Microscopic three-point bending test to probe plate-like silicon particles from AlSi alloys

The strength of composite materials and alloys is often governed by the intrinsic strength of brittle microscopic particles that they contain. Approaches to measure the strength of alloy or composite reinforcing particles are usually indirect: average microscopic mechanical properties are estimated via a given model from macroscopic mechanical data conducted on the composite or alloy. Here we present an approach to directly measure the local strength of microscopic second-phase particles of high aspect ratio, by means of which one can identify and understand the role of their strength-limiting defects. The approach is a microscopic three-point bending test conducted directly on individual microscopic plate-like silicon particles, which constitute, together with aluminium, the eutectic microconstituent in AlSi alloys. Silicon particles are extracted from the alloy by deep-etching and spread on a steel substrate. Focused Ion Beam (FIB) milling is used to carve a beam out of a silicon particle in such a way that the surface that will later on be subjected to tension upon beam bending, i.e. where strength will be measured, is not affected by the FIB nor by redeposition. The silicon beam is then transported using a micromanipulator over a hole previously produced by FIB milling into the substrate. The test is carried out using a nanoindenter with a custom-made ridge tip. Finite element modeling is used to evaluate the three-point bending tests and to explore the effect of misalignments on the strength uncertainty. Tests are conducted in eutectic silicon plate-like particles both with and free of SEM-visible flaws. Results prove quantitatively that silicon particles can attain very high strengths when they are defect-free, whereas surface flaw-containing particles are much weaker

Measuring the strength of brittle microscopic spheres by means of compression tests

The strength of particles is an important property relevant to many industries; in materials science, a wide range of composites and alloys contain small particulate phases, whose microscale strength distribution affects strongly the macroscopic properties of the composite or alloy in question. Yet, despite its importance, the strength of microscopic particles has remained difficult to measure. The most common method used to measure the strength of individual particles is the compression test, conducted by squeezing individual particles between two parallel platens. Progress in nano- and micromechanical testing in recent years has enabled to expand the application of this method to test hard brittle particles at the microscale. The test is generally realized by using hard platens, typically made of diamond; however, in this configuration the compressed particle often fails predominantly due to presence of microcracks that develop, as a result of high stress concentration, at the contact region between the particle and the platen. Pre-existing intrinsic flaws within the particle or along its surface are therefore not probed, such that it is questionable whether the data generated by the test can represent the true strength of particle. One solution to avoid the development of the microcracks and to reduce the stress concentration at the platen/particle contact is to use elasto-plastic platens, which deform plastically during the test where they contact the particle. This allows particle indentation into the platens and with this an increase of the contact area over which the compressive load is applied. If the platens are hard enough to prevent the particle from completely sinking into the platens, then the tensile stress that develops within the particle and/or along its surface can cause failure from pre-existing flaw in regions of the particle that are not in direct contact with the platens. In this study we present a particle-crushing testing approach in which elasto-plastic platens are used. As a testbench particle material we use fused silica microspheres with diameter between 20‑40 micrometers. We demonstrate the method on a custom-built instrumented crushing apparatus designed to work in displacement-controlled mode (i.e., that has a stiff load train), using adaptations of the test so as to allow for fractography. Using analytical solutions in combination with extensive FEM analysis of the sample as it is compressed by the elasto-plastic platens, we show how this modification of the crushing test can be used as a means of measuring the intrinsic strength of brittle microscopic particles, as dictated by the internal flaws they might contain

