Friction and abrasive wear of UHWMPE sliding on ice

Wear 258 (2005) 26–31 Friction and abrasive wear of UHWMPE sliding on ice S. Ducreta,b,∗ , H. Zahouania , A. Midola,b , P. Lanterib , T.G. Mathiaa a ...
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Wear 258 (2005) 26–31

Friction and abrasive wear of UHWMPE sliding on ice S. Ducreta,b,∗ , H. Zahouania , A. Midola,b , P. Lanterib , T.G. Mathiaa a

Laboratoire de Tribologie et Dynamique des Syst`emes, UMR 5513, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully cedex, France b Sciences Analytiques, UMR 5180, Universit´ e Claude Bernard Lyon 1, 43, Boulevard du 11 Novembre 1918, 69100 Villeurbanne cedex, France Received 22 December 2003 Available online 21 November 2004

Abstract The authors present an experimental approach of abrasion resistance of material currently used in ski: ultra high molecular weight polyethylene (UHMWPE) is used as the main element of ski sole: cross country skiing, speed skiing, alpine skiing, . . .. The aim of this work is to characterise abrasive wear and tribological performance of ski sole when one slides on ice in different conditions of loads, ice roughness and temperature. In order to better understand the abrasive action of ice, experiments using ice with different roughness (characterised by different morphological parameters) are performed under different kinematical and thermal conditions. For this purpose, we employed especially designed tribometer in controlled atmosphere enclosure (defined in relation with the use of in situ conditions). In this study, both normal and tangential forces are measured during the friction experiments. To complete the characterisation of the friction process, 3D surfaces topography is measured in order to evaluate abrasive process and consequences on its tribological performance. Furthermore, it improves understanding of abrasion mechanisms of UHMWPE on ice. © 2004 Elsevier B.V. All rights reserved. Keywords: Friction coefficient; Polymer; Wear; Ice; Abrasion

1. Introduction Abrasion resistance and surface damage mechanisms is one of the most important research and development subject in material engineering to optimise skis sliding against ice. Added to difficulties of understanding tribological behaviours, premelting phenomena increase the number of variables giving a system more complex. That is why the field is quite new with few publications. Coupling friction tests with morphological characterisation of rubbed surfaces is a useful process for abrasion resistance analysis and understanding of tribological behaviours. In case of a ski sliding, the energy corresponding to the motion is dissipated into irreversible compression and shearing of snow. This part of energy is converted to heat near the ski/snow interface. This amount of heat generated is often as∗

Corresponding author. Tel.: +33 4 72 18 60 00; fax: +33 4 78 43 33 83. E-mail address: [email protected] (S. Ducret).

0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.026

sumed to cause the local melting and discontinuous thin film creation. We investigated the sliding and wear of hydrophobic polymers on ice as model system for a better understanding of skiing. This paper presents an experimental study of abrasion resistance of ultra high molecular weight polyethylene (UHMWPE) when sliding on ice. UHWMPE is used as the main element of ski sole, because it has a good friction coefficient, which remains the fundamental factor for ski performance [1]. UHWMPE is a thermoplastic polymer with a molecular weight of 6 million g./mol. Its glass transition temperature Tg is 173 K. This paper presents the friction part of the tribological analysis of UHMWPE including wear mechanisms and friction properties. The friction tests performed in this work are used to simulate wear mechanisms. Wear topography measurements are carried out with interferometer in order to analyse the residual morphology of the wear tracks. Morphology analysis is performed under a special topography software analyser.

S. Ducret et al. / Wear 258 (2005) 26–31

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Fig. 1. Principle of sensors’ system.

UHMWPE friction experiments are made in different temperature conditions. Both normal and lateral forces are measured during tests in order to estimate friction coefficient µ and analyse its evolution as a function of different parameters. Many authors [2,3] realised similar friction tests with polymers on ice but in this study, we use different ice roughness, temperatures and loads during test in order to understand and optimise the sliding mechanisms model on ice. Polymer behaviour is usually roughness and load dependent [4]. In this study, in addition to these two parameters, effect of test temperature will be taken into account. The range of temperature used in this work has been defined in relation with the real use of the material in its application. While sliding on ice, the real temperature of surface is between 263 K and 275 K [5]. Abrasion experiments carried out in this study are performed over this range of temperature. In order to better understand abrasive action of ice against ski sole, we will study all experimental results obtained in different conditions. The focus of this study is to analyse the effect of ice on friction coefficient µ.

sentially on rheological and physicochemical state of snow, wax, polymer and morphology of ski bases. It is important to know the abrasive action of snow granularities in order to master the evolution of skis bases for better sliding performance. For this reason, we have built different kind of morphologies (based on corundum particles) with silicon replica having different asperities density. The effect of angle distribution can be analysed by comparing abrasion behaviour of the material sliding on ice morphology. The aim of this analysis is to link attack angle distribution of ice asperities to model the snow/ski interface. To characterise different ice reliefs on replic, topographic software is used to describe the attack angles of ice grains [7]. Fig. 2 shows and summarises the 3D representations and attack angles distributions for both ice roughness. To make ice sample, water is placed in a fridge at 263K with replica on the top of the sample to make ice with roughness R1 or R2 .

