temperature of LaAlO3 from the rhombohedral (R3c) to the ideal cubic (Pm3m) is found. Data reported in this work can be useful for comparing the mechanical and fracture behaviours of different TBCs developed involving LaAlO3 as well as input for numerical simulations.

Bi-doped lanthanum molybdate: Enhancing the anharmonicity and reducing the thermal conductivity using Bi3+ with lone pair electrons

La2Mo2O9 has a considerably low thermal conductivity (0.95 W m−1 K−1) at 1273 K, making it an attractive material for thermal barrier coatings. To further reduce the thermal conductivity of La2Mo2O9, we focused on the anharmonicity of the lattice induced by lone pair electrons. We substituted the La3+ site of La2Mo2O9 by Bi3+, which has lone pair electrons, and studied the effect of bismuth doping on the anharmonicity and thermal conductivity of La2Mo2O9. Bismuth doping enhanced the Grüneisen parameter, which is an indicator of anharmonicity, and reduced the thermal conductivity by 24% with respect to non-doped La2Mo2O9 at 300 K by substituting with 6 mol% Bi. Theoretical predictions based on the Klemens-Callaway model with experimentally obtained Grüneisen parameters were performed to analyze the thermal conductivity; the reduction in thermal conductivity is mainly attributed to the enhanced anharmonicity provided by Bi substitution.

Pronounced enhancement of thermal expansion coefficients of rare-earth zirconate by cerium doping

Rare-earth zirconates (RE2Zr2O7, RE = rare earth elements) are considered as promising thermal barrier coating (TBCs) materials. However, their thermal expansion coefficient (TEC) is relatively low comparing with that of either the state-of-the-art TBCs material Yttria-stabilized Zirconia (YSZ), or the metallic bond-coat. In present research, Cerium was introduced as the substitution of Zirconium to improve the TEC of rare-earth zirconates. A series of Yb2(Zr1−xCex)2O7 solid solutions were synthesized and investigated. Experimental results revealed that their TEC is remarkably increased with the Cerium substitution content. Meanwhile, their thermal conductivity shows no obvious increase compared with that of Yb2Zr2O7due to the phonon scattering effect.

Optimization of Thermal Barrier Coating Material for Diesel Engine Cylinder Liner

Optimization of Thermal Barrier Coating Material for Diesel Engine Cylinder Liner

Hot corrosion behavior of BaLa2Ti3O10 exposed to calcium-magnesium-alumina-silicate at elevated temperatures

Calcium-magnesium-alumina-silicate (CMAS) attack has been regarded as one of the significant failure mechanisms for thermal barrier coatings (TBCs). In this study, CMAS corrosion behavior of BaLa2Ti3O10, a novel TBC material, is investigated at 1300 °C and 1350 °C for 0.5 h, 4 h, 12 h and 24 h. Results reveal that BaLa2Ti3O10 has high resistance to molten CMAS infiltration, attributable to the formation of a dense reaction layer. X-ray diffraction, scanning electron microscope, energy dispersive spectroscope, transmission electron microscope confirm that the layer consists of apatite, celsian and perovskite phases. With increased corrosion duration, the layer retains good phase stability and the thickness increases. The formation of corrosion products and the reaction layer are discussed according to a dissolution-reprecipitation mechanism and the optical basicity theory.

Microstructure and mechanical properties of plasma sprayed Al2O3-YSZ composite coatings

Alumina-yttria stabilized zirconia (Al2O3-YSZ) composite coatings are recognized as a potential alternative material for thermal barrier coatings. In this paper, Al2O3-YSZ composite coatings were deposited using atmosphere plasma spraying. The effects of alumina addition and annealing treatment on the microstructure and mechanical properties of the plasma sprayed Al2O3-YSZ composite coatings were investigated by XRD, SEM and an instrumented indentation testing system. Owing to the existence of YSZ in the composite coatings, the formation of amorphous alumina was suppressed. The quenching stress is high enough to trigger microcracking, these microcracks will further propagate or close under the post deposition thermal mismatch stress. Young's modulus of the coatings keeps an upward tendency from 50.56 GPa to 55.93 GPa and hardness increase from 5.36 GPa to 5.93 GPa with alumina contents varying from 10 wt% to 55 wt%. After an annealing treatment, Young's modulus and hardness increase compared with the as-sprayed ones. Fracture toughness of Al2O3-YSZ composite coatings is higher than the YSZ one, and it increases firstly and then decreases with alumina addition varying from 10 wt% to 55 wt%. The fracture toughness of coatings is related to the amount of heterogeneous specific interface between Al2O3 and YSZ splats and the high microcracks density in the composite coatings.

