Preparation and characterization of aluminum-based coatings deposited by very low-pressure plasma spray

Aluminum-based coatings were manufactured by the very low-pressure plasma spray, using an F4-VB plasma torch under a working pressure of 150 Pa. The morphologies and phase compositions of the resultant coatings were investigated along the radial direction of the plasma jet, which was found that the Al-based coatings exhibited three different deposition states. At the center of the plasma jet, an Al-Fe coating was formed due to the diffusion of the substrate element, which had a packed columnar microstructure that deposited by the vapor and clusters. As the radial distance increases, a pure Al coating was obtained, with a lamellar binary microstructure that mainly formed by spread liquid droplets and plenty of vapor. At the outer edge of the plasma jet, another pure Al coating was achieved, possessed a typical layer microstructure consisted of semi-molten particles, molten particles, and a little vapor. Moreover, the microhardness of the deposits was measured, respectively. The hardness of Al-Fe coating reached 448.74 +/- 23.7 HV0.025, while that of pure Al coating was only 46.37 +/- 3.18 HV0.025. In order to better explain the difference in coating microstructures and properties along the radial direction, a spatial distribution model for the depositions based on the melting state of Al powders was established.