Better thermal spray coatings with HVOF

A Plasma and Thermal Coatings product story

Edited by the Engineeringtalk editorial team Apr 10, 2001

Plasma and Thermal Coatings Ltd, a South Wales based company, is one of the leading exponents in Britain of the developed technique of high velocity oxy-fuel (HVOF) spraying

Thermal spray coating is a technique for enhancing the surface characteristics of a material, or extending its service life.

Finely powdered particles of metal, ceramic or carbide compounds are heated and accelerated by a high-temperature stream of gas to form a high-energy spray of semi-molten droplets which hit the target material, and then adhere to it to form a tough permanent coating.

New advances in the technology have extended its value in different applications ranging from automotive brake discs and piston rings to gas turbine blades, combustion cans and nozzle guide vanes.

Plasma and Thermal Coatings Ltd, a South Wales based company, is one of the leading exponents in Britain of this developed technique of high velocity oxy-fuel (HVOF) spraying, which has a number of important advantages over more traditional thermal spray methods.

Thermal spraying uses a combination of thermal and kinetic energy to ensure the particles form a coating on the target.

Because the target substrate does not reach a very high temperature, there is no distortion of the items being sprayed, so coatings can be applied to high-precision finished parts.

Furthermore, there is no surface oxidation or changes in the microstructure of the parts being sprayed, so substrate materials, which have a relatively low melting point, can be coated safely.

Several hundred different materials can now be sprayed in this way.

They include pure metals like aluminium, nickel and titanium, metallic alloys, ceramics such as oxides of aluminium, chromium and zirconium, cermets (combinations of ceramics with metals and metallic alloys) and mixtures of aluminium and silicone with graphite or polyesters.

Virtually all these materials can be deposited by the plasma spray process, where a stream of inert gas is passed through a constricting nozzle, containing an electric arc.

This heats the gas to a very high temperature, usually well above 16,000 degrees C, causing a huge increase in volume and consequently the velocity of the steam.

The coating powder is injected into the stream, where the particles melt and are accelerated towards the target substrate.

When the molten particles hit the surface, they are flattened and, as they cool and solidify, they shrink into a tough coating with a strong mechanical bond to the substrate.

A modified version of this process, called vacuum plasma spraying, has been developed to eliminate porosity and oxidation in spraying metallic coatings which have to stand up to harsh and corrosive environments.

Plasma and Thermal Coatings Ltd has developed a thermal spray process, based on an advanced technique which matches the toughness and quality of vacuum plasma sprayed coatings, but with significant additional advantages.

The method is called high velocity oxy-fuel coating (HVOF) and uses a combination of a fuel like kerosene, burning with large quantities of oxygen to produce a flame with a gas stream velocity in excess of 2000 metres per second.

This can accelerate the particles of the coating being sprayed to around 600 metres per second, or roughly twice the speed of sound.

Compared with plasma spraying, HVOF supplies the coating particles with most of their energy as kinetic rather than thermal energy, so fewer particles will oxidise in flight.

In addition, the speed with which they hit the target ensures that any particles not fully melted are plastically deformed by the impact, which reduces any porosity in the resulting coating.

The result is a coating with a high corrosion resistance.

The HVOF spray process allows higher output with significant cost savings over vacuum plasma spraying, as there is no need to control the environmental chemistry and pressure around the component, and the cost of the capital equipment is greatly reduced.

The HVOF process can also deal with a wider range of size, weight and geometry of components to be coated, when compared with vacuum spraying, as it is not restricted by the boundaries of a chamber.

Consequently, items like gate valves, ball valves, steel mill rollers, mid-span supports for compressor blades and other high-integrity parts which are exposed to aggressive environments are being coated by HVOF methods, and the simplicity of the process makes it possible to carry out repairs and spraying operations on site.