Influence of Substrate Materials upon Fabrication of Aluminum Nitride Coatings by Reactive RF Plasma Spraying

Abstract

Influence of the substrate materials upon fabrication of aluminum nitride (AlN) coatings by reactive RF plasma spraying was investigated according to spraying onto various substrate materials. Aluminum particles were injected into Ar/N2 plasma and were deposited onto carbon steel (S45C), aluminum alloy (A6063) and quartz substrates. In the case of using carbon steel substrate, fabrication of thick and dense Al based Al/AlN composite coatings was possible. However, higher concentration of AlN phase in the coatings peeled off the coatings during cool down after spraying due to the difference in thermal expansion coefficient between AlN and carbon steel. Therefore, formation of Al interlayer was attempted between AlN layer and carbon steel substrate in order to decrease the thermal stress. As the result, Al interlayer included Al/AlN composite coating was fabricated onto a carbon steel substrate with control the N2 flow rate in plasma gas during spraying. While fabrication of AlN layer onto Al interlayer was possible, it was concerned that formation of Fe-Al intermetallic compound at the interface between the coating and the substrate formed defects. Then not the Al interlayer but aluminum alloy was used as the substrate. However, the aluminum alloy substrate melted during spraying even the substrate was cooled by water-cooled substrate holder. Therefore, it is demanded to the substrate materials have a melting point that is much higher than aluminum. On the other hand, almost completely AlN coatings were fabricated using quartz substrate, which involves lower thermal expansion coefficient and higher melting point. Therefore, the characteristics of low thermal expansion coefficient and high melting point were required for substrate materials in order to fabricate AlN or AlN based Al/AlN composite coatings by reactive RF plasma spraying. Especially, quartz was useful for the substrate material.