Author: Huixing Zhang

Publisher:

Published: 2012

Total Pages:

ISBN-13:

TRISO fuel particles have been developed as nuclear fuels used for a generation IV nuclear reactor: high temperature reactor. Such particle consists of a fuel kernel, pyrolytic carbon (PyC) and silicon carbide (SiC) coatings. This study has been carried out to establish a relationship between mechanical properties and microstructures of SiC coating and PyC coatings produced by fluidized bed chemical vapour deposition. Indentations were used to measure hardness, Young's modulus and fracture behaviour of SiC and PyC coatings. Fracture strength of SiC coatings was measured by crush test. Microstructure of SiC and PyC was mainly characterised by transmission/scanning electron microscopy and Raman spectroscopy. For SiC coatings produced at 1300 oC, Young's modulus is an exponential function of relative density. Hardness of SiC coatings is higher than the bulk SiC produced by CVD, and it is attributed to the high density of dislocations and their interactions. The deformation mechanism of SiC coatings under indentation is explained by presence of defects, such as grain boundaries and nano-pores. The fracture of these coatings beneath the Vickers indentation is the Palmqvist cracks, and indentation fracture toughness was in the range of 3.5-4.9 MPa m1/2. The stress-induced micro-cracks are assumed to be the mechanism for the high indentation fracture toughness. Different hardness and fracture toughness in these SiC coatings are attributed to influences of defects and grain morphology. Measurement of fracture strength was carried out on SiC coatings deposited at 1300-1500 oC. Weibull modulus and fracture strength of the full shell are dominated by the ratio of radius to thickness of coatings, and decrease linearly with the increase of this ratio. The influence of SiC/PyC interfacial roughness on fracture strength of the SiC was quantified by self-affine theory. The fracture strength decreases linearly with the increase of the roughness ratio, which is the long-wavelength roughness characteristic. After thermal treatment at 2000 oC, fracture strength decreased in SiC coatings due to the formation of pores, which are results of diffusion of native defects in as-deposited SiC coatings, and the change of Weibull modulus is related to the size and distribution of pores. For low density PyC coatings, Young's modulus and the mean pressure increase with the increase of the density; however, for high density PyC coatings, they are determined by interstitial defects. The hysteresis deformation behaviour under nano-indenation has been found be affected by density variation and thermal treatment, which is proposed to be due to the disorder structure in PyC coatings.