Search

Many substrates for microelectronic systems contain ceramic/glass layers and metal features (e.g. electrodes, vias, metal pads) built up in a complex 3D architecture. The combination of different materials with distinct thermo-elastic properties may yield significant (local) internal stresses, which are to be superimposed to external thermo-mechanical loads in service. Due to the various material junctions, interfaces, etc, failure of these multilayer systems can hardly be predicted. In this work, a strategy is proposed to quantify the effect of architecture and loading conditions on the mechanical reliability of ceramic-based substrates. Model ceramic structures containing important design features (e.g. inner electrode, via, top metallization) were fabricated and tested in different environments (i.e. humid or dry conditions) under uniaxial as well as biaxial bending. Significant difference in the characteristic strength between ∼260 MPa and ∼620 MPa were measured, associated with the particular architectural feature, type of loading, and/or environment.

Ceramic varistors are electronic components which have a sharp change (over several orders of magnitude) of their electrical resistance at a well-defined voltage (switching voltage). Starting at low voltages, the resistance is large. But by exceeding the switching voltage the resistance drops dramatically and the component acts as a good conductor. Mounted parallel to a consumer, they are used as protection devices against over voltage loading. In the technical routine yearly billions of varistors are used to protect power lines, transformation stations, electronic devices, microelectronic systems or even LEDs. In service large temperature differences may come into existence, which cause severe mechanical stresses that even may destroy the components. The basic principles of the varistor behaviour are explained and examples of mechanical failure are discussed.

Owing to its excellent mechanical and thermal properties such as high strength, high hardness, and good thermal shock resistance, silicon nitride is a material suitable for forming tools. This work analyses the failure of a compaction roll made of Si3N4. One of the two ceramic rings broke during the compaction rolls' operation. Fractographic examinations have shown that very high tensile stresses occurred in the ceramic material during operation. These stresses first led to the formation of cracks and then to the total failure of the roll ring. Failure analysis has shown that roll design and clamping are of major importance. Causes for the generation of tensile stresses are shown and suggestions are made on how to ensure a safe operation of the rolls.

A new method for fracture toughness determination of ceramic balls is presented. The starter crack is introduced into the surface of the ball by a Knoop indentation followed by grinding off the deformed zone. The loading through surface tensile stresses is realized by water quenching, i.e. dropping the heated ball into water. The temperature difference is stepwise increased to find the critical temperature difference for the initiation of crack growth. The geometric factor is calculated in a parametric finite element study, whereas the temperature distribution in the ball was previously determined by using the Biot concept. Combining experimentally measured critical temperature differences for different cracksizes and ball diameters with numerical results of the geometric factor, the fracture toughness of the silicon nitride balls is evaluated. For the evaluation, the knowledge of several material properties (e.g. the CTE) and other parameters is necessary, which have influence on the precision of the measurement. The overall measurement uncertainty is estimated to be about ± 10 %, what roughly corresponds to the value determined with standard measurement procedures. There is an excellent agreement with published fracture toughness results of these balls determined by the modified Surface Crack in Flexure procedure.

This study examines ceramic varistors made from zinc oxide whose electrical resistance drops by many orders of magnitude within a few nanoseconds when a critical voltage threshold is exceeded. These unusual properties are caused by charges which are build into grain boundaries or accumulate near the grain boundaries. Zinc oxide crystals are polar and piezoelectric. Therefore, the charges in the grain boundary area and thus the electrical properties of the varistor depend on the orientation of the crystals towards the grain boundaries and on the mechanical tensions in the individual crystals. In this study, the microstructures of ZnO varistors, particularly the size and distribution of the grains and their orientation towards the grain boundaries will be examined. Among other things, lock-in thermography with a target preparation of strongly active grain boundaries and EBSD analyses in combination with SEM examinations, ion-etching and wet-chemical etching for the determination of crystal orientations are used. For selected grain boundaries, the electrical resistance in dependence of the applied voltage is determined via Micro-4 pole measurements.

In high performance hybrid bearings the balls are conventionally made of silicon nitride ceramics. There are some disadvantages such as costs or the higher stiffness of silicon nitride compared to steel. Therefore, alternative materials are under investigation. The surface strength is one of the most important criteria for the qualification of the spherical components in the application. It has to be evaluated for each new material (or new surface finish). The recently standardised Notched Ball Test (NBT) enables one to determine the surface strength (tensile strength) of balls, which is strongly influenced by the surface finish and volume flaw populations. In this paper the NBT strength of five candidate materials for rolling elements (silicon carbide, silicon nitride, alumina, zirconia and zirconia toughened alumina) is investigated. Fractography is performed to evaluate the direct correlation between the defects found at the surface and the measured strength.

