Influence of microstructure on the thermophysical properties of sintered SiC ceramics

The specific heat, thermal diffusivity and thermal conductivity of porous SiC ceramics sintered using two kinds of SiC powders (fine and coarse) have been investigated for sintering temperatures in the range 1700–2000 °C. Sintered SiC has a porous structure with approximately 30–40 vol.% porosity. Thermal diffusivity was measured by the laser flash method. The thermal diffusivities and thermal conductivities of sintered SiC ceramics increased with increasing sintering temperature. The specific heat decreased slightly with increasing sintering temperature. The thermal diffusivities and thermal conductivities of SiC sintered from coarse powder were higher than those of SiC sintered from fine powder. The thermal conductivity of samples increased markedly with increasing grain size.     

Thermophysical,mechanical and microstructural characterization of aged free-standing plasma-sprayed zirconia coatings

The effect of porosity on the thermal diffusivity and elastic modulus has been studied on artificially aged, free-standing thermal barrier coatings (TBCs) produced by air plasma spray (APS). The activation energy of the sintering phenomenon was estimated from the variation in diffusivity with time and temperature. X-ray diffraction was used to evaluate the phase stability of 7 wt.% yttria partially stabilized zirconia (YPSZ) coatings. The thermal diffusivity and elastic modulus as measured by photothermal techniques and three-point bending, respectively, are reported as a function of the ageing time. Correlations between the thermal and mechanical parameters are investigated by suitable models based on the microstructural features revealed by electron microscopy. The reliability of porosity information provided by image analysis and used as input for the modelling is critically discussed.     

Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons

For thin (< 200 μm) air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) ceramic thermal barrier coatings (TBCs), some non-destructive techniques indicate damage at the bond coat-TBC interface during either ageing or cyclic oxidation tests. However, no technique is available for thick (> 200 μm) APS TBCs.In this work, a semi-quantitative estimation of cracks at the interface of APS TBCs thicker than 300 μm is obtained from thermal diffusivity values measured by using a single side thermographic technique on coupons subjected to thermal cycling.In fact, during thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity.The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface.     

等离子蒸发制备的纳晶镍的压缩变形机理

与传统粗晶金属材料相比,纳晶金属具有特殊的变形机理。为了探索纳晶金属的变形机理,本文以等离子蒸发结合热压烧结制备的块体纳晶镍为研究对象,进行了准静态压缩力学性能测试,并且利用XRD和TEM技术对试样压缩前和压缩后的微结构演化进行了研究。结果表明：块体纳晶镍表现出较高的压缩强度和较好的韧性,且强度和韧性均具有率相关性。同时,纳晶镍压缩变形后晶粒尺寸较压缩前减小,但其微应变增加。结合纳晶镍的力学行为和压缩过程中微结构的演化,本文预测晶界位错运动和晶界滑移的联合机制是块体纳晶镍压缩过程中塑性变形的主要机理。     

Microstructural dependency of thermal expansion and sintering shrinkage in plasma-sprayed zirconia coatings

The effect of microstructure upon thermal expansion and sintering shrinkage in plasma-sprayed zirconia coatings was investigated by an accurate dilatometry. Cut-out samples with different microstructures were prepared from 5.8-mm-thick atmospheric plasma-sprayed (APS) and water-stabilized plasma coatings (WSP). It was quantitatively determined that the samples cut out of different thickness positions had minor differences in microstructure, and these APS samples largely differed from the WSP samples. The thermal expansion behaviors of all the samples coincided after a short annealing time despite their initial structural differences. On the other hand, all the samples showed a significant difference in sintering shrinkages with annealing at 1400 °C. This result was consistent with the theoretical result calculated with Cipitria's sintering model in terms of the relationship between shrinkage and microstructure. It was therefore demonstrated that the initial microstructure, particularly the splat thickness and the inter-splat pore height, exerts a great influence on the sinterability of plasma-sprayed coatings.     

烧结方法对AlN陶瓷微观形貌及热导率的影响

采用真空烧结、N2保护无压烧结、放电等离子烧结等方法对AlN粉末进行烧结,研究烧结方法对粉体烧结行为以及产物物相组成、微观形貌及热导率的影响。结果表明:真空烧结会显著降低AlN材料的脱氮分解温度,无法实现其致密化;而通过N2保护无压烧结和放电等离子烧结的方法均能得到结构致密、热导率较高的AlN陶瓷,其中后者的烧结温度更低、制得陶瓷样品的致密度和热导率更高,在1650℃保温10min即可烧结得到热导率为121.5W·m-1·K-1的AlN陶瓷。     

Non-destructive thermal property measurements of an APS TBC on an intact turbine blade

