Why is there no plasticity in ceramics




The designation Ceramics comes from ancient Greek. "Keramos“Was the name for clay and the dimensionally stable products made from it by firing. The production of ceramics is one of the oldest cultural techniques known to man. Its oldest use seems to have been in the semi-settled hunter cultures in the Upper Nile region. However, it originally owes its enormous spread to the expanded possibilities for storing (stockpiling) food as it was necessary with the Neolithic. Ceramic plays an essential role in the determination of Neolithic cultures. The source material clay, however, also offered incentives for artistic design very early on. For many decades, ceramics have gained great importance in technical applications.

Concept and subdivision

Today the term is broader. Ceramics are largely made of inorganic, fine-grained raw materials with the addition of water at room temperature and then dried, which are sintered to hard, more durable objects in a subsequent firing process above 900 ° C. The term also includes materials based on metal oxides. Ceramic is now increasingly used for technical purposes (technical ceramics) and manufactured in similar processes, but mostly at higher sintering temperatures. In the field of fiber-reinforced ceramics, silicon-containing organic polymers (polycarbosilanes) are also used as starting materials for the production of amorphous silicon carbide ceramic material. They transform from polymer to amorphous ceramic in a pyrolysis process.

A clear systematics of ceramic materials - such as with metal alloys - is difficult because there are fluid transitions with regard to the raw material composition, the firing process and the design process. Ceramic products are therefore often differentiated according to the aspects that are in the foreground, for example regional types of ceramics such as Westerwald ceramics, Bunzlau ceramics or majolica and faience, for technical ceramics according to the raw materials used (e.g. oxide ceramics and non-oxide ceramics ) or according to the intended use (e.g. functional ceramics, utility ceramics, sanitary ceramics and structural ceramics).

The division into coarse and fine ceramics is also common. The first includes the large group of building ceramics (for example building and roof tiles, sewer pipes); these products are thick-walled, often inhomogeneous, and often of a random color. Fine ceramics, on the other hand, are fine-grained (grain size less than 0.05 mm), of a defined color (e.g. white for household ceramics, tableware and sanitary ware); this also includes artistic products. Fine ceramics require considerably more care with regard to the preparation of the raw material, the shaping, drying and firing than is necessary in the manufacture of coarse ceramics. The properties of ceramic products are determined by the type and amount of crystals they contain and the glazing that acts as a bond (so-called glass phases). Ceramics are dimensionally stable, generally hard (there are exceptions: e.g. pyrolytic boron nitride (hexagonal) is flexible due to its layered structure) and heat-resistant.

Ceramic raw materials

Silicate raw materials

This area generally includes all raw materials which [SiO4]4 --Tetrahedra built into the crystal structure.

volume

Clay is a hydrous aluminum silicate. See also clay mineral. A distinction is made between the primary tone and the secondary tone.

The selection and mixing of the raw materials must meet the following requirements: good malleability of the mass, low shrinkage during drying and firing, high stability during firing, little or no discoloration of the end product.

Kaolins

Kaolin, also known as china clay, is a weathered product of feldspar. It consists largely of kaolonite, a hydrated aluminosilicate, accompanied by quartz sand, feldspar and mica. The latter components are removed by sludging and sieving, the end product must be as plastic as possible, dimensionally stable when drying and white after firing. To achieve the desired properties, kaolins of different origins are mixed (mineral dressing); In order to achieve good pouring behavior, plasticizers such as water glass and / or soda are also added.

Clay minerals and their mixtures

Clays and loams are also weathering products of feldspar and related minerals. The main components are illite, montmorillonite and kaolonite, the grain sizes range down to 0.002 mm. Depending on the intended use, these raw materials are divided into stoneware clays, earthenware clays, pottery clays and loam. Marl clays have a high content of lime, which has a strong liquefying effect. Since lead and tin glazes adhere very well to such shards, they are often used for tiles. Bentonites are a weathering product of volcanic origin, they have a very strong plasticizing effect even with small additions, improve the malleability and stability during the drying process. The high water absorption of the bentonites in the molding process results in enormous shrinkage even during drying. The enormous drying shrinkage causes cracks in the molding

