Multilayered thermal barrier coatings

A novel non-destructive inspection technique was developed to measure the residual stresses in TBCs by using Eu3+ photoluminescence piezo-spectroscopy. The relationship between the strongest peak of 5D0→7F2 and stress was determined by the high-pressure experiments and used to evaluate the residual stresses. When the top ceramic coat LZ7C3 in the double-layered coating LZ7C3/YSZ:Eu spalled, the inner YSZ:Eu could produce visible luminescence under UV illumination, providing an indication of the spallation location. The LZ7C3/YSZ:Eu coating spalled bit by bit from LZ7C3 to YSZ:Eu during thermal cycling. The similar thermal expansions of LZ7C3 and YSZ:Eu prolonged the thermal cycling life of the coating

Correlation between porosity, amorphous phase and CMAS corrosion behaviour of LaMgAl11O19 thermal barrier coatings

Calcium-magnesium-alumino-silicate (CMAS) attack is one of the significant failure mechanisms of thermal barrier coatings (TBCs), which can facilitate TBC’s degradation at elevated temperatures. To clarify the correlation between the porosity, CMAS corrosion behaviour, lanthanum magnesium hexaluminate (LaMgAl11O19, LMA) TBCs were prepared by atmospheric plasma spraying (APS) and then heat-treated at 1173K and 1523K, respectively. For comparison, LMA tablets were prepared by mechanical and cold isostatic pressing. CMAS attack at 1523K was carried out both for LMA tablets and LMA coatings. Their microstructure, phase composition, and crystallization behavior after CMAS attack were investigated using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction as well as differential scanning calorimetry (DSC). The results indicated that CMAS attack was arrested for LMA tablets due to the formation of a dense crystalline layer induced by the chemical interactions between LMA and CMAS glass, while the as-sprayed LMA coatings were completely penetrated by molten CMAS due to the presence of amorphous phase and connected pores. Although the isothermal heat-treatment promoted a crystallization of LMA coatings, much vertical cracks formed during heat-treatment. The heat-treated LMA coatings suffered a severer CMAS attack than the as-sprayed one, since the vertical cracks inevitably provided efficient infiltration paths for molten CMAS

Calcium-magnesium-alumino-silicate induced degradation of La2(Zr0.7Ce0.3)2O7/YSZ double-ceramic–layer thermal barrier coatings prepared by electron beam-physical vapor deposition

During last decades, much effort has been made to develop new alternative thermal barrier coating (TBC) to traditional YSZ for applications above 1250oC. La2(Zr0.7Ce0.3)2O7(LZ7C3) is deemed as a very promising TBC candidate for advanced gas turbine because of its extremely low thermal conductivity, high sintering resistance and phase stability from room temperature to 1600oC. Thermal cycling with a gas burner showed that the LZ7C3/YSZ double-ceramic-layer (DCL) coatings prepared by electron beam-physical vapor deposition (EB-PVD) or atmospheric plasma spraying had a much longer lifetime than that of YSZ coating at 1250± 50oC.The use of the new TBC can allow higher gas temperatures, resulting in further improved thermal efficiency and engine performance. However, at these high operating temperatures, TBCs become susceptible to attack by calcium-magnesium-alumino-silicate (CMAS, relative to the main chemical components Ca, Mg, Al and Si) deposits resulting from the ingestion of siliceous minerals (dust, sand, volcanic ash, runway debris) with the intake air. CMAS becomes molten at temperatures above 1200oC and then rapidly penetrates the TBCs by capillary force, resulting in the loss of strain tolerance and premature failure of the coatings. In this paper, CMAS induced degradation of LZ7C3/YSZ DCL coatings prepared by EB-PVD method were investigated. Hot corrosion tests were performed at 1250oC at durations varying from 0.5 h to 24 h. It is observed that the infiltration of CMAS in the intercolumnar gaps was largely suppressed in the case of EB-PVD LZ7C3 coating. The penetration depth rarely exceeded 40 μm below the original surface even after 24 h exposure at 1250oC. This was ascribed to rapid dissolution of the LZ7C3 and essentially concurrent formation of a sealing layer made of crystalline apatite and fluorite phases, which is consistent with the observation on Gd2Zr2O7. However, large vertical cracks would form in the EB-PVD LZ7C3 coating during thermal cycling as a result of re-crystallization, sintering and thermal expansion mismatch between ceramic coating and substrate. These vertical cracks can also act as channels to CMAS melt infiltration. Since the kinetics of the dissolve-reprecipitation reaction was slower than the infiltration rate of CMAS in the vertical crack, the majority of vertical cracks were not sealed. As a result, CMAS flowed down to the LZ7C3/YSZ interface along the vertical cracks, and then easily penetrated the YSZ buffer layer by capillary force. Chemical interaction also occurred in the YSZ buffer layer. What\u27s more, the YSZ layer in the DCL coating even underwent a severer CMAS attack than the single YSZ coating. After 4 h CMAS exposure, the YSZ layer of the LZ7C3/YSZ bilayer coating was totally dissolved by molten CMAS followed by precipitation of a large number of globular ZrO2 particles, while the single YSZ coating just suffered a slight degradation in the same experimental conditions and still kept its columnar structure. The probable reason was that the CMAS melt in the YSZ layer of the DCL coating had a higher CaO/SiO2 ration than the original CMAS composition due to the formation of apatite phase when CMAS reacted with the upper LZ7C3 layer. The initial Si: Ca ratio (Si: Ca≈1.4) in CMAS melt is less than the corresponding apatite (Si: Ca≈3), leading to progressive CaO enrichment during apatite crystallization. For this reason, it is suggested that the effectiveness of the CMAS mitigation strategy for YSZ TBCs by adopting a so-called CMAS-resistant top layer needs to be assessed in the context of more realistic conditions. If the formation of large vertical cracks in TBCs was not avoided, this CMAS mitigation approach may not as effective as expected

