Material list

Metals


Steel:

Steel is an alloy whose major component is iron with a carbon content that is usually less than 2 %. Varying the amount of alloying elements (such as chrome or manganese) and their distribution in the steel controls qualities such as the hardness, elasticity, ductility and tensile strength of the resulting steel. There are various procedures for manufacturing steel which, however, will not be discussed here. RETSCH use the following types of steel for their instruments and accessories:

  1. Stainless steel
  2. Hardened steel (“chrome steel”)
  3. Steel for grinding without heavy metal contamination
  4. Manganese steel (“precision-cast manganese steel”)

Stainless steel

The term “stainless steel” refers to corrosion-resistant steel. The corrosion resistance of this type of steel is due to a very thin, not visible oxidic protective film which is formed when the chrome content is >12 %. The corrosion resistance increases with the chrome content. However, if the chrome content is more than 20 %, the steel loses its hardenability and thus its wear resistance. Constant subjection to mechanical stress, as is the case with grinding tools, can lead to the destruction of the protective film and can also roughen the surface. This can result in cross contamination as well as the forming of corrosive spots on the material. These are not a sign of low quality but are caused by wear. To remove corrosion stains, dirt or rust from the grinding tools standard brushes or cleaning agents for metals can be used. After wet cleaning it is recommendable to clean the parts with isopropanol or acetone and to dry them thoroughly.

Hardened steel (“chrome steel”)

Hardened steel, just like stainless steel, belongs to the group of chrome steels. Its chrome content however is not more than 12 %. Consequently, corrosion resistance is not the strong point of this type of steel but its great hardness.

Manganese steel (“Precision-cast manganese steel”)

Manganese steel is used for the breaking jaws in jaw crushers as it is ideal for wear-resistant tools which are subjected to pressure.


Tungsten Carbide (Hard Metal):

Tungsten carbide is a hard metal. Pure tungsten carbide, a mixed crystal of tungsten and carbon, is very brittle. Therefore, 6 - 10 % of cobalt is added which increases the toughness of the material and reduces abrasion. The appropriate material composition of this hard metal depends on the technical requirements of the application. Tungsten carbide is produced by sinter ing tungsten and carbon. The components are heated under high pressure and carbon is incorporated into the crystal structure of the tungsten. The resulting carbide increases the melting point andthe hardness of the metal.The extreme hardness (even at high temperatures) and wear resistance are characteristic for hard metals. Depending on the composition, hard metal can be as hard as diamond. Therefore, it is frequently used for chipping tools and non-cutting moulding tools. It is also used for tools which are subjected to frictional abrasion. These include grinding tools for mechanical size reduction as used in RETSCH mills and crushers.


Cast Iron / Malleable Iron:

Cast iron is a ferrous alloy with a carbon content of >2 % and a silicon content of >1.5 %. In the so-called grey cast iron carbon is present in the form of fine irregularly distributed graphite flakes. The term “cast iron” refers to the production process of this material which involves casting a metal melt into forms. As opposed to cast iron malleable iron is tempered for several days at a temperature of approx. 1000 °C in oxidizing atmosphere.


Titanium:

Titanium (Ti) is a chemical element with the atomic number 22. It belongs to the group of transition metals and is very strong despite its relatively low density. Titanium forms a protective oxide coating which makes it resistant to corrosion. In its purest form it is ductile. In order to improve its mechanical properties (embrittlement), other elements such as oxygen and nitrogen can be added. Titanium is one of the most abundant elements in the Earth's crust but it mostly occurs in low concentrations in the form of oxides. It is mainly extracted from the minerals rutile (TiO2) and ilmenite (FeTiO3).



Ceramics

Zirconium Oxide:

The main component for the production of zirconium oxide (ZrO2) is the mineral zircon (ZrSiO4). By melting it with coke and lime (reduction of SiO4) ZrO2 is won on an industrial level. The resulting powder is mixed with aggregates and dry pressed into the desired form. The formed part is transferred to the raw product by sintering and is then abraded and polished according to its intended purpose. The sintering process can be carried out under atmospheric pressure as well as under high pressure. The formed parts receive their actual properties during the sintering. Zirconium oxide occurs in different crystal modifications, depending on the temperature. These have different volumes. By adding yttrium oxide, zirconium oxide is prevented from turning into the modification which is more stable at room temperature – it is thus kept in the partially stabilized form. If there's a micro crack, the zirconium oxide changes at that specific point by volume extension into the more stable modification. This effect causes the crack to close immediately. Zirconium oxide is traditionally used as a refractory ceramic. Due to its properties and high bio-compatibility zirconium oxide is increasingly used as a ceramic for implants and dental prostheses. In addition, there are many technical applications for partially stabilized zirconium oxide. It is highly resistant to thermal, chemical and mechanical influences which makes it very suitable for grinding tools.


