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·m1/2, 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.