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Tribological behavior of W-DLC coated rubber seals
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Surface & Coatings Technology 202 (2008) 1869 – 1875 www.elsevier.com/locate/surfcoat
Tribological behavior of W-DLC coated rubber seals Y.T. Pei a , X.L. Bui a , X.B. Zhou b , J.Th.M. De Hosson a,⁎ a
Department of Applied Physics, the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands b Department of Science and Technology, SKF Research and Development B.V., Kelvinbaan 16, 3430 DT Nieuwegan, The Netherlands Received 17 June 2007; accepted in revised form 10 August 2007 Available online 17 August 2007
Abstract Tungsten-containing diamond-like carbon (W-DLC) coatings have been deposited on FKM (fluorocarbon) and HNBR (hydrogenated nitrile butadiene) rubbers via unbalanced magnetron reactive sputtering from a WC target in a C2H2/Ar plasma. The surface morphology and fracture cross sections of uncoated and coated rubbers have been characterized with high resolution scanning electron microscopy (SEM). The tribological behavior of uncoated and coated rubbers has been investigated with ball-on-disc tribotest under dry sliding condition against a 100Cr6 ball. The coefficient of friction (CoF) of uncoated rubbers is very high (N1). Equally a relatively high CoF of W-DLC coated FKM (about 0.6) is observed due to the gradual failure and delamination of the coatings. On the contrary, W-DLC coated HNBR rubber exhibits a superior tribological performance with a very low CoF of 0.2–0.25. The latter is comparable to that of Me-DLC coatings deposited on steel substrates. After 10,000 sliding laps almost no damage of the coatings is observed on the wear tracks. In fact the network of micro-cracks as deposited facilitates the flexibility of the coatings. The different surface roughness and mechanical properties of the rubber substrates explain the differences in the tribological performances of the coated rubbers. © 2007 Elsevier B.V. All rights reserved. Keywords: W-DLC coating; Rubber substrates; Surface morphology; Coefficient of friction; Adhesion 1
1. Introduction Rubber seals are commonly used in lubrication system to prevent dirt and water entering the system and to avoid leakage of lubricants. Dynamic rubber seals operate in sliding contact mode at a relatively high speed and under no or only marginal lubrication condition. Under such tough operational conditions, contact seals are the major sources of friction in lubrication systems or bearings. Furthermore, rubber seals are subjected to severe wear leading to an increase of clearance, which is often the cause of loss of the function and failure of the lubrication system. Therefore, an advanced solution for dynamic rubber seals is stringent for bearings and automotive industries. Diamond-like carbon (DLC) coatings have been widely recognized as a tribo-coating material acting like a wear-
⁎ Corresponding author. Tel.: +31 50 363 4898; fax: +31 50 363 4881. E-mail address: [email protected] (J.Th.M. De Hosson). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.08.013
resistant solid lubricant with low friction coefficient. Processing techniques of DLC coatings, such as PVD and PECVD, have been well developed and industrialized for deposition on metallic substrates [1,2]. However, high residual stresses and brittleness of DLC coatings limit their thickness, adhesion and load-bearing capability. Doping with metal atoms [1,2] and especially the introduction of ceramic nanocrystallites to form DLC-based nanocomposites [3] can efficiently solve some of the drawbacks. Only very recently, few research efforts have been made to deposit DLC coatings on rubber [4–7] or polymer substrates [8–10]. Due to the difficulty in measuring the coefficient of friction (CoF) of rubber material, the experimental data of CoF were rather contradictory. One experiment indicated that the DLC coated rubber has a CoF close to 1 in comparison with that of rubber surfaces, approaching 6 [6]. Other work showed that the CoF of DLC coated polymers is typically 0.25–0.4 in contrast with 0.7–1.5 of uncoated polymers [8,10]. As the rubber is soft and elastic, it is difficult to characterize the
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Table 1 Properties of FKM and HNBR rubber Rubber Color Specific gravity
Max. continuous working temperature (°C)
Max. intermittent working temperature (°C)
Coefficient of thermal expansion (×10− 6 K− 1)
Indentation depth at 5 N Modulus load (mm) (MPa)
FKM Red 1.9 HNBR Black 1.0
205 125
300 150
160 230
155 ± 4 115 ± 3
mechanical properties and the microstructure of coated rubbers. The greatest challenge in the development of DLC-based coatings on rubber seals is to keep the coating flexible as well as adhered to rubber surface under large elastic deformation and dynamic straining. One design proposal to solve this problem is based on a concept of segment-structured DLC coatings formed through a mesh mask [7]. However, the mesh size was limited to sub-millimeter due to technical complexity. In this work, W-DLC coatings were deposited on FKM and HNBR rubbers via reactive unbalanced magnetron sputtering. The tribological properties of coated rubbers sliding against 100Cr6 steel balls have been studied in terms of the tribomechanical responses of a system, in comparison with that of uncoated rubber substrates. The role of a chromium interlayer that is commonly used for the deposition of DLCbased coatings on metallic substrates has been also examined for coatings on soft and flexible substrates. The results are striking in the sense of reducing friction of rubber seals and coating design for soft and flexible substrates such as rubbers.
