Stable room-temperature micron-scale crack growth in single-crystalline silicon
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oom-temperature fracture along the (111) plane of silicon is probed at the micron-scale using chevron notched cantilever beams that enable stable crack growth before unstable fracture in successful tests. The main experimental observation is that a growing crack can extend and arrest at different stress intensity factor values within the same specimen. The present data thus provide evidence of variations in the effective Si fracture toughness along the path of a growing crack. This effect could be explained by variations in the extent of limited cracktip plasticity along the crack path. The present work also shows that the microscopic chevron notch test is, from an experimental point of view, an inconvenient method to probe the fracture toughness of silicon because it is difficult with silicon to nucleate a crack at the chevron tip at loads low enough to allow for subsequent stable crack growth.
I. INTRODUCTION
Silicon’s overwhelming technical importance has prompted numerous studies of its mechanical properties. Much has thus been written on its fracture behavior, which is subject to several complexities, e.g., fracture anisotropy,1–3 a brittle-to-ductile transition (BDT),4–7 initiation fracture toughness8,9 and associated environmental effects,10 slow crack growth,11 dynamic fracture and instabilities,12,13 fractal fracture surface characteristics,14 or failure mechanisms specific to applications such as in lithium-ion batteries.15 Moreover, its availability in virtually defect-free single-crystalline highpurity form has made silicon an oft-used model material for the general study of brittle fracture (despite the fact that its fracture characteristics are highly complex). A bird’s eye view of the literature on the fracture of silicon reveals much scatter and inconsistency across the fracture toughness values reported. DelRio et al.16 provide a critical, in-depth, survey of published measurements of silicon’s fracture toughness at room temperature. Focusing on silicon’s lowest energy planes, i.e., the (111) and the (110) planes, reported values pffiffiffiffi cover the ranges KIcð111Þ p ¼ffiffiffiffi0:65 1:7 MPa m and KIcð110Þ ¼ 0:7 2:5 MPa m; for comparison, values that have been predicted from bond energy and elastic pffiffiffiffi moduli considerations are KIcð111Þ ¼ 0:72 MPa m and
Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: martin.mueller@epfl.ch DOI: 10.1557/jmr.2017.238
pffiffiffiffi KIcð110Þ ¼ 0:73 0:82 MPa m.16 Such a degree of scatter is impressive, particularly if one recalls that the surface energy is ultimately proportional to KIc 2 . As discussed in sections C6–C8 of Ref. 16, a number of studies must be excluded due to their lack of accuracy arising from basic flaws of methodology (notably data gleaned from nanoindentation cracking or from prenotched specimens). Specifically, it is argued in Ref. 16 that specimens in which a notch is introduced either by chemical etching or by ionbeam milling can lead to large overestimations of the fracture toughness in geometries where fracture is u
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