ABSTRACT Surface nucleation and inward crystal growth in 45% SiO 2 – 27.5% Al 2 O 3 – 27.5 SrO (in mol%) glass can be used to create a composite structure with an amorphous core and crystalline clad. The lack of expansion mismatch between the amorphous precursor glass (5.76 ppm/°C) and the fully crystalline material (5.75 ppm/°C) facilitates the formation of mechanically robust structures having a crystalline clad and glassy core. Vickers indentation is used to show that the columnar crystals in the surface clad layer provide a mechanically advantaged structure, i.e., a tortuous path for radial crack extension with crack deflection angles approaching 90°. The optical properties are tunable within this composite material by limiting the depth of the crystalline clad layer to maximize the volume of optically advantaged amorphous material. In an 800 µm thick specimen with a crystalline cladding layer thickness of 130 µm, the axial and diffuse transmittance approach the values obtained for a fully amorphous sample.
Gross et al. (Sun,) studied this question.