Tensile behavior of As-fabricate and burner-rig exposed SiC/SiC composites with Hi-Nicalon type-S fibers

摘要

Tensile stress-strain curves were measured at room temperature and 1315℃ for 2D-woven SiC/BN/SiC ceramic matrix composites (CMC) reinforced by two variations of Hi-Nicalon Type-S SIC fibers. These fibers, which contained a thin continuous carbon-rich layer on their as-produced surface, provided the as-fabricated CMC with good composite behavior and an ultimate strength and strain of 350 MPa and 0.5%, respectively. However, after un-stressed burner-rig exposure at 815℃ for 100 hrs, CMC tensile specimens with cut edges and exposed interphases showed a significant decrease in ultimate properties with effectively no composite behavior. Microstructural observations show that the degradation was caused by internal fiber-fiber oxide bonding after removal of the carbon-rich fiber surface layer by the high-velocity combustion gases. on the other hand, SiC/BN/SIC CMC with Sylramic-iBN fibers without carbon-rich surfaces showed higher as-fabricated strength and no loss in strength after the same burner rig exposure. Based on the strong role of the carbon layer in these observations, a process method was developed and demonstrated for achieving better strength retention of Hi-Nicalon Type-S CMC during burner rig exposure. Other general approaches for minimizing this current deficiency with as-produced Type-S fibers are discussed.