9.8 15.2
on the rubber substrates prior to the deposition of the WC loadbearing layer. The maximum substrate temperature during depositions was measured on the rear side of the rubber substrates to be not higher than 150 °C. It should be indicated that there was no observable damage to the HNBR rubber substrates after deposition, even though the maximum deposition temperature was slightly higher than the maximum continuous working temperature of the rubber (see Table 1). Cross sections of the coated rubbers were made by fracturing after cooling in liquid nitrogen for 10 min. The surface morphology and wear track of the uncoated and coated rubbers were characterized with a scanning electron microscope (Philips FEG-XL30). The uncoated and coated rubber sheets were glued onto ø30 mm polished M2 steel discs for tribotests that were performed at room temperature (22 °C) on a CSM high temperature tribometer with a ball-on-disc configuration. The
2. Experimental FKM (fluorocarbon) and HNBR (hydrogenated nitrile butadiene) rubber sheets of 2 mm thickness were used as substrates. For determination of the rigidity of rubber materials, the rubber sheets were glued onto polished M2 steel discs and indented by a ø6 mm silicon nitride ball with a CSM Revetest scratch tester. The indentation depth at 5 N normal load was considered as a measure of the rigidity of the rubbers. The properties of the rubber materials are listed in Table 1. The rubber sheets were cut into rectangular pieces of size 100 × 70 mm2 for the coating process. Prior to deposition, the rubbers were washed and ultrasonically cleaned in ethanol for 30 min. The deposition of commercial W-DLC coatings was carried out by unbalanced reactive magnetron sputtering from WC targets in Ar and C2H2 plasma. The setup of the sputtering system was similar to the one described in [11]. The W-DLC coatings included two layers: a load bearing WC layer of thickness of about 300 nm capped by a W-DLC top layer with thickness of about 700 nm. The W content of the W-DLC layer was about 20 at.% for all the samples, measured by energydispersive X-ray spectroscopy (EDS). The hydrogen content of the W-DLC layer was 18–20 at.%, comparable with that of most commercial W-DLC coatings [12], analyzed by means of elastic recoil detection (ERD) with a He-ion beam of 2.3 MeV energy. For a comparative study, the coatings were deposited on the rubber substrates without or with a Cr interlayer of about 120 nm thickness, respectively. The Cr interlayer was deposited
Fig. 1. Surface morphology of FKM (a) and HNBR (b) rubbers.
Y.T. Pei et al. / Surface & Coatings Technology 202 (2008) 1869–1875
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counterpart was a ø6 mm commercial 100Cr6 steel ball of hardness HRC 60–62. The tribotest conditions were 1 N normal load, 10 cm/s sliding speed and 35% relative humidity that was kept constant for all the tests with a humidity regulator. For a detailed setup of the tribological testing reference is made to [13].
into the elastic modulus of the rubber substrates via the following equation [14]: 3PR 1=3 ð1Þ a¼ 4E4
3. Results and discussion
a ¼ 2Rh h , one obtains
The surface morphology of the rubber substrates is shown in Fig. 1. Much denser structures and smoother surfaces are observed on FKM rubber (Fig. 1a), in comparison with HNBR rubber (Fig. 1b). The latter exhibits a powdery morphology and rough surface. This observation is consistent with an almost doubled specific gravity of FKM (1.9) compared to that of HNBR (1.0) (Table 1). However, the indentation depth of ø6 mm Si3N4 ball at 5 N normal load on HNBR (115 μm) is much smaller than that on FKM (155 μm), indicating a much higher rigidity of HNBR rubber under loading contact. Such a difference in mechanical response of the rubber substrates is expected to dramatically affect the tribological performances of the coated rubbers though the coating system is the same. According to Hertzian elastic theory of ball-on-flat contact, in fact, the measured indentation depth can be readily translated
With 2
E2 ¼
1 E4
¼
1v21 E1
þ
1v22 E2
and the radius of contact area
2
3PRE1 1 v22
4E1 ð 2Rh h2 Þ3=2 3PR 1 v21
c
3PR 1 v22
4ð 2Rh h2 Þ3=2
ð2Þ
where P is the indentation load, R the radius of the indenting ball and h indentation depth measured under the applied load. E1, v1 and E2, v2 are the elastic modulus and Poisson's ratio of Si3N4 ball used and rubber substrates, respectively. Because the modulus of indenting ball is very large in comparison with that of rubbers and thus the second term of the denominator negligibly small, the approximation at the right end of Eq. (2) is reached. With the known elastic properties E1 = 310 GPa, v1 = 0.278 [15] and v2 = 0.5 [16], the modulus of HNBR and FKM rubber is estimated to be 15.2 MPa and 9.8 MPa, respectively. An overview of the surface morphologies of W-DLC coated FKM and HNBR rubbers is shown in Fig. 2. Cracking is always
Fig. 2. Overview of the surface morphology of W-DLC coatings deposited on FKM rubber (a — without Cr interlayer; b — with Cr interlayer) and HNBR rubber (c — without Cr interlayer; d —with Cr interlayer).
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growing interface during the deposition of Cr interlayer. In addition, opened cracks are observed in the W-DLC coatings on HNBR (Fig. 2c and d), especially in the case without Cr interlayer. This can be attributed to the higher thermal expansion of HNBR (230 × 10− 6 K− 1) compared to that of FKM (160 × 10− 6 K− 1) (Table 1). The large shrinkage of HNBR substrates during cooling from the deposition temperature to room temperature makes the borders of existing cracks of the coatings press each other, leading to further breaking at the vicinities of the borders and delamination due to bending of HNBR substrates during unloading and handling. Consequently, the cracks become further widened and more open. The fracture cross sections of the W-DLC coatings without Cr interlayer are revealed in Fig. 3. There is no essential difference in the fracture morphology of the W-DLC coatings on FKM and HNBR substrates. Both the load bearing WC layer and the W-DLC top layer exhibit columnar microstructures. However, there is a significant interruption between the two layers. Interfacial delamination often occurs between these two layers during fracture, leaving a sharp step along the interface on the fracture cross sections (see Fig. 3). Although it is a common practice to employ a load-bearing layer beneath the top
Fig. 3. SEM micrographs showing the fracture cross section of W-DLC coatings without Cr interlayer: (a) on FKM rubber and (b) on HNBR rubber.
