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:

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
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.

Liquid Phase Sintering

For materials which are hard to sinter a process called liquid phase sintering is commonly used. Materials for which liquid phase sintering is common are Si₃N₄, WC, SiC, and more. Liquid phase sintering is the process of adding an addative to the powder which will melt before the matrix phase. The process of liquid phase sintering has three stages:

Rearrangement - As the liquid melts capillary action will pull the liquid into pores and also cause grains to rearrange into a more favorable packing arrangement.
Solution-Precipitation - In areas where capillary pressures are high(particles are close togeather) atoms will preferentially go into solution and then precipitate in areas of lower chemical potential where particles are non close or in contact. This is called "contact flattening" This densifies the system in a way similar to grain boundary diffusion in solid state sintering. Oswald ripening will also occur where smaller particles will go into solution preferentially and precipitate on larger particles leading to densification.
Final Densification - densification of solid skeletal network, liquid movement from efficiently packed regions into pores.

For liquid phase sintering to be particle the major phase should be at least slightly soluble in the liquid phase and the addative should melt before any major sintering of the solid particulate network occurs otherwise rearrangement of grains will not occur.

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.

Densification and Grain Growth

Sintering in practice is the control of both densification and grain growth. Densification is the act of reducing porosity in a sample thereby making it more dense. Grain growth is the process of grain boundary motion and oswald ripening to increase the average grain size. Since many properties (Mechanical strength, electrical breakdown strength, etc.) benefit from both a high relative density and a small grain size being able to control these processing is of high technical importance. Since densification of powders requires high temperatures grain growth naturally occurs during sintering. Reduction of this process is key for many engineering ceramics.

Sintering Mechanisms

Sintering occurs by diffusion of atoms through the microstructure. This diffusion is caused by a gradient of chemical potential – atoms move from an area of higher chemical potential to an area of lower chemical potential. The different paths the atoms take to get from one spot to another are the sintering mechanisms. The six common mechanisms are:

Surface diffusion - Diffusion of atoms along the surface of a particle
Vapor transport – Evaporation of atoms which condense on a different surface
Lattice diffusion from surface – atoms from surface diffuse through lattice
Lattice diffusion from grain boundary – atom from grain boundary diffuses through lattice
Grain boundary diffusion – atoms diffuse along ground boundary
Plastic deformation – dislocation motion causes flow of matter

Also one must distinguish between densifying and non-densifying mechanisms. 1-3 above are non-densifying – they take atoms from the surface and rearrange them onto another surface or part of the same surface. These mechanisms simply rearrange matter inside of porosity and do not cause pores to shrink. Mechanisms 4-6 are densifying mechanisms – atoms are moved from the bulk to the surface of pores thereby eliminating porosity and increasing the density of the sample.

Grain Growth

Grain growth happens due to motion of atoms across a grain boundary. Convex surfaces have a higher chemical potential then concave surfaces therefore grain boundaries will move toward their center of curvature. As smaller particles tend to have a higher radius of curvature and this results in smaller grains losing atoms to larger grains and shrinking. This is a process called Oswald ripening. Large grains grow at the expense of small grains. Grain growth in a simple model is found to follow:

$G^{m}=G_{0}^{m}+Kt$

Where $G$ is final average grain size, $G_{0}$ is the initial average grain size, $t$ is time, $m$ is a factor between 2 and 4, and $K$ is a factor given by:

$K=K_{0}*exp(-Q/RT)$

Where $Q$ is the molar activation energy, $R$ is the ideal gas constant, $T$ is absolute temperature, and $K_{0}$ is a material dependent factor.

Reducing Grain growth

Solute ions If a dopant is added to the material (Example: Nd in BaTiO3) the impurity will tend to stick to the grain boundries. As the grain boundary tries to move (as atoms jump from the convex to concave surface) the change in concentration of the dopant at the grain boundary will impose a drag on the boundary. The original concentration of solute around the grain boundary will be asymmetrical in most cases. As the grain boundary tries to move the concentration on the side opposite of motion will have a higher concentration and therefore have a higher chemical potential. This increased chemical potential will act as a backforce to the original chemical potential gradient that is the reason for grain boundary movement. This decrease in net chemical potential will decrease the grain boundary velocity and therefore grain growth.

Fine Second Phase Particles If particles of a second phase which are insoluble in the matrix phase are added to the powder in the form of a much finer powder than this will decrease grain boundary movement. When the grain boundary tries to move past the inclusion diffusion of atoms from one grain to the other will be hindered by the insoluble particle. More complicated interactions which slow grain boundary motion include interactions of the surface energies of the two grains and the inclusion and are discussed in detail by C.S. Smith[reference].

References

^ "sintering furnace".
^ "endo gas".

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

External links

[1] "sintering furnace".

[2] "endo gas".

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