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-Al 2O3-MgO-ZnO-TiO2 system. Phase assemblages include those based primari ly on spinel crystals as well as those based on enstatite and spinel; common accessory phases include Mg-petalite, Mg,Al-titanate solid solu tions, quartz solid solutions and rutile. These glass-ceramics can hav e ultra-fine grain sizes, with spinel crystals less than 25 nm and ens tatite crystals less than 100 nm in size. Such nanocrystalline microst ructures are unique in the case of an enstatite glass-ceramic and prov ide extremely smooth perished surfaces, with average roughness of abou t 5-10 Angstrom (0.5-1.0 nm). These materials provide strength and tou ghness values higher than might otherwise be expected from such a fine -grained microstructure, due in part to the toughening effect of the l amellar twinning in the enstatite phase. The morphology of the initial phase separation provides a template for the ultimate crystalline mic rostructure, essentially locking in its nanocrystalline nature such th at regardless of heat treatment crystals grow no larger than 200 nm. ( C) 1997 Elsevier Science B.V.