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

Abstract

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 \mbox{Fe-Cr} alloy particles having either a thin (1 µm) or a thick (10 µm) oxide layer on their surface. It has been found that for the typical particle size of 120 µm in diameter, the cooling rates, under the assumed conditions, are 105-106 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.

Microstructure and Tribological Properties of Ni-Base Coatings under Different Lubrication Conditions

“…According to kinematical scattering, the reasons are mainly associated with two aspects, one is the crystallites become smaller and the other is that large amount of lattice defects are present in coating after spraying [21]. In addition, the plasma spraying is a rapidly cooling process, which is benefited for the formation of fine grain or amorphous phase in Ni-based coatings [22][23][24]. High Performance Structural Material The typical features of the as-sprayed coating are shown in Fig.…”

Microstructural Characterization of Shrouded Plasma-Sprayed Titanium Coatings

“…The first factor is the inadequate amount of oxygen during the build-up of the coating due to the shielding effect of the shroud attachment and the inert gas shroud. The other is the rather short time that the titanium particle spent at a high temperature during the plasma spraying process, which was followed by a very fast cooling rate (10 5 -10 6 Ks −1 range) [21,22] when the particles impacted on the substrate. Therefore, it resulted in the rapid formation of TiO other than TiO 2 in the as-sprayed titanium coatings.…”

Enhancement of metallic glass properties of Cu-based BMG coating by shroud plasma spraying

“…Since these BMGs do not contain dislocations or grain boundaries, they have unique properties including high strength, large elastic strain rates, high corrosion resistance, and wear resistance [1][2][3][4]. Due to the high cooling rate (10 6-7 K/s) seen in thermal plasma, thermal plasma spraying can be used to deposit glass forming materials that retain the amorphous structure of the feedstock [5][6][7][8]. However, BMGs are crystallized to a considerable degree after coating formation by the atmospheric plasma spraying (APS) process which is elucidated in our previous study [9].…”