The Wall Effect in the Flow of Commercial Lubricating Greases

The results of the studies on the formation of surface and boundary layers in commercial lithium (LT4-S3) and calcium (STP) greases near the walls of six different materials are presented. Two elastomeric materials (nitrile-butadiene rubber (NBR), silicone rubber (MVQ/VMQ)), two thermoplastic materials (polyoxymethylene (POM), polyethylene (PE)), and two metal (copper C11000 and steel 304) alloys were used in the tests. The tests were carried out using a rotational rheometer operating in the plate/plate configuration. Structural viscosity–shear rate curves were determined and dynamic oscillatory tests were carried out. The tests have shown that the metal alloys have the highest capacity to adsorb grease thickener particles on their surface. The elastomeric materials have the smallest effect on the change in structural viscosity in the vicinity of the wall, which indicates their low capacity to form a surface layer in the tested commercial greases.