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Characterisation of cohesion, adhesion, and tackiness of lubricating greases using approach–retraction experiments
Tribology International 44 (2011) 1127–1133
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Characterisation of cohesion, adhesion, and tackiness of lubricating greases using approach–retraction experiments S. Achanta a,n, M. Jungk b, D. Drees a a b
Falex Tribology N.V., Wingepark 23B, B3110 Rotselaar, Belgium Dow Corning GmbH, Wiesbaden, Germany
a r t i c l e i n f o
abstract
Article history: Received 25 November 2009 Received in revised form 15 April 2011 Accepted 20 April 2011 Available online 1 May 2011
Greases are widely used in machinery and automotive components to protect components from frictional and wear losses. As a result, interaction properties like adhesion to the substrate, cohesion or consistency, and tackiness become crucial factors and often dictate their performance. All these properties are related to microstructural aspects of grease like thickener network, wetting agents, and additives. The aim of this paper is to use approach–retraction experiments for qualitative/quantitative determination of the above mentioned properties and also understand the influence of grease constituents on these properties. It was found that among all the grease constituents, the thickener in particular dictates the cohesiveness and tackiness of a grease with less influence on the adhesion. The effect of thickener on the lubricating properties of the greases was not clear, which supports the notion that oil in the grease provides the lubrication and thickener to be a carrier. The data on cohesiveness/consistency obtained from this approach–retraction experimental method correlate with the traditional cone penetration tests thus validating this methodology. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Lubricating greases Interaction properties Adhesion Tackiness
1. Introduction Lubricating greases are widely used in many applications ranging from electrical contacts [1], wire-rope applications [2], and largely on machinery where lubricants cannot be successfully applied, e.g. on unsealed or non-enclosed components. The main constituents of lubricating grease are the oil and the thickener which in most cases is metal based soaps [3]. The function of lubricating grease is analogous to a wet sponge. The oil retained in the thickener network is released based on several equilibrium conditions such as temperature, shear stress, and mechanical pressures [4]. Some lubricating greases contain additives for friction reduction, anti-wear properties, temperature stability, etc. [5]. One of the desired properties of greases is stickiness to the surface, which is determined by factors such as cohesiveness, adhesion to a surface, and tackiness or formation of thread like structures. By varying the composition of the constituents the properties of greases can be adjusted. For example, high viscous polymers (called tackifiers) are added to increase tackiness or thread formation. Most experimental methods to characterize lubricating greases with respect to their cohesive and adhesive properties are rather empirical in nature. The cohesiveness historically has been
n
Corresponding author. E-mail address: [email protected] (S. Achanta).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.04.019
qualitatively measured by cone penetration [6] and oil separation measurements [7], while more recently rheological measurements provide some scientific insight [8]. In cone penetration the consistency or cohesiveness of grease is measured by dropping a cone of known mass into grease filled trough and recording the penetration depth of the cone. For instance, a low depth of penetration means firm grease with good consistency. The adherence of greases to the substrate is characterised by water spray off or water wash out measurements [8]. In water spray test, the greased metallic surface is subjected to direct water jet and the adherence is measured from the mass of grease lost as a function of time [9,10]. Another method used for quantifying adherence is by subjecting greased cylinders to centrifugal forces. Depending on the amount of grease mass lost, the adhesion strength is ranked [11]. In most cases, failure occurs within the grease, which technically is cohesive failure. A further improvement in adhesion measurement is done by simultaneous conduction current measurements during centrifuging. With this technique the adhesion of grease to the substrate is defined in terms of coverage area or ratio of metallic to greased area after centrifuging [11]. The above mentioned test methods suffer from issues like poor precision and limited correlation to industrial practise [12]. Another interesting aspect of grease is tackiness (ability to form strings), which is desirable in some applications and unwanted in some others (like gears, wire ropes, etc.). The measurement of tackiness is still very subjective.
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In this paper, we explore the possibility of using approach– retraction experiments to characterise properties of greases like adherence, cohesion, and tackiness (ability to forms threads before separation). The importance of various constituents of grease like oil, thickener, and additives on the above properties will be explored. Moreover, the illustrated method can be an useful tool for the selection of appropriate grease or in the research and development.
2. Experimental The approach–retraction test method is simple in which a greased substrate is moved towards a ball attached to a flexible cantilever. On establishing the contact, the greased substrate is moved further until a certain contact load is achieved. On reaching the target contact load, the greased surface is moved away from the ball (retraction) until complete physical separation. The basic technique is analogous to pull-off force experiments done with an atomic force microscope for studying physical interactions [13]. After this approach–retraction cycle, when the deflection force is plotted as a function of moved distance, important information on the properties of greases can be obtained. A schema of the experimental set-up is shown in Fig. 1a, which consists of steel ball attached to a sensitive cantilever with a low spring constant of 80 N/m and can deflect in the direction parallel to the motion table. The deflection on the cantilever is measured using fibre optic sensor technology (microtribometer [14], Falex NV, Belgium). The sensor used in this study has bundle of emitters and receivers arranged randomly and has characteristic intensity to distance relationship as shown in Fig. 1b. The optical fibre is aligned perpendicular facing a mirror (approximately 1 mm
Cantilever block
Mirror
Fibre optic sensor
Grease
Steel ball
Reciprocating motion table
1.2 Normalised intensity (I/I0)
calibration curve for sensor
1.0 0.8 0.6
away) attached to the cantilever deflection axis (see Fig. 1a). The optical fibre emits certain intensity of light, which is reflected back from the mirror, I0. When there is a deflection, the reflected light intensity changes and the deflection can be determined from the calibration curve shown in Fig. 1b. The obtained deflection in mm is then multiplied with the spring stiffness to precisely determine the deflection forces. The desired load between the ball and the greased surface can be controlled by constantly monitoring the deflection force on the cantilever through a closed feedback loop system. The same set-up can also be used to measure frictional forces by simply changing the cantilever which can deflect in both perpendicular and parallel to the motion table. 2.1. Materials The greases used in this work were supplied by Dow Corning GmbH, Wiesbaden, Germany. The commercial names of the greases are not revealed due to confidentiality. The microstructure of the greases is shown in Fig. 2. The investigated greases are Grease 1 (Fig. 2a) is silicone oil based containing lithium soap thickener (15–20 w%) with a specific gravity of 1.05. Grease 2 is mineral oil based with 3–7 w% lithium soap thickener along with solid lubricant micro-particles (2–10 mm in size) and has a specific gravity of 0.93 (Fig. 2b). Solid lubricant particles are commonly used as additives and remain dispersed structure. Grease 3 is mineral oil based with only solid lubricant particles in the range of 2–10 mm (minimum 30% by weight) with a specific gravity of 1.4 (Fig. 2c). Micro-particles in Grease 3 were polarised. The polarisation of micro-particles improves their adhesion to the substrate. It can be seen that grease 1 has a spongy structure and appears distinct from other two greases. Grease 2 and 3 bear some similarity because of similar type of oil and solid lubricant particles. A polyoxymethylene (POM) polymer substrate with a 200 mm depression was chosen for this investigation. To have a consistent and repeatable layer, grease was first applied on this depression and the excess was scrapped off leaving behind 200 mm thick layer. A bearing steel ball 3 mm in diameter (Stainless steel ISO 3290 grade with HRc 60) was chosen as a counterbody. Prior to the experiments, the surface of the steel counterbody was cleaned with acetone and ethanol. A cantilever with a stiffness of 80 N/m but extremely rigid in normal direction was used. A fresh substrate was used for each variety of grease in order to avoid contamination. A contact load of 14 mN was applied as target load corresponding to a maximum Hertzian stress of 20 MPa [15]. On each grease, 25 approach–retraction cycles were done, i.e. the motion table (LMS in Fig. 1a) is moved 25 times towards and away from the steel ball. The approach–retraction speed was kept constant at 500 mm/s.
