The quantitative method for measuring grease tackiness

Emmanuel Georgiou, Dirk Drees, Michel De Bilde Falex Tribology N.V., Wingepark 23B, B3110, Rotselaar, Belgium

Industrial need to quantify tackiness Lubricating greases are used in various industrial fields ranging from food, transportation, aeronautical, construction, mining and the steel industry. The aim is to decrease frictional forces and to protect industrial components from wear and/or corrosion damage. Their performance depends on interaction properties like adherence to the substrate, cohesion or consistency, and tackiness. Tackiness is defined as the ability of the grease to form threads when pulled, so that an easy transfer of grease to contacting areas can be achieved. In some cases, high tack is desired, but sometimes not.

This issue is attracting more industrial interest because greases are continuously evolving, making it harder for the developers and end-users to differentiate between available greases in research or market and to select the one that fits better to their application. However, up to date there is no established quantitative methodology that can be easily applied to efficiently and accurately evaluate the adhesion and tackiness of a grease.

A first development was made in 2011 in cooperation with the Dow Corning company, resulting in a scientific publication about the measurement principle1. Recently, we have transferred this research methodology to a routine test instrument and focussed on the robustness of the method.

Falex tribology NV adopts new grease adhesion methodology The upgraded Basalt-N2 with light load sensor is used to apply the methodology that was previously developed with a research instrument (Falex_MUST tester). A view on the test instrument in Figure 1 shows the tester and the light load sensor assembly.

Figure 2: Typical approach-retraction curve for a greased contact.

From the approach-retraction curve,several numbers can be extracted. The indentation requires an amount of energy or work (integration of area A in Figure 2=compression of grease). Pulling off the indenter requires energy represented by area B or a maximum

Falex-MUST are mostly related to its improved user friendliness and range of parameters. Indentation experiments can be done easily, repetitions and testing at different locations is programmable, so the tester can collect a lot of data automatically.

In this methodology a grease layer of 200 µm is applied on a standardized Cr6 steel substrate plate. A user selectable indenter body (ball, pin) gradually approaches the grease layer until they come into contact, then the indenter body keeps moving down until a pre-set contact load is reached (Figure 2). Then, the indenter body moves away from the greased substrate under well controlled conditions, until complete physical separation. During this approach- retraction cycle, the force on the load sensor is measured as a function of time and distance moved. This technique is the same as pull-off force experiments with an atomic force microscope for studying physical interactions at smaller scale.

force called the ‘Pull-off force’) and the tackiness is then defined by the work still needed to fully separate both bodies after the highest pull off force has been reached. Approach-retraction test can be performed multiple times either on the same spot or a different one, to get information on the repeatability and to perform statistical analysis of obtained data.

Industrial use Figure 3 shows representative pull-off curves using a 3mm diameter copper ball and two different greases applied on the Cr6 steel substrate, 10 cycles for every experiment. The structure and nature of the grease dictates the appearance and the shape of these curves. It can be seen, for instance, that the rate of force reduction past the pull-off force is different for both greases. Grease 2 (in the bottom of the graph) has a faster return of force to 0, indicating less tackiness than grease 1 (in the top of the graph). Also, grease 1 has a lower pull-off force than grease 2. Sometimes, a higher pull-off force can be seen in the first cycle, attributed to the fact that during the first cycle the surface of the indenter body is still ‘clean’, whereas from the second cycle onwards, a thin grease layer can be present on the indenter. This finding illustrates the sensitivity and precision of this test technique.

Figure 3: Approach-retraction curve for two industrial greases.

The advantages of this tester over the Figure 1: (left) N2 Basalt tester and (right) Millinewton sensor (developed by TETRA). 18

From these indentation cycles, , the pull-off force (Figure 4a) and integrated


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