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Influence of surface treatment of elastomers on their frictional behaviour in sliding contact
Wear 266 (2009) 468–475
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of surface treatment of elastomers on their frictional behaviour in sliding contact B. Verheyde a,∗ , M. Rombouts a , A. Vanhulsel a , D. Havermans a , J. Meneve a , M. Wangenheim b a b
VITO, Flemish Institute for Technological Research, Mol, Belgium IDS - University of Hannover, Institute of Dynamics & Vibrations, Hannover, Germany
a r t i c l e
i n f o
Article history: Received 1 October 2007 Received in revised form 29 February 2008 Accepted 7 April 2008 Available online 16 June 2008 Keywords: Atmospheric plasma treatment Organosiloxanes Elastomers Laser processing Solid lubricant Friction coefficient
a b s t r a c t To reduce friction of elastomer parts moving against a metal counter body in dry conditions, two different surface treatment techniques were applied on elastomer parts: laser cladding and plasma treatment at atmospheric pressure. Polyamide 11 (PA 11) based coatings were produced on thermoplastic polyurethane (TPU) substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS2 coating: friction of the TPU substrate was reduced with 40%. The incorporation of 15 wt% PTFE in the PA 11 coating resulted in a further decrease of the frictional force. A reduction of 80% of the frictional force of the TPU substrate was measured. The surface of the coatings before and after tribotesting was analysed. The plasma treatment of HNBR was done using a Plasmaspot® to form a plasma polymerised coating based on two different types of siloxanes. A reduction of 74–80% of the initial friction coefficient was measured in two different tribotest rig configurations: ball-on-disc and disc-on-disc. The resulting wear tracks were analyzed by SEM and EDX. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In addition to their traditional roles as seals and diaphragms, components made of rubber-like materials are increasingly replacing traditional metallic components due to their ease of manufacture, lightness and cost. However, elastomers have a high dry sliding friction, which forces the use of lubrication. The subject of this study is the application of different coatings on elastomers with the aim to reduce or eliminate lubrication. Different polymers like polyamides (PA), polyimides, polytetrafluorethylene (PTFE) and polysiloxanes are known to have a better friction behaviour compared to elastomers [1]. Moreover a vast literature is available on the exploration of polymer composites for improving the tribological performance of such engineering polymers in sliding contact [2–9]. These composites mostly contain a friction-reducing phase and a strengthening phase. The commonly used frictionreducing agents in polymers are ultra high molecular weight polyethylene, stearic acid, graphite, PTFE and molybdenum disulfide (MoS2 ). A reduced friction coefficient is usually observed but some researchers did not detect a beneficial effect of solid lubri-
∗ Corresponding author. Tel.: +32 14 335607; fax: +32 14 321186. E-mail address: [email protected] (B. Verheyde). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.040
cants in polymers like epoxy [10,11] and polyamide [12,13]. The presence of fillers often deteriorates the bulk properties such as mechanical strength and temperature stability. Altering the tribological performance of polymers by surface treatment has from this perspective the advantage that the material can be tailored in terms of tribological properties while its bulk properties are not significantly affected. This study deals with the frictional behaviour of coated elastomers in dry sliding contact against a steel counterface. Two types of coatings are produced: thick polymer composite coatings by laser cladding and thin hybrid SiOC coatings by atmospheric plasma treatment. Laser cladding is a technology that is used in industry to coat metallic components with a metal or metal matrix composite material [14,15]. This technique is adapted to coat thermoplastic polyurethane substrates with thick polyamide 11 (PA 11) composites containing MoS2 or PTFE as filler material. Plasma treatment at atmospheric pressure is a surface treatment which can be seen as environmental friendly as no solvents are needed during the process. Often, dielectric barrier discharge (DBD) plasma systems are used to modify the surface of different types of substrates by introducing functional groups [16,17] or to deposit a plasma polymerized coating on top of them [18]. Using a plasma torch configuration (Plasmaspot® ) [19] this technique has the flexibility to treat complex three-dimensional components.
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Fig. 1. Schematic overview of (a) the laser cladding setup and (b) the Plasmaspot® setup.
2. Experimental setup 2.1. Materials and characterisation In this study laser cladding is applied to coat flat thermoplastic polyurethane (TPU) substrates (thickness 6 mm) with polyamide and polyamide based composite materials. During laser cladding (see Fig. 1a), polymer-based powder is transported in an argon gas stream through a nozzle to the TPU substrate, where it is heated by a moving laser beam (laser wavelength 940 nm). The flat TPU substrates have a thickness of 10 mm. After powder deposition, a laser beam reheats the coatings above the melting temperature of the polymer. Powder mixtures based on PA 11 (Rilsan EC Gris from Arkema; particle size 20–100 m) are used as feedstock for the
coating process. PA 11 is selected because of its inherent low friction characteristic. Furthermore, the low melting point compared to high temperature resistant thermoplastics with outstanding resistance to sliding wear such as poly ether ether ketone (PEEK) and the absence of charring upon thermal degradation are favorable for the laser cladding process. In addition, the ductility and toughness of polyamides are usually higher than of high-temperature resistant thermoplastics, which means a better ‘property match’ with elastomers. Besides PA 11 powder, mixtures of PA 11 powder with 9 wt% MoS2 powder (from Sigma–Aldrich; particle size <2 m) or 15 wt% PTFE powder (from UPM Kunststoffen; particle size 10–100 m) have been used as feedstock. Due to the difference in thermal and structural properties between the fillers and the PA 11 base material, different process parameters (temperature, laser scan speed,
Fig. 2. Schematic diagrams of used tribometer testrigs. (a) Disc-on-disc (rotational) and (b) ball-on-disc (rotational).