3. Results and discussion
0.4 0.2 0.0 0
500
1000
1500
2000
Distance from the mirror (μm) Fig. 1. (a) Experimental set-up for approach–retraction experiments; (b) characteristic normalised intensity to distance curve for a fibre optic sensor. (LST curve).
A typical plot of cantilever deflection vs. distance moved during approach–retraction cycle for greased and ungreased POM substrate against a steel ball is shown in Figs. 3 and 4, respectively. For an ungreased POM substrate in Fig. 4, both approach (dashed line) and retraction (black) sections of the curve overlap whereas for a greased substrate the two sections of the curve are clearly different. The curve on the greased contact appears like a hysteresis loop, which has to be further analysed to extract useful information (Fig. 3). A schema of events taking
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Fig. 2. Microstructural details of greases recorded with an optical microscope at 100 magnification: (a) Grease 1 (b) Grease 2, and (c) Grease 3.
15
3 A - Compression work on grease B- Pull-off work C- Work of Tackiness
10
Cantilever deflection force (mN)
Cantilever deflection force (mN)
15
5 1
0
7
2
A
4
C B
6
Pull-off force
-5
10 Slope α = K
5
α
0
-5
5
-10
-10 0.0
0.4 0.2 0.6 0.8 Displacement of the table (mm) 1
2
1.0
0.0
0.2 0.4 0.6 0.8 Displacement of the table (mm)
1.0
Fig. 4. Approach–retraction curve on POM surface without grease. Note that slope of the loading part is equal to cantilever stiffness.
3
Approach
4
5
6
Retraction
Fig. 3. Typical approach–retraction curve for greased POM substrate (top) and schema of contact events during approach–retraction (bottom).
place is shown in the bottom part of Fig. 3. Before contact (point 1 in Fig. 3), the cantilever is stationary and reciprocating table moves with a fixed speed. On touching the grease layer (point 2), the cantilever starts to deflect but the deflection on the cantilever will be smaller than the distance moved by the table.
This is because of some distance accommodation within the grease layer. On the contrary, the cantilever would deflect same amount as the distance moved by the table on an ungreased substrate (Fig. 4). Depending on the consistency of the grease, a non-linear trend is observed in the approach part of the curve in a greased contact (segment 2–3). This loading section of the curve is crucial to characterise the consistency or cohesiveness of grease. On retraction, the curve is linear bearing a slope equal to cantilever stiffness ( 80 N/m) because the spring is stuck to the table due to grease adhesion (point 3–4). If there was no adhesion the cantilever would separate from the greased substrate at deflection force equal to zero (point 4). Instead, some additional force is required to start the separation. This additional force is a competition
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3.1. Consistency, adherence, and tackiness of the greases The representative approach–retraction curves between steel ball and each of the greases (namely Grease 1, Grease 2, and Grease 3) applied on POM substrate for 25 test cycles are shown in Fig. 5. The Grease 1 containing silicone oil and Li soap thickener (Fig. 5a) exhibits a non-linear section in the loading section of the curve indicating good consistency of the grease. This grease has a sponge like structure that offers more resistance to the steel ball. There is also a well-defined pull-off point, and finally tackiness indicated by the non-linear section at the closing section of the curve. The approach–retraction curve of Grease 2 containing mineral oilþLi thickenerþmicro-particles differs from Grease 1 especially in the separation part of the curve. Although there is a clear pull-off point, the separation occurs instantly where the ball is released suddenly leading to oscillations of the cantilever. This instability causes fluctuations as shown in Fig. 5b and is an indication of poor tackiness. Finally, Grease 3 which has mineral oilþmicro-particles (Fig. 5c) results in an entirely different curve with small pull-off and tackiness sections. From these approach–retraction curves, the energies required for cohesive, adhesion, and tackiness can be extracted as explained in Fig. 3. The cohesive work Eres, the pull-off work Eadh, and the work against tackiness Etac for the greases are shown in Fig. 6. The amount of Eres is largest for Grease 1 (Fig. 7a) followed by Grease 2 that also contains thickener in the microstructure. This can be explained by the fact that Grease 1 has about 5 times more thickener content than Grease 2. The higher content of fibrous network present resists the ball movement thus needing more energy. On the contrary, Grease 3 has practically no Eres indicating poor consistency. Grease 3 is just particle dispersion in oil and under the applied load the micro-particles can move and compact easily offering little resistance. These factors strengthen the idea that the thickener is necessary for the cohesiveness of grease. The adhesion of grease to the substrate depends on the surface tension of the oil. On compression against the steel ball, the oil oozes out of the thickener. The silicone oil contained in Grease 1 has surface tension of 16–21 mN/m [17] whereas mineral oil in Grease 2 and 3 has slightly higher value of 30 mN/m [18]. With low surface energy the silicone oil can wet the steel surface better than the mineral oil.