observed on the W-DLC coatings in all the cases. Random crack network is a typical feature of hard coatings deposited on soft and flexible substrates such as rubber and polymer [5].During deposition and especially cooling down of the coated rubber substrates afterwards, cracks may initiate and randomly propagate in the coatings due to the huge difference in the coefficient of thermal expansion and elasticity between W-DLC coatings and rubber substrates. W-DLC coatings deposited without a Cr interlayer exhibit cauliflower-like morphologies with irregularly elongated branches separated by clear boundaries in both the cases of FKM and HNBR substrates (see the insert of Fig. 2a and c). Employing a Cr interlayer on FKM substrate, the WDLC coating surface becomes much smoother and shows fine dome-shaped branches that are often observed on W-DLC coatings deposited on Si wafers or well polished metallic substrates. However, the fragments of the cracked coatings severely bulge outward and as a result, the crack network forms in the grooves of winkled surface (Fig. 2b). It is most likely attributed to the interface stresses between the W-DLC coating and the interlayer, and promoted by the low rigidity of FKM rubber. In contrast, a large amount of micrometer sized droplets are observed on the W-DLC coating deposited on HNBR with a Cr interlayer (Fig. 2d). It is correlated to the powdery surface of HNBR rubber, which leads to an enhanced instability of
Fig. 4. SEM micrographs showing the fracture cross section of W-DLC coatings with Cr interlayer on FKM rubber (a) and on HNBR rubber (b). The inserts show the characteristic of interfacial bonding of the coatings with higher magnification.
Y.T. Pei et al. / Surface & Coatings Technology 202 (2008) 1869–1875
Fig. 5. Coefficient of friction of coated and uncoated rubbers.
coating on metallic substrates, its effects for coatings on soft and elastic substrates are insignificant, if not even negative according to this observation. The fracture cross sections of W-DLC rubbers with a Cr interlayer clearly demonstrate the negative effects of the interlayer on the interfacial adhesion and growth stability of the coatings (Fig. 4). On FKM rubber, the coating suffers delamination between the loading support WC layer and the Cr interlayer. Due to shrinkage of the rubber substrate during cooling that generates significant interface stresses, the center of cracked coating fragments bulges outwards and the crack edges bend inwards and often insert into the rubber surface, leading to the formation of bow-like open cracks (Fig. 4a). In contrast, no obvious interfacial delamination is observed in W-DLC coated HNBR rubber with a Cr interlayer. Rather, curving of the Cr interlayer under cyclic stresses (mainly compressive) due to temperature fluctuations enlarges the roughness of the powdery surface of HNBR rubber, resulting in instability of the growing interface. As a result, a large amount of micrometer sized bumps are formed on the coating (Fig. 4b). These bumps seem to have positive effects on stress release and interface adhesion. The tribotest results of uncoated and coated rubbers sliding against 100Cr6 steel ball are shown in Fig. 5. Without W-DLC coating, the CoF of the rubbers is very high. At the beginning of the sliding, the CoF is about 1.2 and 1.9 for FKM and HNBR rubber, respectively. After 6000 laps, the CoF reaches the steady-state values of 1.0 for FKM rubber and 1.3 for HNBR rubber. Such a decrease in friction with sliding is likely correlated to the effect of flash temperature rising on the contact area [17]. These measurements of friction are in good agreement with the values reported in literature, which showed that the coefficients of friction of rubbers sliding against steel varied from 1 to 6 depending on the type of rubbers [6]. With W-DLC coatings, the CoF of coated rubbers is drastically reduced. With W-DLC coating on FKM in both the cases with and without Cr interlayer, the initial coefficient of friction is about 0.2 at the beginning of sliding and gradually increases to the value of 0.57 at the end of the test for the coating with a Cr interlayer and 0.63 for that without a Cr interlayer. The fluctuation peaks in the recorded CoF curves together with the gradual increase in the
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CoF indicate a gradual damage of the W-DLC coatings during the tribotests. The CoF of W-DLC coated HNBR rubbers is about 0.22 without Cr interlayer and 0.25 with Cr interlayer. Especially, the frictional behavior of W-DLC coated HNBR rubbers is very stable during the entire period of tribotests. Such low friction is comparable to that of metal-doped DLC coatings deposited on steel substrates for tribological applications [18,19]. SEM observations on the wear tracks of W-DLC coated FKM rubbers reveal the fracture of W-DLC coatings after tribotests, as shown in Fig. 6. Without Cr interlayer, most of the newly fractured fragments of the coating are still adhered onto the rubber surface, but many of them become pulverized (Fig. 6a). With a Cr interlayer, however, a large amount of coating material is removed from the wear track (Fig. 6b). Evidently, the Cr interlayer does not enhance the adhesion of the coatings on FKM rubber as already revealed by SEM observation on the fracture cross sections (Fig. 4). The adhesion strength and thus the loadbearing capability of W-DLC coating on FKM are weakened when Cr interlayer is applied. This is different from the situation of W-DLC coatings deposited on steel substrates where Cr
Fig. 6. SEM micrographs showing the wear tracks on W-DLC coated FKM rubbers: (a) without Cr interlayer and (b) with Cr interlayer. The inserts show the central area of the wear track with higher magnification.
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the design of the coating systems. It has been shown that the physical linking by interface broadening and interlocking contribute an important part to the adhesion of a coating on the substrate [20]. The rough powdery surface of HNBR rubber, as seen in Fig. 1a, apparently assigns stronger interfacial adhesion for coatings compared to the case of FKM rubber. Especially, higher rigidity or modulus of HNBR rubber leads to a smaller surface deformation under the applied load during tribotest. Therefore, the (bending) strain of the coating fragments under ball loading contact is much lower. All these effects result in a significantly enhanced load-bearing capability and thus little damage of W-DLC coatings on HNBR rubber compared to that on FKM. As a consequence, stable frictional behavior with a low CoF of 0.2–0.25 is achieved with W-DLC coated HNBR rubbers. The wear scar of 100Cr6 balls after sliding 10,000 laps against W-DLC coated FKM and HNBR rubbers is shown in Fig. 8. Corresponding to the different widths of the wear tracks, the wear scar formed with the coated FKM rubber is larger than that coupled with the coated HNBR rubber. The maximum width (perpendicular to the sliding direction) and the maximum length (along the sliding direction) of the wear scars are 1097 × 810 μm when sliding against the coated FKM rubber and
Fig. 7. SEM micrographs showing the wear tracks on W-DLC coated HNBR rubbers: (a) without Cr interlayer and (b) with Cr interlayer.
