Aus der Praxis für die Praxis 37 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0012 2 Introduction Greases are used widely in industrial and technological applications to decrease frictional forces and to protect components from wear damage. Examples can be found in machinery equipment, automotive components and electrical contacts. The performance of greases depends on its lubricating quality but also on interaction properties, such as its adherence to a substrate, its cohesion or consistency and its tackiness. Conventionally, ‘grease tackiness’ is 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. This is firstly because it is not a real physical property, and secondly because it strongly depends on application conditions, such as speed, temperature, load etc. Up to now, in order to evaluate the thread forming part of the tackiness, the ‘finger test’ is used. A grease film is applied between thumb and index finger and then the fingers are pulled apart, resulting in the formation of grease threads as shown in Figure 1. A comparison is made based on the length of these threads, the longer the threads the higher the tackiness of the grease. However, these tests are subjective since their outcome depends on the ‘user’ and on how ‘fast or slow’ did he/ she perform the retraction of the fingers, the quantity and distribution of grease applied, whether it is worked prior to separating. Furthermore, these measurements do not provide any quantitative value, so a quantitative and operator independent ranking between different greases is not possible. The first attempt to quantify tackiness was made by the development of a ‘tack tester’. In this method a grease Quantitative approach to measuring the adhesion and tackiness of industrial greases Emmanuel P. Georgiou, Dirk Drees, Michel De Bilde, Michael Anderson* Recently a new method to measure the adhesion and tackiness of industrial greases, based on analyzing indentation/ retraction curves, has been optimized. It replaces the highly subjective finger tests that are still commonly used in the industry. This method is more versatile, it allows to use different contact geometries, material compositions and measurement parameters, whereas it provides accurate and quantitative measurements. A high precision force sensor that operates in the mN range allows to record the necessary curves. This work presents a thorough overview of this novel adhesion/ tackiness measurement method. The physical meaning of indentation/ retraction curves is described, as well as the importance of the contact conditions (retraction speed, applied load, grease temperature, film thickness and contact geometry). We want to highlight how to use this new method to evaluate and compare the adhesion and tackiness of various industrial greases in an efficient, unambiguous and adequate way. Keywords Greases; Adhesion; Tackiness; Methodology; Indentation-retraction Abstract * Emmanuel P. Georgiou 1,2 Dirk Drees 1 Michel De Bilde 1 Michael Anderson 3 1 Falex Tribology N.V., Wingepark 23B, B3110, Rotselaar, Belgium 2 Department of Materials Engineering (MTM), K.U. Leuven,Kasteelpark Arenberg 44, 3001 Leuven, Belgium 3 .Falex Corporation, 1020 Airpark Drive, Sugar Grove, IL 60554, USA Figure 1: Comparison of greases with the finger test T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 37 In our procedure, a grease layer of about 200 μm is spread out on a standardized DIN 6325 steel plate, Figure 3a. The user can select an ‘indenter body’ or ‘tool’ (ball, pin, different alloys and dimensions), this is gradually approaching the grease layer until it comes into contact, then the indenter body keeps moving down until a preset contact load is reached (Figure 4). Then, the indenter body moves away again from the greased substrate under Aus der Praxis für die Praxis 38 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 film was applied between two plates, being pulled apart with a constant low retraction speed. During each test, the pull-off force is recorded and is used as a ranking of tackiness. However, this method also has certain drawbacks as the pull-off force relates more to the adhesion of the grease to the substrate (plates) rather than the formation of threads. Thus, this technique cannot be used to quantify tackiness. In addition, it does not allow to vary testing conditions such as speed or temperature, which are known influencing factors. Achanta et al. [1] and Georgiou et al. [2] developed a new test method, based on analyzing the measurement curve from an indentation/ retraction sequence. The test instrument allows to measure with high precision both pull-off force and thread formation of greases. In this article, a thorough overview of this method will be presented, and its advantages will be highlighted. The authors consider that this topic is of high importance to the industry, recently a lot of new applications for greases have emerged that require special formulation. The lubrication of electrical motors in electric cars is particularly demanding as it covers a wide temperature regime but also the lubrication of pitch bearings in wind turbines requires special attention. Greases emerge more and more from their former volume production, all-purpose formulations to higher specialization. This makes it harder for the end-users to differentiate between many available greases in the market and to select the one that fits best to their application. This trend is confirmed by the increasing production and use of grease worldwide, as confirmed by the National Lubricating Grease Institute (NLGI) [3]. 