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.

Internal Friction Analysis of CaO-Doped Silicon Carbides

“…Internal friction peaks, Q −1 P , were obtained by subtracting an exponential-like background from the experimental curves, according to a procedure shown in detail elsewhere. 1,10) …”

“…4. These relaxation peak components were extracted by subtracting their respective exponentiallike background components from the experimental curves (this procedure is shown in detail elsewhere 1,10) ). The peaks arose from a sliding process along glassy grain boundaries within the polycrystal.…”

“…Recently, we have shown that the inherent rheological behavior of the grain-boundary glass phase in polycrystalline ceramics can be quantitatively evaluated by means of internal friction measurements. [1][2][3][4] The use of internal friction to quantitatively evaluate the intrinsic viscosity of residual glasses segregated to ceramic grain boundaries has been first proposed by Mosher et al, 5,6) based on the anelastic relaxation mechanism discussed by Zener and Ke. [7][8][9] In the internal friction behavior (as a function of temperature) of glassy bonded ceramic polycrystals, a relaxation peak systematically appears at relatively high temperature, which arises from the anelastic phenomenon of viscous slip along grain boundaries.…”

“…5,10) When a glassy film continuously encompasses the grains as, for example, in Si 3 N 4 or SiC ceramics, the peak is sufficiently intense to allow a quantitative analysis and, thus, the evaluation of the activation energy for viscous flow of the intergranular glass and the related viscosity magnitude. Reliable activation energy analyses can be performed by applying the peak-shift method 1,6,10) that makes use of the peak shift occurring upon varying the damping frequency.…”

How do the grains slide in fine-grained zirconia polycrystals at high temperature?

“…The phase lag φ or tanφ (true for a small phase angle φ), which appears between the stress and strain is a measure of the internal-friction mechanism that dissipates the mechanical energy by motion of point defects, twin walls, grain-boundary interfaces, dislocations, relaxation into intergranular amorphous phases. [3][4][5] The evolution of crystal defects from one equilibrium state to a new one during the stress perturbation is ascribed to a peak appearance in tanφ whose position and height account for the relaxation time τ and the relaxation strength of relaxing defects, respectively. From a microscopic point of view the relaxation strength is proportional to the concentration of crystal defects, while the relaxation time measures their mobility.…”