Fractography of Advanced Ceramics II

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Authors: George D. Quinn, Lewis K. Ives, S. Jahanmir
Abstract: Grinding may create flaws that control strength and limit the performance of finished ceramics. Machining cracks sometimes have been difficult or impossible to find especially in toughened ceramics with interlocking grain microstructures that create rough fracture surfaces. Our fractographic examinations show that machining damage leaves telltale markings on fracture surfaces that may be easily detected using common fractographic techniques. A comprehensive study with over 400 ground rods and rectangular bars was conducted on several commercial silicon nitrides to study the effects of various machining conditions. Similarities and differences in behavior were observed. A paradoxical finding was that tougher silicon nitrides developed deeper grinding cracks. Machining crack size and shape strongly depended on the grinding wheel grit size.
Authors: Roger Morrell
Abstract: The process of edge flaking of brittle materials is a significant limitation in design, handling and use of components. Simple quasistatic tests to identify resistance to edge flaking can be based on near-edge indentation and scratching towards an edge, and these produce rankings of materials that broadly correlate with GIc or KIc. However, most edge damage occurs in practice by impact. Using a drop-weight impact tester, edge chipping tests have been performed dynamically on a range of brittle materials, using repeated impact with step-wise height increments until fracture. It has been found that when impact energy rather indentation force is used as the correlative parameter against distance of the impact site from the edge of the test-piece, a similar relationship to that of quasistatic indentation is found. The shapes of edge flakes produced may also similar. This suggests that even when a relatively blunt impactor is used, compared with conventional indenters, the mechanics of the failure are similar. However, the occurrence of ring cracks can lead to unusual flake shapes. It follows that simpler-to-perform quasistatic tests can model the less well-defined dynamic impact situation in terms of testing for the effects of geometry or for comparing performance of different material types.
Authors: Hatsuhiko Usami, Tomoya Kadomae, Daisuke Igimi, Mineo Mizuno
Abstract: The present study describes the effect of indenter materials on Hertzian cracking behavior of alumina ceramics in sphere indentation. Numerical analysis (FEM) was carried out to investigate the influence of the frictional resistance at the interface due to the elastic mismatch between the indenter and the flat specimen on the stress distribution near the contact area. Two kinds of alumina ceramics with different mechanical properties were used for the flat specimen. Materials of the sphere indenter were cemented carbide, silicon nitride, alumina and hardened steel. The indenter was penetrated into the specimen with an electro-mechanical testing apparatus at a constant cross-head-speed in air. The indentation fracture was monitored with the acoustic emission signal. The numerical analysis revealed that the frictional resistance had influenced on the stress distribution. The experimental data showed that the ring crack radius, the crack pass below the contact zone and the indentation strength were different depending on the indenter material. From the comparison between the numerical analysis and the experimental data, it was found that the minimum value of the ring crack radius of the low density alumina flat specimen corresponds to the position of the maximum tensile stress.
Authors: V. Le Houérou, J.-C. Sanglebœuf, Tanguy Rouxel
Abstract: Grinding and polishing are widely used for glass machining with fine finished surfaces. These processes result from abrasion due to repeated contacts between hard sliding particles and the glass surface. The study of contact mechanics problem is of fundamental interest to understand the process of material removal in glasses. In order to get insight into this problem, an experimental set up was designed which allows a monotonic loading of the indenter combined with a controlled sliding of the specimen to simulate a slow abrasive machining process. In addition, the experiments are conducted with an in-situ video monitoring that allows for the observation of the different fracture phenomena beneath the indenter. Fracture surfaces were also studied using SEM and AFM for multi-scale investigation. Fracture analysis was carried on a standard float glass, four different SLS glasses and a fused silica glass. The observed phenomena were discussed in the light of the influence of the normal load and the chemical composition.
Authors: Zdenĕk Jonšta, Jana Horváthová, Kateřina Blažková, Karel Mazanec
Abstract: The work is devoted to the evaluation of indentation fracture toughness in ceramic materials and to the comparison of these data with the results of this characterisation obtained by the application of fracture mechanics method (chevron notched beam specimen – CVNB). The indentation fracture toughness measurement is realized using the indentation load lying in the loading extent between 50-300 N. The evaluation of basic parameters of lateral crack initiation resulting in chipping formation in the vicinity of indent is performed. This measuring is based on the finding of relationship between the indent load level and the number of loading cycles leading to chips formation. The geometry of lateral cracks and chipping morphology are described. This evaluation is complemented with the analysis of chip fracture surfaces.