3 Experimental procedure A modified Basalt-N2 surface tester with a light load sensor was used to develop this methodology, Figure 2a. The force sensor is a cantilever - based on patented dual leaf spring (Patent by TETRA GmbH Ilmenau) as shown in Figure 2b. Applied load and potential tangential forces are picked up by a high precision displacement measurement of the spring elements, measured by capacitance sensors. DOI 10.30419/ TuS-2019-0012 Figure 2: (a) Basalt N2 tester and (b) schematic of dual leaf cantilever. Figure 3: (a) Grease layer applied on a steel plate and (b) countermaterials used in these tests. Figure 4: Schematic of an indentation-retraction curve for a greased contact and measured values [2] (a) (b) T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 38 well-controlled conditions, until complete physical separation has taken place. In this work, we used 3 mm Ø copper balls and Cr6 steel balls as well as a 6 mm x Ø 6 mm Cr6 steel cylinder, Figure 3b. During the approach-retraction cycle, the force on the load sensor is measured continuously at high data acquisition rate. This technique is similar to pull-off force experiments with an atomic force microscope for studying physical interactions, but operates in a different force range (mN vs. nN). From this approach-retraction curve, a few characteristic numbers can be extracted. The indentation requires an amount of energy or work to compress the grease layer (integration of area A in Figure 4). Pulling off the indenter requires an energy represented by area B or a maximum force called the pull-off force (Fa), we further define as separation energy (Se): the energy needed to fully separate both bodies from the position of highest force (pull-off force). The displacement from the pulloff force until total separation, characterized by a return of the force to 0 mN, represents length of the thread (l), Figure 4. For each set of approach-retraction parameters (notably temperature and retraction speed), ten repeats are performed on the same location, in order to get information on the repeatability of the process and to perform statistical analysis of obtained data. 4 Results and discussion 4.1 Effect of testing conditions It is known from field practice that the performance of greases strongly depends on the contact conditions and environment under which they operate [4]. The main advantage of this method is that it allows to perform multiple measurements of adhesion and tackiness under varying conditions automatically. By changing the applied Aus der Praxis für die Praxis 39 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0012 Figure 5: Effect of (a) retraction speed and (b) temperature on the pull-off force and separation energy of a grease. 1 0 1 5 2 0 4 0 5 0 0 , 0 0 , 2 0 , 4 0 , 6 0 , 8 1 , 0 1 , 2 1 , 4 1 , 6 1 , 8 A p p l i e d l o a d ( m N ) Thread length (mm) 1 0 m N 1 5 m N 2 0 m N 4 0 m N 5 0 m N Figure 6: Effect of applied load on the thread formation of grease A. load, retraction speed and temperature, mapping of these properties is possible. A typical example of the effect of retraction speed and temperature on the pull-off force and separation energy of an example grease, is shown in Figure 5. In Figure 5a, it is evident that the increase of retraction speed leads to a higher pull-off force and higher separation energy, indicating that grease sticks more to the tool and the energy to physically separate the two grease covered surfaces is also the highest. On the other hand, an increase of temperature resulted in a significant decrease of both the pull-off force and separation energy, Figure 5b. This observation is linked to the viscous and viscoelastic response of the grease, which can change drastically with a change of temperature [5-7]. In addition, phase transitions within the microstructure of the grease T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 39 measurement, at least in the range of 100 to 500 µm. Despite having an excess of grease in the contact, the ‘true’ thickness of the grease that interacts in these experiments is mostly determined by the tool size and the contact conditions. These experiments also show that the method is tolerant to the adhesion of possible excess grease around the ball, outside of the center of the contact. It suggests that the tackiness is affected mainly by the ‘direct formation’ of threads at the central area of contact, as shown in Figure 8. It should be mentioned that, for the selected system (3 mm Ø ball vs steel plate at 20 mN), the interaction radius would be in the range of 6.5 µm (calculated by Hertzian contact formulae). 