Characterization of zirconia specimens fabricated by ceramic on-demand extrusion

The Ceramic On-Demand Extrusion (CODE) process is a novel additive manufacturing method for fabricating dense (~99% of theoretical density) ceramic components from aqueous, high solids loading pastes (>50?vol%). In this study, 3?mol% Y₂O₃ stabilized zirconia (3YSZ) specimens were fabricated using the CODE process. The specimens were then dried in a humidity-controlled environmental chamber and afterwards sintered under atmospheric conditions. Mechanical properties of the sintered specimens were examined using ASTM standard test techniques, including density, Young’s modulus, flexural strength, Weibull modulus, fracture toughness, and Vickers hardness. The microstructure was analyzed and grain size measured using scanning electron microscopy. The results were compared with those from Direct Inkjet Printing, Selective Laser Sintering, Lithography-based Ceramic Manufacturing (LCM), and other extrusion-based processes, and indicated that zirconia specimens produced by CODE exhibit superior mechanical properties among the additive manufacturing processes. Several sample components were produced to demonstrate CODE’s capability for fabricating geometrically complex ceramic components. The surface roughness of these components was also examined.     

Mechanical properties of thin and thick coatings applied to various substrates. Part II. Young's modulus determination of coating materials*

To determine Young's modulus of coating materials when they are applied to substrates, theoretical and experimental analyses are performed. Significant residual stresses are generated within thin and thick coatings applied to substrates. As a result of these stresses, the bi-material strip assumes a certain curvature. The curved beam theory was used to establish the equivalent bending stiffness of bi-layer materials as functions of (a) the initial radius of curvature generated by residual stresses, (b) the mechanical radius of curvature during flexure testing, and (c) mechanical (Young's moduli) and geometrical (widths and thicknesses) characteristics of bi-layered systems. The relevant expression was transformed to a second- or third-order equation in order to calculate Young's modulus of the coating undergoing residual stresses (using models developed in Part I and by Stoney, Röll, and Inoue).     

Temperature dependence of Young’s modulus and damping of partially sintered and dense zirconia ceramics

Young’s modulus and damping of partially sintered and almost fully dense zirconia ceramics (tetragonal zirconia polycrystals with 3 mol.% yttria), obtained by firing to different temperatures (range 1000–1400°C), have been determined via impulse excitation, and the evolution of Young’s modulus and damping of partially sintered zirconia with temperature has been monitored from room temperature to 1400°C and back to room temperature. The room-temperature Young’s modulus of the partially sintered materials obeys the Pabst-Gregorová exponential prediction, which is relatively unusual for partially sintered materials. With increasing temperature Young’s modulus decreases, until the original firing temperature is exceeded and sintering (densification) continues, resulting in a steep Young’s modulus increase. During heating and cooling the temperature dependence obeys a master curve with a typical inflection point at approximately 200 °C, the temperature where damping (internal friction) exhibits a maximum. The reasons for this characteristic behavior of doped zirconia are recalled.     

Young’s modulus evolution during heating,re-sintering and cooling of partially sintered alumina ceramics

The Young modulus of partially and fully sintered alumina ceramics, obtained by firing to different temperatures (range 1200–1600°C), has been determined via impulse excitation, and the evolution of Young’s modulus of partially sintered alumina with temperature has been monitored from room temperature to 1600°C. As expected, the room-temperature Young modulus of the partially sintered materials is lower than all theoretical predictions. With increasing temperature Young’s modulus decreases, until the original firing temperature is exceeded and sintering (densification) continues, resulting in a steep Young’s modulus increase. During heating and cooling the temperature dependence obeys a master curve for alumina, unless the temperature of the original firing is excessively low.     

Phase mixture modeling of the grain size dependence of Young’s modulus and thermal conductivity of alumina and zirconia ceramics

The grain size dependence of Young’s modulus and thermal conductivity of alumina and zirconia ceramics is predicted via phase mixture modeling, using both analytical and numerical approaches. Using typical values for the thickness and properties of the grain boundaries, the equivalent volume fraction of “grain boundary phase” is calculated for a given grain shape. Based on this volume fraction estimate and a rough estimate of the grain boundary properties, the effective properties of the polycrystalline materials are calculated and compared in terms of volume-equivalent sphere diameters. For grains of cubic and tetrakaidecahedral shape excellent agreement is found between numerical calculations and analytical predictions based on the lower Hashin-Shtrikman bound. The grain size dependence is extremely weak for Young’s modulus, but can be more significant for thermal conductivity, especially when the intrinsic conductivity of the material is high. The predictions are compared to literature data.     

