Suspension Thermal Spraying – the Potential of a New Spray Technology

Thermal spraying is an established surface technology, with high flexibility in terms of processes and possible coating materials. Atmospheric Plasma Spraying (APS) and High Velocity Oxy-Fuel Spraying (HVOF) are among the most applied spraying techniques to produce coatings with typical thicknesses in the range 100 – 500 µm. In order to form a coating, powders of different materials with a particle size in the range 10-50 µm, are used.

The substitution of the conventional spray powders by suspensions offers new possibilities, such as the direct use of finely dispersed powders, and thus saving in the production of specific thermal spraying feedstock powder, and the production of nanostructured coatings. The advantages and application potential of suspension sprayed coatings are multitudinous, one of the main aspects being the possibility of preparing coatings with thicknesses of a few µm and high bond strength. With suspension spraying, the technological gap in terms of achievable coating thicknesses between traditional spray coatings and thin film surface technologies (CVD/PVD) is bridged.

The development of suspension sprayed aluminium oxide and titanium oxide coatings, and their specific properties, will be summarized below.

The production of suspension sprayed coatings requires on one hand the modification of the APS and/or HVOF equipment, including the development of appropriate feeding systems, and on the other hand the production of suitable aqueous or alcoholic suspensions. Coatings based on aluminium oxide and titanium oxides are multifunctional. For the suspension sprayed coatings composed of both oxides is the long-term stability of the electrical properties particularly interesting, while the highlight for the titanium oxide coatings is their photocatalytic properties.

Suspension sprayed coatings can be produced by APS as well as by HVOF. In comparison to conventional thermally sprayed coatings, they differ in their phase composition and microstructure. When used the appropriate feedstock powder for suspension preparation, coatings with nanoscale structure are obtained. In the case of titanium oxide, the coatings are mainly comprised of a photocatalytic active anatase phase. The photocatalytic activity of a titanium oxide coating, tested in a methylene blue solution, is shown in the graphic.

The use of special aluminium oxide suspension allows the production of coatings with a high amount of corundum without the need of stabilization additives. This discovery granted a patent in Germany. The long-term stability of the electrical properties is currently under investigation.

For both materials, adherent coatings with thicknesses in the range 10-50 µm are easily obtained. The hardness of the suspension sprayed coatings is, when that is relevant for the application, comparable with those obtained by conventional thermal spraying. Also, the economic parameters together with the deposition efficiency of the process are now appealing.