Corrosion and wear behavior of alumina coatings obtained by various methods
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CORROSION AND WEAR BEHAVIOR OF ALUMINA COATINGS OBTAINED BY VARIOUS METHODS T. Lampke,1 D. Meyer,1,2 G. Alisch,1 B. Wielage,1 H. Pokhmurska,1 M. Klapkiv,3 and M. Student3 Alumina coatings are widely used in a range of industrial applications to improve corrosion protection, wear and erosion resistances, and thermal insulation of metallic surfaces. Refined alumina surfaces with long-term use are obtained from various efficient and adjustable processes. It can be seen that costefficient arc-sprayed Al coatings post-treated by plasma-electrolytic oxidation (PEO) form Al 2 O 3 -layers with remarkable corrosion protection, hardness, bonding strength, and abrasion resistance, as well as with the extended service time. The properties of these coatings are compared with alumina coatings obtained by flame spraying and atmospheric plasma spraying. Keywords: plasma electrolytic oxidation, arc spraying, atmospheric plasma spraying, flame spraying, Al 2 O 3 .
The application of technical components under the extreme operating conditions often demands highly corrosion- and wear-resistant coatings to ensure the long-term functionality of technical systems. Thermal spraying of Al 2 O 3 via atmospheric plasma spraying (APS) or flame spraying are commercially used methods to create coatings meeting these requirements. Corrosion and wear resistance of thermally sprayed Al 2 O 3 -coatings are primarily determined by the phase composition and porosity. Low porosity indicates high melting rates of the corundum spray particles. Quick solidification of the molten alumina particles leads to the formation of metastable -Al 2 O 3 and amorphous Al 2 O 3 , which exhibit lower hardness as compared with corundum (-Al 2 O 3 ) . On the other hand, lower melting rates result in higher porosity and lower coating cohesion. The values of microhardness from 750 to 1650 HV (depending on the parameters of the process of thermal spraying) can be found in the literature. Another method used to produce ceramic coatings is plasma-electrolytic oxidation [PEO also called microarc oxidation (MAO) or spark-discharge anodizing]. It is based on the anodic oxide-film formation on valve metals, such as aluminum, magnesium, titanium, zirconium and their alloys under plasma conditions in lowconcentrated alkaline electrolytes [1, 2]. This method is an alternative to electrochemical anodizing especially because of the very high hardness of layers due to crystalline microstructure. The PEO-treated aluminum parts show improved corrosion and wear resistance. The PEO process can also be applied as a post-treatment of thermally sprayed aluminum coatings to improve their performance characteristics. The paper presents the results concerning the correlations between the production, microstructure and functional properties of ceramic coatings obtained by different methods of thermal spraying and PEO treatment of 1
Chemnitz University of Technology, Institute of Materials Science and Engineering, Chemnitz, Germany. Corresponding author; e-mail: [email protected].
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