sintering

Sintering is a method for making objects from powder, by heating the material in a sintering furnace^[1] 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.

The word "sinter" comes from the Middle High German Sinter, a cognate of English "cinder".

Advantages

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 binding contact between segregated powder particles or inclusions (called stringering), as often occurs in melt processes
5. no requirement for deformation to produce directional elongation of grains
6. the possibility of creating materials of uniform controlled porosity.

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.

To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using Ceramic Powder Separator Sheets. These sheets are available in various materials such as alumina, zirconia and magnesia. They are also available in fine medium and coarse particle sizes. By matching the material and particle size to the ware being sintered, surface damage and contamination can be reduced while maximizing furnace loading.

Sintering of metallic powders

Most, if not all, metals can be sintered. This applies especially to pure metals produced in vacuum which suffer no surface contamination. Sintering under atmospheric pressure requires the usage of a protective gas, quite often endo gas.^[2] 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 E_n 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:

E_n / E = (D / d)^3.4

where D is the density, E is Young's modulus and d is the maximum density of iron.

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.

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.

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 PTFE 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.

Separation of items within the furnace is achieved using sheets similar to those described in the ceramic process above.

Plastics sintering

Plastic materials are formed by sintering for applications that require materials of specific porosity. Sintered plastic porous components are used in filtration and to control fluid and gas flows. Sintered plastics are used in applications requiring wicking properties, such as marking pen nibs. Sintered ultra high molecular weight polyethylene materials are used as ski and snowboard base materials. The porous texture allows wax to be retained within the structure of the base material, thus providing a more durable wax coating.

Pressureless sintering

Pressureless sintering is the sintering of a powder compact (sometimes at very high temperatures, depending on the powder) without applied pressure. This avoids density variations in the final component, which occurs with more traditional hot pressing methods.

The powder compact (if a ceramic) can be created by slip casting into a plaster mould, then the final green compact can be machined if necessary to final shape before being heated to sinter.

See also

• Selective laser sintering, a rapid prototyping technology.
• Spark plasma sintering
• Frit
• Yttria-stabilized zirconia
• High-temperature superconductors

References

Bibliography

• Kang, Suk-Joong L. (2005), Sintering (1st ed.), Oxford: Elsevier, Butterworth Heinemann, ISBN 0-7506-6385-5 
• Kingery, W. David; Bowen, H. K.; Uhlmann, Donald R. (April 1976), Introduction to Ceramics (2nd ed.), John Wiley & Sons, Academic Press, ISBN 0-4714-7860-1 
• Chiang, Yet-Ming; Birnie, Dunbar P.; Kingery, W. David (May 1996), Physical Ceramics: Principles for Ceramic Science and Engineering, John Wiley & Sons, ISBN 0-4715-9873-9 

Further reading

• Green, D.J.; Hannink, R.; Swain, M.V. (1989). Transformation Toughening of Ceramics. Boca Raton: CRC Press. ISBN 0-8493-6594-5. 

External links

Look up sintering in Wiktionary, the free dictionary.

• Particle-Particle-Sintering - a 3D lattice kinetic Monte Carlo simulation
• Sphere-Plate-Sintering - a 3D lattice kinetic Monte Carlo simulation
• Thick Film Technologies- A Manufacturer of Ceramic Sintering Separator Sheets