Effect of five kinds of pores shape on thermal stress properties of thermal barrier coatings by finite element method

Thermal ablation is a very important technique to characterize the thermal properties of coating systems. On account of the concentration of thermal stress, thermal barrier coatings (TBCs) often break off from the substrate partly or completely during the thermal erosion. In this paper, the thermal erosion simulation of finite element geometric models based on the possible pore shapes were implemented, especially, the influence of pore shapes on the distribution of coating temperature, X component of stress, Y component of stress, XY-shear stress and von-Mises stress were focused on. The effects of the different porosity of square pore coatings on thermal insulation properties and thermal stresses were discussed in term of the simulation results. The simulation results indicate that different shape pores not only affect the thermal stress distribution above the contact area between the bond coating and top coating surface, but also affect the plastic deformation behavior of TBCs. The micromechanism was discussed in details in this study. The computed results verified that, the computational method can successfully predict thermal shear, crack initiation and normal failure mode of the studied TBCs. All the results are in good agreement with the corresponding experimental findings. The failure mechanism factors in this paper are of great importance to explain the failure micro-mechanism of TBCs.     

Optimal design on the mechanical and thermal properties of porous alumina ceramics based on fractal dimension analysis

In order to investigate the relationship between pore structure and thermal conductivity as well as mechanical strength, porous alumina ceramics (PAC) with various pore structures were fabricated, using starch as the pore‐forming agent. Fractal theory was employed to characterize the pore size distribution more accurately than ever used parameters. The results show that the increase in starch content in PAC leads to an enhanced porosity, a higher mean pore size, and reduced fracture dimension, thermal conductivity and strength. The fractal analysis indicated that the fractal dimension decreases gradually and reaches its minimum value with increasing the starch content up to 25 wt%, but the further incorporation results in an opposite trend. It is suggested from micro‐pore fractographic analysis that the optimization of both thermal insulation performance and mechanical strength are positively correlated with the increase in the mean pore size and proportion of 2‐14 μm pores but negatively corrected with the porosity. These results provide a new perspective and a deeper understanding for fabrication of PAC with both excellent thermal insulation and mechanical performance.     

Plasma-sprayed nanostructured scandia-yttria and ceria-yttria codoped zirconia coatings: Microstructure,bonding strength and thermal insulation properties

Microstructure, thermal insulation behavior and bonding strength of nanostructured 5%scandia, 0.5%yttria codoped zirconia (5.5 ScYSZ) and 25%ceria, 2.5%yttria co stabilized zirconia (27.5%CYSZ) coatings were investigated. ScYSZ and CYSZ nanostructured granules (as a top coat) and commercial NiCrAlY powder (as a bond coat) were thermal sprayed on an Inconel 738 substrate. The results revealed ScYSZ coating to have higher bonding strength. Nevertheless, due to higher porosity and the presence of local strains, the thermal insulation of nanostructured CYSZ coating was higher than for ScYSZ coating.     

不同造孔剂对钢包永久衬用隔热浇注料的影响

钢水冶炼过程中通常利用保温隔热材料来提高钢包周转过程中的保温性能,以保证连铸钢水温度的变化适应炉外精炼过程和连铸过程的工艺需要.为了研制高气孔率、高强度的保温隔热材料,以烧结莫来石为骨料,研究了四种不同造孔剂(菱镁矿粉、聚苯乙烯球、塑料颗粒和漂珠)对浇注料显气孔率及强度的影响.实验结果表明添加菱镁矿粉作造孔剂时,与未添加或添加其他造孔剂相比,烧后试样显气孔率较低,而且当加入量超过10%时,试样强度出现显著提升;添加聚苯乙烯球、塑料颗粒或漂珠做造孔剂均能够明显提升烧后试样的显气孔率,其中添加聚苯乙烯球、塑料颗粒的试样随着造孔剂引入量的增加强度出现明显降低,而添加漂珠的试样随漂珠引入量的增加强度变化不明显;复合添加10%菱镁矿粉和5%塑料颗粒时试样气孔率为29.34%、耐压强度为42 MPa,体积密度为1.98 g·cm-3,综合指标最高,最适合于做为莫来石质钢包保温隔热浇注料的造孔剂.     

Improved mechanical strength and thermal resistance of porous SiC ceramics with gradient pore sizes

Thermal insulation applications of porous SiC ceramics require low thermal conductivity and high mechanical strength. However, low thermal conductivity and high mechanical strength possess a trade-off relationship, because improving the mechanical strength requires decreasing the porosity, which increases the thermal conductivity. In this study, we established a new strategy for improving both the mechanical strengths and thermal resistances of porous SiC ceramics with micron-sized pores by applying a double-layer coating with successively decreasing pore sizes (submicron- and nano-sized pores). This resulted in a unique gradient pore structure. The double-layer coating increased the flexural strengths and decreased the thermal conductivities of the porous SiC ceramics by 24–70 % and 29–49 % depending on the porosity (48–62 %), improving both their mechanical strengths and thermal resistances. This strategy may be applicable to other porous ceramics for thermal insulation applications.     

