Tribological Applications of Recycled and Waste Materials: A Review of Recent Advances and Future Directions

Abstract

Conventional tribological materials such as metals, ceramics, and synthetic polymers demand energy-intensive processing and create end-of-life waste. This motivates the search for more sustainable alternatives. Recent research demonstrates that agricultural residues, industrial by-products, post-consumer waste, and recycled polymers can be engineered into tribological systems that provide competitive wear resistance, stable friction, and multifunctional benefits, including thermal dissipation and vibration damping. This review summarizes progress across these material categories, highlighting how fillers like rice husk ash, fly ash, tire-derived carbon black, and reprocessed plastics transition from low-value waste into high-performance tribomaterials. System-level strategies such as interface engineering, hybrid reinforcement, and advanced processing are essential for overcoming material variability and achieving reliable tribological performance. In parallel, optimization approaches, including predictive modeling and smart material design, are increasingly enabling improved consistency, reproducibility, and scalability. Applications in automotive braking systems, recycled carbon black composites, acoustic damping structures, coatings, and reinforced polymers confirm the industrial viability of waste-derived materials. While challenges remain in feedstock variability, standardization, and long-term durability, these developments point to waste-based tribology as a practical pathway toward circular economy solutions that unite sustainability with engineering performance.

1. Introduction

Traditionally, high-performance tribological components have relied on virgin metals, synthetic polymers, and engineered ceramics. However, these materials often involve energy-intensive processing and generate waste at the end of life. In response, researchers are turning toward waste-derived and recycled materials as functional replacements in tribological applications [1,2,3]. The intersection of tribology and sustainability has created the field of green tribology, which seeks to minimize environmental impact without sacrificing performance [4]. This shift encourages the use of agricultural waste, industrial by-products, and post-consumer plastics to develop composites, coatings, and lubricants with wear and friction characteristics [5]. These recycled materials reduce reliance on virgin resources and offer new functionalities when appropriately processed, such as porosity-enhanced heat dissipation or in situ lubrication through carbon residue [6,7].

Recent research has demonstrated that composites reinforced with waste fillers from rice husk ash and coconut shells to fly ash, tire rubber, and packaging cardboard exhibit wear resistance and coefficient of friction values competitive to traditional materials under specific load conditions [8,9]. Nanostructured additives derived from waste (e.g., carbon black from tires or agro-waste converted into nano-biochar) have enabled surface reinforcement, reduced abrasive wear, and improved load-bearing properties when dispersed in polymeric matrices [10,11]. These developments suggest that it is a viable route to next-generation tribomaterials with tunable properties. Simultaneously, innovations in tribological modeling and material selection methods help engineers navigate trade-offs between thermal stability, wear resistance, recyclability, and manufacturing feasibility [12]. As environmental regulations tighten and materials engineering continues to evolve, understanding how recycled inputs can be functionally integrated into tribological systems is becoming a core area of sustainable design.

This review aims to provide a systematic and mechanistic assessment of tribological materials derived from recycled and waste resources, with the objective of clarifying their performance potential, limitations, and pathways toward practical adoption. Rather than presenting a purely descriptive survey, the article critically examines how waste-derived fillers and matrices influence friction, wear, and durability through mechanisms such as transfer film formation, third-body effects, tribochemical reactions, and load-bearing plateau development. The review is structured to (i) outline the core tribological mechanisms and performance metrics relevant to waste-based materials, (ii) analyze and compare agricultural residues, industrial by-products, post-consumer wastes, and recycled polymers as functional tribological constituents, and (iii) highlight system-level design strategies, including processing routes, interfacial engineering, and hybrid reinforcement, that enable consistent performance. Finally, the review addresses application-specific challenges, data comparability, life-cycle considerations, and barriers to industrial implementation, thereby defining future research directions needed to transition waste-derived tribomaterials from laboratory demonstrations to reliable engineering solutions.

