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Abstract
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 α-alumina Sumicorundum® particles 15–30 µm 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.
Highlights
The approach is inspired from the parallel problem of mechanical characterization of ceramic bearings (Strobl et al, 2014; Supancic et al, 2009; Wereszczak et al, 2007a, 2007b), which we have recently extended to the microscale and demonstrated by measuring the local strength of strong and brittle reinforcing fibers embedded in a metal (Žagar et al, 2015)
We show that random grain boundaries are deleterious to the strength of alumina particles, leading to an overall conclusion that alumina has great potential as a particulate reinforcement in composite materials provided it is (i) smooth in shape and (ii) single- or nano-crystalline
The deep-etching sample preparation procedure provided a number of partially embedded particles situated far enough from neighboring particles to be readily available for focused ion beam machining (FIB) machining
Summary
Particle fracture is well-known to be one of the primary causes for the premature failure of particulate composites or multiphase alloys (Babout et al, 2004; Beremin, 1981; Brechet et al, 1991; Ghosh and Moorthy, 1998; Gurland, 1972; Lewandowski et al, 1989; Li et al, 1999; Llorca et al, 1993; Mummery et al, 1993; Mummery and Derby, 1994; Pandey et al, 2000; Scarber and Janowski, 2001) It is known, from both experiment (Bonderer et al, 2008; Bouville et al, 2014; Hauert et al, 2009a; Kouzeli et al, 2001; Krüger and Mortensen, 2014; Le Ferrand et al, 2015; Miserez et al, 2006, 2004a, 2004b; Miserez and Mortensen, 2004; Munch et al, 2008) and micromechanical theory (Hauert et al, 2009b; Tekoglu and Pardoen, 2010), that stronger, more ductile, and tougher particulate composites are produced if the intrinsic strength of their particulate reinforcement is increased. To date the strength distribution of particulate reinforcements has generally been estimated indirectly, by coupling more or less elaborate micromechanical models of twophase composite material behavior with direct or indirect observations of particle fracture (Babout et al, 2004; Brockenbrough and Zok, 1995; Caceres and Griffiths, 1996; Eshelby, 1957; Hauert et al, 2009a; Kiser et al, 1996; Lewis and Withers, 1995; Li et al, 1999; Majumdar and Pandey, 2000; Mochida et al, 1991; Wallin et al, 1986; Wang, 2004; Wang et al, 2003) or using X-ray and neutron diffraction to estimate average strains in reinforcing particles at the onset of particle cracking (Coade et al, 1981; Finlayson et al, 2007; Mueller et al, 2008)
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