Microstructure–Property Correlations in Industrial Thermal Barrier Coatings

This paper describes the results from multidisciplinary characterization/scattering techniques used for the quantitative characterization of industrial thermal barrier coating (TBC) systems used in advanced gas turbines. While past requirements for TBCs primarily addressed the function of insulation/life extension of the metallic components, new demands necessitate a requirement for spallation resistance/strain tolerance, i.e., prime reliance, on the part of the TBC. In an extensive effort to incorporate these TBCs, a design-of-experiment approach was undertaken to develop tailored coating properties by processing under varied conditions. Efforts focusing on achieving durable/high-performance coatings led to dense vertically cracked (DVC) TBCs, exhibiting quasi-columnar microstructures approximating electron-beam physical-vapor-deposited (EB-PVD) coatings. Quantitative representation of the microstructural features in these vastly different coatings is obtained, in terms of porosity, opening dimensions, orientation, morphologies, and pore size distribution, by means of small-angle neutron scattering (SANS) and ultra-small-angle X-ray scattering (USAXS) studies. Such comprehensive characterization, coupled with elastic modulus and thermal conductivity measurements of the coatings, help establish relationships between microstructure and properties in a systematic manner.     

Macroporous mullite materials prepared by novel shaping strategies based on starch thermogelation for thermal insulation

A rigorous microstructural analysis of porous mullite materials developed using novel shaping strategies based on the starch consolidation casting, and their thermal properties in relation to the processing and starch type were accomplished in view of their use as thermal insulators. In order to characterize the size and morphology of pore, basic size and 2D shape factors, and global 3D stereological parameters were determined using microscopy techniques. Results indicated that the porosity volume, pore connectivity degree, and mean free path were the determining factors of the lowest heat transfer by conduction registered in materials prepared with cassava starch. This material is the best candidate to be used in thermal insulation.     

Enhancing thermal insulation and mechanical strength of porous ceramic through size-graded MA Hollow Spheres

In this study, a series of porous ceramics were prepared using different ratios of small and large size MA hollow ceramic spheres as pore-forming agents, and their thermal insulation properties were investigated. The results showed that increasing the proportion of small size hollow ceramic spheres could effectively decrease the thermal conductivity and improve the compressive strength of the porous ceramics. The optimal porous ceramic was prepared with a ratio of 10∼50 of small and large size hollow ceramic spheres, which had a thermal conductivity of 0.368 W/(m·K) at 800 °C and a compressive strength of 22.43 MPa. Microscopic analysis indicated that the enhanced thermal insulation and mechanical properties were due to the improved pore structure and the enhanced bonding strength between the ceramic spheres and the matrix. The findings provide valuable insights for the development of high-performance thermal insulation materials.     

Formation of vertical cracks in air plasma sprayed YSZ coatings using unpyrolyzed powder

Thick thermal barrier coatings (TTBCs) with vertical cracks deposited by air plasma spray (APS) and solution precursor plasma spray (SPPS) techniques have been widely investigated to achieve good thermal insulation along with reasonable service life. In this study, synthesized unpyrolyzed YSZ powder was air plasma sprayed in order to produce segmentation crack TTBCs. The microstructure and hardness of the deposits were then compared with those of the conventional TTBCs and dense vertically cracked (DVC)TTBCs. In this regard, spraying parameters were optimized to achieve deposits with the appropriate amount of unpyrolyzed particles in them to assist inducing vertical cracks in the deposited layers. The effect of the unpyrolyzed particles on microstructure, porosity, and microhardness of plasma sprayed coatings were also evaluated and compared. The new fabricated coating showed a bimodal structure combining non-molten sub-micron size particles and conventional splats along with segmentation cracks with higher amount of porosity and lower hardness compared to those of the DVC coatings. The results implied that, depositing unpyrolyzed powder by APS, as a new approach for achieving segmentation crack TTBCs, is very promising.     

