Fabrication And Thermal Shock Resistance Of (ZrB₂+ZrO₂)/BN Composites

(ZrB₂+ZrO₂)/BN composites were successfully fabricated by hot-pressing with h-BN, ZrO₂ and ZrB₂ powders as the starting materials. Effects of the sintering aids and reinforcement on microstructure and properties of the composites have been systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), three-point bending test, single edge notched beam method, etc. In addition, the thermal shock resistance behavior of BN composites was focused on, and the corresponding thermal shock resistance mechanism was revealed.The results showed that B₂O3, SiO₂ and their mixtures as sintering aids can effectively promote the densification of the composites. Among them, SiO₂ enables to achieve a higher densification (96.2%). When keeping the volume fraction of sintering aids constant, the flexural strength and fracture toughness of the composites increase with the increasing SiO₂ concentration, and getting the maximum values of 256MPa and 3.5MPa?m1/2. The composite mainly consists of BN,ZrB₂,t-ZrO₂ and m-ZrO₂, which is independent on the sintering aids.With the increase of ZrB₂ content, when retaining the volume fraction of reinforcement constant, the mechanical properties of composite are remarkably improved, and reach 1.97 GPa, 132 GPa, 334 MPa and 4.6MPa?m1/2 for Vickers's hardness, elastic modulus, flexural strength and fracture toughness in 30vol%ZrB₂/BN composite. The (ZrB₂+ZrO₂)/BN composites, with the special microstructure of h-BN skeleton, and the stiff ZrB₂ and ZrO₂ enhanced powders filling into the pores of the skeleton, have a good load-bearing effect. The thermal expansion coefficient and specific heat capacity of the composites increase, while the thermal diffusivity and thermal conductivity decrease with the increasing temperature.(ZrB₂+ZrO₂)/BN composites have a good thermal shock resistance. The residual strength after thermal shock decreases firstly, then increases, and reaches a minimum value within 900¹000oC. The increasing strength is attributed to the formation of the dense oxide film on the surface during the high temperature holding. The residual strength ratio of 30vol%ZrO₂/BN and (10%ZrB₂+20vol%ZrO₂)/BN composites is higher than 100% after one thermal shock (ΔT=1400oC). The residual strength after thermal shock increases firstly then decreases with the increasing numbers of thermal shock cycling, and has a maximum at 5 times. The composites with high ZrO₂ content have a better thermal shock resistance than those with high ZrB₂ content, while the thermal shock damage resistance changes oppositely. The main thermal shock resistance mechanisms of the composites involve microcrack, surface residual compress stress and phase transformation toughening.