Machinability and Strength of Machinable Glass Ceramics
The strength and machinability of machinable glass ceramics is a product of its crystalline microstructure. Read on to find out more.
Machinable glass ceramics, like Corning’s pioneering Macor, have become very popular in many industrial and medical applications because they can be machined using standard metal working tools rather than specialist equipment.
Using tools with standard carbide tips rather than a diamond cutter or a high-temperature kiln, the ceramic materials can be turned into any shape desired. They have a very precise tolerance up to 10 microns and can produce surface finishes of less than 0.5 microns
Their outstanding strengths, high electrical insulation properties as well as their machinability are due to their particular structure of small crystals that are embedded within a glassy matrix.
In chemical terms, machinable glass ceramics are called polycrystalline materials. This microstructure is created through a process known as controlled crystallisation, where small crystals, or crystallites, are grown in large numbers rather than a few large crystals.
Such a method requires a nucleating agent – such as magnesium aluminium silicate – that the glass crystals grow around. The glass itself is rich in fluorine and is essentially the chemical compound trisilicic fluorphlogopite. The formation of the fluorphlogopite is the crucial factor in providing the material’s strength and machinability.
The machinable glass ceramics are prepared by mixing powdered constituents of the final product in given proportions. Such constituents include silica (silicon dioxide), aluminium oxide, magnesium carbonate, magnesium fluoride, potassium carbonate and boron oxide. These are melted in a furnace to about 1,500 degrees Celsius and kept in this state for between one to two hours. The molten mass is then transferred to an annealing furnace for several hours and later cooled slowly at room temperature.
As it cools, it separates into fluorine-rich droplets that make the cooling melt shimmer like an opal, a process called glass-in-glass separation. Further stages of heat treatment, firstly to between 550 and 600 degrees C and then to between 950 and 1,000 degrees C will devitrify, or recrystallise, the melt.
At this point, the melt cools to form the fluorphlogopite phase that is an interlocking microstructure of plates, or sheets of mica. The average size of these crystals is 20 microns. The micro-plates lock together rather like a house of cards. A crack in one crystal propagates easily via the microstructure into another crystal. This is the specialist property that provides the ceramic’s strength and machinability.
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