Fracture toughness measurement with microscopic chevron-notched specimens

The fracture toughness of a material is, in theory, a rather well defined mechanical material property; for ideally brittle material, it is the stress intensity of a sharp crack present in the material at the moment when crack propagation becomes unstable. Measuring the fracture toughness of materials, however, has always been difficult, mainly because meeting the conditions under which the property is well defined can be a challenge. In testing of macroscopic samples the validity of measurements is most often deduced by confronting results of various established testing protocols and sample configurations with criteria derived on the basis of analysis and extensive experimentation, including round-robin test campaigns. At microscopic scales, where specimen dimensions are typically of few micrometers and defined sample geometries more difficult to produce, there are few established test protocols, a fact which is evidenced by significant variations in measured values of the toughness of various given materials. Yet determining the fracture toughness of materials at the microscopic scale is an important issue, which is driven for example by the need to conduct failure analysis and prevention in modern “small scale” technologies, thin film industry and/or micro- or nano-electro-mechanical systems (MEMS/NEMS). The principal tool for shaping microscopic specimens for fracture toughness measurement out of virtually any piece of material is the focused ion beam (FIB). For some materials it is possible to use alternative methods, e.g., lithography, to shape the general specimen; however, the fabrication of sharp notches to serve as a precrack in the specimen is almost regularly done with the FIB. This approach has drawbacks, however, because FIB milling is well known to alter materials along machined surfaces, and also because FIB milled notches are not sharp cracks but always have a finite tip radius. To circumvent these problems associated with FIB-milling of fracture toughness specimens, we explore testing configurations that contain a chevron notch: beam-like samples with a notch that contains a triangular ligament and subject to bending, such that the stress concentration at the apex of the triangle is first able to initiate the crack at a load that is lower than that corresponding to the onset of crack instability. Subsequently, by increasing the load, the initiated crack is driven to traverse the notch. Initially, the crack does so in stable manner due to its increasing crack front width, up to a moment of instability at which the sample suddenly breaks. Provided there is limited plasticity in the sample, its fracture toughness is evaluated at the moment of crack instability (corresponding to the peak load in the test), at which point preceding stable crack growth ensures the existence of a sharp crack. The method has been successfully demonstrated by testing chevron notched micro-cantilever beams with rectangular cross sections FIB-milled into fused quartz or nanocrystaline alumina [1]. We present this work, together with recent developments of the method in which newer sample configurations are explored, aiming to ease sample preparation and testing. Efforts to extend the method towards the testing of anisotropic materials, including single crystal silicon and single crystal TiC are presented. The lack of a need for pre-cracking, and the fact that the measurement is obtained at a moment when the unstable, sharp crack presents a majority of its front remote from FIB- induced damage, are advantages that make this technique rather attractive for measuring the fracture toughness of brittle materials at the micro-scale

Towards long lasting zirconia-based composites for dental implants: Transformation induced plasticity and its consequence on ceramic reliability

Zirconia-based composites were developed through an innovative processing route able to tune compositional and microstructural features very precisely. Fully-dense ceria-stabilized zirconia ceramics (84 vol% Ce-TZP) containing equiaxed alumina (8 vol%Al2O3) and elongated strontium hexa-aluminate (8 vol% SrAl12O19) second phases were obtained by conventional sintering. This work deals with the effect of the zirconia stabilization degree (CeO2 in the range 10.0\u201311.5 mol%) on the transformability and mechanical properties of Ce-TZP-Al2O3-SrAl12O19 materials. Vickers hardness, biaxial flexural strength and Single-edge V-notched beam tests revealed a strong influence of ceria content on the mechanical properties. Composites with 11.0 mol% CeO2 or above exhibited the classical behaviour of brittle ceramics, with no apparent plasticity and very low strain to failure. On the contrary, composites with 10.5 mol% CeO2 or less showed large transformation-induced plasticity and almost no dispersion in strength data. Materials with 10.5 mol% of ceria showed the highest values in terms of biaxial bending strength (up to 1.1 GPa) and fracture toughness (>10 MPa 1am). In these ceramics, as zirconia transformation precedes failure, the Weibull modulus was exceptionally high and reached a value of 60, which is in the range typically reported for metals. The results achieved demonstrate the high potential of using these new strong, tough and stable zirconia-based composites in structural biomedical applications

Composites in the Alumina-Zirconia system: an engineering approach for an effective tailoring of microstructural features and performances

