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To better understand the high temperature properties of the ESIS SiliconNitride Reference Material, the secondary phases it contains are investigated using X-raydiffraction, Differential Scanning Calorimetry and optical microscopy image analysis. TheImpulse 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 thehigh temperature fracture strength of the silicon nitride, as determined by other partners inthe Reference Material Testing Program. It is also shown that the amorphous intergranularphase has limited or no tendency to crystallise, which will facilitate interpretation of timeandloading-rate-dependent and long term behaviour at elevated temperature.Differences between surface and core of the sintered plates are observed. The content ofa crystalline iron phase and the lattice parameters of the â-Si3N4 are larger in the corethan in a surface region of about 1mm thickness. Concurrently the near surface samplesshow a higher Young's modulus. These observations will be taken into account whenfurther assessing the structural integrity of the ESIS Silicon Nitride Reference Material.