Manufacturing: Surface Finishing

Thermal spraying

Thermal sprayed coatings are used extensively for a wide range of industrial applications. The technique generally involves the spraying of molten powder or wire feedstock, the melting being achieved by oxy-fuel combustion or an electric arc (plasma). The molten particles are accelerated by the flame and impact onto a properly prepared substrate, usually metallic. Solidification occurs rapidly so the as-sprayed deposit is ultra-fine grained. The materials which are sprayed include most metal alloys and ceramics.

Thermal spray processing is a well established means of forming coatings of thicknesses greater than about 50 micrometers: so-called "thick coatings". A wide range of materials can be thermal sprayed for a variety of applications, ranging from gas turbine technology (heat engines) to the electronics industry. Thermal sprayed coatings have been produced for at least 40 years, but the last decade has seen a virtual revolution in the capability of the technology to produce truly high performance coatings of a great range of materials on many different substrates. This enhancement of the technology has been achieved largely through the introduction of new spray techniques, the enhancement of spray process controls, by employing state-of-the-art methods of feedstock materials production, and through the use of modern techniques of quality assurance.

Process

The basic steps involved in any thermal coating process are substrate preparation, masking and fixturing, coating, finishing, inspection, and stripping (when necessary). Substrate preparation usually involves scale and oil/grease removal, as well as surface roughening. Roughening is necessary for most of the thermal spray processes to ensure adequate bonding of the coating to the substrate. The most common method is grit blasting usually with alumina.

There are three basic categories of thermal spray technologies:

1. Combustion Torch (flame spray, high velocity oxy-fuel, and detonation gun)
2. Electric (wire) Arc
3. Plasma Arc

Thermal Spray

This process is basically the spraying of molten material onto a surface to provide a coating. Material in powder form is melted in a flame (oxy-acetylene or hydrogen most common) to form a fine spray. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a coating. This flame spray process carried out correctly is called a "cold process" (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.

The main advantage of this flame spray process over the similar Combustion wire spray process is that a much wider range of materials can be easily processed into powder form giving a larger choice of coatings. The flame spray process is only limited by materials with higher melting temperatures than the flame can provide or if the material decomposes on heating.

Electric (wire) Arc

In the Arc Spray Process a pair of electrically conductive wires are melted by means of an electric arc. The molten material is atomised by compressed air and propelled towards the substrate surface. The impacting molten particles on the substrate rapidly solidify to form a coating. This arc spray process carried out correctly is called a "cold process" (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.

Electric arc spray coatings are normally denser and stronger than their equivalent combustion spray coatings. Low running costs, high spray rates and efficiency make it a good tool for spraying large areas and high production rates.

Disadvantages of the electric arc spray process are that only electrically conductive wires can be sprayed and if substrate preheating is required, a separate heating source is needed.

The main applications of the arc spray process are anti-corrosion coatings of zinc and aluminium and machine element work on large components.

Plasma Arc

The Plasma Spray Process is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools forming a coating. This plasma spray process carried out correctly is called a "cold process" (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.

The plasma spray process is most commonly used in normal atmospheric conditions and referred as APS. Some plasma spraying is conducted in protective environments using vacuum chambers normally back filled with a protective gas at low pressure, this is referred as VPS or LPPS.

Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconia unlike combustion processes. Plasma sprayed coatings are generally much denser, stronger and cleaner than the other thermal spray processes with the exception of HVOF and detonation processes. Plasma spray coatings probably account for the widest range of thermal spray coatings and applications and makes this process the most versatile.

Disadvantages of the plasma spray process are relative high cost and complexity of process.