Interlamellar Pores Research Articles

Microstructural evolution of 7wt.% Y2O3–ZrO2 thermal barrier coatings due to stress relaxation at elevated temperatures and the concomitant changes in thermal conductivity

The purpose of this study was to evaluate the combined effect of stress and temperature on the microstructure of air plasma-sprayed 7 wt.% Y{sub 2}O{sub 3}-ZrO{sub 2} thermal barrier coatings, and relate microstructural changes to the thermal conductivity, k{sub th}. To simulate TBC service conditions, stand-alone tubes of YSZ were stress relaxed, starting from a compressive stress of 60 MPa, at temperatures of 1000 C or 1200 C. The duration of the stress relaxation test was either 5 min or 3 h. Detailed scanning electron microscopy (SEM) and Porod's specific surface area (SSA) analysis of small angle neutron scattering (SANS) results were used to determine which void systems, either interlamellar pores or intralamellar cracks, contributed to the observed relaxation of stress in the coatings. SEM investigations revealed closure of intralamellar cracks located perpendicular to the stress direction. For thinner YSZ coatings, SANS measurements indicated a statistically significant reduction in the total SSA and SSA associated with intralamellar cracks after stress relaxation at the times, temperatures, and stress investigated compared to those samples that were exposed to identical times and temperatures, but no stress. The SSA associated with the interlamellar pores was not significantly smaller in YSZ coatings stress relaxed frommore » 60 MPa at 1200 C for 3 h compared to as-sprayed coatings. The thermal conductivity of the coatings was strongly influenced by stress, with increases in kth observed after only 5 min at 60 MPa and 1200 C. Reductions in the total SSA were directly linked to increases in k{sub th}.« less

Assessing Coating Reliability Through Pore Architecture Evaluation

Plasma-sprayed thermal barrier coatings (TBCs) exhibit many interlamellar pores, voids, and microcracks. These microstructural features are primarily responsible for the low global stiffness and the low thermal conductivity commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400 °C, for times of up to 20 h) on the pore architecture of detached YSZ top coats with different impurity levels have been characterized by mercury intrusion porosimetry and gas-sorption techniques. Stiffness and thermal conductivity were also monitored to assess the effect of change in pore architecture on properties. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (surface-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high-treatment temperatures (~1400 °C). These changes are thought to be associated with intrasplat microcrack healing, improved intersplat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Increase in thermal conductivity and stiffness were found to be more pronounced for coatings with higher impurity content (particularly alumina and silica). Reliability issues surrounding such increase in thermal conductivity and stiffness are discussed along with a brief note on the effect of impurities on TBC life.

Process-Controlled Plasma-Sprayed Yttria-Stabilized Zirconia Coatings: New Insights from Ultrasmall-Angle X-ray Scattering

A multicomponent microstructure model is applied in ultrasmall-angle X-ray scattering studies of two groups of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs). One group was sprayed from a single powder feedstock using controlled processing conditions. The other group included three different feedstock morphologies (obtained from different manufacturing methods), each with a similar particle size distribution and sprayed under the same average controlled processing conditions. The microstructure is quantitatively related to the feedstock morphology and processing conditions. Relationships are explored among these microstructures and the coating properties (e.g., thermal conductivity, elastic modulus). The degree of microstructural anisotropy is demonstrated to be pore-size dependent, being more pronounced for larger pores, and more sensitive to feedstock morphology (powder processing) than to spray processing. The microstructure analysis indicates two broad distributions of interlamellar pores, which combined, account for 70%–80% of the pore volume. The total porosity is found to increase with decreasing particle temperature or velocity. For all coatings, a negative linear relationship exists between thermal conductivity and total porosity. Comparison of the new analysis is made with earlier small-angle neutron scattering results, and implications are considered for a more general application of this metrology in TBC microstructure design.

