Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB2–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB2–SiC–BN (40 vol%ZrB2–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB2–SiC (80 vol%ZrB2–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB2–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB2–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB2–20 vol%SiC and 43.1 vol%ZrB2–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm2, respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB2–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB2–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB2–SiC–BN.