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Table 1 Quick test procedure used in rotational disc-on-disc test rig Period
Conditions
a
Run-in
b
Velocity influence
c
Velocity influence
d
Repeatability
e
Load influence
f
Static friction
g
Repeatability
vmax = 100 mm/s; FN = 10 N; t = 300 s vmax = 10 mm/s; FN = 10 N; t = 60 s vmax = 100 mm/s; FN = 10 N; t = 60 s vmax = 10 mm/s; FN = 10 N; t = 60 s vmax = 10 mm/s; FN = 40 N; t = 60 s tstick = 60 s; FN = 10 N; then vmax = 10 mm/s; FN = 10 N; t = 15 s vmax = 100 mm/s; FN = 10 N; t = 60 s
powder feed rate) were applied during laser cladding of the various powders. The plasma treatments are carried out using the Plasmaspot® equipment from VITO which is a plasma torch working at atmospheric pressure [19]. The used power supply consists of a rectifier with a DC output which is converted to an AC signal with a frequency of 75 kHz. A high voltage is created using a transformer. Power is set to 450 W and carrier gas flow is maintained at 80 slm N2 using a mass flow controller. Two hybrid organic/inorganic siloxane precursors have been studied: (3-aminopropyl)-triethoxysilane (APEO) purchased from Sigma–Aldrich and (3-glycidoxypropyl)trimethoxy-silane (GLYMO) purchased from ABCR. They are atomized using a TSI 3076 atomiser and the aerosol is introduced into the afterglow via the central hollow electrode. Flat samples with a thickness of 2 mm of Hydrogenated Nitril Butadiene Rubber (HNBR) are chosen as substrate material. They were cleaned with isopropanol prior to coating deposition. Treatment time in each experiment is 15 s/cm2 . The structure of the coatings before and after tribotesting is analyzed by scanning electron microscopy (JEOL JSM-6430F FEG). Energy dispersive X-ray analysis is performed to reveal the chemical composition of the material. The roughness of the laser cladded coatings is obtained by UBM contact profilometry with a spherical diamond contact stylus with a radius of 10 m according to DIN 4776 standard.
Fig. 4. Roughness of untreated TPU and TPU with laser cladded PA 11, PA 11 + 15% PTFE and PA 11 + 9% MoS2 coatings.
botests are performed at room temperature in dry conditions. The untreated surfaces are cleaned with isopropanol while the coated ones are not cleaned prior to testing. In the disc-on-disc setup (available at IDS) a flat on flat contact is made between a rubber sample of 1 cm2 and a fine grinded stainless steel disc. A X20Cr13 stainless steel disc with a roughness value Ra of 0.21 m is used as counterface material (Fig. 2a). The square shaped flat rubber samples have a side length of 10 mm and a thickness of 2 mm. They are glued to a sample holder. The normal load is applied by a mass and a pulley. A quick test procedure is written to evaluate the coatings (see Table 1). By this means, the friction behaviour is tested for a measuring time t with respect to an influence of
2.2. Tribological testing Two different tribological test setups are used to evaluate the frictional characteristics of the coated substrates (Fig. 2). All tri-
Fig. 5. Friction coefficient during rotational ball-on-disc testing of untreated and laser cladded TPU.
Fig. 3. SEM image of top surface of (a) PA 11 + 15 wt% PTFE coating and (b) PA 11 + 9% MoS2 coating. The light grey phase in figure (b) corresponds to MoS2 particles.
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Fig. 6. SEM image of the wear track of TPU coated with PA 11 + 9 wt% MoS2 after rotational ball-on-disc testing at 100 mm/s and 1 N. The sliding direction is from bottom to top. The light phase corresponds to MoS2 particles.
the sliding velocity vmax and the force FN . Furthermore the static friction after a sticking time tstick is observed. The stability of the coating and its short distance wear resistance is investigated by a continuously measured run-in test and a few repeatability tests. Between the different test conditions a stand still of 2 min is set. During these two minutes there is no contact between the rubber
sample and the counterpart. Only before the static friction test, an additional stand still of 1 min is kept. In the rotational ball-on-disc setup (CSM tribometer available at VITO) the counterbody is a 100Cr6 steel ball with a diameter of 10 mm (Fig. 2b). Linear velocity is set to 100 mm/s and a load of 1 N is applied. The radius of the wear track is 5 mm.
Fig. 7. SEM image of the wear track of TPU coated with PA 11 + 15 wt% PTFE after rotational ball-on-disc testing at 100 mm/s and 1 N. The sliding direction is from bottom to top.
Fig. 8. Counterface after rotational ball-on-disc testing at 100 mm/s and 1 N for (a) untreated TPU, (b) PA 11 coating on TPU and (c) PA 11 + 15% PTFE coating on TPU.
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Fig. 9. SEM-analysis (above) of flat HNBR samples coated with (a) APEO and (b) GLYMO; EDX-spectra (below) taken on top of the coated surfaces.
3. Results and discussion 3.1. Laser cladding The coatings produced by laser cladding have a thickness of 100–200 m. Inside the coatings spherical pores, which are the result of thermal degradation during laser treatment, are often present. Their amount depends on the coating material and the process parameters. The top surface of the coatings containing 9% MoS2 and 15% PTFE is shown in Fig. 3. The SEM images show a homogeneous distribution of fillers at the surface. The PTFE particles did not melt upon laser treatment and result clearly in a rough
Fig. 10. Roughness of untreated flat HNBR samples and HNBR samples plasma coated with GLYMO or APEO.