The ability to measure the properties of thermal barrier coatings (TBCs) applied to engine components is challenging due to the complex geometry of parts and the difficulty of preparing samples suitable for conventional techniques. As a result, there is a shortage of information related to the morphology and thermal properties of coatings on engine components. Phase of photothermal emission analysis (PopTea) is a relatively new non-destructive technique that is suitable for measuring the thermal properties of coatings on serviceable engine parts. To demonstrate this capability, measurements are performed on an intact turbine blade coated with air plasma sprayed (APS) 7 wt.% Y₂O₃-stabilized ZrO₂ (7YSZ). The average thermal diffusivity of the coating applied to the blade was ~ 0.5 mm²/s which is typical for thermal diffusivity previously measured on 7YSZ APS coatings made on test coupons with PopTea and laser flash. Furthermore, trends in thermal properties over the blade are studied and compared. It is discovered that variations in thermal properties are the result of differences in coating porosity.     

Thermo-physical properties of plasma sprayed Yttria Stabilized Zirconia Coatings

This article investigates the thermo-physical properties of plasma sprayed zirconia coatings produced by agglomerates of submicron size particles as the feedstock. The microstructure of these deposits is consisted of splats and non-molten particles. This bimodal structure generally shows a better performance than conventional coatings. The agglomerated feedstock with internal submicron size porosity may significantly affect porosity related properties, such as the thermal diffusivity. In this study different process parameters were used to deposit yttria stabilized zirconia coatings with conventional and bimodal structures. Results showed a good correlation between the shape and distribution of pores and thermal diffusivity of the coatings.     

Toward Highly Sintering-Resistant Nanostructured ZrO2-7wt.%Y2O3 Coatings for TBC Applications by Employing Differential Sintering

There are still concerns in the scientific community about the stability of nanostructured YSZ coatings at high temperatures. Questions have been raised about the possibility of accelerated sintering of these ultrafine materials and the associated changes in properties that could accompany this sintering. In this work, nanostructured YSZ coatings were engineered to counteract sintering effects by tailoring the coatings to exhibit a bimodal microstructure formed by (i) a matrix of dense YSZ zones (produced from molten YSZ particles) and (ii) large porous nanostructured YSZ zones (produced from semimolten nanostructured YSZ particles) that were embedded in the coating microstructure during thermal spraying. These coatings were subjected to heat treatment in air at 1400 °C for 1, 5, and 20 h. The superior driving force for sintering exhibited by the porous nanozones, when compared to that of the dense zones, caused the nanozones to shrink at much faster rates than those exhibited by the denser matrix zones (i.e., differential sintering), thereby creating a significant network of voids in the coating microstructure. Due to these effects, after 20 h exposure at 1400 °C, the thermal conductivity and elastic modulus values of the conventional coatings were approximately two times higher than those of the nanostructured ones.     

Temperature profile optimization for microwave sintering of bulk Ni–Al2O3 functionally graded materials

Bulk multilayer graded Ni–Al₂O₃ samples have been sintered under heating by millimeter-wave radiation using a gyrotron system for high-temperature materials processing. By using a purposely designed thermal insulation arrangement, the temperature profile has been adjusted along the concentration gradient to accommodate for different sintering temperatures of the components. The sintered samples have flat boundaries between layers, and their microstructure is free from cracks and delamination. In addition to metal-ceramic graded transitions, metal-ceramic-metal graded insulator structures have also been fabricated.     

Effect of processing parameters on the microstructure and thermal conductivity of diamond/Ag composites fabricated by spark plasma sintering

Diamond/metal composites with 50 vol.% diamond have been produced by spark plasma sintering (SPS) using pure Ag as a matrix and diamond particles as reinforcement. Three kinds of powder mixing processes were used to prepare the mixture of diamond/Ag powders: dry mixing without milling medium, wet mixing and magnetic blending. Subsequently, they were all consolidated by SPS at various processing parameters to produce bulk diamond/Ag composites. Then samples were heat treated in order to obtain a higher thermal conductivity. The effect of processing parameters on the morphologies of the mixed powders, the microstructure and the thermal conductivity of the composites were investigated by comparing the experimental data. It reveals that particles were easy to agglomerate and the distribution of mixed powders was inhomogeneous by dry mixing method, and wet mixing method is too complex. The most favorable mixing process is magnetic blending by which the powders can be homogenously mixed and the composites prepared by optimized SPS processing parameters can obtain the highest relative density and the best thermal conductivity among the composites prepared by different processes. The magnetic blending diamond/Ag composites even have a 23% increase in thermal conductivity compared with pure silver sintered by SPS.     

