Structure-property Relations in Monolithic and Fibrous Monolithic ZrB2-based Ceramics

The microstructure and mechanical properties of conventional ZrB2 based ceramics and ZrB2-based fibrous monolithic ceramics have been investigated. For conventional ZrB2 -based ceramics, the microstructure and properties have been studied as a function of different processing conditions. Initially, ZrB2 and ZrB2 containing 10, 20, or 30 volume percent of particulate additions were prepared by hot pressing at 1900DGC. Strength was found to increase as SiC content increased from -550 MPa for ZrB to over 1000 MPa for ZrB2 containing 30 vol.% SiC. More detailed investigations of the effect of hot pressing time and temperature revealed that strength was a strong function of grain size. The strength of ZrB2-30% SiC decreased from -1050 MPa when grain size was -1.9 mm to -800 MPa when grain size was -3.3 gm. Other properties such as fracture toughness, elastic modulus, and hardness have been investigated along with oxidation behavior. Beyond traditional structure-property studies, co-extrusion forming has been employed to fabricate ZrB2-based ceramics with engineered architectures. Fibrous monolithic structures composed of high strength ZrB2 - 30 vol.% SiC cells have been combined with relatively weak cell boundary phases composed of either porous ZrB2 or ZrB2-graphite to control fracture behavior and promote non-catastrophic failure. The microstructure and strength will be reported and fracture behavior will be analyzed using failure criteria established with crack deflection models.