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V 2O 5 reaction and melt infiltration in plasma-sprayed 7 wt% Y 2O 3–ZrO 2 (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V 2O 5 with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV 2O 7 (for reactions at 750 °C and below), m-ZrO 2 and YVO 4. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V 2O 5 was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V 2O 5 at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V 2O 5.

Comparative studies on catalytic properties of immobilized Candida rugosa lipase in ordered mesoporous rod-like silica and vesicle-like silica

A lipase from Candida rugosa (CRL) has been immobilized in mesoporous rod-like silica and vesicle-like (or onion-like) silica supports by physical adsorption. The mesostructures and characteristics of the two mesoporous materials before and after adsorption of CRL were characterized by HRTEM, FESEM, SAXRD and N 2 adsorption–desorption experiments. The pore sizes of the rod-like silica and the vesicle-like silicas were determined to be ca. 11 and 12 nm, respectively, larger than enough to accommodate lipase molecules (∼4 nm). The structural integrity of the rod-like silica as well as the vesicle-like silica is retained after adsorption of CRL into the mesopores. The catalytic activity, thermostability, and operational stability of the immobilized CRL were measured in phosphate buffer solution by hydrolysis of tributyrin. When the lipase immobilized in the vesicle-like silica was used as biocatalyst for hydrolysis of tributyrin in a batch process, the hydrolysis activity decreased only slightly from 74% to 60% over 10 cycles of reuse, indicating no obvious leaching out of the lipase from the round channels. The activity of vesicle-like silica immobilized CRL for catalytic hydrolysis of tributyrin is 21% and 14% higher than that of rod-like silica immobilized CRL at incubation temperature of 60 °C at 30 min and 90 min, respectively. The results demonstrate that the lipase immobilized in vesicle-like silica is superior to the rod-like silica in terms of higher activity, reusability and thermal stability. The unique curvature and the large size of the interlamellar pores of vesicle-like silica are suggested to be possible factors in achieving the higher activity and stability. Such a distinctive mesoporous geometry of the vesicle-like silica promises technological applications in protein adsorption and catalysis.

Optical Coherence Tomography for the Inspection of Plasma-Sprayed Ceramic Coatings

Optical coherence tomography (OCT) is evaluated as a potential technique for microstructure characterization of plasma-sprayed ceramic coatings. OCT combines the principles of low-coherence interferometry and optical heterodyne detection to obtain both a high axial resolution and a high sensitivity to weakly backscattered light. It can be used to accurately locate interfaces where the refractive index changes abruptly within translucent materials. Therefore, OCT should be sensitive to interlamellar pores and splat interfaces within plasma-sprayed ceramic coatings. In the present work, OCT cross-sectional images of thin yttria-stabilized zirconia (YSZ) coatings are considered. The interferogram envelopes forming the collected images are analyzed individually to successfully gather information related to light attenuation inside the coatings. This light attenuation is shown to be related to the density of interfaces within the coatings as well as to the material oxidation state. The envelope analysis also allows the evaluation of the refractive index of the YSZ nontransformable tetragonal phase.

Mechanical properties and oxidation behaviour of plasma sprayed functionally graded zirconia–alumina thermal barrier coatings

The microstructures of thermally sprayed coatings usually incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects various coating properties such as elastic modulus, thermal conductivity and dielectric behaviour. This study is conducted to improve the image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for TBCs deposited with partially stabilized zirconia (PSZ), alumina and zirconia–alumina composite coatings by gas tunnel type plasma spraying. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for porosity analysis agreed with coating qualities and the influences of the Vickers hardness. The paper also discusses the thermal behaviour and high temperature oxidation resistance of the Al 2O 3 coatings as compared to composite coatings at the interface. This interlayer is preferred to minimize the detrimental effect of phase transformation of γ-Al 2O 3 to α-Al 2O 3.