Phase stability, thermo-physical properties and thermal cycling behavior of plasma-sprayed CTZ, CTZ/YSZ thermal barrier coatings

ZrO2 co-stabilized by CeO2 and TiO2 with stable, nontransformable tetragonal phase has attracted much attention as a potential material for thermal barrier coatings (TBCs) applied at temperatures > 1200 °C. In this study, ZrO2 co-stabilized by 15 mol% CeO2 and 5 mol% TiO2 (CTZ) and CTZ/YSZ (zirconia stabilized by 7.4 wt% Y2O3) double-ceramic-layer TBCs were respectively deposited by atmospheric plasma spraying. The microstructures, phase stability and thermo-physical properties of the CTZ coating were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric-differential scanning calorimeter (TG-DSC), laser pulses and dilatometry. Results showed that the CTZ coating with single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300 °C. The CTZ coating had a lower thermal conductivity than that of YSZ coating, decreasing from 0.89 W m−1 K−1 to 0.76 W m−1 K−1 with increasing temperature from room temperature to 1000 °C. The thermal expansion coefficients were in the range of 8.98 × 10−6 K−1 – 9.88 ×10−6 K−1. Samples were also thermally cycled at 1000 °C and 1100 °C. Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate, the severe coating sintering and the reduction-oxidation of cerium oxide. The thermal durability of the TBCs at 1000 °C can be effectively enhanced by using a YSZ buffer layer, while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at 1100 °C was still unsatisfying. The thermal shock resistance of the CTZ coating should be improved; otherwise the promising properties of CTZ could not be transferred to a well-functioning coating.

Experimental analysis on thermally coated diesel engine with neem oil methyl ester and its blends

Depletion of fossil fuel has created a threat to the nation’s energy policy, which in turn led to the development of new source renewable of energy. Biodiesel was considered as the most promising alternative to the traditional fossil fuel. In the present study, raw neem oil was considered as a principle source for the production of biodiesel and converted into Neem Oil Methyl Ester (NOME) using two stage transesterification process. The chemical compositions of NOME was analysed using Fourier Transform Infra-Red Spectroscopy (FTIR) and Gas Chromatography- Mass Spectrometry (GC-MS). Baseline readings were recorded with diesel, 25NOME (25% NOME with 75% diesel) and 50NOME (50% NOME with 50% diesel) in a direct injection, four stroke, water cooled diesel engine. Thermal Barrier Coating (TBC) was considered as a better technique for emission reduction in direct injection diesel engine. In the present study, Partially Stabilized Zirconia (PSZ) was used as a TBC material to coat the combustion chamber components like cylinder head, piston head and intake and exhaust valves. In coated engine, 25NOME showed better brake thermal efficiency and declined brake specific fuel consumption than 50NOME. Decreased exhaust emissions like CO, HC and smoke were observed with 25NOME in coated engine except NOx.

Integrated testing approach using a customized micro turbine for a volcanic ash and CMAS related degradation study of thermal barrier coatings

A volcanic ash (VA) test stand has been built with a precise particle feeder, concentration measuring unit and a mini turbine with an integrated thermal barrier coating (TBC) system on the blisk for the first time in a laboratory scale. This setup has allowed the testing of melting, sticking, corrosion and erosion behaviour of volcanic ash in-situ at high temperature in a realistic manner in a gas turbine. Moreover, this mini engine runs with kerosene which could also bring all the impurities such as sulphur in to system which might change the reaction mechanisms of TBC and VA. The particle size influence on the sticking/escape behaviour in the engine components was tested but a clear statement on the particle size versus its sticking behaviour could not be made, although the particles in the size range of 1–10 μm are more susceptible to melting in the given conditions. The blisks were coated with two different TBC materials such as 7 wt% yttria stabilised zirconia (7YSZ) and gadolinium zirconate (GZO) using electron beam physical vapour deposition (EB-PVD) technique. Both coatings were tested in corrosion-erosion regime under VA at high temperature and it was found out that the coating with higher toughness (7YSZ) has performed better than the other coating under the used testing conditions.

Intermetallic Al-, Fe-, Co- and Ni-Based Thermal Barrier Coatings Prepared by Cold Spray for Applications on Low Heat Rejection Diesel Engines

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bond coat and a ceramic heat insulating topcoat. They possess the desired low thermal conductivity, but at the same time they are very brittle and sensitive to thermal shock and thermal cycling due to the inherently low coefficient of thermal expansion. Recent research activities are focused on the developing of multilayer TBC structures obtained using cold spraying and following annealing. Aluminum intermetallics have demonstrated thermal and mechanical properties that allow them to be used as the alternative TBC materials, while the intermetallic layers can be additionally optimized to achieve superior thermal physical properties. One example is the six layer TBC structure in which cold sprayed Al-based intermetallics are synthesized by annealing in nitrogen atmosphere. These multilayer coating systems demonstrated an improved thermal fatigue capability as compared to conventional ceramic TBC. The microstructures and properties of the coatings were characterized by SEM, EDS and mechanical tests to define the TBC material properties and intermetallic formation mechanisms.