Young’s Modulus of Dry-pressed Ceramics: The Effect of the Binder

Measurements of Young’s modulus of green compacts prepared from spray-dried alumina powders containing two binders: polyvinyl alcohol or polyethylene glycol are reported. The variations of Young’s modulus with forming pressure are compared to those of mechanical strength and discussed in terms of glass transition temperature (T_g). When the T_g of the polymer is lower than the pressing temperature (case of PEG and moisture-plasticized PVA), the variation of the Young’s modulus is related to the evolution of the binder film covering the surface of primary particles inside the granules. Microcracking of the brittle external polymer-rich layer of granules seems to be responsible for a different evolution of Young’s modulus of green compact in the case of a binder with a T_g higher than the forming temperature (dry PVA).     

Experimental investigation and optimization of printing parameters of 3D printed polyphenylene sulfide through response surface methodology

Today fused filament fabrication is one of the most widely used additive manufacturing techniques to manufacture high performance materials. This method entails a complexity associated with the selection of their appropriate manufacturing parameters. Due to the potential to replace poly-ether-ether-ketone in many engineering components, polyphenylene sulfide (PPS) was selected in this study as a base material for 3D printing. Using central composite design and response surface methodology (RSM), nozzle temperature (T), printing speed (S), and layer thickness (L) were systematically studied to optimize the output responses namely Young's modulus, tensile strength, and degree of crystallinity. The results showed that the layer thickness was the most influential printing parameter on Young's modulus and degree of crystallinity. According to RSM, the optimum factor levels were achieved at 338°C nozzle temperature, 30 mm/s printing speed, and 0.17 mm layer thickness. The optimized post printed PPS parts were then annealed at various temperatures to erase thermal residual stress generated during the printing process and to improve the degree of crystallinity of printed PPS's parts. Results showed that annealing parts at 200°C for 1 hr improved significantly the thermal, structural, and tensile properties of printed PPS's parts.     

Temperature Effect on the Mechanical Properties of Electrospun PU Nanofibers

Polyurethane (PU) nanofibers were prepared from electrospun method. Atomic force microscopy (AFM) was employed to characterize the mechanical properties of electrospun PU nanofibers. The impact of temperature on the mechanical behavior of PU nanofibers was studied using three-point bending test based on AFM. A Young’s modulus of ~?25?GPa was obtained for PU nanofibers with diameter at ~?150?nm at room temperature. With decrease in nanofiber’s diameter, the increasing Young’s modulus can be due to the surface tension effect. The Young’s modulus of the PU nanofiber decreased linearly while the fibrous morphology was maintained with the increase of temperature.     

Effect of composition and high-temperature annealing on the local deformation behavior of silicon oxycarbides

Silicon oxycarbides with varying compositions were investigated concerning their elastic and plastic properties. Additionally, the impact of thermal annealing on their elastic properties was assessed. Phase separation of SiOC seems to have no significant impact on Young’s modulus (high values of β-SiC compensate the low values of the vitreous silica matrix) and hardness. However, it leads to an increase in Poisson’s ratio, indicating an increase in the atomic packing density. The phase composition of SiOC significantly influences Young’s modulus, hardness, brittleness and strain-rate sensitivity: the amount of both β-SiC and segregated carbon governs Young’s modulus and hardness, whereas the fraction of free carbon determines brittleness and strain-rate sensitivity. Thermal annealing of SiOC glass-ceramics leads to an increase in Young’s modulus. However, the temperature sensitivity of Young’s modulus and Poisson’s ratio is not affected, indicating the glassy matrix being stable during thermal annealing. A slightly improved ordering of the segregated carbon and the β-SiC nanoparticles upon thermal annealing was observed. It is suggested that this is responsible for the increase in Young’s modulus.     

Computer modeling of systematic processing defects on the thermal and elastic properties of open Kelvin-cell metamaterials

Open-cell metamaterials prepared by additive manufacturing or replica techniques are typically prone to processing defects resulting from limited resolution, strut cross-section variations or internal strut porosity. These defects are expected to cause deviations from the ideal (CAD-based or template-based) target microstructures and thus from the envisaged properties. This paper investigates some of these effects in a quantitative manner. Based on computer-generated open Kelvin-cell (tetrakaidecahedral) alumina-based metamaterials, the effective thermal conductivity and elastic constants, mainly Young’s modulus, are calculated in dependence of the voxel size, strut thinning and strut wall thickness. It is shown that the porosity dependence of smooth, straight and full struts agrees closely to the Gibson-Ashby prediction for open-cell foams, while limited resolution and strut thinning leads to property values that tend to be lower and hollow struts lead to higher property values. The Pabst-Gregorová cross-property relation gives an excellent prediction of the conductivity-modulus correlation in all cases.     

