- Email: [email protected]
Tribological behavior of thermoplastic polyurethane elastomers
Materials & Design Materials and Design 28 (2007) 824–830 www.elsevier.com/locate/matdes
Tribological behavior of thermoplastic polyurethane elastomers Riadh Elleuch a
a,*
, Khaled Elleuch a, Boudour Salah a, Hassan Zahouani
b
Laboratoire des Syste`mes Electro-me´caniques (LASEM), Ecole Nationale dÕInge´nieurs de Sfax, B.P. W3038-Sfax, Tunisie Laboratoire de Tribologie et de Dynamique des Syste`mes (LTDS), (UMR CNRS 5513) Ecole Centrale de Lyon, France
b
Received 18 July 2005; accepted 4 November 2005 Available online 6 January 2006
Abstract This study is concerned with the thermoplastic polyurethane elastomers (TPU) in the form of capsules used for handling in textile industry applications. The analysis of static mechanical characteristics constitutes an interesting tool for understanding the tribological behavior of TPU in sliding contact. In order to study such behavior, experiments are conducted using indentation tests with spherical steel indenter for the purpose of determining the material characterization as elastic modulus and adhesion force (E, Fad) and evaluating the viscoelastic behavior of TPU. Comparison of experimental results obtained from indentation tests with analytical simulations based on Hertz theory confirms that Hertz theory can be used to model the indentation for load condition lower than the plastic flow load of TPU. This zone is governed by elastic behavior. Friction steel-TPU tests are conducted in order to quantify the monotonous and cyclic friction phenomena. Damage of TPU manifested by wear scars and wear particles are characterized after a short number of friction cycles. A close correlation between friction and wear was found. In fact, at low normal load (Fn < 1 N), the stable friction leads to an insignificant wear. Moreover, for Fn > 1 N, the friction decrease is induced by wear particles. The analysis developed in this paper confirms a good wear resistance of the TPU at real loading condition even for high numbers of cycles. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Viscoelasticity; Friction; Indentation; Tribology
1. Introduction Thermoplastic polyurethane elastomers (TPU) belong to thermoplastic elastomers (TPE) that combine the mechanical properties of vulcanized rubber with the process ability of thermoplastic polymers. They can be repeatedly melted and processed due to the absence of the chemical networks that normally exist in rubber and, therefore, are used in many fields [1,2]. Moreover, they are used instead of older polymers and other elastomers because of their moderate cost, excellent mechanical properties (higher elasticity, greater flexibility, toughness, etc.) high resistance to tear, oxidation and humidity [3–6].
*
Corresponding author. Tel.: +216 74 268842; fax: +216 74 267321. E-mail address: [email protected] (R. Elleuch).
0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.11.004
In textile industry, TPU is, therefore, used in machines for handling capsules (Fig. 1) which allow the removal of thread reels. This process consists of two steps: Capsule inflation which assures the contact between the reel and the capsule. This operation can be simulated by indentation tests on TPU by a hard body. Rise of the arm manipulator for exerting an axial effort to mount the reel. Such a force is a slight friction effort which can be produced between the TPU capsule (external diameter) and the reel (internal diameter) due to an imminent slip. This study underlines that the contact geometry becomes much stiffer as the displacement speed is increased. This is explained by the viscoelastic character of TPU polymer. Special interest is given to mechanical behavior of TPU to gain insight into the resulted damage when it is brought into contact with a hard body, such
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
825
Nomenclature List of notation E* reduced modulus (MPa) E elastic modulus (MPa) Ef elastic modulus of 100Cr6 Steel (MPa) Et elastic modulus of TPU (MPa) fd dynamic friction coefficient fs static friction coefficient Fad adhesion force (N) Fn normal load (N) L sliding distance (mm) N number of cycles P load (N)
Pp R V Vp Wt Wv Y d mf mt rp
plastic flow (N) radius of the spherical 100Cr6 Steel (mm) sliding speed (lm/s) loading speed (mN/s) total energy (mJ) viscous energy (mJ) sliding amplitude (mm) penetration (lm) PoissonÕs coefficient of 100Cr6 Steel PoissonÕs coefficient of TPU plastic flow stress (MPa)
Table 1 Properties of TPU Density (mg/m3)
Shore hardness (shore A)
Elastic modulus (E) (MPa)
Plastic flow stress (rp) (MPa)
1.2
90
15
1.6
based TPU, supplied in form of translucent, colorless or slightly yellowish pellets. This material can be extruded and injectionmoulded. Physical and mechanical properties of TPU are presented in Table 1. 2.2. Methods Fig. 1. Used capsule.
2.2.1. Indentation test
as steel [7]. Analytical and experimental works [8–11] were conducted to characterize the TPU elastic behavior and damage. Modeling TPU behavior under contact solicitation has been already done taking into account viscoelastic phenomenon [11]. In addition, several studies discovered that environmental exposure has a significant effect on TPU behavior [12–16]. As a result, structural changes occur and may lead to deterioration of physical and mechanical properties. For this, many degradation processes are proposed in [17,18]. This paper deals with contact mechanics and wear analysis of steel/(TPU). This study investigates: (i) the indentation behavior of TPU and (ii) the TPU friction and wear behavior under reciprocating sliding conditions.
