Glass ceramics, machinable

Machinable glass-ceramics are based on internally nucleated fluoromica crystals in glass (Beall 1971a). One commercial material has been marketed for 20 years under the trademark MACOR and has found wide application in such diverse and speciality areas as precision electrical insulators, vacuum feedthroughs, windows for microwave-type parts, samples holders for field-ion microscopes, seismograph bobbins, gamma-ray telescope frames, and 

Minimum hoie and siot sizes (mm) Minimum hole and siot depth (mm) Minimum hole and slot distances (mm) 

KAlSi20g represents the glassy droplet phase, having near leucite composition. 

The mica grows in a preferred lateral direction because the residual glass is fluidized by the B2O3 flux and is also designed to be deficient in the crosslinking species potassium. 

More than 200 special parts of the U.S. space shutde orbiter are made of this glass-ceramic. These parts include rings at all hin points, windows, and doors, 

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Cere Vita Mark It Machinable glass ceramic Vita Zahnfabrik, Bad Sackingen, Germany... 

Glass-ceramics are also used for dental prosthetics. These materials are primarily based on either lithium disilicate or calcium phosphate glasses. Machinable glass-ceramics based on fluoromica phases have also been developed for machined inlays. All of these materials must be stained to match the surrounding teeth to be acceptable to the patient. 

Brittle fracture is used for shaping and machining ceramics after they have been fired. Ceramics can be modified to make them machinable this is controlled fracture and is the approach we adopt with machinable glass-ceramics such as Macor (Chapter 26). Of course, many ceramics already are machinable and can be shaped into intricate and beautiful forms as illustrated in the carved marble sculpture shown in Figure 18.1. 

Borosilicate glass matrix composites with chopped graphite fibres are highly machine-able materials [29]. Machinable glass-ceramics containing interlocking platelet-like and... 

The material chosen as a test material is MACOR. MACOR is a machinable glass ceramic and is a fluorine rich glass with a composition approaching trisilicic fluorphlogopite mica (KMg3AlSi30ioF2) (6). The material was acquired in a rod form. 

Dicor. Tradename. A castable, machineable glass-ceramic based on mica (tetra-silicic fluormica) with up to 7% added Zr02 to improve chemical durability and translucency for dental applications. (P.J. Adair, Dentsply International and Corning Glass. U.S. Pat, 4431420 1984)... 

Figure 2-19 Microstructure formation in machinable glass-ceramics of the phlogopite-type...

Therefore, if the iron-oxide content (sum of FeO and Fe203) is higher than 6 mol% and MgO is higher than 15 mol%, a ferrimagnetic and machinable glass-ceramic material may develop following an additional heat treatment of 1180 C for 60 min. The chemical composition is shown in Table 2-22 and the corresponding microstructure is demonstrated in Fig. 2-44. 

Vitron SpezialwerkstofFe GmbH, Jena, Germany, manufactures a machinable glass-ceramic called Vitronit . It is used for technical applications. 

Two types of DICOR glass-ceramics are produced for dental applications. The first type is a castable glass-ceramic with which dental restorations are fabricated in the dental laboratory. The second type is a machinable glass-ceramic with which dental restorations are machined using CAD/CAM technology in the dental clinic. 

Grossman D.G., "Machinable Glass-Ceramics Based on Tetrasilicic Mica," /. Am. Ceram. Soc., 55, 446-49 (1972). 

Holand W., Vogel W., Mortier W.J., Duvigneaud P.H., Naessens and Plumat E., "A New Type of Phlogopite Crystal in Machinable Glass-Ceramics," Glass Technol.., 24, 318-22 (1983a). 

Sumikin Photon Ceramic Co. Machinable Glass-Ceramic products brochure, 1998. 

Property Units MACOR machinable glass ceramic Boron nitride 96% BN Alumina nominally 94% AI2O3 Valox thermoplastic polyester... 

For these applications the original idea to produce a machinable glass ceramic has been further extended to materials which are at the same time biocompatible or bioactive. 

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