Tribological studies have always focused on the frictional properties of ball bearings and their components due to the fact that lower friction results in lower energy consumption. However, it is necessary to protect the bearings from penetration of impurities in order to keep this low coefficient of friction (CoF) To achieve this, rubber seals are employed to prevent the entry of impurities into the ball bearings as well as to maintain lubricants inside the bearing.
Many studies have been exclusively performed to characterize the CoF of the ball bearings themselves but only a little attention has been paid to study the frictional properties of these rubber seals. The CoF of rubber seals influences the overall friction of a ball bearing, but high CoF values can cause premature damage of the seal due to frictional heat and rubber degradation. Hence, considerable effort has been taken to improve the frictional properties of rubber seals by applying surface coatings with low CoF.
Diamond-like carbon (DLC) thin films are considered to provide an optimal solution for lowering the CoF of rubber seals. DLC thin films have to show good adhesion to the rubber material used in addition to being very flexible. Using appropriate deposition techniques such as optimized plasma-assisted chemical vapor deposition (PACVD)1 can help impart good flexibility to DLC films, while tribological testing has been used to characterize the friction properties. Also, tribological tests should be performed at different loads to gain insights into the mechanisms of friction/adhesion in respect to various loads and DLC structures.
This article discusses a study of frictional properties of 300 nm thin DLC coatings applied over alkyl acrylate copolymer (ACM rubber) under different loads. Thanks to the ability of the NTR2 Nanotribometer to apply a wide range of normal loads and pressures, the study results revealed the variations between friction and deformation mechanisms during the tribological contact.
Deposition of DLC Thin Films
Here, the PACVD method was used to deposit the DLC thin films on the ACM rubber. A pulsed DC power unit was employed as a substrate bias source, operating at 250 kHz with a pulse off time of 500 ns and voltages in the range of 300-600 V. In each batch, two pieces of ACM rubber with dimensions of 50×50×2 mm were coated. Before depositing the DLC thin films on the rubber substrates, they were cleaned by two subsequent wash procedures to achieve a good film adhesion. The first procedure involved five cycles of ultrasonic washing in a 10 vol. % solution of detergent in demineralized water at a temperature of 60 °C for 15 minutes. The second wash procedure involved five cycles of ultrasonic washing in boiling demineralized water for 15 minutes in each cycle.
The deposition method was a two-step process, where the ACM samples were first etched for ~30 minutes in argon plasma to further clean the contaminations on the surface and then in a plasma mixture of argon and hydrogen for ~10 minutes to further improve the adhesion of the subsequently deposited DLC thin films. The second treatment involved replacement of hydrogen with acetylene, followed by deposition of the DLC films.
This resulted in the preparation of two types of samples:
- Uncoated ACM Rubber with a thickness of 2 mm
- ACM rubber with homogeneous DLC coating with a thickness of 300 nm
Figure 1 shows the surface and the cross-section of the DLC coating.
Figure 1. The morphology of the DLC coated ACM rubber on top surface (a) and on cross-section (b). The scale bars represent 50 µm and 5 µm respectively.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
For more information on this source, please visit Anton Paar GmbH.