The Si3N4/SiC and Si3N4/SiC/TiO2 composite ceramics were sintered at 1850 °C, 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 100 ㎛. Crack-healing behavior depends on healing temperature. The optimum healing condition of Si3N4/SiC and Si3N4/SiC/TiO2 composite ceramics are well known for 1,300 °C, 1h in air environment. So Smooth and as-cracked specimens were heated in this condition. To investigate temperature-dependence of the SiO2 gel coated specimen, four kinds of temperatures (800, 900, 1000, 1,300 °C) were tested. And to investigate SiO2 gel coating time-dependent, one and three times were coated. SiO2 gel coated specimens indicated increase of bending strength as the temperature goes up. Moreover the bending strength of SiO2 gel coating specimens was similar to the bending strength of healed smooth specimen at 900 °C.

The optimized conditions of pressureless sintering were investigated in order to obtain the bending strength and the elastic wave signal of Al2O3 composite ceramics for textiles machinery. As sintering conditions, a temperature range from 1400°C to 1700°C and time from 30 minutes to 150 minutes were applied. Three-point bending tests were conducted on the sintered materials to obtain the strength property. From the test results, the optimum sintering condition was 1600°C, 100 minutes. Al2O3 composite ceramics showed that the elastic wave signal characteristics had a regular correlativity between the optimum sintering temperature and time as well as the maximum bending strength.