All-ceramic bearings offer high temperature capability, longer fatigue lifetimes, and lower weight compared to conventional steel bearings. However, the low fracture toughness of the ceramic material (typically silicon nitride) restricts the allowable hoop stress, and therefore severely limits their use for potential applications. Reinforcement of the ceramic matrix is a viable means to increase fracture toughness, although reinforcement of the bearing surface may induce fatigue crack initiation, result in differential wear rates and reduce attainment of fine surface finish. Consequently, a ceramic matrix composite (CMC) design with a monolithic bearing surface case and a higher toughness core region fabricated into a bearing race geometry is desirable. In this Phase I effort, techniques to fabricate silicon nitride with selective reinforcement (graded CMC) are conceptualized, demonstrated and evaluated by fabricating materials in selected geometries and thereafter characterized with respect to physical and mechanical properties. Fabrication techniques to be considered for producing selectively graded reinforced ceramic bearing races include: 1) centrifugal spinning of matrix based slurries containing particulate reinforcement(s), 2) selected high pressure molten metal infiltration to produce graded-tough cermets and 3) incorporation of refractory fibers for selected reinforcement.
