Damage tolerant oxide/oxide fiber laminate composites

Oxide-fiber/oxide-matrix composites were developed using non-infiltrated woven fiber layers between matrix-infiltrated fiber layers in order to achieve damage tolerant behavior. A fiber interface coating was not used. This technique enables damage tolerance in materials with strong fiber-matrix bonding and under oxidizing conditions. Fabrication of composites was carried out through a slurry infiltration technique. Slurries for fiber (Nextel™ 720, 3M) infiltration were prepared using a submicron α-Al2O3 powder coated with an amorphous SiO2-layer through a sol–gel process. Hot-pressing was used to densify and bond the laminate layers together, followed by pressureless heat-treatment to allow mullite to form. Room temperature three-point bending tests were performed on as-received samples and on samples which underwent long-term annealing at high temperatures (1200–1300°C) in air. Subsequent examination revealed that due to the lack of a fiber interface coating, matrix-infiltrated fiber layers behaved in a quasi-monolithic manner with little or no crack deflection. Layers of non-infiltrated fibers, however, provided damage tolerance by deflecting cracks in the plane of the laminate and by serving as a mechanical bond between matrix-infiltrated layers. The laminate composites demonstrate reasonable room-temperature fracture strength both in the as-received state (88 MPa) and after exposure to 1300°C air for 200 h (72 MPa) along with extensive fracture deflection through the layers of non-infiltrated fiber. Composite properties, specifically fracture strength and damage tolerance, can be tailored by varying lay-up and processing parameters such as fiber-matrix ratio and type of fiber weave.