This paper presents the development of a new thermal spray gun for the so-called warm spraying process in which powder particles are not melted but heated to temperatures much higher than those typically found in a cold spray process. The increased heating leads to a reduction in the particle impact velocity required to deposit the coating and hence reduces operating cost. The new gun utilizes methane-oxygen combustion for particle heating and features a swirl-type combustion chamber to create a turbulent mixture of the fuel and oxidizer for efficient combustion. Powder can be fed axially or radially into the gun. To control particle temperature independently, combustion gases are diluted by adding nitrogen gas through axial or radial ports provided in the gun. A converging-diverging nozzle with a downstream cylindrical barrel accelerates the burnt gases to supersonic velocities. The design of the nozzle and barrel was optimized using numerical simulations. Mass flow rates of methane, oxygen, and nitrogen were calculated using energy balance, stoichiometric combustion, and nozzle flow rate equations. The gun is designed to operate up to 200 kW and is water-cooled. Experiments were conducted to test the performance of the new gun in which tungsten carbide coatings were deposited on aluminum substrates. Coatings were analyzed using standard methods and showed promising results.

Splats formed during a thermal spray process may be either highly fragmented or intact and disk-like. To predict this change in splat morphology, a dimensionless solidification parameter (Θ), which takes into account factors such as the substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, has been defined. Θ is the ratio of the thickness of the solid layer formed in the splat while it is spreading, to the splat thickness. The value of Θ can be calculated from simple analytical models of splat solidification and spreading. If the solid layer growth is very slow (Θ << 1), the droplet spreads out to a large extent. Once it reaches maximum spread it becomes so thin that it ruptures, producing fragmented splats. If, however, the solid layer thickness is significant (Θ ~ 0.1 – 0.4), the droplet is restricted from spreading too far and does not become thin enough to rupture. Under such circumstances, disk-type splats are expected. When the solid layer growth is rapid (Θ~1), spreading of the droplet is significantly obstructed by the solid layer, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data.