Functional Gradient Ceramics, and Thermal Barriers

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Authors: Lisa Palmqvist, Karin Lindqvist, Chris Shaw
Abstract: Porous piezoelectric ceramics are of interest for hydrophones and medical imaging because of their enhanced coupling with water or biological tissue due to acoustic impedance matching. Multilayer lead zirconate titanate (PZT) substrates with dense and porous interlayers were produced by tape casting of aqueous PZT slips with high solids contents. The use of latex binders with low viscosity enabled addition of starch as a fugitive additive to create air/ceramic composites with ´3-3´connectivity. Microstructures, piezoelectric and mechanical properties of sintered, poled laminates were evaluated. The relative permittivity, ε33, decreased by 40% for laminates with porous interlayers compared to dense ones, whereas the relative decrease in piezoelectric longitudinal coefficient, d33, was 35%. Laminates with porous interlayers maintained 72% of their bending strength compared to dense ones.
Authors: Jonas Gurauskis, Antonio Javier Sanchez-Herencia, Carmen Baudín
Abstract: The production of multilayer ceramics by laminating stacked green ceramic tapes is one of the most attractive methods to fabricate layered materials. In this work, a new lamination technique was employed to obtain laminated ceramic structures in the aluminazirconia system with residual stress compression at the outer layers. This reinforcement mechanism would lead to ceramics with changed material properties and R-curve behaviour. The optimization of processing parameters for fabrication of defect free monolithic and laminated structures is described. The residual stresses developed in the laminated structures are discussed in terms of the results obtained from piezo-spectroscopic technique measurements and finite element method calculations.
Authors: Jana Andertová, Jiří Havrda, Radek Tláskal
Abstract: The work deals with preparation of functional gradient alumina ceramics with controlled porosity by slip casting method of aqueous alumina suspension containing pore-generating agent. The sol-gel transition of AlO(OH) was employed to stabilize pore-generating agent in the suspension. The composite bodies with layers of variable porosity were prepared. Based on dilatometer measurement the admission difference of irreversible dilatation changes #αirr between compounded layers was determined in order to prepare defect free bi-layer bodies. The dependence of physical and mechanical properties of as fired composite bodies on the porosity value was expressed.
Authors: Valeria Cannillo, Luca Lusvarghi, Tiziano Manfredini, M. Montorsi, Cristina Siligardi, A. Sola
Abstract: The present work was focused on glass-alumina functionally graded materials. The samples, produced by plasma spraying, were built as multi-layered systems by depositing several layers of slightly different composition, since their alumina and glass content was progressively changed. After fabricating the graded materials, several, proper characterization techniques were set up to investigate the gradient in composition, microstructure and related performances. A particular attention was paid to the observation of the graded cross sections by scanning electron microscopy, which allowed to visualize directly the graded microstructural changes. The scanning electron microscopy (SEM) inspection was integrated with accurate mechanical measurements, such as systematic depth-sensing Vickers microindentation tests performed on the graded cross sections.
Authors: Shui Gen Huang, Omer Van der Biest, Jef Vleugels, Kim Vanmeensel, Lin Li
Abstract: In this study, 2 mol% Y2O3 (2Y), 1 mol% Y2O3+6 mol% CeO2 (1Y6Ce) and 12 mol% CeO2 (12Ce)-doped tetragonal ZrO2 ceramics were made by spark plasma sintering (SPS) for 2 min at 1450°C under a pressure of 60 MPa. The influence of stabilizers on microstructure, phase and mechanical properties of the ZrO2 ceramics was investigated. After sintering, the 2Y and 1Y6Ce were intact, containing full tetragonal ZrO2 phase on the polished cross-sectioned surface, whereas the 12Ce exhibited macro-cracks, corresponding to a large amount of monoclinic ZrO2 phase. Graded microstructure and mechanical properties were observed in the 1Y6Ce, showing a gradually decreased fracture toughness from sample edge towards centre, together with the slight decreased hardness. The 2Y had a uniform microstructure and mechanical properties. The formation of the graded structure and toughness profile was explained in terms of the gradual CeO2 reduction to Ce2O3 in the Y2O3+CeO2 doped ZrO2 ceramics.
