Is the structure of anisotropic pyrolytic carbon a consequence of growth by the Volmer-Weber island growth mechanism?

Pyrolytic carbon deposits, formed on flat zirconia surfaces exposed to the oxypyrolysis of CH4 at 1673 K, were examined by SEM and found to resemble Johnson-Mehl tessellations. Carbon deposits were also observed that were isolated on the zirconia surface. These deposits were either hemispheres or resembled Johnson-Mehl tessellations with rounded edges. Examination of cross-sections and internal structures of the carbon hemispheres, produced by both FIB milling and mechanical damage, showed that the hemispheres were comprised of concentric layers of carbon. These observations are consistent with growth by the Volmer-Weber 3D island mechanism, in which isolated deposits grow and impinge upon one another, and then coalesce to eventually form a continuous film. A simple mathematical model of Volmer-Weber growth was used to demonstrate that this mechanism can produce the structures observed in this study, and more generally can provide insights into some aspects of pyrolytic carbon structure and growth. In particular, columnar structures, tessellated surfaces and growth cones, all known for anisotropic pyrolytic carbons, are consistent with growth by the Volmer-Weber 3D island mechanism.