Tribological evaluation of plasma sprayed coatings for high temperature sliding surface against cordierite

A study has been carried out to develop a high temperature sliding seal for the ceramic gas turbine engine's (CGT) regenerator. The solid-lubricant coatings, which consisted of the following combinations: NiO, Ni/Cr, Cr3Cr2/NiCr, ZrO2/CaO,CaF2, BaF2, Ag2O and graphite, were synthesized by low pressure plasma spraying. Friction and wear characteristics were evaluated at temperatures from 50 to 1000 degrees C by rubbing the coatings against a cordierite disk. After the sliding tests, the worn surfaces were analyzed by SEM-EDX and XRD. The analytical results showed that the tribological properties of coatings were significantly related to the surface layers. Formation of a CaF2/BaF rich layer led to a low friction and a low wear property. On the other hand, formation of solid-state reaction product (Mg0.4Ni0.6O) between the coating and the cordierite increased the friction coefficients, and the transfer of the cordierite debris caused a seizure and/or a severe wear. Among the sprayed coatings, the Cr3C2/NiCr based coating containing CaF2/BaF2 and Ag2O showed excellent tribological properties, which should satisfy the specifications for the basic design of a CGT engine.