Elementary Mechanisms behind the High‐Temperature Deformation Behavior of Lutetium‐Doped Silicon Nitride

Abstract

Intergranular sliding and diffusive mechanisms behind the deformation behavior of a commercially available lutetium‐doped silicon nitride were investigated and discussed. A method of locating and separating phenomena critical for mechanical relaxation at elevated temperatures was applied; the method was based on low‐frequency forced‐vibration damping measurements. The potentiality of lutetium addition for improving the deformation resistance of silicon nitride was clearly reflected in the high‐temperature damping behavior of the investigated polycrystal. Softening of intergranular lutetium silicate phases located at multigrain junctions, which resulted in a grain‐boundary sliding peak, occurred at remarkably high temperatures (>1725 K). This phenomenon, partly overlapping diffusional flow, was followed by further damping relaxation with the melting of the lutetium silicates. Subsequent grain growth was also detected at temperatures >2100 K. Torsional creep results, collected up to 2100 K, consistently proved the presence of a “locking” effect by lutetium silicates with the sliding of silicon nitride grain boundaries below 1873 K.