2. Fundamentals of Recycled Material Integration

Integration of recycled and waste-derived materials into tribological systems is a viable path toward sustainability without compromising performance. These materials, when properly selected and processed, can enhance wear resistance, thermal stability, and frictional behavior under load, all of which are key factors in determining the longevity and reliability of tribological interfaces.

Conventional tribological materials often rely on steel, ceramics, and synthetic polymers due to their consistent mechanical properties and compatibility with lubrication systems. However, these materials are non-renewable, expensive to process, and difficult to recycle. In contrast, waste-derived materials, including post-consumer plastics, industrial residues, agricultural biomass, and scrap rubber, can be ground, carbonized, or modified to serve as reinforcements in tribological matrices.

Material Categories that have shown particularly strong potential:

• Agricultural wastes such as rice husk ash, coconut shell powder, and banana peel char are used as biogenic fillers in bio-based or thermoplastic composites [13,14]. When properly treated, they demonstrate improved abrasion resistance, better heat dispersion, and in some cases, self-lubricating properties through carbonaceous phases [15,16].
• Industrial by-products like fly ash, red mud, steel slag, and aluminum dross are being incorporated into polymer and resin-based tribo-composites [17,18,19]. These fillers can significantly improve dry sliding performance by enhancing surface hardness and resisting thermal softening during extended contact cycles [20].
• Post-industrial rubber waste, including tire crumb and pyrolyzed carbon black, has also been studied as a tribological additive. These materials exhibit morphology-dependent friction behavior; smaller particles typically yield more uniform reinforcement and energy dissipation, while larger particles can introduce abrasive micro-cutting effects [21].
• Recycled polymers like polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) can be reused directly or as matrices for composite formation. Their tribological performance is often enhanced through the addition of fine mineral or carbon-based waste particles, yielding improved wear resistance and lower COF values under dry and lubricated conditions [22,23,24,25].

Table 1. Summary of recycled and waste-derived material categories used in tribological systems.

Category	Examples	Role	Tribological Effect	Processing Sensitivities	References
Agricultural Waste	Rice husk ash, coconut shell powder, banana peel/biochar	Biogenic filler in bio-based or thermoplastic matrices	Increase wear resistance; improved heat dispersion; occasional self-lubrication	Benefit depends on ash/carbon content, particle size, and surface activation; moisture removal and de-ashing help	[26,27,28,29,30,31]
Industrial Byproducts	Fly ash, red mud, steel slag, aluminum dross	Ceramic/mineral filler in polymer/resin tribo-composites	Decrease dry sliding wear via increased hardness; better thermal stability	Uniform dispersion and interface coupling are critical; high loadings may embrittle the matrix	[18,32,33,34,35,36,37,38,39,40,41]
Post-industrial Rubber Waste	Tire crumb, pyrolyzed carbon black	Solid particles or nano-carbon additive	Morphology-dependent friction: finer particles mean smoother reinforcement & energy dissipation	Control particle size/surface chemistry to avoid abrasive wear and agglomeration	[10,11,42,43,44,45,46,47,48]
Recycled Polymers	PE, PP, PET (often with mineral/carbon waste fillers)	Matrix polymers, sometimes hybridized	Decrease COF, decrease wear; can match or exceed virgin grades	Performance hinges on filler dispersion, compatibilization, and recycling history	[22,49,50,51,52]

provides a summary of the main categories of recycled and waste-derived fillers mentioned above. It gives examples, outlining their sources, functional roles in composites, and observed tribological effects. This table highlights the diversity of available waste streams and underscores how material type and processing route dictate performance outcomes.

