Hardness and toughness control of brittle boron suboxide ceramics by consolidation of star-shaped particles by spark plasma sintering

High‐Temperature Strength of Boron Suboxide Ceramic Consolidated by Spark Plasma Sintering

“…This underlines the problem in the densification of boron suboxide: the oxygen deficiency in the rhombohedral cell at the 6 c position affects the “sinterability” of as‐synthesized boron suboxide powder . The results of Ref.…”

“…Two commercially available powders, amorphous boron (aB, 97%; Wako Pure Chemical Industries, Ltd., Osaka, Japan) and B 2 O 3 powder (99.0%; Kanto Chemical Co., Inc., Tokyo, Japan), were used as the starting materials. The mixing procedure for the B 2 O 3 and aB raw powders was the same as that in our previous works . The raw powders were mixed in 6:1 and 10:1 aB:B 2 O 3 mole ratios (B 6 O‐6:1 and B 6 O‐10:1 ceramics, respectively).…”

“…Furthermore, as this boron suboxide powder has a low oxygen deficiency level in its B 6 O x cells (i.e., x = 0.88), the hardness of a bulk sample subjected to SPS was reported to be comparable to that of ceramic tiles synthesized at a high pressure . A high macroscopic hardness exceeding 40 GPa was reported along with a high indentation fracture toughness of 4.2 MPa/m 2 , which was attributed to numerous crack deflections at the sharp‐edged B 6 O subgrains . These boron suboxide subgrains originated from multilayer B 6 O particles; the broken parts of the star‐shaped particles with sharp angular edges promote the additional densification of the B 6 O ceramic owing to the additional accommodation of grains and subgrains.…”

“…One of the key reasons for this is the use of Ta foil, which prevents carbon diffusion into the B 6 O lattice, allowing the initial low oxygen deficiency level of the as‐synthesized powder to be preserved . Furthermore, as this boron suboxide powder has a low oxygen deficiency level in its B 6 O x cells (i.e., x = 0.88), the hardness of a bulk sample subjected to SPS was reported to be comparable to that of ceramic tiles synthesized at a high pressure . A high macroscopic hardness exceeding 40 GPa was reported along with a high indentation fracture toughness of 4.2 MPa/m 2 , which was attributed to numerous crack deflections at the sharp‐edged B 6 O subgrains .…”

“…These boron suboxide subgrains originated from multilayer B 6 O particles; the broken parts of the star‐shaped particles with sharp angular edges promote the additional densification of the B 6 O ceramic owing to the additional accommodation of grains and subgrains. It was concluded that the highly hierarchical microstructure inside bulk boron suboxide tiles shall be the origin of the high strength.…”