Strain-induced multiscale structural changes in lamellar thermal barrier coatings

The thermal mismatch stress, as well as residual stress, in coating/substrate systems often leads to structural changes and subsequent coating debonding in the systems. This study focused on the changes induced in the microstructure and properties of lamellar yttria-stabilized zirconia coatings upon heating, with the aim of elucidating their starting microstructure prior to sintering. The results showed that the combined effect of the residual stress and the thermal mismatch stress results in scale-sensitive changes in the properties of the coatings. The macroscale properties changed significantly, while the microscale properties changed only slightly. Structural characterization revealed that a certain degree of expansion at the tips of both the intersplat pores and the intrasplat cracks occurs, contributing to the microscale structural changes observed in most regions. Moreover, a few mesoscale cracks covering several layers were also observed. A lamellar structural model was developed to correlate the multiscale structural changes observed with those in the properties. Finally, this study revealed that the actual starting structure of plasma-sprayed thermal barrier coatings prior to sintering is different from that in the as-deposited state. This should aid in obtaining an in-depth understanding on the microstructural and properties evolution of the constrained coatings under actual service conditions.     

Multi‐scale analysis of acoustic emission signals in dense‐phase pneumatic conveying of pulverized coal at high pressure

Acoustic emission technique in conjunction with multiscale processing method has been utilized to investigate the flow behavior of the dense‐phase pneumatic conveying system at high pressure. A clearly defined classification of microscale, mesoscale, and macroscale signals has been put forward with the aid of wavelet transform and V statistics analysis. The detailed signals d₁–d₄, d₅–d₇, d₈–d₁₀ were recomposed into the microscale, mesoscale, and macroscale signals, respectively, which represent microscale particle‐wall interactions, mesoscale interaction between gas phase and solid phase (such as bubbles, plugs, dunes), and macroscale flow‐induced pipe vibration. Further analysis shows that as the mass flow rate of pulverized coal increases, the energy fraction (energy of detailed signal divided by the energy of original signal) of microscale signals decreases while that of mesoscale signals increases, which indicates that particles are more likely to move as particle aggregates than individual particles when mass flow rate increases. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2635–2648, 2016     

A novel method for fabricating brick-mortar structured alumina-zirconia ceramics with high toughness

The present work reports a novel and simple approach to prepare alumina-zirconia composites with superior toughness. Alumina microspheres were innovatively used as the raw materials, followed by coating zirconia and hot-pressing sintering to fabricate alumina-zirconia ceramics. The resultant ceramics are given a unique brick-mortar microstructure, in which the zirconia “mortar” layers continuously distribute around the alumina “brick” matrix, leading to outstanding fracture toughness of 7.34 MPa·m^1/2 and high strength of 635.84 MPa when prepared with zirconia contents of 10 wt%. The major explanation could be ascribed to that crack tips in sintered samples tend to propagate along the zirconia “mortar” layer, accompanied by deflection and branching, which effectively improve the fracture toughness of composites. The uniformity and integrity of the brick-mortar structure could be well tuned by varying the amount of zirconia. This method has reference significance for the preparation of high toughness alumina-based multiphase ceramics.     

Effect of binder system on the thermophysical properties of 3D-printed zirconia ceramics

Fabrication of 3D-printed ceramic parts with high complexity and high spatial resolution often demands low wall thickness as well as high stiffness at the green state, whereas printing simpler geometries may tolerate thicker, more compliant walls with the advantage of a rapid binder-burn-out and sintering process. In this work, the influence of the binder system on the thermophysical properties of 3D-printed stabilized zirconia ceramics was investigated. Samples were fabricated with the lithography-based ceramic manufacturing (LCM) technology using two different photosensitive ceramic suspensions (LithaCon 3Y230 and LithaCon 3Y210), with the same ZrO₂ powder. A significant difference in stiffness in the green state (~3 MPa vs. ~32 MPa for LithaCon 3Y230 and LithaCon 3Y210, respectively) was measured, associated with a rather loose or a linked network formed in the binder due to photopolymerization. Both materials reached high relative densities, that is, >99%, exhibiting a homogeneous fine-grained microstructure. No significant differences on the coefficient of thermal expansion (11.18 ppm/K vs. 11.17 ppm/K) or Young's modulus (207 GPa vs. 205 GPa) were measured, thus demonstrating the potential of tailoring binder systems to achieve the required accuracy in 3D-printed parts, without detrimental effects on material's microstructure and thermophysical properties at the sintered state.     

