Mica glass-ceramics machinability

Beall (1971) determined that different microstructures can be produced, depending on the composition crystallization process used. Furthermore, the chemical stability of the respective glass-ceramics also changes. The machin-ability of this mica glass-ceramic proved to be very favorable. A ceramic containing one-third by volume mica was shown to demonstrate satisfactory machinability. Optimum machinability was obtained at two-thirds by volume. 

Various investigations have been conducted in an effort to combine two specific properties in the fluorosilicate glass-ceramic system and in the mica glass-ceramic system in particular, that is, high strength and machinability as a result of mica precipitation. 

Machinable ceramics Mica glass-ceramics Mica ceramics (Homasald, 1988, 1996)... 

Hamasaki T, Eguchi K., Koyanagi Y., Matsumoto A., Utsunomiya T, Koba K. Preparation and characterization of machinable mica glass-ceramics by the sol-gel process. J. Am. Ceram. Soc. 1988 71 1120-1124... 

Hamasaki T. Development of machinable mica glass-ceramics by the sol-gel method. New Ceram. 1990 4 38 9... 

Phospha.tes. Many phosphates cl aim unique material advantages over siUcates that make them worth the higher material costs for certain apphcations. Glass-ceramics containing the calcium orthophosphate apatite, for example, have demonstrated good biocompatibiUty and, in some cases even bioactivity (the abiUty to bond with bone) (25). Recent combinations of fluorapatite with phlogopite mica provide bioactivity as well as machinability and show promise as surgical implants (26). 

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)... 

The particularly favorable machinable properties of this glass-ceramic must be attributed to the fact that mica crystals are readily cleaved. The bonds between the three-layer units of the mineral (see Section 1.3.1.) can be broken relatively easily by external force. In addition, the cracks propagate along the crystal boundaries. Thus, machining of the material is made possible without causing the glass-ceramic to fracture. In addition to machinability, this material also exhibits the following favorable properties heat resistance... 

Specifically, it was demonstrated that the new glass-ceramics can be machined more rapidly than glass-ceramics with plane mica crystals. 

Fluorapatite and mica crystallization are examples of a twofold controlled mechanism to develop a glass-ceramic having a combination of diflFerent properties, such as machinability and bioactivity. The microstructure of the final glass-ceramic comprising mica and apatite crystals embedded in a glass matrix is shown in Fig. 2-32. 

Figure 2-44 Microstructure of machinable fenimagnetic glass-ceramic (see the composition in Table 2-22) after heat treatment (1180 C/1 h). Cubic spinel crystal of the ferrite-type and mica crystals characterize the morphology of the glass-ceramic (SEM).

Figure 3-10 House-of-cards structure in machinable fluormica glass-ceramic. Note phase-separated residual borosilicate glass and affinity of siiiceous droplets for mica flakes. Black bar = 1 pm.

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