VC, Cr3C2 and NbC doped WC–Co cemented carbides prepared by pulsed electric current sintering

WC–12 wt.% Co grade cemented carbides doped with 0.9 wt.% VC, NbC or Cr₃C₂ grain growth inhibitor were consolidated by pulsed electric current sintering (PECS), also known as spark plasma sintering (SPS), in the solid state at 1240 °C for 2 min. The microstructure and properties of the PECS material grades are compared with those of pressureless sintered grades, liquid phase sintered at 1420 °C for 1 h. Microstructural and hardness characterization revealed that both the chemical composition and sintering technique play an important role on the WC grain growth and final mechanical properties. To obtain a nanometer sized WC–Co microstructure, it is essential to carefully select the grain growth inhibitor in addition to the application of a fast thermal densification cycle by means of spark plasma sintering.     

Fabrication of sintered tungsten by spark plasma sintering and investigation of thermal stability

Tungsten has been considered as the most promising candidate for plasma-facing materials (PFMs) in a next generation fusion reactor. It is well known that commercialized ITER (International Thermonuclear Experimental Reactor) grade tungsten is manufactured by the mechanical processing at high temperature after sintering to ensure a high density with an improved structural stability. In this study, in order to obtain the high-density sintered tungsten with more enhanced structural stability, spark plasma sintering (SPS) method was employed, where a pulsed direct electric current was applied during heat treatment of powders with a pressure in the specimen. It is well known that by utilizing SPS, high-density sintered materials at a relatively lower temperature for a shorter time could be achieved compared to the other conventional sintering methods. In particular, in this study, reduction in H₂ atmosphere and two-step sintering were introduced to remove the residual oxygen and achieve the full densification with suppressed grain growth at relatively low operating temperature. In an optimized condition, a fully densified sintered tungsten with a relative density of 99.9% and an average grain size of 4.4 μm was fabricated. The thermal stability of tungsten specimens was evaluated by high heat flux (HHF) test, where the surface temperature was set up to 2300 °C by nitrogen plasma. Then, the microstructural changes of the specimen surface have been examined after the HHF test. As a result, it was confirmed that the high-density sintered tungsten samples fabricated by SPS show an excellent microstructural stability for PFMs.     

3D Analysis of Porosity in a Ceramic Coating Using X-ray Microscopy

Suspension plasma spraying (SPS) is a new, innovative plasma spray technique using a feedstock consisting of fine powder particles suspended in a liquid. Using SPS, ceramic coatings with columnar microstructures have been produced which are used as topcoats in thermal barrier coatings. The microstructure contains a wide pore size range consisting of inter-columnar spacings, micro-pores and nano-pores. Hence, determination of total porosity and pore size distribution is a challenge. Here, x-ray microscopy (XRM) has been applied for describing the complex pore space of the coatings because of its capability to image the (local) porosity within the coating in 3D at a resolution down to 50 nm. The possibility to quantitatively segment the analyzed volume allows analysis of both open and closed porosity. For an yttria-stabilized zirconia coating with feathery microstructure, both open and closed porosity were determined and it could be revealed that 11% of the pore volumes (1.4% of the total volume) are closed pores. The analyzed volume was reconstructed to illustrate the distribution of open and closed pores in 3D. Moreover, pore widths and pore volumes were determined. The results on the complex pore space obtained by XRM are discussed in connection with other porosimetry techniques.     

Recent Trends in Newly Developed Plasma-Sprayed and Sintered Coatings for Implant Applications

The current paper aims to review recent trends (2011 to 2015) in newly developed plasma-sprayed and sintered coatings for implant applications. Recent developments in plasma-sprayed and sintered coatings have focused on improving biological performance, bacterial growth resistance, and mechanical properties, predominantly of HA and glass ceramics. The majority of these improvements are attributed to the addition of dopants. To improve biological performance, trace elements, such as Zn and Mg, both of which are found in bone, were added to replicate the functions they provide for the skeletal system. Though bacterial growth resistance is traditionally improved by Ag dopant, the addition of new dopants such as CeO₂ and Zn were explored as well. Great effort has also been made to improve coating adherence and reduce stresses by minimizing coefficient of thermal expansion mismatch between the coating and substrate through the addition of elements such as Zn and Mg or the inclusion of a buffer layer. For sintering process in particular, there was an emphasis on reducing sintering temperature through modification of 45S5 Bioglass. New plasma spray and sintering technologies aimed at reducing high-temperature exposure are briefly introduced as well. These include microplasma spray and spark plasma sintering.     

The relationship between the microstructure and thermal diffusivity of plasma-sprayed tungsten coatings

Tungsten and tungsten alloy coatings are candidate materials for plasma facing components of divertor plates in future fusion reactors. In normal operation, the sprayed coatings will be submitted to intense heat fluxes and particle bombardment. This work investigated the relationship between the microstructure of plasma-sprayed tungsten coatings and their thermal diffusivity as determined by the laser flash method. The microstructural investigation was carried out on copper-infiltrated coatings. Such a preparation technique permitted the measurement of the total true contact area between the lamellae within the tungsten coatings. The spraying atmosphere was found to strongly influence the interfacial contact between lamellae and coating thermal diffusivity.     