Mechanical properties of cellular ceramics obtained by gel casting: Characterization and modeling

Dense and cellular ceramics were produced from yttria partially stabilized zirconia powders by gel-casting, using agar as a gelling agent and polyethylene spheres (125–300 μm diameter) as volatile pore forming agent to create 50–65 vol.% spherical macropores, uniformly distributed in a microporous matrix.The mechanical properties of both dense and porous samples were investigated at the microscale by nanoindentation testing. The influence of micro-porosity on the mechanical properties of samples was evaluated by the analysis of hardness and modulus depth profiles, coupled with FIB-SEM section observations of selected indentation marks. The intrinsic elastic modulus of the zirconia phase resulted to be of the order of 220 GPa. Mechanical characterization at the macroscale consisted of uniaxial compression tests and four point bending tests. Elastic moduli of about 170 GPa were measured for about 93% dense ceramics, lowering down to 44 and 13 GPa with the addition 50 and 65 vol.% macropores, respectively. Digital image based finite element analysis (DIB-FEA) procedures were implemented in order to verify their applicability for the prediction of mechanical behavior of this type of cellular materials: results confirmed that a very good match between measured and calculated values of elastic modulus can be achieved, provided that the effects of micro-porosity are considered by the proper choice of the elastic properties to be assigned to each individual phase identified by Image Analysis.     

Bubbling fluidized bed methanation study with resolving the mesoscale structure effects

To elucidate the influence of the changes in the mesoscale structure caused by microscale chemical reactions on the macroscale properties, continuity equations, simplified momentum equations, and pressure-drop balance equation were introduced into the conventional reactor model to accurately analyze the mesoscale phenomenon in the fluidized bed methanation system, thereby avoiding the limitation due to the use of empirical correlations to describe the mesoscale structure. The reduction of gas volume and the variations of composition and physical properties due to methanation reactions were considered in this model, and the effects of these changes on the mesoscopic flow structures and the macroscopic properties of the entire reactor were further investigated. Several experiments of fluidized bed methanation were also carried out to verify the effectiveness of the proposed reactor model. The results showed that the suggested model can successfully predict the hydrodynamics and reaction behaviors under various operating conditions.     

Microstructure and Young’s modulus evolution during re-sintering of partially sintered alumina-zirconia composites (ATZ ceramics)

Partially and fully sintered alumina-zirconia composites (ATZ ceramics), with porosities decreasing from 53.5 to 1 %, have been prepared by uniaxial pressing and firing at 1000–1500 °C and characterized by the Archimedes method and mercury intrusion porosimetry. Young’s modulus has been measured via the impulse excitation technique at room temperature, resulting in an almost exponential porosity dependence (which is unusual for partially sintered ceramics in which the microstructure is dominated by concave pore surfaces), and at elevated temperatures up to 1500 °C during heating and cooling, resulting in a temperature master curve with a low-temperature inflection point around 200 °C (accompanied by a damping maximum). Both results confirm previous findings for zirconia and are typical for zirconia-containing ceramics. When the original firing temperature is exceeded, sintering and densification continues, albeit with a temperature lag when the sintering activity (specific surface area) is reduced as a consequence of previous firing.     

Preparation of Alumina-Zirconia Powders by Evaporative Decomposition of Solutions

Fine-grained, homogeneously dispersed alumina-zirconia and zirconia powders were prepared by evaporative decomposition of solutions. The pure metastable tetragonal zirconia powder transformed to the monoclinic form when it was heated to 1150°C. The zirconia in the alumina-zirconia powder, which was also in the tetragonal form, did not transform when the powder was heated to 1150° C. This result is explained in terms of inhibition of coarsening of the zirconia grains by the alumina particles.     

