4. Future Perspectives

This entry indicated that industrial and agricultural wastes are mainly used for tribological applications. Among the agricultural wastes, the most frequently described wastes are various natural fibers of plant origin (in a polymer matrix) and RHA (in the metal matrix), whereas among industrial wastes, FA, RM, and various slags and sludges, are the most commonly used. It seems that at least some of them have a high potential to reduce frictional resistance and wear.

Aluminum alloy metal matrix composites can be a good example here. The specific configuration of their properties, such as high mechanical strength and corrosion resistance with low weight, predispose them as a hypothetical material for manufacturing of many frictionally working parts like pistons, plane bearings, sliders, brake rotors, etc. Unfortunately, the problem is often the high production costs of parts based on such composites. That is why low-cost materials with comparable characteristics are in demand. In our review, we indicate applications in which the use of RHA as a reinforcing filler for a composite with an AlSi10Mg alloy matrix significantly improved its wear resistance ^[7]. Another example would be the use of RHA as a filler for the composite with the AA6351 matrix. In this case, only a slight decrease in wear resistance was observed compared to the reference material. Therefore, it is possible to use agricultural waste fillers for composites with an aluminum matrix without adversely affecting their tribological properties.

Even greater possibilities apply to composites with typically plastic matrixes. There are also great possibilities in the application of waste fillers to compose the structure of composites with typically polymer matrixes. The results of the scratch tests presented in Ariharan et al. ^[8] indicate that the resistance to wear of an RHA-reinforced PP composite exhibits the same wear resistance as the reference material (PP composite without RHA). Therefore, it can be assumed that PP-RHA composites will be suitable for tribological applications of classic PP composites. In practice, these can be parts operating under light to medium-heavy sliding friction conditions, e.g., plain bearings or gaskets and inner and outer races of rolling bearings (balls and rollers are steel or glass). Another example would be the use of snail shells (as a filler) in PE matrix composites. In this case, it was observed that the addition of waste filler increased the hardness and wear resistance compared to the reference composite (PE composite without snail shells) ^[9]. PE composites are widely used in modern technics. Therefore, their versions with waste fillers may prove to be an interesting alternative for less responsible applications, e.g., low-speed bearings, runners for bottling in filling lines, chain guides in plate conveyors, etc. The responsible application of these composites as camshafts in engines, valve trains, or biomedical bearing material (joint prostheses) can also be considered. PA composites are another widely used group of polymeric materials in which waste fillers can be used. Biocarbon obtained from the pyrolysis of polymer wastes can be an example of it. In the investigation carried out in Myalski et al., the wear of the PA composite with this filler was characterized by much lower wear in the sliding test on cast iron than standard PA6 and other PA composites ^[10]. This “paves the way” for PA composites with waste fillers for the production of plain bearings as well as gears. Recycled carbon fibers can also be used as a filler for PEEK-based composites. According to Lin and Schlarb, this type of composite is characterized by excellent friction and wear performance ^[11]. PEEK is one of the polymers with the best tribological properties, including high operating temperature, resistance to most chemical reagents, and suitability for work under high contact stresses. The disadvantage is the high production cost. Therefore, the use of waste filler can be economically advantageous without losing strength properties. The application of such composites can be wide—from the previously mentioned bearings and gears to friction pairs in aviation and aerospace, working at temperatures exceeding even 200 °C.

Although the heavy industry is the largest producer of potentially hazardous waste material, the agricultural byproducts seem to be an interesting alternative since they can deliver various materials and substances that can be reused. For example, the total production value of food waste in the world is estimated at $1 trillion, whereas about one-third of food is wasted globally ^[12]. Fruit seeds seem a unique waste material that cannot only be used to produce bioactive compounds ^[13][14] but also offers a potentially valuable fiber. Although there are some studies dealing with the disposal of fruit seeds (e.g., from Jatropha curcas L. or Phoenix dactylifera L.) for tribological applications ^[15][16], the highest seeds waste is generated from other, more popular fruits ^[13]. Poland is one of the biggest fruit producers in Europe (behind Spain, Italy, France and Greece), and at the same time is the second biggest sour cherry producer in the world ^[17]. As sour cherries are mostly being processed and not consumed fresh, there is a high need to utilize the generated seeds for various applications, e.g., for tribological materials.

Postconsumer wastes seem to also be promising, albeit still not so popular, filler materials. Although this review indicates the potential possibilities of their use in tribological materials, it is still a research area to be expanded, especially regarding the fact that the disposal of textile products or plastics at the end of their life is a huge environmental problem. The global market of composites made from polymers and textile fibers is expected to increase by 40% from 2014 to 2020, which results from many desirable properties, such as high-strength values and corrosion resistance ^[18]. However, these types of materials require a special pretreatment, including the process of recovery of fibers from textile materials by mechanical, chemical, or biochemical methods. These methods should also be optimized to be as eco-friendly and material-efficient as possible. To gain a maximal environmental effect, there is still a need for the development and selection of the optimal treatment methods for all types of wastes. However, to select the best waste material for a particular application and the best methods of its treatment, various environmental tools, such as Life Cycle Assessment, should be used. It is also crucial to utilize the most abundant materials in the first place (e.g., FA) still considering their possible migration from the composite (and possible toxic or ecotoxic effect), but also to reuse wastes available locally (e.g., sour cherries seeds in Poland) to avoid transport-related emissions. Only taking these parameters into account will allow for the production of truly green materials exhibiting desirable tribological properties.

5. Conclusions

• This entry has shown that agricultural, industrial and postconsumer waste is a promising filler material for composites based on both polymer and metal matrices;

• Its use as reinforcement for composites exhibiting desirable tribological properties only gained in popularity in recent years (especially after 2015);

• The reuse of wastes for tribological applications has advantages, such as environmental and economic benefits, an improvement of mechanical and tribological properties and a reduction in the overall weight of the material;

• Wherever the wear of the produced composites is not worse than the reference materials, their application should be considered for ecological and economic reasons;

• There is a great need to find further wastes that can potentially be useful in improving the tribological properties of the currently used materials;

• Future research should focus not only on the waste that is generated in large quantities but also on that available locally to minimize the environmental impact related to the life cycle of tribological materials.