sintering - Ceramic sintering

One of the techniques of powder metallurgy. Metal parts are made by forming a shape in metal powder and then holding it for several hours just below the melting point of the metal, or (if an alloy) at that of the higher melting component. It is an economical process for making small parts, and is necessary for metal which has so high a melting point that it cannot easily be cast. It can produce porous structures, advantageous in lubrication.

Sintering is a method for making objects from powder, by heating the material (below its melting point) until its particles adhere to each other. Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.

Sintered bronze in particular is frequently used as a material for bearings, since its porosity allows lubricants to flow through it. In the case of materials with high melting points such as Teflon and tungsten, sintering is used when there is no alternative manufacturing technique. For instance, the tensile strength En 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 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 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.

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

There are two types of sintering: with pressure (also known as hot pressing), and without pressure.