Microstructure Characteristics Related to the High Temperature Fracture Resistance of the ESIS Silicon Nitride Reference Material

摘要

To better understand the high temperature properties of the ESIS Silicon Nitride Reference Material, the secondary phases it contains are investigated using X-ray diffraction, Differential Scanning Calorimetry and optical microscopy image analysis. The Impulse Excitation Technique was used to determine the elastic and damping properties, both at room and elevated temperature. Tests revealed the presence of a substantial amount of amorphous intergranular phase, which passes a glass transition around 950°C. This observation is used to interpret the high temperature fracture strength of the silicon nitride, as determined by other partners in the Reference Material Testing Program. It is also shown that the amorphous intergranular phase has limited or no tendency to crystallise, which will facilitate interpretation of time-and loading-rate-dependent and long term behaviour at elevated temperature. Differences between surface and core of the sintered plates are observed. The content of a crystalline iron phase and the lattice parameters of the β-Si_3N_4 are larger in the core than in a surface region of about 1mm thickness. Concurrently the near surface samples show a higher Young's modulus. These observations will be taken into account when further assessing the structural integrity of the ESIS Silicon Nitride. Reference Material.