surface (Fig. 3a). This is also reflected by the roughness value Rz , which is globally higher than for the PA 11 coating (Fig. 4). It should be noted that the roughness value Rz of the coatings is more than one order of magnitude higher than of the untreated TPU substrate. The direction of coating deposition does not have a significant effect on the roughness apart from the PA 11 + 9% MoS2 coating. The evolution of the coefficient of friction of untreated and laser cladded TPU during ball-on-disc testing up to 1 km running distance is given in Fig. 5. The friction coefficient of the TPU substrate reaches a maximal value of 1.5 at the beginning of the test, drops with 30% and remains after a sliding distance of about 400 m constant. The frictional behaviour of the laser cladded samples during sliding is clearly different. There is no clear maximum in frictional force at low travelling distances. The PA 11 and PA 11 + 9% MoS2 coatings have a similar friction characteristic. The coatings both reduce the friction of TPU with 50%. The presence of MoS2 does not have a beneficial effect on the frictional behaviour under the applied tribotest conditions. This is also observed for PA 6 nucleated with MoS2 and 5% dye [3] and for epoxy mixed with 0–30 vol% of MoS2 [10]. Liu et al. pointed out that MoS2 was not very effective for reducing friction of nylon 6 [20]. The PA 11 + 15% PTFE coating yields clearly the largest reduction in friction. A stable friction coefficient of about 0.2 is obtained. The low friction characteristic of PTFE is attributed to its smooth molecular structure, which leads to easy formation of a thin transfer film on the counterface material. Its beneficial effect as a filler has been reported for PEEKPTFE composites at relatively modest bulk concentration of PTFE (∼5–10%) [3] and for PA 66 + 20 wt% PTFE in unlubricated conditions [13]. However, no beneficial effect of PTFE when added to
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Fig. 11. SEM-analysis of the surfaces of flat HNBR samples coated with (a) APEO and (b) GLYMO. Cracks are found in both cases.
nylon 6 was observed in [3]. Samyn et al. investigated the tribological behaviour of various PA 6 based materials [2]. The addition of 8 wt% wax, which contained both MoS2 and PTFE, proved to stabilise and lower friction due to the formation of a coherent and thin transfer layer. The wear track and counterface material after tribotesting were analysed. Without coating no transfer layer on the steel ball is formed (Fig. 8a). PA 11 instead forms a transfer film on the counterface (Fig. 8b). The wear track cannot be clearly distinguished on the PA 11 coating. On the PA 11 + 9% MoS2 coating grooves along the sliding direction are present on the wear track (Fig. 6). This feature was not observed on the untreated TPU substrate or the PA 11 coating after contact. As can be seen at a higher magnification, MoS2 particles are displaced along the sliding direction and result in this way in grooves (Fig. 6b). The concentration of MoS2 on the wear track appears to be much lower than in the untested surface area (see Fig. 6a). High MoS2 concentrations remain only in local depressions present on the wear track. Fig. 7 shows the wear track of the PA 11 + 15% PTFE coating. The PTFE particles are clearly smoothened (see Fig. 3a and Fig. 7). Microscopic analysis shows the presence of a transfer film on the counterface material. EDX analysis points out that the transfer film contains a large amount of fluor, which confirms that it originates from the PTFE particles.
3.2. Plasma coating Two type of plasma coatings are reported here: GLYMO and APEO. At first a study of the coating morphology and thickness was performed using FE-SEM. Fig. 9 shows SEM-pictures of 25◦ tilted cross-sections of coated HNBR samples. Cross-sections were prepared by cryogenic breaking of the flat samples. Both coatings have a smooth surface and the morphology is much differing from the rough rubber substrate material. As can be seen in Fig. 10, both coatings have low roughness values Ra and Rz which are comparable to the roughness of the uncoated substrate material. This suggests that the coating tends to follow the initial roughness of the elastomer substrate material. Comparing the thickness of both coatings it seems that APEO (1110 nm) is forming a much thicker coating than GLYMO (350 nm). As the plasma process parameters (treatment time, power, gas flow) with both precursors were the same, it can be said that APEO has a much higher polymerisation speed compared to GLYMO. On the coating surface an EDX-spectrum was taken to have an idea of the chemical composition of the coating. A strong signal of Si was found, which was not present in the untreated HNBR sample and which is an evidence of the presence of a siloxane based coating. It can be seen that the signal for Si is much stronger for the APEO coating than for the GLYMO coating. Also a small signal for zinc was noticed in the spectrum of the GLYMO coating. This is due
to zinc containing components present in the bulk material which are measured together with the thinner GLYMO coating. In the case of the thicker APEO coating no zinc was detected. As can be seen in Fig. 11 a number of cracks appeared on top of the coated surface directly after coating deposition. These cracks have a width of about 2–3 m and have a dendritic structure with main cracks and side cracks. Most probably they are due to a difference in the elastic behaviour of the siloxane coating and the rubber substrate material. It must be said that they were not noticed when the same coating was applied on Si wafers or glass substrates. The exact origin of these cracks will be further studied in future. The tribological properties of both coatings were evaluated and compared to the untreated HNBR substrate material to have an idea of the improvement of the friction coefficient and the tribological behaviour of the coating. Two types of tribotest rigs were used: a disc-on-disc configuration (Fig. 2a) and a rotational ball-on-disc test configuration (Fig. 2b). Tribometer quick tests of both untreated and plasma coated flat HNBR samples performed by IDS are shown in Fig. 12. In comparison to the untreated HNBR sample, the general level of friction is lower on the GLYMO coated samples. Furthermore, the dependence of the coefficient of friction on the sliding velocity is smaller. The repeatability of the resulting friction coefficient is good in terms of the level of friction force for repeated measurements under the same conditions (comparison of periods b and d as well as periods c and g). APEO coated HNBR samples show an even smaller level of friction. In this case, a dependence of the friction on the sliding
Fig. 12. Friction coefficient of coated HNBR samples during a disc-on-disc test configuration. Both APEO and GLYMO are decreasing the friction coefficient, respectively, with about 80% and 58%, respectively. The tribotest conditions are given in Table 1.
