Vacuum plasmaspraying

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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.

1 Process
2 Changing effects with plasma
3 Uses
4 Safety in Metal Spraying
5 See also
6 External links

[edit] 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.

[edit] 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.

[edit] Uses

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

[edit] Safety in Metal Spraying

METAL SPRAYING need not be a dangerous process, if the equipment is treated with care, and correct spraying practices are followed. As with any industrial process, there are a number of hazards, of which the operator should be aware, and against which specific precautions should be taken.

Ideally, equipment should be operated automatically, in enclosures specially designed to extract fumes, reduce noise levels, and present direct viewing of the spraying head. Such techniques will also produce coatings that are more consistent. There are occasions when the type of components being treated, or their low production levels, require manual equipment operation. Under these conditions, a number of hazards, peculiar to thermal spraying, are experienced, in addition to those commonly encountered in production or processing industries.

NOISE

Metal spraying equipment uses compressed gases, which create noise. Sound levels vary with the type of spraying equipment, the material being sprayed, and the operating parameters. Typical sound pressure levels taken 1 meter behind the arc.

LIGHT

Combustion spraying equipment produces an intense flame, which may have a peak temperature more than 3,100°C, and is very bright. Electric arc spraying produces ultra-violet light, which may damage delicate body tissues. Spray booths, and enclosures, should be fitted with ultra-violet absorbent dark glass. Where this is not possible, operators, and others in the vicinity, should wear protective goggles containing BS grade 6 green glass. Opaque screens should be placed around spraying areas. The nozzle of an arc pistol should never be viewed directly, unless it is certain that no power is available to the equipment.

DUST AND FUMES

The atomization of molten materials, produces a certain amount of dust and fumes. Proper extraction facilities are vital, not only for personal safety, but to minimize entrapment of re-frozen particles in the sprayed coatings. The use of breathing masks, fitted with suitable filters, is strongly recommended, where equipment cannot be isolated. Certain materials offer specific known hazards.

1. Finely divided metal particles are potentially pyrophorric and none should be allowed to accumulate.

2. Certain materials e.g. aluminum, zinc and other base metals may react with water to evolve hydrogen. This is potentially explosive and special precautions are necessary in fume extraction equipment.

3. Fumes of certain materials, notably zinc and copper alloys are unpleasant to smell, and, in certain individuals, may cause a fever-type reaction. This may occur some time after spraying and usually subsides rapidly. If it does not, medical advice must be sought.

HEAT

Combustion spraying guns use oxygen and fuel gases. The fuel gases are potentially explosive. In particular, acetylene may only be used under approved conditions. Oxygen, while not explosive, will sustain combustion, and many materials will spontaneously ignite, if excessive oxygen levels are present. Care must be taken to avoid leakage, and to isolate oxygen and fuel gas supplies, when not in use.

ELECTRICITY

Electric arc guns operate at low voltages (below 45 dc), but at relatively high currents. They may be safely hand-held. The power supply units are connected to 440 volt AC sources, and must be treated with caution.

COMPRESSED AIR

The air supply, to spray guns, is at high pressure. It should not be directed towards people. The motor air supply is lubricated, and should never be fitted to a breathing apparatus. Any breathing equipment, used with the thermal spraying process, must be supplied with air of breathing quality.