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Synthetic lubricants are coming into prominence for lubrication of aircraft turbine engines, because of the stringent operating temperature requirements. Because many of the surfaces to be lubricated in the turbine engine operate under conditions of boundary or “thin-film” lubrication, the friction and surface-failure properties of the lubricant under these conditions are of extreme importance. In consequence, an investigation was made at the NACA laboratories of the friction properties of several classes of synthetic lubricants over a wide range of sliding velocities. Most synthetics including a diester, a polyether, a silicate ester and a phosphonate ester are more effective boundary lubricants at high sliding velocities than petroleum oils of comparable viscosity at 100° F. The breakdown of effective lubrication takes place at a much higher sliding velocity with these synthetic fluids than with the petroleum oils.