Formation of Metal -Ceramic Plasma Coatings with Aluminum Hydroxide Addition

Abstract

Thermal spraying due to slight heating, minimizing thermal deformation and the absence of structural changes in the surface of the products allows to effectively form protective composite coatings of various compositions. One of the most promising is the coating with aluminum oxide, which is characterized by high hardness, heat resistance, chemical inertia and high thermal protection and electrical insulation properties. Among other thermal spraying methods, plasma spraying is the most versatile and effective. As a result of the combination of wire (wire brand ER5356) plasma spraying with simultaneous supply of powder material, a composite coating consisting of an aluminum matrix and particles of aluminum hydroxide was formed. It is shown that the microstructure of the coating has a characteristic lamellar structure with a porosity of about 15 %. The microhardness of the aluminum matrix is 0,32 GPa. The particles of aluminum hydroxide and its decomposition products are partially exposed in the preparation of cross-sections, which is explained by the formation of conglomerates and insufficient wetting of aluminum particles in the formation of the coating. It is established that during the spraying active release of water vapor due to the process of thermal dehydration of aluminum hydroxide is observed. Further formation of oxide ceramics in the coating structure is proposed by the heat treatment at 900 °C for 2 hours. Metallographic analysis of samples showed the formation of aluminum particles at the site of aluminum hydroxide (α and γ) with a microhardness of 5,2… 7,2 GPa. Significant decrease in the amount of metal light phase compared to the condition after spraying is noted, which is explained by the oxidation of aluminum by the products of decomposition of aluminum hydroxide in the process of heat treatment. The boundary of the "coating-substrate" separation is characterized by the presence of a diffusion zone with a depth of about 80 microns. X -ray analysis determined that the coating after heat treatment contains: 52,2 % of corundum (α-Al₂O₃), 4,5 % γ-Al₂O₃, 28,2 % aluminum, 13,7 % of iron, 1,5 % silicon.