Thermal barrier coatings on polymer materials

Polyimide matrix composite (PIMC) has been widely used to replace metallic parts due to its low density and high strength. It is considered as an effective approach to improve thermal oxidation resistance, operation temperature and lifetime of PIMC by depositing a protection coating. The objective of the research was to fabricate a series of thermal barrier coatings (TBCs) on PIMC by a combined sol-gel/sealing treatment process and air plasma spraying (APS). By optimizing the experimental parameters, thermal shock resistance, thermal oxidation resistance and thermal ablation resistance of PIMC could be improved significantly. The ZrO2 sol was prepared by sol-gel process and the effects of the different organic additions on phase structure, crystallite size and crystal growth behavior of the ZrO2 nanocrystallite were investigated. The addition of HAc and DMF were beneficial to decrease the crystallite size and alter the activation energy for crystal growth, further inducing the crystallization of ZrO2 nanocrystallite at low temperature (300ºC) and the stability of tetragonal ZrO2 at 600ºC. Based on the optimized parameters of the sol preparation, the ZrO2/phosphates duplex coating was fabricated on PIMC via a combined sol-gel and sealing treatment process. The sealing mechanism of the phosphates in the duplex coating was primarily attributed to the adhesive binding of the phosphates and the chemical bonding between the sealant and the coating. It was demonstrated that the duplex coating exhibited excellent thermal shock resistance and no apparent delamination or spallation occurred. Relatively, the duplex coating with the thickness of 150 μm provided excellent thermal oxidation and thermal ablation resistance for the polymer substrate. However, the presence of cracks and delamination in the coatings provided the channels for oxygen diffusion, causing the final failure of the protection coating. Figure 4 – TBCs on CFPI The Zn/YSZ and Al/YSZ coating systems were successfully deposited on PIMC by APS. Metals with comparatively low melting point as the bond coats (Cu, Al, Zn) were beneficial to increase thermal shock resistance of the coating systems. In comparison with the Al/YSZ coating system, the Zn/YSZ coating exhibited the better thermal shock resistance, which was ascribable to the lower residual stress in the Zn layer after deposition and the lower thermal stress induced during thermal shock test. For these coatings, the increase in surface toughness of the substrate as well as the decrease in thickness of metal layer favored the improvement of thermal shock resistance of the coatings. With the temperature increases, thermal shock lifetime of the coatings decreased disastrously. However, the difference was that the slight increase of the thickness of YSZ layer favored the increase in thermal shock resistance of the Al/YSZ coatings, while for the Zn/YSZ coating systems the increase in the thickness of YSZ layer made thermal shock resistance weaken. Owing to the protection of Zn/YSZ and Al/YSZ coating systems, the time for 5 wt% weight loss of the sample was prolonged from 16 h to 50 h when oxidation at 400ºC; as the oxidation temperature increased to 450ºC, the time for 5wt% weight loss was extended from 5 h to 13 h. By depositing different coatings, the anti-ablation property of PIMC was significantly improved. During property testing, the formation of cracks and delamination in the coating and the occurrence of the spallation led to the failure of the coating systems, which was mainly due to the residual stress during the deposition process, thermal stress induced by the mismatch in thermal expansion coefficient and further oxidation of the substrate. Please click Additional Files below to see the full abstract

Thermal cycling behavior and hot corrosion performance of the plasma sprayed Er2Si2O7 coatings deposited on Cf/SiC composites

