Characterization of thermally sprayed coatings for high temperature wear protection applications

Abstract

Under normal high temperature gas-cooled reactor (HTGR) operating conditions, faying surfaces of metallic components under high contact pressure are prone to friction, wear and self-welding damage. Component design calls for coatings for the protection of the mating surfaces. Anticipated operating temperatures of up to 850–950 °C (1562–1742 °F) and a design life of 40 years require coatings with excellent thermal stability and adequate wear and spallation resistance. In addition, they must be compatible with the HTGR coolant helium environment. Plasma- and detonation-gun-deposited chromium-carbide-base and stabilized zirconia coatings are under consideration for wear protection of reactor components such as the thermal barrier, heat exchangers, control rods and turbomachinery. Programs are under way to investigate the structural integrity, helium compatibility and tribological behavior of relevant sprayed coatings. In this paper the need for the protection of critical metallic components and the criteria for the selection of coatings are discussed. The technical background to coating development and the experience with the steam cycle HTGR are commented on. Coating characterization techniques employed at the General Atomic Company are presented, and the progress of the experimental programs is briefly reviewed. In characterizing the coatings for HTGR applications, it is concluded that a systems approach to establish correlation between coating process parameters and coating microstructural and tribological properties for design consideration is required.