Physical vapour deposition (PVD) is a thin-film coating process which produces coatings of pure metals, metallic alloys and ceramics with a thickness usually in the range 1 to 10µm. Physical vapour deposition, as its name implies, involves physically depositing atoms, ions or molecules of a coating species on to a substrate.
There are three main types of PVD, all of which are undertaken in a chamber containing a controlled atmosphere at reduced pressure (0.1 to 1 N/m 2):
- thermal evaporation
- sputtering
- ion plating
Thermal evaporation uses the heating of a material to form a vapour which condenses on a substrate to form the coating. Heating is achieved by various methods including hot filament, electrical resistance, electron or laser beam and electric arc.
Sputtering involves the electrical generation of a plasma between the coating species and the substrate.
Ion plating is essentially a combination of thermal evaporation and sputtering.
All three techniques can be used for the direct deposition of a material or for 'reactive' use in which chemical reaction occurs in the vapour/plasma phase between atoms of the coating material and 'reactive' gases.
The temperature of the substrate being coated is typically in the range of 200-400°C, considerably lower than temperatures associated with CVD (chemical vapour deposition, the other thin-film process). PVD is a line-of-sight process and requires the substrate surface to be easily accessible. Some components are rotated to produce even coatings.
PVD is a batch coating process with typical cycle times of 1 to 3 hours, depending on the material being deposited and the desired coating thickness. Common coating rates are 50 to 500µm/hr depending on the technique. Coated components do not require additional machining or heat treatment.
Applications
There are many uses for coatings prepared by PVD; applications include aluminium tracks and ceramic resistors for electronic circuitry; anti-reflective ceramic coatings for optics; decorative coatings on plastics; corrosion resistant coatings on gas turbine blades; and wear prevention coatings for machine and press tools.
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