Mechanical and structural characterization of sapphire ribbons and continuous filaments

Abstract

The mechanical and structural characterization of sapphire ribbons and filaments is presented. Ribbons were grown in short (12 in.) lengths which were cut and tested in quarterpoint loading. The ribbons had thicknesses in the range 0.007 to 0.024 in., and widths in the range 0.027 to 0.260 in. and displayed a maximum average failure strength of 300 ksi. Filaments were grown in essentially continuous lengths with a circular cross-section nominally 0.010 in. in diam. The average tensile strength was 400 ksi. Both ribbons and filaments were annealed at 1860°C for various times. Strength enhancements are measured in annealed ribbons and the increases are compared with calculated values obtained by analysis based on the edge curvature of split as-grown ribbons. No evidence of strength enhancement was noted in annealed filament. Instead, strength degradations were measured in annealed filaments and are related to the introduction of stress concentrators through thermal etching. The strength-controlling feature of filament is discussed in terms of a void-free shell at the surface.