Comparison of Classical and Axial Injection Torches for Spraying Alumina Coatings

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Abstract
A conventional and a hollow cathode dc spraying plasma torch are compared by spraying fused and crushed alumina particles with two size distributions : −45, +22 μm and −90, +45 μm. The conventional plasma torch was studied for 3 nozzle internal diameters (internal diameter): 6, 7, and 8) mm. The characterization of the plasma jets has shown that the main effect of the arc current I or total gas flow rate [mdot]increase was to raise the gas velocity with a very little increase of the jet length, as confirmed by measurements of the velocity and surface temperature distributions of the particles in flight. Finally, the coatings sprayed with a 7 mm internal diameter nozzle, at P=35 kW, [mdot]=60 slm and 25 vol% H2 and a stand off distance of 100 mm correspond for the −45, +22 um powder to a deposition efficiency pD = 64%, a ratio a/(a+g) = 2% and a Vickers Hardness HV5=1040. For the −90, +45 μm powder;r pD= 47%, α/(a+v) = 9% and HV5 = 800 (where α and γ correspond to the equilibrium phase of alumina and a non-equilibrium metastable one respectively). The hollow cathode torch exhibits much lower velocities for the plasma gas exiting the nozzle but higher velocities and surface temperatures of the particles in flight. When comparing the coatings sprayed with a stand-off distance of 70 mm with P = 15·6 kW, m°= 46 slm and 24 vol% of H2, the coating properties are quite similar to those obtained with the conventional torch but with slightly higher deposition efficiencies in spite of the power level of less than half.