This is study analyzed the surface morphology of Si3N4 composite ceramics as functions of the heat-treatment temperature and the additive SiO2. The additive colloidal SiO2 and a coating of the colloidal SiO2 on cracks was found to significantly increase the bending strength. Moreover, the presence of the additive TiO2 resulted in decreasing the crack healing temperature by 300 K. Meanwhile, the bending strength of a specimen with the additive colloidal SiO2 was revealed to be higher than that without colloidal. In addition, the bending strength of a specimen coated with colloidal SiO2 was found to be much higher that of a non-coated specimen. During an in-situ observation, a fog-like phenomenon appeared oil the surface of the crack-healed specimen at 1,573 K. When using an SPM (Scanning Probe Microscope), both of the crack-healed specimens, i.e. the one coated and non-coated with the colloidal SiO2, were found to have completely healed at 1,273 K. However, cracks existed in both cases at 1,573 K. This call be regarded as the result of the evaporation of the SiO2 at high temperatures. Crystallized Y2Si2O7, Y2Ti2O7 and SiO2 were employed as the crack-healing materials of Si3N4 composite ceramics. A large amount of Si and O, and a minute amount of C were detected using in EPMA (Electron Probe X-Ray Micro Analyzer). The heat treatment temperature was found to result in an increase in Si and O and a concurrent decrease in C. Higher levels of Si and O were detected on the specimens with the SiO2 additive than in ones when the additive SiO2 was not present. Meanwhile, significantly higher levels of O were detected in the specimen coated with the colloidal SiO2.

Keywords: Si3N4 ceramics; SiO2 colloidal coating; Hybride binder; In-situ observation; Crack-healing; Morphology