Wear resistant coatings for reactor components in liquid sodium environments

Rubbing surfaces of critical components for the coming generation of the nation's liquid–metal-cooled fast-breeder reactors must withstand severe environments of high temperature, liquid–metal corrosion, and nuclear irradiation. For some component requirements, the use of the better known bearing materials and hardfacings has been shown to be inadequate or impractical. For several applications, bulk (> 25-μ thick) coatings have provided the only successful solution to friction problems in high-temperature sodium. Coating processes that have been or are being evaluated include plasma spray, detonation gun, spark-discharge, sputtering, and diffusion coating processes. Testing included friction and wear tests in up to 650 °C sodium, sodium corrosion tests for up to one year, thermal cycling, bond strength tests, and irradiation tests in a fast reactor. For one particularly severe application, a specially modified commercial coating of chromium carbide in a 15 vol% nichrome binder, applied by a commercial detonation gun process, has so far been found to be most acceptable in meeting the friction, corrosion, and irradiation damage criteria. Much of the test data reported is on this coating and its antecedent modifications.