Coating by plasma polymerization with low-pressure plasma

Coating by plasma polymerization with a low-frequency, or high-frequency generator

Coating by plasma polymerization with microwave plasma

1. How does coating by plasma polymerization work?

For coating with low-pressure plasma, monomers are introduced (either gaseous or liquid), which then polymerize under the influence of the plasma. The coating thicknesses achieved with plasma polymerization are in the range of one micrometre. The adhesion of the coatings on the surface is very good.

The process is much more complex than for activating and degreasing.

For example, barrier coatings can be produced in fuel tanks, scratch-resistant coatings on headlights and CDs, PTFE-like coatings, hydrophobic coatings, etc.

There are three coating methods which have become established on a large scale:

Hydrophobic coatings	Monomers: e.g. hexamethyldisiloxane (HMDSO)
PTFE-like coatings	Monomer: fluorine-containing process gases - see also epilamization
Hydrophilic coatings	Monomers: Vinyl acetate, hexamethyldisiloxane defined mixed with oxygen (significantly more HMDSO than O₂)

Application	Group	Material	Type of gas	Pressure [mbar]	Performance [%]	Time [Min]
Coating process	Metal	Aluminium	HMDSO + O₂	0.1 - 0.5	50 - 100	1 - 60
		Stainless steel	HMDSO + O₂	0.1 - 0.3	50 - 100	1 - 60
	Plastic	Plastic	HMDSO + O₂	0.1 - 0.5	50 - 100	1 - 60
		Plastic	C₄F₈	0.1 - 0.5	50 - 100	1 - 60
	Glass	Glass	C₄F₈	0.1 - 0.5	50 - 100	30 - 120
	Special process
	Special process	1. Activation	5 minutes O₂
		2. Coating process	5 minutes HMDSO
		3. "Etching"	12 seconds O₂
		4. Coating process	5 minutes HMDSO
		5. "Etching"	12 seconds O₂
		6. Coating process	5 minutes HMDSO

2. What effects does the coating of metals by means of plasma polymerization permit?

The coating of metals by plasma polymerization allows various effects such as one to several weeks sustained activation and functional coatings in addition to the application of decorative coatings.

A sputter system is generally suitable, especially for metals.

Group	Material	Type of gas	Pressure [mbar]	Performance [%]	Time [Min]
Metal	Aluminium	HMDSO + O₂	0.1 - 0.5	50 - 100	1 - 60
	Stainless steel	HMDSO + O₂	0.1 - 0.3	50 - 100	1 - 60

(The process parameters including performance and time must be adjusted to take account of the equipment specification (generator type and strength, electrode assembly) as well as the material properties of the workpiece).

Mix the gases as follows for a durable hydrophilic coating with HMDSO: HMDSO : O₂ = 1:4

Mix the gases as follows for a durable hydrophobic coating with HMDSO: HMDSO : O₂ = 4:1

3. Can plastics be coated by plasma polymerization?

Plastics can generally be coated by plasma polymerization in an uncomplicated way.

So CDs and DVDs, for example, can be given a scratch-resistant coating without their quality suffering.

PTFE-like coatings can be applied to increase the low-friction characteristics of the treated products. Furthermore, it is therefore possible to attach functional groups to the plastic surface (e.g. amino groups for bio-analytical applications).

Group	Material	Type of gas	Pressure [mbar]	Performance [%]	Time [Min]
Plastic	Plastic	HMDSO + O₂	0.1 - 0.5	50 - 100	1 - 60
	Plastic	C₄F₈	0.1 - 0.5	50 - 100	1 - 60

The polymerization can also be carried out in several stages:

Example:

1.	Activation	5 minutes O₂
2.	Coating process	5 minutes HMDSO
3.	"Etching"	12 seconds O₂
4.	Coating process	5 minutes HMDSO
5.	"Etching"	12 seconds O₂
6.	Coating process	5 minutes HMDSO

Then possible curing of the coating with a few seconds of oxygen plasma. In this step however, the coating becomes hydrophilic!

4. Can glass and ceramics be coated by plasma polymerization?

The only difficulty when coating glass and ceramics is to prepare the surface accordingly (see Activation and Etching). Once this difficulty has been overcome, there is nothing to prevent applying a variety of coatings. The respective adhesion of the coating must be checked in each case. If "incompatibility" occurs between the coating and substrate material, intermediate layers may possibly have to be applied as a bonding agent. An example of a successful hydrophobic and oleophobic coating is shown in the image.

Group	Material	Type of gas	Pressure [mbar]	Performance [%]	Time [Min]
Glass	Glass	C₄F₈	0.1 - 0.5	50 - 100	30 - 120

5. Can textiles be coated by plasma polymerization?

Textiles can be very well coated in plasma. The difficulty remains in achieving long-term stability of the coatings against surfactants. Hydrophobic coatings can be achieved by the application of fluorine-containing gases/monomers.

6. Which applications are possible?

For more information, see "Applications".

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