The main vapor-phase coating processes
Vapor deposition belongs to the so-called dry coating category. Their aim is to improve the properties of materials by functionalizing their surface to improve :
- Their aesthetic appearance: different possible colours such as black, grey, brown, light yellow to gold, with a matt or glossy appearance.
- Their mechanical properties: wear resistance, friction behaviour and hardness.
- Their corrosion resistance
- Their electrical properties: conduction, insulation.
- Their optical properties: reflection, transmission, radiation detection.
- Their biocompatibility
These types of processes are complementary to thermal spraying in the sense that they allow the production of so-called thin films with a thickness ranging from a few nanometres to a few tens of micrometres.
All vapor deposition systems use the injection of plasmagen gases (except in the case of evaporation) in addition to which reactive gases (N2, O2) or precursors can be used (the precursors are often hydrocarbons such as C2H2, CH4, C6H6, C7H8 or liquids carried by a carrier gas such as argon or TMA and TMS). In addition, these processes are mostly used under partial or high vacuum, although the use of certain sub-categories of these techniques tends to be developped at atmospheric pressure.
Two main categories can be distinguished within vapor deposition:
– CVD processes (Chemical Vapor Deposition).
– PVD processes (Physical Vapor Deposition)
These two technologies use three essential components:
- a source of material to be deposited (metallic or ceramic target and precursors containing the elements to be deposited)
- a part to be coated on which the deposit from the source will condense (after recombination or not)
- a medium in which the material transfer takes place (seat of the physical or chemical phenomenon involved).
These two coating methods differ greatly in many aspects. The obtaining of vapor in CVD is essentially of a chemical type, whereas in PVD coating it is essentially physical. (It is however possible to have both chemical and physical phenomena at the same time when using certain sub-categories of processes such as so-called reactive PVD or PECVD (Plasma Enhanced Chemical Vapor Deposition)).
In CVD, gaseous species will react after thermal decomposition in a homogeneous or heterogeneous phase to form a third body which will constitute the coating that will be chemically bonded to the substrate surface.
In the case of PVD, the material that will constitute the deposit is initially in the form of a massive part with a precise geometry (cylindrical, wire-like) called a target or in the form of balls, chips, etc. and of different possible types: metallic, alloys, ceramics (mainly carbide, nitride, carbonitride). This material is then bombarded by ions resulting from the implementation of a plasma or evaporated by heating (other means of sublimation of the material can be implemented) within the vacuum chamber. Thus, atoms are pulled out of the target following the bombardment or evaporated and come to be deposited on the surface of the part to be coated. This type of coating can be assimilated to a condensation phenomenon on the surface of the part to be coated.
For PECVD, the vapor of the species to be deposited is generated from gaseous precursors. A plasma is used to decompose the gas in order to generate chemically active species which participate in the formation of the film. The thermal energy required for a CVD reaction is replaced by the electronic energy of the plasma. This allows materials to be deposited by chemical reaction at moderate temperatures (from room temperature to 250 °C).
Surface preparation and masking
The parts to be coated are prepared in such a way as to ensure that the good adhesion of the coating : it is necessary to have a substrate that is free of all pollution (grease, atmospheric residues) and free of surface oxides.
Some applications require a specific surface finish. For this, parts are polished using a dedicated polishing machine according to specific ranges in the case of simple geometries (test parts in particular) or by mass finishing (obtaining a mirror polish) for parts with complex geometries.
Prior to deposition, the substrates are first cleaned with solvents (ethanol, acetone) or ultrasonic detergents with or without temperature. Then, a heating and stripping phase is generally used during the process to clean the various surfaces.
Masking processes can be developed using either specific adhesives for use under secondary vacuum and temperature or by making a specific substrate holder integrating masking zones. This allows certain areas requiring no coating to be spared.
The structure and properties of the coatings depend on the parameters used during the development phase, which are adapted according to the elements defined in the specifications, as well as the nature of the substrates.
The choice between the various processes is based on numerous criteria such as the composition and properties of the desired coating, the nature of the substrates used and the desired thicknesses.