Sintering

Sintering is a method for making objects from powder, increasing the adhesion between particles as they are heated. Sintering traditionally serves for manufacturing ceramic objects, and has also found use in such fields as powder metallurgy. Sintering relates to diffusion.

The word "sinter" comes from the German Sinter, an analogue of English "cinder".

Sintered bronze in particular frequently serves as a material for bearings since its porosity allows lubricants to flow through. In the case of materials difficult to melt, such as Teflon and tungsten, sintering serves for want of an alternative manufacturing techniqe, so that very low porosity is desirable and is often achieved.

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. Surface tension serves as the driving force for this movement.

Metallurgists can sinter most, if not all, metals. This applies especially to pure metals produced in vacuum which suffer no surface contamination. Many nonmetallic substances also sinter, such as glass, alumina, silica, magnesia, lime, beryllia, ferric oxide, and various organic polymers. Sintering with subsequent reworking can produce a great range of material properties. Changing density, alloying, or heat treatments can alter the physical characteristics of various products. For instance, the tensile strength E_n of sintered iron powders remains insensitive to sintering time, alloying, or particle size in the original powder, but depends upon the density (D) of the final product according to E_n/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:

1. the possibility of very high purity for the starting materials and their great uniformity
2. preservation of purity due to the restricted nature of subsequent fabrication steps
3. stabilization of the details of repetitive operations by control of grain size in the input stages
4. absence of stringering of segregated particles and inclusions (as often occurs in melt processes)
5. no requirement for deformation to produce directional elongation of grains

Many literary references exist on sintering dissimilar materials for solid/solid phase compounds or solid/melt mixtures in the processing stage. Any substance which melts may also become atomized using a variety of powder production techniques. When working with pure elements, one can recycle scrap remaining at the end of parts manufacturing through the powdering process for reuse.

Ceramic sintering

Ancient sintering techniques for the making of pottery and ceramic art objects remain in wide use to this day, but research has also led to more advanced techniques which work for a wider array of ceramics. Most ceramic materials have a lower affinity for water and a lower plasticity index than clay, requiring organic additives in the stages before sintering. The general procedure of creating ceramic objects via sintering of powders includes:

• Mixing water, binder, anti-flocculant, 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 item)
• Heating the green body at low temperature to burn off the binder
• Sintering at a high temperature to fuse the ceramic particles together

de:Sintern ja:焼結