3. Result 2. Experimental details Specific friction tester is developed to perform friction experiment under various loads at various temperatures. It is important to note that a calibration of the friction tester is necessary to work at negative temperature. In addition to the normal load Fn , the tangential force Ft resulting from the sliding motion of the sole sample across ice surface was measured during experiment by strain gauge [6]. Components and friction test device are monitored with a special software (Fig. 1). Friction test is placed in a hermetic enclosure in order to easily choose the temperature (253–303 K). 2.1. Morphology of ice The spatial distribution of the contact pressure between ski base and snow is very heterogeneous and depends es-

Increasing scientific research in field of premelting phenomena allows to demonstrate that ice friction is full of unresolved inconsistencies. For example, there is a spread of three orders of magnitude in reported temperature-dependent thickness of liquid film for different measurement techniques [8]. At temperature close to the melting point Tm (T > 0.9Tm ), ice forms a liquid layer at the surface. Premelting of ice surface was first discussed by Faraday, relating to geology of polar ice, environmental science and tribology [9]. Recently, it has been experimentally demonstrated that a lubricating liquid film of thickness above 50 nm is necessary for measuring friction coefficient of about 0.03 at 271 K. This result obtained with modern running bases is correlated with data mentioned above on continuous or discontinuous films [5]. In this study, UHMWPE behaviour is analysed over a wide range of strain, stress and temperature conditions

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Fig. 2. 3D surface ice topography and attack angle statistics.

during experiments conducted with different ice roughness and normal load at different temperatures (263 K and 278 K). 3.1. Study of friction coefficient as a function of attack angle β (ice roughness) We use different ice roughness (Fig. 2) in order to approach the real slope roughness. In mountain, Olympic slope are made by quantity of ice particles. To understand the effect of abrasive particles, we decide to build different ice roughness with different topography. Subsequently, friction coefficient becomes the most important parameter. We will now develop it. The apparent friction coefficient µ is defined by: µ=

Ft Fn

(1)

where Ft : tangential force, Fn : normal force applied to ice surface. With this definition of friction coefficient, it is necessary to define the attack angle. It constitutes the interface between sole and ice [10]. Attack angle β is defined as the angle between the sample surface and indenter as shown in Fig. 3. The relationship between β and α is given by the following equation: β=

π −α 2

Indenter used in this definition is represented by ice roughness and we tried to find correlation between conical indenter with variable angles and ice asperity. Fig. 4 shows evolution of the apparent friction coefficient µ as a function of attack angle β (ice asperity) for UHMWPE at temperature 268 K. According to the theory and as shown on Fig. 4, friction coefficient increases with ice roughness increasing [11]. For a given normal load (10 N) (Fn ) and a constant velocity (2500 ␮m/s), friction coefficient is directly proportional to the tangential force (Ft ), which is dependent upon the tangential projected contact area (At ) and the tangential strain τ (Ft = At τ). For a perfect cone (ice asperity), contact radius (R) is proportional to the penetration δ. As demonstrated by Briscoe [12] with Eq. (3): µ=

2 tan β π

(3)

Friction coefficient is proportional to the indenter attack angle. This equation illustrates that the assumption in this model that contact pressure in normal direction is equivalent to contact pressure in tangential direction and that there is little effect of the actual friction between sole and ice asperity. Adhesive part is verify with a comparison between apparent friction result (Ft /Fn ) and ploughing part Eq. (3).

(2)

Fig. 3. Definition of ice attack angle β.

Fig. 4. Variation of friction coefficient µ vs. ice roughness in a constant temperature (268 K).

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3.4. Wear morphology after friction on different roughness

Fig. 5. Variation of apparent friction coefficient vs. normal load (N) on R1 ice roughness sample.

3.2. Variation of friction coefficient versus normal load (Fn ) Fig. 5 shows that friction coefficient stay similar with variable normal load for a given ice roughness (new sample of ice roughness at each experiment) and a given temperature (268 K). Result found for both roughness (R1 , R2 ) are similar. This result confirms that contact geometry is independent of the loads and that we can neglect the rounding at the extremity of ice. 3.3. Variation of friction coefficient with different temperatures Fig. 6 shows evolution of friction coefficient used in different conditions of temperatures (258 K; 263 K; 268 K; 273 K; 278 K). We note that the friction coefficient decreases as the temperature increases. The coefficient is optimised till a specify temperature (268 K). Above, we note a fall of friction. Below 273 K, contact between ice and sole is describe as a solid/solid contact but lubricated by water [13]. Above 273 K, ice becomes very soft and both forces signal decrease. The friction coefficient falls directly. NB: Polymers are made with flexible macromolecules. When temperature decreases to 263 K, polymers chains become less flexible and more contracted which will be more difficult to scratch.