Subcritical crack growth (SCCG) in glasses and ceramics causes a decrease of strength with time under applied load. The dependence of the crack growth rate with the stress intensity factor can be approximated by a power law, with n being the SCCG exponent. In this work the effect of the inherent scatter of strength as well as measurement uncertainties on the determination of n is analysed. Constant stress rate tests as proposed in the EN 843-3 standard are simulated using Monte Carlo techniques. It is shown that the large scatter of strength causes large uncertainties on the determination of n , yielding a significant influence of the sampling procedure especially for small samples. Measurement uncertainties have a surprisingly low influence on the determination of n with exception of a special situation that may arise during long time testing campaigns.

Objective Strength is one of the preferred parameters used in dentistry for determining clinical indication of dental restoratives. However, small dimensions of CAD/CAM blocks limit reliable measurements with standardized uniaxial bending tests. The objective of this study was to introduce the ball-on-three-ball (B3B) biaxial strength test for dental for small CAD/CAM block in the context of the size effect on strength predicted by the Weibull theory. Methods Eight representative chairside CAD/CAM materials ranging from polycrystalline zirconia (e.max ZirCAD, Ivoclar-Vivadent), reinforced glasses (Vitablocs Mark II, VITA; Empress CAD, Ivoclar-Vivadent) and glass-ceramics (e.max CAD, Ivoclar-Vivadent; Suprinity, VITA; Celtra Duo, Dentsply) to hybrid materials (Enamic, VITA; Lava Ultimate, 3M ESPE) have been selected. Specimens were prepared with highly polished surfaces in rectangular plate (12 × 12 × 1.2 mm3) or round disc (O = 12 mm, thickness = 1.2 mm) geometries. Specimens were tested using the B3B assembly and the biaxial strength was determined using calculations derived from finite element analyses of the respective stress fields. Size effects on strength were determined based on results from 4-point-bending specimens. Results A good agreement was found between the biaxial strength results for the different geometries (plates vs. discs) using the B3B test. Strength values ranged from 110.9 MPa (Vitablocs Mark II) to 1303.21 MPa (e.max ZirCAD). The strength dependency on specimen size was demonstrated through the calculated effective volume/surface. Significance The B3B test has shown to be a reliable and simple method for determining the biaxial strength restorative materials supplied as small CAD/CAM blocks. A flexible solution was made available for the B3B test in the rectangular plate geometry.

The strength of Low Temperature Co-fired Ceramics (LTCC) can be affected by the environmental conditions under which the material is loaded. In this work, the strength degradation associated with Subcritical Crack Growth (SCCG) mechanisms is investigated in several multilayer LTCC architectures designed with surface compressive residual stresses. The magnitude of the residual stresses was tailored combining two different LTCC materials. Biaxial strength measurements using the ball-on-three-balls method performed at room temperature in water (as reference environment) showed a clear increase in the characteristic strength with the compressive residual stress in the surface layer. The strength distribution in dependence of the surface stresses in the outer layer can be represented by a three-parameter Weibull distribution, thus providing a “threshold strength” (i.e. minimum strength) for the material. In addition, the use of compressive stresses in LTCCs introduces a threshold intensity factor for the SCCG behaviour, below which no environmental assisted cracking can occur.

Ceramic materials fail from defects which are distributed on the surface or in the bulk of specimens and components. The more dangerous surface flaws are often caused by improper machining or handling. In this investigation two possible ways to improve the surface quality and, as a consequence, increase the strength of a commercial silicon carbide ceramic are shown. In bending tests which apply a uniaxial stress state on the specimens only cracks perpendicular to the stress direction are dangerous but in biaxial tests cracks of any orientation may become fracture origins. Therefore, for this investigation, the biaxial Ball-on-Three Balls (B3B) test was used for the strength tests. To investigate the influence of surface finishing 6 samples of specimens were machined in different ways and tested with the B3B-test afterwards. It could be shown that a better surface finishing leads to a higher strength. In order to heal surface defects and to find the best healing conditions 5 samples of specimens were heat treated under different conditions and then tested with the B3B-test. For a better comparison of the obtained results all heat treated samples had the same surface finishing (ground with a grit size of 46 μm). Microscopic investigations showed that defects are healed by the formation of a glassy layer. The layer fills cracks and pores (at least partially) and causes a significant increase of strength.