In order to characterize surface materialÕs mechanical properties, the equipment used in this study employs a fully automated tribological device based on a static contact between a spherical indenter (100Cr6 Steel) of 6.35 mm radius and the flat TPU (see schematic of configuration for indentation test, Fig. 2). The tested samples have a cylindrical shape of a 20 mm diameter and 18 mm in depth. Prior to each test, the balls were washed by acetone and isopropyl alcohol. The indentation test allows the following of the normal load (Fn) evolution versus the penetration depth in real time. In order to simulate industrial conditions (compression air = 0.5 MPa) the load domain of Fn is ranged from 20 to 80 mN.
Unloading Loading
FN 2. Experimental approach R
2.1. Materials Thermoplastic polyurethane elastomers (TPU) belong to thermoplastic elastomers (TPE). The studied material is (PEARLTHANE D11T92EM) which is a polycaprolactone-copolyester
a
Fig. 2. Schematic diagram of indentation test.
826
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
3. Results and discussion
16
Stress σ (MPa)
14
3.1. Monotonous compression test
12 10 8 6 4 2 0 0
20
40
60
Strain ε (%) Fig. 3. Compressive stress–strain curve related to TPU with speed V = 12 mm/min.
2.2.2. Monotonous friction test A frictional tribometer test was used to carry out friction tests (Fig. 3). The TPU sample was plate with 2 mm thick. Spherical indenter (100Cr6 Steel) of 2.25 mm radius, tangents the surface, after indentation, it carries out a scratch at a constant normal load. The sensor system measures the normal and tangential forces in real time. For these tests, we can control the normal force Fn, the sliding speed (V), the loading speed (Vp) and the sliding distance (L). Friction tests were carried out at a loading speed (Vp) ranging from 50 to 200 mN/s and the sliding speed (V) ranging from 50 to 250 lm/s under normal load (Fn) ranging from 0.5 to 5 N. The sliding amplitude was approximately equal to Y = 2 mm per half cycle. 2.2.3. Cyclic friction test The sample is the same as the static friction except that the indenter makes forward-backward movement on the same line in mono-pass mode or multi- passes. For these two working/functioning mode, the normal load is kept constant. We justify the use of the friction mode by a direct application in the wear domain of the samples to be analyzed. Such tests are carried out in the air with the help of the friction tribometer (see Fig. 4), where the normal load is Fn = 0.5, 1, 2, and 5 N, the sliding speed is V = 250 lm/s, the number of cycles is N = 10, 50, 100, 1000, 5000 and 10,000 cycles and the sliding amplitude L = 2 mm.
Standard compression tests were selected to investigate the monotonic responses of TPU materials. Cylindrical specimens with standard dimensions (17 mm diameter and 25 mm height) are considered. The axis of the cylinder was taken in the same direction as the depth of the TPU specimen. In order to ensure an accurate measurement of compression displacement on the specimen, a new experimental device is developed. Such a device is fixed on the plates of the universal tensile testing machine to allow the use of an extensometer. This latter eliminates errors introduced by the load frame deformation and the drive system. Therefore, precise measurement of the deformation on the specimen is guaranteed. For each compression test, a minimum of five samples were tested, and the monotonic compressive curve of TPU was conducted at 12 mm/mn (200 lm/s). A typical stress–strain curve is shown in Fig. 3. Values of elastic modulus E (slope at the origin stress–strain curve) and plastic flow stress rp (stress at the beginning of non linear part of stress–strain curve) are presented in Table 1. 3.2. Indentation test 3.2.1. Influence of the applied normal load The analysis of the indentation–load curve allows the determination of the mechanical properties of the materialÕs surface. In particular, the influence of the applied normal load Fn = 20, 40 and 80 mN is shown in Fig. 5, and the mechanical properties of the TPU surface are illustrated in Table 2. According to Fig. 5, it can be deduced that the normal load has no significant impact on the adhesion force (Fad = 0 mN) during the contact between the TPU and steel. It can also be noted that the hysteresis increases with the normal load, and thus, the viscous energy (Wv) increases and the viscoelastic behavior becomes more significant. In addition, the
80 Fn 20 mN Fn 40 mN Fn 80 mN
Normal load (mN)
70 60 50 40 30 20 10 0 0
5
10
15
Penetration depth (µm) Fig. 4. Friction tribometer.
Fig. 5. Influence of the applied normal load, Fn = 20, 40 and 80 mN (Vp = 50 mN/s and R = 6.35 mm).
R. Elleuch et al. / Materials and Design 28 (2007) 824–830 Table 2 Mechanical properties of the TPU surface for Fn = 20, 40 and 80 mN 20 12 0 6.1 49.7 17.4
40 12 0 9.7 159.7 24.6
Normal load (mN)
Fn (mN) E (MPa) Fad (mN) dmax (lm) Wt (mJ) % Wv
20
80 12.5 0 14.6 456.8 32
Vp (200 mN/s)
10 5
0
2
4
6
Penetration depth (µm)
20
Normal load (mN)
Vp (50 mN/s) Vp (100 mN/s)
15
0 Exp Vp=(200 mN/S) HERTZ (E=15MPa)
Fig. 7. Effect of the loading speed Vp (Fn = 20 mN and R = 6.35 mm).