interlayer promotes the interfacial adhesion. It is understandable that DLC-based coatings can better adhere directly onto rubber surface due to stronger chemical affinity via C–C or C–H bonds, provided no foreign material such as a metallic interlayer is introduced in between. While the coatings on FKM rubbers are all seriously damaged during tribotests, the coatings on HNBR rubber hardly show further damage and still adhere very well on the substrate as shown in Fig. 7, even if dense crack networks exist as deposited. The wear tracks of W-DLC coated HNBR rubbers do not show any delamination or further fracture of the coatings after tribotests, no matter whether a Cr interlayer is employed or not. A small amount of debris in dark color is observed on the wear track (Fig. 7a), which is likely attributed to the large opening cracks of the W-DLC coatings without Cr interlayer. The wear tracks are almost undetectable on the W-DLC coatings with Cr interlayer (Fig. 7b), where the crack networks have little opening. The preceding results of the same top coating deposited on different rubber substrates indicate that the surface morphology, chemical and mechanical properties of the rubber substrates have great effects on the tribological performance and thus on
Fig. 8. Optical micrographs showing the wear scar of 100Cr6 balls after sliding 10,000 laps against W-DLC coated rubber (without Cr interlayer): (a) FKM and (b) HNBR. An arrow indicates the sliding direction of the balls themselves.
Y.T. Pei et al. / Surface & Coatings Technology 202 (2008) 1869–1875
890 × 690 μm against the coated HNBR rubber, respectively. Especially, there are many deep scratches formed on the wear scar when sliding against the coated FKM rubber, resulted from the fractured W-DLC coating fragments. These fragments become abrasive particles and cause severe wear and high friction. In contrast, the wear scar formed on the balls sliding against coated HNBR rubber appears matte and is likely covered with a thin transfer film. It is thus an indication of mild wear on the ball counterpart, supported by a low friction as measured. 4. Conclusions W-DLC coatings deposited on FKM and HNBR rubber substrates show dense crack networks of micrometer size, which facilitate flexibility for coatings to comply large deformations applied on rubber substrates. A Cr interlayer that is commonly used for metallic substrates exhibits only negative effects for W-DLC coatings on rubber substrates, i.e. in promoting interface delamination or growth instability. Uncoated rubbers possess very high coefficients of friction (N1). Relatively high coefficient of friction of W-DLC coated FKM rubber are attributed to coating damage, resulting from interface delamination and low rigidity of the rubber substrates. In contrast, W-DLC coated HNBR rubber exhibit superior tribological performances with a coefficient of friction of 0.2∼0.25. Due to the rough surface and high modulus of HNBR rubber, although open cracks break the continuity of the coating, the coating fragments still adhere well to HNBR rubber and almost no damage was observed on the wear tracks after tribotest. The rigidity of rubbers is crucial in determining the tribological performances of coated rubbers and thus in the design of coating systems. As a prospect, interface modification of rubber seals prior to deposition may play an important role in
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promoting the flexibility and adhesion of sputtered coatings on soft and elastic substrates. Acknowledgements Financial support from The Netherlands Institute for Metals Research is gratefully acknowledged. References [1] S. Yang, A.H.S. Jones, D.G. Teer, Surf. Coat. Technol. 133–134 (2000) 369. [2] C. Strondl, G.J. Van der Kolk, T. Hurkmans, W. Fleischer, T. Trinh, N.M. Carvalho, J.T.M. De Hosson, Surf. Coat. Technol. 142–144 (2001) 707. [3] Y.T. Pei, D. Galvan, J.T.M. De Hosson, Acta Mater. 53 (2005) 4505. [4] N. Miyakawa, S. Minamisawa, H. Takikawa, T. Sakakibara, Vacuum 73 (2004) 611. [5] H. Takikawa, N. Miyakawa, S. Minamisawa, T. Sakakibara, Thin Solid Films 457 (2004) 143. [6] T. Nakahigashi, Y. Tanaka, K. Miyake, H. Oohara, Tribol. Int. 37 (2004) 907. [7] Y. Aoki, N. Ohtake, Tribol. Int. 37 (2004) 941. [8] N.K. Cuong, M. Tahara, N. Yamauchi, T. Sone, Surf. Coat. Technol. 174–175 (2003) 1024. [9] D. Hegemann, H. Brunner, C. Oehr, Surf. Coat. Technol. 174–175 (2003) 253. [10] K. Donnelly, D.P. Dowling, M.L. McConnell, R.V. Flood, Diamond Relat. Mater. 8 (1999) 538. [11] C. Strondl, N.M. Carvalho, J.Th.M. De Hosson, G.J. Van der Kolk, Surf. Coat. Technol. 162 (2003) 288. [12] C. Strondl, PhD thesis, University of Groningen, the Netherlands, 2007. [13] Y.T. Pei, P. Huizenga, D. Galvan, J.T.M. De Hosson, J. Appl. Phys. 100 (2006) 114309. [14] K.L. Johnson, Contact Mechanics, Cambridge University Press, 1996, p. 427. [15] S. Sakaguchi, N. Murayama, Y. Komada, F. Wakai, J. Mater. Sci. Lett. 10 (1991) 282. [16] J.A. Rinde, J. Appl. Polym. Sci. 14 (1970) 1913. [17] B.N. Persson, J Phys.: Condens. Matter 18 (2006) 7789. [18] S. Zhang, X.L. Bui, J. Jiang, X. Li, Surf. Coat. Technol. 198 (2005) 206. [19] O. Wanstrand, M. Larsson, P. Hedenqvist, Surf. Coat. Technol 111 (1999) 247. [20] G.K. Wolf, Surf. Coat. Technol. 43–44 (1990) 920.