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Fig. 13. Friction coefficient of coated HNBR samples during a ball-on-disc test configuration. Both APEO and GLYMO are decreasing the friction coefficient, respectively, with about 74% and 55%.
velocity can be recognized. Again the repeatability test was good for this type of coating. The static friction of the contact partners is checked as well. Large differences between the level of static and dynamic friction can advance undesired stick-slip vibrations of the system. In contrast to the uncoated samples, the GLYMO as well as the APEO coated samples did not show any difference between the maximum static friction and the dynamic friction. Stick-slip vibrations are observed in the friction force signal. In comparison to a quasi-stationary measurement, this can lead to an underestimation
of the level of the coefficient of friction, cp. [21]. This effect however is not considered in the evaluation of the friction levels of different coated samples, because a qualitative comparison is aspired. In Fig. 13 the evolution of the friction coefficient in a ballon-disc test configuration is shown for a total running distance of about 9 km. After a running-in period of some hundreds of meter, a decrease of 55% in the friction coefficient is found for the GLYMO coating compared to the untreated HNBR. Applying an APEO-coating on the elastomer substrate resulted in even lower values (decrease with 80%). Because of the nature of the HNBR rubber, the frictional behaviour of untreated HNBR is quite unstable. A stabilisation of the friction coefficient over time is noticed when the HNBR substrate is plasma coated with both APEO as well as GLYMO. The wear track after performing a tribotest in the ball-on-disc configuration was evaluated using FE-SEM and EDX analysis. Differences were found between the APEO and the GLYMO coating. Although the friction level remained stable for the full duration of the tribotest, the GLYMO coating was partially removed from the substrate material as can be seen in Fig. 14b. This is probably due to a bad adhesion of the coating to the substrate material. For the APEO coating this local delamination was not observed. The same cracking behaviour, which was already noticed on the coated samples directly after coating, was still found after the tribotest. Moreover, deformations during the tribotest seem to enhance the degree of cracking. Compared to the GLYMO coating, it can be concluded that the APEO coating is showing a better adhesion to HNBR.
Fig. 14. SEM–EDX analysis of the wear track after a ball-on-disc tribotest for (a) APEO coated HNBR and (b) GLYMO coated HNBR. Sliding direction is from top to bottom.
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From the point of view of the friction, the stable level of friction over a distance of 8.5 km has to be emphasized. While the original HNBR sample shows changes in the value of friction due to variations of the contact area because of thermal and wear effects, the coated samples do not show such behaviour. Although an analysis of the wear track by different characterisation techniques showed cracks and delamination of the coating layer, the level of friction remains low. The smooth surfaces of the coatings support the stable low friction. The roughness asperities of the untreated HNBR result in increased local thermal effects and wear, which comes along with higher friction forces. 4. Conclusions Surface treatment of elastomers by laser cladding and atmospheric pressure plasma coating has been performed. The results of tribological testing of these treated samples are very promising as a significant reduction of the coefficient of friction against steel is demonstrated. Polyamide 11 based coatings were produced on thermoplastic polyurethane substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS2 coating: friction of the TPU substrate was reduced with 40%. A thick transfer film on the counterface was formed. The MoS2 particles were displaced during sliding contact and gave rise to grooves parallel to the sliding direction. The incorporation of 15 wt% PTFE in the PA 11 coating reduced the frictional force of the PA 11 coating with 60%. Two different siloxane based precursors (APEO and GLYMO) were used to deposit a plasma polymerized coating on top of flat HNBR samples. Compared to untreated HNBR, APEO was decreasing the friction coefficient with about 80% in a ball-on-disc test setup as well as in a disc-on-disc test setup with flat on flat contact. The effect of a GLYMO coating was less pronounced but still a stable friction reduction of about 58% was obtained. The wear track was analyzed by SEM and EDX. Apart from cracks already present after coating deposition, the APEO coating was still completely covering the HNBR surface after a ball-on-disc tribotest whereas a strong delamination in the case of the GLYMO coating was noticed. However it did not influence the level of the friction coefficient which remained constant for the full duration of the tribotest. With the presented dry friction tests, a preselection of promising surface treatments is possible, especially for technical applications where lubricants are undesired, like in food or clean room industry. Acknowledgements The authors wish to acknowledge the support of the Partners of the Kristal project and in particular Merkel Freudenberg and
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Macpuarsa, and the European Commission for their support in the integrated project “Knowledge-based Radical Innovation Surfacing for Tribology and Advanced Lubrication” (EU Project Reference NMP3-CT-2005-515837). References [1] H.A. Unal, A. Mimaroglu, Friction and wear behaviour of unfilled engineering thermoplastics, Mater. Des. 24 (2003) 183–187. [2] P. Samyn, P. de Baets, G. Schoukens, I. Van Driessche, Friction, wear and transfer of pure and internally lubricated cast polyamides at various testing scales, Wear 262 (2007) 1433–1449. [3] K. Friedrich, Friction and Wear of Polymer Composites, Elsevier Press, 1986. [4] H. Unal, U. Sen, A. Mimaroglu, Dry sliding wear characteristics of some industrial polymers against steel counterface, Tribol. Int. 37 (9) (2004) 727–732. [5] J. Khedkar, I. Negulescu, E. Meletis, Sliding wear behaviour of PTFE composites, Wear 252 (2002) 361–369. [6] D.K. Baek, M.M. Khonsari, Friction and wear of a rubber coating in fretting, Wear 258 (2005) 898–905. [7] H. Unal, A. Mimaroglu, U. Kadioglu, H. Ekiz, Sliding friction and wear behavior of polytetrafluoroethylene and its composites under dry conditions, Mater. Des. 25 (2004) 239–245. [8] P. Samyn, G. Schoukens, J. Quintelier, P. De Baets, Friction, wear and material transfer of sintered polyimides sliding against various steel and diamond-like carbon coated surfaces, Tribol. Int. 39 (6) (2006) 575–589. [9] M. Masuko, H. Miyamoto, A. Suzuki, Tribological characteristics of selfassembled monolayer with siloxane bonding to Si surface, Tribol. Int. 40 (2007) 1587–1596. [10] X. Li, Y. Gao, J. Xing, Y. Wang, L. Fang, Wear reduction mechanism of graphite and MoS2 in epoxy composites, Wear 257 (2004) 279–283. [11] O. Jacobs, R. Jaskulka, F. Yang, W. Wu, Sliding wear of epoxy compounds against different counterparts under dry and aqueous conditions, Wear 256 (2004) 9–15. [12] J. Wang, M. Gu, B. Songhao, S. Ge, Investigation of the influence of MoS2 filler on the tribological properties of carbon fiber reinforced nylon 1010 composites, Wear 255 (2003) 774. [13] Y.K. Chen, S.N. Kukureka, C.J. Hooke, M. Rao, Surface topography and wear mechanisms in polyamide 66 and its composites, J. Mater. Sci. 35 (2000) 1269– 1281. [14] J. de Damborenea, Surface modification of metals by high power lasers, Surf. Coat. Technol. 