The deformation and twinning of graphite crystals under a point load

The deformation produced by point loading on the cleaved basal surfaces of graphite crystals has been studied in the scanning electron microscope. The experiments were of two types, viz. sliding and static contact. In the first a small tungsten stylus (tip radius ∼ 1 μm) was slid on the graphite surface inside the scanning electron microscope and the resulting damage observed as it occurred. The marked anisotropy and the limited number of slip systems of the graphite structure result in a characteristic deformation mode which involves the cleavage of thin flakes followed by folding of the partially separated layers. In the second type of experiment the deformation produced by a diamond indenter was studied. These experiments showed that the hardness value of c. 4.8 × 107 N/m2 for naturally-occurring graphite and 22 × 107 N/m2 for neutron-irradiated graphite are determined by a cleavage and folding process and are not related to the basal plane shear strength. It is suggested that the mechanism of folding in both types of experiment is one of mechanical twinning. An important feature of the deformation process is that the dislocation walls forming the twin boundaries are mobile under the applied stress and are free to run in the newly formed chips and flakes in a direction parallel to the basal plane. The greatly increased hardness of neutron-irradiated graphite is explained by the reduction in mobility of the twin boundaries due to the high point defect concentration.