Non-plastic raw materials

In comparison to lime, feldspars are also good fluxing agents, but the higher the firing temperature they cause the products to become more compact. The drying shrinkage is reduced, but the shrinkage during firing increases. As a leaning agent, quartz lowers drying and burning shrinkage, but worsens plasticity. Quartz is used as fine-grain sand or as ground gangue, it must be as pure as possible to avoid undesirable discoloration. Lime is used as slurried chalk or as ground limestone. As a leaning agent, it supports dimensional stability when drying, and when firing it acts as a flux. However, its sintering and melting points are close to one another, so there is a risk of deformation if the firing temperature is too high. Fireclay, as ground fired clay or slate, is a leaning agent that increases porosity and reduces drying and firing shrinkage. Magnesium minerals (talc, magnesite) give the products high resistance to temperature changes; they are preferred for electrotechnical products.

Oxidic raw materials

The oxide ceramics listed below are used to manufacture oxide ceramics that are used in many technical ceramics applications. Some are synthetic raw materials.

Alumina

Alumina ceramics are based on α-Al2O3, the corundum. They serve, for example, as grinding and polishing agents and are also used as a carrier material for integrated circuits. Refractory products can be made from sintered corundum or fused corundum. Aluminum oxide products can contain a glass phase; a high proportion of glass phase lowers the sintering temperature, but also the strength and temperature resistance.

To further increase the strength, zirconium oxide can be added: this particularly tough ceramic is known as ZTA (Zirconia toughened alumina).

Beryllium oxide

Sintered beryllium oxide (BeO) is used to make crucibles for chemical reactions at very high temperatures. Furthermore, electrically insulating, but highly thermally conductive chip carriers were produced from BeO in order to conduct the resulting thermal energy to a heat sink. Because of its high price and toxicity, BeO has increasingly been replaced by other materials, e.g. aluminum oxide or the more expensive aluminum nitride for heat dissipation and graphite for high-temperature laboratory vessels.

Zirconium oxide

Zirconium oxide

Aluminum titanate

Aluminum titanate

Non-oxidic raw materials

The non-oxide raw materials listed below are used to manufacture non-oxide ceramics, which have become established in many technical applications (see technical ceramics). In practice, all of these raw materials are artificially produced.

Silicon carbide

Silicon carbide (SiC) is mainly used as an abrasive and in plain bearings for chemical pumps because it is very hard and thermally and chemically resistant. Another use is rings in mechanical seals.

The most important production (Acheson process) takes place from quartz sand and coke at 2,200 ° C according to:

SiO2 + 3 C → SiC + 2 CO

It is comparable to the reduction of quartz to raw silicon; however, an excess of carbon is used. Manufacture from molten silicon and carbon is suitable for better shaping. Charcoal, which has been brought to the right shape beforehand, has proven itself here. Silicon can be absorbed through the pores and then react to form SiC. This creates a special form of silicon carbide, the so-called SiSiC (silicon-infiltrated SiC), which still contains a few percent of unreacted silicon, which has a negative impact on corrosion resistance.

SiC is rarely found in nature. It is then referred to as moissanite.

Silicon nitride

Silicon nitride

Aluminum nitride

Aluminum nitride

Boron nitride

Since boron nitride (BN) has a similar structure to graphite under normal conditions and is also very temperature-resistant (it only reacts with air at 750 ° C), it is suitable as a high-temperature lubricant. The diamond-like modification borazon is the hardest material after diamond.

The hexagonal crystal structure can be derived by alternately replacing the carbon in graphite with boron and nitrogen. In other words, it consists of planes of borazine rings condensed on all sides. Boron nitride is not electrically conductive like graphite because the electrons are more strongly localized on the nitrogen atoms.

At 60-90 kbar and 1,500-2,200 ° C, BN transforms into cubic borazon, which crystallizes in the zinc blende structure analogous to diamond. Borazon is just as hard as diamond, but more resistant to oxidation and is therefore used as an abrasive.

Boron carbide

Boron carbide (B.13C.3) is another very hard material (third after diamond and borazon). It is used as an abrasive for armor plates and sandblasting nozzles. The production takes place at 2,400 ° C from B2O3 and carbon.