100–101 (1998) 377–382. [15] C.L. Sexton, G. Byrne, K.G. Watkins, Alloy development by laser cladding: an overview, J. Laser Appl. 13 (1) (2001) 2–11. [16] C. Liu, N. Cui, N.M.D. Brown, B.J. Meenan, Effects of DBD plasma operating parameters on the polymer surface modification, Surf. Coat. Technol. 185 (2-3) (2004) 311–320. [17] M. Noeske, J. Degenhardt, S. Strudthoff, U. Lommatzsch, Plasma jet treatment of five polymers at atmospheric pressure: surface modifications and the relevance for adhesion, Int. J. Adhes. Adhes. 24 (2) (2004) 171–177. [18] S. Paulussen, R. Rego, O. Goossens, D. Vangeneugden, K. Rose, Plasma polymerization of hybrid organic–inorganic monomers in an atmospheric pressure dielectric barrier discharge, Surf. Coat. Technol. 200 (1-4) (2005) 672–675. [19] R. Rego, D. Havermans, J. Cools, Patent WO2006/081637 Atmospheric Plasma Jet. [20] W.M. Liu, C.X. Huang, L. Ling, Study of the friction and wear properties of MoS2 filled nylon 6, Wear 151 (1991) 111–118. ¨ [21] M. Lindner, M. Kroger, K. Popp, M. Gimenez, Stick-slip behaviour of seals with respect to time dependent friction forces, in: Proceedings of the 2004 ASME International Mechanical Engineering Congress and RD + D Expo, Anaheim, USA, 2004.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of surface treatment of elastomers on their frictional behaviour in sliding contact B. Verheyde a,∗ , M. Rombouts a , A. Vanhulsel a , D. Havermans a , J. Meneve a , M. Wangenheim b a b
VITO, Flemish Institute for Technological Research, Mol, Belgium IDS - University of Hannover, Institute of Dynamics & Vibrations, Hannover, Germany
a r t i c l e
i n f o
Article history: Received 1 October 2007 Received in revised form 29 February 2008 Accepted 7 April 2008 Available online 16 June 2008 Keywords: Atmospheric plasma treatment Organosiloxanes Elastomers Laser processing Solid lubricant Friction coefficient
a b s t r a c t To reduce friction of elastomer parts moving against a metal counter body in dry conditions, two different surface treatment techniques were applied on elastomer parts: laser cladding and plasma treatment at atmospheric pressure. Polyamide 11 (PA 11) based coatings were produced on thermoplastic polyurethane (TPU) substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS2 coating: friction of the TPU substrate was reduced with 40%. The incorporation of 15 wt% PTFE in the PA 11 coating resulted in a further decrease of the frictional force. A reduction of 80% of the frictional force of the TPU substrate was measured. The surface of the coatings before and after tribotesting was analysed. The plasma treatment of HNBR was done using a Plasmaspot® to form a plasma polymerised coating based on two different types of siloxanes. A reduction of 74–80% of the initial friction coefficient was measured in two different tribotest rig configurations: ball-on-disc and disc-on-disc. The resulting wear tracks were analyzed by SEM and EDX. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In addition to their traditional roles as seals and diaphragms, components made of rubber-like materials are increasingly replacing traditional metallic components due to their ease of manufacture, lightness and cost. However, elastomers have a high dry sliding friction, which forces the use of lubrication. The subject of this study is the application of different coatings on elastomers with the aim to reduce or eliminate lubrication. Different polymers like polyamides (PA), polyimides, polytetrafluorethylene (PTFE) and polysiloxanes are known to have a better friction behaviour compared to elastomers [1]. Moreover a vast literature is available on the exploration of polymer composites for improving the tribological performance of such engineering polymers in sliding contact [2–9]. These composites mostly contain a friction-reducing phase and a strengthening phase. The commonly used frictionreducing agents in polymers are ultra high molecular weight polyethylene, stearic acid, graphite, PTFE and molybdenum disulfide (MoS2 ). A reduced friction coefficient is usually observed but some researchers did not detect a beneficial effect of solid lubri-
∗ Corresponding author. Tel.: +32 14 335607; fax: +32 14 321186. E-mail address: [email protected] (B. Verheyde). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.040
cants in polymers like epoxy [10,11] and polyamide [12,13]. The presence of fillers often deteriorates the bulk properties such as mechanical strength and temperature stability. Altering the tribological performance of polymers by surface treatment has from this perspective the advantage that the material can be tailored in terms of tribological properties while its bulk properties are not significantly affected. This study deals with the frictional behaviour of coated elastomers in dry sliding contact against a steel counterface. Two types of coatings are produced: thick polymer composite coatings by laser cladding and thin hybrid SiOC coatings by atmospheric plasma treatment. Laser cladding is a technology that is used in industry to coat metallic components with a metal or metal matrix composite material [14,15]. This technique is adapted to coat thermoplastic polyurethane substrates with thick polyamide 11 (PA 11) composites containing MoS2 or PTFE as filler material. Plasma treatment at atmospheric pressure is a surface treatment which can be seen as environmental friendly as no solvents are needed during the process. Often, dielectric barrier discharge (DBD) plasma systems are used to modify the surface of different types of substrates by introducing functional groups [16,17] or to deposit a plasma polymerized coating on top of them [18]. Using a plasma torch configuration (Plasmaspot® ) [19] this technique has the flexibility to treat complex three-dimensional components.