Authors: Peter Supancic, Z. Wang, Walter Harrer, K. Reichmann, Robert Danzer
Abstract: Modern low-voltage piezoelectric actuators consist of a stack of piezoceramic layers (PZT) with metallic electrodes in between. Due to the use of these parts in automotive applications, a big but sensitive market is opened. During application mechanical stresses are an inherent loading of these electro-mechanical converter components. Therefore some strength of the actuators is necessary to guarantee a demanded life time. Bending and tensile tests were performed on commercial components to measure the strength in axial direction. Fracture surfaces were investigated with the methods of fractography to get information about the weakest links in the microstructure.
Authors: A. Platzer, Peter Supancic, C. Lembacher, U. Theiszl, Robert Danzer
Abstract: PTCs are electrical resistors (thermistors) with a positive temperature coefficient. They change their resistivity up to seven orders of magnitude within a certain temperature range. Though these parts are only loaded electrically, they often fail due to thermo-mechanical stresses caused by Joule self heating. With the aid of an infrared camera system the temperature distributions of PTCs in service were investigated. They show a big variety in appearance, often strongly differing from the temperature distribution predicted by a model-calculation using homogenous material properties. The temperature distributions measured with the infrared system give information about gradients in material properties. Performing destructive tests by high electrical loading lead to fractures, which are initiated in the most stressed regions. Fractography was used to identify fracture origins. With the information of the fractography and thermal analysis of the infrared camera the FEM-model could be modified in order to understand different kind of fracture modes.
Authors: George D. Quinn, William N. Sharpe, Glenn M. Beheim, Noel N. Nemeth, Osama Jadaan
Abstract: Direct tension strength tests were conducted on chemical vapor deposited silicon carbide microspecimens. Three types of specimens were used: straight gage section, tapered gage section, and notched gage section. The average strengths and standards deviations were: 0.42 GPa ± 0.13 GPa; 0.47 GPa ± 0.16 GPa; and 0.68 GPa ± 0.19 GPa, respectively. The fracture origins were identified by fractographic analysis and were cracks in large grains next to surface grooves from the deep reactive ion etch (DRIE) process used to fabricate the specimens.
Authors: Junichi Tatami, Tomoko Ohbuchi, Katsutoshi Komeya, Takeshi Meguro
Abstract: Observation of fracture surfaces in ceramics is useful for improving their mechanical properties. In this study, fracture surfaces of polycrystalline alumina were observed using scanning-probe microscopy (SPM) on a nanoscale, also called “nano-fractography.” The average grain size of polycrystalline alumina specimen used in this study was 4.5µm, and the fracture toughness was 3.0MPa・m-1/2. The fracture mode was found to be a mixture of intergranular and transgranular fractures. The fracture surface of intergranular fractures consisted of smooth and rough areas composed of very small steps, whose detection was impossible using scanning electron microscopy. Cleavage and non-cleavage fractures were observed in transgranular fracture grains. The fracture surface of single-crystalline alumina, which is the typical model of the transgranular fracture, was also observed by SPM. The cleavage plane of alumina macroscopically exhibited a very smooth, glass-like surface. However, sub-nano meter steps can be observed on the cleavage fracture surface and appear to be formed by plastic deformation during crack propagation because the size of the step nears that of the Burgers vector.
Authors: Jakob Kübler, R. Baechtold, Gurdial Blugan, K. Lemster, S. Fuso
Abstract: Modern waste burners are equipped with catalysts to reduce NOX. During operation the catalysts and their performance are influenced by, for example rapid temperature changes, facility vibrations, cleaning procedures and unwashed exhaust gas (when the catalyst is installed before the washer). The catalyst discussed in this paper comprised over 2’500 extruded elements of 150 x 150 x 770 mm3. During commissioning the catalyst elements exhibited a tendency to spall off pieces, thus preventing acceptance of the plant. For the failure analysis, one element from the catalyst was removed and two spare elements were selected. The investigation comprised a visual check, fractography, measurement of the most important physical, mechanical and chemical properties, and a microstructural analysis. It could be shown that: − Processing defects, such as extrusion defects, were the source of cracks which led to pieces spalling off; − Hot steam, used for periodic cleaning, reduced the structural strength by half; − The elements were exposed to higher mechanical loads than expected during transport; − The cleaning process, in combination with particles transported by the exhaust gas, has an erosive effect on the catalyst material; − Blockages, caused by catalyst material among other things, could be found at different depths; − The chemical composition did not significantly differ between used and unused elements. In summary, the failure analysis led to an understanding of the failure mechanism and to a set of recommendations for improvements whose implementation ultimately led to the plant being cleared for operation.

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