Tungsten carbide

Tungsten carbide

manipulated ceramic raw materials

Originally called "metallic-ceramic raw materials" here. As a rule, ceramics have nothing to do with metallic materials. Since metals can be used in shaping processes similar to that of ceramic raw materials, this category was probably misleadingly named so. It is about dry pressing, slip casting, or plastic shaping using binders. This part of the production of metallic materials is called powder metallurgy. The finest grains are used.

  • Dry pressing: The ceramic powder is pressed dry in a steel matrix by pressure from a lower and an upper punch with pressures of over 1 t / cm². Cold isostatic pressing is also possible. The ceramic powder is filled into a rubber mold and pressed evenly from all sides by means of liquid pressure (usually oil). After shaping, the workpiece is burned or sintered. With the shaping process of cold isostatic pressing, compared to dry pressing, uniform properties are possible throughout the workpiece.
  • Slip pouring: The ceramic powder is brought into suspension with water and a suitable liquefier (electrolyte) at a low viscosity. It is possible to reduce the viscosity of the suspension by using peptization aids so that as much solid / volume as possible can be introduced into the suspension / slip. By pouring the slip into plaster of paris molds, the plaster of paris mold absorbing the water from the slip, a plastic skin is formed on the edge of the mold. When the "superfluous" slip (in the truest sense of the word) is poured off, the actual product remains in the mold. After the subsequent drying and sintering, the end product is manufactured.
  • Plastic shaping: By adding so-called plasticizers to the ceramic powder, the material is malleable. These plasticizers are often of organic origin. They cure through polycondensation or through polymerization, so that they cure through the complete reaction of the plasticizer and obtain sufficient strength. The shaping itself is done either by extrusion or by pressing in molds. The organic additives burn later in the sintering process. Addendum: This material combination is now also used in a more fluid form in rapid prototyping (3D printing).

Other additives

Flux in the glass industry

Plasticizers / flocculants

Organic plasticizers are, for example, glue, waxes, gelatine, dextrin, gum arabic and paraffin oil. They improve the malleability and burn during the firing process.

Liquefiers / peptizers

Other aids

Finely ground burnout agents such as sawdust and cork flour, starch, coal dust and styrofoam balls make the shards porous and light and can create interesting surface effects; they also burn in the fire. So-called porosity agents have the main purpose in the brick industry, whereby they reduce the density and the thermal conductivity of the bricks.

Processing of raw materials

In industrial ceramics production, after the components have been partially pre-fired, they are finely ground together in drum mills according to the recipe. After sludging with the addition of water, this is largely removed again in filter presses. The remaining filter cake is dried and ground again. In this form, the raw mass is either stored or immediately kneaded in machines with the addition of water and liquefying auxiliaries and, if necessary, deaerated. In addition, semi-wet and dry processing has recently gained importance in industrial production. In some parts of the pottery workshop, this process is still carried out by hand today. Since grinders are often not available, sludging is of great importance. The homogenization of the mass was achieved through laborious kneading. Today, machines are mostly available for this. The aim is to create a working mass that is as homogeneous, pliable and bubble-free as possible.

The shaping

In the case of heavy ceramics, the shaping takes place, among other things, by extrusion (e.g. tubes and rods) or by compression molding. Fine ceramics are formed (analogous to historical development) using the following processes:

 

  • Model
  • Extrude
  • Construction work from individual strands (e.g. for hollow vessels)
  • Plate technology
  • Rotating rotationally symmetrical hollow vessels on the potter's wheel
  • Pour low-viscosity mixtures into split plaster molds that soak up the water
  • Shaping on motor-driven disks in hollow forms with the help of templates (so-called screwing in and turning over)
  • Pressing and punching or milling
  • Injection molding and temperature inverse injection molding
  • Foil casting

Semi-dry and dry shaping has gained industrial importance because the drying times are very shortened and the products have a high degree of dimensional accuracy. However, impurities such as soluble salts cannot be separated off, which is why this process is not so well suited for the production of fine ceramics such as porcelain.


