Direct Basal-Plane Shear in Single-Crystal Graphite

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Abstract
The basal‐plane shear stress‐strain behavior of small, highly anisotropic‐annealed natural graphite single crystals was studied at room temperature. A static uniaxial‐shear stress was applied directly along the basal plane with a minimum of normal force, incorporating a technique to detect translations down to ∼40 Å. Comparative measurements were also made on compression‐annealed pyrolytic graphite. The staticshear modulus G was also corroborated by a modified ultrasonic transit‐time method. Basal‐plane dislocation systems strongly reduce the measured G, where values of (0.013–0.14)×1011 dyn/cm2 were observed. This reduction is found to be caused primarily by a dislocation concentration of ∼2×106 cm−2. The average critical‐resolved shear stress σc was 0.29×106 dyn/cm2, and an analysis of the relation between σc and the critical breakaway stress for dislocation pinning shows that dislocation line segments l≃120–310 μ are operative. Plastic curvature of the stress‐strain curves shows the effect of very sensitive creep and glissile slip. Laminar flow in this principal slip direction produces a plastic strain ε* = Aσ4.2 analogous to easy glide in hcp metals along the close‐packed direction. Classical Andrade t1/3 creep was observed at higher stresses and approached a logarithmic creep behavior with decreasing stress. A saturation effect seen in the shear‐strength σs with shear‐fracture cycling leads to a resultant σs of (2.5–7.5)×106 dyn/cm2. Possible contributions to the elastic shear strain, including dislocations, grain boundaries, hard inclusions, delamination voids, and the Fermi‐level shift, are considered as they affect the measured shear modulus. Dislocation pinning by boron ions, in the dilute concentration range from 7 ppm B up to 1500 ppm B, was found to produce a large increase in G with eventual saturation. After accounting for all known shear‐strain components, this saturation value leads to an intrinsic single‐crystal graphite C44 value of (0.45±0.06)×1011 dyn/cm2.

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