Machinable Glass‐Ceramics Based on Tetrasilicic Mica

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Abstract
Tetrasilicic‐mica glass‐ceramics are formed from glasses in the simple quaternary system K 2 O‐MgF 2 ‐MgO‐SiO 2 . The mica phase crystallizes as small (400‐Å) spherical grains at #625°C and recrystallizes into larger booklets at 940°C. The mica platelets grow by a dissolution and reprecipitation mechanism, thereby increasing the aspect ratio during growth. Two regions of strength dependence on grain size are found. When the mica platelets are very small, the strength increases as the 1/5 power of the mica‐plate diameter. Above a critical plate diameter, the strength decreases, being inversely proportional to the first power of the mica‐plate diameter. In the second region, the strength appears to be controlled by the interfacial shear stresses developed as a result of the thermal expansion mismatch between the mica platelets and the residual glass.