Microstructure control and toughening of ZrB2-SiC/Zr-Al-C composite ceramics by selecting additional powders mixed with ZrB2 in ball milling for spark plasma sintering

ZrB2-SiC composite ceramics were fabricated by spark plasma sintering SiC powders with various mixtures of ZrB2, Zr, Al and graphite components, toughening the ceramics through the in-situ synthesis of Zr-Al-C microstructures. Different microstructures of Zr-Al-C toughened ZrB2-SiC (ZSA) composite ceramics were formed during the sintering process by varying the components ball milled with the ZrB2 powders prior to sintering. When the milled ZrB2-based powders contained Al, the major Zr-Al-C phase changed into Zr3Al4C6 from the designed Zr2Al4C5, and the layered Zr-Al-C grains formed with a large aspect ratio in the ZSA ceramics due to the formation of an Al-based coating layer covering the ZrB2 powders during milling process. The Zr and Al co-milled ZrB2-based powders further improved the toughness of composite ceramics through a more uniform distribution and the larger aspect ratio of Zr-Al-C grains. As a result, the ZSA ceramic made using the milled powders of ZrB2, Zr and Al showed the highest fracture toughness of 5.96 MPa.m(1/2), about 10% higher than that of the ceramic made using milled ZrB2 and Zr powders. The toughening mechanisms are shown to be crack deflection and bridging caused by Zr-Al-C grains. This work points to a possible pathway to control the microstructure of Zr-Al-C grains for toughening ZrB2-SiC composite ceramics. (C) 2018 The Ceramic Society of Japan. All rights reserved.