Microstructure in Silicon Nitride Containing β‐Phase Seeding: III, Grain Growth and Coalescence

Abstract

The mechanical properties of Si 3 N 4 materials depend mainly on the microstructure, which originates during the densification process. The microscopic evidence indicates that ␤-Si 3 N 4 seeds incorporated in the starting powders play an important role in microstructural development, especially in the heterogeneous grain growth of ␤-Si 3 N 4 grains during sintering. The growth of ␤-grains is initiated from the ␤-seeds, resulting in a core/shell microstructure. The presence of Moiré fringes and dislocations is attributed to misfit strain and compositional differences between the core and the shell. Coalescence can occur at the final stage of sintering.

Thermal conductivity improvement in silicon nitride ceramics via grain purification

“…2 O 3 dissolved into the host β-Si 3 N 4 to afford metastable solid solution or the present high-temperature sintering at 1950 • C allowed physical incorporation of the Mg-Gd-Si-O-N liquid into the host β-Si 3 N 4 as previously suggested for the liquid phase sintered β-Si 3 N 4 -based ceramics 7,8,13,14. …”

“…11,12 Despite the short sintering period, glassy precipitates with diameters less than 100 nm were observed at the boundary between the core originating from the β-Si 3 N 4 seed and the shell epitaxially grown onto the β-Si 3 N 4 seed via the solution-precipitation of some α-phase particles initially located around the β-Si 3 N 4 seed. 13 It was suggested that the glassy inclusions were trapped by rapid β-Si 3 N 4 grain growth during the initial stage of the α-⇀ β-Si 3 N 4 phase transformation via the solution-reprecipitation through the liquid phase formed in situ or segregated and remained after the increase in sintering temperature and time. 13 Yokota et al 14 also suggested that a portion of the liquid phase was trapped by rapid β-Si 3 N 4 growth during the α-⇀ β-Si 3 N 4 phase transformation and remained as intragranular nanoprecipitates after homogenization: either hexagonal or spherical precipitates with a diameter of 10-30 nm were observed within the β-Si 3 N 4 grains of Si 3 N 4 -Yb 2 O 3 (10 wt%)-ZrO 2 (2 wt%) ceramics sintered at 1900 • C for 8 h under N 2 pressure at 0.9 MPa.…”

“…13 It was suggested that the glassy inclusions were trapped by rapid β-Si 3 N 4 grain growth during the initial stage of the α-⇀ β-Si 3 N 4 phase transformation via the solution-reprecipitation through the liquid phase formed in situ or segregated and remained after the increase in sintering temperature and time. 13 Yokota et al 14 also suggested that a portion of the liquid phase was trapped by rapid β-Si 3 N 4 growth during the α-⇀ β-Si 3 N 4 phase transformation and remained as intragranular nanoprecipitates after homogenization: either hexagonal or spherical precipitates with a diameter of 10-30 nm were observed within the β-Si 3 N 4 grains of Si 3 N 4 -Yb 2 O 3 (10 wt%)-ZrO 2 (2 wt%) ceramics sintered at 1900 • C for 8 h under N 2 pressure at 0.9 MPa. TEM-EDS analysis revealed that the nanoprecipitates were composed of small microcrystalline and amorphous phases having elemental components of Yb, O, and N, which indicated that the nanoprecipitates originated from one kind of the Yb-Si-O-N and/or Zr-Yb-O liquid phase formed during the 1900 • C-sintering.…”

Effect of Heating Rate on Spark Plasma Sintering of a Nanosized β-Si3N4-Based Powder

“…42 Figure 12(a) shows the BF/TEM micrograph in the middle of the large, elongated grain. An APB occurred when the atoms sheared through a displacement and formed an y ABBA y arrangement, which offers possible evidence of grain coalescence during sintering.…”

“…An APB occurred when the atoms sheared through a displacement and formed an y ABBA y arrangement, which offers possible evidence of grain coalescence during sintering. 42 Figure 12(a) shows the BF/TEM micrograph in the middle of the large, elongated grain. The figure reveals that a periodically arranged lattice fringe (subgrain X) is involved in the large, elongated grain.…”