Understanding high velocity oxygen fuel spraying

Understanding high velocity oxygen fuel spraying

Understanding high velocity oxygen fuel spraying

Keywords Metallisation, Thermal spray, Coatings

High velocity systems are becoming popular in the thermal spraying market, according to Metallisation Limited.

Their popularity is growing among engineers for more wear resistant or corrosion resistant coatings, as high velocity coatings give harder, denser coatings such as tungsten carbide compared to more familiar well-established processes.

Unlike the more traditional methods of thermal spraying in which feedstock is melted and projected onto the substrate, high velocity systems (Plate 1) simply soften the powder before projecting it.

Plate 1 High velocity spraying of pinch roll from a decoiling machine using Metallisation HVOF equipment

The superior quality claimed is said to arise from using a combustion chamber and accelerating nozzles which produce a particle velocity of 1,100m/sec, resulting in high impact energy and reduced porosity levels.

A process which produces hard-wearing corrosion resistant and low porosity coatings is particularly appropriate for the aerospace industry.

"Densities of most high velocity metallic coatings will be more than 99.5 per cent of the theoretical density, with above average micro hardness and bond strength", says Dr Terry Lester of Metallisation, "Carbide coatings are not only harder and denser, but are also more ductile, and can be applied to a much greater thickness than normal".

"Plasma type coatings normally fail through break-up of the coating rather than any wear on the particles. As a result, high velocity coatings are significantly more wear resistant, especially where loads are high or erosion is prevalent.

"In corrosion resistance applications, a resistant barrier layer provides the corrosion protection, which is why the density of the coating is so important, but it is also crucial that there is no degradation of the alloy. High velocity systems are able to ensure this because the temperatures involved are relatively low", said Dr Lester.

Applications for high velocity coating systems are wide, varied, and growing all the time. Used, for example, as a replacement application process for hard chrome plating, it reportedly has led to radical performance improvements in terms of simple wear and in corrosion resistance.

Aluminium can be sprayed very densely using high velocity systems. Coatings of 7mm thickness are said to have been achieved and, subsequently, finely machined.

Other coating materials such as copper, nickel alloys, steels, tungsten carbide/cobalt/chromium etc. are also said to yield enhanced performances after being applied using the Metallisation high velocity system process.

Further details are available from Metallisation Ltd. Tel: +44 (0)1384 252464; Fax: +44 (0)1384 237196; E-mail: spray@metallisation.com