Coatings have
conventionally made to offer security from corrosion and erosion that is to
secure the material from chemical and physical attack with its service media.
Corrosion and wear issues still occur at the large scale in the diverse
industrial applications and products and cause degradation and eventual damage
of components and devices in processing and manufacturing industries. Different
techniques can be used to coat the suitable surface protection that can prevent
corrosion in specific conditions. These are often recognized by coating thickness,
deposit of thin layers in micro-meters and deposit of thick layers.
The thick layers
usually developed at atmospheric pressure have thickness above 30 ยตm to few mm and are used when performance and service life of a
component depend on the thickness of a protective layer.
Coating
technology can be divided into two different categories- wet and dry coating
methods, the essential difference between the condition in which the material
is processed. Wet group includes electroplating, electroless plating and hot
dip galvanizing whilst dry group includes other methods, vapor deposition,
thermal spray techniques, brazing and weld overlays.
Here we discuss
the thermal spray coatings that includes different coating processes in which
finely divided metallic materials are deposited in a molten form to develop a
coating. Commonly used coating processes are:
- Direct current arcs or radio frequency discharge generated plasmas
- Plasma transferred arcs
- Wire arcs
- Flames
- High velocity oxy fuel flames
- High velocity air fuel flames
- Detonation gun
- Cold gas dynamic spray technology
Most coating
processes are used at atmospheric pressure in air. The coating material used
here is arc spray wire to offer sufficient corrosion and wear resistance
to the component. The central part of the system is a torch transforming the
supplied energy into a stream of hot gases. The coating material is heated to
molten form and accelerated by elevated temperature, high speed gas to the
substrate. It impacts on the substrate in the stream of drops that are produced
by melting of the tips of wires in the high energy gas stream. The droplets
flatten on the substrate and produce lamelle named as splats. The accumulation
of several layered splats makes the coating.
Thermal spray processes
are now commonly used to spray coating to offer protection from wear and
corrosion and also create a thermal barrier and for various functional
purposes. The choice of the deposition process is firmly based on the desired
coating properties for the and coating deposition cost. Coating
characteristics are specified by the coating material, form in which is it is
used and by specific factors followed to execute the deposition process.
Thermal spray
coatings are normally featured by lamellar structure and contact between splats
and substrate or the accumulated layers assess to a large level the coating characteristics
for example heat conductivity and Young’s modulus. The contact area varies
between 20 – 60% of the coating surface parallel to the substrate. It improves
the impact velocities of particles offered that the latter are not extensively
superheated or below their melting points.