The aim of this PhD study is the development of composites in the alumina-zirconia system through a powder engineering approach which allows tailoring the compositional and microstructural features, and, as a consequence, the properties of the final materials. The experimental activities refer to two different projects. The first, named MITOR project, was devoted to the elaboration and mechanical characterization of macro-porous Alumina-Zirconia composites. It dealt with the development of a new method for the elaboration of composite cellular ceramics and with the investigation of the role of zirconia and its toughening mechanisms in porous materials, thus filling a gap in the scientific literature. The latter, a European Project named Longlife, was dedicated to the preparation and characterization of zirconia (stabilized with ceria)-based composites for dental and spine implants. The major aim was to overcome the drawbacks proper of yttria-zirconia-based materials, concerning their stability in moisture atmosphere as well as their low toughness. Ceria-zirconia-based composites should benefit from phase transformation toughening still keeping high strength. In addition, they should not suffering of surface degradation in presence of water. So, materials characterized by high strength, high toughness with a perfect reliability and a lifetime longer than 60 years were investigated. The first chapter collects a literature overview of the most relevant zirconia-containing materials, either dense and macro-porous, with particularly emphasis on the transformation toughening proper of such materials and its effects on the mechanical properties and stability. It is described the role of the elaboration route used to prepare the composite powders and the role of some parameters (such as nature of the dopant, microstructural features and phase composition) on the final properties. A brief introduction of the mechanical models, particularly the Gibson-Ashby model, and of the influence of the total porosity and pore size on the mechanical behavior of porous ceramics is also illustrated. The second chapter deals with the set-up of the elaboration process of composite powders through the surface modification of commercial powders with inorganic precursors of the secondary phases. This innovative approach insures a high degree of control of the size and distribution of the second-phase grains on the surface of the parent material. Alumina-based composite powders containing 10 vol% of un-stabilized zirconia as well as tri-phasic zirconia-based composite powders containing 8 vol% of alumina and 8 vol% of an aluminate phase, were developed and characterized in terms of phase evolution and thermal behaviour. In addition, the adopted elaboration route allowed tailoring the ceria amount inside the zirconia grains: four different zirconia stabilization degree were thus investigated. It was shown that the deep knowledge of all the involved mechanisms (such as raw powders dispersion, pH suspension, powder thermal treatments) is crucial for achieving a full control of the powders features and, consequelntly, of the final microstructures. The third chapter is related to the MITOR project and deals with the development of macro-porous alumina-zirconia bodies through a modified gel-casting method in which a sacrificial phase was used as pore former. The selected pore former agent allows tailoring the porosity features, such as the amount of porosity, the pores shape and size distribution. Bodies with porosity amounts ranging from about 60% to 80 vol% were produced and characterized in terms of their microstructures and mechanical properties. Their properties were compared with those obtained on pure alumina components produced by the same way. The compressive strength decreased with decreasing the relative density and, from a compositional point of view, the porous composites showed higher strength values as compared to the pure alumina ones. The well-known zirconia toughening mechanisms (transformation and microcracking toughening mechanisms) were investigated, revealing a poor influence on the mechanical properties. The improvement of the compressive strength in the composite materials can be reasonably due to their finer microstructure, being characterized by smaller grains and pores. The last part of this thesis, related to the Longlife project, describes the development of dense Ce-TZP tri-phasic composites by slip casting and pressureless sintering. Here, the main results of a full characterization in terms of phase composition, microstructure, mechanical (hardness, fracture strength and toughness) as well as physical properties (aging behaviour, transformability, optical properties) are presented. The adopted surface modification technique of a commercial Ce-TZP powder was successful in developing composites having highly homogeneous and complex microstructures characterized by a very good distribution of the secondary phases (round-shaped alumina and elongated aluminate grains) inside a fine zirconia matrix. A strong influence of the composition and sintering cycle on the microstructure and, consequently, on the mechanical properties was revealed. In particular, two completely different mechanical behaviors were observed: in some composites (when the strontium-aluminate phase is present), the strength was transformation driven and the t-m transformation phase took place well before failure. Instead, when a magnesium-aluminate is present, the tetragonal-monoclinic phase transformation took place only around the fracture surface where weak transformation bands can be observed. Two very promising composites with high fracture strength, of about 900 MPa, and high crack resistance were found. Furthermore, the investigated composites showed high transformability and no low temperature degradation in moisture atmosphere in the time-scale of medical applications. It was shown that these properties are strongly affected by the zirconia stabilization degree: it is necessary to carefully investigate the relationship between the final properties and the composition/microstructure architecture in order to reach the desired propertie

Composites dans le système alumine-zircone : Une approche d’ingénierie des poudres permettant de façonner les caractéristiques microstructurales et compositionnelles