The strength of materials suffering from subcritical crack growth (SCCG) can be affected by the loading conditions and environment. This crack growth can also occur during the measurement of the material’s fracture toughness, K Ic , yielding lower measured (apparent) values. In this work the K Ic measurements of two different brittle materials was compared in different environments ( i.e., water, ambient air and argon) using three testing methods: (i) Single Edge V-Notch Beam, (ii) Chevron Notch Beam, and (iii) Ball-on-three-Balls-KIc. A strong effect of the environment was found on the measured K Ic values using all three methods. Low “ K Ic values” were measured in water, i.e., ∼1.3–1.7 MPa m 1/2 depending on the material, whereas higher “toughness values” were obtained in argon, i.e., ∼2.0–2.3 MPa m 1/2 . This is explained with a model accounting for the SCCG during the fracture toughness experiment. Recommendations on the assessment of fracture toughness in brittle materials are given.

For the proper function of solid oxide fuel cells (SOFC) their structural integrity must be maintained during their whole lifetime. Any cell fracture would cause leakage and partial oxidization of the anode, leading to a reduced performance, if not catastrophic failure of the whole stack. In this study, the mechanical strength of a state of the art SOFC, developed and produced by Hexis AG/Switzerland, was investigated with respect to the influence of temperature and ageing, whilst for the anode side of the cell the strength was measured under reducing and oxidizing atmospheres. Ball-on-3-Ball bending strength tests and fractography conducted on anode and cathode half-cells revealed the underlying mechanisms, which lead to cell fracture. They were found to be different for the cathode and the anode side and that they change with temperature and ageing. Both anode and cathode sides exhibit the lowest strength at T = 850 °C, which is greatly reduced to the initial strength of the bare electrolyte. This reduction is the consequence of the formation of cracks in the electrode layer which either directly penetrate into the electrolyte (anode side) or locally increase the stress intensity level of pre-existing flaws of the electrolytes at the interface (cathode side).

Functional components are commonly fabricated combining a ceramic substrate with external and/or internal metallization (e.g. metal electrodes, vias, contact pads, etc.) using a tape casting process. Different layers are printed and/or fired (e.g. up to 800°C) onto the ceramic part to provide the component with a (certain) functionality. As a result of the combination of different materials (e.g. ceramic, glass, metal alloys) with different coefficients of thermal expansion, internal stresses may arise during the different fabrication steps. Although some of these tensile residual stresses may relax due to plastic deformation of metallic materials, stress concentrations generated in material junctions or terminations (imposed by geometrical constrains) may lead to cracks and/or reduce the component strength.In this work different architectures combining metal and glass layers on the surface of ZnO substrates were investigated experimentally and numerically in order to identify weak points in commercial components. Mechanical testing using three-point-bending was performed on samples taken after different steps. A FE model was developed to (i) calculate residual stresses generated during the manufacturing process, and (ii) simulate the propagation of initial crack/defect during manufacturing. Experimental results were compared with numerical predictions. These results in combination with fractographic analyses were used to validate the finite element model in order to assess location of failure.

In many ceramic components for electric applications, small defects in the microstructure may cause an anomalous current flow. Especially in components exhibiting nonlinear electro-thermal properties, such as varistors, PTC or NTC resistors, inhomogeneities lead to high concentrations of current and localized heat generation due to positive feedback effects. While the integral behavior of those components is often nearly not affected by these defects, the local load at these hot spot regions is considerably high and can even lead to the complete failure of the device. In this work micro-analytical tools are combined to identify the location and type of defects which may cause leakage currents, overheat or other reasons for inhomogeneous current distributions in functional ceramic components. Lock-in thermography is used to localize dominating current paths. This is even possible if the heat sources are in the micrometer range or less. Additionally, micro-sectioning techniques (e.g. Focus Ion Beam) are utilized to expose such regions to investigate features of the microstructure and causes for the enhanced current flows. Examples are shown in PTC resistor components as well as in LTCC based modules.

