Probing the micromechanical strength of oxide ceramic composite reinforcements

This work shows how one can probe the micromechanical strength of ceramic reinforcements used in metal matrix composites, which greatly influences the mechanical performance of the composite material yet has seldom been quantified with precision. More specifically, this study presents two methods by means of which one can measure the statistical strength distribution of microscopic, low-aspect-ratio, ceramic particles. Additionally, the study reveals the nature of specific defects that weaken such ceramic reinforcements and shows that, when those defects are absent, one can produce particles of near-theoretical strength, which have the potential to produce remarkably strong and tough metal matrix composites. In one developed method called here the Meridian Crack Test, individual spherical particles are compressed uniaxially between a pair of parallel elasto-plastic platens. It is shown that, by tailoring the platen hardness one can control the relative area of particle-to-platen contact during the test, thereby eliminating the initiation of contact microcracks that are often found to influence particle fracture when hard platens are used. It is shown how this method, coupled with the mathematics of statistical survival-analysis, can give unambiguous access to the particle statistical tensile strength as governed by surface flaws. The method is first demonstrated using microscopic fused quartz spheres 40±20µm in diameter and is then used to measure the strength controlled by surface and subsurface flaws in plasma-sprayed spherical amorphous and nanocrystalline near-eutectic "Eucor" alumina-zirconia-silica ceramic particles of diameter near 30 µm. Results show that nanocrystalline Eucor particles exhibit a characteristic Weibull strength of 1490 MPa, which is approximately 30% higher than in corresponding amorphous particles. The second developed method, called here the C-shaped sample test, combines focused ion beam milling, loading using a nanoindentation device, and bespoke finite element simulations to measure the local strength of ceramic reinforcements free of artifacts commonly present in micromachined specimens. The method is first demonstrated on Nextel 610TM nanocrystalline alumina fibres embedded in aluminium. Results reveal a size effect that does not follow, across size scales, the Weibull statistical strength distribution that is measured by tensile testing macroscopic samples of the fibres. This indicates that, in micromechanical analysis of multiphase materials, highly localized events such as the propagation of internal damage require input data that are measured at the same, local, micro- scale as the event. Finally, we implement the C-shaped sample test method with additional micro-cantilever beam testing to measure the local strength of vapour-grown ¿-alumina Sumicorundum® particles 15 to 30 µm in diameter, known to be attractive reinforcing particles for aluminium. Results show that, provided the particle surface is free of readily observable defects such as pores, twins or grain boundary grooves, the particles can achieve local strength values that approach those of high-perfection single-crystal alumina whiskers, on the order of 10 GPa. 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

Compression testing spherical particles for strength: Theory of the meridian crack test and implementation for microscopic fused quartz

AbstractWe show that uniaxial compression testing of spherical particles can give unambiguous access to their tensile strength as governed by surface flaws if one uses pairs of elasto-plastic platens, tailoring their hardness in order to control the relative area of particle-to-platen contact during the test. This eliminates the development of contact microcracks that are typically found to govern particle fracture when hard platens are used. We show that, if the platen materials are well chosen, one can probe a range of stress states for which it is known that particle failure was initiated along the surface, under elevated hoop stress within a region situated remote from the points of load application. Specifically, platens must be chosen such that particles tend to fracture when the ratio of projected contact area radius to particle radius exceeds a specific value that depends on the Poisson ratio of the particles. With fused quartz of Poisson ratio 0.17, this specific ratio value equals 0.65. We demonstrate the approach using microscopic fused quartz spheres 40±20µm in diameter as a testbench material; with those particles hardened steel serves as an appropriate platen material. Their strength values are statistically distributed; this is addressed using several platen materials. The resulting bank of data is interpreted using established survival-analysis methods, namely the non-parametric product-limit estimator. We also give a maximum likelihood estimation of the particle strength Weibull distribution parameters derived from the ensemble of data after left-truncation and/or right-censoring of data points situated inside of the range of unambiguous surface fracture strength measurement for each platen material. This gives a Weibull modulus of 6.3 and characteristic strength of 890MPa for the fused quartz particles. These values are significantly lower than what is produced in high-strength fused quartz fibers of comparable diameter; the difference is most likely a result of surface damage caused during powder storage and manipulation in the absence of a protective coating

Microscopic chevron-notch fracture test of hard second phases

It is well understood that the intrinsic mechanical properties of reinforcements and second phases strongly influence those of composite materials and alloys; however, these properties are challenging to determine due to the small size and irregular shape of the particles. We present a method for the measurement of the fracture toughness of micron-sized specimens based on Focused Ion Beam (FIB) milling, combined with micro-mechanical testing techniques and Finite Element (FE) simulation. Microscopic chevron-notched cantilever beams are first machined. These beams are subsequently loaded using a nanoindentation testing apparatus. It is found that the crack initiates at the chevron notch tip under a load that is in most tests lower than that corresponding to the onset of crack instability. Some degree of stable crack growth can thus be observed and meaningful toughness measurements gleaned on these small tested volumes. The measurements are furthermore minimally influenced by milling-induced defects given that, at the onset of unstable growth, most of the crack front is situated away from the FIB-machined surface. Test data are interpreted using compliance calibration curves determined by three‑dimensional FE simulation of each beam, after measurement of its dimensions using electron microscopy. The method is applied to nanocrystalline Nextel ™ 610 alumina fibres, which are used in aluminium-matrix composites. Results are consistent with expected values for the material at hand, suggesting that the technique is reliable despite the small specimen size, and that it can be transposed to other reinforcements. The influence on the test of environmentally induced subcritical crack growth, known to be operative with alumina in air, is also examined

