82 research outputs found
In situ stable fracture of ceramic interfaces
The fracture toughness of ceramics is often dominated by the structure of their grain boundaries. Our capacity to improve the performance of ceramic components depends on our ability to investigate the properties of individual grain boundaries. This requires development of new fracture testing methods providing high accuracy and high spatial resolution. Recently, several techniques have been developed using small scaled mechanical testing, based within a nanoindenter, using a variety of tip and sample geometries, including: micropillar compression, microcantilever bending and double-cantilever compression. However, the majority of the published work relies on load-displacement curves for the identification of crack initiation and the geometries can result in a complex analysis of force distribution and stress intensity factor.
Our approach uses a double cantilever geometry to obtain stable crack growth and we calculate the fracture energy under a constant wedging displacement. The tests are carried out within an SEM, this has two benefits: the sample is well aligned for a controlled test and images are recorded during the test for later analysis. Crucially this allows us to use beam deflection and crack length rather than critical load to measure fracture toughness. Our tests have proved it is possible to initiate and stably grow a crack in a controlled manner in ceramic materials for several microns. This approach has been validated on SiC where it gives a good approximation of the surface energy and then extended to SiC bi-crystals along with Ni-Al2O3 interfaces where crack blunting and bridging mechanism can be observed and measure
Using in-situ microLaue diffraction to understand plasticity in MgO
The present study investigates the micromechanical modes of deformation in MgO prior to cracking at room temperature. A combination of time resolved white beam Laue diffraction technique and in-situ nano-indentation of large single crystal micropillars provides a unique method to study the operating mechanisms of deformation in this otherwise brittle oxide ceramic. Upon indenting an [100]-oriented MgO micropillar, rotation and streaking of Laue spots were observed. From the streaking of the Laue spots, differential slip on orthogonal {110} slip planes was inferred to take place in adjacent areas under the indent - this was consistent with the results from the transmission electron microscopy studies. Upon cyclic loading of the pillar, subsequent stretching and relaxation of peaks was hypothesised to happen due to pronounced mechanical hysteresis commonly observed in MgO. Also, time-resolved spatial mapping of the deformation gradients of the area under the indent were obtained from which the strain and rotation components were identified
Accessing slip activity in high purity tin with electron backscatter diffraction and measurement of slip strength
Beta-tin has been used widely as an interconnect in modern electronics. To
improve the understanding of the reliability of these components, we directly
measure the critical resolved shear stress of individual slip systems in
beta-tin using micropillar compression tests at room temperature with crystal
orientations near-[100] and [001] in the loading direction within a large grain
high purity tin (99.99%) sample. This activates the (110)[1-11]/2,
(110)[1-1-1]/2, (010)[001] and (110)[001] slip systems. Analysis of the slip
traces and load-displacement curves enables measurement of the critical
resolved shear stress for epsilon=10^(-4) of
tau_(CRSS)^({110}/2)=10.4+/-0.4 and tau_(CRSS)^({010})=3.9+/-0.3
MPa
Analyses of microstructural and elastic properties of porous SOFC cathodes based on focused ion beam tomography
Mechanical properties of porous SOFC electrodes are largely determined by
their microstructures. Measurements of the elastic properties and
microstructural parameters can be achieved by modelling of the digitally
reconstructed 3D volumes based on the real electrode microstructures. However,
the reliability of such measurements is greatly dependent on the processing of
raw images acquired for reconstruction. In this work, the actual
microstructures of La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF) cathodes sintered at an
elevated temperature were reconstructed based on dual-beam FIB/SEM tomography.
Key microstructural and elastic parameters were estimated and correlated.
Analyses of their sensitivity to the grayscale threshold value applied in the
image segmentation were performed. The important microstructural parameters
included porosity, tortuosity, specific surface area, particle and pore size
distributions, and inter-particle neck size distribution, which may have
varying extent of effect on the elastic properties simulated from the
microstructures using FEM. Results showed that different threshold value range
would result in different degree of sensitivity for a specific parameter. The
estimated porosity and tortuosity were more sensitive than surface area to
volume ratio. Pore and neck size were found to be less sensitive than particle
size. Results also showed that the modulus was essentially sensitive to the
porosity which was largely controlled by the threshold value.Comment: 21 pages, 14 figures, 2 tables, journal paper published in Journal of
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Small-scale testing of ceramic matrix composites
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Shear and delamination behaviour of basal planes in Zr3AlC2 MAX phase studied by micromechanical testing
The mechanical properties of layered, hexagonal-structured MAX phases often
show the combined merits of metals and ceramics, making them promising material
candidates for safety critical applications. While their unique mechanical
performance largely arises from the crystal structure, the effect of chemistry
on the properties of these materials remains unclear. To study this, here we
employed two in situ electron microscope small scale testing approaches to
examine the micromechanical properties of Zr3AlC2, and compared the results
with the properties of Ti3SiC2: we used micropillar compression tests to
measure basal slip strength, and double cantilever beam splitting tests to
evaluate fracture energy for basal plane delamination. We observed distinct and
systematic differences in these measured properties between Zr3AlC2 and
Ti3SiC2, where Zr3AlC2 appeared to be stronger but more brittle at the
microscale, and discussed the implications of the results in the selection,
design, and engineering of MAX phases for targeted engineering applications
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