15
Table 3 Mechanical properties of the TPU surface for Fn = 20 mN, Vp = 50, 100 and 200 mN/s
Pp 10
Vp (mN/s) E (MPa) Fad (mN) dmax (lm) Wt (mJ) % Wv
5
0 0
2
4
6
Penetration depth (μm) Fig. 6. Hertz theory and experimental confrontation for steel spherical cap on flat TPU (Fn = 20 mN, Vp = 200 mN/s and R = 6.35 mm).
elastic modulus is observed to be constant when the normal load increases. 3.2.2. Validation of the indentation test In Fig. 6, a theoretical load–penetration curve is plotted for Hertz elastic theory [19] using the following expression: P¼
827
4E R1=2 3=2 d ; 3
ð1Þ
where P, d, R and E* are, respectively, the load, the penetration, the radius of the spherical 100Cr6 Steel, and the reduced modulus given by: 1 ð1 m2t Þ ð1 m2f Þ þ ¼ E Et Ef
ð2Þ
in which Et = 15 MPa is the elastic modulus of TPU measured with compression test, mt = 0.45 the PoissonÕs coefficient of TPU, Ef = 210 GPa the elastic modulus of 100Cr6 Steel, and mf = 0.3 the PoissonÕs coefficient of 100Cr6 Steel. It can be observed that two domains can be identified in Fig. 5. In the first domain, the experimental indentation curve agrees with the theoretical one. This corresponds to loads that are lower than that of the plastic flow at Pp = 10 mN. This critical load was estimated by the application of the Von Mises criteria to the stress field provided by Hertz theory using the plastic flow stress of TPU measured with the compression test. The second domain corresponds to loads that are higher than Pp and for which the discrepancy between theoretical and experimental curve is apparent.
50 12 0 6.1 49.7 17.4
100 13.4 0 5.4 46 15
200 15 0 4.9 55 13
3.2.3. Influence of the loading speed Vp The influence of the loading speed (Vp = 50, 100 and 200 mN/s) is investigated in Fig. 7 from which the mechanical properties of the TPU surface as Vp varies are illustrated in Table 3. It can be seen that the mechanical behavior of TPU under indentation test depends on the loading speed. For a given penetration depth, the measured load increases as the loading speed increases. This can be explained by the fact that increasing the loading speed yields an increase of the TPU elastic modulus, and thus, the contact becomes much stiffer. Therefore, it can be seen that elastic modulus increases with the loading speed. The increase in hardness implies that the penetration depth decreases for high loading speeds. In conclusion, the viscoelastic behavior of TPU becomes more sensitive to the increase of the loading speed and less sensitive to the increase of the normal load (see Table 4). 3.3. Monotonous friction tests 3.3.1. Influence of the normal load (Fn) Monotonic friction tests were carried out at different normal loads (Fn = 0.5 N, Fn = 2 N and Fn = 5 N) Fig. 8. The friction between the steel and the TPU increases rapidly at the beginning of the test until reaching the full sliding contact (static friction) after which the friction coefficient reaches a constant value fd = 0.5 (dynamic friction). Two types of TPU responses are recorded: Table 4 Variation of average friction coefficient in relation to the normal load Fn (V = 250 lm/s) Fn (N)
0.5
2
5
Dynamic friction coefficient (fd)
0.5
0.53
0.53
828
R. Elleuch et al. / Materials and Design 28 (2007) 824–830 Table 5 Variation of average friction coefficient in relation to the number of cycles
0.8 0.7 0.6
Ft / Fn
0.5
Fn (N)
0.5
1
2
5
Dynamic friction coefficient (fd) at 1 cycle Dynamic friction coefficient (fd) at 1000 cycles
0.5 0.5
0.5 0.45
0.53 0.4
0. 53 0.35
0.4 0.3
Fn 0,5 N
0.2
Fn 2 N
0.1
Fn 5 N
3.4. Cyclical friction tests
0 0
0.5
1
1.5
2
Sliding distance (mm) Fig. 8. Influence of applied normal load of testing on friction with sliding distance at V = 250 lm/s.
Table 5 presents the variation of the dynamic friction coefficient on the sliding contact in terms of the number of cycles N (1 and 1000 cycles) at different applied normal loads Fn (0 N, 1 N, 2 N and 5 N). For Fn = 0.5 N, the friction coefficient remains constant during 1000 cycles (f = 0.5). For Fn > 0.5 N, the friction coefficient decreases
for low normal loads Fn < 1 N, the friction coefficient is stable, for high normal loads Fn > 1 N, the friction coefficient presents a peak, which is normally attributed to full sliding condition (Ft = fFn), followed by a decrease until a stable value at the end of the friction test (fd = 0.5). This variation will be explained later where the wear damage and its link with friction are studied.
3.3.2. Influence of the friction speed The friction tests were carried out at various sliding speeds V = 50, 100 and 250 lm/s. These sliding speeds present no significant impact on the friction coefficient in the studied domain. The evolution of the friction response in relation to the sliding speed (for Fn = 5 N and R = 2.25 mm) is illustrated by the curves in Fig. 9. According to these curves, we notice that the sliding speed has no impact on the TPU friction response. In the next section, cyclical friction tests at a 250 lm/s speed are studied.
Fig. 10. Topographic characterization of TPU under Fn = 0.5 N, N = 1000 cycles, V = 250 lm/s, L = 2 mm.
0.8
Ft / Fn
0.6 0.4 V=50µm/s V=100µm/s V=250µm/s
0.2 0 0
0.5
1
1.5
2
Sliding distance (mm) Fig. 9. Influence of the friction speed of testing on friction with sliding distance under Fn = 5 N.
Fig. 11. Topographic characterization of TPU under Fn = 2 N, N = 100 cycles, V = 250 lm/s, L = 2 mm.