In order to identify wear mechanisms undergone by UHMWPE with various roughness, we analysed scratch morphology made after friction experiments (200 cycles; 268 K; 10 N) (Fig. 7). We note a significant difference between material behaviour according to the roughness. These behaviour relate mechanism presented by Briscoe [12]. Fig. 2 shows the attack angles βmax for R2 (1.51 rad) and for R1 (1.44 rad). The difference between angles for both ice roughness is tiny and may not be sufficient to explain the difference of friction coefficient value. However, it is significant to note that, in spite of a weak variation of βmax , we obtain quite different coefficients of friction (Fig. 4). The difference between the mean values of attack angles is more significant. βmean represents the mean attack angle for both surfaces. Values for R2 is 0.95 rad and for R1 is 0.66 rad. When we compare the result, the difference of 0.29 rad in βmean can explain the difference between two friction coefficients and two wear behaviours. For morphology sliding on R1 , surface undergo smooth scratch (ductile). Scratch is very straight and parallel. For morphology sliding on R2 , we note important fractures deeper than those measured on R1 [14]. With R2 , literature says that contact surfaces are less significant, consequently friction generated should be weaker than coefficient calculated with R1 . But in experiments made with R2 , each top of ice asperity represents an indenter and as the result, friction coefficient and abrasive wear increase. So we observe different wear morphology on surface, due to the difference of ice asperities distribution. As for UHWMPE surface measurement, it would have been interesting to carry out the type of topography for scratching that is carried out in the cold. For technical reasons, it was impossible for us to measure 3D topography at low temperature and obtain coherent measurement.

Fig. 6. Variation of friction coefficient for five temperatures on R1 ice roughness (10 N; 2500 ␮m/s). Comparison with ski regime of friction.

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Fig. 7. Surfaces topography of polymer UHMWPE after friction (10 N) on ice (R1 –R2 ) at 268 K.

Fig. 8. SEM analyse (ski sole; R1 ; 10 N; 268 K).

3.5. SEM characterisation To complete our data about surface behaviour, we have made supplementary measurements with scanning electronic microscopy (SEM) on soles before and after cycles wear on ice (Fig. 8). Figures represent an analyse by SEM of ski sole with a wet roughness (track under the arrow) before (a) and after (b) wear cycles on R1 at 268 K. We note difference between surface. Before sliding there are particles on picture (a). During sliding, particles are scratched and flatted by ice aperities as it is seem on the figure (b). It remains the same morphology without particles for each surface [10].

4. Discussion The experiment carried out with different ice state roughness at different temperatures provide very interesting information about UHWMPE abrasion. This study shows the possibility of working on friction test. However a lot of works have been carried out on friction of ice but not with variation of parameters as temperature and roughness [2–4]. The control of temperature during friction tests gives an additional dimension to the protocol which allows the researchers to obtain more behaviour information. Moreover, to approach concrete phenomena such as the ski contact on the track, we chose to develop a new friction protocol using of multi ice asperity [15]. We have thus compared different types of friction protocol with multi ice asperities at different temperatures. It is interesting to note that, with the surface morphology soft-

ware, statistics of local granular geometry allows to calculate the real contact area (ice angles distribution/polymer), the contact pressure and material behaviour during scratch tests. The combination of friction coefficient and scratched topography of surface can give some information about polymers behaviour. We note several significant things concerning the friction coefficient. In this case, friction coefficient depends of attack angle of ice asperity. The enclosure temperature is also an essential parameter because it can modify the result of friction coefficient. When temperature starts at 258 K and increases, there is a decrease of friction coefficient till an optimum. After 273 K, friction falls because it is function of ice hardness which decreases highly above 276/278 K. Concerning finally material behaviour, we note a great difference related to ice roughness. It is possible to induce various phenomena of abrasion from smoothing (R1 and weak attack angle) to fracturing (R2 and great attack angle). In summary, discussion is primarily about the different behaviours and morphology results that we could find between R1 and R2 asperities in various sliding conditions.

5. Conclusion In this study, the principal difficulty is to establish efficient bridge between research of physical and chemical characteristics such as polymer structure, wax physical, rheological behaviours, ice premelting, morphology of ski base offering specific and final tribological performances not only in the lab but also on skiing sites.

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This purpose allows to study and understand wear behaviour of UHMWPE on ice. Another work completed during this experimentation represents a part of thesis, treating abrasion of skis soles directly on slope in mountain. As the ice presents broken and varied relief, a preliminary study on abrasion in laboratory enables us to understand phenomena which soles of skis (UHMWPE) during a world cup downhill are confronted to. We want that the final results obtained in friction of the UHMWPE in situ gives us same wear faces to those obtained during laboratory experimentation. Then, it will be possible to model ice attacks on ski soles with our tool. That remains very interesting for industrial and professional of ski with ski races objective.

Acknowledgement Authors would like to thank V. Dumollard.

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