The following special processes are also used in technical ceramics:

  • CVD (Chemical Vapor Deposition, or in German: chemical vapor deposition)
For example, for the production of pyrolytic boron nitride, several gases (e.g. boron trichloride and ammonia, plus various auxiliaries) react under low pressure and high temperature and boron nitride is deposited in layers on a graphite cylinder, which defines the shape.
  • CVI (Chemical Vapor Infiltration, or in German: chemical gas phase infiltration). It is
the shape, however, is already given by a part to be infiltrated (preprag) e.g. a fabric made of carbon fibers. This is then infiltrated with methane, which pyrolyzes under high temperature, exclusion of air and an argon atmosphere and leaves graphite in the molded part.
  • PVD (Physical Vapor Deposition, or in German: physical vapor deposition)

The drying

After shaping, the blank is moist through

  1. mechanically trapped water in the cavities
  2. physicochemically bound water (adhesion, capillary water)
  3. chemically bound water (crystal water)

The drying speed depends not only on the surrounding climate, but also on the recipe of the raw mass. In order to keep the drying speed low to avoid cracks, the blanks can be covered. Industrial drying takes place in air-conditioned rooms. The water mentioned under no. 2, but in particular under no. 3, was only driven out by the fire.

The three stages of drying

  1. Hard as leather: The broken glass can no longer be deformed, but it still has enough moisture to decorate it.
  2. Air dry: The broken glass no longer gives off any moisture at room temperature and feels cool to the touch.
  3. Ready to burn: The broken glass no longer feels cool, but proves to be conditionally absorbent. (Experiment: tongue sticks to broken glass.)

The burning process

The firing process (raw or biscuit firing) - called sintering - is very demanding and requires careful control. In the resulting “body”, crystals close together at the grain boundaries (crystal growth) and are cemented (if contained) by glassy parts. The proportion and type (grain size distribution, textures, etc.) of the crystal and glass phase as well as the pores determine the properties of the fired product. The temperatures used (up to around 1,400 ° C; also considerably higher for special ceramics) depend on the raw mixture, i.e. on the product to be produced, and in many cases must be varied during the firing process (temperature profile). In addition, it is often important that the process takes place temporarily in a reducing atmosphere (e.g. avoiding yellowing due to iron contamination in white dishes or in sanitary ware). Chamber, tunnel, ring and roller ovens are used (for flat products such as tiles). The oven types are divided into periodic and continuous ovens. Electric stoves or stoves fired with fossil fuels are used in craft businesses. A distinction must be made here between open systems in which the combustion gases (with different flames) come into direct contact with the goods and muffle furnaces in which the combustion gases indirectly heat the goods.

Glazes

Glazes are thin, glass coatings that meet two essential requirements. On the one hand, they make the porous clay body almost waterproof and give it an easy-to-clean surface. On the other hand, they enable a varied, decorative design of the ceramics. Glazes can be colored, transparent or opaque, glossy, semi-matt or matt. They can be soft and low-melting (max. 1,000 ° C) or hard and high-melting (over 1,200 ° C). Depending on their chemical composition, a distinction can be made between, for example, borosilicate, feldspar, salt and lead-containing glazes. The glazes are often (e.g. pottery) only applied after the biscuit firing of the goods (dipping, spraying, brushing, stamping) and then glazed in a new firing process (smooth firing), which, however, must be below the firing temperature of the blank.

Orienting classification of ceramic bodies

  1 earthenware

1.1 Building ceramics

  • Not fireproof. Bricks, shaped stones (1,200 - 1,350 ° C), clinker, drainage pipes (1,000 - 1,150 ° C), roof tiles

1.2 Refractory masses

  • Fireclay bricks for stoves, ovens (1,300 ° C). Sillimanite, magnesite, e.g. for lining industrial furnaces in the iron and cement industry (1,500 ° C)

1.3 Other earthenware

1.3.1 Earthenware

  • Pure white to ivory colored, porous body with a transparent glaze. Raw spirit 1,150 - 1,250 ° C; Glaze firing> 960 ° C, but below the raw firing temperature; mostly translucent or colorless.

1.3.1.1 Limestone or soft stone

  • Clay, kaolin, quartz, lime. Firing temperature 1,120 - 1,150 ° C. Particularly suitable for underglaze painting.

