A study of the microstructure and mechanical properties of SiC coatings on spherical particles

We have investigated the effect of the microstructure on the mechanical properties of three nearly stoichiometric SiC coatings (SiC, SiC + C and SiC + Si coating), which were coated onto spherical particles as simulated nuclear fuel particles by fluidized-bed chemical vapour deposition (FBCVD). The mechanical properties of the SiC coatings were studied using micro- and nano-indentation. The microstructure was characterised using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM was also used to elucidate the deformation behaviour under the indentation. The FBCVD SiC coatings studied exhibited a higher hardness than conventional CVD SiC coatings, and SiC coating gave the highest hardness among the three coatings. TEM confirmed that the presence of pores affect the Young's modulus of SiC coatings. The high hardness was attributed to the high density of dislocations and their interactions. The initiation and propagation of micro cracks under the confined shear stress was found to be responsible for the mechanism of plastic deformation. Based on this hardness-related plastic deformation mechanism, the variation of hardness in the three types of SiC coating was due to different grain morphologies.