What is Sintering? (A Definitive Guide) (2023)

Sintering, which is also called ‘frittage,’ is the process of forming a solid mass of material through heat and pressure without melting to the point of liquefaction. This process involves the atoms in materials diffusing across the particle boundaries and fusing together into one piece. Sintering occurs naturally in mineral deposits, and is used as a manufacturing process for materials including ceramics, metals and plastics.

Because the sintering temperature doesn’t reach the materials’ melting point, it is often used for materials with high melting points, such as molybdenum and tungsten.

Powder metallurgy is the study of sintering using powder-related processes.

Contents

Click the links below to skip to the section in the guide:

  • What is the sintering process?
  • How does it work?
  • Types
  • Advantages
  • Where is it used?
  • FAQs
  • Conclusion

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(Video) What does sintering mean? Sintering process easily explained

What is the Sintering Process?

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.

(Video) Conventional Press-and-Sinter Powder Metallurgy

How Does it Work?

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.

Types

There are several types of sintering, depending on the material being joined or the specific sintering process, as follows:

Ceramic Sintering

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 Sintering

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.

(Video) Sintering Zirconia: What You Need to Know

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.

Pressureless Sintering

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.

Microwave Sintering

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.

Advantages

While the different methods and materials offer a range of benefits, there are a number of general advantages associated with sintering:

(Video) Powder Metal 101

  1. Purity: Sintering offers high levels of purity and uniformity in the starting materials, which can be maintained due to the simple fabrication process
  2. Repeatable: Controlling the grain size during input allows for highly repeatable operations
  3. No Binding Contact / Inclusions: Unlike with some melting processes, sintering will not cause binding contact between powder particles or inclusions (aka ‘stringering’)
  4. Uniform Porosity: Create materials with a uniform, controlled porosity
  5. Nearly Net-Shaped Objects: Sintering can create nearly net-shaped objects
  6. High Strength Materials: Sintering can create high strength items such as turbine blades
  7. High Mechanical Handling Strength: The sintering process improves the mechanical strength for handling
  8. 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

Where is it Used?

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.

FAQs

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.

Conclusion

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.

(Video) How it Works: Direct Metal Laser Sintering (DMLS)

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.

Related Frequently Asked Questions (FAQs)

FAQs

What is the meaning of sintering? ›

Sintering, which is also called 'frittage,' is the process of forming a solid mass of material through heat and pressure without melting to the point of liquefaction. This process involves the atoms in materials diffusing across the particle boundaries and fusing together into one piece.

What are steps of sintering process? ›

ChinaSavvy's metal sintering process, also commonly known as the powder metallurgy process, is divided into three main steps: Blending. Compaction.
...
Controlled atmospheres used in the furnace for sintering:
  1. A nitrogen – hydrogen atmosphere.
  2. A hydrogen atmosphere.
  3. Vacuum.
  4. Dissociated Ammonia.
  5. An Endothermic gas atmosphere.

What are the types of sintering process? ›

Basically, sintering processes can be divided into two types: solid state sintering and liquid phase sintering.

What is metal sintering process? ›

Metal sintering is the process of fusing metal powders to create a solid object and involves heat and pressure. The result is a metal object with a near-net shape, meaning it only requires minimal finishing work before being used.

Is sintering a heat treatment process? ›

Sintering is a heat treatment applied to a powder compact in order to impart strength and integrity. The temperature used for sintering is below the melting point of the major constituent of the Powder Metallurgy material.

Why does sintering happen? ›

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.

What are the 3 principal sintering processes? ›

Having them, the basic process to follow for producing a sintered part consists of three operations, i.e., powder mixing, compacting, and sintering.
  • RAW MATERIAL. The raw material of a sintered component is always a metallic powder. ...
  • MIXING. ...
  • TOOLING. ...
  • COMPACTING. ...
  • SINTERING.

What material is used in sintering? ›

Some of the materials / powders used in metal sintering includes: Iron and Carbon Steels. Iron-Copper and Copper Steels. Iron-Nickel and Nickel Steels.