Microstructural Evolution in Some Silicate Glass–Ceramics: A Review

Just as the microstructures in glass–ceramics encompass the range from nanocrystalline transparent materials to microcrystalline tough materials, so the paths of microstructural evolution in glass–ceramics vary widely. Evolution can proceed in numerous ways, their genesis being a perturbation of some type, including the surface nucleation used in glass frit processing, crystallization of the primary phase or phases upon distinct crystalline nuclei, and nucleation promoted by nano- or microscale amorphous phase separation in the parent glass. Examples of the crystallization history of several glass–ceramic materials are described, with emphasis on how their microstructural evolution influences their ultimate physical and optical properties.     

Micro- and macroscale coefficients of friction of cementitious materials

Millions of metric tons of cementitious materials are produced, transported and used in construction each year. The ease or difficulty of handling cementitious materials is greatly influenced by the material friction properties. In the present study, the coefficients of friction of cementitious materials were measured at the microscale and macroscale. The materials tested were commercially-available Portland cement, Class C fly ash, and ground granulated blast furnace slag. At the microscale, the coefficient of friction was determined from the interaction forces between cementitious particles using an Atomic Force Microscope. At the macroscale, the coefficient of friction was determined from stresses on bulk cementitious materials under direct shear. The study indicated that the microscale coefficient of friction ranged from 0.020 to 0.059, and the macroscale coefficient of friction ranged from 0.56 to 0.75. The fly ash studied had the highest microscale coefficient of friction and the lowest macroscale coefficient of friction.     

Sintering Behavior of Nanocrystalline Zirconia Doped with Alumina Prepared by Chemical Vapor Synthesis

Powders of nanocrystalline zirconia doped with 3–30 mol% alumina have been synthesized using chemical vapor synthesis (CVS). Dense or mesoporous ceramics of small and narrowly distributed grain and pore sizes in the nanometer range are obtained via pressureless vacuum sintering. The microstructural development of the doped samples is strongly dependent on the alumina content. Sintering of zirconia samples with 3 and 5 mol% alumina at temperatures of 1000°C for 1 h results in fully dense, transparent ceramics with grain sizes of 40–45 nm and homogeneous microstructures.     

Three scale thermomechanical theory for swelling biopolymeric systems

A three-scale theory for the swelling polymeric/biopolymeric media is developed via the hybrid mixture theory. At the microscale, the solid polymeric matrix interacts with the solvent through surface contact. At the mesoscale, the homogeneous mixture of vicinal fluid and solid polymers exchanges thermodynamic properties with two bulk fluids, one of which is of the same type as the vicinal fluid. The relaxation processes within the polymeric matrix are incorporated by modeling the solid phase as viscoelastic and the solvent phases as viscous at the macroscale. We obtain novel equations for the total stress tensor, chemical potential of the solid phase, heat flux and Darcy's law all at the macroscale. Viscoelastic stress components in Darcy's law make it applicable for both Fickian and non-Fickian fluid transport. The form of the generalized Fick's law is similar to that obtained in earlier works involving colloids. Thermoviscoelastic and thermoviscous effects are incorporated by coupling thermal gradients with strain-rate tensors for the solid phase and the deformation-rate tensors for the liquid phases.     

Zirconia ceramics in metal-free implant dentistry

Because of their outstanding mechanical properties, chemical stability, and biocompatibility, 3-mol % yttria-stabilised tetragonal zirconia polycrystals (3Y-TZP), known as zirconia ceramics in dentistry, are an important choice for various types of prosthesis. In addition to extensive use for crown and bridge construction, considerable interest has been generated for applications in implant dentistry, including full-contour zirconia crowns as supra-constructions, zirconia abutments, and novel zirconia implants. However, their use among dentist and researchers is controversial, especially compared with the well-established implants made of titanium alloys. As a latecomer, the merits and limitations of 3Y-TZP are awaiting careful investigation. Design, manufacturing, and clinical operation guidelines are urgently needed. The aim of this review was to address the present status of the application of zirconia ceramics related to implant dentistry by analysing the published data from both in vitro and in vivo studies. Suggestions are provided for potential improvements and suitable applications of zirconia ceramics in metal-free implant dentistry.     

Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia

In this research, the comparison between microwave sintering and conventional sintering on the mechanical properties and microstructural evolution of 3?mol% yttria-stabilised zirconia were studied. Green bodies were compacted and sintered at various temperatures ranging from 1200?°C to 1500?°C. The results showed that microwave assisted sintering was beneficial in enhancing the densification and mechanical properties of zirconia, particularly when sintered at 1200?°C. It was revealed that as the sintering temperature was increased to 1400?°C and beyond, the grain size and mechanical properties for both microwave- and conventional-sintered ceramics were comparable thus suggesting that the sintering temperature where densification mechanism was activated, grain size was strongly influenced by the sintering temperature and not the sintering mode.     

Flash microwave sintering of zirconia by multiple susceptors cascade strategy

In the field of flash sintering, microwave energy represents an interesting way to densify ceramics complex shapes, thanks to a contactless volumetric heating. Attaining a fast and homogeneous heating is a critical parameter and hybrid heating, using silicon carbide susceptors, is generally used. In this study, an original multiple susceptors cascade strategy is developed, using both SiC and 3D-printed ZrO₂ susceptors. This novel configuration follows perfectly the flash heating scheme, even for high heating rates up to 1000 K.min^-1 and leads to a high stability of the “flash” hybrid heating. Flash microwave sintering produced dense (97 % relative density) microstructures within 45 s. Based on comprehensive multiphysics simulations of the overall process, in-situ dilatometry measurements, kinetics method analysis and microstructural characterizations, this work highlights the sintering behavior of zirconia and the temperature distribution during flash microwave sintering.     

The role of carbon nanotubes on the stability of tetragonal zirconia polycrystals

The effect of single walled carbon nanotubes (SWNT) at zirconia grain boundaries on the stability of a tetragonal zirconia polycrystalline matrix has been explored in as–sintered composites and after low–temperature hydrothermal degradation (LTD) experiments. For this purpose, highly–dense 3?mol% Y₂O₃–doped tetragonal zirconia polycrystalline (3YTZP) ceramics and SWNT/3YTZP composites were prepared by spark plasma sintering (SPS). Quantitative X–ray diffraction analysis and microstructural observations point out that an increasing amount of well–dispersed SWNT bundles surrounding zirconia grains decreases the metastable tetragonal phase retention in the ceramic matrix after sintering. In contrast, the tetragonal ceramic grains in composites with SWNTs are less sensitive to the presence of water, i.e. to undergo a martensitic transformation under LTD conditions, than monolithic 3YTZP ceramics. The SWNT incorporation diminishes micro–cracking due to tetragonal to monoclinic ZrO₂ phase transformation in the composites.     

Numerical calculation of particle transport in turbulent wall bounded flows

The past 25 years has seen particle technology grow from an under-funded and widely scattered research enterprise to a thriving globally recognized engineering discipline. Despite this change in the research environment, design and analysis of industrial particulate processes remain rooted in empiricism. Scale-up is largely heuristic, and quantitative design methods are non-existent. This is in stark contrast to fluid-phase systems, for which accurate scale-up and design methods have existed for decades.Why is this? One explanation lies in the traditional approaches to studying particle technology operations. Typically, these are studied at two length scales: the macroscale (unit operation level) and the microscale (particle level). Relatively little attention has been directed at an intermediate length scale, the mesoscale, which is characteristic of a “homogeneous” powder. This situation is analogous to ignoring classical thermodynamics and transport properties of fluids in the analysis of fluid-phase operations, instead trying to model unit operation performance directly using statistical mechanics or molecular dynamics.Advances in the design and analysis of particle technology operations requires filling of this “scale gap.” This can be accomplished by studying particulate systems in an analogous way to that used to study fluid-phase unit operations. This will require detailed investigation of mass, momentum, and energy transport in existing unit operations; development of experimental systems with well-defined and characterized flow fields; measurement of powder properties (transport properties and transformation kinetics) in these “simple” experimental systems; validation of theory and simulation against these data; and integration of these theories and simulations into system-level models.     

