Thermally sprayed hard metal coatings are the industrial standard solution for numerous demanding applications. Often the performance of thermally sprayed coatings is improved by using finer particle sizes due to improved surface finish and decreased defect sizes. In the aim of utilizing finer particle and primary carbide sizes in thermal spraying of hard metal coatings, several approaches have been studied to control the spray temperature. The most viable solution is to use the modern high velocity air-fuel (HVAF) spray process, which has already proven to produce high quality coatings with dense structures. In HVAF spray process, the particle heating and acceleration can be efficiently controlled by changing the nozzle geometry. In this study, fine WC-10Co4Cr powder (-25+5 µm) was sprayed with three nozzle geometries to investigate their effect on the particle temperature, velocity and coating microstructure. The study demonstrates that the particle melting and resulting W2C formation can be efficiently controlled by changing the nozzle geometry from cylindrical to convergent-divergent. Moreover, the average particle velocity was increased from 780 to over 900 m/s. This increase in particle velocity significantly improved the coating structure and density while deposition efficiency decreased slightly. Further evaluation was carried out to resolve the effect of particle in-flight parameters on coating structure and cavitation erosion resistance, which was significantly improved with the increasing average particle velocity.

One important trend in thermal spraying is the application of novel Fe-based corrosion/wear protection coating systems. A typical field of application for such corrosion and abrasive wear protection coatings are rotary dryers of paper machines. At the moment, these cylinders are coated by wire arc spraying. A disadvantage of the wire arc sprayed coatings is their high thickness, which has a heat-insulation effect, and their high roughness. Therefore, an expensive post production grinding process is necessary in order to achieve the required surface quality. The goal is to develop a HVAF process that enables the production of thin, dense and near net shape corrosion/wear protection coating systems, which significantly reduce the post-production time and costs. In this study, the HVAF coating process and a novel Fe-based feedstock material are investigated. In the first step the Fe-based powder is analysed thermally using differential scanning calorimetry, to investigate the solidification and melting temperature of the feedstock material. Furthermore, the influence of the spraying distance and the powder feed rate on the microstructure and porosity of the resulting coatings is investigated using light microscopy. Furthermore, the deposition efficiency of HVAF coatings is analysed regarding their economic efficiency.

The HVAF process provides a slightly different (to HVOF and other thermal spray processes) and unique combination of thermal and kinetic energy, which presents a technical challenge to material manufacturers who have over the past years optimized their material offerings to the demands and characteristics of plasma, combustion and more recently the various HVOF processes. The recent development of HVAF ID torches, capable of applying the highest quality coatings into dimensions of less than 100mm (4”), has provided a further need to optimize spray materials for the best possible interaction between the spray device and the feedstock in order to meet developing coating specifications from various industries. This paper will discuss the methodologies utilized from the points of view of torch design, material manufacture, spray parameter and procedure optimization, as a means of producing the highest performing coatings to meet specific industry aspirations.