One of the most versatile methods of altering the surface
properties of engineering machine elements is by the application
of plasma sprayed coatings.
Plasma
coatings are applied using a specially designed 'gun' wherein
a pressurised and high velocity inert gas, usually nitrogen or
argon, is electrically excited to form a self contained plasma
arc which, with the addition of hydrogen or helium secondary
gas, creates a high energy heat source.
This energy is exploited by the precise feeding into the plasma
arc of powered materials which melt or soften and are propelled
onto the workpiece by the plasma gas steam.
Any material that melts when heated can thus be plasma sprayed
to form coatings ranging from 15 - 25 microns to unlimited thickness.
Since the plasma arc is self contained within the plasma 'gun',
heat transfer to the workpiece is insignificant and therefore
plasma spraying can be classed as a cold process. An immediate
benefit of this feature is that parts made from soft, lower melting
point materials, such as aluminium and its alloys, can be easily
hardfaced with such differing materials as hard steel, tungsten
carbide or chrome oxide ceramic.
Prior to receiving the coating the workpiece is chemically
cleaned and mechanically roughened so that the spray of molten
or semi-plasticised material freezes instantly on impact and
interlocks by mechanically keying to the workpiece. The coatings
thus formed can be broadly categorised as follows.
- Thin carbide and ceramic hardfacings for resistance to abrasive
particle wear, sliding and fretting wear.
- Ceramic coatings to resist chemical and atmospheric corrosion.
- Softer 'friable' ceramic coatings that form thermal barrier
heat shields.
- Coatings for electrical conductivity or dielectric properties.
- Sacrificial coatings designed to be easily abraded for turbomachinery
clearance control applications.
Coatings can be easily finish machined, ground or brushed
to impact desired surface characteristics.
Plasma coatings are used throughout manufacturing industry
in OEM and repair and maintenance applications where the above
features greatly extend component operating life, reduce down-time
and increasing machinery service interval periods.
Typical examples include aero, industrial and marine gas turbines
parts, automotive engine parts, textile and yarn processing plant,
pump sleeve seals and wear rings, gas and oil pipeline testing
equipment, molten metal casting moulds and pouring troughs, power
station boiler tubes and core burner pipes.
