| The ability to lubricate gas turbine bearings at temperatures beyond the limits of liquid lubrication has been a research objective for more than four decades. This research investigated several classes of synthetic lubricants for potential use as vapor phase lubricants in gas turbines. Specific lubricants investigated were: a tertiary-butyl phenyl phosphate (TBPP); a cyclophosphazene (X-1P); a 2 cSt polyalphaolefin blended with TBPP; a polyphenylether (5P4E); and a pefluoroalkylether (Z). Preliminary tests were conducted in a ball-on-rod tester modified for high temperature operation. The lubricants containing phosphorus demonstrated the best high temperature performance, with sustained operation at temperatures above {dollar}600spcirc{dollar}C in an air environment.; After test, bearing surfaces were analyzed using {dollar}mu{dollar}-FTIR, Raman spectroscopy, SEM, EDS, and AES. Condensed phosphates and graphite were identified on the bearing surfaces lubricated with the TBPP and X-1P vapor. Abrasive iron oxides with significantly less graphite were identified on surfaces lubricated with 5P4E vapor. The difference in deposition films is attributed to the phosphate structure formed by the organophosphorus lubricants.; Additionally, AES depth profiles indicated that mass diffusion was involved in the formation of vapor deposition films for several of the lubricants tested. Diffusion coefficients of the order of {dollar}rm 10sp{lcub}-14{rcub} cmsp2/s{dollar} were detected on T15 bearing steel at bearing temperatures of {dollar}370spcirc{dollar}C when lubricated by TBPP vapor. This value is six orders of magnitude larger than the self-diffusion coefficient for BCC iron at {dollar}370spcirc{dollar}C. The difference is attributed to subsurface stress gradients and chemical reactions with vapor lubricants.; The candidate lubricants were also evaluated in full-scale bearings at gas turbine engine conditions. The TBPP lubricant, T15 bearing steel, {dollar}rm Sisb3Nsb4{dollar} rolling elements, C-C composite bearing cages, and oil-mist delivery provided the best performance, demonstrating sustained operation at temperatures up to {dollar}450spcirc{dollar}C, stress loads of 1.8 GPa, and bearing speeds of 1.3 MDN. These conditions represent the limitations of the bearing tester, not the technology itself. The use of C-C bearing cages and oil-mist delivery were important for thermal management in high speed bearings. |