Functional components such as multilayer low temperature co-fired ceramics are examples of the combination of a ceramic-based substrate with internal electrodes as well as surface features (e.g. metallization, contacting pads, cylindrical vias, etc) employed to provide the component with a given functionality. Another example is that of functionalized silicon chips to be embedded into polymer circuit boards in order to enhance integration and save costs. The functionality of the system can be influenced by the mechanical reliability of the different components. Due to miniaturization and design complexity, no standard methods for mechanical testing can be applied for the characterization of these brittle components. In this work, an experimental approach is presented, which enables determining the strength distribution in functional components (e.g. rectangular plates as small as 2 × 2 × 0.1 mm3) in different environments at different temperatures. The method is based on localized biaxial testing using a ball-on-three-balls fixture. The high accuracy of the test allows quantifying the effect of surface quality, surface features and/or metallization (e.g. contact pads or cylindrical vias) on the component strength distribution. Experimental findings show that the strength distribution of ceramic components can be affected by environmental degradation, whereby subcritical crack growth phenomena can be enhanced in environments with high relative humidity. In addition, metallization at the surface subjected to tensile stresses can even raise the strength of the component, acting as a protective layer against environmental degradation, whereas cylindrical vias can become weak points in the design. It is shown that functionalized layers such as those used in silicon chips can have a significant effect on the strength parameters, thus influencing the lifetime of the device.

Strength degradation in glass and ceramic materials is related to subcritical crack growth mechanisms acting at the crack tip during mechanical loading. In this work the effect of humidity and temperature on the strength of a commercial low temperature co-fired ceramic was investigated using a biaxial testing procedure. Experiments were performed in argon and in air at different stress rates between 25 and 125 °C. The effect of humidity on strength was assessed at room temperature varying only the relative humidity. The sole effect of temperature was evaluated in argon at high stress rates. The combined effect of humidity and temperature was determined in air, testing at different temperatures. Results showed the existence of an inert strength of the material at room temperature. Measurements in ambient air showed a counterbalance effect of temperature and humidity yielding an almost constant strength for this material between 25 and 125 °C.

Owing to its excellent mechanical and thermal properties, silicon nitride is an increasingly used tool material in the metallurgical industry. Some of its applications involve high thermal cycling loads, which may lead to tool failure. The failure of ceramic materials initiates from defects in the volume or on the surface of components and samples. Surface defects, often introduced during finishing, are considerably more harmful than defects in the volume. It was possible to heal surface defects by applying a specifically developed heat treatment – provided the defects are not too large. In water quenching experiments on samples and components, it could be proven that the strength increase induced by the heat treatment also results in an improved thermal shock resistance. Due to high processing costs and in some cases due to the components' geometry, enhancing the surface quality of components is not possible. The proposed heat treatment is an alternative to increase strength, thermal shock resistance (and thus the reliability and service life of silicon nitride components) which requires little effort.

Double Schottky barriers in ZnO are modified piezotronically by the application of mechanical stresses. New effects such as the enhancement of the potential barrier height and the increase or decrease of the natural barrier asymmetry are presented. Also, an extended model for the piezotronic modification of double Schottky barriers is given.

Zirconia tapes are established for the application as electrolytes in solid oxide fuel cells. One of their functional tasks is to provide the structural integrity of the cell. The mechanical failure of the electrolyte would cause leakage between the electrodes, leading to a reduction of the performance of a whole fuel cell stack. In order to assess the reliability of a certain zirconia tape with respect to a specific stress distribution and environment, the strength has to be properly characterised. In the present study a selection of several commercial highly ionic conductive scandia doped zirconia tapes was investigated regarding their strength under inert and humid conditions at room temperature, which required the determination of the elastic constants and the fracture toughness as well. The strength of the different zirconia tapes ranks according to their fracture toughness, while the strength distributions typically exhibit a Weibull modulus of m = 10. It was found that all tapes suffer from sub critical crack growth, which leads to a limited lifetime at a given static load and a drastic reduction of the room temperature strength at time scales relevant for actual operation.

Solid-Oxide-Fuel-Cell systems are efficient devices to convert the chemical energy stored in fuels into electricity. The functionality of the cell is related to the structural integrity of the ceramic electrolyte, since its failure can lead to drastic performance losses. The mechanical property which is of most interest is the strength distribution at all relevant temperatures and how it is affected with time due to the environment. This study investigates the impact of the temperature on the strength and the fracture toughness of different zirconia electrolytes as well as the change of the elastic constants. 3YSZ and 6ScSZ materials are characterised regarding the influence of sub critical crack growth (SCCG) as one of the main lifetime limiting effects for ceramics at elevated temperatures. In addition, the reliability of different zirconia tapes is assessed with respect to temperature and SCCG. It was found that the strength is only influenced by temperature through the change in fracture toughness. SCCG has a large influence on the strength and the lifetime for intermediate temperature, while its impact becomes limited at temperatures higher than 650 °C. In this context the tetragonal 3YSZ and 6ScSZ behave quite different than the cubic 10Sc1CeSZ, so that at 850 °C it can be regarded as competitive compared to the tetragonal compounds.

