Improving the tribological properties of polymer nanocomposites
There are a number of ways in which the addition of a particle to a polymer can improve its friction and wear properties and all of these have been exploited in the production of nanocomposites. One approach is to reduce the adhesion of the composite to its sliding counterface by adding friction-modifying reinforcements which may also reduce the heat generated in a sliding contact. These friction modifiers may be in the form of low-friction particles which form low-friction transfer layers at the interface between the composite and its sliding counterface when the surface layers are disrupted by sliding. The fillers may also modify the properties of the transfer layer which forms, reacting chemically with the matrix and counterface materials to produce a low shear material. Another approach is to increase the hardness, stiffness and strength of the material using reinforcement particles as discussed previously. Yet another approach is to use fillers to control the heat transfer through the material in order to dissipate the heat generated by friction. Not all fillers increase the wear resistance of the composite; in the case where the filler decomposes during sliding and the reaction products strongly adhere to
the counterface (Briscoe, 1993) or where the adhesion between the filler and matrix is poor the wear rate will increase (Dasari et al., 2005).
To modify friction and adhesion, solid lubricants such as graphite or polytetrafluoroethylene (PTFE) are used. These are either layered structures (e.g. graphite or MoS2) with low shear strength planes within their structure or smooth molecular species (PTFE) which can become aligned in the sliding direction in a similar manner to combed hair. In both cases these form a modified transfer film on the surface during wear, e.g. a PTFE film (Hager and Davies, 1993). One of the problems with the layered lubricants is the fact that chemical bonding at the edge of the low shear planes can lead to increases in the shear stress required to maintain sliding and a consequent increase in friction. A recent development which addresses this problem is the production of fullerene-like MoS2 nanoparticles where the low-friction basal planes are wrapped into an approximately spherical shell with a much reduced number of chemical bonding sites, usually at kinks in the planes (Rapoport et al., 2005a). This type of material shows low friction and improved life and load bearing capacity compared with conventional flake particles both in composites and as an addition to grease (Rapoport et al., 2005b). Another approach is to soak lubricants into a nanoporous composite (Ahn et al., 2003).
The reinforcement material composition affects the composition and properties of the transfer layer that forms during sliding which controls friction and wear behaviour in most nanocomposite systems. A uniform, tenacious transfer film is produced in clay-nylon-6 nanocomposites, which leads to a reduction in friction and a lower wear rate (Srinath and Gnanamoorthy, 2005). Similar results are observed for silica nanoparticle reinforcement (Garcia et al., 2004).
The use of reinforcement particles that improve the mechanical properties of the composite in order to improve its wear performance is well documented for a range of polymer matrix materials. As the particle size is reduced the improvement in tribological properties tends to increase as smaller regions of matrix material are exposed to the sliding counterface during wear. For instance the wear performance of silica reinforced epoxy composites increases as the particle size is reduced from 500 to 120nm (Xing and Li, 2004). However, opposite trends were observed for glass reinforced nylon in abrasive wear tests (Friedrich, 1986b). In fact the behaviour is often a complex function of particle size with an increase in wear rate at intermediate size and a reduction at the smallest sizes (Fig. 9.1). For composites with nanoscale particle reinforcement (i.e. less than 100 nm diameter) there is generally an improvement in wear performance as the particle size is reduced (Wang et al., 1996, Bahadur and Sunkara, 2005). Optimum wear performance is usually obtained with a fixed volume fraction of filler (Cai et al., 2003). For a particular particle size the wear resistance increases with volume fraction up
Particle size [nm]
9.1 Effect of reinforcement diameter on wear performance for silica— polyurethane nanocomposites (12 vol%).
Particle size [nm]
9.1 Effect of reinforcement diameter on wear performance for silica— polyurethane nanocomposites (12 vol%).
to a maximum, usually when the filler particles start to interact with each other. The maximum in performance thus represents a limit imposed by the dispersion of the particles in the matrix. In cases where particle interactions occur, the wear can be very severe and apparently abrasive in nature whereas at lower reinforcement volume fractions the wear mechanism is different (e.g. adhesive and fatigue wear). A strong adhesion between reinforcement and matrix is necessary to prevent pull-out and high wear in nanocomposites designed for tribological applications. This has driven the development of coupling agents and other chemical modifications to improve performance (Zhang et al, 2002a,b).
An added advantage of nanocomposites manufactured with small diameter reinforcements is the reduction in surface roughness of the sliding surface. This generally leads to a reduction in the contact stresses at asperity contacts and a resultant reduction in damage to the composite system. The roughness of the counterface is also critical because this dictates the nature of the initial contact with the nanocomposite surface and controls the mechanics of formation of the transfer film (Friedrich, 1986b) - when the roughness of the counterface is less than the particle size, the presence of the nanoparticles tends to increase the wear rate because detached particles get trapped between the sliding surfaces and cannot be taken out of the contact by moving into the roughness valleys. In this case three body abrasion occurs before a protective transfer film forms and the wear rate increases.
Improvements in friction and wear have also been reported with carbon nanotube reinforced polymer composites (Igarashi et al., 2005). This is due to both increases in strength of the material and the modification of the transfer layer by fragments of nanotube which can reduce friction. There is an optimum nanotube composition (Cai et al., 2004) typically about 10% by weight, for the best wear performance (Werner et al., 2004).
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