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Fig. 1. Schematic overview of (a) the laser cladding setup and (b) the Plasmaspot® setup.
2. Experimental setup 2.1. Materials and characterisation In this study laser cladding is applied to coat flat thermoplastic polyurethane (TPU) substrates (thickness 6 mm) with polyamide and polyamide based composite materials. During laser cladding (see Fig. 1a), polymer-based powder is transported in an argon gas stream through a nozzle to the TPU substrate, where it is heated by a moving laser beam (laser wavelength 940 nm). The flat TPU substrates have a thickness of 10 mm. After powder deposition, a laser beam reheats the coatings above the melting temperature of the polymer. Powder mixtures based on PA 11 (Rilsan EC Gris from Arkema; particle size 20–100 m) are used as feedstock for the
coating process. PA 11 is selected because of its inherent low friction characteristic. Furthermore, the low melting point compared to high temperature resistant thermoplastics with outstanding resistance to sliding wear such as poly ether ether ketone (PEEK) and the absence of charring upon thermal degradation are favorable for the laser cladding process. In addition, the ductility and toughness of polyamides are usually higher than of high-temperature resistant thermoplastics, which means a better ‘property match’ with elastomers. Besides PA 11 powder, mixtures of PA 11 powder with 9 wt% MoS2 powder (from Sigma–Aldrich; particle size <2 m) or 15 wt% PTFE powder (from UPM Kunststoffen; particle size 10–100 m) have been used as feedstock. Due to the difference in thermal and structural properties between the fillers and the PA 11 base material, different process parameters (temperature, laser scan speed,
Fig. 2. Schematic diagrams of used tribometer testrigs. (a) Disc-on-disc (rotational) and (b) ball-on-disc (rotational).
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Table 1 Quick test procedure used in rotational disc-on-disc test rig Period
Conditions
a
Run-in
b
Velocity influence
c
Velocity influence
d
Repeatability
e
Load influence
f
Static friction
g
Repeatability
vmax = 100 mm/s; FN = 10 N; t = 300 s vmax = 10 mm/s; FN = 10 N; t = 60 s vmax = 100 mm/s; FN = 10 N; t = 60 s vmax = 10 mm/s; FN = 10 N; t = 60 s vmax = 10 mm/s; FN = 40 N; t = 60 s tstick = 60 s; FN = 10 N; then vmax = 10 mm/s; FN = 10 N; t = 15 s vmax = 100 mm/s; FN = 10 N; t = 60 s
powder feed rate) were applied during laser cladding of the various powders. The plasma treatments are carried out using the Plasmaspot® equipment from VITO which is a plasma torch working at atmospheric pressure [19]. The used power supply consists of a rectifier with a DC output which is converted to an AC signal with a frequency of 75 kHz. A high voltage is created using a transformer. Power is set to 450 W and carrier gas flow is maintained at 80 slm N2 using a mass flow controller. Two hybrid organic/inorganic siloxane precursors have been studied: (3-aminopropyl)-triethoxysilane (APEO) purchased from Sigma–Aldrich and (3-glycidoxypropyl)trimethoxy-silane (GLYMO) purchased from ABCR. They are atomized using a TSI 3076 atomiser and the aerosol is introduced into the afterglow via the central hollow electrode. Flat samples with a thickness of 2 mm of Hydrogenated Nitril Butadiene Rubber (HNBR) are chosen as substrate material. They were cleaned with isopropanol prior to coating deposition. Treatment time in each experiment is 15 s/cm2 . The structure of the coatings before and after tribotesting is analyzed by scanning electron microscopy (JEOL JSM-6430F FEG). Energy dispersive X-ray analysis is performed to reveal the chemical composition of the material. The roughness of the laser cladded coatings is obtained by UBM contact profilometry with a spherical diamond contact stylus with a radius of 10 m according to DIN 4776 standard.
Fig. 4. Roughness of untreated TPU and TPU with laser cladded PA 11, PA 11 + 15% PTFE and PA 11 + 9% MoS2 coatings.
botests are performed at room temperature in dry conditions. The untreated surfaces are cleaned with isopropanol while the coated ones are not cleaned prior to testing. In the disc-on-disc setup (available at IDS) a flat on flat contact is made between a rubber sample of 1 cm2 and a fine grinded stainless steel disc. A X20Cr13 stainless steel disc with a roughness value Ra of 0.21 m is used as counterface material (Fig. 2a). The square shaped flat rubber samples have a side length of 10 mm and a thickness of 2 mm. They are glued to a sample holder. The normal load is applied by a mass and a pulley. A quick test procedure is written to evaluate the coatings (see Table 1). By this means, the friction behaviour is tested for a measuring time t with respect to an influence of
2.2. Tribological testing Two different tribological test setups are used to evaluate the frictional characteristics of the coated substrates (Fig. 2). All tri-
Fig. 5. Friction coefficient during rotational ball-on-disc testing of untreated and laser cladded TPU.
Fig. 3. SEM image of top surface of (a) PA 11 + 15 wt% PTFE coating and (b) PA 11 + 9% MoS2 coating. The light grey phase in figure (b) corresponds to MoS2 particles.
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Fig. 6. SEM image of the wear track of TPU coated with PA 11 + 9 wt% MoS2 after rotational ball-on-disc testing at 100 mm/s and 1 N. The sliding direction is from bottom to top. The light phase corresponds to MoS2 particles.
the sliding velocity vmax and the force FN . Furthermore the static friction after a sticking time tstick is observed. The stability of the coating and its short distance wear resistance is investigated by a continuously measured run-in test and a few repeatability tests. Between the different test conditions a stand still of 2 min is set. During these two minutes there is no contact between the rubber
sample and the counterpart. Only before the static friction test, an additional stand still of 1 min is kept. In the rotational ball-on-disc setup (CSM tribometer available at VITO) the counterbody is a 100Cr6 steel ball with a diameter of 10 mm (Fig. 2b). Linear velocity is set to 100 mm/s and a load of 1 N is applied. The radius of the wear track is 5 mm.