AbstractRare-earth based disilicates are promising coating materials for Cf/SiC composites. In this study, Er2Si2O7 coating was deposited on the Cf/SiC substrate by atmospheric plasma spraying using the solid-state reaction synthesized powders. Sintered Er2Si2O7 coupons derived from the synthesized powders were subjected to dilatometric studies showing the average value of 4.3ppm/°C for the coefficient of thermal expansion. The coated Cf/SiC coupons were thermally cycled between 400°C and 1500°C on a burner rig facility until failure of coating. Meanwhile, hot corrosion performance of the free standing plasma sprayed Er2Si2O7 coating coupon was evaluated by using molten vanadate salt with loading of 15mg/cm2 at 800°C. Analytical techniques such as X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy were applied to study the microstructural characteristics of the powders and coatings before and after thermal cycling and hot corrosion tests. Coatings’ degradation as a result of thermal cycling and hot corrosion tests was investigated in terms of microstructure and microchemistry transformations

An in-situ method for assessing soil aggregate stability in burned landscapes

Due to soil repellency in burned areas, slope runoff and soil erodibility escalates following forest fires, increasing the vulnerability to post-fire debris flows. Soil aggregate stability is a critical determinant of soil infiltration capacity and erosion susceptibility. The prevalent method of assessing soil aggregate stability in burned areas, the counting the number of water drop impacts (CND) method, is time-intensive and impractical for in-situ measurements. In response, this study introduces a novel technique based on the shock and vibration damage (SVD) effect for evaluating soil aggregate stability in burned areas. Thirteen distinct soil aggregate types were meticulously prepared for indoor simulated fire testing, with due consideration to factors such as bulk weight, organic matter content, and water repellency, which influence stability of soil aggregates. Employing a custom-built test apparatus, the mass loss rate (MLR) of soil aggregates was determined through orthogonal experiments using the SVD method and compared against the standard CND technique's quantification of water droplet-induced aggregate destruction. The findings demonstrated that SVD method, employing Test Scheme 6 (testing 20 aggregates, 1-meter impact height, 40% water content, and five impacts), exhibits excellent agreement (Kendall coefficient = 0.797) and correlation (R2 = 0.634) with CND method outcomes. This testing scheme, characterized by rapid determination and effective discrimination, is identified as the optimal testing approach. The SVD testing apparatus is straightforward, portable, and easily disassembled, rendering it suitable for on-site use. It can be used to distinguish the stability level of soil aggregates swiftly and quantitatively under various fire intensities in burned areas in situ, which is an important guiding significance for the study of soil erosion, erosion control, and post-fire debris flow initiation mechanism in burned areas

Zinc Finger-Homeodomain Transcriptional Factors (ZHDs) in Upland Cotton (Gossypium hirsutum): Genome-Wide Identification and Expression Analysis in Fiber Development

Zinc finger-homeodomain (ZHD) genes encode a family of plant-specific transcription factors that not only participate in the regulation of plant growth and development but also play an important role in the response to abiotic stress. The ZHD gene family has been studied in several model plants, including Solanum lycopersicum, Zea mays, Oryza sativa, and Arabidopsis thaliana. However, a comprehensive study of the genes of the ZHD family and their roles in fiber development and pigmentation in upland cotton has not been completed. To address this gap, we selected a brown fiber cultivar for our study; brown color in cotton is one of the most desired colors in the textile industry. The natural colored fibers require less processing and little dying, thereby eliminating dye costs and chemical residues. Using bioinformatics approaches, we identified 37 GhZHD genes from Gossypium hirsutum and then divided these genes into seven groups based on their phylogeny. The GhZHD genes were mostly conserved in each subfamily with minor variations in motif distribution and gene structure. These genes were largely distributed on 19 of the 26 upland cotton chromosomes. Among the Gossypium genomes, the paralogs and orthologs of the GhZHD genes were identified and further characterized. Furthermore, among the paralogs, we observed that the ZHD family duplications in Gossypium genomes (G. hirsutum, G. arboreum, and G. raimondii) were probably derived from segmental duplication or genome-wide duplication (GWD) events. Through a combination of qRT-PCR and proanthocyanidins (PA) accumulation analyses in brown cotton fibers, we concluded that the candidate genes involved in early fiber development and fiber pigment synthesis include the following: GhZHD29, GhZHD35, GhZHD30, GhZHD31, GhZHD11, GhZHD27, GhZHD18, GhZHD15, GhZHD16, GhZHD22, GhZHD6, GhZHD33, GhZHD13, GhZHD5, and GhZHD23. This study delivers insights into the evolution of the GhZHD genes in brown cotton, serves as a valuable resource for further studies, and identifies the conditions necessary for improving the quality of brown cotton fiber

