Nanotribological properties of Diamond like carbon thin flexible films on ACM rubber
Friction properties of ball bearings and their parts have always been in the focus of tribological studies because lower friction leads to lower energy consumption. However, to maintain this low coefficient of friction (CoF), the bearings have to be protected against penetration of impurities. For this purpose, rubber seals are used to prevent impurities to enter the ball bearing and also to keep lubricants inside the bearing. Many studies have been dedicated to characterize the coefficient of friction of the ball bearings themselves; in contrast, only a little attention has been paid to determination of frictional properties of the rubber seals. The CoF of rubber seals contributes to overall friction of ball bearing but high values of CoF can lead to premature damage of the seal due to frictional heat and rubber degradation. Therefore there has been considerable effort in improving the frictional properties of rubber seals by depositing surface coatings with low CoF. Diamond like carbon (DLC) thin films seem to be an optimal solution for decreasing the CoF of rubber seals. These coatings have to be very flexible and exhibit good adhesion to rubber. While good flexibility can be achieved by appropriate deposition methods such as optimized plasma-assisted chemical vapor deposition (PACVD) , friction properties have to be characterized using tribological testing. Furthermore, it is necessary to perform the tribological tests at various loads to understand the mechanisms of friction/adhesion in respect to different loads and DLC structures.
This Application report presents a study of frictional properties of 300 nm thin DLC coatings on alkyl acrylate copolymer (ACM rubber) under various loads. Thanks to application of a large range of normal loads and pressures of the NTR2 Nanotribometer, the results showed differences between friction and deformation mechanisms during the tribological contact.
Deposition of DLC thin films
The diamond like carbon(DLC) thin films were deposited on acrylic (ACM) rubber by PACVD. A pulsed DC power unit was used as a substrate bias source, operating at 250 kHz with a pulse off time of 500 ns and voltages between 300 V and 600 V. Two pieces of ACM rubber (50×50×2 mm) were coated in each batch. Before deposition the rubber substrates were cleaned by two subsequent wash procedures in order to achieve good film adhesion. The first treatment comprised of five cycles of ultrasonic washing in a 10 vol. % solution of detergent in demineralized water at 60°C for 15 minutes; the second treatment consisted of five cycles of ultrasonic washing in boiling demineralized water for 15 minutes in each cycle.
The deposition process was composed of two steps. In the first one, the ACM samples were etched for ~30 minutes in argon plasma in order to further clean the surface from contaminations, followed by ~10 min in a plasma mixture of argon and hydrogen which was used to further improve the adhesion of the subsequent deposited DLC film. In the second treatment, hydrogen was replaced by acetylene and deposition took place. As a result, two types of samples were prepared:
- Uncoated ACM Rubber with thickness of 2 mm
- ACM rubber with homogeneous DLC coating (thickness 300 nm)
The surface and the cross-section of the DLC coating is shown in Fig. 1.
Fig. 1: The morphology of the DLC coated ACM rubber on top surface (a) and on cross-section (b). The scale bars represent 50 mm and 5 mm respectively.
The nanotribological experiments were performed using Anton Paar Pin-on-disk Nanotribometer (NTR2, Fig. 2) at four loads: 1 mN, 10 mN, 100 mN and 1000 mN. The NTR2 is a unique instrument allowing tests at such large range of loads and contact pressures. The NTR2 is using active force feedback to ensure precise control of normal load under various conditions. Its concept with easily exchangeable double cantilevers allows maintaining excellent force and displacement resolution in the range of 0.005 mN to 1000 mN. The counterbody in all tribological experiments was stainless steel ball with diameter of 2 mm. The stainless steel ball can easily be fixed on specially designed support shaft, which allows also attachment of other customer made counter bodies. The tests at each load were done with wear track radii of 4 mm, 6 mm, 8 mm and 10 mm to ensure sufficient distance between individual wear tracks. The linear speed was set to 5 cm/s and the total number of laps was set to 10‘000 for all tests. The coefficient of friction (CoF) was recorded.
Fig. 2: The Anton Paar Nanotribometer. System of easily interchangeable cantilever allows to span loads from 0.005 mN up to 1000 mN.