During the last fifty years the mechanical properties of ceramic materials have been greatly improved, their toughness and strength have been increased and the scatter of strength decreased. Adequate statistical design procedures for brittle materials exist but cracking and brittle fracture of ceramic components still occur very often. In this review the theory of brittle fracture and the underlying assumptions are critically discussed and the measurement procedures of strength are reviewed. It is shown that the strength of materials, the strength of specimens and the strength of components are often quite different properties. Three main factors are identified which – in order to avoid unexpected failure of components – have to be considered much more than in the past: (i) hidden stresses, i.e. stresses caused by thermal strain mismatch, by contact (for example in joints) and internal stresses, (ii) the quality of the component's surfaces and edges and (iii) proper handling of ceramic materials and components. It can clearly be stated that the mechanical properties of many ceramic materials are appropriate even for applications under severe loading conditions but bad or incomplete mechanical design, insufficient surface finish and mishandling are the main reasons for unexpected failure of ceramic components.

Silicon nitride rolling elements for hybrid bearings (rollers, two sizes of balls and plate material as a reference) made from the same material lot and using the same sintering route are investigated in regard to their microstructure and their mechanical properties. For the first time, a direct comparison of the surface strength of the components as well as of their fracture toughness is enabled by recently developed testing methods for balls and rollers. It was found that the microstructure of these material variants influences the fracture toughness up to about 10%. In addition, due to the different surface finish, the surface strength of the different rolling elements (made of the same material) is very different. The differences may reach up to 100% and more.

The strength of ceramic materials is limited by flaws which are distributed in the volume or on the surface of the material. Commonly, fractographic investigations are performed after the strength tests to interpret the strength values. The relatively new Ball-on-Three Balls (B3B)-bending test applies a biaxial stress state (which is more searching for cracks than a uniaxial stress state) on the specimen. To identify typical fracture initiating flaws and to get a better understanding of the fracture behaviour of B3B-specimens a systematic fractographic investigation was performed on 260 silicon nitride specimens divided into batches with different surface qualities. It could be shown that in most cases (at least those in which origins could be clearly identified) surface or near surface located defects were responsible for failure. On specimens with poor surface qualities, surface defects were introduced through machining. On specimens with a better surface quality, volume defects, which were exposed on the surface by polishing, could be identified as fracture origins. In only a few cases defects in the bulk were fracture origins.

A new tensile strength test for ceramic rollers (cylinders) is presented, following the idea of the previously published and since that time standardized notched ball test. The specimen is prepared by cutting a long narrow notch along the symmetry plane of the roller, perpendicular to the end faces. By applying compressive forces perpendicular to the notch, tensile stresses are induced in the outer surface region of the roller opposite to the notch, mainly pointing in tangential direction with respect to the roller axis. The stress amplitude is proportional to the applied force and depends also on the test geometry and the Poisson’ ratio, which has been analyzed carefully with FE methods. Strength tests have been performed on two different qualities of commercial silicon nitride rollers of 10 mm diameter and length. According to their specification a significant difference in the strength has been obtained.

The varistor effect in ZnO ceramics is triggered by the behavior of the grain boundaries. To understand this effect in detail we have electrically characterized individual grain boundaries. Here, we apply the micro four-point probe method to measure the electrical properties of individual grain boundaries in a polycrystalline ZnO varistor ceramic with a mean grain size of 10 μm. The investigation revealed a wide spread in electrical properties, like nonlinearty exponents from 10 to 150 and switching voltages from 2.3 V to 3.6 V.

For solid oxide fuel cells (SOFCs) one key aspect is the structural integrity of the cell and hence its thermo mechanical long term behaviour. The present study investigates the failure mechanisms and the actual causes for fracture of electrolyte supported SOFCs which were run using the current μ-CHP system of Hexis AG, Winterthur – Switzerland under lab conditions or at customer sites for up to 40,000 h. In a first step several operated stacks were demounted for post-mortem inspection, followed by a fractographic evaluation of the failed cells. The respective findings are then set into a larger picture including an analysis of the present stresses acting on the cell like thermal and residual stresses and the measurements regarding the temperature dependent electrolyte strength. For all investigated stacks, the mechanical failure of individual cells can be attributed to locally acting bending loads, which rise due to an inhomogeneous and uneven contact between the metallic interconnect and the cell.

