MICA-BASED LITHIUM CONTAINING MACHINABLE GLASS-CERAMICS

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In recent years, patient demand for esthetics has stimulated the development of new materials for all ceramic dental restorations. Glass- ceramics belong to this category and offer many advantages over conventional feldspathic porcelain for dental applications. However, several factors limit their use: 1) the fracture strength of the dental glass-ceramic systems presently available does not allow for the fabrication of fixed partial dentures, 2) the glass-ceramic process is time consuming and requires expensive equipment, 3) crystallization defects are frequent and render the restoration unesthetic. The overall objective of the proposed research to develop a new machinable glass-ceramic material dental applications. This material will be processed at lower temperature and in less time that currently available dental glass-ceramics. It will have improved homogeneity and mechanical properties compared to currently available dental glass-ceramics. The strategy is to evaluate the effect of CaF2, A1PO4 and Zr02 additives on the thermal, optical and mechanical properties of a basic glass-ceramic composition in the system Li2O-MgO-K2O-SiO2-F. The hypotheses to be tested are that additives such as CaF2 or A1PO4 will induce phase separation and promote the nucleation and growth of mica crystals within the glass- ceramic. Tetragonal zirconia nanoparticles will induce a unique microstructure of nanoparticles embedded in mica crystals without effecting the translucency of the glass-ceramic since their size is less than the wavelength of light. The scattering and absorption coefficients will be calculated from reflectance measurements using a dual-beam spectrophotometer. The thermal properties will be studied by DTA. The crystalline phases and microstructure will be characterized by x-ray diffraction and Scanning Electron Microscopy. The elastic constants, flexural strength, fracture toughness and Hertzian indentation response of the five glass-ceramic materials will be evaluated in the second part of the proposed research. The hypotheses to be tested are first that the presence of taeniolite, a lithium containing tetrasilicic fluormica characterized by dense and compact crystals, will insure favorable mechanical properties for the base composition material. Secondly, the addition of CaF2 or AIPO4 will help develop an homogeneous microstructure with improved mechanical properties with regard to the base composition material. Thirdly, zirconia will be retained in the tetragonal form after crystallization and strengthen the glass-ceramic via transformation toughening.