Effect of BN whisker-like additive on the flexural strength of Si3N4 ceramics prepared by gel casting method

The gel casting method was used for the production of Si3N4 porous ceramics with 25 vol% solid loading, and the effect of adding 1 and 3 wt.% of BN whisker-like (synthesized in the laboratory) on the flexural strength of the composites was investigated. The gel-cast samples were prepared using an AM-MBAM monomer system and deionized water solvent. The gel cast bodies were dried at ambient temperature and humidity for 48hrs. The burnout process was carried out at 600 degrees C for 14hrs in the air. The samples were then subjected to heat treatment at 1650 degrees C for 2 hrs in a bed of 50% Si3N4-50% BN. Results showed that BN whisker-like were agglomerated in deionized water. To solve this problem and to properly disperse the whisker-like in the slurry, active surface agent groups were employed. Adding BN whisker-like to ceramic bodies, increased the porosity from 33% in samples without BN to 46% in samples containing 3 wt% BN whisker-like, resulting in a decrease in the density from 1.8 g/cm(3) to about1.61 g/cm(3). Since the Si3N4 ceramics require aids for the sintering, SiO2 and Y2O3 additives were used with a ratio of 1 to 2, respectively, which led to the formation of Si2N2O phase. Si2N2O is similar to the Si3N4 phase in terms of the thermal expansion coefficient and chemical properties. Because of its rod shaped morphology, it has a similar function to beta-Si3N4 phase in improving the mechanical properties of the ceramic. Although porosity in the ceramics containing 3 wt.% BN whisker-like was higher than those containing 1 wt.%, the porosity created vacant spaces for further growth of Si2N2O-rod shaped grains and in spite of higher porosity, better flexural strength was achieved for bodies with 3 wt.% BN whisker-like. Also, in bodies without BN whisker-like, the flexural strength was 88 MPa, due to the low porosity and the presence of stretched Si2N2O grains.