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
Fig. 14. Topographic characterization of N = 10,000 cycles, V = 250 lm/s, L = 2 mm.
Fig. 12. Topographic characterization of TPU under Fn = 2 N, N = 1000 cycles, V = 250 lm/s, L = 2 mm.
829
TPU
under
Fn = 5 N,
Loading conditions on the applied normal force and the total number of cycles were varied. We notice that at 0.5 N, the TPU presents no wear damage as shown in Fig. 10. However, degradation is noticed for Fn > 1 N in relation to the number of cycles as illustrated in Figs. 11 and 12, For Fn = 5 N and N = 10,000 cycles wear debris were detected near the contact edges confirming a serious wear (Fig. 14). Based on topography investigation of TPU damage, close correlation between friction and wear is observed. A good wear resistance of the TPU at real pressure (0.5 MPa) even for high number of cycles is noticed. 4. Conclusion
in terms of the number of cycles. Such decrease is more noticeable when the normal load increases (Fn = 5 N, N = 1000 cycles, fd = 0.35). In both monotonous and cyclical cases, friction coefficient is more important when the normal load exceeds 0.5 N. Topography analyses of these surfaces, given in the next section, become necessary in order to study the correlation between friction and wear mechanisms.
In this study, tribological behavior of TPU used as capsules for handling in textile industry applications, under indentation and friction tests was investigated. Viscoelastic behavior has been evaluated using indentation tests with spherical steel indenter. Indentation of a flat TPU elastomer with spherical steel can be modeled by the Hertz theory for loads lower than the plastic flow load Pp. Over this critical load provided by Hertz theory, the discrepancy between theoretical and experimental curve is observed. Monotonous and cyclic friction steelTPU tests were performed using a friction tribometer. Damage of TPU manifested by wear scars and wear particles are characterized after a short number of friction cycles. Close correlation between friction and wear have been confirmed from topography investigation of TPU damage. The analysis developed in this paper confirmed a good wear resistance of the TPU at real pressure (0.5 MPa) even for high number of cycles.
3.5. Wear analyses
References
Fig. 13. Topographic characterization of TPU under Fn = 5 N, N = 5000 cycles, V = 250 lm/s, L = 2 mm.
In order to gain insight into the evolution of the damage caused on the test-pieces, we conduct observations on the tested surfaces with an optical microscope. The quantification of the sample degradation is considered by examining the topography of surfaces before and after each cyclic friction test (100, 1000, 5000 and 10,000 cycles) (Figs. 10–14).
[1] Beck RA, Truss RW. Effect of chemical structure on the wear behaviour of polyurethane-urea elastomers. Wear 1998;218: 145–52. [2] Frick A, Rochman A. Characterization of TPU-elastomers by thermal analysis (DSC). Polym Test 2004;23:413–7. [3] Dombrow BA. Polyurethanes. New York: Reinhold Publishing Corporation; 1957.
830
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
[4] Saunders JH, Frisch KC. Polyurethanes: chemistry and technology. Part II: technology. New York: Interscience Publishers; 1964. [5] Ulrich H. Polyurethanes in modern plastics. Encyclopedia, vol. 60. New York: McGraw-Hill; 1983. p. 76–84. [6] Gary TH. Biodegradation of polyurethane. Int Biodeter Biodegr 2002;49:245–52. [7] Salah B, Elleuch R, Elleuch K, Zahouani H. Characterisation of the mechanical and frictional properties of thermoplastic polyurethane elastomer. In: Second international conference on advances in mechanical engineering (ICAME), Sousse, Tunisia; 2004. [8] Zhang LJ, Lin Z. Constitutive modelling and material characterisation of polymeric foams. J Eng Mater Technol 1997;119:284–91. [9] Simo JC. On a fully three-dimensional finite strain viscoelastic damage model: formulation and computational aspects. Comput Method Appl Mech Eng 1987;60:153–73. [10] Govindjee S, Reese S. A presentation of two large deformations visco-elasticity models. J Eng Mater Technol 1997;119: 251–5. [11] Burr A. Continuum description of damage in ceramic–matrix composites. Eur J Mech A Solids 1997;16(1):53–78.
[12] Rek V, Bravar M, Jocic T, Govorcin E. Ang Macromol Chem 1988;158/159:247–56. [13] Govorcin Bajsic E, Rek V, Sendijarevic A, Sendijarevic V, Frish KC. The effect of different molecular weight of soft segments in polyurethanes on photooxidative stability. Polym Degrad Stabil 1996;52: 223–33. [14] Van Der Ven LGJ, Bos MLM, Van Dijk JH. Adv Org Coat Sci Technol Ser 1990;12:162–9. [15] Herrera M, Matuschek G, Kettrup A. Thermal degradation of thermoplastic polyurethane elastomers (TPU) based on MDI. Polym Degrad Stabil 2002;78:323–31. [16] Chandra R, Thapliyal BP, Soni RK. Stabilization of polyurethane films against thermal and photo-oxidative degradation. Polym Degrad Stabil 1993;39:93–101. [17] Oprea S, OpreaV. Mechanical behaviorduringdifferentweatheringtests of the polyurethane elastomers films. Eur Polym J 2002;38:1205–10. [18] Beck RA, Truss RW. Effect of chemical structure on the wear behavior of polyurethane-urea. Wear 1998;218:145–52. [19] Johnson KL. Contact mechanics. Cambridge: Cambridge University Press; 1985.
