Sintering is a process for manufacturing or changing materials . Fine-grain ceramic or metallic materials are heated - often under increased pressure - but the temperatures remain below the melting temperature of the main components, so that the shape of the workpiece is retained. As a rule, shrinkage occurs because the particles of the starting material compact and pore spaces are filled. A basic distinction is made between solid phase sintering and liquid phase sintering, which also results in a melt. Sintering processes are of great importance in ceramic production ("sintered glass ceramic ") and in metallurgy("Sintered metals" and " Powder metallurgy "). The temperature treatment of the sintering turns a fine or coarse-grained green body that was formed in a previous process step - for example by means of extrusion - into a solid workpiece. The sintered product only receives its final properties through the temperature treatment, such as hardness, strength or thermal diffusivity, which are required in the respective application.
During sintering, granular or powdery substances are usually mixed and then connected or compressed by heating. In contrast to pure melt, none or at least not all of the starting materials are melted. In other words, the starting materials are "baked together" in colloquial terms. It is therefore categorized under the two manufacturing processes, archetype and changing material properties .
Sintering makes use of the fact that powders have a large surface area and thus a large surface energy, but every system strives to adopt a state of the lowest free enthalpy . During sintering, the individual grains enlarge, so that the surface energy decreases. At the same time, the proportion of saturated chemical bonds increases, so that the body as a whole solidifies.
The powder masses are first brought into the shape of the desired workpiece. This is done either by pressing the powder masses (manufacture of technical products) or by molding and subsequent drying (for example in the manufacture of stoneware or clay ). At least a minimal cohesion of the powder particles must be given here. If this cohesion is not given, a binding agent must be used, e.g. B. when pouring cold . This so-called green compact or green body is then compressed and cured by heat treatment below the melting temperature .
Sintering has been used since the invention of ceramics and the process has since been refined empirically. The firing of porcelain is one of the oldest applications. During the Second World War, the originally copper bullet guide rings were replaced by sintered iron ("SiFe") in Germany (because of the copper shortage). That was the first large-scale use of sintered metal. However, systematic research into the sintering process did not begin until the 1950s with the development of powder metallurgy , when the production of metal components from powder moldings began. The phenomena underlying sintering could be more easily researched because of the simpler chemical properties of metals. The knowledge gained was then transferred to the handling of high-performance ceramics. In the field of silicate ceramics, the kinetics of sintering in particular has been investigated since then .
Advantages and disadvantages
The greatest advantage of sintering is that it brings together raw materials that can only be combined with great difficulty or not at all to form a new material.
From a chemical point of view, sintered materials mostly still consist of microscopic particles of the starting materials. So they are not as homogeneous as z. B. Metal alloys . For this reason, they can also have a porosity which can make the material permeable for liquids and gases. With certain exceptions, e.g. B. in the production of filters and catalyst components and slide bearings, apart from this, this permeability is generally undesirable. Therefore, in the production of components, there is sometimes considerable scrap , the proportion of which is then caused by post-treatment, such as B. impregnation, must be reduced.
The sintering process takes place in three stages, during which the porosity and the volume of the green body are significantly reduced. In the first stage, only the green compact is compacted, whereas in the second stage the open porosity is significantly reduced. The strength of the sintered body is based on the sinter necks formed in the third stage, which are created by surface diffusion between the powder particles.
In some cases, the workpiece is calibrated after the last process, usually when a very high degree of dimensional accuracy is required, which cannot be achieved by pure sintering due to the volume loss that cannot be precisely calculated. The quasi-finished workpiece is pressed into a mold again under high pressure. In this way, a high degree of dimensional accuracy or, for example, compliance with technical tolerances (shape and position tolerance) is possible.
Today sintering is used in the field of minerals ( ceramics ), technical ceramics and metals (powder metallurgy). It is also used in the field of nanotechnology, in which the layer resistance of polysilicon can be significantly reduced by sintering in a metal layer (silicidation with, for example, tungsten, titanium or tantalum (MOS field effect components).
Important sintered products
- Soft magnetic ferrites for transformers and chokes in power electronic devices e.g. B. Switching power supplies as well as filters, transformers and antennas from the audio frequency to the MHz range
- Components such as cutting ceramics, electroporcelain or ceramic magnets
- Metallic molded and finished parts in large series ,
- the dental technology used sintering for the manufacture of ceramic teeth and veneers.
The development and optimization of new metal powders is constantly expanding the areas of application for sintered molded parts (automobile construction, household, do-it-yourself and office machines).
Sintering in iron and steel metallurgy
The aim of sintering in iron metallurgy is to agglomerate fine ores and concentrates that are not ready for immediate use and thus feed them into the blast furnace process. Historically, the basic idea behind this was to feed the fines (fine ore) produced by sieving the lump ore for processing. The aim is to create a Möller that is as self- sufficient as possible .