Effect of initial powder morphology on thermal and mechanical properties of stand-alone plasma-sprayed 7 wt.% Y 2O 3–ZrO 2 coatings

The effects of starting powder morphology on the thermal and mechanical properties of stand-alone plasma-sprayed 7 wt.% Y 2O 3–ZrO 2 (YSZ) coatings were studied. Two powder morphologies were investigated: an agglomerated and sintered powder (referred to presently as “AS”) and a powder manufactured using plasma spheroidization to create hollow spheres (referred to presently as “HS”). Coatings made from AS powders contained 0.21 wt.% SiO 2 impurity, twice as much as observed in coatings made from HS powders. Properties of coatings made from each powder type were compared in the as-sprayed state and after 50 h heat treatments at temperatures ranging from 1000 to 1400 °C. SEM microstructural investigations revealed significant differences in the porosity and distribution of pores in the coatings. In coatings made with AS powders the majority of the high aspect ratio pores were located between lamella (interlamellar porosity). In addition to interlamellar pores, coatings made with HS powders demonstrated 1.5 times more spherical-shaped globular pores by number located within lamella. Globular pores were shown to still exist in coatings made with HS powders after 50 h heat treatments at 1400 °C. Archimedes porosity measurements showed that coatings made with AS powders typically contained 4–5% less total porosity than coatings made with HS powders. Thermal conductivity experiments using laser flash showed that there was no difference in the thermal conductivity of coatings made from either powder type in the as-sprayed state despite higher porosity in the coatings made from HS powders. After a 50 h heat treatment at 1000 and 1200 °C, coatings made from both powder types still demonstrated statistically similar thermal conductivities. However, after a 50 h heat treatment at 1400 °C the thermal conductivity of coatings made from AS powders was found to be 0.3 W/m/K higher than coatings made from HS powders. Microstructural differences in the coatings made from the two powder types that affected sintering rates, including the increased level of SiO 2 impurities and fewer globular pores, were used to explain variations in properties. In uniaxial compression tests on stand-alone samples at 1000, 1100, and 1200 °C, coatings made from AS powders typically relaxed more stress than coatings made from HS powders. Differences in coating relaxation became more apparent as the test temperature increased through 1200 °C. The higher percentage of impurity SiO 2 in the AS coatings is believed to contribute to increased relaxation via formation of a glassy phase at the grain boundaries which assists grain boundary sliding during elevated temperature deformation.

Grain‐Boundary Grooving of Plasma‐Sprayed Yttria‐Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma‐sprayed 7 wt% Y2O3–ZrO2 thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma‐sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain‐boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat‐treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.

Quantitative analysis of pores of two types in a plasma-sprayed coating

Quantitative microstructural analyses of thermally sprayed coatings are reviewed. Then a ceramic plasma-sprayed coating is analysed using light microscopy and image analysis. Globular pores and interlamellar flat pores are reconstructed from serial sections of a specimen and their volume and surface area histograms are estimated. The spatial distribution of pores is described using a method based on 3D distances and testing of complete spatial randomness is performed. Interpretations of results in terms of the physical background of the material are discussed.

Application of image analysis for characterization of porosity in thermal spray coatings and correlation with small angle neutron scattering

The microstructures of thermally sprayed coatings are very complex and incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects a wide range of coating properties such as elastic modulus, thermal conductivity and dielectric behavior. Various methods are employed for quantitative measurement of porosity, which forms an important and integral part of microstructural characterization of thermal spray coatings. Studies have been conducted here to establish image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for diverse coatings deposited with distinct spray processes and material feedstock characteristics. The IA methods put forth have been successfully applied to a variety of thermally sprayed coatings; materials of interest in this study being produced from partially stabilized zirconia (PSZ, ZrO 2+8% Y 2O 3) and alumina ceramics. The results have been complemented with the microstructural information obtained using advanced characterization techniques, such as small-angle neutron scattering (SANS) and computer micro-tomography (CMT) carried out on similar coatings. The trends shown by these different methods for variation in porosity with respect to feedstock characteristics were in agreement. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for the characterization of porosity.