Erosion of atmospheric plasma sprayed rare earth oxide coatings under air suspended corundum particles

Atmospheric plasma sprayed (APS) zirconium oxide based coatings are used widely in aero engine components for providing thermal insulation, improving the corrosion and oxidation resistance. Despite its wide spread industrial use, little is known about the basic erosion behaviour and the mechanisms by which such coatings erode. In this paper, the erosive wear behaviours of Yttria Stabilized Zirconia (YSZ) coatings; Lanthanum Zirconate (LZ) coatings and Inconel 738 base material (BM) were studied and compared under air jet erosion conditions with corundum particles as erodent material. The erosion behaviour was studied with respect to the different porosity volume percentages of the coatings and the changes in velocity of erodent, impact angle of erodent and erodent particle flux. It was found that in solid particle erosion, the wear resistances of YSZ and LZ coatings were the best at their lowest porosity volume and it decreased with the increase in the percentage volume of porosity. There was a linear increase in the wear resistance with the increase in hardness. Further, relationships among the erosion parameters with respect to erosive wear loss were derived by using the response surface methodology and the erosion mechanisms were discussed adequately.     

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

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.     

再生保温混凝土抗冻性能的细观研究

再生粗骨料和玻化微珠保温骨料的加入,会导致再生保温混凝土(RATIC)内部形成复杂的孔隙结构。为了进一步揭示冻融作用下再生保温混凝土的劣化机理,在宏观试验的基础上,通过CT扫描细观试验分析了不同再生粗骨料取代率的RATIC试件在冻融作用下的裂缝发展规律和孔隙特征。试验结果表明:新旧水泥砂浆界面过渡区是冻融循环作用下RATIC的薄弱环节,试件内部多数裂缝产生于新旧水泥砂浆界面过渡区;试件的相对孔隙率是定量反映RATIC试件内部冻融破坏规律的重要指标。     

Evolution of porosity in suspension thermal sprayed YSZ thermal barrier coatings through neutron scattering and image analysis techniques

Porosity is a key parameter on thermal barrier coatings, directly influencing thermal conductivity and strain tolerance. Suspension high velocity oxy-fuel (SHVOF) thermal spraying enables the use of sub-micron particles, increasing control over porosity and introducing nano-sized pores. Neutron scattering is capable of studying porosity with radii between 1 nm and 10 μm, thanks to the combination of small-angle and ultra-small-angle neutron scattering. Image analysis allows for the study of porosity with radii above ～100 nm. For the first time in SHVOF 8YSZ, pore size distribution, total porosity and pore morphology were studied to determine the effects of heat treatment. X-ray diffraction and micro-hardness measurements were performed to study the phase transformation, and its effects on the mechanical properties. The results show an abundant presence of nano-pores in the as-sprayed coatings, which are eliminated after heat treatment at 1100 °C; a transition from inter-splat lamellar to globular pores and the appearance of micro-cracks along with the accumulation of micro-strains associated with the phase transformation at 1200 °C.     

Evolution of thermal insulation of plasma-sprayed thermal barrier coating systems with exposure to high temperature

The thermal insulation potential of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings is generally assessed via the evaluation of the ceramic layer. However, ageing of the complete system leads to microstructural transformations that may also play a role in the heat transport properties. This study thus investigated the microstructure-heat insulation relationships of different TBC systems in their as-deposited state and when aged under various conditions, through the systematic analysis of both microstructure and thermal diffusivity. The latter was measured from room temperature up to 1100 °C using the laser-flash technique, while the porous microstructure was assessed using image analysis. The different coatings exhibited relatively similar thermal diffusivity values that were shown to be mostly influenced by the thin porosities in contrast to larger defects. The thermal insulation of the TBC systems after exposure to high temperature was shown to be stable despite the microstructural variations introduced by cracks, oxidation and chemical degradations.     

Model-based analysis of thermal insulation coatings

Thermal insulation properties of coatings based on selected functional filler materials are investigated. The underlying physics, thermal conductivity of a heterogeneous two-component coating, and porosity and thermal conductivity of hollow spheres (HS) are quantified and a mathematical model for a thermal insulation coating developed. Data from a previous experimental investigation with hollow glass sphere-based epoxy and acrylic coatings were used for model validation. Simulations of thermal conductivities were in good agreement with experimental data. Using the model, a parameter study was also conducted exploring the effects of the following parameters: pigment (hollow spheres) volume concentration (PVC), average sphere size or sphere size distribution, thermal conductivities of binder and sphere wall material, and sphere wall thickness. All the parameters affected the thermal conductivity of an epoxy coating, but simulations revealed that the most important parameters are the PVC, the sphere wall thickness, and the sphere wall material. The model can be used, qualitatively, to get an indication of the effect of important model parameters on the thermal conductivity of an HS-based coating and thereby be used as a specification tool or as a help in the planning of relevant experiments to conduct. Further work with the model must involve additional experiments to secure a general verification of important underlying model assumptions.     

