Sintering is a method for making objects from powder, increasing the adhesion between particles as they are heated. It is used with ceramic powders and in powder metallurgy. Sintering is related to diffusion.
In most cases the density of a collection of grains increases as material flows into voids causing a decrease in overall size. Mass movements which occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation with diffusion. In the final stages metal atoms move along crystal boundaries to the walls of internal pores, redistributing mass from the internal bulk of the object and smoothening pore walls.
Most, if not all, metals may be sintered. This is especially true of pure metals produced in space which suffer no surface contamination. Many nonmetallic substances also sinter, such as glass, alumina, silica, magnesia, lime, beryllia , ferric oxide, and various organic polymers. A great range of material properties can be obtained by sintering with subsequent reworking. Physical characteristics of various products can be altered by changing density, alloying, or heat treatments. For instance, the tensile strength En of sintered iron powders is insensitive to sintering time, alloying, or particle size in the original powder, but is dependent upon the density (D) of the final product according to En/E = (D/d)3.4, where E is Young's modulus and d is the maximum density of iron.
Particular advantages of this powder technology include:
- the possibility of very high purity for the starting materials and their great uniformity
- preservation of purity due to the restricted nature of subsequent fabrication steps
- stabilization of the details of repetitive operations by control of grain size in the input stages
- absence of stringering of segregated particles and inclusions as often occurs in melt processes
- no deformation is required to produce directional elongation of grains
There are many literary references on sintering dissimilar materials for solid/solid phase compounds or solid/melt mixtures in the processing stage. Any substance which can be melted may also be atomized using a variety of powder production techniques. Finally, when working with pure elements, scrap remaining at the end of parts manufacturing may be recycled through the powdering process for reuse.
The general procedure of creating ceramic objects via sintering is:
- Mixing water, binder, antiflocculant , and ceramic powder to form a slurry
- Spray drying the slurry
- Putting the spray dried powder into a mold and pressing it to form a Green Body (an unsintered ceramic material)
- Heating the Green Body at low temperature to burn the binder off
- Sintering at a high temperature to fuse the ceramic particles together