Crystalline Structure of Machinable Glass Ceramics
Machinable glass ceramics, such as Corning’s Macor, have numerous applications in the aerospace and energy industries. An earlier variant, Dicor, now little used, was a pioneering material in the 1980s for restorative dentistry.
These materials have outstanding strengths and electrical insulation characteristics. Macor can be used continuously up to temperatures of 800 degrees C, matching most metals as well as sealing glasses. It has zero porosity, is non-wetting and, unlike all other ductile materials, does not deform. It can also be used as an excellent insulator at very high voltages and will not outgas when placed in a vacuum environment.
However, it is the machinability of these glass ceramics that has made them popular. Macor has a very tight tolerance of up to 10 microns (0.0005 inches) and can be machined to produce surface finishes lower than 0.5 microns. All of this can be achieved using standard cutting tools rather than specialist diamond cutters or high-temperature kilns.
These machinable glass ceramics are essentially polycrystalline materials with a very fine micro-structure that is produced using a process of controlled crystallisation. Macor is a glass rich in the element fluorine and is chemically the compound trisilicic fluorphlogopite mica.
When it cools from a melt, this compound separates into droplets that are fluorine-rich and give the glass an opal-like appearance. This process is called glass in glass separation. Its control is crucial to crystal formation and the creation of the material’s outstanding properties.
Subsequent heat treatment devitrifies, or recrystallises, these droplets to create interlocking sheets of the mica. The mean size of crystals is about 20 microns. The collective micro-structure of these crystals resembles a house of cards and produces the characteristic sheet-like structure of mica.
Any cracks that appear in one crystal can propagate through the micro-structure to another crystal. This is the property that is easily exploited to produce machinable glass ceramics.
The sheets, or platelets, of mica usually have straight boundaries. However, recent technological innovations have produced curved platelets.
This curvature is created by ensuring that a preliminary crystalline phase does not form and the heating process moves on to the final crystallisation. Here the mica is allowed to grow into separate crystalline sheets that curve around each other like the head of a cabbage or lettuce. These curved glass ceramics also have outstanding machining properties.
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