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.
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.
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
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.
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.
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
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.
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.
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.
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
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.