The best-known sintering process for raw ores is the Dwight-Lloyd process :
For sintering, a mixed material is produced that consists of fine ores, cycle materials, fuel (coke breeze), sand-lime brick and sintered sifting. This mixture is mixed with water and layered on an endless grate belt. The coke contained in the mixture is then ignited by natural gas / furnace gas flames. The induced draft fan located under the grate belt now pulls the burning front through the mixture, so that the sinter cake is completely burned through at the discharge point of the belt.
The heat generated in the process melts the surface of the fine ores so that their grains form a solid bond. After the sinter cake is broken, it is cooled and classified. So-called grate deposits and sintered residues remain in the sintering plant, the finished sinter is fed to the blast furnace .
The advantages of sintering, in addition to the smelting of fine ore and cycle material, are the elimination of loss on ignition and the pre-reduction of the ores.
Sintering in the ceramic
Ceramic is one of the oldest materials and is used in many ways. Already in ancient times, clay and later porcelain played an important role in everyday life. Today, in addition to the classic, the so-called "technical ceramics" are also of great importance. These are ceramic products that are manufactured for technical applications.
Ceramics can be assigned to the groups of silicate , oxide and non-oxide ceramics .
- Among the silicate ceramics include, for example technical porcelains, steatite , cordierite and mullite ceramics. The main components are clay and kaolin, as well as feldspar and soapstone as silicate carriers.
- Oxide ceramics mostly contain over 90% single-phase or one-component metal oxides. The most important representatives are aluminum oxide (Al 2 O 3 ), magnesium oxide (MgO), zirconium oxide (ZrO 2 ), aluminum titanate (Al 2 TiO 5 ) and piezoceramics.
- The group of non-oxide ceramics includes carbides ( silicon carbides with different manufacturing processes, boron carbide ) and nitrides ( silicon nitride , aluminum nitride , silicon aluminum oxynitride). A high proportion of covalent bonds gives these materials very good mechanical properties even at high temperatures - see temperature resistance .
Individual ceramic products have very diverse properties. As a result, there are also numerous manufacturing processes - depending on the requirement profile of the component, type of material, price and number of pieces. Overall, however, the production processes can be summarized in two large steps - shaping and compression.
When shaping, the aim is to achieve a homogeneous packing density, that is to say a uniform mass distribution in the entire green compact. In addition, the choice of the shaping process is also influenced by factors such as the type of material, shape of the desired component, intended number of pieces and costs. There are basically three types of shaping available:
Dry pressing and wet pressing
Regarding moisture, there are two possibilities for shaping by pressing: dry pressing and wet pressing.
When dry pressing, the water content of the raw material is less than 7%. This method is particularly suitable for the production of large quantities. The molding tools are very expensive and are usually only worthwhile for large series. In addition to possible differences in density (internal inhomogeneities, voids ), complex powder preparation and limitations in the component geometry represent further disadvantages. However, there are also advantages such as good reproducibility , high dimensional accuracy and an automatic process sequence.
The alternative to dry pressing is wet pressing with a water content of over 12%. This type of shaping enables complex geometries and a more uniform density distribution. However, the green body must be dried . In addition, pressed parts made of crumbly moist granulate have a lower compression and thus a lower strength than dry pressed parts.
The method of uniaxial pressing is often used to produce, for example, plate-shaped bodies. The pressure is only applied to the body in one direction. The flowability of the powder (grain shape, grain distribution function ) is of great importance as the compaction properties depend on it. Pressing aids such as oils and waxes improve the lubricity and compressibility.
The shrinkage behavior during drying and firing is mainly influenced by the homogeneity of the compaction. Different densities over the sample cross-section often arise due to the friction of the mass on the mold. Therefore, with an increasing ratio L / D (body thickness L , sample diameter D ) counterpressure or the flying jacket is required , with the walls moving with it.
Another problem arises when the pressure is too high. When the compact is released , local tensile stresses can occur, which after relaxation often lead to crater-shaped cracks in the green compact. Nevertheless, uniaxial pressing has developed into a process suitable for large-scale production.
With isostatic pressing, the pressing pressure is the same in all directions. This method is well suited for small parts with high isotropy and uniform compression and is also favorable for demanding prototypes and production in small series .
In addition to pressing, casting is also used as a shaping process. This requires pourable slips (with organic additives) that have a water or solvent content of over 30%. Hollow bodies with a uniform wall thickness are produced by hollow casting . The core casting (for example in plaster molds) is suitable for solid components .
For special applications, the processes of die casting , injection molding (see also powder injection molding ) and foil casting are used, with which workpieces with special geometries can be produced, and possibly with specific properties.
The plastic shaping process is often useful for components that are to have a very complex geometry. This includes, for example, extrusion or powder injection molding: a heated screw presses raw material into the shaping end piece or negative tool. The base material used for this process is very different from normal sintered powder.
With laser sintering, a laser beam heats the raw material point by point, which is then applied in layers. This enables very free shaping, but production is time-consuming.
