Coating
Under coating (english coating) is understood in the manufacturing technology a major group of the manufacturing method according to DIN 8580, which are used for applying an adherent layer of formless substance on the surface of a workpiece. The corresponding process, as well as the coating layer itself is also referred to as a coating. With a coating, it may be a thin layer or a thick layer as well as several self contiguous layers, the distinction is not well defined and is based on the coating method and application. The coating methods differ by the type of coating application in chemical, mechanical, thermal and thermo-mechanical process.
Materials
As coating materials and support materials (substrates), almost all known ( fixed ) can be used materials, i.e. metals, insulators, semiconductors, crystalline or amorphous materials, fabrics, etc. ( wovens, nonwovens, knits ) films. However, can not be applied to any substrate materials all materials. For example, reducing the adhesion of the coating layer voltages, so that these peels off partially or completely. For a good adhesion of the coating to the substrate (substrate), a trouble-free process, uniform application and good resistance of the layer against environmental influences, the workpieces usually before coating are pretreated mechanically and / or chemically in all coating processes. The values of the interfacial layer (interface) is the coating method and the substrate pre-treatment ( for example, grinding, shot-peening, etching, silanization or silicatization etc. ) dependent.
Method
Coating processes can be distinguished by
- The initial state of the material to be applied (see table),
- The type of carrier material ( substrate) and
- The type of bonding agent (primer ).
- Chemical vapor deposition (also CVD of Engl. chemical vapor deposition) Flame coating (C- CVD), an atmospheric pressure CVD method
- Sputtering ( sputtering )
- Painting or Färbeln
- Paint
- Spin coating
- Spray-painting
- Thermal spraying
- Plasticizing
- Dip coating ( anodic or cathodic )
- Hot dipping
- Enamel
- Slot- die coating
- Blade coating
- Dip coating
- Spray
- Roll coating
- Multiple coating by cascade or curtain coating
- Electroplate
- Chromating
- Galvanize
- Phosphating
- Tin
- Sol-gel
- Electroless Nickel
- Lüstersud
- Mechanical galvanizing
- Thermal spraying
- Powder coating
- Solder coating
- Plasma - arc welding
- Hardfacing
- Fluidized bed
Applications
In addition to the materials engineering alloy, doping and surface structuring the method of coating is important in order to influence the physical, electrical and / or chemical properties of metallic or semi -metallic materials.
The coating can be carried out for example over baths or spraying deposition of a further material or by immersing it in appropriate ( galvanic ). Some methods include an additional thermal treatment such as nitriding, in which the so-called link layer grows from the surface. The different coating processes, as methods for patterning and modification of surfaces, part of the surface treatment or heat treatment.
Properties and their measurability
To assess the quality of a coating various properties are used, eg:
- Adhesion
- Layer thickness
- Corrosion resistance
To measure individual characteristics, there are some in each case to many different methods. We distinguish between destructive and nondestructive methods.
Measurement of bond strength
There are many methods for measuring adhesion strength. A widely used is the cross-cut test ( ISO 2409 ). For this purpose, the coating is scratched crosswise and thereby divided into a checkerboard -like individual segments. An adhesive tape is stuck and peeled off again. The adhesive strength of the coating is then determined based on the number of lost segments.
Parameters determining the adhesion of a layer on the substrate:
- Purity
- Roughness
Measurement of coating thickness
To determine the layer thickness, there are also a variety of measurement methods. Uniform in the field of painting and electroplating of metallic base materials are the eddy current and magnetic induction method ( Magnetic induction method). Other measurement methods are: X-ray fluorescence techniques, microscopic micrograph analysis ( etched section), weighting, ellipsometry and Others
Corrosion protection
An important task of coatings is the protection of metals against corrosion. Decisive for the corrosion protection is the adhesion of the coating to the substrate, therefore mostly special pre-treatment is carried out, or are used prior to coating adhesion promoter. The coating compositions may also contain corrosion-inhibiting substances themselves.
The corrosion protection of steel structures by protective paint systems is treated in the standard EN ISO 12944 comprehensively.
The corrosion resistance can be tested in open- field tests. Since these experiments are very lengthy, they are usually replaced by various shorter tests that simulate accelerated the specific stresses. Spray tests the corrosion susceptibility can test by salts, mostly trial periods on the order 1000-3000 hours are recognized. In air tests, the test objects are exposed in a climatic chamber changing temperatures and humidities; typical test periods are in the range of 7-30 days. The resistance to UV radiation can be tested in special irradiation chambers (eg rain ) simulate the partially also weathering; typical test periods are in this case 1000 hours or more. The results are then evaluated, for example, according to EN ISO 6270-2 ( formerly DIN 50017 ).
The corrosion protection is carried out at base metals such as aluminum, iron or zinc by application of a passivation. Here, the applied passivation layer separating the metal from the atmosphere, and thereby prevents the corrosion of the material.