Tribological Evaluation of Plasma Sprayed Coatings for High Temperature Sliding Surface Against Cordierite

Abstract

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, Cr₃ Cr₂,/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^°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 (Mg_0.4Ni_0.6O) 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 Cr₃Cr₂/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.