Application of a novel microwave plasma treatment for the sintering of nickel oxide coatings for use in dye-sensitized solar cells

In this study the use of microwave plasma sintering of nickel oxide (NiO_x) particles for use as p-type photoelectrode coatings in dye-sensitized solar cells (DSSCs) is investigated. NiO_x was chosen as the photocathode for this application due to its stability, wide band gap and p-type nature. For high light conversion efficiency DSSCs require a mesoporous structure exhibiting a high surface area. This can be achieved by sintering particles of NiO_x onto a conductive substrate. In this study the use of both 2.45 GHz microwave plasma and conventional furnace sintering were compared for the sintering of the NiO_x particles. Coatings 1 to 2.5 μm thick were obtained from the sintered particles (mean particle size of 50 nm) on 3 mm thick fluorine-doped tin oxide (FTO) coated glass substrates. Both the furnace and microwave plasma sintering treatments were carried out at ~ 450 °C over a 5 min period. Dye sensitization was carried out using Erythrosin B and the UV–vis absorption spectra of the NiO_x coatings were compared. A 44% increase in the level of dye adsorption was obtained for the microwave plasma sintered samples as compared to that obtained through furnace treatments. While the photovoltaic performance of the DSSC fabricated using the microwave plasma treated NiO_x coatings exhibited a tenfold increase in the conversion efficiency in comparison to the furnace treated samples. This enhanced performance was associated with the difference in the mesoporous structure of the sintered NiO_x coatings.     

Microwave sintering of large size pieces with complex shape

In this study, large size pieces of kaolin with a complex shape were successfully sintered in a microwave multimode cavity. The challenge was to obtain a sample homogeneously sintered on a large surface. For this purpose, prior to sintering, a finite element modelling study was carried out to determine the optimal experimental set-up configuration permitting to have an electromagnetic field as homogenous as possible around the sample. In this case, it was pointed out that the hybrid heating configuration (use of susceptors) was the most suitable configuration.The samples experimentally obtained were structurally and mechanically characterized. Therefore, it was highlighted that the microwave sintered samples present a microstructure and mechanical properties similar to those of samples conventionally processed for a lower sintering temperature.Finally, the microwave sintering permit to significantly reduce the energy consumption required for the production of sintered ceramic pieces, which is crucial for sustainable development.     

Investigation of Amorphous/Nanocrystalline Iron-Based Thermal Barrier Coatings

Because of their favorable thermophysical properties, good machinability and low material costs, iron-based coatings which exhibit a highly amorphous/nanocrystalline microstructure are currently in the focus of research. Considering the crystallization temperature of the material, iron-based coatings might be the next generation of thermal barrier coatings (TBCs) for low-temperature systems, reducing thermal losses. The objective of this research project is the development of highly amorphous, iron-based coatings. For this purpose, amorphous feedstock materials with different chromium contents have been developed and characterized regarding their microstructures, phase compositions, crystallization temperatures and amorphous content. The results show that the amorphous content is reduced with increasing particle size and chromium content. The coatings were deposited by air plasma spraying (APS) and high-velocity oxygen fuel spraying (HVOF). It is shown that all coatings exhibit amorphous structures. HVOF coatings show a smaller amount of amorphous content compared to the feedstock materials, indicating crystallization occurring in not fully melted particles or insufficient rapid cooling. The APS process can increase the amount of amorphous content compared to the feedstock material, as shown for x _Cr = 15%. All coatings proof good thermal shock behavior. Lowest thermal diffusivity values were determined for APS coatings, which confirms the potential of iron-based TBCs.     

Thermal Conductivity in Suspension Sprayed Thermal Barrier Coatings: Modeling and Experiments

Axial suspension plasma spraying (ASPS) can generate microstructures with higher porosity and pores in the size range from submicron to nanometer. ASPS thermal barrier coatings (TBC) have already shown a great potential to produce low thermal conductivity coatings for gas turbine applications. It is important to understand the fundamental relationships between microstructural defects in ASPS coatings such as crystallite boundaries, porosity etc. and thermal conductivity. Object-oriented finite element (OOF) analysis has been shown as an effective tool for evaluating thermal conductivity of conventional TBCs as this method is capable of incorporating the inherent microstructure in the model. The objective of this work was to analyze the thermal conductivity of ASPS TBCs using experimental techniques and also to evaluate a procedure where OOF can be used to predict and analyze the thermal conductivity for these coatings. Verification of the model was done by comparing modeling results with the experimental thermal conductivity. The results showed that the varied scaled porosity has a significant influence on the thermal conductivity. Smaller crystallites and higher overall porosity content resulted in lower thermal conductivity. It was shown that OOF could be a powerful tool to predict and rank thermal conductivity of ASPS TBCs.