Disintegration behavior of silicon nitride ceramic by hydrothermal treatment in aqueous HF solution

The possibility of recycling silicon nitride ceramic by hydrothermal treatment was investigated. A Si3N4 sintered compact with 0.9 mass% SrCO3, 3.6 mass% MgO and 4.5 mass% CeO2 as sintering aids was used as a specimen for treatment. Hydrothermal treatment was performed using aqueous HF solution at 70-150degreesC for 0-120 h. The dense sintered compact was easily disintegrated into powdery particles and a core fragment by the combination of hydrothermal treatment and subsequent sonication treatment. The treated powder was identified by X-ray diffraction (XRD) analysis as beta-Si3N4 containing CeF3 and MgF2, which were formed in aqueous HF solution during hydrothermal treatment. These fluorides were completely removed by rinsing in HNO3 solution. Then, a powder consisting only of needle like beta-Si3N4 crystals was recovered by a series of these treatments. X-ray photoelectron spectroscopy (XPS) revealed that the surface of the recovered Si3N4 powder was oxidized very slightly. Hydrothermal treatment at 120degreesC for 24 h gave approximately 30% yield of powdery beta-Si3N4 particles, excluding the amount of sintering aid components. In the case of hydrothermal treatment for shorter terms or at lower temperatures, the corrosion behavior of the Si3N4 ceramic is linearly dependent on treatment time and is controlled by surface chemical reaction. The activation energies for the dissolution of the Si3N4 ceramic, and the grain boundary components of strontium, magnesium and cerium were estimated as 31.9 +/- 0.7, 54.4 +/- 3.6, 74.3 +/- 1.4 and 51.7 +/- 3.9 kJ/mol, respectively.