Deflection on cantilever (mN)
12
Cycle number 2 5 10 15
8
25
4 0 -4 -8 0.0
12
Deflection on cantilever (mN)
between the adhesion force (substrate-grease) and cohesive force of the grease (point 5). The pull-off force indicated by point 5, marks the onset release and is analogous to the force needed to separate two glued surfaces (or measure of grease adherence). The eventual physical separation of the ball from the greased substrate happens when the meniscus like structures formed by the grease are broken (segment 5–6–2). The formation of meniscus structures or tackiness is an important characteristic of lubricating grease [16]. During retraction the viscous nature of the grease decelerates the release, which causes a non-linear section in the retraction curve after pull-off force point (point 6 in Fig. 3 bottom schema). Based on this analysis, the obtained hysteresis loop on a greased surface can be divided into three segments. The area represented by section A (enclosed by points 2–3–4–2) in the Fig. 3 is the amount of work performed against the resistance or consistency of the grease (Eres) under compression. The area represented as B (enclosed by points 4–5–7–4) is the work required to start the separation event or pull-off work, Eadh. Finally, tackiness can be estimated from the area enclosed by points 5–6–7–5 or represented by area C (Etac), which is equivalent to the work done to break the grease threads under tension.
0.2
0.4 0.6 Displacement (mm)
Cycle number 2 5 10 15
8 4
0.8
1.0
0.8
1.0
0.8
1.0
25
Fluctuations
0 -4 -8 0.0
0.2
0.4
0.6
Displacement (mm)
12
Deflection on cantilever (mN)
1130
Cycle number 2 5 10 15
8
25
4 0 -4 -8 0.0
0.2
0.4 0.6 Displacement (mm)
Fig. 5. Approach–retraction curves for 25 cycles recorded on different greases applied as 200 mm layer on POM substrate: (a) Grease 1 (b) Grease 2, and (c) Grease 3.
The lower surface tension allows the oil to penetrate the inter asperity gaps providing a thorough wetting. This could be the reason for the highest pull-off energy recorded on Grease 1 during the first cycle when the steel surface is still fresh. From second cycle, some
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0.5
Work of compression (μJ)
Cycle 1
Cycle 25
0.4
Pop-in
0.3
0.2
Cycle 1 0.1
Grease 1
Grease 2
Grease 3
1.0 0.9
Cycle 1
Cycle 25
pull-off work (μJ)
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Grease 1
Grease 2
Grease 3
0.20 Cycle 1
Cycle 25
0.15
0.10
0.05
0.00
Cycle 25
Fig. 7. Pop-in feature noticed in the Grease 3 due to polarised micro-particles.
0.0
Tackiness energy (μJ)
Pop-in
Grease 1
Grease 2
Grease 3
Fig. 6. (a) Energy of compression Eres, (b) pull-off work Eadh, and (c) tackiness energy Etac, recorded on the tested greases.
grease gets transferred to the ball and thereon contact occurs between two greased surfaces and the wetting event becomes less significant. Grease 2 with mineral oil (higher surface tension) does not exhibit a high pull-off work in the first cycle. The pull-off work remains nearly constant for the entire 25 cycles. Although Grease 2 has lesser thickener content, the pull-off work is same as Grease 1 at higher test cycles. It is believed that both the oil and the polarised particles in Grease 2 contribute to its adhesion.
Grease 3 with no thickener recorded smallest pull-off work. This suggests that the presence of polarised particles and the oil is not sufficient in providing adhesion, and the presence of thickener leads to some mechanism which promotes adhesion. A hypothesis could be that thickeners trap and release the oil at various locations in the contact leading to multiple wetting contact points. The oils surface tension will provide sufficient adhesion through capillary bridges which is obviously more effective for adhesion than one large single capillary [19]. On combining all the pull-off work results, it can be said that the presence of thickener is important for better adhesion whereas the adhesion will not rise indefinitely with increase in thickener content. Although Grease 3 exhibited poor cohesion and adhesion, there was something interesting in the approach–retraction curves. On zooming, a pop-in or sudden dip was noticed just before the ball and greased surface contact point (Fig. 7). The polarised micro- particles in Grease 3 have an affinity towards the metallic surface (due to electrostatic forces) which results in such a dip or pop-in [20]. This event was quite repeatable where the magnitude of the force during the first cycle was 100 mN. However, after the first cycle there is a transfer of grease on to the steel ball, and due to particle–particle interaction from the second cycle a larger dip of 400 mN is noticed. This reassures that it is the ability of a thickener to release the oil and the surface tension of the oil which contribute mostly to the adhesion. In terms of tackiness, Grease 1 once again showed the highest tackiness energy (about 10 times more than Greases 2 and 3). During retraction of the ball, grease threads formation was observed in the case of Grease 1 which delayed the ball release. The grease in this final stage is subjected to a tensile force and being a viscoelastic material, non-linear section is obtained from the pull-off point until complete separation. In Grease 2, such threads were hardly visible which resulted in spontaneous release of the steel ball. This proves that grease tackiness is predominantly controlled by the thickener content. Some grease manufacturers also add polymers called tackifiers (e.g., poly-isobutene) to further increase tackiness of the grease. In summary, it is interesting to notice that Grease 2 (oilþ3–5% thickenerþsolid lubricants) has pull-off force similar to Grease 1 (oilþ15–20% thickener), tackiness similar to Grease 3 (oilþsolid particles) and cohesion in between Greases 1 and 3. In summary, for good cohesion and tackiness the presence of thickener is important. In order to investigate the importance of thickener further, two variations of silicone oil based Grease 1 namely, Grease 1 low (low thickener content) and Grease 1 high (high thickener content) were tested. Typical characteristic approach–retraction curves for these two greases (200 mm in thickness on POM) with 500 mm/s speed are shown in Fig. 8. A summary of Eres, Eadh, and Etac extracted from Fig. 8 is given in Table 1. The Grease 1 high
S. Achanta et al. / Tribology International 44 (2011) 1127–1133
Cantilever deflection (mN)
15 Grease with high thickener % Grease with low thickener %
10
5
0
-5
-10 0.4
0.6
0.8
1.0
1.2
LMS displacement (mm) Fig. 8. Approach–retraction curves on Grease 1 containing high and low thickener content.