Once shaping and drying have been completed, the green compacts are treated at high temperatures . With silicate ceramics the temperatures are 800 ° C to 1400 ° C, with technical ceramics up to 2500 ° C. During this process, which is known as sintering, a linear shrinkage of 10 to 25% occurs, depending on the porosity of the solid (30 to 60% by volume). There are also non-shrinking sintering mechanisms (e.g. surface diffusion). This increases density (except in the case of non-shrink sintering) and strength. Note that the sinterability increases as the particle radius decreases. Depending on the proportion of the melt phase in the material, a distinction is made between different types of sintering:
Solid phase sintering
Solid phase sintering takes place at temperatures below the melting point of the lowest melting component. Any eutectics that may be present , which greatly reduce the liquidus temperature, must be taken into account - these temperatures must not be exceeded if the composition is appropriate. All components must be in a solid state.
Liquid phase sintering and reaction sintering
In liquid phase sintering, components with a higher melting point are also involved, so that some of the starting materials melt when heated. Another possibility is sintering by means of viscous flow, in which case there is a high proportion of melt. In contrast to metallic melts, silicate melts, such as those found in ceramics, generally have a high viscosity.
In reaction sintering, a chemical reaction is intentionally superimposed on the sintering process. Another new phase arises.
The sintering process is essentially divided into three sintering stages:
- Starting area
- Particle rearrangement occurs here, with initial contacts being formed. Necks are created. These lower the total energy by breaking down the surface. The change in density in this initial stage is about + 10%, while the shrinkage is less than 5%. The material transport takes place on the basis of gas diffusion , surface diffusion , grain boundary diffusion and volume diffusion. It should be noted that the surface diffusion is much faster than the grain boundary diffusion or volume diffusion.
- Intermediate area
- Neck growth and gradual formation of grain boundaries follow . It is characterized by the formation of a pore channel system through which the gases can escape. This prevents the workpiece from bursting, and atoms can diffuse more easily or more quickly from the outside to the inside. The change in density makes its biggest jump in this area (+ 30%), while the shrinkage progresses by around 10%. The material transport now only takes place through grain boundary and volume diffusion.
- End area
- In the end area there is further grain growth, with large grains sometimes “swallowing” small grains (giant grain growth, which is usually not desired). Pores are reduced or eliminated. The body shrinks by less than 3% and the density increases by another 5%.
Structure of sintered products
The technical-industrial name for ceramic materials is " sintered goods ". Their basic properties , which are important for technical applications, such as strength , fracture toughness , wear and thermal shock resistance as well as corrosion resistance, can be applied to the " structure ", i. H. can be traced back to the microstructural structure of the material. The following factors determine the structure:
Chemical-mineralogical processes when sintering porcelain
Numerous chemical processes take place when clay is heated. The clay substance (Al 2 O 3 · 2 SiO 2 · 2 H 2 O) produces metakaolin Al 2 O 3 · 2 SiO 2 + 2 H 2 O by releasing water , from which the flaky (primary) mullite (Al 2 O 3 · 2 SiO 2 ). From a temperature of 950 ° C., the feldspar K 2 O · Al 2 O 3 · 6 SiO 2 , Na 2 O · Al 2 O 3 · 6 SiO 2 or CaO · Al 2 O 3 · 2 SiO 2 melts . The alkali oxides, which serve as flux, can thus get into the mass. This creates a highly viscous melt and the existing quartz dissolves in it, which leads to an increase in toughness. This ensures that the body remains stable even when the temperature increases and does not collapse. The melt also has the advantage that it can partially close the pores. When this melt cools down, a glass phase is created in which mullite and cristobalite partially crystallize as the main phases.
There are also some applications in the field of plastics , but these are not always accepted. Product advertising creates expectations that can turn out to be incorrect in practice: for example, when mass production was introduced around 1970, sintered, cloudy, translucent ski surfaces made of colorless polyethylene were developed for which ski wax was no longer required. In the course of time it has been found that, depending on the environmental conditions, ski wax allows an adaptation that is not provided by the ski base alone.
Polytetrafluoroethylene was hailed as Teflon as a coating for frying pans . However, it is decomposed by heating from around 260 ° C, producing toxic fumes. This effect caused uncertainty among consumers. In the meantime, the properties of the material can be changed during production through additives ( compounding ) during sintering.
Quality assurance procedures
The thermogravimetric analysis (TGA) is a method in which is measured as the mass of a sample as a function of temperature changes. The sample is placed in an oven with a built-in balance. Measurements are made with constant or alternating temperature, which is referred to as a static or dynamic analysis.
Furthermore, the measurement can take place under a defined atmosphere in order to be able to determine the course of the reaction (e.g. the oxidation of a material).
Another analytical method is dilatometry . The change in length of a sample is determined as a function of the temperature. The differential thermal analysis was mentioned measures in one energy changes depending on the temperature.
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