Nanocrystalline non-alkali glass-ceramics

Nanocrystalline glass-ceramics with high elastic modulus and moderate strength and toughness can be produced from a wide area in the SiO2-Al2O3-MgO-ZnO-TiO2 system. Phase assemblages include those based primarily on spinel crystals as well as those based on enstatite and spinel; common accessory phases include Mg-petalite, Mg,Al-titanate solid solutions, quartz solid solutions and rutile. These glass-ceramics can have ultra-fine grain sizes, with spinel crystals less than 25 nm and enstatite crystals less than 100 nm in size. Such nanocrystalline microstructures are unique in the case of an enstatite glass-ceramic and provide extremely smooth perished surfaces, with average roughness of about 5-10 Angstrom (0.5-1.0 nm). These materials provide strength and toughness values higher than might otherwise be expected from such a fine-grained microstructure, due in part to the toughening effect of the lamellar twinning in the enstatite phase. The morphology of the initial phase separation provides a template for the ultimate crystalline microstructure, essentially locking in its nanocrystalline nature such that regardless of heat treatment crystals grow no larger than 200 nm. (C) 1997 Elsevier Science B.V.