Table 1 Summary of Eadh, Etac, and Eres for Grease 1 with high and low thickener content. Experiments done with steel ball on 200 mm layer grease applied on POM substrate. Energy loss (lJ)
High thickener
Low thickener
Compression or cohesive work (Eres) Pull-off work (Eadh) Tackiness work (Etac)
0.65 0.52 0.35
0.32 0.40 0.10
exhibits about 100% increase in Eres than Grease 1 low. This finding is in good agreement with the conventional cone penetration test result. For example, Grease 1 high has NLGI grade 2–3 corresponding to a cone penetration depth of 24–28 mm whereas Grease 1 low has NLGI grade 1–2 which counts to 29–33 mm cone penetration depth [21,22]. This difference can be attributed to the dense network structure of the high thickener grease. Compared to cohesion, the adhesion or pull-off work changed only by only 30% (Table 1). From Fig. 8, it can be seen that the absolute pull-off forces are quite close to 7.5 mN for high thickener grease and 7 mN for low thickener grease. This means that increasing thickener content will not significantly influence the adhesion aspect of a grease. Therefore the difference in pulloff work between greases with and without thickener is much significant than between greases containing the thickener. It could be possible that beyond certain thickener content, the amount of oil wetting the surface will be similar depending upon the equilibrium conditions as applied stress, temperature, etc. This dependence of adhesion with increasing thickener content needs further investigation. From a tackiness point-of-view, Grease 1 high results in a higher energy loss than Grease 1 low. With more thickener content the threads formed in the contact also increase. Finally, the denser structure will require more energy for breaking the threads.
of the contact is high and the Hertzian contact pressures also reach high values. The experimental parameters used for ball-on-flat sliding experiments are given in Table 2. By increasing the applied load, the contact is pushed into boundary lubrication regime as explained by Stribeck curves [23]. All the tests were performed for short test duration of 50 cycles under ambient conditions of 23 1C and 50% RH. The evolution of the coefficient of friction with the test duration for different loads with Grease 1 is shown in Fig. 9. It is interesting to notice that the experiment at 25 mN or 270 MPa has a high coefficient of friction of 0.21. At other loads namely 100, 300, and 600 mN, Grease 1 exhibits a consistent and lower coefficient of friction of 0.125 typically observed for boundary lubricated contacts. This could mean that at low contact pressures the release of lubricating oil from the grease is not effective. Eventually, the high coefficient of friction at 25 mN could have originated from the viscous drag caused by the thickener on the ball. The evolution of coefficient of friction with test duration for different loads when lubricated with grease 2 is shown in Fig. 10. Even at 25 mN this grease exhibits a low coefficient of friction compared to Grease 1. This grease exhibited an unusually low coefficient of friction below 0.06 at 100 mN (470 MPa). It is probable that the solid lubricants present in the grease are responsible for this low friction especially when the pressures are insufficient to release oil from the thickener. Even under more boundary conditions, i.e. at higher loads of 300 and 600 mN, there could be a combined action of oil and solid lubricants which yield a coefficient of friction of 0.09. In comparison with Grease 1, Grease 2 gives a lower coefficient of friction at all loads. The Grease 3 (without thickener) shows decreasing coefficient of friction with increasing load as shown in Fig. 11. At maximum applied pressure of 870 MPa (600 mN), the coefficient of friction was lowest at 0.07. Especially at higher loads, this oil exhibits lower Table 2 Experimental parameters chosen for reciprocating sliding experiments on greased substrates in ball-on-flat contact configuration. Load (mN)
Max. Hertzian contact pressure (MPa)
Sliding distance (mm)
Sliding speed (mm/s)
25 100 300 600
270 470 670 850
2
2
0.30 0.25
Coefficient of friction
1132
0.20 0.15 0.10
3.2. Tribological behaviour of greases
0.05
The tribological properties of the greases were studied by performing a series of reciprocating sliding experiments. In order to generate realistic contacts like in sliding bearings, the grease was applied on AISI52100 steel disc. The counterbody used was 3.175 mm 100Cr6 ball same as one used for approach–retraction experiments. By choosing steel–steel contact the effective modulus
0.00
25 mN 100 mN 300 mN 600 mN
0
10
20
30
40
50
Number of sliding cycles Fig. 9. Evolution of coefficient of friction with test duration for different loads for Grease 1 when sliding against 100Cr6 steel counterbody.
S. Achanta et al. / Tribology International 44 (2011) 1127–1133
0.30 25 mN 100 mN
0.25 Coefficient of friction
300 mN 600 mN
0.20
0.15
0.10
0.05
0.00 0
10
20
30
40
50
Number of sliding cycles Fig. 10. Evolution of coefficient of friction with test duration for different loads for Grease 2 when sliding against 100Cr6 steel counterbody at 23 1C and 50% RH.
0.30 25 mN 100 mN
Coefficient of friction
0.25
300 mN
1133
substrate wetting by the oil. However, for a grease without any thickener the adhesion was poor. The cohesion on greases measurements were in good correlation with conventional cone penetration tests data which validates this experimental method. The approach–retraction technique provides sufficient data in one single experiment (three interaction properties like adherence, consistency or stiffness, and tackiness in one test) unlike conventional grease tests. In general, the effect of thickener on the lubricating properties of the greases was less clear unlike on interaction properties. Grease 1 gave highest coefficient of friction at low pressures whereas under high loads consistent lubrication behaviour was observed possibly due to better oil release. The presence of solid lubricants reduces the coefficient of friction at least under the boundary lubrication conditions. Despite similar composition, Grease 3 without thickener offered lower friction than Grease 2. The former also exhibited poor adhesion, cohesion, and tackiness properties. The good lubricating property could be attributed to the strong adsorption of polarised solid lubricants in the absence of thickener. This indicates that thickener reduces the efficacy of solid lubricants which was clearly observed during approach–retraction curves with Grease 2 (no pop-in) and Grease 3 (pop-in). This supports the notion that oil and solid lubricants in the grease provide the lubrication and thickener just carries them. In the future work, in-situ methods like on-line microscopy and contact resistance measurements will be integrated to further investigate the structure-property relationship of greases.
600 mN
References
0.20 0.15 0.10 0.05 0.00 0
10
20 30 Number of sliding cycles
40
50
Fig. 11. Evolution of coefficient of friction with test duration for different loads for Grease 3 when sliding against 100Cr6 steel counterbody at 23 1C and 50% RH.
coefficient of friction than Greases 1 and 2. It is believed that the polarised solid lubricants in the absence of thickener can anchor firmly to the substrate. The attractive force of these particles to the steel ball was also confirmed by pop-in behaviour in the approach– retraction curves of Grease 3 whereas this electrostatic attraction event was not observed in the similar composition Grease 2 with thickener. Thus, the adsorption event could be the most probable lubrication mechanism for this grease at high loads.