Ion concentration caused by an external solution into the porewater of compacted bentonite

The concentrations caused by the external solution into the porewater were studied with compacted bentonite (MX-80), from which easily dissolving components had been removed in order to ensure that the ions in the porewater came from the external solution. The dry densities of the samples varied from 700 to 1700 kg/m 3 and NaCl solutions of 0.1–3 M were used as the external solution for saturation. The concentrations in the porewater were determined by the direct analysis of the squeezed porewaters and by dispersing the sample in deionized water. At high concentrations, the Donnan model can predict the concentrations in the porewater rather well. At low concentrations, where the ion exclusion is stronger, the measured concentrations are clearly higher than the modelled values. One possible explanation for this discrepancy is the microstructure of the bentonite, and an attempt to couple the effects of the microstructure and the Donnan model was made. It was assumed that there are two pore types, interlamellar pores in the montmorillonite stacks and large pores in the gel between the stacks. The dimensions of the microstructure were obtained from SAXS and BET(N 2) measurements. In this case, the fitting is much better, which supports the assumption of different pore types in bentonite.

Small-angle neutron scattering study of the role of feedstock particle size on the microstructural behavior of plasma-sprayed yttria-stabilized zirconia deposits

The microstructures of thick plasma-sprayed yttria-stabilized zirconia (8% mass fraction yttria) deposits were studied in a series of Porod small-angle neutron scattering (SANS) and multiple small-angle neutron scattering (MSANS) experiments. Three main void components were identified in the deposits: intrasplat cracks, interlamellar planar pores, and globular pores. The SANS and MSANS measurements were analyzed using the traditional theory for Porod scattering and a recently developed three-component model for MSANS evaluation. The average size, volume fraction, internal surface area, and orientation distribution for each void component within the deposits were determined. This study focused on gaining a better understanding of the effects of initial feedstock particle size and annealing temperatures on the microstructure of deposits sprayed under equivalent particle-impact conditions. Quantitative results are presented for each of four deposit samples: one prepared using the as-received feedstock particle wide-size distribution and three prepared from feedstock powder of different and relatively narrow particle size ranges with average sizes of 32, 47, and 88 μm. Except for the coarse (88 μm) feedstock powder, only mild monotonic variations were found in the microstructural anisotropies, the porosities (13 ± 1%), and the internal surface areas in the as-sprayed deposits. The internal surface area was independent of the feedstock particle size, even with the coarse feedstock. When the deposits were annealed at high temperatures (1100 and 1400 °C), the microstructures were altered with a reduction of the total internal surface area and a mild coarsening of the voids. These changes in the microstructural evolution were well-captured and described by the three-component model. The results were compared and related with those obtained from scanning electron microscopy images and elastic moduli measurements.

Transport of Moist Air Through Microporous Polyolefin Films

The technological revolution and busy schedule compel human society to search for better civic amenities. This leads to the use of modern personal care goods such as disposable diapers and sanitary napkins. Presently, all of the goods use microporous polyolefin films. The high-strength microporous polyolefin films also find applications in packaging industries. The polyolefins create a liquid barrier due to their hydrophobic character, whereas the microporosity allows permeation of moist air, thereby providing comfort. The introduction of microporosity in the polyolefin film is rather an art than a science. One of the techniques of generating microporosity is by the pulling of a polyolefin at the interface of a solid inclusion. The other technique includes pulling of pure polyolefin films, which are mainly used for packaging, to create interlamellar voids or pores in the films. This review article discusses the theory of diffusion through the microporous film and the various factors of microporosity such as the addition of filler, its nature (organic or inorganic), size and quantity, film drawing temperature, stretch ratio, its direction (uniaxial and biaxial), and above all, the type of polyolefins used. The aim of this review is to introduce readers to the fascinating science and technology of breathable microporous polyolefin films.

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y_2O_3-ZrO_2 (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y_2O_3 stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y_2O_3 in the t-zirconia began to decrease as predicted by the Y_2O_3-ZrO_2 phase diagram. The c-ZrO_2 phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO_2, which has a higher thermal conductivity than tetragonal or cubic zirconia.

