Sintering is a heat treatment process where loose material is subjected to high temperature and pressure in order to compact it into a solid piece. This is similar to when ice cubes adhere together in a glass of water due to the temperature difference between the ice and the water, or when you push snow together to form a compact snowball.
The heat and pressure required for the sintering process is less than the material’s melting point.
Just as a material has a melting point, it will also have a desirable sintering point, at which the heat and pressure are enough to reduce the porous spaces between the material’s particles and squeeze loose material together into a solid lump.
This use of pressure and heat takes place naturally in mineral deposits within the Earth as well as in glacial formations.
Sintering is used to increase material properties, including thermal and electrical conductivity, material strength and integrity, and translucency.
There are several types of sintering, depending on the material being joined or the specific sintering process, as follows:
Sintering is used in the manufacture of ceramic objects including pottery. Because some ceramic raw materials have a lower plasticity index and affinity for water than clay, they need organic additives adding ahead of sintering. The process is associated with material shrinkage as the glass phases flow once the transition temperature has been reached and the powdery structure of the material consolidates, reducing the material porosity. The process is driven through the use of high temperatures, although this can be coupled with other forces such as pressure or electrical currents. Pressure is the most common additional factor, although ‘pressureless sintering’ is possible with graded metal-ceramic composites along with a nanoparticle sintering aid and bulk moulding technology. Hot isostatic pressing is a variant of sintering that is used for creating 3D shapes.
Metallic Powder Sintering
Most metals can be sintered, particularly pure metals in a vacuum where surface contamination cannot occur. When sintering a metal powder, such as iron powder, under atmospheric pressure a protective gas should be used. Sintering can cause a reduction in the overall volume of material as the density increases and material fills voids before the final stages see metal atoms travel along crystal boundaries and smooth out the pore walls due to surface tension. Liquid state sintering is when at least one (but not all) of the materials are in a liquid state. Still considered powder metallurgy, this technique is used to make tungsten carbide and cemented carbide. Sintered metal powder is used for a range of applications from making bearings and jewellery to heat pipes and even shotgun shells. Sintering is also one of the few viable options for manufacturing with materials that have high melting points, such as carbon, tanatalum and tungsten.
Plastic items that need specific material porosity are formed by sintering, including for applications such as filtration units and the control of fluid and gas flows. Other applications for sintered plastics include inhaler filters, lining on packaging materials and the nibs for whiteboard markers. Sintered plastics are also used as the base materials in skis and snowboards.
Liquid Phase Sintering
This process is used for materials that are difficult to sinter. Liquid phase sintering involves the addition of an additive to the powder to be sintered. This additive melts and the liquid is pulled into the pores and cause the grains to be rearranged into a more favourable packing arrangement. Where the capillary pressures are high and the particles are close together, the atoms go into solution and precipitate into areas of lower chemical potential in what is called ‘contact flattening.’ This is similar to grain boundary diffusion in solid state sintering. To be effective, the additive needs to melt before the sintering occurs.
Permanent Liquid Phase Sintering
This process is similar to regular liquid phase sintering, except it promotes capillarity to attract the liquid into open pores leading to grain movement and improved packing.
Transient Liquid Phase Sintering (TLPS)
This bulk material forming process is used for ceramics, metals and metal matrix-ceramic materials. These materials need to be mutually soluble with the liquid wetting the solid and creating a high diffusion rate.
Electric Current Assisted Sintering
First patented in 1906 by A.G. Bloxam, this process uses electric currents to drive or enhance sintering. The process was developed further over the ensuing years, including combining electric currents with pressure, which was found to be beneficial for sintering refractory metals and conductive nitride and carbide powders. There have been over 640 electric current sintering-related patents since 1906, including resistance sintering (aka hot pressing).
Spark Plasma Sintering
This type of sintering uses pressure and an electric field to enhance the density of ceramic and metallic powder compacts. By using the electric field and hot pressing to improve densification, this process allows lower sintering temperatures and less time for the process. However, the name is slightly misleading as research showed that there is no plasma used and so alternative names such as Field Assisted Sintering Technique (FAST), Electric Field Assisted Sintering (EFAS), and Direct Current Sintering (DCS) have come into use.
