The primary factors which determine the microstructures of pyrolytic carbons deposited in a tumbling bed

Abstract

Microstructures of pyrolytic carbons deposited in a tumbling bed from propane gas have been examined by polarized light microscopy, scanning electron microscopy, and transmission electron microscopy. Isotropic, laminar, and columnar structures were obtained. These structural varieties, which could be produced by changes in the deposition conditions and the substrate geometry, Were investigated to select more reasonable deposition mechanism. Two essential factors are found directly related to determine the microstructure of pyrolytic carbon. [)ne is the degree of supersaturation of pyrolysis products in a gas phase. The other is the gas flow pattern of recirculation within a deposition zone. The overall deposition process of pyrolytic carbon can be explained by the factors independent of various coating systems. This paper gives the experimental evidences that the above two factors are inter-related and simultaneously contribute to form pyrolytic carbons, and the cracks formed in deposits are closely related to their fracture mechanism. Due to the cracks the fractured surfaces of isotropic deposits are round and those of columnar deposits are laminated. Laminar deposits have both round and laminated fracture surfaces.