Sintered Aluminium Oxide:

Sintered aluminium oxide (Al2O3) is a synthetic ceramic material.In nature Al2O3 occurs in form of corundum, the second hardest mineral after diamond. Due to its hardness Al2O3 often is used as abrading medium. When producing sintered aluminium oxide, pressed alumina powder is fired to 1300 °C. The powder is an intermediate product in the process of winning aluminium from bauxite. The main reasons for using sintered aluminium oxide in grinding tools are its considerable hardness and purity but also the reasonable price. Due to the relatively low density of sintered aluminium oxide and the low energy input which results from this, the sample material is hardly subjected to thermal stress during grinding. This can be an advantage with sensitive materials such as plants.


Hard Porcelain:

Hard porcelain is a silicate ceramic which is composed of the raw materials kaolin (non-ferrous clay), potash, feldspar andquartz. It consists of 25 - 70 % Al2O3 and 30 - 75 % SiO2. Hard porcelain is fired to a temperature of approx. 1400 °C. In the middle of the 19th century, porcelain was first used as engineering ceramics, mainly in the electronic industry where it remains an important material until today (e.g. as insulators).


Silicon Nitride:

Silicon nitride is a non-oxidic ceramic which stands out byextreme fracture toughness and wear resistance. The material does not occur naturally and is produced in different ways. One of the most popular manufacturing methods is reaction bonding: a compact of silicon powder is heated in a nitrogen atmosphere. The reaction in which the silicon powder is forming nitrides starts at 1200°C. Since silicon nitride decomposes at standard pressures and temperatures above 1700°C the powder has to be sintered under very high pressures. Silicon nitrides are characterized by a homogeneous grey colour. By adding sinter additives (AL2O3 and Y2O3) the material hardness can be even increased whilst tensile strength remains. These ceramics are then termed SiAlONs. Because of its high abrasion resistance silicon nitride is particularly well suited for the use in bearing technology and as cutting tool.



Other Material

Agate (natural stone):

Agate is a quartz-mineral (silicon dioxide SiO2) and belongs to the group of semi-precious stones. It is formed in cavities of volcanic rocks through precipitation of SiO2 from solutions. Agate was named from the river “Achates” (today “Drillo”) in Sicily where it was found for the first time. Today's habitatsinclude Bohemia, Brazil, Sicily and Uruguay. At the beginning of the 20th century agate was first used as a technical material. Pharmacists and homeopaths soon recognized that agate hand mortars offered considerable advantages for the trituration of powders and pastes due to their pure SiO2 composition and their wear resistance. Agate does not cause abrasion and is easy to clean due to its smooth surface which also helps to avoid cross contamination. Theseproperties distinguish agate from brass or hard porcelain mortars. The procedures for processing agate have improved through the years by using diamond tools which finally allowed for its use in a variety of grinding instruments. As a consequence, hard porcelain is gradually losing its significance as a material for mechanical size reduction.


Glass:

Glass is an amorphous, i.e. not crystalline, substance. It is usually produced from a melt which cools down very rapidly without sufficient time for a regular crystal lattice to form. Glass mainly consists of SiO2. The properties of glass can be modified with the help of various additives. Laboratories mostly use borosilicate glass due to its high chemical and temperature resistance. The chemical resistance is determined by the contents of boron oxide which amounts to approx. 13 %. Borosilicate glass is also known as “Jenaer Glass” or “Duran Glass”.


PTFE (Plastic):

PTFE or Teflon is a thermoplastic (polymer). The abbreviation PTFE stands for polytetrafluoroethylene, a fluorine-carbon compound. The term “Teflon” is a trade mark of the company DuPont. PTFE is practically inert which means that it is resistant to many chemicals such as acids, bases, alcohol and benzine. Therefore, PTFE coatings are often used as anticorrosive against aggressive substances. Further applications of PTFE are non-stick coatings (for pots and pans), high performance fabrics (Gore-Tex), sealing materials and medical technology (implants). PTFE was developed in the 1930s while searching for a new cooling agent for refrigerators.


Polypropylene (Plastic):

Polypropylene (PP) is a thermoplastic resin (polymer) which is produced by polymerization from the hydrocarbon propene. Polypropylene is frequently used in the laboratory sector. It isresistant to almost any organic solvent and fat as well as acids and bases.


Polycarbonate (Plastic):

Polycarbonate is a thermoplast (polymer), which is produced by the esterification of carbonic acids with diols. Due to its scratch and shock resistance polycarbonate is used e. g. as a protective film for data layers on CDs and DVDs. It is resistant to mineralic acids and most unpolar solvents, but not to acetone.