Which is the first step of the sintering cycle? ›

Binder Removal

The first step in the sintering cycle is to remove the binder from the part. Depending on the specific binder being used, this is done via degradation and/or evaporation.

How do you determine sintering temperature? ›

The sintering temperature of laser ceramics compacts is usually about 200–300°c below the melting point of the corresponding composition. The choice of sintering temperature is determined by the growth of ceramics grains because there is a temperature point where the rate of grain growth increases sharply.

What is sintering heat? ›

Sintering is a thermal process of converting loose fine particles into a solid coherent mass by heat and/or pressure without fully melting the particles to the point of melting.

What is sintering and why is it necessary for some ores? ›

The sintering process is used to agglomerate a mix of iron ores (blend), return fines, fluxes and coke, with a particle size of less than 10 mm, so that the resulting sinter, with a screened size of 10 mm to 30 mm, can withstand pressure and temperature conditions in the blast furnace.

What happens to the metal during sintering? ›

Sintering is done by putting your parts through a rather toasty furnace. The goal is to control the temperature so it reaches just below the melting point of your parts' primary metal … but not quite. The sintering process causes the metal particles to bond together so you get a part that does what you need it to do.

What are the benefits of metal sintering? ›

Sintering allows to obtain complex shapes directly from the compacting tooling, without any machining operation, like teeth, splines, profiles, frontal geometries, etc.

What happens after sintering? ›

After sintering, it is possible to improve the properties of MIM components using various methods which include heat treatment. The treatment applied to the sintered components will depend on the application. Post-sintering processes include assembling of parts, heat treatment, densification and finishing.

Is sintering the same as melting? ›

Sintering combines materials by heat and pressure, without melting involved. Melting combines particles by heating them till they liquify and combine as one material. Sintering can occur at low temperatures if enough pressure is applied.

How many stages are there in sintering *? ›

(a) bound growth starting (initial stage), (b) shrinkage of pores volume (intermediate stage), (c) formation of grains boundaries (final stage) [18].

What is powder sintering process *? ›

Sintering of powder metals is a process in which particles under pressure chemically bond to themselves in order to form a coherent shape when exposed to a high temperature. The temperature in which the particles are sintered is most commonly below the melting point of the main component in the powder.

What is two step sintering? ›

Two‐stage sintering (TSS) is an effective way to achieve fine‐grained microstructured ceramics with high densification and relatively low cost. TSS method is successfully applied for all types of ceramics such as structural ceramics, bioceramics, ferrites, piezoelectric ceramics and electrolyte ceramics.

What are the required properties of sinter? ›

Sinter is subjected to in-depth characterisation in terms of chemical and granulometric analysis, determination of the mineral phases in its structure and of quality indices such as reducibility, low temperature degradation, reduction degradation, and tumbler strength.

What is the difference between sintering and firing? ›

Firing is the high-temperature treatment in ceramic technology, by which the consolidated powder compact (green body) is transformed into a rigid ceramic (dense or porous) and sintering is the corresponding process occurring in the material.

Is sintering physical or chemical change? ›

The nature of sintering process changes according to kind of heterogeneous catalyst and temperature as following [19-21]: ➢ Sintering on bulk catalysts is normally physical rather than chemical phenomena.

Why does sintering increase strength? ›

Early in heating sinter bond growth by surface diffusion adds strength faster than the parent material loses strength due to thermal softening. Consequently, the powder compact strengthens faster than the parent material is weakening.

Is sintering a casting process? ›

Casting sintering technology is the heat treatment of a powder or compacted metal or ceramic by pressure with the subsequent passage through a controlled atmosphere oven and at a temperature below the melting temperature of the mixture, approximately 70% -80% thereof.

How does sintering increase strength? ›

Sintering reaction behaviors result in good metallic bonding during the Cu-sintering process. The shear strength increased with an increase in sintering pressure and temperature.