Fig. 7. SEM image of the wear track of TPU coated with PA 11 + 15 wt% PTFE after rotational ball-on-disc testing at 100 mm/s and 1 N. The sliding direction is from bottom to top.
Fig. 8. Counterface after rotational ball-on-disc testing at 100 mm/s and 1 N for (a) untreated TPU, (b) PA 11 coating on TPU and (c) PA 11 + 15% PTFE coating on TPU.
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Fig. 9. SEM-analysis (above) of flat HNBR samples coated with (a) APEO and (b) GLYMO; EDX-spectra (below) taken on top of the coated surfaces.
3. Results and discussion 3.1. Laser cladding The coatings produced by laser cladding have a thickness of 100–200 m. Inside the coatings spherical pores, which are the result of thermal degradation during laser treatment, are often present. Their amount depends on the coating material and the process parameters. The top surface of the coatings containing 9% MoS2 and 15% PTFE is shown in Fig. 3. The SEM images show a homogeneous distribution of fillers at the surface. The PTFE particles did not melt upon laser treatment and result clearly in a rough
Fig. 10. Roughness of untreated flat HNBR samples and HNBR samples plasma coated with GLYMO or APEO.
surface (Fig. 3a). This is also reflected by the roughness value Rz , which is globally higher than for the PA 11 coating (Fig. 4). It should be noted that the roughness value Rz of the coatings is more than one order of magnitude higher than of the untreated TPU substrate. The direction of coating deposition does not have a significant effect on the roughness apart from the PA 11 + 9% MoS2 coating. The evolution of the coefficient of friction of untreated and laser cladded TPU during ball-on-disc testing up to 1 km running distance is given in Fig. 5. The friction coefficient of the TPU substrate reaches a maximal value of 1.5 at the beginning of the test, drops with 30% and remains after a sliding distance of about 400 m constant. The frictional behaviour of the laser cladded samples during sliding is clearly different. There is no clear maximum in frictional force at low travelling distances. The PA 11 and PA 11 + 9% MoS2 coatings have a similar friction characteristic. The coatings both reduce the friction of TPU with 50%. The presence of MoS2 does not have a beneficial effect on the frictional behaviour under the applied tribotest conditions. This is also observed for PA 6 nucleated with MoS2 and 5% dye [3] and for epoxy mixed with 0–30 vol% of MoS2 [10]. Liu et al. pointed out that MoS2 was not very effective for reducing friction of nylon 6 [20]. The PA 11 + 15% PTFE coating yields clearly the largest reduction in friction. A stable friction coefficient of about 0.2 is obtained. The low friction characteristic of PTFE is attributed to its smooth molecular structure, which leads to easy formation of a thin transfer film on the counterface material. Its beneficial effect as a filler has been reported for PEEKPTFE composites at relatively modest bulk concentration of PTFE (∼5–10%) [3] and for PA 66 + 20 wt% PTFE in unlubricated conditions [13]. However, no beneficial effect of PTFE when added to
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Fig. 11. SEM-analysis of the surfaces of flat HNBR samples coated with (a) APEO and (b) GLYMO. Cracks are found in both cases.
nylon 6 was observed in [3]. Samyn et al. investigated the tribological behaviour of various PA 6 based materials [2]. The addition of 8 wt% wax, which contained both MoS2 and PTFE, proved to stabilise and lower friction due to the formation of a coherent and thin transfer layer. The wear track and counterface material after tribotesting were analysed. Without coating no transfer layer on the steel ball is formed (Fig. 8a). PA 11 instead forms a transfer film on the counterface (Fig. 8b). The wear track cannot be clearly distinguished on the PA 11 coating. On the PA 11 + 9% MoS2 coating grooves along the sliding direction are present on the wear track (Fig. 6). This feature was not observed on the untreated TPU substrate or the PA 11 coating after contact. As can be seen at a higher magnification, MoS2 particles are displaced along the sliding direction and result in this way in grooves (Fig. 6b). The concentration of MoS2 on the wear track appears to be much lower than in the untested surface area (see Fig. 6a). High MoS2 concentrations remain only in local depressions present on the wear track. Fig. 7 shows the wear track of the PA 11 + 15% PTFE coating. The PTFE particles are clearly smoothened (see Fig. 3a and Fig. 7). Microscopic analysis shows the presence of a transfer film on the counterface material. EDX analysis points out that the transfer film contains a large amount of fluor, which confirms that it originates from the PTFE particles.
