This study analyzed the crack-healing behavior of Si3N4 composite ceramics based on variations in the heat-treatment temperature and the amount of colloidal SiO2 added. Semi-elliptical cracks about 100 μm length were obtained from a Vickers indenter using a load of 24.5 N. The results showed that the use of an optimum amount of added colloidal SiO2 and the coating of colloidal SiO2 on the cracks could significantly increase the bending strength. Meanwhile, the heat-treatment temperature has a profound influence on the extent of crack healing and the degree of strength recovery. The optimum heat-treatment temperature is dependent on the amount of added colloidal SiO2 employed. The crack healing strength was far greater in the case of the cracked specimen when colloidal SiO2 had been coated on the crack surfaces. When scanned with a microscope probe, the cracks were found to have almost entirely disappeared when heat treated at a temperature of 1,273 K. The bending strength of the SSTS-1 cracked specimen not coated with the colloidal SiO2 recovered to the same extent as the smooth specimen at the optimum healing temperature of 1,273 K. However, the bending strength of the cracked specimen with the colloidal SiO2 coating increased by up to 140%. The optimum amount of the added colloidal SiO2 was found to be 1.3wt.%. Moreover, the bending strength of the crack-healed specimen with the colloidal SiO2 coating was up to 160% greater than that of the smooth specimen with 0.0 wt.% of the added colloidal SiO2 coating. Crack closure and the rebonding of the cracks caused by the oxidation of cracked surfaces were thus identified as the dominant healing mechanism of Si3N4 composite ceramics.

Keywords: Si3N4 composite ceramics, SiO2 colloidal, Additive, Crack-healing, Bending strength.