Due to the scatter of properties of individual grain boundaries, a scatter of the behavior of low-voltage varistor components and asymmetric current–voltage characteristics with respect to the electrical field direction is observed. Using complementary methods (thermography, scanning surface probe microscopy, electron back-scattered diffraction), we investigated the grain boundary behavior of Praseodymium type varistors. Measurements on the relevant grain boundaries revealed a newly observed effect: dependent on the field direction, the main current flows via different spatial paths.

The biaxial strength of Low Temperature Co-fired Ceramics was determined using the ball-on-three-balls test in several environments (dry oil, air and water). Subcritical crack growth phenomenon was observed, activated by the prolonged presence of humidity at the specimen surface. Whereas high strength values were reached during high-rate testing in dry oil, up to a 50% lower strength was measured on specimens immersed in water tested for longer periods. Experiments in a relative dry environment have shown for the first time evidence of two different crack growth mechanisms in this material. A model has been implemented to interpret the experimental results.

The Newman and Raju formula for the stress intensity factor of a semi-elliptical surface crack loaded in uniaxial tension or in bending has been developed about 30 years ago using an FE-analysis for several geometric parameters and fitting an empirical equation to the data points. The Poisson's ratio analyzed was 0.3. In this paper a reassessment of the Newman and Raju formula is made, where all relevant geometric parameters of crack and specimen and the Poisson's ratio are considered. The deviations of the old formula from the new results are up to 21%, if the full range of Poisson's ratio is taken into account. Furthermore the influence of the crack-surface intersection angle is discussed. The results of this work are important for more precise fracture toughness measurements in brittle materials and give a practical guidance for appropriate specimen preparation for fracture toughness measurements, which is also considered here.

A new toughness test for ball-shaped specimens is presented. In analogy to the “Surface Crack in Flexure”-method the fracture toughness is determined by making a semi-elliptical surface crack with a Knoop indenter into the surface of the specimen. In our case the specimen is a notched ball with an indent opposite to the notch. The recently developed “Notched Ball Test” produces a well defined and almost uniaxial stress field. The stress intensity factor of the crack in the notched ball is determined with FE methods in a parametric study in the practical range of the notch geometries, crack shapes and other parameters. The results correlate well with established calculations based on the Newman–Raju model. The new test is regarded as a component test for bearing balls and offers new possibilities for material selection and characterisation. An experimental evaluation on several ceramic materials will be presented in a consecutive paper.

An experimental approach is proposed, based on localized biaxial testing of brittle samples at several locations under different environments, to quantify the effect of surface features or metallization (e.g. contacting pads or cylindrical vias) on the strength distribution of functional ceramic components. It is found that cylindrical vias can act as weak points in the design (i.e. lower strength and reliability), whereas contacting pads can raise the strength of the component acting as a protective layer against environmental degradation.

The influence of measurement uncertainties (MU) on the determination of the parameters of a distribution function has been analyzed using a Monte Carlo simulation technique on the example of the Weibull distribution, which describes the strength of brittle materials. It is shown that in the parameter range which is relevant for strength testing of brittle materials (e.g. ceramics) very high precision measurements are necessary if the Weibull modulus of the parent distribution is m ≥ 20. In that case the MU should be lower than ±2% of the measured value to obtain the same confidence level compared to the ideal case (MU = 0). Otherwise a significant underestimation of m becomes very probable. However, if m ≤ 10, even relatively large MU (up to ±10%) are tolerable. In summary, the precision of the measurements is acceptable as long as the width of the error distribution be much smaller than the width of the parent distribution.

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components. For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data. The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was – depending on the machining conditions – between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased. The improvement was caused by the formation of a thin (0.5–2 μm) glassy layer which filled surface cracks and surface related pores during annealing.

Details of surface finish affect significantly the surface quality of ceramic specimens (e.g. surface roughness, residual stress). To analyse the influence of surface quality on strength five samples of silicon nitride specimens were machined applying different surface finishing procedures. Then they were strength tested using the biaxial Ball-on-three Balls (B3B) test. The test results were compared to those of conventional bending tests. A fractographic analysis was performed on selected specimens to identify the fracture origins. It could be shown that a better surface quality leads to an increased strength but increases the scatter of strength data too.