Why does sintering need to occur at an elevated temperature? ›

High sintering temperatures are generally needed to process refractory metals due to their high melting temperatures, but specific conditions depend on the particular alloy, particle size and impurities. The tendency of refractory metals to oxidize requires reducing atmospheres or vacuum for sintering.

Can stainless steel be sintered? ›

Sintered stainless steel is used in systems requiring high corrosion resistance at both ambient temperature and high temperature. The typical materials used to make sintered stainless steel parts are: Austenitic stainless steels (300 series). These have the greatest corrosion resistance.

Does sintered steel rust? ›

Corrosion and mechanical properties of sintered materials are typically worse than their wrought counterparts. In the case of stainless steels, corrosion rates can be two or more orders of magnitude worse for sintered materials.

Can plastic be sintered? ›

Sintered porous plastics are created using a combination of heat and pressure to bond the materials together. The objective is not to melt or change the main properties of the material, but to create a porous material suitable for the particular use.

How accurate is sintering? ›

Excess powder material can be returned to the powder feed cartidges for reuse. Different materials can be sintered for different applications using this process. Accuracy as good as + / - 0.05 inches across the part cylinder in the X, Y, and Z coordinate.

Does sintering change density? ›

4, the density increases markedly with increasing sintering temperatures. With increasing sintering time, higher density is obtained at lower sintering temperatures. When the sintering was performed for 10 and 30min, respectively, high densities of more than 98% theoretical density were obtained in all samples.

Is sintering sustainable? ›

Using renewable energy sources like hydroelectricity, wind, and solar means the high electricity consumption of sintering processes has a low environmental impact compared to alternative manufacturing processes.

What is sintering in snow? ›

Sintering, or pressure sintering, is the physical process of bond formation between grains in the snowpack. It usually accompanies the rounding process. Sintering occurs when water vapour is deposited at the contact points between snow grains, forming necks.

What is the difference between sintering and annealing? ›

Sintering is the process of welding together small particles of a metal by applying heat below the melting point of the metal. Annealing is a heat treatment process in which we have to heat a metal to a predominant temperature, hold for some time and then cool it down in order to improve ductility.

What is sintering in 3d printing? ›

Selective laser sintering (SLS) is a 3d printing process (additive manufacturing) that uses high-powered lasers to sinter, or bind, finely powdered material together into a solid structure.

Is sintering the same as firing? ›

Firing is the high-temperature treatment in ceramic technology, by which the consolidated powder compact (green body) is transformed into a rigid ceramic (dense or porous) and sintering is the corresponding process occurring in the material.

What is the sintering temperature? ›

For example, the sintering of Fe takes place at 1000 °C–1200 °C, and for Cu at 700 °C–900 °C. For sintering of mixed metal powders, the sintering temperature is generally lower than that of the main composition, or a little above the low melting eutectic temperature according to the phase diagram.

What is the effect of sintering temperature? ›

By increasing the sintering temperature, the higher density was obtained. When the sintering temperature increases from 1000 °C to 1050 °C, a sharp increase in density occurs because of the activation of liquid phase sintering and after that by increasing the temperature, the increasing rate of density is lower.

What are the three steps in the sintering cycle? ›

Having them, the basic process to follow for producing a sintered part consists of three operations, i.e., powder mixing, compacting, and sintering.
  • RAW MATERIAL. The raw material of a sintered component is always a metallic powder. ...
  • MIXING. ...
  • TOOLING. ...
  • COMPACTING. ...
  • SINTERING.

What are the advantages of sintering? ›

Sintering allows to obtain complex shapes directly from the compacting tooling, without any machining operation, like teeth, splines, profiles, frontal geometries, etc.

How many stages are there in sintering? ›

During all three stages, atoms move by several transport mechanisms to create the microstructure changes, including surface diffusion and grain boundary diffusion. Sintering models include parameters such as particle size and surface area, temperature, time, green density, pressure and atmosphere.

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