Grain‐Boundary Relaxation in High‐Purity Silicon Nitride

Abstract

Internal friction, torsional creep, and shear modulus relaxation experiments were conducted on a model Si3N4 polycrystalline material, which contained a continuous amorphous film of pure SiO2 at the grain boundary. Internal friction experiments were performed in the frequency range between 3 and 13 Hz, in 5 Pa of nitrogen atmosphere. Very high temperatures (up to 2000°C) could be applied for the first time by using a newly developed torsional pendulum apparatus. This apparatus was also capable of precise torsional strain measurements under static‐load conditions. The internal friction curves at various frequencies were generally found to consist of a grain‐boundary peak super‐imposed on an exponential‐like background. The peak, of anelastic diffusive origin, was centered in the temperature range of 1612–1710°C depending on the frequency of the measurement, namely within an interval of about 100°C below the nominal melting point of the pure SiO2 phase (i.e., ∼ 1730°C). The background was instead found to be of viscoelastic nature. A common micromechanical origin between the creep plastic strain and the internal friction background curves was identified and the data could be fitted by the same Arrhenius plot. Structural and chemical characterization of internal grain boundaries was performed by high‐resolution electron microscopy (HREM) in addition to electron energy‐loss spectroscopy (EELS). A small amount of nitrogen was detected within the amorphous residue along grain boundaries. According to the above set of microstructural/chemical and mechanical data, the viscosity properties of the intergranular phase were assessed and the sliding mechanism between adjacent Si3N4 grains was modeled.