The growth process of pyrolytic carbon (PyC) over zirconia microspheres was investigated at length scales starting from 10⁻¹⁰ m to 10⁻⁶ m using Transmission and Scanning Electron Microscopy, Raman spectroscopy, Small Angle X-ray and Neutron Scattering. The extent of multiphase reactions which evolved through the formation of nano-crystalline carbon have been studied here right from the formation of the disc shaped crystallites of about 3–4 nm in diameter to fused lumps comprising of spherical stacks ca. of 100 nm diameter. The observations have been correlated with the deposition conditions, which affect reaction mechanisms. Based on these observations, a plausible mechanism for the deposition of PyC coatings at different length scales has been proposed with a novel concept of role of thermal boundary layer adjacent to the deposition surface. Pore volume measurement further validated our proposed growth mechanism at a length scale of 10⁻⁶ m. Analysis of residence time of the acetylene precursor within the solid bed of zirconia microspheres has shown to have great potential in exploring growth mechanism of multiphase reactions.