3.2. Plasma coating Two type of plasma coatings are reported here: GLYMO and APEO. At first a study of the coating morphology and thickness was performed using FE-SEM. Fig. 9 shows SEM-pictures of 25◦ tilted cross-sections of coated HNBR samples. Cross-sections were prepared by cryogenic breaking of the flat samples. Both coatings have a smooth surface and the morphology is much differing from the rough rubber substrate material. As can be seen in Fig. 10, both coatings have low roughness values Ra and Rz which are comparable to the roughness of the uncoated substrate material. This suggests that the coating tends to follow the initial roughness of the elastomer substrate material. Comparing the thickness of both coatings it seems that APEO (1110 nm) is forming a much thicker coating than GLYMO (350 nm). As the plasma process parameters (treatment time, power, gas flow) with both precursors were the same, it can be said that APEO has a much higher polymerisation speed compared to GLYMO. On the coating surface an EDX-spectrum was taken to have an idea of the chemical composition of the coating. A strong signal of Si was found, which was not present in the untreated HNBR sample and which is an evidence of the presence of a siloxane based coating. It can be seen that the signal for Si is much stronger for the APEO coating than for the GLYMO coating. Also a small signal for zinc was noticed in the spectrum of the GLYMO coating. This is due
to zinc containing components present in the bulk material which are measured together with the thinner GLYMO coating. In the case of the thicker APEO coating no zinc was detected. As can be seen in Fig. 11 a number of cracks appeared on top of the coated surface directly after coating deposition. These cracks have a width of about 2–3 m and have a dendritic structure with main cracks and side cracks. Most probably they are due to a difference in the elastic behaviour of the siloxane coating and the rubber substrate material. It must be said that they were not noticed when the same coating was applied on Si wafers or glass substrates. The exact origin of these cracks will be further studied in future. The tribological properties of both coatings were evaluated and compared to the untreated HNBR substrate material to have an idea of the improvement of the friction coefficient and the tribological behaviour of the coating. Two types of tribotest rigs were used: a disc-on-disc configuration (Fig. 2a) and a rotational ball-on-disc test configuration (Fig. 2b). Tribometer quick tests of both untreated and plasma coated flat HNBR samples performed by IDS are shown in Fig. 12. In comparison to the untreated HNBR sample, the general level of friction is lower on the GLYMO coated samples. Furthermore, the dependence of the coefficient of friction on the sliding velocity is smaller. The repeatability of the resulting friction coefficient is good in terms of the level of friction force for repeated measurements under the same conditions (comparison of periods b and d as well as periods c and g). APEO coated HNBR samples show an even smaller level of friction. In this case, a dependence of the friction on the sliding
Fig. 12. Friction coefficient of coated HNBR samples during a disc-on-disc test configuration. Both APEO and GLYMO are decreasing the friction coefficient, respectively, with about 80% and 58%, respectively. The tribotest conditions are given in Table 1.
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Fig. 13. Friction coefficient of coated HNBR samples during a ball-on-disc test configuration. Both APEO and GLYMO are decreasing the friction coefficient, respectively, with about 74% and 55%.
velocity can be recognized. Again the repeatability test was good for this type of coating. The static friction of the contact partners is checked as well. Large differences between the level of static and dynamic friction can advance undesired stick-slip vibrations of the system. In contrast to the uncoated samples, the GLYMO as well as the APEO coated samples did not show any difference between the maximum static friction and the dynamic friction. Stick-slip vibrations are observed in the friction force signal. In comparison to a quasi-stationary measurement, this can lead to an underestimation
of the level of the coefficient of friction, cp. [21]. This effect however is not considered in the evaluation of the friction levels of different coated samples, because a qualitative comparison is aspired. In Fig. 13 the evolution of the friction coefficient in a ballon-disc test configuration is shown for a total running distance of about 9 km. After a running-in period of some hundreds of meter, a decrease of 55% in the friction coefficient is found for the GLYMO coating compared to the untreated HNBR. Applying an APEO-coating on the elastomer substrate resulted in even lower values (decrease with 80%). Because of the nature of the HNBR rubber, the frictional behaviour of untreated HNBR is quite unstable. A stabilisation of the friction coefficient over time is noticed when the HNBR substrate is plasma coated with both APEO as well as GLYMO. The wear track after performing a tribotest in the ball-on-disc configuration was evaluated using FE-SEM and EDX analysis. Differences were found between the APEO and the GLYMO coating. Although the friction level remained stable for the full duration of the tribotest, the GLYMO coating was partially removed from the substrate material as can be seen in Fig. 14b. This is probably due to a bad adhesion of the coating to the substrate material. For the APEO coating this local delamination was not observed. The same cracking behaviour, which was already noticed on the coated samples directly after coating, was still found after the tribotest. Moreover, deformations during the tribotest seem to enhance the degree of cracking. Compared to the GLYMO coating, it can be concluded that the APEO coating is showing a better adhesion to HNBR.
Fig. 14. SEM–EDX analysis of the wear track after a ball-on-disc tribotest for (a) APEO coated HNBR and (b) GLYMO coated HNBR. Sliding direction is from top to bottom.
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From the point of view of the friction, the stable level of friction over a distance of 8.5 km has to be emphasized. While the original HNBR sample shows changes in the value of friction due to variations of the contact area because of thermal and wear effects, the coated samples do not show such behaviour. Although an analysis of the wear track by different characterisation techniques showed cracks and delamination of the coating layer, the level of friction remains low. The smooth surfaces of the coatings support the stable low friction. The roughness asperities of the untreated HNBR result in increased local thermal effects and wear, which comes along with higher friction forces. 4. Conclusions Surface treatment of elastomers by laser cladding and atmospheric pressure plasma coating has been performed. The results of tribological testing of these treated samples are very promising as a significant reduction of the coefficient of friction against steel is demonstrated. Polyamide 11 based coatings were produced on thermoplastic polyurethane substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS2 coating: friction of the TPU substrate was reduced with 40%. A thick transfer film on the counterface was formed. The MoS2 particles were displaced during sliding contact and gave rise to grooves parallel to the sliding direction. The incorporation of 15 wt% PTFE in the PA 11 coating reduced the frictional force of the PA 11 coating with 60%. Two different siloxane based precursors (APEO and GLYMO) were used to deposit a plasma polymerized coating on top of flat HNBR samples. Compared to untreated HNBR, APEO was decreasing the friction coefficient with about 80% in a ball-on-disc test setup as well as in a disc-on-disc test setup with flat on flat contact. The effect of a GLYMO coating was less pronounced but still a stable friction reduction of about 58% was obtained. The wear track was analyzed by SEM and EDX. Apart from cracks already present after coating deposition, the APEO coating was still completely covering the HNBR surface after a ball-on-disc tribotest whereas a strong delamination in the case of the GLYMO coating was noticed. However it did not influence the level of the friction coefficient which remained constant for the full duration of the tribotest. With the presented dry friction tests, a preselection of promising surface treatments is possible, especially for technical applications where lubricants are undesired, like in food or clean room industry. Acknowledgements The authors wish to acknowledge the support of the Partners of the Kristal project and in particular Merkel Freudenberg and
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