The influence of hafnium removal on the microstructure and properties of 8YSZ ceramics

The effects of hafnium removal on the sinterability, phase composition, and microstructural, mechanical, and electrical properties of 8YSZ (8 mol% yttrium stabilized zirconia) were investigated using SEM, XRD, Raman spectroscopy, EBSD, three-point bending, Vickers indentation, and impedance spectroscopy. The 8YSZ and 8YSZ₀ (8 mol% yttrium-stabilized hafnium-free zirconia) ceramics were prepared via dry pressing and atmospheric sintering, respectively. The overall mechanical properties of the 8YSZ₀ ceramic were poor. However, at a sintering temperature of 1450°C, the relative density of 8YSZ and 8YSZ₀ ceramics was almost identical. 8YSZ₀ had a slightly smaller grain size and activation energy, and its electrical properties were slightly better than those of the 8YSZ ceramics. The presence of tetragonal secondary phases in the cubic structure of 8YSZ ceramics inhibited crack propagation and led to an increase in the mechanical properties and a decrease in the ion conductivity. In terms of the crystal structure, the increase in the cubic phase lattice parameters and tetragonal phase c/a values of the 8YSZ₀ ceramics was attributed to the larger Zr⁴⁺radius, reduced local lattice distortion, and increased matrix oxygen vacancy concentration and cubic phase content. The EBSD analysis results indicated that there was no significant difference in grain orientation between the two types of ceramics, but the content of 8YSZ ceramics in large angle grain boundaries was slightly higher, especially in special grain boundaries Σ3 and Σ9. Therefore, this material can be used as a solid-state electrolyte candidate.     

Flash Spark Plasma Sintering of 3YSZ: Modified sintering pathway and impact on grain boundary formation

Yttria stabilized zirconia (3 mol% YSZ) ceramics were prepared by Flash-SPS, while allowing high heating rates up to 200 °C/s, which led to the extremely fast densification within a few seconds. The high heating rates had strong impact on sintering mechanisms, in terms of densification and grain growth. While the specimens ended with 5–15 vol% porosity and limited grain growth (< 350 nm), their hardness is higher than fully dense counterpart SPSed ceramics. Using the sintering trajectories, microstructural observations, and impedance spectroscopy, we highlight altered sintering mechanism which resulted in very thin grain boundaries compared to SPS. It appears that densification is largely advanced at grain boundary interfaces, with no residual nano-pores at the grain junctions, where some pores with size comparable to grain size were present. This opens up opportunities for the fabrication of porous lightweight ceramics with good mechanical properties.     

Facile processing of oriented macro-porous ceramics with high strength and low thermal conductivity

Unidirectionally oriented architectures demonstrate a notable efficiency in enhancing the properties of macro-porous materials, yet are difficult to construct in a time- and cost-effective fashion. Here a facile approach was exploited for fabricating oriented macro-porous ceramic materials by employing natural graphite flakes as a fugitive material and preferentially aligning the flakes within ceramic matrices using accumulative rolling technique. Flaky to near-ellipsoid shaped pores with a homogeneous distribution were created in macro-porous zirconia ceramics with their porosity and microstructural characteristics adjustable by controlling the additive amounts of graphite flakes. The resulting materials exhibited a good combination of properties with high compressive strength up to over 1.5 GPa, which exceeds those of most other porous zirconia ceramics with similar porosities, along with low thermal conductivity of 0.92–1.85 Wm^?1·K^?1. This study offers a simple means for developing new oriented macro-porous materials with enhanced properties, and may promote their application by allowing for easy mass production.