Influence of the concentration and nature of carbon nanotubes on the mechanical properties of AA5083 aluminium alloy matrix composites

Carbon nanotubes were shown to reinforce aluminium alloy composites prepared by powder metallurgy and hot-extrusion techniques. We study the influence of the carbon nanotube concentration on the composite Young’s modulus and yield strength, which leads to an estimation of the nanotubes Young’s modulus and aspect ratio using analytical models. We compare the reinforcement achieved with single-walled and multi-walled carbon nanotubes. Finally, we discuss how to optimise the mechanical properties of carbon nanotube-reinforced aluminium alloy composites.     

Comparison of test methods estimating the stiffness of ultrathin coatings

A key engineering parameter of thin coatings is their stiffness. Stiffness characterization of ultrathin coatings with a nanometer scale thickness is experimentally challenging. In this work, three feasible methods have been used to estimate the Young’s modulus of metal coatings on polymer films. The methods are: (1) nanoindentation, (2) strain-induced elastic buckling and (3) peak-force measurements integrated in atomic force microscopy. The samples were prepared by atomic layer deposition of TiO₂ (6 and 20 nm thick) and mixed oxides of TiO₂ and Al₂O₃ (4 and 20 nm thick). The differences in estimated Young’s modulus are interpreted in terms of the underlying assumptions and test conditions. Their specific advantages and drawbacks are also compared and discussed. In particular, the nanoindentation necessitates a sufficiently sharp indenter tip to make localized measurements dominated by the coating. The strain-induced elastic buckling method is simple in practice, but showed a large scatter due to variation in local coating thickness and irregular deformation patterns. The stiffness characterization using atomic force microscopy gave the most consistent results, due to a sharp tip with a radius comparable to the thinnest coating thickness. All methods gave a higher Young’s modulus for the TiO₂ coating than for the mixed oxide coating, with a variation within one order of magnitude between the methods.     

Numerical prediction of elastic properties for alumina green parts printed by stereolithography process

Stereolithography is a process based on the photopolymerization of a UV-reactive system consisting of ceramic particles dispersion in a curable resin. A key issue of this process is the control of the rigidity of green parts, which are strongly related to UV light exposure. This work is focused on the numerical prediction of green part stiffness according to stereolithography manufacturing parameters. A first macroscopic approach, based on the modelling of ceramic suspension polymerization, makes it possible to establish a relationship between the exposure and the Young's modulus. A second microscopic approach, using a periodic homogenization technique based on the strain energy, is applied to a 2D finite element model to evaluate the effective elastic properties. Numerical results show that macroscopic model is able to provide a Young’s modulus with a good level of accuracy. The modelling results from the microscopic model demonstrate an acceptable convergence with the experimental Young’s modulus.     

On the prediction of graphene’s elastic properties with reactive empirical bond order potentials

The elastic properties of graphene as described by the reactive empirical bond order potential are studied through uniaxial tensile tests calculations at both zero temperature, with a conjugate gradient approach, and room temperature, with molecular dynamics simulations. A perfect linear elastic behavior is observed at 0 K up to ≈0.1% strain. The Young’s modulus and Poisson’s ratio obtained with this potential are of ≈730 GPa and 0.39, respectively, with little chirality effects. These values differ significantly from former estimations, much closer to experimental values. We show that these former values have certainly been obtained by neglecting the effect of atomic relaxation, leading to a severe inaccuracy. At larger strains, an extended apparent linear domain is observed in the stress–strain curves, which is relevant to Young’s modulus calculations at finite temperature. Our molecular dynamics simulations at 300 K have allowed obtaining the following, chirality dependent, apparent Young’s moduli, 860 and 761 GPa, and Poisson’s ratios, 0.12 and 0.23, for armchair and zigzag loadings, respectively.     