4.3 Effect of substrate roughness Since adhesion is a surface phenomenon, the surface roughness/ topography of the steel plates should be considered. Three distinctly different roughnesses were prepared by mechanical grinding with SiC paper and polis- Aus der Praxis für die Praxis 40 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 can also influence its yield strength [8] and subsequently the energy required for the deformation and separation of this layer. Contrary to the strong effects of retraction speed and temperature, the applied load during approach does not appear to have a significant influence on the measured properties of the grease, e.g. Figure 6 for thread length. Evidently, this method is not very sensitive to load fluctuations (e.g. brought about by vibrations). In any case, our experiments have shown that adhesion and tackiness of a grease are not a single value, but depend on the test conditions. So recording adhesion and tackiness can only be done in terms of a specified method. 4.2 Effect of film thickness We have varied the thickness of the applied grease film, to measure its influence on the retraction values. Figure 7, shows that the thickness has little effect on the DOI 10.30419/ TuS-2019-0012 Figure 7: Effect of grease film thickness on the (a) pull-off force and (b) separation energy of a grease. 2 0 0 µ m 4 0 0 µ m 6 0 0 µ m 2 3 4 Pull-off force (mN) 2 0 0 µ m 4 0 0 µ m 6 0 0 µ m 2 0 0 µ m 4 0 0 µ m 6 0 0 µ m 2 4 6 Separation energy (µJ) 2 0 0 µ m 4 0 0 µ m 6 0 0 µ m Figure 8: Thread formation during retraction of the copper ball. hing with 3 µm diamond paste for the lowest roughness (LR) of 0.06 µm Ra. The medium roughness (MR) was 0.39 µm Ra and high roughness (HR) 0.68 µm Ra. In Figure 9 the effect of the plate roughness on the pulloff force and separation energy is presented. Despite having a very localised contact, the surface roughness of the plate does not have any significant effect on adhesion and separation energy. The only noticeable difference is that at the highest roughness the spread of values appears larger, which is possibly the result of a less homogeneous distribution of the grease film on these plates. 4.4 Effect of countermaterial composition This test method allows the use of different tool materials. It is important because greases can be used in various systems with widely varying material combinations. Depending on the material, different adhesion could result. T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 40 An example of the effect of the tool material is shown in Figure 10. Tests were done with both a copper and a bearing steel ball. Some small differences in the pull-off force and separation energy can be seen, particularly a slightly higher pull-off force and separation energy with a steel ball, but also a higher spread in the values. Because of the better repeatability, copper was considered as the reference material for all further testing where only a relative comparison between greases is envisaged. 4.5 Effect of contact geometry (point vs area) Intuitively, it can be imagined that the contact geometry can have a strong influence on the indentation/ retraction measurement. As an example, differences between a ball contact and an area contact are evaluated (Figure 11a). The relevant indentation/ retraction curves show significant differences in the morphology of the curves, Figure 11b, c. The pull-off force (Figure 11d) and separation energy (Figure 11e) are higher in the case of the area contact, which is to be expected due to the formation of more threads over a larger interaction area, than with the ball contact. However, the main drawback of the area contact is the high variation between repeat measurements, whereas the ball contact leads to a much more stable behavior. The area contact variation is thus less appropriate for comparison of greases and standardization purposes, because the larger variation makes it more difficult to recognize significant differences between greases. 4.6 Repeatability of method One of the key questions in every new method is ‘how repeatable is it? ’. Triplicate tests on new sample sets showed that the indentation/ retraction method is very repeatable, Figure 12. Further experimental determination of repeatability and -in a later stage reproducibilityare planned, but these measurements indicate a high potential for having a satisfactory repeatability of the test method. 4.7 Ranking of industrial greases The ultimate goal of this methodology is to allow a ranking of industrial greases in terms of their adhesion and tackiness characteristics. Because we evaluate tackiness or adhesion over a range of test parameters, a comparison of greases can be done by plotting two dimensional graphs, defined as ‘grease experimental tackiness’ Aus der Praxis für die Praxis 41 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0012 Figure 9: Effect of steel plate roughness on the (a) pull-off force and (b) separation energy of a grease. L R M R H R 2 4 6 Pull-off force (mN) L R M R H R LR MR HR 2 4 6 Separation energy (µJ) LR MR HR Figure 10: Effect of ball composition on the (a) pull-off force and (b) separation energy of a grease. C u C r 6 s t e e l 0 2 4 6 Pull-off force (mN) C u C r 6 s t e e l C u C r 6 s t e e l 0 2 4 6 Separation energy (µJ) C u C r 6 s t e e l T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 41 X-axis with a cone penetration measurement, and sliding velocity with retraction speed, Figure 13. In this way the adhesion and tackiness can be plotted as a function of combined working conditions. Aus der Praxis für die Praxis 42 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 (GET) curves. The idea is based on the concept of Stribeck curves [9], but replacing the coefficient of friction in the Y-axis with a tackiness parameter, e.g. separation energy or pull-off force, and replacing the viscosity in DOI 10.30419/ TuS-2019-0012 Figure 11: (a) Selected contact geometries. Indentation retraction curves for (b) point contact and (c) area contact. Effect of contact geometry on (d) pull-off force and (e) separation energy. B a l l _ T h i n f i l m C y l i n d e r _ T h i n f i l m 0 1 0 2 0 3 0 4 0 5 0 Pull-off force (mN) B a l l _ T h i n f i l m C y l i n d e r _ T h i n f i l m B a l l _ T h i n f i l m C y l i n d e r _ T h i n f i l m 0 1 0 2 0 3 0 4 0 5 0 Separation energy (µJ) B a l l _ T h i n f i l m C y l i n d e r _ T h i n f i l m Figure 12: Evolution of (a) pull-off force and (b) separation energy in triplicate indentation/ retraction tests. 0 2 4 6 8 10 12 0 2 4 6 8 10 12 Pull-off force (mN) Cycles Grease A_Repeat 1 GreaseA_Repeat 2 Grease A_Repeat 3 Grease B_Repeat 1 Grease B_Repeat 2 Grease B_Repeat 3 0 2 4 6 8 10 12 0 2 4 6 8 10 12 Separation energy (µJ) Cycles Grease A_Repeat 1 Grease A_Repeat 2 Grease A_Repeat 3 Grease B_Repeat 1 Grease B_Repeat 2 Grease B_Repeat 3 T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 42 An example of ranking industrial greases based on GET curves is given in Figure 14. This figure clearly shows that the working conditions determine the result: the adhesion and tackiness values always have to be linked to the test conditions. It is also clear that a better separation is reached for the higher value of the working conditions, in our experiments notably for the higher retraction speeds. Besides this 2D representation, a 3D mapping of greases is also possible by considering two main variables, for instance temperature and retraction speed [2]. These 3D maps can be further modified to include other variables (e.g. contact pressure, humidity level etc.) which can be more closely related to the specific needs of the end user or grease producer. In the end, it will be possible to map the operational windows of greases in terms of required tackiness, related to many operational parameters. 5 Conclusions This work presents an overview of the new indentation/ retraction method. This technique allows to measure quantitively and with high precision adhesion and tackiness of greases. Due to the versatility of the setup, many factors that might influence the adhesion and tackiness can be separately studied. From the experimental examples presented in this work, it is evident that adhesion and tackiness are not a single material value, but depend on the test conditions. The method is not very sensitive to the grease film thickness and the load during approach, making it a very robust method. The results show however that the tackiness depends a lot on retraction speed and temperature, as can be intuitively understood or demonstrated by the finger test. In this sense, the method correlates well with experience, but adds on the possibility to attach an independent and real quantitative value on the tackiness of a grease. Finally, analysis and comparison of industrial greases is now easy, either by plotting them in 2D grease experimental tackiness (GET) curves and/ or 3D maps. Currently, the authors work on standardization of this method and equipment, to be used to create a database for industrial greases. References [1] S. Achanta., M. Jungk, D. Drees, Characterisation of cohesion adhesion, and tackiness of lubricating greases using approach retraction experiments. Tribol. Int. 44 (2011) 1127-1133 [2] E.P. Georgiou, D. Drees, M. De Bilde, M. Anderson, Can we put a value on the adhesion and tackiness of greases? Tribol. Lett. 66 (2018) 60 [3] NLGI Annual Grease Survey. 2005 [4] P.M. Lugt, Grease lubrication in rolling bearings. Wiley, 1 st Ed. London, UK (2013) [5] B.P. Williamson, K. Walters, T.W. Bates, R.C. Coy, A.L. Milton. The viscoelastic properties of multigrade oils and their effect of journal-bearing characteristics. J. Non- Newtonian FluidMech. 73 (1997) 115-126 [6] M.A. Delgado, C. Valencia, M.C. Sanchez, J.M. Franco, C. Callegos. Thermorheological behaviour of a lithium lubricating grease. Tribol. Lett. 23 (2006) 47-53 [7] M.A. Delgado, C. Valencia, M.C. Sanchez, J.M. Franco, C. Callegos. Influence of soap concentration and oil viscosity on the rheology and microstructure of lubricating greases. Ind. Eng. Chem. Res. 45 (2006) 1902-1910 [8] F. Cyriac, P.M. Lugt, R. Bosman. On a new method to determine the yield stress in lubricating grease. Tribol. Trans. 58 (2015) 1021-1030 [9] I. Hutchings, P. Shipway, Friction and wear of engineering materials, 2nd Ed. Butterworth-Heinmann, Elsevier, Oxford, UK (2013) Aus der Praxis für die Praxis 43 Tribologie + Schmierungstechnik · 66. Jahrgang · 2/ 2019 DOI 10.30419/ TuS-2019-0012 kinematic viscosity → replace by cone penetration sliding speed → replace by retraction speed Tackiness ! ! " #$%& % & #$%& % ' #$%& % ( #$%& % ) #$%& % % #$%& % * #$%& % # Figure 14: GET curves showing influence of retraction speed on the separation energy of a number of industrial greases. Figure 13: Grease Experimental Tackiness (GET) curves based on modified Stribeck curve. T+S_2_2019.qxp_T+S_2018 16.04.19 13: 48 Seite 43