Tribological behavior of thermoplastic polyurethane elastomers Riadh Elleuch a
a,*
, Khaled Elleuch a, Boudour Salah a, Hassan Zahouani
b
Laboratoire des Syste`mes Electro-me´caniques (LASEM), Ecole Nationale dÕInge´nieurs de Sfax, B.P. W3038-Sfax, Tunisie Laboratoire de Tribologie et de Dynamique des Syste`mes (LTDS), (UMR CNRS 5513) Ecole Centrale de Lyon, France
b
Received 18 July 2005; accepted 4 November 2005 Available online 6 January 2006
Abstract This study is concerned with the thermoplastic polyurethane elastomers (TPU) in the form of capsules used for handling in textile industry applications. The analysis of static mechanical characteristics constitutes an interesting tool for understanding the tribological behavior of TPU in sliding contact. In order to study such behavior, experiments are conducted using indentation tests with spherical steel indenter for the purpose of determining the material characterization as elastic modulus and adhesion force (E, Fad) and evaluating the viscoelastic behavior of TPU. Comparison of experimental results obtained from indentation tests with analytical simulations based on Hertz theory confirms that Hertz theory can be used to model the indentation for load condition lower than the plastic flow load of TPU. This zone is governed by elastic behavior. Friction steel-TPU tests are conducted in order to quantify the monotonous and cyclic friction phenomena. Damage of TPU manifested by wear scars and wear particles are characterized after a short number of friction cycles. A close correlation between friction and wear was found. In fact, at low normal load (Fn < 1 N), the stable friction leads to an insignificant wear. Moreover, for Fn > 1 N, the friction decrease is induced by wear particles. The analysis developed in this paper confirms a good wear resistance of the TPU at real loading condition even for high numbers of cycles. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polyurethane; Viscoelasticity; Friction; Indentation; Tribology
1. Introduction Thermoplastic polyurethane elastomers (TPU) belong to thermoplastic elastomers (TPE) that combine the mechanical properties of vulcanized rubber with the process ability of thermoplastic polymers. They can be repeatedly melted and processed due to the absence of the chemical networks that normally exist in rubber and, therefore, are used in many fields [1,2]. Moreover, they are used instead of older polymers and other elastomers because of their moderate cost, excellent mechanical properties (higher elasticity, greater flexibility, toughness, etc.) high resistance to tear, oxidation and humidity [3–6].
*
Corresponding author. Tel.: +216 74 268842; fax: +216 74 267321. E-mail address: [email protected] (R. Elleuch).
0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.11.004
In textile industry, TPU is, therefore, used in machines for handling capsules (Fig. 1) which allow the removal of thread reels. This process consists of two steps: Capsule inflation which assures the contact between the reel and the capsule. This operation can be simulated by indentation tests on TPU by a hard body. Rise of the arm manipulator for exerting an axial effort to mount the reel. Such a force is a slight friction effort which can be produced between the TPU capsule (external diameter) and the reel (internal diameter) due to an imminent slip. This study underlines that the contact geometry becomes much stiffer as the displacement speed is increased. This is explained by the viscoelastic character of TPU polymer. Special interest is given to mechanical behavior of TPU to gain insight into the resulted damage when it is brought into contact with a hard body, such
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
825
Nomenclature List of notation E* reduced modulus (MPa) E elastic modulus (MPa) Ef elastic modulus of 100Cr6 Steel (MPa) Et elastic modulus of TPU (MPa) fd dynamic friction coefficient fs static friction coefficient Fad adhesion force (N) Fn normal load (N) L sliding distance (mm) N number of cycles P load (N)
Pp R V Vp Wt Wv Y d mf mt rp
plastic flow (N) radius of the spherical 100Cr6 Steel (mm) sliding speed (lm/s) loading speed (mN/s) total energy (mJ) viscous energy (mJ) sliding amplitude (mm) penetration (lm) PoissonÕs coefficient of 100Cr6 Steel PoissonÕs coefficient of TPU plastic flow stress (MPa)
Table 1 Properties of TPU Density (mg/m3)
Shore hardness (shore A)
Elastic modulus (E) (MPa)
Plastic flow stress (rp) (MPa)
1.2
90
15
1.6
based TPU, supplied in form of translucent, colorless or slightly yellowish pellets. This material can be extruded and injectionmoulded. Physical and mechanical properties of TPU are presented in Table 1. 2.2. Methods Fig. 1. Used capsule.
2.2.1. Indentation test
as steel [7]. Analytical and experimental works [8–11] were conducted to characterize the TPU elastic behavior and damage. Modeling TPU behavior under contact solicitation has been already done taking into account viscoelastic phenomenon [11]. In addition, several studies discovered that environmental exposure has a significant effect on TPU behavior [12–16]. As a result, structural changes occur and may lead to deterioration of physical and mechanical properties. For this, many degradation processes are proposed in [17,18]. This paper deals with contact mechanics and wear analysis of steel/(TPU). This study investigates: (i) the indentation behavior of TPU and (ii) the TPU friction and wear behavior under reciprocating sliding conditions.