Evolution of the void structure in plasma-sprayed YSZ deposits during heating

The evolution of the anisotropic void microstructure of plasma-sprayed yttria-stabilized zirconia (YSZ) deposits has been observed as a function of temperature by small-angle neutron scattering. Scattering experiments were carried out in-situ, in a furnace between 600 and 1400°C. The terminal slope (Porod scattering) of the scattering spectra was used to derive the specific surface area of the voids. For samples with sufficient scattering anisotropy, the two major void systems — intersplat (inter-lamellar) pores and intrasplat cracks — could be characterized separately. The pore and crack specific surface areas were found to depend on temperature differently. The specific surface area of the intrasplat cracks decreased markedly at temperatures below 1000°C, whereas the specific surface area of the intersplat pores began to decrease above 1000°C. This indicates important differences in the sintering of these two void systems probably related to their size and shape. Changes in the void surface were observed at temperatures as low as 800°C, a temperature comparable to, or less than, the usual operational temperature for this material.

Anisotropic Microstructure of Plasma-Sprayed Deposits

The microstructure of plasma sprayed deposits (PSD) is dominated by two void systems—interlamellar pores and intralamellar cracks—each with a different anisotropy. Anisotropy of these void systems and varying crack-to-pore ratios within PSDs are responsible for the anisotropic properties observed within deposits. While it is difficult to apply standard porosity measurement techniques to the assessment of anisotropic microstructures, novel techniques utilizing different approaches have recently emerged. Image analysis (IA) of impregnated PSD samples is the most direct technique. The structure is stabilized by impregnation and then polished and imaged. The limitations of IA lie in the impregnation process and in the subsequent polishing. Also, the images produced from anisotropic materials can be difficult to interpret quantitatively. The technique of small-angle neutron scattering (SANS) has recently been applied to the study of PSDs. The major advantages of SANS are that it does not require sample preparation and that quantitative information can be obtained concerning the separate crack and pore systems, including their distinctive anisotropies. However, the relationship between the SANS results and the underlying structure is more complex and less intuitive than for IA, and the availability of the SANS technique is limited by the need to have access to a powerful neutron source, such as a reactor. Also, the two techniques present different views of the microstructure because of the different sensitivities in different parts of the size range. This article compares results from IA and SANS for a set of thick plasma-sprayed ceramic deposits possessing a range of crack/pore microstructures and discusses how the two techniques might complement one another.

Influence of Spray Angle on the Pore and Crack Microstructure of Plasma‐Sprayed Deposits

To understand the influence of processing parameters on the microstructure of plasma‐sprayed deposits, smallangle neutron scattering measurements were made of the processing‐parameter‐dependent specific surface area of the voids in gray alumina deposits. These studies indicate that the voids are in the form of pores between the splats and around inclusions or unmelted particles, and are also in the form of cracks within the splats which may develop during cooling. The porous volume increases as the angle between the spray gun and the substrate (the “spray angle”) decreases. This study also indicates that the cracks are preferentially oriented, and that the crack orientation also depends on the spray angle. The interlamellar pores, however, are preferentially oriented parallel to the substrate surface, and the orientation of the pore is independent of the spray angle.

Interlamellar and Multilayer Nitrogen Sorption by Homoionic Montmorillonites

AbstractNitrogen sorption by various homoionic montmorillonites was studied at 78°K. The adsorption isotherms in the relative pressure range P/P0= 0.05-0.25 were found to be either Type I or Type II in the BET classification. The nitrogen sorption process was considered to be predominantly interla-mellar when described by a Type I isotherm. With a Type II isotherm, the adsorption was assumed to be predominantly on those surfaces not in the interlamellar regions.It was concluded that only cations within a certain size range promote significant interlamellar nitrogen penetration in montmorillonites. The role of the smaller cations in nitrogen sorption by montmorillonites seems to be their influence on the external aggregate structures. Very large cations tend to clog up the interlamellar pores as well as some of the external voids.