| Machinable glass ceramics (MGC) can be machined with conventional tools. In biomedical engineering field, CAD/CAM techique combined with MGC bring a reformatin in dental restorative manufacture for tailoring prothesis rapidly. The relatively low strength of currently available MGC restricts their use to conservative veneers, inlays and onlays, but the materials cannot be used as bridge restoration that requires higher strength.In the conventional dental mica-based MGC, the weak bonding between tetrahedron-octahedron-octahedron at layers of mica leads to low strength. In the paper on the base of materials design and calculation in mica MGC system, the partly-substitution K+ by Ca2+ doublely strengthens bonding force between layers, then increases the strength of mica crystal in itself. At the same time, high strength mica-based MGC can be prepared by doping zirconia to strengthen and toughen. The study on preparation process, microstructure, strengthening and toughening mechanisms of MGC mainly containing Ca-mica were investigated, together with mechanical properties, machinability, and preliminary biological evaluation.Crystallization mechanisms were probed in the CaO-MgO-AhOs-SK^-F system in the study. Spherulite was firstly observed in the system and the formation, growth and evolvement of spherulite were its crystallization behavior, which was different from other mica-based glass-ceramic systems. Expital growth also was one of the crystallization behaviors. K+ promoted mica's basic unit [AlSiOa] to form; zirconia provided nucleating site for planar like mica crystal. It is confirmed byisothermal and non-isothermal crystallization test that jumping across the interface controlled the growth of Ca-mica.The influence of crystalline phase and microstructure on mechanical properties were analyzed, together with strengthen and toughen mechanisms. The glass-ceramics containing large-number less crystals showed higher strength. The fracture toughness of the material improved with the increase in aspect ratio of mica crystal, and several existing toughening mechanisms were the phase transformation from t-ZrO2, the crack deflexion and crystal acting as debonding tore-out fiber. The short-fiber bridge model of critical aspect ratio was firstly introduced in the mica-based MGC system to discuss strengthening mechanism and calculate the critical aspect ratio. The better mechanical properties attributed to the reinforcement and the crack deflexion of crystals with aspect ratios over 8. On the base of the model of residual stress, the stress state between matrix and crystal was analyzed. With the introduction to the theory of crack deflexion, the effect of different aspect ratios of crystals on toughen was discussed. It is determinated by the experimental that the main toughening mechanism of the materials with large aspect ratios was debonding tore-out crystals like fiber.With the special design, the MGC mainly containing Ca-mica crystals still has not been reported in China, was preparared using two-stage heat treatment; the highest bending strength is 235MPa and the corresponding toughness is 2.17 MPa.m1/2, which was better than that of currently dental MGC. The results of machinability evaluation test showed that the material is easy to machine; a core was successfully fabricated by the material with dental CAD/CAM system. It was showed during preliminary biological evaluation that the material had excellent biocompatibility and good resistantance to corrosion. The fabrication by CAD/CAM system and biological evaluation in the Ca-mica MGC system still has not reported. |
PDF Full Text Request