In order to characterize surface materialÕs mechanical properties, the equipment used in this study employs a fully automated tribological device based on a static contact between a spherical indenter (100Cr6 Steel) of 6.35 mm radius and the flat TPU (see schematic of configuration for indentation test, Fig. 2). The tested samples have a cylindrical shape of a 20 mm diameter and 18 mm in depth. Prior to each test, the balls were washed by acetone and isopropyl alcohol. The indentation test allows the following of the normal load (Fn) evolution versus the penetration depth in real time. In order to simulate industrial conditions (compression air = 0.5 MPa) the load domain of Fn is ranged from 20 to 80 mN.
Unloading Loading
FN 2. Experimental approach R
2.1. Materials Thermoplastic polyurethane elastomers (TPU) belong to thermoplastic elastomers (TPE). The studied material is (PEARLTHANE D11T92EM) which is a polycaprolactone-copolyester
a
Fig. 2. Schematic diagram of indentation test.
826
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
3. Results and discussion
16
Stress σ (MPa)
14
3.1. Monotonous compression test
12 10 8 6 4 2 0 0
20
40
60
Strain ε (%) Fig. 3. Compressive stress–strain curve related to TPU with speed V = 12 mm/min.
2.2.2. Monotonous friction test A frictional tribometer test was used to carry out friction tests (Fig. 3). The TPU sample was plate with 2 mm thick. Spherical indenter (100Cr6 Steel) of 2.25 mm radius, tangents the surface, after indentation, it carries out a scratch at a constant normal load. The sensor system measures the normal and tangential forces in real time. For these tests, we can control the normal force Fn, the sliding speed (V), the loading speed (Vp) and the sliding distance (L). Friction tests were carried out at a loading speed (Vp) ranging from 50 to 200 mN/s and the sliding speed (V) ranging from 50 to 250 lm/s under normal load (Fn) ranging from 0.5 to 5 N. The sliding amplitude was approximately equal to Y = 2 mm per half cycle. 2.2.3. Cyclic friction test The sample is the same as the static friction except that the indenter makes forward-backward movement on the same line in mono-pass mode or multi- passes. For these two working/functioning mode, the normal load is kept constant. We justify the use of the friction mode by a direct application in the wear domain of the samples to be analyzed. Such tests are carried out in the air with the help of the friction tribometer (see Fig. 4), where the normal load is Fn = 0.5, 1, 2, and 5 N, the sliding speed is V = 250 lm/s, the number of cycles is N = 10, 50, 100, 1000, 5000 and 10,000 cycles and the sliding amplitude L = 2 mm.
Standard compression tests were selected to investigate the monotonic responses of TPU materials. Cylindrical specimens with standard dimensions (17 mm diameter and 25 mm height) are considered. The axis of the cylinder was taken in the same direction as the depth of the TPU specimen. In order to ensure an accurate measurement of compression displacement on the specimen, a new experimental device is developed. Such a device is fixed on the plates of the universal tensile testing machine to allow the use of an extensometer. This latter eliminates errors introduced by the load frame deformation and the drive system. Therefore, precise measurement of the deformation on the specimen is guaranteed. For each compression test, a minimum of five samples were tested, and the monotonic compressive curve of TPU was conducted at 12 mm/mn (200 lm/s). A typical stress–strain curve is shown in Fig. 3. Values of elastic modulus E (slope at the origin stress–strain curve) and plastic flow stress rp (stress at the beginning of non linear part of stress–strain curve) are presented in Table 1. 3.2. Indentation test 3.2.1. Influence of the applied normal load The analysis of the indentation–load curve allows the determination of the mechanical properties of the materialÕs surface. In particular, the influence of the applied normal load Fn = 20, 40 and 80 mN is shown in Fig. 5, and the mechanical properties of the TPU surface are illustrated in Table 2. According to Fig. 5, it can be deduced that the normal load has no significant impact on the adhesion force (Fad = 0 mN) during the contact between the TPU and steel. It can also be noted that the hysteresis increases with the normal load, and thus, the viscous energy (Wv) increases and the viscoelastic behavior becomes more significant. In addition, the
80 Fn 20 mN Fn 40 mN Fn 80 mN
Normal load (mN)
70 60 50 40 30 20 10 0 0
5
10
15
Penetration depth (µm) Fig. 4. Friction tribometer.
Fig. 5. Influence of the applied normal load, Fn = 20, 40 and 80 mN (Vp = 50 mN/s and R = 6.35 mm).
R. Elleuch et al. / Materials and Design 28 (2007) 824–830 Table 2 Mechanical properties of the TPU surface for Fn = 20, 40 and 80 mN 20 12 0 6.1 49.7 17.4
40 12 0 9.7 159.7 24.6
Normal load (mN)
Fn (mN) E (MPa) Fad (mN) dmax (lm) Wt (mJ) % Wv
20
80 12.5 0 14.6 456.8 32
Vp (200 mN/s)
10 5
0
2
4
6
Penetration depth (µm)
20
Normal load (mN)
Vp (50 mN/s) Vp (100 mN/s)
15
0 Exp Vp=(200 mN/S) HERTZ (E=15MPa)
Fig. 7. Effect of the loading speed Vp (Fn = 20 mN and R = 6.35 mm).
