Mechanism of flaking in pyrolytic graphite on ion bombardments. I

Elastic fields in the sub-surface layer of pyrolytic graphite due to a penny-shaped crack on the basal plane associated with internal gas pressure are calculated theoretically by the 'inverse method'. Extraordinarily large stress concentrations appear in the stress components, such as sigmasub(rr) and sigmasub(phiphi), specified in cylindrical coordinates with the z-axis perpendicular to the basal plane, when the depth of the crack from the free-surface becomes smaller than its radius. Small stress concentrations appear in sigmasub(zz) and sigmasub(rz). These stress components change their signs depending both on the depth and the radial coordinate around the crack. Therefore, the sub-surface layer behaves like an elastic plate associated with bending moments whose signs depend on the radial coordinate. Both tensile and compressive hydrostatic pressures also appear around the crack. These stress components are briefly discussed in relation with the flaking mechanism. (Auth.)