Unique nanoindentation damage for highly textured pyrolytic carbon

We observed unique convex damage patterns when performing nanoindentation normal to the deposition plane of ∼1.5 mm thick pyrolytic carbon produced by chemical vapor deposition. The damage consisted of a combination of delamination cracks parallel to the surface and mismatched cracks normal to the surface resulting in raised features where the indenter contacted the surface with no damage extending past the indentation contact area. This pattern is significantly different than the radial cracks typically observed for brittle materials. The damage was observed for a cube corner indenter and a 3 μm radius cono-spherical indenter but was not observed for a Berkovich indenter. We were not able to cause any cracking or induce any damage when indenting in the deposition plane which is unexpected because the deposition plane is known to have lower fracture toughness. We attribute this inability to cause cracking to accommodation of the stresses by nanobuckling of the graphite-like planes when loaded in compression.