A widely diversified process range using physical, electrochemical and chemical coating processes makes it possible to use application and requirement-appropriate coatings. A proper comprehension of the system forms the basis for a correct choice.
PVD / PACVD
PVD (Physical Vapour Deposition) is the deposition of ceramic hard coatings in a plasma process. The metallic components of the layers consist of titanium, chromium or aluminium. Under the influence of a plasma and at temperatures of 150 to 500°C, compounds can be produced that contain nitrogen and carbon and thus form a hard ceramic. The hardness of the layers produced in this way is in the range of 1000 to 3500 HV, and depending on the application, the layer thickness is between 1 and 10 µm.
In sputtering (also called cathode sputtering), particles are removed from the surface by ion bombardment. This process can be used to remove oxides or water from the surface, for example. In thin-film technology, this physical process is also used to atomise material from the target and thus transfer it into the gas phase. The resulting gaseous material is then fed onto the substrate to be coated and condenses there. The coating thickness is controlled by the coating time. The coating temperature here is below 200°C.
Depending on the reactive gas composition, PVD coatings produce nitrides and carbides or mixtures of the two as layers, which offer very good wear resistance in addition to high hardness.
DLC (diamond like carbon) coatings can be produced by combined sputtering and PACVD (Plasma Assisted Chemical Vapour Deposition).
These coatings combine a very low coefficient of friction with good wear resistance against abrasion, adhesive wear and surface fatigue. Targeted process control here produces a spectrum of surface properties such as sliding friction, wear and wetting and adhesion behaviour.
One of the most important process groups in coating technology is electroplating. The history of electroplating, as it is colloquially known, goes back to the Italian physician Luigi Galvani, who discovered galvanism, named after him, on 6 November 1780.
It includes all processes for the electrochemical deposition of electrically conductive layers (usually metals) on a metallic substrate. For this purpose, the substrate is immersed in an electrolytic bath and an electrical voltage is applied. An electric current flows in the resulting electric circuit, which is formed in the electrolyte primarily by the movement of positive metal ions. When the voltage is applied, the dissolved metal ions move to the negative pole (cathode), the substrate to be coated, and deposit there.
Technically important functional coatings are hard chrome (including repair chrome plating, mass chrome plating), pearl chrome, bright chrome, nickel, nickel sulphamate, tin, silver, hard anodising, anodising and also combinations of these.
A wide range of materials can be used as substrates. However, the most important prerequisite is at least a low electrical conductivity at the surface. In general, the applied layer thickness depends on the current intensity used and the process duration as well as the bath composition. The deposition in holes and trenches can also be influenced via bath additives.
In addition to the coating processes, one may also mention ablative processes here. This includes electropolishing as well as pickling, in which the surface layers are removed and subsequently processed. In addition to coating, passivation is also of great importance here.
Electroless nickel coatings are deposited without external current, i.e. without the supply of electrical energy from a suitable bath electrolyte.
These electroless nickel coatings produce uniform coating thicknesses that are characterised by high hardness and wear resistance. While other processes can only be used to a limited extent for components with complicated shapes and holes, cavities and undercuts, electroless nickel plating is excellently suited for coating them.
With specific pre-treatment methods, aluminium alloys as well as ceramics and glass can be coated in addition to steels and copper alloys.
A special class of these electroless coatings are the dispersion coatings, whereby solid particles are introduced into a metallic matrix in order to suitably influence the coating behaviour. The particles used here are diamond, carbides, PTFE and nitrides.
Hybrid coatings are the combination of different technologies, e.g. galvanic, chemical or PVD processes for the deposition of a multi-phase coating package with special functional properties.
These properties can be adapted to the respective requirement profile and generate added value that would not be achievable with monotechnological processes.