Direct Basal-Plane Shear in Single-Crystal Graphite

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.