Background

Tribology is the science of friction, wear and lubrication; it originates from a Greek word, from tribos, which means rubbing [1]. Due to the relative motion of interacting surfaces, several machine components are influenced by friction and wear, which decrease the lifetime of these components and increase their operation and maintenance costs. Therefore well-designed tribo-materials and tribo-components have substantial economic and ecological importance.

For the development of tribo-materials the designers have to consider an important aspect.

Friction and wear are not simple material properties; they are system properties, as many factors influence their values. These factors can be the contact type and geometry, the sliding/rolling speed, the contact pressure, the counterface material, the surface roughness and pattern, the atmosphere, the environmental temperature and the relative humidity.

The word polymer is derived from Greek words as poly (many) and meres (parts). Synthetic polymers are produced via polymerisation of monomers making a long chain molecule with a large number of repeating units and having primary covalent bonds in between. Polymers are beneficial in those applications where vibration absorption, impact and shock load withstanding are needed, or in other words, where customers require high internal damping capacity.

Focusing on tribological applications, polymers can also be used in a dirty and dusty environment, and a further advantage is that most of them have quiet running. Polymers have some other beneficial features as well, such as low density, anti-corrosive nature and chemical resistivity [2-5]. Polymers can be classified into two main groups based on their thermal processing behaviour such as thermoplastics and thermosets/crosslinked elastomers.

Thermoplastics have secondary bonds between the macromolecules, in this way, these materials have the ability to re-melt after they have solidified. Thermosets and crosslinked elastomers have primary bonds between the molecular chains (covalent bonds). They solidify (cure) via chemical reaction; therefore they cannot be re-melted after solidification.

Thermoplastics based on their structure are divided into amorphous and semi-crystalline thermoplastics. In amorphous thermoplastics, the molecules solidify in a random arrangement, while in semi-crystalline ones there are crystalline domains with three-dimensional order. In these materials, both crystalline and amorphous structures can be found (Figure 1.1) [6].

Besides the neat form of these materials, it is also possible to develop polymer composites.

These composites involve two or more constituent materials (matrix and fillers) to achieve better performance. The matrix is the base material, while the fillers are usually particles and fibres. The fillers' role is to improve some of the properties of the neat polymer, for example, to enhance the mechanical, the physical, the thermal and/or the tribological features. If the applied fillers are proposed to increase mechanical or tribological properties, the fillers are also referred to as reinforcement materials or tribo-fillers, respectively.

Figure 1.1. Illustration of semi-crystalline thermoplastics [6].

Both semi-crystalline thermoplastics and thermosets are used in tribological applications as bearings, bushings, seals or gears. Traditionally thermosets are beneficial at higher loads, and thermoplastics at higher speeds but new research and developments were continuously expanding their limits. Additional advantages of thermosets are the good creep resistance and high dimensional stability. They can withstand heavy loads and they are proper materials in heavy-duty applications [7]. Thermosets in tribo-systems are only secondary materials due to their high surface energy and low deformation capability. Another challenge with thermosets is the lack of self-lubrication, the inability to form an adequate (uniform and durable) transfer layer because of thermal degradation. In self-lubrication, the transfer layer is formed by the deposits from the applied polymer samples, they are locked in the asperities of the steel counterfaces.

During the transfer layer formation, the amount of the deposits increases in the asperities (running-in period), filling the surface roughness valleys and in ideal case forming a thin layer (~few μm) on the steel counter surface (steady-state condition). This phenomenon results in polymer – partly polymer contact instead of the original polymer-metal contact. Functional fillers such as polytetrafluoroethylene (PTFE), molybdenum disulphide (MoS2) and graphite can support the transfer layer formation to make it more uniform and durable. In recent years, new research has introduced these solid lubricants as an integral part of polymer composites [8, 9]. It is important to understand the relation between the formed transfer layer and the observed tribological characteristics. With this knowledge we can achieve a better material design for tribo-composites. The main reasons for the use of semi-crystalline thermoplastics are the self-lubrication nature and the uniform transfer layer formation, both of them originating from their melting behaviour. The ability to form an adequate uniform transfer layer is the key factor in optimising friction and wear characteristics. As a protective agent, this transfer layer

has a positive influence on the friction and wear characteristics of the materials. In the polymer-metal pair, the transfer layer fills the depressions of the polymer-metal counterface, decreases its surface roughness, and alters the polymer-metal contact to polymer-polymer contact. In this way, the abrasive wear can be reduced or fully eliminated. The quality, uniformity and durability of this transfer layer have a serious influence on reducing friction and wear properties [10-13].

Besides the brief discussion of the self-lubrication and transfer layer formation, it is also important to mention the third-body concept, which further explains these phenomena. This approach introduces the dominant role of the wear particles in dry condition and sliding motion.

The solid third-body concept was formulated and introduced by Maurice Godet and further developed by Yves Berthier. The so-called third-body include all of the interfacial particles and elements between the contact surfaces, these elements separate the interacting surfaces (first-bodies). The third-body approach's three main steps are the following: wear debris detachment by e.g. adhesion or abrasion, debris circulation, and debris ejection. During the debris circulation, the wear particles are trapped between the contact surfaces reducing their interaction. At this stage, wear debris accumulation can be observed. During the last step, the wear debris is ejected, increasing the interaction between the contact surfaces, and the cycle starts again with the first step. The wear rate of a material is influenced by the balance between the detachment and the elimination of these wear debris. In other words, the wear particles can be even recycled and lost from the contact giving a dynamic balance for the wear rate.

The detached wear particles trapped between the contact surfaces can even participate in load-carrying during the wear process, in this way, similarly to fluid bodies, the solid third-bodies also have load-carrying properties [14-16].

Regarding the friction of polymers, two main components have to be considered, such as deformation and adhesion. The deformation component comes from the resistance of the polymer to the interpenetrating of the asperities of the steel counter surface. The adhesion component originates from the adhesive junctions between the real contacting surfaces and it is related to the relative surface energy of the interactive surfaces. It is supposed that the adhesion is the dominant component of solid friction [17, 18]. The surface energy of the interactive surfaces can be calculated measuring their wettability by e.g. sessile drop test.

PTFE is a widely used material in tribology, as it has good chemical resistance, broad service temperature range, low coefficient of friction and self-lubrication nature. In industry, PTFE composites are widely used as rolling / sliding bearings, seals, guideways and linear slides if the requested mechanical load is very low. This material is also considered in case of specific requirements (e.g. strong need for chemical resistance and/or high thermal stability).