Determining acute ischemic stroke onset time using machine learning and radiomics features of infarct lesions and whole brain

Accurate determination of the onset time in acute ischemic stroke (AIS) patients helps to formulate more beneficial treatment plans and plays a vital role in the recovery of patients. Considering that the whole brain may contain some critical information, we combined the Radiomics features of infarct lesions and whole brain to improve the prediction accuracy. First, the radiomics features of infarct lesions and whole brain were separately calculated using apparent diffusion coefficient (ADC), diffusion-weighted imaging (DWI) and fluid-attenuated inversion recovery (FLAIR) sequences of AIS patients with clear onset time. Then, the least absolute shrinkage and selection operator (Lasso) was used to select features. Four experimental groups were generated according to combination strategies: Features in infarct lesions (IL), features in whole brain (WB), direct combination of them (IW) and Lasso selection again after direct combination (IWS), which were used to evaluate the predictive performance. The results of ten-fold cross-validation showed that IWS achieved the best AUC of 0.904, which improved by 13.5% compared with IL (0.769), by 18.7% compared with WB (0.717) and 4.2% compared with IW (0.862). In conclusion, combining infarct lesions and whole brain features from multiple sequences can further improve the accuracy of AIS onset time

Development of New Thermal Barrier Coatings for Gas Turbines

Keywords: Thermal Barrier Coatings, Materials, Lanthanum Zirconate, Lanthanum Cerate, Spray-Drying ; New thermal barrier coating (TBC) materials for gas turbines were studied in this work. There are numerous factors to determine the application lives of TBCs, and those factors can be basically grouped into: 1) Properties of the material: melting point, thermal expansion coefficient, thermal conductivity, phase stability between room temperature and the application temperature, sintering ability, chemical stability, purity of the material, etc. 2) Preparation technique of TBCs: there are several techniques to prepare TBCs, such as Electron Beam-Physical Vapor Deposition (EB-PVD), Plasma Spraying (PS) and High Velocity Oxyfuel (HVOF). Due to the special columnar structure of TBCs made by EB-PVD, the as-resulted coatings have much longer thermal cycling lives than those made by other techniques. The preparation procedure of each technique also has great influence on the coating quality. 3) Microstructures of TBCs : distribution of the micropores, direction of the microcracks (vertical or parallel to the coating surface), thickness of the coating, graded distribution of the compositions, etc. 4) Other factors: properties of the bond coat, pre-treatment of the metal substrate, morphology of the powder for the coating, etc. In this thesis, I concentrated my energy on the improvement of two of the most important properties of the material for TBCs, i.e. thermal expansion coefficient and phase stability, and also the powder preparation by spray-drying. Following are the main points of the work: 1.1. Spray-drying is the process by which a fluid feed material is transformed into a dry powder by spraying the feed into a hot medium. The feed materials are either water-based suspensions with air as the drying gas or ethanol-based suspensions with nitrogen as drying-gas. In general, there are five effects to influence the characteristics of spray-dried powders, i.e. type of atomizer, solid content of the suspension, feeding speed, drying temperature and feeding pressure. Powder characteristics such as size distribution, shape, density and purity have a significant influence on the thermal spraying process and the coating properties. Spray-dried powders offer an advantage in the application in TBCs where a high porosity (10-20%) is necessary. On the other hand, particles of the spray-dried powders are usually spherical and free-flowing, the powder can be easily fed by the plasma gun. However, not all the spray-dried powders are suitable for thermal spraying. The powder should be dense enough and have a proper size distribution. If the powder is too fine and too porous, the powder can not enter the centre of the plasma flame and will float on the flame surface or be evaporated by superheating before sputtering on the substrate, resulting in a poor deposition efficiency and poor coating-bond strength. In our case, the powder with high density and large particle size is preferred for the plasma spraying of coatings. In this work, we tried to improve the quality of TBCs by using the spray-dried powders and look for new candidates for high temperature TBCs (> 1523 K). The drying machine used in this work was Mobile Minor$^{TM'2ooo'}$ Ex Model H (Niro A/S), pneumatic nozzle and a drying capacity 5 Kg(water)$\cdot$h$^{-1}$. The inner diameter $\varnothing_{inner}$ of the drying chamber was 0.85 m. The two most important properties of spray-dried powders to determine the coating quality are density and particle size. Polyethyleneimine (PEI) acts as both an organic binder and a dispersant giving low viscosity in the suspension. The optimized suspension composition is : $\geq$ 33.6 vol% powder + 1.8 wt% PEI + ethanol, and operational parameters of the drying machine: drying temperature 448 K, feeding rate 55 cm$^{3}$ $\cdot$ min$^{-1}$, feeding pressure 1.013 x 10$^{4}$ Pa. [...