Low temperature co-fired ceramics (LTCCs) are multilayered ceramic based components, which can be used as high precision electronic devices in highly loaded environments. In many applications, LTCC end components are exposed to mechanical stresses, which may yield different types of failure coming from different locations, thus decreasing the mechanical reliability of the device. The aim of this work is to assess the mechanical strength of LTCC parts and investigate the influence of the metal internal structure (supporting the maximum load) on the local fracture response. Strength of different positions (e.g. near vias, metal-pads, ceramic layers) has been measured under biaxial loading and compared with a reference bulk LTCC. The strength results were interpreted in the framework of Weibull theory. Fractographic analyses revealed a significant effect of the first metallisation layer below the tensile surface on the strength reliability of the structure, which should be considered to optimise LTCC designs.

Single-crystalline silicon chips are widely employed in printed circuit boards (PCBs) as embedded components. Their design often requires one side patterned with metal contacts, whereas the opposite one is constituted by pure silicon. These components must possess a minimum strength to withstand the loads occurring during both production and operation of the board. In this work, the strength and fracture behaviour of miniaturised Si chips (dimensions: 2 mm × 2 mm × 0.125 mm) has been assessed under biaxial loading on both the pure silicon side and the metal-patterned side by means of a miniaturised ball-on-three-balls (B3B) fixture. Experimental results showed significant difference in the characteristic fracture load between the silicon-side ( P 0 = 21.2 N, Weibull modulus m ≈ 2.6) and the metal-patterned side ( P 0 = 8.6 N, m ≈ 12.3). Fracture mechanics and fractographic analyses, together with FE simulations of the loading process, helped clarifying the effect of the metal contacts on the overall fracture behaviour of the Si-chips.

Rolls for rolling of metal wires are traditionally made from cemented carbides, but they suffer from roughening of the working surfaces and severe wear which restrict the roll lifetimes. Silicon nitride ceramics are feasible alternative materials for rolls (and other components in rolling mills) due to their good combination of properties, e.g. high strength and hardness, excellent high-temperature properties, oxidation resistance and good thermal shock behaviour. Increasingly the switch is being made to such ceramics for extension of life and improvements in surface quality of the rolled product. In a collaborative project between universities, industrial partners and research centres, rolls made of silicon nitride, supported and clamped by steel rings were tested at the rolling mill of Bohler-Edelstahl in Kapfenberg, Austria. Under low and medium hard loading conditions the suitability of these composite rolls has been very satisfactorily demonstrated. However, under very severe loading conditions one pair of silicon nitride rings failed. In this paper, the failure of these ceramic rings is analysed, and suggestions for further improvements are presented.

Direct inkjet printing (DIP) allows the production of small ceramic specimens with special geometries starting from high solids content suspensions. In this work, thin (300m thickness) 3Y-TZP specimens were produced with the DIP technique as model materials for microelectronic applications. The mechanical strength of the printed specimens was evaluated under biaxial loading, and the results were interpreted within the framework of the Weibull theory. Hot-pressed 3Y-TZP specimens with the same geometry and dimensions were tested for comparison. The fracture surfaces were subsequently examined using scanning electron microscopy (SEM). The inkjet printed materials revealed high mechanical reliability (m ≈ 10 for σ0 ≈ 1400 MPa), which was ascribed to the uniform and defect free microstructure generated by the DIP technique. © 2010 Elsevier Ltd. All rights reserved.

The study of the close relationship between crack propagation and domain switching is of extreme importance in the reliability of PbZr1−xTixO3 (PZT) devices. We performed domain imaging in the neighbourhood of indentation cracks on poled and non-poled PZT bulk specimens using polarized micro-Raman spectroscopy. Non-destructive Raman measurements helped to correlate the indentation-induced domain texture with the crack growth resistance on both samples. Hence, polarized micro-Raman spectroscopy offers the potential to locally investigate regions of interest in complex piezoelectric devices.

Diamond composite materials prepared as thin layers on tungsten carbide substrates for applications such as rock cutting present characterisation challenges. As a consequence of the production route, the availability of only small pieces of diamond composite material has required some ingenuity in devising testing methods which are appropriate but which also yield accurate data to enable differences between grades of material to be characterised. A number of additional problems associated with their hardness and stiffness, coupled with difficulties of accurate machining, have to be overcome. This paper reports some of the techniques attempted to measure flexural strength, fracture toughness, elastic modulus, fatigue behaviour and edge chip resistance, and assesses their success.

