For most people ceramics are hard and brittle materials which are used to manufacture tiles, bricks and pottery. These functional and predominantly clay-based materials are our most common everyday contact with ceramics.
The more specialist technical ceramics that are used in extreme environments and applications are less frequently encountered and have properties that make them unique. Some of these have an even rarer capability; in this world of hard and ultra-hard technical ceramics, they are called ‘machinable’.
In terms of technical ceramics, ‘machinable’ is a term meaning that the material can be turned, milled, drilled and sawn without diamond tooling when sintered.
The vast majority of sintered ceramics can only be processed with diamond tooling as the materials themselves are nearly as hard as actual diamond making machinable ceramics less common. However, by sacrificing the super powers of hardness, these materials offer a broader range of alternate benefits.
Machinable ceramics tend to be fabricated in large blocks or billets for consistency and ease of manufacture. The more technical materials have their structure controlled through hot pressing or HIPping. The hot press process is not cheap but imparts a consistency greatly appreciated by engineers. By pressing the ceramic material at controlled temperatures, crystal growth is restricted, preventing large crystals forming and leaving a fine structure. This allows fine particles to be plucked out of the surface of the material by carbide tools leaving a smooth surface during the machining process.
The billet size of machinable ceramics is in most cases significantly greater than blocks of sintered hard technical ceramics. Hard technical ceramics densify during the sintering process, mostly at atmospheric pressure and generally with 20% shrinkage , this shrinkage is reasonably predictable but this still leaves a variable result within +/-1%
Machinable ceramics come from billets where the shrinkage tolerance is not an issue as the parts are machined to an exact size from the already sintered billet. A typical machined tolerance is +/-0.004” all over. This is a time-consuming process but in small volumes, it is still more practical than the time it would take to produce and sinter a blank with the possible additional diamond grinding adding to the lead time and cost.
Non-machinable technical ceramics are fabricated to an estimated size and any tight tolerances then require additional diamond grinding.
Hot pressing is not the only route to producing machinable ceramics as Corning discovered in the development of Macor, a unique machinable glass ceramic emulating the machinable and naturally occurring properties of mica. Macor rapidly gained favour and found fame in NASA’s Space Shuttle Programme with large pieces machined to exact sizes helping to thermally insulate the cabin from the alternating hot and cold endured in an orbiting spacecraft.
In summary, machinable ceramics cover a wide range of materials including Macor machinable glass ceramic, Shapal Hi-M Soft machinable aluminium Nitride and a large array of boron nitrides through to natural materials such as Mica and Lava, each material offers a unique solution to fast response issues and technical problems that these impossible materials can solve.