Papers by Author: Raúl Bermejo

Abstract: The contribution deals with the issue of residual stresses in particulate ceramic composites used in microelectronics. Residual stresses are developed in the composite due to cooling during manufacturing process. Different coefficients of thermal expansions of particles and matrix cause important residual stresses in the composite influencing their mechanical behaviour. The main aim of the paper is to determine influence of particle properties on magnitude and distribution of residual stresses in the composite matrix. Three dimensional numerical model was developed and finite element method (FEM) was used for numerical simulations. Results obtained contribute to a better understanding of residual stresses distribution and fracture processes in the studied type of composite.

Abstract: The aim of the present work is to analyze the influence of residual stresses in the particulate ceramic composite on the crack propagation. The crack propagation direction was estimated using Sih’s criterion based on the strain energy density factor. A two-dimensional finite element model was developed for determination of crack path. The residual stresses resulting from the mismatch of coefficients of thermal expansion during the fabrication process of the composite were implemented to the computational model. The effect of the particles shape on the crack propagation was investigated. Conclusions of this paper can contribute to a better understanding of the propagation of micro-cracks in particulate composites in the field of residual stresses.

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: Layered ceramics are foreseen as possible substitutes for monolithic ceramics due to their attractive mechanical properties in terms of strength reliability and toughness. The different loading conditions to which ceramic materials may be subjected in service encourage the design of tailored layered structures as function of their application. The use of residual stresses generated during cooling due to the different thermal strain of adjacent layers has been the keystone for the improvement of the fracture response of many layered ceramic systems, e.g. alumina-zirconia, alumina-mullite, silicon nitride-titanium nitride, etc. In this work, the fracture features of layered ceramics are addressed analysing two multilayered structures, based on the alumina-zirconia system, designed with tailored compressive residual stresses either in the external or internal layers. Contact strength and indentation strength tests have been performed to investigate the response of both designs to crack propagation. The experimental findings show a different response in terms of strength and crack growth resistance of both designs. While layered structures with compressive stresses at the surface provide a better response against contact damage compared to monoliths, a flaw tolerant design in terms of strength and an improved toughness through energy release mechanisms is achieved with internal compressive stresses. The use of layered architectures for automotive or biomedical applications as substitutes for alumina-based ceramics should be regarded in the near future, where reliable ceramic designs are needed.

Abstract: Alumina-zirconia multilayered ceramics have been proposed as an alternative for the design of structural ceramics with improved fracture toughness and strength reliability. During the processing of these laminates, significant residual stresses may arise due to the thermal expansion mismatch between adjacent layers. The correct evaluation of such stress distribution in the laminate may determine its range of application. In this work, the residual stress state in a layered material designed with five thick alumina layers of approximately 650 microns alternated with four thin alumina-zirconia layers of approximately 140 microns was estimated using different methods. A finite element analysis (FEM) was performed for stress evaluation in the bulk and an indentation method and X-Ray diffraction to account for stresses at the surface. Experimental findings show a constant stress distribution within the bulk for each layer, while at the surface stress position dependence is observed in the alumina layers, being the maximum tensile stresses near the layer interface. The accuracy of the results provided by each technique is discussed.

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: In order to understand the mechanical behavior of layered composites with compositional gradient, it is necessary to determine their state of residual stresses. Compositionally graded materials can offer the advantage of eliminating abrupt changes in composition between layers having different thermal expansion coefficients. The existence of a compositional gradient may reduce discontinuities in thermal residual stresses, something beneficial from the point of view of the mechanical properties. We present here a study of the state of the residual stresses in a layered material made of thicker (several mm) homogeneous layers of alumina and alumina-20%zirconia separated by a thinner (less than 300 µm) intermediate graded alumina-zirconia layer, obtained by controlled deposition of powders from a solution using an electrophoretic deposition method. The thermal residual stresses generated during cooling after sintering were measured in the homogeneous layers at each side, and at steps of about 30 µm in the graded layer along the direction of the compositional gradient, by using fluorescence ruby luminescence piezo-spectroscopy. Results show that the hydrostatic stresses on alumina grains vary continuously, indicating the absence of discontinuities in thermal residual stresses along the compositionally graded layer and at the interfaces of the homogeneous layers.

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.