Processing parameters greatly affect performance. Particle size distribution, dispersion homogeneity, filler–matrix interfacial bonding, and post-treatment (e.g., thermal stabilization or plasma activation) all play important roles in dictating tribological properties. Figure 1 illustrates a mechanism-oriented framework linking processing parameters of waste-derived fillers to real contact conditions and the resulting dominant wear mechanisms. For instance, a uniform dispersion of nanostructured waste fillers, such as tire-derived carbon black or agro-based biochar, can reduce micro-cracking and delamination under repeated sliding [53]. Porosity is another important attribute. Many recycled fillers, especially those derived from organic or fibrous sources, create microscopic holes that affect interlocking and heat conduction across the interface. In certain applications, such as rail–wheel adhesion or dry brake pads, porosity can enhance performance by facilitating debris evacuation or thermal relief.

Wear modes such as adhesive, abrasive, fatigue, and corrosive wear occur depending on whether the interface is dominated by sliding, rolling, or particle impacts. Adhesive wear originates from asperity junction formation and rupture and is strongly moderated by transfer-layer development. Carbonaceous and polymer-rich waste fillers can reduce adhesion by forming low-shear interfacial films, whereas poor dispersion or weak filler–matrix bonding leads to unstable transfer layers and accelerated material pull-out. Abrasive wear is governed by the hardness, angularity, and retention of filler particles and wear debris. Irregular or poorly bonded recycled fillers may act as active abrasives, promoting micro-cutting and ploughing, while uniformly dispersed fine particles can function as load-bearing plateaus that redistribute contact stresses and suppress abrasion. Under cyclic loading, fatigue wear becomes dominant, with crack initiation and propagation driven by stress concentrations around agglomerates, pores, or stiff filler–matrix interfaces. Improved dispersion and interfacial compatibility mitigate fatigue by enabling more homogeneous stress transfer across the contact. In chemically active or humid environments, corrosive wear couples mechanical damage with surface degradation through oxidation or tribochemical reactions. Residual inorganic phases or moisture-sensitive constituents in waste fillers may exacerbate this process, whereas hydrophobic or chemically inert fillers can stabilize the interface and limit reaction-assisted wear. In addition, recycled fillers with irregular morphology or heterogeneous chemistry often promote dynamic third-body formation, which may either reduce wear through load sharing and surface smoothing or accelerate damage if the debris becomes abrasive. Consequently, tribological responses such as friction oscillation, transfer-layer stability, and wear-rate sensitivity to load and sliding speed are strongly influenced by filler dispersion quality and interfacial bonding. These characteristics highlight that tribological performance in waste-derived systems is governed not only by the prevailing wear mechanism but also by the evolution of interfacial conditions under service, reinforcing the need for mechanism-informed material design rather than simple property-based comparisons.

Alongside experimental tribological testing, modern evaluations are incorporating lifecycle assessment and multi-criteria decision-making (MCDM) models. These tools help quantify wear performance and environmental and economic trade-offs associated with using recycled materials. The techniques, such as Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and the Combined Compromise Solution (CoCoSo), are ranking-based decision tools that enable the selection of waste-derived materials based on conflicting priorities like COF minimization, heat resistance, and recyclability.

Waste and recycled materials in tribology are not passive cost-saving substitutes. They offer new design options for tuning frictional properties and material sustainability while reducing dependency on virgin resources. Successful applications require understanding the complex interactions between filler properties, interfacial behavior, and system-level operating conditions. These fundamentals now form the basis for ongoing innovations in green tribological engineering.

3. Advancements in Waste Materials

Waste-derived materials have emerged as promising candidates for tribological systems, driven by increasing demands for sustainability, cost reduction, and resource efficiency. Tribological data for waste-derived materials are reported using diverse test configurations, including different tribometers, loads, sliding speeds, environments, and counterface materials. As a result, direct quantitative comparison of wear rate and friction values across studies is inherently limited. This section reviews recent advancements in waste-derived tribomaterials, with emphasis on how material origin, processing route, and incorporation strategy govern friction and wear performance.

3.1. Agricultural Residues

Agricultural residues are often treated as low-value waste. Recently, they have been increasingly recognized as reinforcements in tribological systems due to their abundance, low cost, and intrinsic mineral and carbon content. Materials such as plantain peels, palm fruit bunches, rice husk ash, coconut shells, and pistachio shells have demonstrated noticeable benefits when incorporated into lubricants and solid composites, contributing both to performance and sustainability [27,54,55].

