Vacuum plasmaspraying

  Vacuum plasmaspraying (VPS) is a technology for etching and surface modification to create porous layers with high reproductibility and for cleaning and surface engineering of plastics, rubbers and natural fibres as well as for replacing CFCs for cleaning metal components.

This surface engineering can improve properties such as frictional behaviour, heat resistance, surface electrical conductivity, lubricity, cohesive strength of films, or dielectric constant, or it can make materials hydrophilic or hydrophobic.

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Process

Vacuum plasma treatment is a process typically operating at 39–120 °C to avoid thermal damage. The process can induce non-thermally activated surface reactions, causing surface changes which cannot occur with molecular chemistries at atmospheric pressure.

Plasma processing is done in a controlled environment inside a sealed chamber at a medium vacuum, around 13–65 Pa. The gas or mixture of gases is energised by an electrical field from DC to microwave frequencies, typically 1–500 W at 50 V. The treated components are usually electrically isolated. The volatile plasma by-products are evacuated from the chamber by the vacuum pump, and if necessary can be neutralised in an exhaust scrubber.

In contrast to molecular chemistry, plasmas employ:

• Molecular, atomic, metastable and free radical species for chemical effects.
• Positive ions and electrons for kinetic effects.

Plasma also generates electromagnetic radiation in the form of vacuum UV photons to penetrate bulk polymers to a depth of about 10 µm. This can cause chain scissions and cross-linking.

Plasmas affect materials at an atomic level. Techniques like X-ray photoelectron spectroscopy and scanning electron microscopy are used for surface analysis to identify the processes required and to judge their effects. As a simple indication of surface energy, and hence adhesion or wetability, often a water droplet contact angle test is used. The lower the contact angle, the higher the surface energy and more hydrophilic the material is.

Changing effects with plasma

At higher energies ionisation tends to occur more than chemical dissociations. In a typical reactive gas, 1 in 100 molecules form free radicals whereas only 1 in 10⁶ ionises. The predominant effect here is the forming of free radicals. Ionic effects can predominate with selection of process parameters and if necessary the use of noble gases.

Uses

• Components for high-temperature fuel cells like SOFC.
• Coating of titanium or titanium alloys for implants.

See also

• Plasma spraying