Surface & Coatings Technology 202 (2008) 1869 – 1875 www.elsevier.com/locate/surfcoat
Tribological behavior of W-DLC coated rubber seals Y.T. Pei a , X.L. Bui a , X.B. Zhou b , J.Th.M. De Hosson a,⁎ a
Department of Applied Physics, the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands b Department of Science and Technology, SKF Research and Development B.V., Kelvinbaan 16, 3430 DT Nieuwegan, The Netherlands Received 17 June 2007; accepted in revised form 10 August 2007 Available online 17 August 2007
Abstract Tungsten-containing diamond-like carbon (W-DLC) coatings have been deposited on FKM (fluorocarbon) and HNBR (hydrogenated nitrile butadiene) rubbers via unbalanced magnetron reactive sputtering from a WC target in a C2H2/Ar plasma. The surface morphology and fracture cross sections of uncoated and coated rubbers have been characterized with high resolution scanning electron microscopy (SEM). The tribological behavior of uncoated and coated rubbers has been investigated with ball-on-disc tribotest under dry sliding condition against a 100Cr6 ball. The coefficient of friction (CoF) of uncoated rubbers is very high (N1). Equally a relatively high CoF of W-DLC coated FKM (about 0.6) is observed due to the gradual failure and delamination of the coatings. On the contrary, W-DLC coated HNBR rubber exhibits a superior tribological performance with a very low CoF of 0.2–0.25. The latter is comparable to that of Me-DLC coatings deposited on steel substrates. After 10,000 sliding laps almost no damage of the coatings is observed on the wear tracks. In fact the network of micro-cracks as deposited facilitates the flexibility of the coatings. The different surface roughness and mechanical properties of the rubber substrates explain the differences in the tribological performances of the coated rubbers. © 2007 Elsevier B.V. All rights reserved. Keywords: W-DLC coating; Rubber substrates; Surface morphology; Coefficient of friction; Adhesion 1
1. Introduction Rubber seals are commonly used in lubrication system to prevent dirt and water entering the system and to avoid leakage of lubricants. Dynamic rubber seals operate in sliding contact mode at a relatively high speed and under no or only marginal lubrication condition. Under such tough operational conditions, contact seals are the major sources of friction in lubrication systems or bearings. Furthermore, rubber seals are subjected to severe wear leading to an increase of clearance, which is often the cause of loss of the function and failure of the lubrication system. Therefore, an advanced solution for dynamic rubber seals is stringent for bearings and automotive industries. Diamond-like carbon (DLC) coatings have been widely recognized as a tribo-coating material acting like a wear-
⁎ Corresponding author. Tel.: +31 50 363 4898; fax: +31 50 363 4881. E-mail address: [email protected] (J.Th.M. De Hosson). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.08.013
resistant solid lubricant with low friction coefficient. Processing techniques of DLC coatings, such as PVD and PECVD, have been well developed and industrialized for deposition on metallic substrates [1,2]. However, high residual stresses and brittleness of DLC coatings limit their thickness, adhesion and load-bearing capability. Doping with metal atoms [1,2] and especially the introduction of ceramic nanocrystallites to form DLC-based nanocomposites [3] can efficiently solve some of the drawbacks. Only very recently, few research efforts have been made to deposit DLC coatings on rubber [4–7] or polymer substrates [8–10]. Due to the difficulty in measuring the coefficient of friction (CoF) of rubber material, the experimental data of CoF were rather contradictory. One experiment indicated that the DLC coated rubber has a CoF close to 1 in comparison with that of rubber surfaces, approaching 6 [6]. Other work showed that the CoF of DLC coated polymers is typically 0.25–0.4 in contrast with 0.7–1.5 of uncoated polymers [8,10]. As the rubber is soft and elastic, it is difficult to characterize the
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Table 1 Properties of FKM and HNBR rubber Rubber Color Specific gravity
Max. continuous working temperature (°C)
Max. intermittent working temperature (°C)
Coefficient of thermal expansion (×10− 6 K− 1)
Indentation depth at 5 N Modulus load (mm) (MPa)
FKM Red 1.9 HNBR Black 1.0
205 125
300 150
160 230
155 ± 4 115 ± 3
mechanical properties and the microstructure of coated rubbers. The greatest challenge in the development of DLC-based coatings on rubber seals is to keep the coating flexible as well as adhered to rubber surface under large elastic deformation and dynamic straining. One design proposal to solve this problem is based on a concept of segment-structured DLC coatings formed through a mesh mask [7]. However, the mesh size was limited to sub-millimeter due to technical complexity. In this work, W-DLC coatings were deposited on FKM and HNBR rubbers via reactive unbalanced magnetron sputtering. The tribological properties of coated rubbers sliding against 100Cr6 steel balls have been studied in terms of the tribomechanical responses of a system, in comparison with that of uncoated rubber substrates. The role of a chromium interlayer that is commonly used for the deposition of DLCbased coatings on metallic substrates has been also examined for coatings on soft and flexible substrates. The results are striking in the sense of reducing friction of rubber seals and coating design for soft and flexible substrates such as rubbers.
