When a solid is cleaved, energy must be supplied to separate one part of the crystal from another. The molar energy required to separate adjacent structural units in two different solids is listed below.
• Solid X (diamond) across a {111} plane: 13 kJ mol⁻¹
• Solid Y (graphite) between adjacent layers: 0.30 kJ mol⁻¹
A materials engineer is considering the use of one of these substances as a dry lubricant inside a moving mechanical joint that experiences both shear and compressive forces.
(a) Calculate the ratio of the cleavage energy of diamond to that of graphite for the planes described.
(b) Identify which substance would serve as the more effective lubricant under shear and justify your choice with reference to the calculated ratio and bonding dimensionality.
(c) The joint periodically experiences high compressive stress perpendicular to the sliding surfaces. Predict how this stress is likely to affect the electrical resistance of graphite within the joint and provide a molecular-level explanation.