4. Conclusions The approach–retraction experiments were used to understand the effect of grease constituents on their interaction characteristics like adhesion, cohesion/consistency, and tackiness. Among all the constituents, the thickener has the highest influence on the cohesion and tackiness (thread formation ability). The effect of thickener content on the adhesion was less important between the greases containing the thickener. This indicates that most of the adhesive interaction is provided by the release and
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Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Characterisation of cohesion, adhesion, and tackiness of lubricating greases using approach–retraction experiments S. Achanta a,n, M. Jungk b, D. Drees a a b
Falex Tribology N.V., Wingepark 23B, B3110 Rotselaar, Belgium Dow Corning GmbH, Wiesbaden, Germany
a r t i c l e i n f o
abstract
Article history: Received 25 November 2009 Received in revised form 15 April 2011 Accepted 20 April 2011 Available online 1 May 2011
Greases are widely used in machinery and automotive components to protect components from frictional and wear losses. As a result, interaction properties like adhesion to the substrate, cohesion or consistency, and tackiness become crucial factors and often dictate their performance. All these properties are related to microstructural aspects of grease like thickener network, wetting agents, and additives. The aim of this paper is to use approach–retraction experiments for qualitative/quantitative determination of the above mentioned properties and also understand the influence of grease constituents on these properties. It was found that among all the grease constituents, the thickener in particular dictates the cohesiveness and tackiness of a grease with less influence on the adhesion. The effect of thickener on the lubricating properties of the greases was not clear, which supports the notion that oil in the grease provides the lubrication and thickener to be a carrier. The data on cohesiveness/consistency obtained from this approach–retraction experimental method correlate with the traditional cone penetration tests thus validating this methodology. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Lubricating greases Interaction properties Adhesion Tackiness
1. Introduction Lubricating greases are widely used in many applications ranging from electrical contacts [1], wire-rope applications [2], and largely on machinery where lubricants cannot be successfully applied, e.g. on unsealed or non-enclosed components. The main constituents of lubricating grease are the oil and the thickener which in most cases is metal based soaps [3]. The function of lubricating grease is analogous to a wet sponge. The oil retained in the thickener network is released based on several equilibrium conditions such as temperature, shear stress, and mechanical pressures [4]. Some lubricating greases contain additives for friction reduction, anti-wear properties, temperature stability, etc. [5]. One of the desired properties of greases is stickiness to the surface, which is determined by factors such as cohesiveness, adhesion to a surface, and tackiness or formation of thread like structures. By varying the composition of the constituents the properties of greases can be adjusted. For example, high viscous polymers (called tackifiers) are added to increase tackiness or thread formation. Most experimental methods to characterize lubricating greases with respect to their cohesive and adhesive properties are rather empirical in nature. The cohesiveness historically has been
n
Corresponding author. E-mail address: [email protected] (S. Achanta).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.04.019
qualitatively measured by cone penetration [6] and oil separation measurements [7], while more recently rheological measurements provide some scientific insight [8]. In cone penetration the consistency or cohesiveness of grease is measured by dropping a cone of known mass into grease filled trough and recording the penetration depth of the cone. For instance, a low depth of penetration means firm grease with good consistency. The adherence of greases to the substrate is characterised by water spray off or water wash out measurements [8]. In water spray test, the greased metallic surface is subjected to direct water jet and the adherence is measured from the mass of grease lost as a function of time [9,10]. Another method used for quantifying adherence is by subjecting greased cylinders to centrifugal forces. Depending on the amount of grease mass lost, the adhesion strength is ranked [11]. In most cases, failure occurs within the grease, which technically is cohesive failure. A further improvement in adhesion measurement is done by simultaneous conduction current measurements during centrifuging. With this technique the adhesion of grease to the substrate is defined in terms of coverage area or ratio of metallic to greased area after centrifuging [11]. The above mentioned test methods suffer from issues like poor precision and limited correlation to industrial practise [12]. Another interesting aspect of grease is tackiness (ability to form strings), which is desirable in some applications and unwanted in some others (like gears, wire ropes, etc.). The measurement of tackiness is still very subjective.
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In this paper, we explore the possibility of using approach– retraction experiments to characterise properties of greases like adherence, cohesion, and tackiness (ability to forms threads before separation). The importance of various constituents of grease like oil, thickener, and additives on the above properties will be explored. Moreover, the illustrated method can be an useful tool for the selection of appropriate grease or in the research and development.
2. Experimental The approach–retraction test method is simple in which a greased substrate is moved towards a ball attached to a flexible cantilever. On establishing the contact, the greased substrate is moved further until a certain contact load is achieved. On reaching the target contact load, the greased surface is moved away from the ball (retraction) until complete physical separation. The basic technique is analogous to pull-off force experiments done with an atomic force microscope for studying physical interactions [13]. After this approach–retraction cycle, when the deflection force is plotted as a function of moved distance, important information on the properties of greases can be obtained. A schema of the experimental set-up is shown in Fig. 1a, which consists of steel ball attached to a sensitive cantilever with a low spring constant of 80 N/m and can deflect in the direction parallel to the motion table. The deflection on the cantilever is measured using fibre optic sensor technology (microtribometer [14], Falex NV, Belgium). The sensor used in this study has bundle of emitters and receivers arranged randomly and has characteristic intensity to distance relationship as shown in Fig. 1b. The optical fibre is aligned perpendicular facing a mirror (approximately 1 mm
Cantilever block
Mirror
Fibre optic sensor
Grease
Steel ball
Reciprocating motion table
1.2 Normalised intensity (I/I0)
calibration curve for sensor
1.0 0.8 0.6
away) attached to the cantilever deflection axis (see Fig. 1a). The optical fibre emits certain intensity of light, which is reflected back from the mirror, I0. When there is a deflection, the reflected light intensity changes and the deflection can be determined from the calibration curve shown in Fig. 1b. The obtained deflection in mm is then multiplied with the spring stiffness to precisely determine the deflection forces. The desired load between the ball and the greased surface can be controlled by constantly monitoring the deflection force on the cantilever through a closed feedback loop system. The same set-up can also be used to measure frictional forces by simply changing the cantilever which can deflect in both perpendicular and parallel to the motion table. 2.1. Materials The greases used in this work were supplied by Dow Corning GmbH, Wiesbaden, Germany. The commercial names of the greases are not revealed due to confidentiality. The microstructure of the greases is shown in Fig. 2. The investigated greases are Grease 1 (Fig. 2a) is silicone oil based containing lithium soap thickener (15–20 w%) with a specific gravity of 1.05. Grease 2 is mineral oil based with 3–7 w% lithium soap thickener along with solid lubricant micro-particles (2–10 mm in size) and has a specific gravity of 0.93 (Fig. 2b). Solid lubricant particles are commonly used as additives and remain dispersed structure. Grease 3 is mineral oil based with only solid lubricant particles in the range of 2–10 mm (minimum 30% by weight) with a specific gravity of 1.4 (Fig. 2c). Micro-particles in Grease 3 were polarised. The polarisation of micro-particles improves their adhesion to the substrate. It can be seen that grease 1 has a spongy structure and appears distinct from other two greases. Grease 2 and 3 bear some similarity because of similar type of oil and solid lubricant particles. A polyoxymethylene (POM) polymer substrate with a 200 mm depression was chosen for this investigation. To have a consistent and repeatable layer, grease was first applied on this depression and the excess was scrapped off leaving behind 200 mm thick layer. A bearing steel ball 3 mm in diameter (Stainless steel ISO 3290 grade with HRc 60) was chosen as a counterbody. Prior to the experiments, the surface of the steel counterbody was cleaned with acetone and ethanol. A cantilever with a stiffness of 80 N/m but extremely rigid in normal direction was used. A fresh substrate was used for each variety of grease in order to avoid contamination. A contact load of 14 mN was applied as target load corresponding to a maximum Hertzian stress of 20 MPa [15]. On each grease, 25 approach–retraction cycles were done, i.e. the motion table (LMS in Fig. 1a) is moved 25 times towards and away from the steel ball. The approach–retraction speed was kept constant at 500 mm/s.
