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PhD Defence Muhammad Khafidh

friction, wear and noise of short-cut aramid fibre reinforced elastomers in sliding contacts

Muhammad Khafidh is a PhD student in the Department of Mechanics of Solids, Surfaces & Systems (MS3). His supervisor is prof.dr.ir. D.J. Schipper from the Faculty of Engineering Technology (ET).

Many types of elastomer based products are found in daily life, such as tyres, v-belts and wiper blades. In applications, several reinforcing materials are usually added into the elastomers to increase their mechanical and tribological properties. The examples of these reinforcing materials are carbon black, silica and fibres. Short-cut aramid fibre is a relatively new high-performance material that can be used to reinforce elastomers. However, the interaction between fibre and elastomer matrix is still a problem to be solved. Moreover, friction, wear and friction-induced noise of short-cut aramid fibre reinforced elastomers is not well known. Understanding friction, wear and friction-induced noise will lead to a better design, so that the lifetime of the elastomers can be prolonged.

This research is conducted within the project FINE-FIT (Fibres IN Elastomer For Improved Tribology), which is a collaboration between the Surface Technology and Tribology (STT) group and the Elastomer Technology and Engineering (ETE) group at the University of Twente. An optimized formulation of composites to improve the interaction between the fibres and elastomer matrix was investigated by the ETE group, while the investigation of the tribological behaviour of short-cut aramid fibre reinforced elastomers was conducted by the STT group. The short-cut aramid fibre reinforced elastomers used in this thesis are based on the optimized formulation of the ETE group.

Tribological phenomena of elastomers during sliding friction were studied, such as the contact area, the formation of a modified surface layer and the occurrence of a wavy wear track. The size and shape of the contact area of elastomers during sliding change in comparison with the static condition. The contact area depends on the sliding velocity and the mechanical properties of the elastomers, such as storage modulus.

During sliding contact, the composition and the mechanical properties of the elastomer surface may change. These surface alterations will lead to a change of the tribological behaviour of elastomers. The existence of a modified surface layer is influenced by the competition between formation and wear, which depends on the contact pressure, sliding velocity and sliding distance. Another phenomenon during sliding friction is a macro surface irregularity at the wear track, called a wavy wear track. In application, the wavy wear track needs to be avoided because it will reduce the performance of the sliding system and generate vibrations and noise. The occurrence of the wavy wear track depends on the mechanical properties of the elastomer, the operating conditions (such as sliding velocity and force), the inertia mass of the counter surface frame and the circumferential length of the wear track.

Friction, wear and friction-induced noise of short-cut aramid fibre reinforced elastomers were investigated by using two types of short-cut aramid fibres, namely non-coated fibre (NF) and epoxy-coated fibre (EF). The wear mechanism during sliding contact greatly influences the frictional behaviour of the composites. For a long sliding distance, the presence of fibres on the wear track reduces the coefficient of friction and friction-induced noise drastically. The presence of fibres on the wear track causes the composites to follow Amontons’ law when the applied contact pressures are below a certain threshold value. Once the contact pressure is higher than the threshold value, Amontons’ law is no longer valid. The threshold contact pressure of composites containing EF is higher than those containing NF. Furthermore, the effect of fibre direction and fibre amount in the composites on friction and wear were studied. Elastomers reinforced with silica and short-cut aramid fibres were also studied to investigate the effect of short-cut aramid fibres. The coefficient of friction and wear of elastomers containing EF is lower than those containing NF.

During sliding contact, noise generation due to sliding friction between the composites and counter surface was investigated. Adding short-cut aramid fibres into the elastomers reduces the friction-induced noise in comparison with the unreinforced elastomers. The friction-induced noise was found to increase with increasing sliding velocity and contact pressure. Moreover, the friction-induced noise of the composites containing EF is lower than those containing NF. The presence of fibres on the wear track reduces the friction-induced noise. The increase of noise is caused by vibrations of the pin holder and motor noise. The noise can be reduced by two ways: (1) reduce the amplitude of friction force and (2) reduce the level of friction force.