Cooling rates of plasma-sprayed metallic particles in liquid and gaseous nitrogen

Plasma spraying of metallic particles in the ambient atmosphere is accompanied by their oxidation. The oxides formed on the particle surfaces are often unstable at room temperature. Fast cooling of the particles may conserve these oxides, leaving thus the particles heterogeneous and consisting of at least two materials-the oxide layer on the surface and the metal in the centre. The times required to cool the particles below the melting points of the two materials are estimated for slow and fast cooling in gas and liquid nitrogen, respectively. The estimates are carried out numerically by solving a one-dimensional heat transfer equation for solidifying spherical iron and Fe-Cr alloy particles having either a thin (1 mum) or a thick (10 mum) oxide layer on their surface. It has been found that for the typical particle size of 120 mum in diameter, the cooling rates, under the assumed conditions, are 10(5)-10(6) K s(-1) and it takes 10-1 ms to completely solidify the initially liquid particles. The temperature histories of the particles without any oxide layer and with a thin oxide layer are almost identical.