Tribological performance of DLC films deposited on ACM rubber by PACVD

https://doi.org/10.1016/j.surfcoat.2011.04.072Get rights and content

Abstract

In this paper the tribological and adhesive performance of DLC films prepared by plasma assisted chemical vapor deposition on acrylic rubber (ACM) are studied. The effect of applied load and sliding velocity on the coefficient of friction and wear rate has been investigated. Effects of the rubber substrate and of ageing of the coated samples have also been explored. In addition, the adhesion of the DLC films to the rubber substrates is evaluated via stretch tests and the measured adhesion strength is larger than 40 MPa, indicating a superb adhesion to the substrate. It is shown that the tribological performance is greatly influenced by the viscoelastic properties of the substrate, and higher coefficients of friction are obtained at higher loads and velocities. The wear followed a similar trend, although very low in all the cases.

Research highlights

► The protection of rubber with DLC films is very effective. ► The adhesion of the DLC films to the rubber is superb. ► The CoF is low, and depends on the patch size. ► The CoF is influenced by the rubber viscoelasticity, and depends on the test conditions. ► The wear is neglibigible, although higher for higher test velocity and load.

Introduction

Diamond-like carbon (DLC) films are an optimal solution as protective coating for many applications, due to the combination of relatively high hardness, chemical inertness, low friction coefficient and low wear rate [1]. Their chemical composition, which is mainly composed by C and H, suggests a good compatibility with rubber materials. Rubber seals are incorporated in ball bearings widely used in many technical fields, like aerospace or automotive industries [2], in order to prevent the leakage of lubricant and the entrance of dirt. However, under operation dynamic rubber seals suffer from severe wear and cause high friction, leading to an ultimate failure of the bearing. Therefore, a protective coating with optimal tribological performance is of much interest in the respects of energy saving and environment protection. The primary requirements for a rigid film to be coated on compliant substrates such as rubbers are film flexibility and strong adhesion.

Table 1 summarizes some previous work regarding the preparation and tribotesting of DLC films deposited on rubber. As in the present case, all were prepared by plasma assisted chemical vapor deposition (PACVD), although using rf power sources. In two cases CH4 was used as a precursor gas instead of C2H2. Aoki et al. [3] observed low coefficients of friction (CoF) in the range of 0.1–0.2 for loads up to 2 N, although the films showed a fast failure and a deep increase of CoF for larger loads. Nakahigashi et al. [4] reported a strong reduction of CoF after deposition of the DLC film when compared to unprotected rubbers, although the lowest CoF reached under their low-load tribotest condition was around ~ 0.7. In contrast, Martínez et al. [5] studied much stronger conditions (10 and 40 N load), reaching CoFs of 0.44 and 0.23 as lowest values, respectively.

In previous papers [6], [7] we have demonstrated the possibility of tailoring the microstructure and flexibility of DLC films deposited with plasma assisted chemical vapor deposition (PACVD) by defining the growth conditions in terms of bias voltage and treatment time. As a result, the temperature variation during film growth (ΔT) could be controlled, which defined the thermal stresses in the system and thus the density of crack network. In addition, it was demonstrated that bias voltage did not produce a major influence on the chemical bonding in the DLC films, and the differences in tribological performance among the films were mainly related to the different microstructure. The aim of this work is to study the tribological properties of DLC films deposited on alkyl acrylate copolymer (ACM) rubber in detail. The effect of different test conditions, such as speed, length and load, are explored. The influence of the substrate, in terms of quality and mechanical properties, is studied. The adhesion and wear of the films are also examined.

Section snippets

Experimental details

DLC thin films were deposited on ACM rubber by means of PACVD in a Teer UDP/400 close field unbalanced magnetron sputtering rig, with all the magnetrons powered off. A pulsed DC (p-DC) power unit (Advanced Energy) was used as substrate bias source, operating at 250 kHz with a pulse-off-time of 500 ns and voltages between 300 and 600 V. Before deposition, the rubber substrates were cleaned by two subsequent wash procedures using a detergent solution and boiling water, in order to improve the film

Adhesion

Strong adhesion between a film and the substrate is a principal requirement in the case of protective applications. We have investigated the adhesion of the DLC films to the ACM substrate by using the method described by Ollivier et al. [9], which consists on studying the patch size of the film during the strain of the flexible substrate. During straining, the film cracks and the patch size is reduced until the maximum value of adhesion is reached, and then the patch size becomes constant.

Conclusions

The adhesive and tribological properties of DLC films deposited on ACM rubber by PACVD have been evaluated. The adhesion strength of the DLC films to the substrate is higher than 40 MPa, indicating a superb interfacial adhesion that ensures the excellent tribological performance of the coated rubber. The tribological performance is greatly influenced by the viscoelastic properties of the substrate, and higher CoF values are observed at higher loads and velocities. The wear of DLC films is

Acknowledgements

This research was carried out under project number MC7.06247 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl).

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