Microstructure characterization of precursor-derived Si-C-N ceramics before and after creep testing Original Research Article

Precursor-derived Si-C-N ceramics before and after creep testing were characterized by means of X-ray diffractometry (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The surface of Si-C-N ceramics was found to be amorphous after annealing at 1400 °C in argon, whereas it was composed of α-cristobalite crystals after creep testing in air. Before and after creep testing, there was no significant compositional change in the intrinsic Si-C-N ceramics where a large number of homogeneously precipitated β-SiC nanocrystallites were observed. Creep testing for a longer time or at a higher temperature resulted in a higher crystallinity of the Si-C-N ceramics. Between α-cristobalite and crept Si-C-N ceramic, an intermediate zone was found in which Si2N2O nanocrystallites precipitated homogeneously. Moreover, three kinds of nano-sized porosity (closed, quasi-open and open) in the crept Si-C-N ceramics were identified, and a two-step oxidation mechanism of Si-C-N ceramics was finally proposed.