Reactive atmospheric plasma spray of Al powder

Abstract

Reactive plasma spraying (RPS) is based on using reactive plasma to react with feedstock powders. RPS is a promising technology of material processing and coating manufacturing, especial for ceramic coatings like AlN which is expected to contribute to semiconductor equipments and so on, due to its outstanding properties [1]. However, fabrication of AlN coatings by conventional thermal spray processes was impossible due to thermal decomposition of AlN powder during spraying. In our previous study we fabricated AlN coatings through nitriding of Al powder in evacuated Radio Frequency (RF) plasma spray equipments using Ar/N 2 mixture as a plasma gas [2,3]. In this study we fabricated AlN coatings through nitriding of Al powder in the air by Atmospheric plasma spray (APS) process, using N 2 /H 2 as plasma gases. Al particles are injected in the plasma jet created by the passage of N 2 /H 2 gases through an electric arc under high current conditions. Al particles continuously heated up (At high temperatures: above its melting point) and interacts with the plasma jet during flight to reach a certain energy state which is composed of thermal energy and kinetic energy. Al particles are fused and simultaneously accelerated by the plasma flow and gain their velocity and temperature by thermal and kinematic transfers towards SS400 substrate to be coated. Firstly we used carrier gas Ar and we found that the spray distance is an important parameter for nitriding reaction of Al powder. At small spray distance the coatings contain both Al and AlN but by increasing the spray distance the amount of AlN increases gradually, due to increase the reaction time so the possibility of Al nitriding increases, that at 300 mm spray distance the coatings is pure AlN. However, the coatings thickness decreased gradually from 100 to 20 µm with increasing the spray distance from 150 to 300 mm, thus particles temperature decrease by increasing the spray distance so its deposition efficiency decreased. To fabricate thick AlN coatings we used N 2 as carrier gas and the coating mainly AlN with thickness 100 µm at spray distance 100 mm.