The Gas Dynamics of High-Velocity Oxy-Fuel Thermal Sprays

An experimental study of the gas dynamics of the High-Velocity Oxy-Fuel (HVOF) thermal spray process has been performed. With this process, a hot, combustion-driven, supersonic jet is used to propel particles onto a surface, thus forming metal coatings that provide wear, temperature, and corrosion resistance. The fundamental physics of the spray process were studied and several key areas of interest were identified for in-depth study. Optical diagnostic techniques, including microsecond -exposure schlieren and shadowgraph imaging, were used to visualize the hot supersonic jet produced during the spray process. Energetic turbulent mixing of the jet with the surrounding atmosphere was observed. Measurements of oxide levels in aluminum and mild steel coatings sprayed for a range of conditions indicated that the turbulent mixing influences coating oxidation. However, experiments conducted with a low-speed coaxial shroud of inert gas demonstrated that coating oxide formation can be effectively controlled during the spray process. A simple numerical model was developed to predict the behavior of a spray particle in the HVOF jet. The results of computations indicated that independent control of spray particle velocity and temperature was possible through systematic variations in combustion chamber pressure and particle injection location within the nozzle. This hypothesis was confirmed through a series of experiments in which stainless steel particle velocity and temperature were measured using trace velocimetry and two-color radiative pyrometry, respectively. Combustion chamber pressure had a strong effect on particle velocity. Injection location was used to control the residence time of a particle within the flow, thus allowing manipulation of particle temperature without a measurable effect on velocity. Thus, the results of these experiments revealed that the gas dynamics--the behavior of the compressible gas flow--of the HVOF spray process strongly influenced spray particle properties, which, in turn, affect coating properties.