The Thermal Spray Process
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"Thermal Spraying is the process by which, a coating is produced by impacting accelerated molten particles onto a prepared surface”.
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The Thermal Spray process dates back to C.1909 in Switzerland. The ballistics engineer Max U. Schoop developed the first known Thermal Spray gun, surprisingly enough called, the “Schoop Gun”.
The story goes that Dr. Schoop stumbled across the idea whilst he was setting the sights on a long barrelled gun. Using a brick wall as a target, he was unhappy with the resulting damage to the wall after each test firing. He decided to secure a metal plate against the wall to protect it from further damage. The “shot” from the gun was said to “stick” to the metal plate on impact and so the concept of Thermal Spraying was born.
Whether this is a true account of those events is irrelevant because the story holds up and we can see from the account that the process relies upon two fundamental energies; Thermal and Kinetic (Heat and Speed).
We do know that the first Thermal Spray patent was filed by Dr. Schoop on March 30th 1915 and that his original Schoop gun bares a remarkable resemblance to the modern day Combustion Wire guns.
We also know that in 1917 Dr. Schoop was presented with the “John Scott Award” for “The Schoop Metal Spraying Process”. John Scott was an Edinburgh druggist who wanted to create a method of recognising ingenious or unique inventions. He chose the City of Philadelphia in the USA to administer the annual awards.
In 1922 Metallisation Ltd started trading in the UK after buying the rights to the Schoop patent.
In 1933 Metco Inc started trading in the USA, again after buying the rights to the Schoop patent.
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With any of the Thermal Spray processes, a material (usually in wire or powder form) is fed into a heat source.
This heat source can vary and an extensive range of materials are used.
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There are three types of heat source commonly used.
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Oxy-Fuel gas
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Electric Arc
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Plasma Flame
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As a result of the introduction of the material into the heat source, molten particles are produced and accelerated and impacted onto a prepared surface (substrate).
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A vast array of materials are now available to the Thermal Spray Industry, ranging from materials such as Zinc and Aluminium for the anti-corrosion industry through to complex Super Alloys, Carbides and Ceramics for Aero Engine applications.
A small number of the coating applications are listed below:
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General reclamation of worn shafts and bearing journals
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Wear resistance
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High temperature oxidation resistance
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Chemical resistance
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Dielectric or electrical insulation coatings
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Electrical conductivity
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Thermal Barrier coatings
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Grip surfaces
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Biomedical implant coatings
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“Abradable” coatings that are designed to be soft, thereby preventing damage to Aero Engine compressor blade tips amongst other applications.
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To a certain degree, the component or substrate material is irrelevant. We look at what we want our surface properties to be and select the coating accordingly. Of course, substrate properties are an important consideration but the overriding principle is that we can select a coating to give us the surface properties that we require on almost any given substrate material.
There are 5 common Thermal Spray Processes which utilise either a wire or powder in combination with one of the three heat sources:
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Combustion Wire
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Combustion Powder
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Electric Arc Wire
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Plasma
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HVOF (High Velocity Oxy-Fuel)
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Generally speaking, the higher the overall values of the Thermal and Kinetic energies (Heat and Speed), the higher the coating quality in terms of bond strength, density and oxide content. There are many provisos to this statement but it holds true in most scenarios.
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The Schoop Gun C.1909
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