Influence of microporosity on fracture stress of pyrocarbon coatings

Coated particles are available with and without a siliconcarbide interlayer (SiC) between pyrocarbon coatings (PyC). The pyrocarbon has to protect the SiC-layer against mechanical damage in the first case and it has to retain the gaseous and solid fission products originating in the kernel due to fuel burnup in absence of SiC. High stresses in the outer PyC coatings are built up by irradiation induced anisotropic dimensional changes which may lead to fracture. As the integrity of the coating is of main importance, many investigations were carried out in order to measure and to maximise its fracture stress. In this paper recent investigations on fracture behaviour of integral PyC-coatings are presented. The fracture stresses of propene, acetylene and methane-derived pyrocarbon are measured as a function of deposition temperature and deposition rate. The variation of fracture stress due to these deposition parameters is high compared with errors due to the rather simple measuring technique. The results are discussed and compared with former fracture stress measurements on PyC rings at KFA and on flat propane PyC strips at GGA. (Auth.)