3. Results and discussion
0.4 0.2 0.0 0
500
1000
1500
2000
Distance from the mirror (μm) Fig. 1. (a) Experimental set-up for approach–retraction experiments; (b) characteristic normalised intensity to distance curve for a fibre optic sensor. (LST curve).
A typical plot of cantilever deflection vs. distance moved during approach–retraction cycle for greased and ungreased POM substrate against a steel ball is shown in Figs. 3 and 4, respectively. For an ungreased POM substrate in Fig. 4, both approach (dashed line) and retraction (black) sections of the curve overlap whereas for a greased substrate the two sections of the curve are clearly different. The curve on the greased contact appears like a hysteresis loop, which has to be further analysed to extract useful information (Fig. 3). A schema of events taking
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Fig. 2. Microstructural details of greases recorded with an optical microscope at 100 magnification: (a) Grease 1 (b) Grease 2, and (c) Grease 3.
15
3 A - Compression work on grease B- Pull-off work C- Work of Tackiness
10
Cantilever deflection force (mN)
Cantilever deflection force (mN)
15
5 1
0
7
2
A
4
C B
6
Pull-off force
-5
10 Slope α = K
5
α
0
-5
5
-10
-10 0.0
0.4 0.2 0.6 0.8 Displacement of the table (mm) 1
2
1.0
0.0
0.2 0.4 0.6 0.8 Displacement of the table (mm)
1.0
Fig. 4. Approach–retraction curve on POM surface without grease. Note that slope of the loading part is equal to cantilever stiffness.
3
Approach
4
5
6
Retraction
Fig. 3. Typical approach–retraction curve for greased POM substrate (top) and schema of contact events during approach–retraction (bottom).
place is shown in the bottom part of Fig. 3. Before contact (point 1 in Fig. 3), the cantilever is stationary and reciprocating table moves with a fixed speed. On touching the grease layer (point 2), the cantilever starts to deflect but the deflection on the cantilever will be smaller than the distance moved by the table.
This is because of some distance accommodation within the grease layer. On the contrary, the cantilever would deflect same amount as the distance moved by the table on an ungreased substrate (Fig. 4). Depending on the consistency of the grease, a non-linear trend is observed in the approach part of the curve in a greased contact (segment 2–3). This loading section of the curve is crucial to characterise the consistency or cohesiveness of grease. On retraction, the curve is linear bearing a slope equal to cantilever stiffness ( 80 N/m) because the spring is stuck to the table due to grease adhesion (point 3–4). If there was no adhesion the cantilever would separate from the greased substrate at deflection force equal to zero (point 4). Instead, some additional force is required to start the separation. This additional force is a competition
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3.1. Consistency, adherence, and tackiness of the greases The representative approach–retraction curves between steel ball and each of the greases (namely Grease 1, Grease 2, and Grease 3) applied on POM substrate for 25 test cycles are shown in Fig. 5. The Grease 1 containing silicone oil and Li soap thickener (Fig. 5a) exhibits a non-linear section in the loading section of the curve indicating good consistency of the grease. This grease has a sponge like structure that offers more resistance to the steel ball. There is also a well-defined pull-off point, and finally tackiness indicated by the non-linear section at the closing section of the curve. The approach–retraction curve of Grease 2 containing mineral oilþLi thickenerþmicro-particles differs from Grease 1 especially in the separation part of the curve. Although there is a clear pull-off point, the separation occurs instantly where the ball is released suddenly leading to oscillations of the cantilever. This instability causes fluctuations as shown in Fig. 5b and is an indication of poor tackiness. Finally, Grease 3 which has mineral oilþmicro-particles (Fig. 5c) results in an entirely different curve with small pull-off and tackiness sections. From these approach–retraction curves, the energies required for cohesive, adhesion, and tackiness can be extracted as explained in Fig. 3. The cohesive work Eres, the pull-off work Eadh, and the work against tackiness Etac for the greases are shown in Fig. 6. The amount of Eres is largest for Grease 1 (Fig. 7a) followed by Grease 2 that also contains thickener in the microstructure. This can be explained by the fact that Grease 1 has about 5 times more thickener content than Grease 2. The higher content of fibrous network present resists the ball movement thus needing more energy. On the contrary, Grease 3 has practically no Eres indicating poor consistency. Grease 3 is just particle dispersion in oil and under the applied load the micro-particles can move and compact easily offering little resistance. These factors strengthen the idea that the thickener is necessary for the cohesiveness of grease. The adhesion of grease to the substrate depends on the surface tension of the oil. On compression against the steel ball, the oil oozes out of the thickener. The silicone oil contained in Grease 1 has surface tension of 16–21 mN/m [17] whereas mineral oil in Grease 2 and 3 has slightly higher value of 30 mN/m [18]. With low surface energy the silicone oil can wet the steel surface better than the mineral oil.
