Strength and Surface Morphology of Si_3N_4 Composite Ceramics Coated with SiO_2 Gel

摘要

The Si_3N_4/SiC and Si_3N_4/SiC/TiO_2 composite ceramics were sintered at 1850 ℃, for 2 h, in air by Hot-Press. A semi-elliptical surface crack was made at the center of the tensile surface of the test specimen using a Vickers indenter at a load of 24.5 N. By this method, the crack length on surface was made the semi-elliptical cracks of about 100μm. Crack-healing behavior depends on healing temperature. The optimum healing condition of Si_3N_4/SiC and Si_3N_4/SiC/TiO_2 composite ceramics are well known for 1,300 ℃, lh in air environment. So Smooth and as-cracked specimens were heated in this condition. To investigate temperature-dependence of the SiO_2 gel coated specimen, four kinds of temperatures (800, 900, 1000, 1,300 ℃) were tested. And to investigate SiO_2 gel coating time-dependent, one and three times were coated. SiO_2 gel coated specimens indicated increase of bending strength as the temperature goes up. Moreover the bending strength of SiO_2 gel coating specimens was similar to the bending strength of healed smooth specimen at 900 ℃.