Fracture modes in micropillar compression of brittle crystals
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This article describes cracking during microcompression of Si, InAs, MgO, and MgAl2O4 crystals and compares this with previous observations on Si and GaAs micropillars. The most common mode of cracking was through-thickness axial splitting, the crack growing downward from intersecting slip bands in pillars above a critical size. The splitting behavior observed in all of these materials was quantitatively consistent with a previous analysis, despite the differences in properties and slip geometry between the different materials. Cracking above the slip bands also occurred either in the side or in the top surface of some pillars. The driving forces for these modes of cracking are described and compared with observations. However, only through-thickness axial splitting was observed to give complete failure of the pillar; it is, therefore, considered to be the most important in determining the brittle-to-ductile transitions that have been observed.
I. INTRODUCTION
Micropillar compression, originally used for studying effects of size in metals,1–3 has the potential to be a technique for studying plastic flow in brittle materials at low temperatures as cracking may be suppressed without confining pressure or material.4–7 Furthermore, by choosing the appropriate pillar orientation, individual slip systems can be interrogated,7 whereas in indentation, multiple slip systems are always activated.8 This may not be important in metals where the deformation often requires only a single type of slip system, but in brittle materials with more complex structures, this is rarely the case.9 Here, the values of hardness tend to be associated with flow on the harder slip system.10,11 To make use of micropillar compression as a test technique, it is necessary to know how small a sample is required to suppress cracking. For this, the ways in which cracking can occur and the extent to which these are affected by scale need to be understood. Although there have been a number of studies looking at micropillar compression of brittle crystalline materials,4–7,12,13 there have been few systematic studies of a size-dependent ductile–brittle transition. In studies of ,100.-oriented micropillars of silicon5 and gallium arsenide,4 cracking occurred predominantly by throughthickness axial splitting in pillars with diameters greater than about 0.3 and 2 lm for the silicon and gallium arsenide micropillars, respectively. At diameters less than these values, splitting was not observed, though Gerberich et al.14–16 have reported cracking in silicon spheres with diameters less than 100 nm, although there was extensive a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.256 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
plastic flow even where cracking occurred in both the micropillars4,5 and the spheres.15,16 There are a number of ways in which size is known to influence cracking. For instance, as the size of the largest defect within a sample is limited by the sample size, decreasing this increases the strength in
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