Anisotropic properties of unidirectionally crystallized calcium metasilicate

Abstract

The anisotropy displayed by materials which contain aligned, fibrous crystals in a crystalline or glass matrix has been investigated. The materials were prepared by the unidirectional crystallization of calcium metasilicate from glasses and melts of approximately metasilicate compositions. Rectangular prism test specimens were fabricated such that the fibre axis ran parallel to the long axis, across the width or through the depth. This enabled the variation of properties with fibre orientation to be established. The high-temperature polymorph,α-CaSiO₃, could be crystallized dendritically such that the primary arms were aligned, and the growth of secondary arms inhibited, by mutual impingement.β-CaSiO₃, the low-temperature polymorph, has a chain silicate structure and crystallized as fibres by the spherulitic crystallization mechanism. These fibres grew in approximately parallel alignment due to transcrystallization. The glass remaining after crystallization of the primary phase was either stable or subsequently crystallized from nuclei on the primary crystals, giving materials with either a glass or crystalline matrix respectively. Thermal expansion data were obtained from an automatic recording dilatometer, and the moduli of rupture and shear strengths were found using three-point bend tests of appropriate span-to-depth ratios. The results of these tests showed that the unidirectionally crystallized materials behaved similarly to conventional brittle matrix / brittle fibre composites.