L’objectif de cette thèse est le développement de composites dans le système alumine-zircone par une approche d’ingénierie des poudres permettant de façonner les caractéristiques microstructurales et compositionnelles, et, par conséquent, les propriétés des matériaux finaux. Les poudres composites ont été elaborée à travers la modification de la surface des poudres commerciales par un précurseur inorganique de les phases secondaires . Cette approche innovante assure un degré élevé du contrôle de la taille et de la distribution des grains de seconde phase sur la surface du matériau parent. Les poudres composites à base d’alumine contenant 10 vol% de zircone non-stabilisée et des poudres composites à base de zircone triphasique contenant 8 vol% d’alumine et 8 vol% de phase aluminate, ont été développées. Leur propriétés physiques, chimiques et mécaniques a été caractérisé. Dans le cas premier, les materials porous ont été développés à travers le technique gel-casting avec sphères de PE. Les mécanismes de ténacité de la zircone ont été étudiés, révélant aucun influence sur les propriété mécaniques. Les composites poreux présentent des valeurs de résistance à la rupture plus élevées, en comparant à des alumines pures, mais cet amélioration peut être raisonnablement dû à leur microstructure plus fine, celle-ci étant caractérisée par des grains et des pores plus petits. Dans le deuxième cas, a partir de la Ce-TZP, connue pour sa ténacité et sa stabilité importantes, un travail d’optimisation de la microstructure a été réalisé afin d’obtenir une résistance à la rupture maximale. Les matériaux présentés dans cette étude ont été développés afin de répondre au triple objectif de ténacité, résistance et stabilité. Deux composites très prometteurs, avec des résistances à la rupture élevées (environ 900 MPa) et ténacités élevées (environ 10Mpa√m), ont été réalisés. De plus, les composites étudiés ont montré de hautes capacités de transformation et pas de faible température de dégradation en atmosphère humide, dans les temps des applications médicales. Il a été démontré que les propriétés mécanique sont vivement affectées par le degré de stabilisation de la zircone. L’étude de la relation entre les propriétés finales et l’architecture de la composition/microstructure, au but d’obtenir les propriétés désirées, se révèle nécessaire.The aim of this PhD study is the development of composites in the alumina-zirconia system through a powder engineering approach which allows tailoring the compositional and microstructural features, and, as a consequence, the properties of the final materials. The experimental activities refer to two different projects. The first, named MITOR project, was devoted to the elaboration and mechanical characterization of macro-porous Alumina-Zirconia composites. It dealt with the development of a new method for the elaboration of composite cellular ceramics and with the investigation of the role of zirconia and its toughening mechanisms in porous materials, thus filling a gap in the scientific literature. The latter, a European Project named Longlife, was dedicated to the preparation and characterization of zirconia (stabilized with ceria)-based composites for dental and spine implants. The major aim was to overcome the drawbacks proper of yttria-zirconia-based materials, concerning their stability in moisture atmosphere as well as their low toughness. Ceria-zirconia-based composites should benefit from phase transformation toughening still keeping high strength. In addition, they should not suffering of surface degradation in presence of water. So, materials characterized by high strength, high toughness with a perfect reliability and a lifetime longer than 60 years were investigated. Alumina-based composite powders containing 10 vol% of un-stabilized zirconia as well as tri-phasic zirconia-based composite powders containing 8 vol% of alumina and 8 vol% of an aluminate phase(Srontiuma nd Magnesium hexaxaluminate), were developed and characterized in terms of phase evolution and thermal behaviour. In addition, the adopted elaboration route allowed tailoring the ceria amount inside the zirconia grains: four different zirconia stabilization degree were thus investigated. Two very promising composites with high fracture strength, of about 900 MPa, and high crack resistance were found. Furthermore, the investigated composites showed high transformability and no low temperature degradation in moisture atmosphere in the time-scale of medical applications. It was shown that the deep knowledge of all the involved mechanisms (such as raw powders dispersion, pH suspension, powder thermal treatments) is crucial for achieving a full control of the powders features and, consequelntly, of the final microstructures

The local strength of individual alumina particles

We implement the C-shaped sample test method and micro-cantilever beam testing to measure the local strength of microscopic, low-aspect-ratio ceramic particles, namely high purity vapor grown a-alumina Sumicorundum (R) particles 15-30 mu m in diameter, known to be attractive reinforcing particles for aluminum. Individual particles are shaped by focused ion beam micromachining so as to probe in tension a portion of the particle surface that is left unaffected by ion-milling. Mechanical testing of C-shaped specimens is done ex situ using a nanoindentation apparatus, and in the SEM using an in-situ nanomechanical testing system for micro-cantilever beams. The strength is evaluated for each individual specimen using bespoke finite element simulation. Results show that, provided the particle surface is free of readily observable defects such as pores, twins or grain boundaries and their associated grooves, the particles can achieve local strength values that approach those of high-perfection single-crystal alumina whiskers, on the order of 10 GPa, outperforming high-strength nanocrystalline alumina fibers and nano-thick alumina platelets used in bio-inspired composites. It is also shown that by far the most harmful defects are grain boundaries, leading to the general conclusion that alumina particles must be single-crystalline or alternatively nanocrystalline to fully develop their potential as a strong reinforcing phase in composite materials. (C) 2017 The Authors. Published by Elsevier Ltd