9.8 15.2
on the rubber substrates prior to the deposition of the WC loadbearing layer. The maximum substrate temperature during depositions was measured on the rear side of the rubber substrates to be not higher than 150 °C. It should be indicated that there was no observable damage to the HNBR rubber substrates after deposition, even though the maximum deposition temperature was slightly higher than the maximum continuous working temperature of the rubber (see Table 1). Cross sections of the coated rubbers were made by fracturing after cooling in liquid nitrogen for 10 min. The surface morphology and wear track of the uncoated and coated rubbers were characterized with a scanning electron microscope (Philips FEG-XL30). The uncoated and coated rubber sheets were glued onto ø30 mm polished M2 steel discs for tribotests that were performed at room temperature (22 °C) on a CSM high temperature tribometer with a ball-on-disc configuration. The
2. Experimental FKM (fluorocarbon) and HNBR (hydrogenated nitrile butadiene) rubber sheets of 2 mm thickness were used as substrates. For determination of the rigidity of rubber materials, the rubber sheets were glued onto polished M2 steel discs and indented by a ø6 mm silicon nitride ball with a CSM Revetest scratch tester. The indentation depth at 5 N normal load was considered as a measure of the rigidity of the rubbers. The properties of the rubber materials are listed in Table 1. The rubber sheets were cut into rectangular pieces of size 100 × 70 mm2 for the coating process. Prior to deposition, the rubbers were washed and ultrasonically cleaned in ethanol for 30 min. The deposition of commercial W-DLC coatings was carried out by unbalanced reactive magnetron sputtering from WC targets in Ar and C2H2 plasma. The setup of the sputtering system was similar to the one described in [11]. The W-DLC coatings included two layers: a load bearing WC layer of thickness of about 300 nm capped by a W-DLC top layer with thickness of about 700 nm. The W content of the W-DLC layer was about 20 at.% for all the samples, measured by energydispersive X-ray spectroscopy (EDS). The hydrogen content of the W-DLC layer was 18–20 at.%, comparable with that of most commercial W-DLC coatings [12], analyzed by means of elastic recoil detection (ERD) with a He-ion beam of 2.3 MeV energy. For a comparative study, the coatings were deposited on the rubber substrates without or with a Cr interlayer of about 120 nm thickness, respectively. The Cr interlayer was deposited
Fig. 1. Surface morphology of FKM (a) and HNBR (b) rubbers.
Y.T. Pei et al. / Surface & Coatings Technology 202 (2008) 1869–1875
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counterpart was a ø6 mm commercial 100Cr6 steel ball of hardness HRC 60–62. The tribotest conditions were 1 N normal load, 10 cm/s sliding speed and 35% relative humidity that was kept constant for all the tests with a humidity regulator. For a detailed setup of the tribological testing reference is made to [13].
into the elastic modulus of the rubber substrates via the following equation [14]: 3PR 1=3 ð1Þ a¼ 4E4
3. Results and discussion
a ¼ 2Rh h , one obtains
The surface morphology of the rubber substrates is shown in Fig. 1. Much denser structures and smoother surfaces are observed on FKM rubber (Fig. 1a), in comparison with HNBR rubber (Fig. 1b). The latter exhibits a powdery morphology and rough surface. This observation is consistent with an almost doubled specific gravity of FKM (1.9) compared to that of HNBR (1.0) (Table 1). However, the indentation depth of ø6 mm Si3N4 ball at 5 N normal load on HNBR (115 μm) is much smaller than that on FKM (155 μm), indicating a much higher rigidity of HNBR rubber under loading contact. Such a difference in mechanical response of the rubber substrates is expected to dramatically affect the tribological performances of the coated rubbers though the coating system is the same. According to Hertzian elastic theory of ball-on-flat contact, in fact, the measured indentation depth can be readily translated
With 2
E2 ¼
1 E4
¼
1v21 E1
þ
1v22 E2
and the radius of contact area
2
3PRE1 1 v22
4E1 ð 2Rh h2 Þ3=2 3PR 1 v21
c
3PR 1 v22
4ð 2Rh h2 Þ3=2
ð2Þ
where P is the indentation load, R the radius of the indenting ball and h indentation depth measured under the applied load. E1, v1 and E2, v2 are the elastic modulus and Poisson's ratio of Si3N4 ball used and rubber substrates, respectively. Because the modulus of indenting ball is very large in comparison with that of rubbers and thus the second term of the denominator negligibly small, the approximation at the right end of Eq. (2) is reached. With the known elastic properties E1 = 310 GPa, v1 = 0.278 [15] and v2 = 0.5 [16], the modulus of HNBR and FKM rubber is estimated to be 15.2 MPa and 9.8 MPa, respectively. An overview of the surface morphologies of W-DLC coated FKM and HNBR rubbers is shown in Fig. 2. Cracking is always
Fig. 2. Overview of the surface morphology of W-DLC coatings deposited on FKM rubber (a — without Cr interlayer; b — with Cr interlayer) and HNBR rubber (c — without Cr interlayer; d —with Cr interlayer).
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growing interface during the deposition of Cr interlayer. In addition, opened cracks are observed in the W-DLC coatings on HNBR (Fig. 2c and d), especially in the case without Cr interlayer. This can be attributed to the higher thermal expansion of HNBR (230 × 10− 6 K− 1) compared to that of FKM (160 × 10− 6 K− 1) (Table 1). The large shrinkage of HNBR substrates during cooling from the deposition temperature to room temperature makes the borders of existing cracks of the coatings press each other, leading to further breaking at the vicinities of the borders and delamination due to bending of HNBR substrates during unloading and handling. Consequently, the cracks become further widened and more open. The fracture cross sections of the W-DLC coatings without Cr interlayer are revealed in Fig. 3. There is no essential difference in the fracture morphology of the W-DLC coatings on FKM and HNBR substrates. Both the load bearing WC layer and the W-DLC top layer exhibit columnar microstructures. However, there is a significant interruption between the two layers. Interfacial delamination often occurs between these two layers during fracture, leaving a sharp step along the interface on the fracture cross sections (see Fig. 3). Although it is a common practice to employ a load-bearing layer beneath the top
Fig. 3. SEM micrographs showing the fracture cross section of W-DLC coatings without Cr interlayer: (a) on FKM rubber and (b) on HNBR rubber.
