The relation between the coating conditions and mechanical pmperties of the isotropic pyrolytic carbons deposited in a tumbling bed have been investigated. At a constant total flow rate of mixed gases and a constant revolution rate of the reaction tube. pure and Si-alloyed pyrolytic carbons were deposited using a wide range of deposition conditions.
The structures of the pyrolytic carbons were characterized by density and crystallite size. The mechanical properties (Young's modulus, fracture strength and strain energy to Fracture) were measured by three point bend tests with a knife edge span of 0.35 in.
In spite of similar density and crystallite size, all of the mechan~ca1 properties of pyrolytic carbons deposited in this process were lower than those of carbons deposited in a fluid&d bed. These differences were caused by soot inclusions which had a non-load bearing character and behaved as cracks under load, which were not present in carbons deposited in a fluidized bed.
By alloying silicon with the carbon, the number of soot inclusions was not changed. and the fracture strength increased linearly with the Sic content.