Application of a high temperature self-lubricating composite coating on steam turbine components

A high temperature self-lubricating composite coating has been successfully applied on steam turbine governor valve lift rods that are subject to metal-to-metal wear and gall at elevated temperature (540 °C). The coating (NASA designation PS304) is a chrome-oxide based plasma sprayed coating with embedded silver and BaF2/CaF2 eutectic compound particles to function as solid lubricants. An inspection of the coated lift rods, after 8500 h of operation, showed that the surfaces were well protected. Further examination indicated that a layer of lubricous glaze, containing chrome-oxide, silver and barium/calcium fluorides, had formed on the coating surface, which effectively protected the rods from high temperature wear and galling damage. Laboratory evaluation of a sample coating, which had been heated to 540 °C for 150 h, demonstrated that the coating was thermally stable. The mechanisms of the solid lubricants transferring to the counterface under different conditions are discussed in the article.