observed on the W-DLC coatings in all the cases. Random crack network is a typical feature of hard coatings deposited on soft and flexible substrates such as rubber and polymer [5].During deposition and especially cooling down of the coated rubber substrates afterwards, cracks may initiate and randomly propagate in the coatings due to the huge difference in the coefficient of thermal expansion and elasticity between W-DLC coatings and rubber substrates. W-DLC coatings deposited without a Cr interlayer exhibit cauliflower-like morphologies with irregularly elongated branches separated by clear boundaries in both the cases of FKM and HNBR substrates (see the insert of Fig. 2a and c). Employing a Cr interlayer on FKM substrate, the WDLC coating surface becomes much smoother and shows fine dome-shaped branches that are often observed on W-DLC coatings deposited on Si wafers or well polished metallic substrates. However, the fragments of the cracked coatings severely bulge outward and as a result, the crack network forms in the grooves of winkled surface (Fig. 2b). It is most likely attributed to the interface stresses between the W-DLC coating and the interlayer, and promoted by the low rigidity of FKM rubber. In contrast, a large amount of micrometer sized droplets are observed on the W-DLC coating deposited on HNBR with a Cr interlayer (Fig. 2d). It is correlated to the powdery surface of HNBR rubber, which leads to an enhanced instability of
Fig. 4. SEM micrographs showing the fracture cross section of W-DLC coatings with Cr interlayer on FKM rubber (a) and on HNBR rubber (b). The inserts show the characteristic of interfacial bonding of the coatings with higher magnification.
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Fig. 5. Coefficient of friction of coated and uncoated rubbers.
coating on metallic substrates, its effects for coatings on soft and elastic substrates are insignificant, if not even negative according to this observation. The fracture cross sections of W-DLC rubbers with a Cr interlayer clearly demonstrate the negative effects of the interlayer on the interfacial adhesion and growth stability of the coatings (Fig. 4). On FKM rubber, the coating suffers delamination between the loading support WC layer and the Cr interlayer. Due to shrinkage of the rubber substrate during cooling that generates significant interface stresses, the center of cracked coating fragments bulges outwards and the crack edges bend inwards and often insert into the rubber surface, leading to the formation of bow-like open cracks (Fig. 4a). In contrast, no obvious interfacial delamination is observed in W-DLC coated HNBR rubber with a Cr interlayer. Rather, curving of the Cr interlayer under cyclic stresses (mainly compressive) due to temperature fluctuations enlarges the roughness of the powdery surface of HNBR rubber, resulting in instability of the growing interface. As a result, a large amount of micrometer sized bumps are formed on the coating (Fig. 4b). These bumps seem to have positive effects on stress release and interface adhesion. The tribotest results of uncoated and coated rubbers sliding against 100Cr6 steel ball are shown in Fig. 5. Without W-DLC coating, the CoF of the rubbers is very high. At the beginning of the sliding, the CoF is about 1.2 and 1.9 for FKM and HNBR rubber, respectively. After 6000 laps, the CoF reaches the steady-state values of 1.0 for FKM rubber and 1.3 for HNBR rubber. Such a decrease in friction with sliding is likely correlated to the effect of flash temperature rising on the contact area [17]. These measurements of friction are in good agreement with the values reported in literature, which showed that the coefficients of friction of rubbers sliding against steel varied from 1 to 6 depending on the type of rubbers [6]. With W-DLC coatings, the CoF of coated rubbers is drastically reduced. With W-DLC coating on FKM in both the cases with and without Cr interlayer, the initial coefficient of friction is about 0.2 at the beginning of sliding and gradually increases to the value of 0.57 at the end of the test for the coating with a Cr interlayer and 0.63 for that without a Cr interlayer. The fluctuation peaks in the recorded CoF curves together with the gradual increase in the
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CoF indicate a gradual damage of the W-DLC coatings during the tribotests. The CoF of W-DLC coated HNBR rubbers is about 0.22 without Cr interlayer and 0.25 with Cr interlayer. Especially, the frictional behavior of W-DLC coated HNBR rubbers is very stable during the entire period of tribotests. Such low friction is comparable to that of metal-doped DLC coatings deposited on steel substrates for tribological applications [18,19]. SEM observations on the wear tracks of W-DLC coated FKM rubbers reveal the fracture of W-DLC coatings after tribotests, as shown in Fig. 6. Without Cr interlayer, most of the newly fractured fragments of the coating are still adhered onto the rubber surface, but many of them become pulverized (Fig. 6a). With a Cr interlayer, however, a large amount of coating material is removed from the wear track (Fig. 6b). Evidently, the Cr interlayer does not enhance the adhesion of the coatings on FKM rubber as already revealed by SEM observation on the fracture cross sections (Fig. 4). The adhesion strength and thus the loadbearing capability of W-DLC coating on FKM are weakened when Cr interlayer is applied. This is different from the situation of W-DLC coatings deposited on steel substrates where Cr
Fig. 6. SEM micrographs showing the wear tracks on W-DLC coated FKM rubbers: (a) without Cr interlayer and (b) with Cr interlayer. The inserts show the central area of the wear track with higher magnification.
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the design of the coating systems. It has been shown that the physical linking by interface broadening and interlocking contribute an important part to the adhesion of a coating on the substrate [20]. The rough powdery surface of HNBR rubber, as seen in Fig. 1a, apparently assigns stronger interfacial adhesion for coatings compared to the case of FKM rubber. Especially, higher rigidity or modulus of HNBR rubber leads to a smaller surface deformation under the applied load during tribotest. Therefore, the (bending) strain of the coating fragments under ball loading contact is much lower. All these effects result in a significantly enhanced load-bearing capability and thus little damage of W-DLC coatings on HNBR rubber compared to that on FKM. As a consequence, stable frictional behavior with a low CoF of 0.2–0.25 is achieved with W-DLC coated HNBR rubbers. The wear scar of 100Cr6 balls after sliding 10,000 laps against W-DLC coated FKM and HNBR rubbers is shown in Fig. 8. Corresponding to the different widths of the wear tracks, the wear scar formed with the coated FKM rubber is larger than that coupled with the coated HNBR rubber. The maximum width (perpendicular to the sliding direction) and the maximum length (along the sliding direction) of the wear scars are 1097 × 810 μm when sliding against the coated FKM rubber and
Fig. 7. SEM micrographs showing the wear tracks on W-DLC coated HNBR rubbers: (a) without Cr interlayer and (b) with Cr interlayer.