A recent study by Abdulazim at Al-Hikmah University developed bio-lubricant greases using plantain peels and palm fruit bunches, transforming these wastes into bio-alkaline precursors for grease thickening. The resulting formulations displayed oxidation stability of up to 35 min at 80 °C, thermal stability beyond 190 °C, and dropping points above 140 °C; performance values comparable to lithium-based commercial greases. Microstructural analyses shown in Figure 2 confirmed porous and crystalline features (graphite, silica) that enhanced load support and lubrication, underscoring the potential of such agro-waste residues as sustainable lubricant feedstocks [56].

A complementary agricultural waste approach focused on solid friction composites, demonstrating how agricultural residues can reinforce high-load components such as brake pads. Shete and Jadhav (2024) blended coconut shell and pistachio shell powders as dual fillers in green friction composites for brake and clutch applications [31]. Coconut shell is known for its heat resistance but has weak friction stability, whereas pistachio shell provides high abrasion resistance but limited thermal stability. Their combination was designed to balance these opposing properties.

Another complementary approach has been reported by Adekunle et al., who evaluated the abrasive wear behavior of epoxy composites reinforced with bio-silica and bio-carbon derived from rice husk [57]. In this study, rice husks were subjected to calcination and carbonization to obtain silica and carbon particulates, which were then incorporated into epoxy at 26 wt.% loadings. Pin-on-disc testing demonstrated that both fillers significantly improved wear resistance compared to neat epoxy, with optimal performance observed at 4 wt.% silica and 4 wt.% bio-carbon. At this concentration, composites exhibited reduced wear loss and stable friction coefficients, attributed to improved filler–matrix interfacial adhesion and the uniform distribution of fine particulates.

Friction Composite Material (FCM) composites, fabricated with 25–35 percent Coconut shell powder (CSP)–pistachio shell powder (PSP) filler, phenolic resin binder, copper reinforcement, and graphite/SiO₂ additives, were evaluated for physical, mechanical, and tribological properties. Among the formulations, FCM3 (25 percent CSP-PSP blend, 30% binder) achieved the most favorable results compared to FCM1 (35 vol.% CSP–PSP, 20% binder) and FCM2 (30 vol.% CSP–PSP, 25% binder), showing low water and oil absorption, higher flame resistance, improved hardness, and wear resistance compared to higher filler loadings. Pin-on-disc testing confirmed that FCM3 exhibited a stable and low wear rate (~28.5 µm) along with a consistent coefficient of friction (mean COF ≈ 0.27) under varying loads and speeds, as shown in Figure 3.

Further examination explored the use of rice husk as a filler in hybrid epoxy composites reinforced with Bauhinia vahlii and sisal fibers. In this study, Kumar et al. found that the addition of rice husk directly influenced sliding wear resistance under pin-on-disc testing [58]. The wear resistance improved steadily with rice husk loading up to 4 wt.%, indicating enhanced load-bearing capacity and improved interfacial stress transfer within the composite. At higher filler contents, however, wear resistance deteriorated due to interfacial debonding, particle fragmentation, and the generation of abrasive debris, which disrupted the stability of the sliding interface. The optimal condition therefore emerged at intermediate loading levels, confirming that rice husk not only improves the mechanical strength of natural fiber composites but also plays a decisive role in reducing wear.

Microstructural analyses revealed that filler particle size and binder content were key in determining wear resistance. The binder effect outweighed filler hardness, as better packing of constituents improved density and reduced wear, even when the hardness differences between samples were minor. Overall, the coconut–pistachio shell composites not only met standard performance ranges for automotive friction materials but also reduced reliance on polluting fillers like fly ash. Similar trends were reported for corncob husk epoxy composites, where reducing particle size (500–425 μm down to 200–125 μm) enhanced interfacial bonding, hardness, and wear resistance [16]. However, further reduction to the nano–micro-range (100–25 μm) caused agglomeration and clustering of fillers, resulting in increased wear despite higher crystallinity and stiffness. These findings reinforce that optimized particle size and dispersion are critical design parameters for agro-waste reinforcements, often being more decisive than intrinsic filler hardness.