1.3.1.2 Feldspar or hard stone

  • Clay, kaolin, quartz, feldspar. Firing temperature 1,220 - 1,250 ° C. Frost-proof wall panels, sanitary ware, dishes.

1.3.1.3 Mixed stone

  • Clay, kaolin, quartz, lime, feldspar. Wall plates, dishes.

1.3.2 Pottery

  • Clays rich in flux, up to 40% lime

1.3.2.1 Unglazed pottery

  • Yellow to red fired weatherproof ceramic. Terracotta (addition of chamotte or brick flour); Figures, everyday objects and ornaments, flower pots.

1.3.2.2 Glazed pottery

1.3.2.2.1 Majolica

  • A reliable differentiation between majolica and faience is not possible because these terms are used alternately in the literature. Originally: colored porous body with an opaque colored glaze.

1.3.2.2.2 Faience

  • A reliable differentiation between majolica and faience is not possible because these terms are used alternately in the literature. Originally: white, yellow-gray or light-red-brown, porous body, white opaque glaze.

1.3.2.2.3 Other pottery

  • White, ocher to red-brown porous body with a matt, fine-grain break. Firing temperature 1,000 - 1,200 ° C. Pottery molded by hand (potter's wheel, casting process) or by means of a press. Crockery, tools for home and garden, decorative ceramics.

2 sintered products

  • Clay or kaolin and optionally quartz and / or feldspar, lime. Non-crystallized, dense masses, not translucent or only translucent at the edges, high strength

2.1 stoneware

  • Dense, not translucent. Shards also colored, but mostly light-colored.

2.1.1 Coarse stoneware (not white-firing)

  • Firing temperature 1,100 - 1,400 ° C. Often clay or blending glaze. Clinker, tiles, troughs, sewer pipes, vessels for chem. Industry.

2.1.2 Porcelain stoneware (white or light-burning, similar to porcelain)

  • Clay, quartz, feldspar. Firing temperature 1,250 - 1,300 ° C (joint raw and glaze firing). Similar to porcelain. Tableware, sanitary ware, chemical devices, mosaics, ornamental vessels. Transitional form to porcelain: porcelain, semi-porcelain, vitreous china.

2.2 porcelain

  • Hard-paste porcelain: dense, transparent body. Kaolin, quartz sand, feldspar. Raw firing 900 ° C, glaze firing 1,400 ° C. Utensils and ornamental crockery Soft-paste porcelain has a similar composition, but a lower temperature for the glaze firing. Preferred for decorative sculptures.

3 Ceramic special sizes

3.1 High-temperature special dimensions (also called mixed ceramics)

  • Highly refractory oxide ceramics with small additions of various metals. The toughness of metals is combined with the corrosion resistance and fire resistance of ceramics. Use as turbine blades or as cutting tools.

3.2 Electrotechnical special dimensions

As Heavy pottery the classes 1.1; 1.2; 2.1.1. All others count towards the Fine ceramics: Selected raw materials, careful preparation of the mixtures, more complex shaping, partly by hand.

literature

  • Small encyclopedia technology, Bibliographisches Institut, Leipzig, 1972
  • Lueger Lexicon of Technology, here Materials and material testing - basics (four volumes), Rowohlt Taschenbuch-Verlag, Reinbek, 2003, ISBN 3499190087
  • P. Rada: The technique of ceramics, Dausien 1989, ISBN 3-7684-1868-5
  • Sven Frotscher: dtv-Atlas ceramics and porcelain, Munich 2003, ISBN 3-423-03258-8
  • R. Schreg: Pottery from southwest Germany. A help for the description, determination and dating of archaeological finds from the Neolithic to modern times. Teaching and working materials on archeology in the Middle Ages and modern times (Tübingen 1998. 3rd edition 2007)
  • Technical Ceramics Breviary, Fahner Verlag, Lauf a.d. Pegnitz, 4th edition 2003, ISBN 3-924158-77-0, Publisher: Verband der Keramischen Industrie e.V., web link see below

See also

  • Artistic ceramics
  • Kalkspatz
  • pottery
  • Raku

Categories: Powder | Material | Ceramic material