Influential Factors on the Combustion of the Polymer Binder System in F-HVOF

Abstract This study investigates the combustion dynamics and decomposition of filled thermoplastic filaments in filament high-velocity oxy-fuel spraying to improve deposition efficiency and coating quality. A quasi-steady-state combustion mechanism is proposed. Filament tip melting, fragmentation, and in-flight decomposition transform the monomodal primary powder with a mean size of 1.8 µm into a trimodal particle size distribution. Peaks occur near 500 nm, 5 µm, and 150-400 µm, with the first two likely from clean melting and decomposition, and the third from material buildup on cooled nozzle walls. Process optimizations varied the stoichiometric ratio ( λ ), feed gas flow rate, and filament diameter, and a copper insert was added to enhance filament decomposition. Coatings were prepared using PBAT filaments containing 70-80 wt.% alumina. Sub-stoichiometric flame conditions, particularly λ ≈ 0.8, gave the highest deposition efficiencies by maximizing flame temperature and promoting polymer decomposition. Reducing the feed gas flow to 5 slpm further improved efficiency, while gas composition had little effect under low flow. Filament diameter influenced efficiency by controlling feed velocity, which in turn affected the filament tip position within the nozzle. The adapted setup and optimized spraying parameters achieved a deposition efficiency of 76%, hardness of 1149 HV0.1, low porosity of 3.0%, and surface roughness values of Ra = 6.9 µm and Rz = 44.8 µm at a feed rate of 4 g/min.