Direct current plasma spraying of mechanofused alumina - steel particles

Stainless steel particles (60 μm in mean diameter) cladded with an alumina shell (2 μm thick and manufactured by mechanofusion) were sprayed with an Ar-H2 (53-7 slm) d.c. plasma jet (I = 500 A, P = 28 kW, th = 56 %). Two main types of particles were collected in flight, as close as 50 mm downstream of the nozzle exit: particles with a steel core with pieces of alumina unevenly distributed at their surface and those consisting of a spherical stainless steel particle with an alumina cap. To understand the phenomena, stainless steel particles were also sprayed in the same conditions. Particles collected contained oxide nodules inside them (due to the convective movement induced by the plasma flow within the liquid particles) and presented also an oxide cap and an oxide shell (only observed between 70 and 100 mm downstream of the nozzle exit). The plasma flow was modeled by a 2D steady parabolic model and a single particle trajectory by using the 3D Boussinesq-Oseen-Basset equation. The heat transfer, within the two-layer, stainless steel cladded by alumina, particle, considered the heat propagation phenomena including phase changes. The models allowed determining the positions, along the particle trajectory, where the convective movement could occur as well as the entrainment of the liquid oxide (the spinel for the pure stainless steel particles and the alumina for the cladded ones) to the leading edge of the in-flight particles. The heat transfer calculations showed the importance of the thermal contact resistance TCR between alumina and steel, which values were varied between 10 and 10 m·K·W because no experimental value was available. However whatever maybe the TCR value, the alumina shell melting occurs always at the position, along the particle trajectory, where the force applied by the plasma flow onto the liquid oxide light layer is sufficient for its entrainment towards the leading edge of the particle (cap formation).