15
Table 3 Mechanical properties of the TPU surface for Fn = 20 mN, Vp = 50, 100 and 200 mN/s
Pp 10
Vp (mN/s) E (MPa) Fad (mN) dmax (lm) Wt (mJ) % Wv
5
0 0
2
4
6
Penetration depth (μm) Fig. 6. Hertz theory and experimental confrontation for steel spherical cap on flat TPU (Fn = 20 mN, Vp = 200 mN/s and R = 6.35 mm).
elastic modulus is observed to be constant when the normal load increases. 3.2.2. Validation of the indentation test In Fig. 6, a theoretical load–penetration curve is plotted for Hertz elastic theory [19] using the following expression: P¼
827
4E R1=2 3=2 d ; 3
ð1Þ
where P, d, R and E* are, respectively, the load, the penetration, the radius of the spherical 100Cr6 Steel, and the reduced modulus given by: 1 ð1 m2t Þ ð1 m2f Þ þ ¼ E Et Ef
ð2Þ
in which Et = 15 MPa is the elastic modulus of TPU measured with compression test, mt = 0.45 the PoissonÕs coefficient of TPU, Ef = 210 GPa the elastic modulus of 100Cr6 Steel, and mf = 0.3 the PoissonÕs coefficient of 100Cr6 Steel. It can be observed that two domains can be identified in Fig. 5. In the first domain, the experimental indentation curve agrees with the theoretical one. This corresponds to loads that are lower than that of the plastic flow at Pp = 10 mN. This critical load was estimated by the application of the Von Mises criteria to the stress field provided by Hertz theory using the plastic flow stress of TPU measured with the compression test. The second domain corresponds to loads that are higher than Pp and for which the discrepancy between theoretical and experimental curve is apparent.
50 12 0 6.1 49.7 17.4
100 13.4 0 5.4 46 15
200 15 0 4.9 55 13
3.2.3. Influence of the loading speed Vp The influence of the loading speed (Vp = 50, 100 and 200 mN/s) is investigated in Fig. 7 from which the mechanical properties of the TPU surface as Vp varies are illustrated in Table 3. It can be seen that the mechanical behavior of TPU under indentation test depends on the loading speed. For a given penetration depth, the measured load increases as the loading speed increases. This can be explained by the fact that increasing the loading speed yields an increase of the TPU elastic modulus, and thus, the contact becomes much stiffer. Therefore, it can be seen that elastic modulus increases with the loading speed. The increase in hardness implies that the penetration depth decreases for high loading speeds. In conclusion, the viscoelastic behavior of TPU becomes more sensitive to the increase of the loading speed and less sensitive to the increase of the normal load (see Table 4). 3.3. Monotonous friction tests 3.3.1. Influence of the normal load (Fn) Monotonic friction tests were carried out at different normal loads (Fn = 0.5 N, Fn = 2 N and Fn = 5 N) Fig. 8. The friction between the steel and the TPU increases rapidly at the beginning of the test until reaching the full sliding contact (static friction) after which the friction coefficient reaches a constant value fd = 0.5 (dynamic friction). Two types of TPU responses are recorded: Table 4 Variation of average friction coefficient in relation to the normal load Fn (V = 250 lm/s) Fn (N)
0.5
2
5
Dynamic friction coefficient (fd)
0.5
0.53
0.53
828
R. Elleuch et al. / Materials and Design 28 (2007) 824–830 Table 5 Variation of average friction coefficient in relation to the number of cycles
0.8 0.7 0.6
Ft / Fn
0.5
Fn (N)
0.5
1
2
5
Dynamic friction coefficient (fd) at 1 cycle Dynamic friction coefficient (fd) at 1000 cycles
0.5 0.5
0.5 0.45
0.53 0.4
0. 53 0.35
0.4 0.3
Fn 0,5 N
0.2
Fn 2 N
0.1
Fn 5 N
3.4. Cyclical friction tests
0 0
0.5
1
1.5
2
Sliding distance (mm) Fig. 8. Influence of applied normal load of testing on friction with sliding distance at V = 250 lm/s.
Table 5 presents the variation of the dynamic friction coefficient on the sliding contact in terms of the number of cycles N (1 and 1000 cycles) at different applied normal loads Fn (0 N, 1 N, 2 N and 5 N). For Fn = 0.5 N, the friction coefficient remains constant during 1000 cycles (f = 0.5). For Fn > 0.5 N, the friction coefficient decreases
for low normal loads Fn < 1 N, the friction coefficient is stable, for high normal loads Fn > 1 N, the friction coefficient presents a peak, which is normally attributed to full sliding condition (Ft = fFn), followed by a decrease until a stable value at the end of the friction test (fd = 0.5). This variation will be explained later where the wear damage and its link with friction are studied.
3.3.2. Influence of the friction speed The friction tests were carried out at various sliding speeds V = 50, 100 and 250 lm/s. These sliding speeds present no significant impact on the friction coefficient in the studied domain. The evolution of the friction response in relation to the sliding speed (for Fn = 5 N and R = 2.25 mm) is illustrated by the curves in Fig. 9. According to these curves, we notice that the sliding speed has no impact on the TPU friction response. In the next section, cyclical friction tests at a 250 lm/s speed are studied.
Fig. 10. Topographic characterization of TPU under Fn = 0.5 N, N = 1000 cycles, V = 250 lm/s, L = 2 mm.
0.8
Ft / Fn
0.6 0.4 V=50µm/s V=100µm/s V=250µm/s
0.2 0 0
0.5
1
1.5
2
Sliding distance (mm) Fig. 9. Influence of the friction speed of testing on friction with sliding distance under Fn = 5 N.
Fig. 11. Topographic characterization of TPU under Fn = 2 N, N = 100 cycles, V = 250 lm/s, L = 2 mm.
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
Fig. 14. Topographic characterization of N = 10,000 cycles, V = 250 lm/s, L = 2 mm.