Probing the microscopic strength of alumina reinforcements

In this work we develop a methodology to probe the strength and toughness of microscopic brittle phases by combining focused ion beam (FIB) milling methods with microtesting techniques and finite element simulation. We present results from tests conducted on alumina fibres and compare data thus obtained with known macroscopic fibre properties derived from tensile tests conducted on long sections of the fibres. The developed methodology is to be transposed towards the microscopic mechanical testing of irregularly shaped second phases in MMCs and alloys

Strong particles in Metal

We all know that strong fibers make most of today¹s composite materials what they are : exceptionally stiff, strong, light and tough materials that serve in a number of demanding structural applications. This presentation will try to make the case for strong particles, proposing that they hold equally high promise as a reinforcing phase in metal ¬ if they can be made to be as strong as today¹s engineering fibres. The talk will be articulated in two parts. In the first, experimental data from the author¹s laboratory, combined with simple micromechanical calculations, will be used to show how strong ceramic particles combined with metal do indeed have the potential to produce highly attractive materials. The second part of the presentation will address the question of measuring particle strength : unlike fibres, which are easily tested in uniaxial tension, miroscopic second phases or particles used for the reinforcement of metals are far more difficult to characterize for strength. A few methods devised to this end in recent work within the author¹s laboratory will be presented, together with experimental results gathered on strong brittle particles that are, or can be, used, for the reinforcement of metals. The presentation will conclude with a few practical implications of this work

Probing the local strength and toughness of microscopic hard second phases in metals and composites

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases, e.g. silicon particles in aluminium alloys, carbides in steels or ceramic particles in metal matrix composites. It is well understood that the mechanical properties of such alloys and composites are strongly influenced by those of their reinforcing particles; yet not much is known of the nature and size of strength-limiting flaws within such microscopic second phases. To investigate the strength and fracture toughness of these microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting techniques and bespoke finite element simulation. Local strength is probed by micromachining individual reinforcing particles in such a way that tensile stresses are produced in a volume of material free of FIB and polishing artifacts upon the application of a load via nanoindenter. Fracture toughness is measured by the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are currently being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, with the objective to measure their local mechanical properties and identify microstructural flaws that govern the strength of such particles, using in-depth electron microscopy characterization combined with microtesting

Probing the local strength and toughness of microscopic hard second phases in alloys and composites

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases. It is known that the mechanical properties of such alloys and composites are strongly influenced by those of those reinforcing phases; however, surprisingly little is known of the toughness, or of strength-limiting flaws within those phases. To investigate the strength and fracture toughness of such microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting and bespoke finite element simulation. Local strength is measured using a nanoindenter probing micromachined samples carved into individual reinforcing particles in such a way that tensile stresses are produced in a volume of material that is free of FIB and polishing artifacts. Fracture toughness is measured using the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are now being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, while other tests are being developed to probe phases the shape of which enables the design of other tailored testing procedures

Untangling the controversy on Ce3+luminescence in LaAlO3crystals

The work was supported by the Czech Science Foundation project no. 18-14789S and by Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760). We acknowledge MAX IV Laboratory for time on Beamline FinEstBeAMS under Proposal 20180572. The research leading to this result was supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. The Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under Grant Agreement No. 739508, Project CAMART2.Aluminum perovskites represent an important group of promising scintillation materials with excellent proportionality and energy resolution, but due to difficulties in crystal growth not much attention has been paid to them. We studied a Ce-doped LaAlO3lanthanum-aluminum perovskite (LaAP) because of its easy crystal growth facilitated by the large La3+cations in the matrix. Moreover, recent observations of intense blue luminescence by some researchers show that the potential of this material could not be ruled out. On the other hand, some reports claim that Ce3+luminescence is completely absent in the LaAP matrix. Therefore, we have decided to study this material in much greater detail using an extended set of correlated experiments to explain the observed discrepancies and underlying phenomena. Crystal growth by the micro-pulling-down method is reported together with the luminescence and scintillation properties. We demonstrate the influence of inclusions of other aluminate phases created during the crystal growth on the luminescence processes. The existence of the phases was simultaneously confirmed by observations using a scanning electron microscope, cathodoluminescence, energy-dispersive X-ray analysis and electron paramagnetic resonance (EPR), which were correlated with photoluminescence and scintillation studies. The EPR evidenced the incorporation of Ce ions in different environments comprising the LaAP matrix and inclusions. Based on these results, the luminescence mechanism is proposed and discussed and the low scintillation efficiency of the Ce-doped LaAP is explained together with the discrepancies in the literature. © 2022 The authors. --//-- Published under the CC BY and CC BY-NC licence.Czech Science Foundation project no. 18-14789S; Ministerstvo Školství, Mládeže a Tělovýchovy 730872, SOLID21 CZ.02.1.01/0.0/0.0/16_019/0000760; CALIPSOplus under the Grant Agreement 730872; The Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under Grant Agreement No. 739508, Project CAMART2