MBe International - What is Thermal Spraying?

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.

There are three types of heat source commonly used:

• Oxy-Fuel gas
• Electric Arc
• Plasma Flame

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).

The Basic Principle of Thermal Spraying

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:

• General reclamation of worn shafts and bearing journals
• Wear resistance
• High temperature oxidation resistance
• Chemical resistance
• Dielectric or electrical insulation coatings
• Electrical conductivity
• Thermal Barrier coatings
• Grip surfaces
• Biomedical implant coatings
• “Abradable” coatings that are designed to be soft, thereby preventing damage to Aero Engine compressor blade tips amongst other applications

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:

• Combustion Wire
• Combustion Powder
• Electric Arc Wire
• Plasma
• HVOF (High Velocity Oxy-Fuel)

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.

Surface preparation

Whichever Thermal Spray process is being used, the preparation of the surface (substrate) is critical so the overall success of the application. The most sophisticated and expensive Thermal Spraying equipment currently available could be utilised but it’s effectiveness would be negated if poor surface preparation techniques were used.

The most common method of surface preparation currently used is grit-blasting. Various types and sizes of grit are utilised depending upon the application. The anti-corrosion industry generally uses low cost abrasive grit such as garnet or copper slag (a by-product from copper smelting). These types of abrasive grits are usually “one-use” grits and not recycled but if reliable collection, filtering and sieving systems are in place, they can sometimes be re-used one or two times.

More information on the methods used for surface preparation within the anti-corrosion industry can be found in the anti-corrosion section.

For reclamation work and all applications on machine elements, the most common abrasive blasting grit is Aluminium Oxide (Al2O3). It is used in blast cabinets and blast rooms (for larger components).

Aluminium Oxide Blast Media

Nb. The brown colouration of the Al2O3 Grit is caused by the use of Titanium Dioxide (TiO2) as a toughener.

The benefits of this blast media are many:

• It can be recycled
• It is almost totally inert so any imbedded particles do not cause corrosive cells
• It is relatively cheap
• It breaks down over time into smaller particles but importantly, these smaller particles remain sharp

Typical Blasting Cabinet

MBe International has over 20 years experience in the techniques and disciplines required to ensure that the surface is prepared correctly prior to the application of any Thermal Sprayed coating.

Some common areas of consideration and evaluation are as follows:

• Component cleanliness prior to blasting process (degreasing)
• Size and quality of blast media
• Condition of blasting equipment
• Correct air pressures
• Quality of compressed air
• Impingement angle of blast media
• Inclusions of grit in the surface of the component (substrate)
• Achieved surface profile
• Operator techniques

Grit blasted surface of mild steel test coupon (X 2,000) G34 Al2O3 Blast Medi

Sulzer Metco® & Metco® are registered trademarks of Sulzer Metco AG Switzerland