Deflection on cantilever (mN)
12
Cycle number 2 5 10 15
8
25
4 0 -4 -8 0.0
12
Deflection on cantilever (mN)
between the adhesion force (substrate-grease) and cohesive force of the grease (point 5). The pull-off force indicated by point 5, marks the onset release and is analogous to the force needed to separate two glued surfaces (or measure of grease adherence). The eventual physical separation of the ball from the greased substrate happens when the meniscus like structures formed by the grease are broken (segment 5–6–2). The formation of meniscus structures or tackiness is an important characteristic of lubricating grease [16]. During retraction the viscous nature of the grease decelerates the release, which causes a non-linear section in the retraction curve after pull-off force point (point 6 in Fig. 3 bottom schema). Based on this analysis, the obtained hysteresis loop on a greased surface can be divided into three segments. The area represented by section A (enclosed by points 2–3–4–2) in the Fig. 3 is the amount of work performed against the resistance or consistency of the grease (Eres) under compression. The area represented as B (enclosed by points 4–5–7–4) is the work required to start the separation event or pull-off work, Eadh. Finally, tackiness can be estimated from the area enclosed by points 5–6–7–5 or represented by area C (Etac), which is equivalent to the work done to break the grease threads under tension.
0.2
0.4 0.6 Displacement (mm)
Cycle number 2 5 10 15
8 4
0.8
1.0
0.8
1.0
0.8
1.0
25
Fluctuations
0 -4 -8 0.0
0.2
0.4
0.6
Displacement (mm)
12
Deflection on cantilever (mN)
1130
Cycle number 2 5 10 15
8
25
4 0 -4 -8 0.0
0.2
0.4 0.6 Displacement (mm)
Fig. 5. Approach–retraction curves for 25 cycles recorded on different greases applied as 200 mm layer on POM substrate: (a) Grease 1 (b) Grease 2, and (c) Grease 3.
The lower surface tension allows the oil to penetrate the inter asperity gaps providing a thorough wetting. This could be the reason for the highest pull-off energy recorded on Grease 1 during the first cycle when the steel surface is still fresh. From second cycle, some
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0.5
Work of compression (μJ)
Cycle 1
Cycle 25
0.4
Pop-in
0.3
0.2
Cycle 1 0.1
Grease 1
Grease 2
Grease 3
1.0 0.9
Cycle 1
Cycle 25
pull-off work (μJ)
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Grease 1
Grease 2
Grease 3
0.20 Cycle 1
Cycle 25
0.15
0.10
0.05
0.00
Cycle 25
Fig. 7. Pop-in feature noticed in the Grease 3 due to polarised micro-particles.
0.0
Tackiness energy (μJ)
Pop-in
Grease 1
Grease 2
Grease 3
Fig. 6. (a) Energy of compression Eres, (b) pull-off work Eadh, and (c) tackiness energy Etac, recorded on the tested greases.
grease gets transferred to the ball and thereon contact occurs between two greased surfaces and the wetting event becomes less significant. Grease 2 with mineral oil (higher surface tension) does not exhibit a high pull-off work in the first cycle. The pull-off work remains nearly constant for the entire 25 cycles. Although Grease 2 has lesser thickener content, the pull-off work is same as Grease 1 at higher test cycles. It is believed that both the oil and the polarised particles in Grease 2 contribute to its adhesion.
Grease 3 with no thickener recorded smallest pull-off work. This suggests that the presence of polarised particles and the oil is not sufficient in providing adhesion, and the presence of thickener leads to some mechanism which promotes adhesion. A hypothesis could be that thickeners trap and release the oil at various locations in the contact leading to multiple wetting contact points. The oils surface tension will provide sufficient adhesion through capillary bridges which is obviously more effective for adhesion than one large single capillary [19]. On combining all the pull-off work results, it can be said that the presence of thickener is important for better adhesion whereas the adhesion will not rise indefinitely with increase in thickener content. Although Grease 3 exhibited poor cohesion and adhesion, there was something interesting in the approach–retraction curves. On zooming, a pop-in or sudden dip was noticed just before the ball and greased surface contact point (Fig. 7). The polarised micro- particles in Grease 3 have an affinity towards the metallic surface (due to electrostatic forces) which results in such a dip or pop-in [20]. This event was quite repeatable where the magnitude of the force during the first cycle was 100 mN. However, after the first cycle there is a transfer of grease on to the steel ball, and due to particle–particle interaction from the second cycle a larger dip of 400 mN is noticed. This reassures that it is the ability of a thickener to release the oil and the surface tension of the oil which contribute mostly to the adhesion. In terms of tackiness, Grease 1 once again showed the highest tackiness energy (about 10 times more than Greases 2 and 3). During retraction of the ball, grease threads formation was observed in the case of Grease 1 which delayed the ball release. The grease in this final stage is subjected to a tensile force and being a viscoelastic material, non-linear section is obtained from the pull-off point until complete separation. In Grease 2, such threads were hardly visible which resulted in spontaneous release of the steel ball. This proves that grease tackiness is predominantly controlled by the thickener content. Some grease manufacturers also add polymers called tackifiers (e.g., poly-isobutene) to further increase tackiness of the grease. In summary, it is interesting to notice that Grease 2 (oilþ3–5% thickenerþsolid lubricants) has pull-off force similar to Grease 1 (oilþ15–20% thickener), tackiness similar to Grease 3 (oilþsolid particles) and cohesion in between Greases 1 and 3. In summary, for good cohesion and tackiness the presence of thickener is important. In order to investigate the importance of thickener further, two variations of silicone oil based Grease 1 namely, Grease 1 low (low thickener content) and Grease 1 high (high thickener content) were tested. Typical characteristic approach–retraction curves for these two greases (200 mm in thickness on POM) with 500 mm/s speed are shown in Fig. 8. A summary of Eres, Eadh, and Etac extracted from Fig. 8 is given in Table 1. The Grease 1 high
S. Achanta et al. / Tribology International 44 (2011) 1127–1133
Cantilever deflection (mN)
15 Grease with high thickener % Grease with low thickener %
10
5
0
-5
-10 0.4
0.6
0.8
1.0
1.2
LMS displacement (mm) Fig. 8. Approach–retraction curves on Grease 1 containing high and low thickener content.
