3D Visualization of Morphological Evolution of Large Defects during Spark Plasma Sintering of Alumina Granules

Abstract

The mechanical reliability of products must be assured for scaling up and production of complex‐shaped components by spark plasma sintering (SPS) of spray‐dried granules. The evolution of morphologies of pores and defects, which control the mechanical strength, is investigated by using synchrotron X‐ray multiscale tomography during SPS of alumina granules at 1300 °C. While large defects arising from the hierarchical granule packing structure cannot be removed by pressureless sintering, crack‐like defects and branched rodlike defects are almost eliminated by SPS at stresses higher than 30 and 50 MPa, respectively. But, small ellipsoidal porous regions, which may arise from aggregates or dimples of granules, cannot be removed even at a pressure of 50 MPa. A very large defect is also found by using micro‐CT. It is supposed that this defect is formed from a large void in loosely packed granules. The shrinkage of large voids and the elimination of crack‐like defects are explained by the theoretical prediction based on the continuum theory of sintering.

Synchrotron X‐ray multiscale tomography: Visualization of heterogeneous microstructures and defects in ceramics

“…We refer to the mutually interconnected pore in Figure 9E with a genus greater than zero as a complex pore. 76 We hypothesize that the complex pores are formed from aggregates present in the raw powder or porous regions at quadruple points of granules in Figure 8B. The ellipsoidal porous region in Figure 9F is a large complex pore with an ellipsoidal shape.…”

“…87,88 The morphological evolution of large defects during SPS was investigated by using the spray-dried DS31 as a model. 76 The relative density increased with increasing the applied stress; 97.59% and 99.47% at 30 and 50 MPa, respectively. Although the branched crack-like defects and the circular crack-like defects could not be eliminated by pressureless sintering (Figure 10A), they disappeared by SPS at a pressure of 30 MPa.…”

“…The SEM micrographs in Figure 9B-F show a variety of types of defects. 76 The origin of these defects is schematically illustrated in Figure 8B; (I) coarse round pores (Type I) distributed randomly (Figure 9B), (II) branched cracklike defects (Type II) along boundaries (Figure 9C), 58,77,78 and (III) linear crack-like defects (Type III) that are perpendicular to the pressing direction (Figure 9D). 58,79 Type II and III defects, which are crack-like in nature, are the defects that limit the strength of the material.…”

“…Shrinkage and deformation of a spherical pore and oblate spheroidal pores (c < a) during spark plasma sintering (SPS). 76 The microstructural control for obtaining smoother and more continuous electrodes is crucial to enhance the performance of MLCCs. The capacitance of MLCC is directly proportional to the continuity and connectivity of the electrode area and then the discontinuity brings about capacitance loss.…”

“…The contrast‐enhanced SEM image in Figure 9A presents the shape, size, and spatial distribution of defects in alumina ( ρ = 97.6%) made by pressureless sintering of DS31 granules. The SEM micrographs in Figure 9B–F show a variety of types of defects 76 . The origin of these defects is schematically illustrated in Figure 8B; (I) coarse round pores (Type I) distributed randomly (Figure 9B), (II) branched crack‐like defects (Type II) along boundaries (Figure 9C), 58,77,78 and (III) linear crack‐like defects (Type III) that are perpendicular to the pressing direction (Figure 9D).…”