September / October 2020 NLGI Spokesman

Grease Adhesion and Tackiness: Do They Influence Friction?

Emmanuel Georgiou(1), Dirk Drees(1), Michel De Bilde(1), Mike Feltman(2), Michael Anderson(2) Falex Tribology N.V., Wingepark 23B, B3110, Rotselaar, Belgium Falex Corporation, 1020 Airpark Drive, Sugar Grove, IL 60554, USA Abstract The performance of greases in the field is strongly dependent on their adhesion to surfaces and their tackiness or ability to form threads. Recently, a new instrumental method was developed to measure with precision the adhesion and tackiness of greases based on repeated indentation and retraction measurements. However, until now, the link between grease adhesion, tackiness, and friction has not been explored. This study focused on investigating the relationship between adhesion and tackiness on friction of greases. Adhesion and tackiness were measured by indentation-retraction and compared with friction data from Mini-Traction (MTM) and TE-77 tests for formulated greases.

Introduction Greases are used widely in industrial and technological applications to decrease friction and to protect components from wear. The performance of a grease depends on its ability to lubricate as well as its adhesion to substrates, its cohesion, its consistency, and its tackiness. Conventionally, ‘grease tackiness’ is understood to be the ability of a grease to form threads when it is pulled apart. However, to define tackiness in terms of numbers is far more complicated because it strongly depends on speed, temperature, load, and other application conditions. At present, the most appropriate method to measure both grease adhesion and tackiness is based on analyzing the measurement curve from an indentation-retraction sequence [1-3].

In this method, a grease layer is applied on a standardized steel holder. An indenter such as a ball or pin is gradually brought into contact with the grease layer, and then pressed into the grease until a preset contact load is reached (maximum force in Figure 1a). Then, the indenter is gradually moved away from the greased substrate under well-controlled conditions until there is complete physical separation.

During the retraction step, the following parameters are measured. The pull-off force (indicated by minimum force in Figure 1a) is the force required to start the physical separation of the indenter from the grease and relates to adhesion. As the indenter is retracted, a grease thread forms (Figure 1b), which resembles the finger test (Figure 1c). A highly sensitive sensor measures the force of the thread pulling against the indenter. After the thread breaks, the force is zero.

In this study, both force and displacement of the indenter were recorded, and tackiness was defined as the displacement of the indenter during the retraction process required for complete physical separation. In addition, the separation energy (Se) was defined as the energy needed to fully separate the indenter from the grease by the pull-off force. Based on this definition, tackier greases formed longer threads as with the finger test.

‘Tacky’ does not necessarily mean strong adhesion. A typical example can be found with glues that show strong adhesion (high pull-off force), but do not form threads.

- 20 NLGI Spokesman | VOLUME 84, NUMBER 4 | September/October 2020