Rheological behavior of greases: Part I - Effects of composition and structure

The aim of this work is to show the correlation between grease rheological behavior and its composition and structure (base oil viscosity, soap nature, and concentration). A rheological investigation was carried out under the controlled stress mode. The results, obtained over different time intervals, show that grease behaves like a non-Newtonian viscoelastic material, with a narrow linear domain. The soap nature and concentration are the dominant parameters when grease is submitted to low shearing. This was confirmed by TEM observations: the closer networks gave the highest rheological properties. Base oil viscosity influenced grease behavior only under high strain rates. Some properties are discussed with respect to their influence on the contact replenishment, which is a critical point for mechanism lifetime prediction. Finally, the authors suggest using rheometry as an alternative technique to standardized methods. Yield stress varies in the same way as penetration does, but is further influenced by the sample macrostructure.