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Sintering
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Sintering is a method for making objects from powder, by heating the material below its melting point (solid state sintering) 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.
A special form of sintering, still considered part of powder metallurgy, is liquid state sintering. In liquid state sintering, at least one but not all elements are in a liquid state.
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Sintering is a method for making objects from powder, by heating the material below its melting point (solid state sintering) 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.
A special form of sintering, still considered part of powder metallurgy, is liquid state sintering. In liquid state sintering, at least one but not all elements are in a liquid state. Liquid state sintering is required for making cemented carbides or tungsten carbide.
The word "sinter" comes from the Middle High German Sinter, a cognate of English "cinder".
Sintered bronze in particular is frequently used as a material for bearings, since its porosity allows lubricants to flow through it or remain captured within it. For materials that have relatively high melting points, by comparison to other materials of the same type, such as Teflon and tungsten, sintering is one of the few viable manufacturing processes. In these cases very low porosity is desirable and can often be achieved.
Sintered bronze and stainless steel are used as filter materials in applications requiring high temperature resistance while retaining the ability to regenerate the filter element. For example, sintered stainless steel elements are used for filtering steam in food and pharmaceutical applications.
Sintering is static when a metal powder under certain external conditions may exhibit coalescence, and yet reverts to its normal behavior when such conditions are removed. In most cases, the density of a collection of grains increases as material flows into voids, causing a decrease in overall volume. Mass movements that occur during sintering consist of the reduction of total porosity by repacking, followed by material transport due to evaporation and condensation from 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 smoothing pore walls. Surface tension is the driving force for this movement.
Most, if not all, metals can be sintered. This applies especially to pure metals produced in vacuum which suffer no surface contamination. Many nonmetallic substances also sinter, such as glass, alumina, zirconia, silica, magnesia, lime, ice, beryllium oxide, ferric oxide, and various organic polymers. Sintering, with subsequent reworking, can produce a great range of material properties. Changes in density, alloying, or heat treatments can alter the physical characteristics of various products. For instance, the Young's Modulus En of sintered iron powders remains insensitive to sintering time, alloying, or particle size in the original powder, but depends upon the density of the final product:
where D is the density, E is Young's modulus and d is the maximum density of iron.
Advantages
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 binding contact between segregated powder particles or inclusions (called stringering), 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. 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.
Ceramic sintering
Sintering is part of the firing process used in the manufacture of pottery and other ceramic objects. Some ceramic raw 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, deflocculant, and unfired 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
All the characteristic temperatures associated to phases transformation, glass transitions and melting points, occurring during a sinterisation cycle of a particular ceramics formulation (i.e. tails and frits) can be easily obtained by observing the expansion-temperature curves during optical dilatometer thermal analysis. In fact, sinterisation is associated to a remarkable shrinkage of the material because glass phases flow, once their transition temperature is reached, and start consolidating the powdery structure and considerably reducing the porosity of the material.
There are two types of sintering: with pressure (also known as hot pressing), and without pressure. Pressureless sintering is possible with graded metal-ceramic composites, with a nanoparticle sintering aid and bulk molding technology. A variant used for 3D shapes is called hot isostatic pressing.
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