sintering

 

process of forming objects from a metal powder by heating the powder at a temperature below its melting point. In the production of small metal objects it is often not practical to cast them. Through chemical or mechanical procedures a fine powder of the metal can be produced. When the powder is compacted into the desired shape and heated, i.e., sintered, for up to three hours, the particles composing the powder join together to form a single solid object.

This article or section includes a list of references or a list of external links, but its sources remain unclear because it lacks in-text citations. You can improve this article by introducing more precise citations.

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.

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. In the case of materials with high melting points such as Teflon and tungsten, sintering is used when there is no alternative manufacturing technique. 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.

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.

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, 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 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

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 due to the fact that 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.

See also

• Selective laser sintering, a rapid prototyping technology.

External Links

• Particle-Particle-Sintering - a 3D lattice kinetic Monte Carlo simulation
• Sphere-Plate-Sintering - a 3D lattice kinetic Monte Carlo simulation

References

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

Donate to Wikimedia