Under coating ( English coating ) is in the manufacturing technology , a main group of the manufacturing method according to DIN 8580 understood that for applying a firmly adhering layer of shapeless material on the surface of a workpiece to be used. The corresponding process, as well as the coated layer itself is also called coating or coating , respectively. A coating can be a thin layer or a thick layer as well as several coherent layers; the distinction is not precisely defined and is based on the coating process and application purpose. The coating processes differ in the type of layer application in chemical , mechanical , thermal and thermomechanical processes .
The associated job description is the process mechanic for coating technology .
Almost all known (solid) materials can be used as coating materials and carrier materials (substrates), i.e. metals, insulators, semiconductors, crystalline or amorphous materials, flat textiles (fabrics, fleeces, knitted fabrics), foils, etc. v. m. However, not all materials can be applied to any substrate. For example, layer stresses can reduce the adhesion of the coating so that it flakes off partially or completely. For good adhesion of the layer to the substrate , a trouble-free process , uniform application and good resistance of the layer to environmental influences, the workpieces are usually mechanically and / or chemically pretreated before coating in all coating processes. The characteristics of the interface depends on the coating process and the substrate pretreatment (e.g. grinding , microblasting , pickling , silanizing or silicating, etc.).
Coating processes can be differentiated according to
- the initial state of the material to be applied (see table),
- the type of carrier material (substrate) and
- the type of adhesion promoter (primer).
In materials technology , in addition to alloying , doping and surface structuring, the coating method is important in order to influence the physical, electrical and / or chemical properties of metallic or semi-metallic materials.
The coating can take place, for example, by vapor deposition or spraying on a further material or by immersion in appropriate (galvanic) baths. Some processes also include heat treatment such as B. the nitriding process , in which the so-called connecting layer grows out of the surface. The various coating processes, such as methods for structuring and modifying surfaces, are part of surface technology or heat treatment .
Properties and their measurability
Various properties are used to assess the quality of a coating, e.g. B:
- Adhesive strength
- Layer thickness
- Corrosion resistance
- Seal qualifications
There are a few or many different methods for measuring the individual properties. A distinction is made between destructive and non-destructive methods.
Measurement of adhesive strength
There are many methods of measuring adhesive strength. A widely used is the cross-cut test (ISO 2409). For this purpose, the coating is scratched crosswise and thereby divided into individual chessboard-like segments. Then an adhesive tape is stuck on and pulled off again. The adhesive strength of the coating is then determined on the basis of the number of torn segments.
Parameters that determine the adhesive strength of a layer on the substrate:
- Condition of the substrate surface:
- Activation energies for surface and volume diffusion
Measurement of the layer thickness
There are also a number of measuring methods for determining the layer thickness . The eddy current and magnetic induction methods ( magnetic induction method ) are common in the field of painting and electroplating of metallic base materials . Other measuring methods are: X-ray fluorescence methods, microscopic microsection analysis (etched section), weighing methods, ellipsometry and the like. a.
An important task of coatings is to protect metals against corrosion. The adhesive strength of the coating on the substrate is decisive for corrosion protection, which is why special pretreatments are usually carried out or adhesion promoters are used before coating. The coating agents themselves can also contain corrosion-inhibiting substances.
A large number of regulations are used for corrosion protection in steel construction, for example
- DIN EN ISO 2063 for thermal spraying with zinc
- DIN EN ISO 1461 for hot-dip galvanizing (batch galvanizing)
- DIN 55633 for powder coating systems
- DIN EN ISO 12944 for wet coating systems.
The corrosion resistance can be tested in free field tests. Since these tests are very tedious, they are usually replaced by various shorter tests that are intended to simulate certain loads more quickly. Spray tests can test the susceptibility to corrosion by salts, with test periods of the order of 1000–3000 hours usually being set. A connection between the corrosion that occurred in the salt spray test and the corrosion to be expected under real conditions is rather rare, as the corrosion mechanisms usually differ. The ASTM Committee G-1 for the Corrosion of Metals, responsible for the ASTM B 117 salt spray standard, has therefore drawn up the following resolution: “The ASTM Committee G-1 for the Corrosion of Metals confirms that the results of salt spray tests according to ASTM B 117 rarely match the behavior in natural surroundings. ”In climatic tests, the test objects are exposed to changing temperatures and humidity in a climatic chamber ; typical test periods are in the range of 7-30 days. The resistance to UV radiation can be tested in special radiation chambers, some of which also simulate weathering (e.g. rain); typical test periods are 1000 hours and more. The results are then e.g. B. rated according to EN ISO 6270-2 (formerly DIN 50017).
In the case of base metals such as aluminum , copper or zinc, corrosion protection occurs through the reaction of the metal with oxygen; this passive oxide layer is also known as the passive layer or patina. The passivating layer prevents the further reaction of the oxygen with the metal surface. Since metals need an electrolyte and oxygen to corrode, the metal can be protected from further corrosion by the passive barrier of the oxide layer.
- Flexible coating
- Manufacturing process according to DIN 8580
- Surface protection systems
- Edge alignment
- DIN EN ISO 14713-1 - Zinc coatings - Guidelines and recommendations for protecting iron and steel structures from corrosion - Part 1: General design principles and corrosion resistance (ISO 14713-1: 2009); German version EN ISO 14713-1: 2009, page 25