Hot-pressed sintered Ca-alpha-Sialon ceramics with grains from short prismatic to elongated morphology synthesized via carbothermal reduction and nitridation

Ca-alpha-Sialon powders was initially synthesized by carbothermal reduction-nitridation (CRN) of SiO2, AIN, CaF2 and carbon black at 1700 degrees C, subsequently was used as the raw material to prepare Ca-a-Sialon ceramic by hot-pressed (HP) sintering at 1900 degrees C. The main intermediatenitridated products were Si2N2O and O-Sialon formed at 1400 degrees C-1600 degrees C. The resultant Ca-alpha-Sialon just had hexagonally short prismatic structure at 1700 degrees C. In order to obtain its elongated crystals, alpha-Sialon seeding was introduced during CRN process. The results revealed that the seeding facilitated crystal refinement of Ca-alpha-Sialon and the increase in its aspect ratios from 2 to 5.45. The resultant HP-sintered Ca-alpha-Sialon ceramic with seeding at 1900 degrees C had compact structure with fine and elongated grains, forming a highly interlocked microstructure. They had a Vickers indentation hardness of similar to 21 GPa independent of the microstructure. The flexural strength and fracture toughness of the Ca-alpha-Sialon ceramics drastically increased with increasing the amounts of seeds, which was in accordance with that in grains aspect ratio of a-Sialon crystals synthesized by CRN as function of amounts of seeds. The improved flexural strength and fracture toughness should be ascribed to the development of elongated alpha-Sialon grains, which could enhance crack deflection and grain pullout. (C) 2018 Elsevier B.V. All rights reserved.