Václav Pejchal
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.
EPFL2017