Authors: Akhilesh Kumar Swarnakar, S. Giménez, Sedigheh Salehi, Jef Vleugels, Omer Van der Biest
Abstract: The Impulse Excitation Technique (IET) is a non-destructive technique for evaluation of the elastic and damping properties of materials. This technique is based on the mechanical excitation of a solid body by means of a light impact. For isotropic, homogeneous materials of simple geometry (prismatic or cylindrical bars), the resonant frequency of the free vibration provides information about the elastic properties of the materials. Moreover, the amplitude decay of the free vibration is related to the damping or internal friction of the material. At present, IET is a well-established non-destructive technique for the calculation of elastic moduli and internal friction in monolithic, isotropic materials. Standard procedures are described in ASTM E 1876-99 and DIN ENV 843-2. IET can also be performed at high temperature (HT-IET) using a dedicated experimental setup in a furnace and constitutes a valuable tool in the field of mechanical spectroscopy. In the present work, the most recent advances in high temperature characterization using IET at K.U. Leuven are presented: the deformation behaviour of WC-Co hard metals, softening phenomena in TiB2, relaxation mechanisms in ZrO2 composites and “in-situ” monitoring of the damage evolution in uniaxially pressed metallic green compacts during delubrication.
Authors: Javier Pascual, Francis Chalvet, Tanja Lube, Goffredo de Portu
Abstract: In asymmetrical 3-layer laminates with constant overall and inner layer thickness, the residual compressive stresses in the two outer layers are not longer the same: compression is higher in the thinner layer.Therefore, it can be expected that the strength also depends on the outer layer thickness. Experimental evidence of this behavior was found by measuring the bending strength of asymmetrical tri-layers. A value of the thermal expansion mismatch was determined by fitting the theoretical expression to the experimental data.
Authors: Lorenzo Micele, Mylene Brach, Francis Chalvet, Goffredo de Portu, Giuseppe Pezzotti
Abstract: To improve mechanical properties of mullite, a mullite-Al2O3\mullite laminate composite was prepared. Lamination generates residual stresses within the structure, measured by piezospectroscopy. A preliminary and complete piezo-spectroscopic characterization of the Al2O3\mullite system was carried out. A method to determine the concentration of Al2O3 in the composite by Raman spectrum was proposed and used to assess the composition of the laminated structure along the cross section. The experimental results evidenced a gradual change of composition and residual stress state between the two layer.
Authors: Alvaro Rico, Miguel Angel Garrido, Enrique Otero, J. Rodríguez
Abstract: An experimental study was performed to evaluate the roughness effect on the determination of hardness and Young´s modulus of ceramic materials from nanoindentation tests. Several specimens polished at various stages were tested at different peak load values. Local roughness measurements have been done by means of atomic force microscopy. Results indicate that roughness and size effects are joined. Proportional Specimen Resistant Modified model (PSRM) was applied to avoid the scale effect, isolating the roughness influence. Indentations where the average roughness-elastic displacement ratio is lower than a critical value are needed to get consistent results.
Authors: Raúl Bermejo, Peter Supancic, Tanja Lube
Abstract: In this work, the geometry effect on the thermal shock behaviour of a nine layered Al2O3- 5%tZrO2/Al2O3-30%mZrO2 ceramic fabricated by slip casting has been studied. A finite element model has been used to estimate the magnitude and location of the maximum thermal stresses in the layered material as well as the influence of the variation of this layered architectural design in the thermal shock crack initiation and extend throughout the specimens of study. Experimental tests on various samples have been carried out to validate the model. The residual stress distribution profile in the laminate, due to the elastic mismatch of the different layers along with the zirconia phase transformation on the Al2O3-30%mZrO2 layers, conditions the thermal shock response of the material. It is demonstrated how the variation of the outer most layer thickness in the laminates modifies the stress state in the surface, affecting the thermal shock crack initiation.

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