Papers by Author: Peter Supancic

Abstract: The fracture of mechanically loaded ceramics is a consequence of material critical defects located either within the bulk or at the surface, resulting from the processing and/or machining and handling procedures. The size and type of these defects determine the mechanical strength of the specimens, yielding a statistically variable strength and brittle fracture which limits their use for load-bearing applications. In recent years the attempt to design bio-inspired multilayer ceramics has been proposed as an alternative choice for the design of structural components with improved fracture toughness (e.g. through energy release mechanisms such as crack branching or crack deflection) and mechanical reliability (i.e. flaw tolerant materials). This approach could be extended to complex multilayer engineering components such as piezoelectric actuators or LTCCs (consisting of an interdigitated layered structure of ceramic layers and thin metal electrodes) in order to enhance their performance functionality as well as ensuring mechanical reliability. In this work the fracture mechanisms in several structural and functional multilayer components are investigated in order to understand the role of the microstructure and layered architecture (e.g. metal-ceramic or ceramic-ceramic) on their mechanical behaviour. Design guidelines based on experiments and theoretical approaches are given aiming to enhance the reliability of multilayer components.

Abstract: Low Temperature Co-fired Ceramics (LTCCs) are layered ceramic based components, which – in recent years - are increasingly used as high precision electronic devices (e.g. mobile and automotive technologies) in highly loaded (temperatures, inertia forces, etc.) environments. They consist of a complex three-dimensional micro-network of metal structures embedded within a glass-ceramic substrate. Even though LTCCs have been used for more than 20 years, there is insufficient understanding of the mechanical loads during processing. In this regard, different types of failure of the end component during service have been reported, coming from different parts within the part. In this work, the influence of the internal architectures in the fracture response of LTCC components during bending has been investigated. Strength has been determined in 10 × 10 mm2 specimens using the ball-on-three-balls test (biaxial loading) and evaluated using Weibull statistics. Fractography of broken specimens has been performed to determine the mode of fracture of the components and the role of the internal architecture in the crack path. Results show strength dependence as a function of the testing position within the part. The influence of the internal architecture and residual stresses is also discussed.

Abstract: For some years ceramic bearing balls based on silicon nitride have been routinely used in technical practice. An important property of bearing balls is their strength, but appropriate testing methods are still missing. In this paper four different methods for strength testing are applied to commercial bearing balls. Each of the tests needs a different type of specimen, their preparation needs a very different effort, and the stress state applied to the specimens is also very different. This causes pros and cons, which are discussed in detail. The conventional 4-point bending test characterises the material in the interior of the balls. The applied stress state is uniaxial. The machining of the bending bars out of the balls is time intensive and costly. The ball on three balls test also characterises interior of the balls. The stress state is biaxial. The machining of the disc shaped specimens out of the balls is less expensive than the production of bending bars, but the finish of the tensile loaded surface needs special care. The data of both types of tests can be converted into each other using Weibull theory. The specimens in the triple ball crush test are as-received bearing balls, which are squeezed together. This causes some kind of contact loading, as will also occur in service. Failure is caused by the creation and growth of contact cracks, followed by a collapse of the compressed and cracked material. A detailed analysis of test results is complicated. It can be speculated that the component’s behaviour is mainly influenced by the toughness of the material and that the flaws in the material or at the component’s surface are of less significance. In the newly developed notched ball test the highest stressed region is a part of the original surface of the balls. Machining of the notch is straightforward. The stress state is almost uniaxial. The strength depends on size of flaws in the surface region. Therefore the notched ball test is a relevant measure to characterize the quality of the bearing balls.

Abstract: The ball-on-three-balls (B3B)-test is a biaxial strength test for brittle materials. The results of B3B-tests are very stable against small geometrical inaccuracies of the specimens or the test support. In contrast to conventional bending tests there exists only a small influence of friction and edge defects are not relevant. These advantages, compared to beam tests, make the testing of mini-specimens with volumes of a few mm3 feasible. For this investigation silicon nitride specimens of different sizes were tested by use of the B3B-test. The maximum tensile stresses and the effective volumes and effective surfaces of the specimens were determined. The obtained results are compared directly and with the results of conventional 4-point-bending tests and are discussed in the framework of the Weibull-Theory. Additionally fracture surfaces of B3B-specimens and bending specimens were investigated fractographically to identify possible fracture origins.

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.

Abstract: Electroceramic varistors, typically based on ZnO, show a strong non-linear currentvoltage characteristics. Actually they can be considered as electrical isolators below a critical field strength and highly conductive above. These components are used as over voltage protection elements in various fields of electric and automotive industry. In testing and in service high power varistors are loaded with µs-voltage pulses and suffer high mechanical loads due to thermal shock and inertia effects. Therefore the reliability in terms of mechanical strength becomes a crucial mark of quality. In this work the microstructure and the strength of commercial high power varistors of three different producers are presented. By a subsequent fractographic analysis fracture initiating defects are identified. It is shown, that defects in the microstructure, which result from the processing of the varistor components, play a significant role for the strength of the ceramics and of course also for the service performance of the components.

Abstract: In this work, the thermal shock behavior of an Al2O3-5%tZrO2/Al2O3-30%mZrO2 multilayer ceramic is studied. On these materials, a tetragonal to monoclinic phase transformation within the Al2O3-30%mZrO2 layers takes place when cooling down from sintering. The latter induces an increase in volume and therefore compressive residual stresses arise in these layers. The residual stress distribution profile in the laminate influences the thermal shock response of the material. A finite element model has been developed to estimate both the thermal strain effects during the sintering process as well as the temperature distribution and stress profile within the laminate during thermal shock testing. Experimental tests on the monoliths and laminates were carried out and compared to the model. It is observed that the presence of the compressive layers within the laminate inhibits the penetration of thermal shock cracks into the body at even more severe conditions than in the monolithic material.

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.

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.