Layered ceramics designed with weak interfaces favour interface delamination, while laminates with strong interfaces show higher strength and enhanced mechanical reliability. In this paper, conditions are reviewed aiming to combine crack bifurcation and interface delamination mechanisms in a unique architecture to design layered ceramics with high failure resistance. Based on a bi-material theoretical approach supported by experiments it is found that interface delamination can be favoured if crack bifurcation occurs in the compressive layers with a low inclination angle. The thickness and stresses of the compressive layers are the key features to optimise the mechanical behaviour in layered ceramics.

For failure assessment of natural cracks caused by surface treatment, the crack resistance for very small crack extensions is necessary. In ceramics with strongly rising R curves, the initial part of a few micrometer crack propagation is of special interest. For this purpose, the use of very small sharp starter notches introduced by a focused ion beam is recommended. As an example, the initial R curve for gas-pressure-sintered silicon nitride at high temperature will be estimated. Whereas for V-notched bending bars a fracture toughness of about 6-6.3 MPa·m 1/2 was determined, the toughness for small strength-relevant cracks was found to be K Ic ≅5.1 MPa·m 1/2 .

For hot rolling, tools are nowadays made of cemented carbides. In service, these rolls suffer from wear and thermal fatigue. Due to the properties of ceramics, their use could cause improvements in tool behaviour. In field tests – when rolling materials with high deformation resistance – cracks developed in the silicon nitride rolls, which grew for a long time period before large parts of the rolls broke apart. In more moderate rolling conditions the rolls operated safely. A FE model is used to analyse the in-service behaviour of cracks in the silicon nitride rolls. For the observed crack path the stress intensity factor of the cracks is determined using the weight function method. It increases up to a crack depth of around 0.35 mm and then decreases again with increasing crack depth. This explains the observed pop-in-type growth of cracks after an overload. Depending on the rolled materials, the popped in cracks have a length of up to 1.2 mm. The further growth of the cracks to a length of several millimetres, which is caused by a fatigue growth mechanism, needs several thousand additional revolutions.

Modern multilayer piezoelectric actuators (MPAs) consist of stacks of piezoceramic layers with interdigitated metallic electrodes in between. The combination of thermal, electrical and mechanical loads during service may affect the structural and functional integrity of such electro-mechanical converters. Therefore, a deep knowledge of the coupling phenomena among the field-type quantities such as mechanical stress, electrical field strength and temperature is absolutely necessary. Theoretical simulations are the only way to obtain these (in detail unknown) physical relevant field quantities within MPAs. In this work, the constitutive laws describing the highly non-linear piezoelectric material behaviour are parameterised by experiments, in order to be implemented into FEA tools.

A new strength test for ceramic spheres (balls) is presented. A long narrow notch is cut in the equatorial plane of the ball and the ball is then loaded in compression perpendicular to the notch. This causes tensile stresses in the outer surface region of the ball opposite to the notch, which are analysed carefully with finite element (FE) methods. The tensile stress amplitude depends on the bending moment in the notch ligament – given by the applied force – and on details of the notch geometry. The stress state in the highly stressed surface is almost uniaxial showing only a slight influence of Poisson's ratio. Numerical solutions for balls with quite different notch geometries are given. Strength tests have been performed on commercial silicon nitride balls of 5 mm diameter. Two sets of specimens having notches of different length have been tested. Although the typical fracture loads in both sets of data are quite different, the tensile strengths are closely similar. This indicates the validity of the data evaluations. Experimental details are discussed and an analysis of the experimental uncertainties on the test results is made. For balls with 5 mm the uncertainties are estimated to be less than ±3% (of the measured value). For balls having a diameter of 10 mm or more the uncertainties are less than ±1%.

Laminates with strong interfaces and having compressive residual stresses in the outer layer are designed to have high strength and high reliability. Bending strength tests on such laminate specimens were performed and compared with tests made on monolithic specimens made from the outer layer material. In both types of specimens fracture started from similar flaws, but the laminate specimens showed a higher strength and Weibull modulus. In the case of the monolithic material the toughness is constant but in the case of the laminates it increases – due to the action of the residual stresses – with crack extension (i. e. it is an apparent toughness). The influence of the apparent toughness curve on the fracture statistics is discussed. It is also shown that the statistics of the laminate can be determined using the statistics of the monolithic material, if the influence of the residual stresses on the toughness and strength is properly taken into account.