Fabrication, thermal shock resistance, and dielectric property of α-Si3N4-based ceramic coating on porous Si3N4 ceramics

A dense alpha-Si3N4-based ceramic protective coating was successfully prepared on porous Si3N4 ceramics by a liquid infiltration and filling method. The coating composed of a primary alpha-Si3N4 phase and secondary O'-Sialon, beta-Sialon, and Y-Si-Al-O-N glass phase. After thermal shock at Delta T = 1000 degrees C for five times, cracks were produced, but the tip of crack stopped inside the coating; so the coated porous Si3N4 ceramics still had a good waterproof ability and its water absorption was only 7%. During thermal shock, toughening mechanisms involving needle-like O'-Sialon particle bridging, crack deflection, and rough fracture, occurred within the cracks, contributing to thermal shock resistance of the coating. The dielectric constant of the coated porous Si3N4 ceramics showed a slow increase trend with increasing temperature, and it reached the maximum value of 3.57 at 1100 degrees C at the frequency of 11 GHz. The dielectric loss increased slowly as the temperature increased from room temperature to 900 degrees C, but it started to increase evidently when the temperature was over 900 degrees C.