Mechanical stability and rheology of lithium–calcium-based grease containing ZDDP

Abstract

Although lubricating greases with zinc dialkyl-dithiophates (ZDDP) have been used widely, the influence of ZDDP on the rheological properties of lithium–calcium-based greases (LCBG) has not been reported. This work investigated the effects of ZDDP on the rheology and mechanical stability of lithium–calcium-based greases (LCBG) by means of scanning electron microscopic (SEM) analysis, rheology characterization and roll stability test. It was found that the thickener's base grease fiber containing ZDDP becomes looser, and the effects of ZDDP on low-alkali LCBG are greater than that on low-acid LCBG. Thereby, the values of the viscoelastic functions in the linear viscoelastic region, the shear stress and mechanical stability of LCBG decrease with the increase in ZDDP concentration. Due to the fibrous instability of the thickener above 60 °C, considerable shortening of the thickener's fibers and increasing penetration values after shearing of the grease in a roll stability tester at 75 °C, as well as transformation from shear thinning to shear thickening at the shear rate of 1800 s⁻¹ and 80 °C, were observed. The maximum recommended ZDDP concentrations related to mechanical stability and resistance under working conditions are below 4 wt% for low-alkali grease and 5 wt% for low-acid grease. Additionally, the saturated adsorption of 3 wt% ZDDP on the soap fibers can lead to sufficient ZDDP migration to the polar metallic surface, which indicates the optimized tribological effect of ZDDP on LCBG. This study provides a promising formulation range of around 3% to 5 wt% ZDDP in low-acid grease.