Curvature and stresses for bi-layer functional ceramic materials

The technical application of layered functional ceramic components is challenged by curvature effects and residual stresses originating mostly from the thermal mismatch or chemical strains of the joined materials. Based on the general solution for elastic deformation of monolithic and multilayered materials the determination of curvature and residual stress for linear elastic bi-material specimens with chemical strains, chemical reduction in stiffness, shape variations, gradients in elastic modulus or thermal expansion is outlined. The use of the relationships is exemplified for ceramic solid oxide fuel cell (SOFC) and ceramic membrane materials. For SOFCs curvature changes are considered resulting from the reduction of the anode and crystallization of a glass–ceramic sealant with semi-spherical shape. For gas separation membranes which currently under development for fossil power plants the effect of chemical strains is assessed. The limits of using analytical relationships are addressed for the warpage of thin, rectangular SOFCs.     

High temperature stiffening of ferroelastic LaCoO3

The cyclic ferroelastic hysteretic behavior of pure LaCoO₃ perovskite ceramic has been studied at different temperatures in four point bending. The stress-strain deformation behavior of LaCoO₃ was analyzed both in the term of the maximum stress in the cycle and in terms of the temperature used when the cyclic testing was performed. The characteristics of the stress-strain hysteresis loops, such as hysteresis loop area and irreversible strain, as well as effective Young’s modulus, were analyzed, and it was established that both the loading and the temperature history have a significant influence on the mechanical behavior of LaCoO₃. Young’s modulus values are reported to be much higher in the 700–900 °C temperature range as compared to the measurements performed in the RT-400 °C temperature range.     

Young’s modulus and thermal conductivity of model materials with convex or concave pores – from analytical predictions to numerical results

The effective Young’s modulus and thermal conductivity of porous materials can be rigorously bounded from above via micromechanical bounds (upper Wiener–Paul bounds and upper Hashin–Shtrikman bounds), and several model relations are commonly used as tentative approximate predictions (Maxwell-type, Coble–Kingery-type, power-law and exponential relations). Based on numerical calculations on computer-generated digital model microstructures, both periodic and random, it is shown that these model relations provide rough approximations that are more or less appropriate for microstructures with essentially convex pores, but are not suitable for microstructures with concave pores. On the other hand, the Pabst–Gregorová cross-property relation provides a very accurate (better than 0.04 relative property units) analytical prediction for the relative Young’s modulus of isotropic porous materials with isometric pores, both convex and concave, when the relative thermal conductivity is known. It is shown that this cross-property relation is the best prediction currently available for isotropic porous materials with isometric pores.     

A novel theoretical model to predict the temperature-dependent fracture strength of ceramic materials

A novel temperature-dependent fracture strength model for ceramic materials is developed, based on a critical fracture energy density associated with material fracture comprising strain energy, the corresponding equivalent potential energy, and kinetic energy of atoms per unit volume. It relates the fracture strength at high temperatures to that at the reference temperature, the temperature-dependent Young’s modulus, the temperature, and the melting point. The model is verified by comparison with experimental data of ceramic materials. The model predictions and the experimental data are in excellent agreement with each other. As the Young’s modulus can easily be obtained by experiments and the melting point can easily be obtained by materials handbook, the model can easily predict the fracture strength of ceramic materials at arbitrary temperatures.     

Mechanical properties of lanthanum zirconate-based composite thermal barrier coatings

Lanthanum zirconate is a promising candidate material for thermal barrier coating (TBC) applications due to its low thermal conductivity and high temperature phase stability. However, its application is limited by thermal durability caused by low fracture toughness and low coefficient of thermal expansion. We recently developed LZ/8YSZ composite TBC systems using blended LZ and 8YSZ powders, which have demonstrated excellent thermal cycling performance. In this study, the mechanical properties of the composite TBCs were characterised using both nanoindentation and Vicker’s microhardness tests. The nanoindentation results show that both Young’s modulus and nanohardness increase with increasing 8YSZ content, suggesting the mechanical properties can be tailored by changing the volume ratio of 8YSZ. The ratios of Young’s modulus to nanohardness remain constant, ～18, irrespective to the coating’s composition. The microhardness results show the same dependence with 8YSZ content, which is confirmed by the analytic models based on composite theory.     

Effects of functional groups on the mechanical and wrinkling properties of graphene sheets

The effects of the degree of functionalization, molecular structure and molecular weight of functional groups on the Young’s modulus of graphene sheets were investigated through molecular dynamics and molecular mechanics simulations. The dependence of shear modulus, strength and critical wrinkling strain of graphene sheets on the chemical functionalization was also examined. It is found that Young’s modulus depends greatly on the degree of functionalization and molecular structure of the functional groups, while the molecular weight of the functional groups plays a minor role in determining Young’s modulus. The chemical functionalization also reduces the shear modulus and critical wrinkling strain. The binding energy between the functional groups and the graphene sheets is mainly responsible for these findings.