Hierarchically porous lanthanum zirconate foams with low thermal conductivity from particle-stabilized foams

Lanthanum zirconate (LZO) ceramic foams with hierarchical pore structure were fabricated by particle-stabilized foaming method for the first time, and the as-prepared ceramics have high porosity of 90.7%-94.9%, low thermal conductivity, and relatively high compressive strength. The LZO powder was synthesized by solid-state method. The porosity of the ceramic foams was tailored by suspensions with different solid loadings (20-40 wt%). The sample with porosity of 94.9% has thermal conductivity of 0.073 W/(m·K) and compressive strength of 1.19 MPa, which exhibits outstanding property of thermal insulation and mechanical performance, indicating that LZO ceramic foam is a promising thermal insulation material in high temperature applications.     

Highly porous YSZ ceramic foams using hollow spheres with holes in their shell for high-performance thermal insulation

Yttria-stabilized zirconia (YSZ) porous ceramic foams were fabricated using YSZ microspheres with holes on the surface to determine their properties as insulation materials. Highly porous YSZ ceramics with bimodal pore structures, such as internal pores in single hollow spheres and external pores between the spheres, were successfully prepared using YSZ spheres as raw materials. Additionally, holes were added to the shells to reduce continuous thermal pathways and significantly enhance the insulation properties. Furthermore, by adding holes on the surface of the sphere, the porous foams using a hollow sphere exhibit a maximized porosity of 80.69%, remarkably enhanced their insulation properties with low thermal conductivity (0.10 W/m-K), and have sufficient compressive strength to protect the green body (5.7 MPa). The mechanical strength of the YSZ porous foam was maintained owing to the uniform arrangement of the supports.     

Investigation on pore properties,thermal conductivity,and compressive behavior of fly ash/slag-based geopolymer foam

Geopolymer foam has emerged as a promising inorganic porous material in the last decade. Despite of the numerous advantages, there are some pending issues to be addressed, on top of that is the low compressive strength. To overcome this, this study synthesizes a high-strength geopolymer foam by the partial substitution of fly ash (FA) with ground granulated blast furnace slag and carries out an intensive investigation into its microstructure, pore properties, thermal conductivity as well as compressive behavior. The microstructure is firstly analyzed by X-ray diffraction and Fourier transform infrared spectroscopy techniques. The pore characteristics are also scrutinized, including pore size distribution, total porosity and water absorption. Then, the thermal conductivity is investigated and the applicability of basic effective thermal conductivity models to characterize the relationship with total porosity is evaluated. Afterward, the compressive strength together with the softening coefficient is examined, and the relationship with total porosity is also studied. Finally, comparisons between the proposed geopolymer foam and other FA-based geopolymer foams in the literature are performed. The results show that the proposed geopolymer foam possesses not only a comparable thermal conductivity but also a far superior compressive strength, which sheds light on the widespread applications in thermal insulation.     

Pore filling behavior of YSZ under CMAS attack: Implications for designing corrosion‐resistant thermal barrier coatings

To understand the pore filling behavior in thermal barrier coatings during calcium‐magnesium‐alumino‐silicate (CMAS) infiltration process, porous yttria‐stabilized zirconia pellets with different sizes of spherical pores were prepared to simulate thermal barrier coatings. The pores (D50 ranging from 6 to 77 μm) were introduced to the pellets using poly methyl methacrylate as pore forming agents. Then the pellets were sintered to remove the pore forming agents and to achieve a similar volume fraction of porosity with thermal barrier coatings. After CMAS infiltration, only some small pores in the CMAS‐infiltrated zones were filled by CMAS, whereas all large pores (larger than 13 μm) remained unfilled; besides, the results also show that even open pores can resist filling by CMAS. The reason may relate to pore diameters; if the diameter of a pore is relatively large, the pore surface will not be completely wetted by liquid CMAS, the liquid meniscus will be discontinuous, and therefore the pore cannot be filled. The key insight gained from this study is that introducing “CMAS‐proof” pores into thermal barrier coatings may be a potential way to mitigate CMAS damage.     

Novel nanometer alumina-silica insulation board with ultra-low thermal conductivity

Advanced nano-porous super thermal insulation materials are widely used in spacecraft, soler-thermal shielding, heat exchangers, photocatalytic carriers due to their low thermal conductivity. In this work, adopting dry preparation technology, nano-Al₂O₃, nano-SiO₂, SiC and glass fibers as raw materials, novel nanometer alumina-silica insulation board (NAIB) were prepared. The preparation process was simple, safe, and reliable. In addition, the NAIB exhibited a high porosity (91.3–92.3%), small pore size (39.83–44.15 nm), low bulk density (0.22–0.26 g/cm³), better volumetric stability, and low thermal conductivity (0.031–0.050 W/(m·K) (200–800 °C)), respectively. The as-prepared NAIB could render them suitable for application as high-temperature thermal insulation materials.