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Thermal Spraying

There are a variety of different thermal spray processes, each aimed at projecting a stream of heated particles at high velocity on to a substrate, i.e. the part being coated. Hence, the process is always line-of-sight. And for the most part the bond between the coating and the substrate is purely physical.

Important plasma spray processes

Wire spraying

Metal or alloy wire (e.g. copper, stainless steel), is fed through an oxy/acetylene flame. The material melts and is blasted on to the part by compressed air.

Powder spraying

Similar to wire spraying, but the metallic material is fed into the oxy/acetylene flame as a fine powder.

Electric arc spraying

The metal feedstock is two wires, crossing at the gun nozzle and with a high voltage electric arc struck between them. The melted feedstock is atomised by compressed air and projected at the part.

Plasma spraying

A compressed gas (argon, hydrogen or nitrogen is ionised (a ‘plasma’) by a high voltage across a copper anode and tungsten cathode. The powder (which can be a ceramic) is heated to very high temperature and blasted at the part by the compressed gas.

HVOF – High Velocity Oxy Fuel

Fuel, consisting of kerosene, acetylene, propylene and hydrogen, ignited with oxygen, is projected by compressed air and compressed through a long barrel. The material is fed into the stream as a powder. It is very high velocity, but relatively cool, so ideal for cermets, not ceramics.

D-Gun – Detonation Gun

A charge of powder is fired at the part by a repeating detonation, about eight times per second. The fuel is acetylene/oxygen, ignited by a synchronised spark, producing extremely high velocity. Like HVOF, it is relatively cool, ideal for cermets and generally considered to produce the densest coatings.

The Apticote 800 Range

The choice of coating material depends on the application and the desired functionality. Poeton provide a wide variety of coatings, each coded with an Apticote 800 number. The list below shows the most common grades.

Uses

Apticote 800 thermal spray coatings are used to combat a wide variety of problems, including:

Low stress abrasive wear

Occurring in industries like textiles, plastics, and food, where micro-particles in the product cause abrasion of machine parts – for instance, a textile guide. The best solution is usually a plasma-sprayed ceramic like chromium oxide.

High stress abrasive wear

Occurring in pumps, valves, conveyors, etc. where aggressive debris or product is heavily loaded against operating surfaces. Only the toughest and hardest high energy thermal sprayed coatings should be specified.

Fretting and surface fatigue

This requires tough, rather than hard coatings, with materials (like nickel based alloys) that resist cracking and oxidation; both characteristics of fretting fatigue. Applications include cam-followers, rocker arms, expansion joints, seals and press-fit spacers.

Erosion

Erosion, either by impinging particles or fluids (perhaps with cavitation) is best resisted by tough, rather than hard, coatings. Applications include exhaust fan blades and seats, turbine nozzles and dust collectors.

Salvage

Parts that have been damaged, worn or eroded can reclaimed by sprayed coatings. The damaged area is cleaned up, blasted and then coated with new material, oversize, before being machined or ground back to size.

The coating must, in general, match the substrate, with regard to the composition and hardness – including their required machining or grinding characteristics.

Corrosion and stress corrosion

Nickel, chrome or cobalt based spray coatings are available for protection, usually sealed to close any residual porosity. On high strength steel or aluminium substrates, a corrosion resistant coating, sometimes followed by shot-peening, can combat stress corrosion and eliminate cracking failures.

Thermal barriers

Ceramic coatings, such as alumina, MCrAlY or ittria stabilised zirconia, provide low conductivity and an oxygen diffusion barrier, protecting vulnerable substrates.

Applications include piston crowns, rocket nozzles, missile nose cones and carburising boxes.

Electrical

Conductivity – sprayed metals like copper for lightening arrestor and ground connectors.

Resistivity – dense, pore-free sprayed ceramics for the highest dielectric constant, for insulation applications. Including heater tubes, soldering tips and electronic parts.

Shielding – sprayed coating to absorb and earth stray RF induction, and others to shield against gamma rays or thermal neutrons. Applications including instrument assemblies and missile systems.

Bedding-in

The classic application of thermal spray coatings is for turbine stators, where an ‘abradable’ coating like Nickel/graphite is applied.

Abrasive coatings on the turbine blade tips then cut the stator surface, creating the perfect dimensional match, with minimum blade/stator clearance.

Efficiency and costs

Powder costs are an important element of the processing economics, so spray efficiency is a crucial aspect. To this end. Poeton employ a Sulzer Metco TriplexPro™- 200 Robot Plasma Spraying System, offering the ultimate in spraying performance.

Additionally, our Sulzer Metco MultiCoat ® High Performance Thermal Spray Controller can simultaneously control up to four thermal spray processes from a single consol.

Quality control

Quality control covers four main properties, overseen by the Poeton laboratory, the thickness, porosity, substrate bonding and coating structure. Central to this is skilful metallography, that is the sectioning, polishing and microscopic examination of sprayed samples, representing the parts. (In some cases, actual parts are sacrificed as representative sample).

Surface preparation and masking

The part is aggressively grit-blasted prior to coating, to provide a high bond-strength mechanical key (typically 50 MN/m2). Very thin sections should be reinforced, as part of the design, since the compressive stress imparted to a surface can sometimes distort the component.

Masking is by mechanical shields and robust tapes – a time consuming step. So it is worth consideration at the design stage – a part that can be coated all over is a much easier and cost-effective task.

Spraying up to or around a sharp corner is difficult, leading to chipping. If possible, corners should be radiused.

Surface finish and thickness

Coating thickness can be up to several mm, particularly for salvage, but most ceramics, cermets and metal/alloy coatings are applied to between 250 microns and 1mm. The as-sprayed finish is rough, around 10 microns Ra. So most parts (excluding for instance, some thermal barriers) will need post-grinding or machining. Poeton will advise on procedures.

Porosity

Metal and alloy coatings exhibit near zero porosity. Ceramics and cermets, applied by high temperature plasma spraying, require the most rigorous quality control of the spay parameters, achieving porosity levels below 2%.

Substrates

Hardened steel or other hard substrates (>450Hv) pose difficult problems in obtaining a good coating bond, and should be avoided.

Plastics, composites, graphite and even wood can, unusually, be used as substrates, but Poeton will advise on the specialist spraying techniques required.

Approvals and Accreditations

Poeton are approved to the following Third Party Quality Systems at their production site at Gloucester.

Specifying the coating

When placing an order for Poeton Apticote 800, be sure to specify your requirements, specifically:

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