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The Technology
Plasma spraying is a technique for producing coatings and free-standing parts using a plasma jet. Deposits having a thickness from micrometers to several millimeters can be produced from a variety of materials, including metals and ceramics.
The material to be deposited, or feedstock, is typically a powder. It is introduced into a high velocity air gas plasma jet (where the temperature is around 10,000°C). The particles are heated and propelled at high velocity to impact onto the workpiece or substrate. On impact the kinetic energy is also released as heat and the particles rapidly form fine molten splats, building up a coating of many layers of splats on the substrate. Free-standing parts can also be produced by removing the substrate.
Air plasma spraying can thus be likened to welding. The nozzle uses high voltage and power (up to 80 kW) and is an anode electrode. The gas mixture can have variable pressures and flow rates and the coating feedstock (metal or ceramic powder) has controlled particle size and feed rate.
A large number of parameters influence the interaction of the particles with the plasma jet and the substrate, and therefore the deposit properties. These parameters include feedstock type; plasma gas composition & flow rate; energy input; torch configuration; torch offset distance and substrate cooling. The gas can be any mixture and any flow rate of argon, nitrogen, hydrogen and helium. Control is exercised over the heat input into the particles and also over the oxidising or reducing environment.
A combination of surface preparation and a bond coat is used before the ceramic thermal barrier coating. Coatings can be applied to a variety of metals and composites and after coating, the components can be finished in a range of different finishes and colours.
Plasma spraying can be used to fabricate complete ceramic components with extremely tight dimensional tolerances. Just one example would be two ceramic tubes, one a sliding fit within the other, 1 m x 200 mm, but with a wall thickness of less than 1 mm!
Applications
Metal wire spraying was introduced as early as 1904, and was used to coat large structures with metal coatings for protection against the environment. Further developments resulted in air thermal plasma spraying (1960). Applications originally included coating aircraft turbine blades with MCrAlY & thermal barrier coatings. These protect the blades against erosion & oxidation and permit operation at surface temperatures exceeding the melting point of the blades themselves. These coating technologies are now used in a variety of market sectors - including automotive, telecomms and orthopaedic implants.
Automotive
The most obvious automotive application of this technology is to coat engine exhaust systems, manifolds and turbochargers (or any hot component) and keep the heat in. This reduces under-bonnet temperatures, improves thermal efficiency and possibly gives a faster warm up time for catalytic converters. A drop in air intake temperature of 30°C can deliver a 6% improvement in power or economy! Lower under bonnet temperatures can also improve the reliability of heat sensitive components, particularly on older cars.
Alternatively the ceramic coating can be applied to heat sensitive components (e.g. carbon fibre bodywork) surrounding the exhaust. Ceramic coatings can also form heat barriers in other areas such as on piston crowns. In Formula One a lightweight thermal barrier is applied to carbon composite wishbones and then a reflective coating added to protect against radiant heat. This allows high strength, lightweight materials to be used where they would otherwise be unsuitable. They're exhibited at the Science Museum (see link below).
Effective thermal management can reduce component damage, reduce running costs and improve driver comfort. Beside thermal barrier coatings plasma spraying can be used to produce engineering surfaces, technical ceramics and other advanced coatings. As well as motorsport the technology is used in performance cars & bikes and classic cars.
There is also an exhibition at the Science Museum called Fast Forward: 20 ways F1 is changing our world. More at: www.sciencemuseum.org.uk/visitmuseum/galleries/fast_forward.aspx
Important note:
The information above is derived from several public sources.It should not be construed as definitive and is presented for background knowledge only.Specific and qualified expert advice must be obtained before reliance is made on the facts presented.
