Ceramic Material Manufacturing Methods

All ceramics start as a mixture of powdered base material (Zirconia, etc.), binders and stabilizers.  This mixture is “formed” into shapes and then fired (sintered) at high temperature to create hard, dense materials.    Forming is done using standard processes such as pressing, extruding, injection molding, tape casting or slip casting.  Ceramics can also be machined prior to being fired using standard machine tools in a process known as “green machining.”  Green machining  is inexpensive because unfired material is soft.  However,  firing causes ceramics to lose 20% to 40% of their volume, therefore green machining followed by firing is suitable only for those applications with loose tolerances (~1% of characteristic lengths).  In contrast, tight tolerance parts must be machined using high speed, diamond tools after ceramics are fired.

Some of the better know ceramic manufacturing processes combined sintering with forming.

Sintering (Firing):  Ceramics are consolidated into dense material by exposing them to 1800°C -  2000°C for days or weeks at a time, depending on the ceramic and process details.  The addition of the thermal energy promotes strong bonds between the raw ceramic particles, leading to densification.   Green machined, near net shapes or raw stock material can be sintered.  Knowledgeable ceramics manufacturers are very adept at accounting for volumetric shrinkage.

Hot Pressing:  Hot pressing combines the forming and firing steps to produce relatively simple geometric shapes.   The ceramic powder is simultaneously subjected to sintering temperatures and uniaxial pressure.  Simple shapes are generated by placing the raw material in a high temperature die while under load.

Hot Isostatic Pressing (HIP):  Hot isostatic pressing is a uniform pressure assisted method of sintering formed ceramics into simple and complex shapes.  The pressure, usually applied via an inert gas like Argon to prevent reactions, significantly reduces porosity and therefore improves physical properties.  Often times, the pressurization process is preceded by evacuating all air to reduce moisture and impurities.  In order for the hot isotatic press process to work, the green ceramic must be placed in a gas tight container.  An alternative method is to pre-sinter the ceramic to remove porosity at the surfaces.  In this way, the ceramic material itself acts as the pressure vessel.   Hot isostatic pressing differs from isostatic pressing in that the former applies uniform pressure to the ceramic during sintering.

Chemical Vapor Deposition (CVD):  Chemical vapor deposition is the process of converting gases (called precursors) into solids by continuously depositing monolayers of material onto a heated substrate.  This is a thermodynamically driven process, so control of substrate temperature and chamber pressure is critical.  Certain ceramic materials, such as Silicon Carbide and Silicon Nitride, can be manufactured using chemical vapor deposition techniques.   Shapes are formed using sacrificial targets premachined into the desired shape of the part.   Although the resulting material is much more expensive than its conventionally made counterparts, the cost is warranted by applications requiring superior physical properties. 

Reaction Bonding:  Reaction bonding uses a chemical reaction to bind ceramic powders into a solid form.  After forming, the binder is burned off to create a porous preform, and then capillary pressure is used to infiltrate liquefied reactants (different reactants for different ceramics) into the preform at temperatures just above the ceramic melting point.  The resulting reaction creates the solid ceramic form.  For example, liquefied Si is used in reaction bonded Silicon Carbide.    The main disadvantage of reaction bonded ceramics is that it leads to relatively high porosity