Sealing of thermal spray coatings by impregnation
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J. Knuuttila, P. Sorsa, and T. Mäntylä (Submitted 3 June 1998; in revised form 12 January 1999) Results from the sealing of porosity by impregnation show that below a certain wetting angle of the sealant, high penetration depths are achieved. However, only sealants with very low curing shrinkages can prevent the transport of electrolyte through the coating. Various sealant types and impregnation methods are discussed, and factors influencing impregnation and sealing ability of sealants are reviewed. Experimental results from the sealing of plasma-sprayed aluminum-oxide coatings are presented.
Keywords
coatings, corrosion, impregnation, sealing, thermal spray
1. Introduction Moderate adhesion and a porous structure are the most often encountered problems in thermal-spray coatings that restrict their use in many applications involving corrosive media. Although most metallic consumables can be sprayed to dense coatings without open, through-coating porosity, ceramic coatings invariably contain open porosity and usually also cracks. These structural flaws not only deteriorate the corrosion resistance of the coating-substrate system but also decrease mechanical properties and consequently the wear resistance of the coating. Although the development of higher velocity processes has decreased coating porosities, the transport of corrosive species to the substrate can still only be prevented by coating posttreatment. Posttreatment of thermal spray coatings to close the surface porosity can be performed by laser- or electron-beam surface melting or alloying (Ref 1, 2). Also hot isostatic pressing (HIP) of the coating-substrate system can be used for densifying (Ref 3). Hot isostatic pressing not only reduces porosity but also improves mechanical properties. It is effective through the whole coating thickness but demands complex and expensive equipment. Obviously the deposition of a dense layer by another method over the porous coating can be used for sealing. Hightemperature chemical vapor deposition (CVD) processes (Ref 4) and the modification, chemical vapor infiltration (CVI), using reactive gases, are other possibilities for porosity sealing. Recently metal-organic chemical vapor deposition (MOCVD) has been used for sealing oxide coatings although the results against high-temperature gaseous corrosion were not encouraging (Ref 5). More often posttreatment is performed by impregnation using polymers, inorganic solutions, or even molten metals. In this article, factors affecting coating porosity and sealing by impregnation are reviewed. Commercial sealants, their properties, and sealing abilities were studied and compared to laboJ. Knuuttila, M. Sc. (Eng.), P. Sorsa, and T. Mäntylä, Tampere University of Technology, Institute of Materials Science, P.O. Box 589, FI-33101 Tampere, Finland. Contact e-mail: [email protected]. J. Knuuttila and P. Sorsa presently at Millidyne Oy, Hermiankatu 8D, FI-33720, Tampere, Finland. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
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