Table 1 Summary of Eadh, Etac, and Eres for Grease 1 with high and low thickener content. Experiments done with steel ball on 200 mm layer grease applied on POM substrate. Energy loss (lJ)
High thickener
Low thickener
Compression or cohesive work (Eres) Pull-off work (Eadh) Tackiness work (Etac)
0.65 0.52 0.35
0.32 0.40 0.10
exhibits about 100% increase in Eres than Grease 1 low. This finding is in good agreement with the conventional cone penetration test result. For example, Grease 1 high has NLGI grade 2–3 corresponding to a cone penetration depth of 24–28 mm whereas Grease 1 low has NLGI grade 1–2 which counts to 29–33 mm cone penetration depth [21,22]. This difference can be attributed to the dense network structure of the high thickener grease. Compared to cohesion, the adhesion or pull-off work changed only by only 30% (Table 1). From Fig. 8, it can be seen that the absolute pull-off forces are quite close to 7.5 mN for high thickener grease and 7 mN for low thickener grease. This means that increasing thickener content will not significantly influence the adhesion aspect of a grease. Therefore the difference in pulloff work between greases with and without thickener is much significant than between greases containing the thickener. It could be possible that beyond certain thickener content, the amount of oil wetting the surface will be similar depending upon the equilibrium conditions as applied stress, temperature, etc. This dependence of adhesion with increasing thickener content needs further investigation. From a tackiness point-of-view, Grease 1 high results in a higher energy loss than Grease 1 low. With more thickener content the threads formed in the contact also increase. Finally, the denser structure will require more energy for breaking the threads.
of the contact is high and the Hertzian contact pressures also reach high values. The experimental parameters used for ball-on-flat sliding experiments are given in Table 2. By increasing the applied load, the contact is pushed into boundary lubrication regime as explained by Stribeck curves [23]. All the tests were performed for short test duration of 50 cycles under ambient conditions of 23 1C and 50% RH. The evolution of the coefficient of friction with the test duration for different loads with Grease 1 is shown in Fig. 9. It is interesting to notice that the experiment at 25 mN or 270 MPa has a high coefficient of friction of 0.21. At other loads namely 100, 300, and 600 mN, Grease 1 exhibits a consistent and lower coefficient of friction of 0.125 typically observed for boundary lubricated contacts. This could mean that at low contact pressures the release of lubricating oil from the grease is not effective. Eventually, the high coefficient of friction at 25 mN could have originated from the viscous drag caused by the thickener on the ball. The evolution of coefficient of friction with test duration for different loads when lubricated with grease 2 is shown in Fig. 10. Even at 25 mN this grease exhibits a low coefficient of friction compared to Grease 1. This grease exhibited an unusually low coefficient of friction below 0.06 at 100 mN (470 MPa). It is probable that the solid lubricants present in the grease are responsible for this low friction especially when the pressures are insufficient to release oil from the thickener. Even under more boundary conditions, i.e. at higher loads of 300 and 600 mN, there could be a combined action of oil and solid lubricants which yield a coefficient of friction of 0.09. In comparison with Grease 1, Grease 2 gives a lower coefficient of friction at all loads. The Grease 3 (without thickener) shows decreasing coefficient of friction with increasing load as shown in Fig. 11. At maximum applied pressure of 870 MPa (600 mN), the coefficient of friction was lowest at 0.07. Especially at higher loads, this oil exhibits lower Table 2 Experimental parameters chosen for reciprocating sliding experiments on greased substrates in ball-on-flat contact configuration. Load (mN)
Max. Hertzian contact pressure (MPa)
Sliding distance (mm)
Sliding speed (mm/s)
25 100 300 600
270 470 670 850
2
2
0.30 0.25
Coefficient of friction
1132
0.20 0.15 0.10
3.2. Tribological behaviour of greases
0.05
The tribological properties of the greases were studied by performing a series of reciprocating sliding experiments. In order to generate realistic contacts like in sliding bearings, the grease was applied on AISI52100 steel disc. The counterbody used was 3.175 mm 100Cr6 ball same as one used for approach–retraction experiments. By choosing steel–steel contact the effective modulus
0.00
25 mN 100 mN 300 mN 600 mN
0
10
20
30
40
50
Number of sliding cycles Fig. 9. Evolution of coefficient of friction with test duration for different loads for Grease 1 when sliding against 100Cr6 steel counterbody.
S. Achanta et al. / Tribology International 44 (2011) 1127–1133
0.30 25 mN 100 mN
0.25 Coefficient of friction
300 mN 600 mN
0.20
0.15
0.10
0.05
0.00 0
10
20
30
40
50
Number of sliding cycles Fig. 10. Evolution of coefficient of friction with test duration for different loads for Grease 2 when sliding against 100Cr6 steel counterbody at 23 1C and 50% RH.
0.30 25 mN 100 mN
Coefficient of friction
0.25
300 mN
1133
substrate wetting by the oil. However, for a grease without any thickener the adhesion was poor. The cohesion on greases measurements were in good correlation with conventional cone penetration tests data which validates this experimental method. The approach–retraction technique provides sufficient data in one single experiment (three interaction properties like adherence, consistency or stiffness, and tackiness in one test) unlike conventional grease tests. In general, the effect of thickener on the lubricating properties of the greases was less clear unlike on interaction properties. Grease 1 gave highest coefficient of friction at low pressures whereas under high loads consistent lubrication behaviour was observed possibly due to better oil release. The presence of solid lubricants reduces the coefficient of friction at least under the boundary lubrication conditions. Despite similar composition, Grease 3 without thickener offered lower friction than Grease 2. The former also exhibited poor adhesion, cohesion, and tackiness properties. The good lubricating property could be attributed to the strong adsorption of polarised solid lubricants in the absence of thickener. This indicates that thickener reduces the efficacy of solid lubricants which was clearly observed during approach–retraction curves with Grease 2 (no pop-in) and Grease 3 (pop-in). This supports the notion that oil and solid lubricants in the grease provide the lubrication and thickener just carries them. In the future work, in-situ methods like on-line microscopy and contact resistance measurements will be integrated to further investigate the structure-property relationship of greases.
600 mN
References
0.20 0.15 0.10 0.05 0.00 0
10
20 30 Number of sliding cycles
40
50
Fig. 11. Evolution of coefficient of friction with test duration for different loads for Grease 3 when sliding against 100Cr6 steel counterbody at 23 1C and 50% RH.
coefficient of friction than Greases 1 and 2. It is believed that the polarised solid lubricants in the absence of thickener can anchor firmly to the substrate. The attractive force of these particles to the steel ball was also confirmed by pop-in behaviour in the approach– retraction curves of Grease 3 whereas this electrostatic attraction event was not observed in the similar composition Grease 2 with thickener. Thus, the adsorption event could be the most probable lubrication mechanism for this grease at high loads.
4. Conclusions The approach–retraction experiments were used to understand the effect of grease constituents on their interaction characteristics like adhesion, cohesion/consistency, and tackiness. Among all the constituents, the thickener has the highest influence on the cohesion and tackiness (thread formation ability). The effect of thickener content on the adhesion was less important between the greases containing the thickener. This indicates that most of the adhesive interaction is provided by the release and
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