There
are many methods to process ceramics. These can vary depending on the desired
properties of the final part.
Three
of the normal methods for processing include:
METHOD
1
Raw
Powder - need to spec properly
Milling
of Powder - to blend binders or sintering aids
Preform
-Die press uniaxially, cold isostatically, slip cast, or injection mold
De-binder
(dewax) - if required, via vacuum or flow through gas
Sinter-
under vacuum or partial pressure (2 to 50 psi) (Zirconia usually air fired)
These
steps may achieve the desired product.
Hot
Isostatically Press - this additional step might be needed to improve properties.
The actual pressure and temperature could vary depending on desired product.
Alumina and Zirconia are typically HIPped at 1500 C, 15,000 psi for 2 hours.
Silicon Nitride is being pressed at 1500 psi, but some are using higher
pressures to 45,000 psi. Higher pressures typically produce better properties,
but often the minimal gain is not cost effective. By Sintering to above
92 percent you would typically achieve a non porous part that will not
require any HIPping membrane (container).
METHOD
2
Raw
Powder
Milling
- if required
Preform
De-binder
- if required
Glass
encapsulation - to provide a barrier membrane
Hot
Isostatically Press
METHOD
3
Raw
Powder
Milling
- if required for sintering aids. Binders not typically required as part
goes from powder to product. Binders are usually added to facilitate handling
of a pre-form.
Hot
Uniaxial Press - in vacuum, or partial pressure.
This
method will produce reasonably good parts that may satisfy part requirements.
Parts may have minimal density variations, and will probably require more
machining than an isostatically pressed component.
For
some ceramic materials, binders can be burned off in low cost ovens at
atmospheric pressure either in air or in a non-oxidizing atmosphere. After
binder removal the work is then transferred to a sinter furnace.