Results and discussion
The average value of CoF varied between 0.7 and 1.3 for the ACM rubber without coating for all loads due to strong adhesive interactions between the uncoated rubber and counterpart (see Fig. 3). The CoF was also varying during each test indicating severe damage of the ACM rubber (see Fig. 4). On the other hand, the CoF of DLC coated samples was much lower than that of uncoated ACM rubber: it varied between 0.05 and 0.5 depending on the applied load. The lowest CoF was obtained on DLC film with load of 10 mN (average CoF value of 0.05). In contrast to the uncoated ACM rubber, the CoF of the DLC film was very stable during the whole duration of the tests (10’000 laps). Such behavior indicates not only good stability but also excellent durability of the DLC coating on rubber during repeated spherical contact even at high loads (see Fig. 4). Nevertheless, a slight increase of the CoF during the tests can be observed, particularly at higher loads (100 mN and 1000 mN). This is because the contact area gradually increases during the tests due to non-recovery of the underlying ACM rubber substrate , which results in increase of the CoF.
Fig. 3: Comparison of coefficient of friction at various loads for the two tested sample: uncoated rubber (a) and DLC coated rubber (b).
The evolution of the CoF with the applied load (Fig. 3) can be explained as a combination of two effects: adhesion of the steel ball to the DLC coating and viscoelastic hysteresis of the underlying ACM rubber substrate due to repeated mechanical loading.
At low loads (1 mN), the effect of adhesion between the steel ball and the DLC surface is important, and leads to high values of CoF. At the 10 mN load, the contribution of adhesion reduces, and the viscoelasctic hysteresis is not as pronounced as at high loads (100 mN and 1000 mN) and so the value of the CoF is very low (~0.05). Figure 5 shows the evolution of CoF in respect to the normal load applied during the nanotribological experiments. The lowest average values of CoF were recorded for the DLC coated rubber at 10 mN.
Fig. 4: Comparison of the wear track after the 1000 mN load pin on disk test on ACM rubber (a) and DLC coated rubber (b). Note deep track on the ACM rubber indicating severe damage.
The possibility to apply broad range of loads provided by the NTR2 Nanotribometer allowed studying of the contact conditions on the transition between the adhesive and hysteresis contributions to the CoF on the uncoated and coated ACM rubber. Also, the DLC coating lead obviously to significant decrease of coefficient friction in respect to uncoated ACM rubber, which is very important for application as rubber seals in ball bearings.
The present study clearly shows that the DLC coating leads to important decrease of coefficient of friction in comparison to uncoated ACM rubber. Furthermore, the DLC coating has high
durability because the CoF remains extremely stable throughout the whole tribological test. The uncoated ACM rubber on the other hand, shows severe damage at 100 mN and 1000 mN, further confirming the protective function of the DLC coating. The broad range of applied loads showed that the frictional behavior of this type of elastic-plastic materials is governed by combination of adhesion (predominant at low loads) and viscoelastic hysteresis of rubber (predominant at high loads).
Fig. 5: Evolution of coefficient of friction for the ACM uncoated rubber and DLC coated rubber as a function of normal load.
The Anton Paar NTR2 Nanotribometer is an excellent tool for such focused research as it provides large range of applied forces together with superior force range and resolution.
 Y.T. Pei, X.L. Bui, J.P. Van der Pal, D. Martinez-Martinez, X.B. Zhou, and J.Th.M. De Hosson, ‘Flexible diamond-like carbon films on rubber: on the origin of self-acting segmentation and film flexibility’, ActaMaterialia 60 5526 (2012).
 D. Martinez-Martinez, J.P. Van der Pal, Y.T. Pei, J.Th.M. De Hosson, ‘Performance of diamond-like carbon-protected rubber under cyclic friction. I. Influence of substrate viscoelasticity on the depth evolution’, Journal of Applied Physics 110, 124906 (2011).
 D. Martinez-Martinez, J.P. Van der Pal, M. Schenkel, K.P. Shaha, Y.T. Pei, and J.Th.M. De Hosson, ‘On the nature of the coefficient of friction of diamond-like carbon films deposited on rubber’, Journal of Applied Physics 111 114902 (2012).
Dr. Jiri Nohava, Anton Paar TriTec SA
Contact Anton Paar
Tel: +41 32 552 1600
Fax: +41 32 552 1610
NTR2 is designed to investigate surface interaction at extremely low contact pressure, especially where soft materials are of interest. NTR3 combines the resolution of an Atomic Force Microscope (AFM) with the stability and robustness of a dual double-beam cantilever transducer, taking the well proven pin-on-disk tribometer testing principle to the new dimension of nano-tribology.