Microscopic strength of silicon particles in an aluminium–silicon alloy

A microscopic three-point bending test that measures the strength of faceted particles of high aspect ratio is developed and used to probe individual coarsened plate-like silicon particles extracted from the eutectic Al-12.6%Si alloy. Focused ion beam milling is used in sample preparation; however, the tapered beam cross-section and multistep preparation procedure used here ensure that the particle surface area subject to tension in mechanical testing is free of ion beam damage. Results show that coarsened silicon particles in aluminium can reach strength values on the order of 9 GPa when they are free of visible surface defects; such high strength values are comparable to what has been reported for electronic-grade silicon specimens of the same size. By contrast, tests on eutectic silicon particles that feature visible surface defects, such as pinholes or boundary grooves, result in much lower particle strength values. Reducing the incidence of surface defects on the silicon particles would thus represent a potent pathway to improved strength and ductility in 3xx series aluminium casting alloys

Flexural strength of micron-scale plate-like silicon in aluminum

Mechanical properties of composite materials and alloys are strongly influenced by the intrinsic mechanical properties of the reinforcing phases they contain; however, due to the irregular shape and small size of particulate reinforcements, measuring their intrinsic properties is a substantial methodological challenge. Here we present a method with which we probe the local flexural strength of individual microscopic plate-like silicon particles, which constitute, together with aluminium, the eutectic microconstituent in AlSi alloys. Silicon particles are extracted from the cast and heat-treated alloy by deep-etching the aluminium matrix. The plates are then are dispersed on a steel substrate, where irregular plate- like particles rest lying on one of their large flat facets. A beam of well-defined dimensions is micro-machined out of individual particles using focused ion beam (FIB) milling perpendicular to the substrate. In this way, the particle surface which is in contact with the substrate and that will later on be subjected to tension upon beam bending, i.e. where strength will be measured, is not affected by the FIB nor by redeposition. The FIB is also used to produce a hole in the steel substrate nearby the micro-machined beam so that the silicon beam can be transported and placed on top of the hole using a micromanipulator. The beam is tested in 3- or 4-point bending until fracture by applying a force with a nanoindenter equipped with a diamond tip featuring one or two small rounded ridges that are longer than the width of the beam. Fractography is carried out after testing to locate and characterize the fracture initiation point. Test results are coupled with Finite Element simulations for interpretation. Error in the measurement, including the influence of possible misalignments, of the measured strength is evaluated

Kinetic processes in the high-temperature pressure-infiltration of Al into Al2O3

We explore the influence (i) of the interaction between aluminium and alumina, and (ii) of sodium impurities present in Bayer alumina, on the pressure infiltration of alumina particle preforms with molten aluminium. At 1000°C or above, although the aluminium/alumina system is non-reactive, capillarity-driven solution-reprecipitation processes cause the liquid-solid interface to become mobile. Data show that this can result in infiltration kinetics that resemble those observed with reaction-driven pressure infiltration, namely a continuously increasing melt saturation under fixed infiltration pressure. Resulting isobaric saturation velocities are measured at 1000°C, 1050°C and 1100°C. The role of alumina particle shape and of Na-containing inclusions is investigated. It is found that the main factors affecting the rate of high-temperature isobaric infiltration in this system is the particle geometry. Measured steady infiltration rates give an activation volume on the order of ≈ 200 nm3 and an activation energy in the range of 300-500 suggesting that isobaric infiltration kinetics are governed by diffusion through the solid alumina. Sodium impurities of Bayer alumina are present within β″-Al2O3. They do not influence steady pressure infiltration but ease initial melt penetration into the preform, possibly because evaporated Na2O alters the oxide skin layer that lines the surface of molten aluminium