Electro Sinter Forging
This electric current-assisted sintering technology is used to produce diamond metal matrix composites and is derived from capacitor discharge sintering. The process is being investigated for use with a range of metals and is characterised by a low sintering time.
As mentioned above, this technique involves sintering without the use of applied pressure, avoiding density variations in the final product as a result. Ceramic powder compacts can be created through cold isostatic pressing, injection moulding or slip casting, following which they are pre-sintered and machined to a final shape before heating. There are three different heating techniques for pressureless sintering - constant-rate of heating (CRH), rate-controlled sintering (RCS), and two-step sintering (TSS). The ceramic microstructure and grain size will vary depending on the material and technique used.
This process can be used to generate heat within the material rather than through the surface from an external heat source. It is suited for small loads where it can offer faster heating, less energy expenditure and improvements in product properties. However, since microwave sintering typically sinters just one compact at a time, the overall productivity can be poor if more are required. In addition, since microwaves only penetrate a short distance for materials with high conductivity and high permeability, the powders must have a particle size similar to the penetration depths of microwaves in that particular material. In addition, some materials fail to couple and others may show run-away behaviour. Because the process and side reactions are several times faster with microwave sintering there can be different properties for the final sintered product. Despite the drawbacks, this technique is quite effective for maintaining fine grain sizes in bioceramics.
While the different methods and materials offer a range of benefits, there are a number of general advantages associated with sintering:
- Purity: Sintering offers high levels of purity and uniformity in the starting materials, which can be maintained due to the simple fabrication process
- Repeatable: Controlling the grain size during input allows for highly repeatable operations
- No Binding Contact / Inclusions: Unlike with some melting processes, sintering will not cause binding contact between powder particles or inclusions (aka ‘stringering’)
- Uniform Porosity: Create materials with a uniform, controlled porosity
- Nearly Net-Shaped Objects: Sintering can create nearly net-shaped objects
- High Strength Materials: Sintering can create high strength items such as turbine blades
- High Mechanical Handling Strength: The sintering process improves the mechanical strength for handling
- Work with Difficult Materials: Sintering allows you to work with materials that cannot be used with other technologies, such as metals with very high melting points
Because sintering can enhance material properties such as electrical and thermal conductivity, strength, and translucency, it has uses in a range of industries and applications. The process of creating metal parts by pressing powders dates back many centuries and has been used to make items from almost every type of ceramic or metal.
Modern uses include the creation of structural steel parts, porous metals for filtering, tungsten wiring, self-lubricating bearings, magnetic materials, electrical contacts, dental products, medical products, cutting tools and more.
What is the Meaning of Sintered?
The word ‘sinter’ came to the English language from German in the late 18th Century and has comparisons to the English word ‘cinder.’ Sintering is a heat treatment process that involves subjecting aggregate material to temperature and pressure in order to compact the loose material into a solid object.
Why is Sintering done and Why is it Important?
Sintering is done to impart strength and integrity to a material as well as reducing porosity and enhancing electrical conductivity, translucency and thermal conductivity. This is important to deliver desirable properties to products and also allows items to be created from metals with high melting points (since the materials do not need to melt when sintering).
How Long does it Take?
Depending on materials and techniques, sintering can take anywhere from a few milliseconds to over 24 hours.
Material differences that effect how long the process may take include the mobility of the atoms, the self-diffusion coefficients, melting temperature, and level of thermal conductivity. In addition, field assisted techniques can reduce sintering times while selective laser sintering (basically, 3D printing for metals) is slower and the traditional oven process is slower still.
The addition of a liquid phase will also speed up sintering times. However, faster sintering times can lead to reduced density and residual porosity.
Sintering works through the diffusion of atoms across particle boundaries before fusing together into one piece under the influence of pressure and/or heat. While this process can occur naturally for mineral deposits, it is also widely used by a range of industries to manufacture items from materials including ceramics, metals and plastics.
Sintering occurs at heats below the melting point of the materials, making it useful for creating items from metals that have high melting points.
There are a range of different techniques depending on factors such as the use of electrical currents, pressure and heat sources as well as the actual materials being sintered.
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