interlayer promotes the interfacial adhesion. It is understandable that DLC-based coatings can better adhere directly onto rubber surface due to stronger chemical affinity via C–C or C–H bonds, provided no foreign material such as a metallic interlayer is introduced in between. While the coatings on FKM rubbers are all seriously damaged during tribotests, the coatings on HNBR rubber hardly show further damage and still adhere very well on the substrate as shown in Fig. 7, even if dense crack networks exist as deposited. The wear tracks of W-DLC coated HNBR rubbers do not show any delamination or further fracture of the coatings after tribotests, no matter whether a Cr interlayer is employed or not. A small amount of debris in dark color is observed on the wear track (Fig. 7a), which is likely attributed to the large opening cracks of the W-DLC coatings without Cr interlayer. The wear tracks are almost undetectable on the W-DLC coatings with Cr interlayer (Fig. 7b), where the crack networks have little opening. The preceding results of the same top coating deposited on different rubber substrates indicate that the surface morphology, chemical and mechanical properties of the rubber substrates have great effects on the tribological performance and thus on
Fig. 8. Optical micrographs showing the wear scar of 100Cr6 balls after sliding 10,000 laps against W-DLC coated rubber (without Cr interlayer): (a) FKM and (b) HNBR. An arrow indicates the sliding direction of the balls themselves.
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890 × 690 μm against the coated HNBR rubber, respectively. Especially, there are many deep scratches formed on the wear scar when sliding against the coated FKM rubber, resulted from the fractured W-DLC coating fragments. These fragments become abrasive particles and cause severe wear and high friction. In contrast, the wear scar formed on the balls sliding against coated HNBR rubber appears matte and is likely covered with a thin transfer film. It is thus an indication of mild wear on the ball counterpart, supported by a low friction as measured. 4. Conclusions W-DLC coatings deposited on FKM and HNBR rubber substrates show dense crack networks of micrometer size, which facilitate flexibility for coatings to comply large deformations applied on rubber substrates. A Cr interlayer that is commonly used for metallic substrates exhibits only negative effects for W-DLC coatings on rubber substrates, i.e. in promoting interface delamination or growth instability. Uncoated rubbers possess very high coefficients of friction (N1). Relatively high coefficient of friction of W-DLC coated FKM rubber are attributed to coating damage, resulting from interface delamination and low rigidity of the rubber substrates. In contrast, W-DLC coated HNBR rubber exhibit superior tribological performances with a coefficient of friction of 0.2∼0.25. Due to the rough surface and high modulus of HNBR rubber, although open cracks break the continuity of the coating, the coating fragments still adhere well to HNBR rubber and almost no damage was observed on the wear tracks after tribotest. The rigidity of rubbers is crucial in determining the tribological performances of coated rubbers and thus in the design of coating systems. As a prospect, interface modification of rubber seals prior to deposition may play an important role in
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promoting the flexibility and adhesion of sputtered coatings on soft and elastic substrates. Acknowledgements Financial support from The Netherlands Institute for Metals Research is gratefully acknowledged. References [1] S. Yang, A.H.S. Jones, D.G. Teer, Surf. Coat. Technol. 133–134 (2000) 369. [2] C. Strondl, G.J. Van der Kolk, T. Hurkmans, W. Fleischer, T. Trinh, N.M. Carvalho, J.T.M. De Hosson, Surf. Coat. Technol. 142–144 (2001) 707. [3] Y.T. Pei, D. Galvan, J.T.M. De Hosson, Acta Mater. 53 (2005) 4505. [4] N. Miyakawa, S. Minamisawa, H. Takikawa, T. Sakakibara, Vacuum 73 (2004) 611. [5] H. Takikawa, N. Miyakawa, S. Minamisawa, T. Sakakibara, Thin Solid Films 457 (2004) 143. [6] T. Nakahigashi, Y. Tanaka, K. Miyake, H. Oohara, Tribol. Int. 37 (2004) 907. [7] Y. Aoki, N. Ohtake, Tribol. Int. 37 (2004) 941. [8] N.K. Cuong, M. Tahara, N. Yamauchi, T. Sone, Surf. Coat. Technol. 174–175 (2003) 1024. [9] D. Hegemann, H. Brunner, C. Oehr, Surf. Coat. Technol. 174–175 (2003) 253. [10] K. Donnelly, D.P. Dowling, M.L. McConnell, R.V. Flood, Diamond Relat. Mater. 8 (1999) 538. [11] C. Strondl, N.M. Carvalho, J.Th.M. De Hosson, G.J. Van der Kolk, Surf. Coat. Technol. 162 (2003) 288. [12] C. Strondl, PhD thesis, University of Groningen, the Netherlands, 2007. [13] Y.T. Pei, P. Huizenga, D. Galvan, J.T.M. De Hosson, J. Appl. Phys. 100 (2006) 114309. [14] K.L. Johnson, Contact Mechanics, Cambridge University Press, 1996, p. 427. [15] S. Sakaguchi, N. Murayama, Y. Komada, F. Wakai, J. Mater. Sci. Lett. 10 (1991) 282. [16] J.A. Rinde, J. Appl. Polym. Sci. 14 (1970) 1913. [17] B.N. Persson, J Phys.: Condens. Matter 18 (2006) 7789. [18] S. Zhang, X.L. Bui, J. Jiang, X. Li, Surf. Coat. Technol. 198 (2005) 206. [19] O. Wanstrand, M. Larsson, P. Hedenqvist, Surf. Coat. Technol 111 (1999) 247. [20] G.K. Wolf, Surf. Coat. Technol. 43–44 (1990) 920.