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

A dense α-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 α-Si₃N₄ phase and secondary O'-Sialon, β-Sialon, and Y–Si–Al–O–N glass phase. After thermal shock at ΔT = 1000°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°C at the frequency of 11 GHz. The dielectric loss increased slowly as the temperature increased from room temperature to 900°C, but it started to increase evidently when the temperature was over 900°C.