Fig. 12. Topographic characterization of TPU under Fn = 2 N, N = 1000 cycles, V = 250 lm/s, L = 2 mm.
829
TPU
under
Fn = 5 N,
Loading conditions on the applied normal force and the total number of cycles were varied. We notice that at 0.5 N, the TPU presents no wear damage as shown in Fig. 10. However, degradation is noticed for Fn > 1 N in relation to the number of cycles as illustrated in Figs. 11 and 12, For Fn = 5 N and N = 10,000 cycles wear debris were detected near the contact edges confirming a serious wear (Fig. 14). Based on topography investigation of TPU damage, close correlation between friction and wear is observed. A good wear resistance of the TPU at real pressure (0.5 MPa) even for high number of cycles is noticed. 4. Conclusion
in terms of the number of cycles. Such decrease is more noticeable when the normal load increases (Fn = 5 N, N = 1000 cycles, fd = 0.35). In both monotonous and cyclical cases, friction coefficient is more important when the normal load exceeds 0.5 N. Topography analyses of these surfaces, given in the next section, become necessary in order to study the correlation between friction and wear mechanisms.
In this study, tribological behavior of TPU used as capsules for handling in textile industry applications, under indentation and friction tests was investigated. Viscoelastic behavior has been evaluated using indentation tests with spherical steel indenter. Indentation of a flat TPU elastomer with spherical steel can be modeled by the Hertz theory for loads lower than the plastic flow load Pp. Over this critical load provided by Hertz theory, the discrepancy between theoretical and experimental curve is observed. Monotonous and cyclic friction steelTPU tests were performed using a friction tribometer. Damage of TPU manifested by wear scars and wear particles are characterized after a short number of friction cycles. Close correlation between friction and wear have been confirmed from topography investigation of TPU damage. The analysis developed in this paper confirmed a good wear resistance of the TPU at real pressure (0.5 MPa) even for high number of cycles.
3.5. Wear analyses
References
Fig. 13. Topographic characterization of TPU under Fn = 5 N, N = 5000 cycles, V = 250 lm/s, L = 2 mm.
In order to gain insight into the evolution of the damage caused on the test-pieces, we conduct observations on the tested surfaces with an optical microscope. The quantification of the sample degradation is considered by examining the topography of surfaces before and after each cyclic friction test (100, 1000, 5000 and 10,000 cycles) (Figs. 10–14).
[1] Beck RA, Truss RW. Effect of chemical structure on the wear behaviour of polyurethane-urea elastomers. Wear 1998;218: 145–52. [2] Frick A, Rochman A. Characterization of TPU-elastomers by thermal analysis (DSC). Polym Test 2004;23:413–7. [3] Dombrow BA. Polyurethanes. New York: Reinhold Publishing Corporation; 1957.
830
R. Elleuch et al. / Materials and Design 28 (2007) 824–830
[4] Saunders JH, Frisch KC. Polyurethanes: chemistry and technology. Part II: technology. New York: Interscience Publishers; 1964. [5] Ulrich H. Polyurethanes in modern plastics. Encyclopedia, vol. 60. New York: McGraw-Hill; 1983. p. 76–84. [6] Gary TH. Biodegradation of polyurethane. Int Biodeter Biodegr 2002;49:245–52. [7] Salah B, Elleuch R, Elleuch K, Zahouani H. Characterisation of the mechanical and frictional properties of thermoplastic polyurethane elastomer. In: Second international conference on advances in mechanical engineering (ICAME), Sousse, Tunisia; 2004. [8] Zhang LJ, Lin Z. Constitutive modelling and material characterisation of polymeric foams. J Eng Mater Technol 1997;119:284–91. [9] Simo JC. On a fully three-dimensional finite strain viscoelastic damage model: formulation and computational aspects. Comput Method Appl Mech Eng 1987;60:153–73. [10] Govindjee S, Reese S. A presentation of two large deformations visco-elasticity models. J Eng Mater Technol 1997;119: 251–5. [11] Burr A. Continuum description of damage in ceramic–matrix composites. Eur J Mech A Solids 1997;16(1):53–78.
[12] Rek V, Bravar M, Jocic T, Govorcin E. Ang Macromol Chem 1988;158/159:247–56. [13] Govorcin Bajsic E, Rek V, Sendijarevic A, Sendijarevic V, Frish KC. The effect of different molecular weight of soft segments in polyurethanes on photooxidative stability. Polym Degrad Stabil 1996;52: 223–33. [14] Van Der Ven LGJ, Bos MLM, Van Dijk JH. Adv Org Coat Sci Technol Ser 1990;12:162–9. [15] Herrera M, Matuschek G, Kettrup A. Thermal degradation of thermoplastic polyurethane elastomers (TPU) based on MDI. Polym Degrad Stabil 2002;78:323–31. [16] Chandra R, Thapliyal BP, Soni RK. Stabilization of polyurethane films against thermal and photo-oxidative degradation. Polym Degrad Stabil 1993;39:93–101. [17] Oprea S, OpreaV. Mechanical behaviorduringdifferentweatheringtests of the polyurethane elastomers films. Eur Polym J 2002;38:1205–10. [18] Beck RA, Truss RW. Effect of chemical structure on the wear behavior of polyurethane-urea. Wear 1998;218:145–52. [19] Johnson KL. Contact mechanics. Cambridge: Cambridge University Press; 1985.