These studies show how agricultural waste can be engineered into bio-lubricant precursors, frictional fillers, and structural reinforcements. Their mineral phases and porous morphologies provide not only cost savings but also measurable performance benefits. Mineral-rich and carbonaceous phases derived from agro-waste act as compliant third bodies that reduce direct asperity contact and promote the formation of load-bearing plateaus, leading to reduced wear and stabilized friction. Porous morphologies further aid oil retention and debris accommodation in lubricated systems. Together, they demonstrate that agricultural residues are not passive substitutes but active contributors to load support, wear reduction, and lubrication stability in tribological systems.

3.2. Industrial Wastes

Industrial waste can be reformed into valuable resources for tribological processes. Unlike agricultural residues, which are largely biogenic, these waste categories encompass inorganic by-products such as fly ash (FA), red mud (RM), and aluminum dross [59]. These materials are generated in vast quantities worldwide, creating environmental challenges regarding disposal but offering unique mineral and structural properties that can be harnessed in wear-resistant composites and lubricants. When used as fillers or reinforcements, industrial wastes have contributed improvements in hardness, thermal stability, abrasion resistance, and friction control, rivaling or surpassing that of virgin materials.

Sydow et al. demonstrated that inorganic industrial wastes such as FA, RM, and aluminum dross can significantly enhance hardness, wear resistance, and friction stability in both polymer and metal matrix composites [1]. For instance, stir-cast aluminum alloys reinforced with 10 to 15 wt.% FA showed up to a 50% reduction in sliding wear rate compared to unreinforced alloys, while simultaneously increasing hardness due to the ceramic oxide phases. Similarly, Al6061 composites containing eighteen volume percent FA reduced the specific wear rate from 411 × 10⁻⁵ mm³/m down to 203 × 10⁻⁵ mm³/m. Red mud (two to six weight percent) added to hybrid aluminum systems also improved wear resistance and maintained consistent coefficients of friction under higher loads, rivaling conventional SiC-reinforced materials. Building on these results, Kumar et al. showed that a composition of Al6061 reinforced with six weight percent FA, three weight percent graphite, and two weight percent magnesium achieved the lowest wear rate under dry sliding (10 N load, 3 m/s, 2 km distance), confirming that moderate FA loadings optimized via Taguchi-ANOVA can yield superior wear resistance while maintaining cost efficiency [33]. Collectively, these studies highlight that waste-derived ceramic fillers can provide both low-cost and high-performance reinforcement in tribological alloys.

As illustrated in Figure 4, the general structure of metal and polymer matrix composites shows how agricultural, industrial, and post-consumer wastes interact with the matrix, where the binder–filler interface and particle morphology are decisive for achieving long-term tribological stability [1].

A notable contribution is the work of Sutar et al., who applied atmospheric plasma spraying (APS) to deposit red mud-based coatings on mild steel, modified with 20–30 percent carbon and 30 percent fly ash [18]. Compared with pure red mud coatings, the hybrid formulations exhibited substantially lower cumulative mass loss during sliding wear, demonstrating that synergistic reinforcement with fly ash and carbon produces a denser interfacial film and stronger particle–matrix bonding. SEM analyses revealed reduced cavitations and more uniform splat morphologies for the RM + 20 percent C + 30 percent FA coating, while XRD confirmed phase transformations leading to manganese aluminate and fayalite structures that enhanced mechanical stability.

Recent work has further expanded industrial waste utilization to metallurgical slags, generated in enormous quantities by steel and ironmaking. Studies of granulated blast furnace slag (GBFS), blast furnace slag (BFS), and steel furnace slag (SFS) in copper-free friction composites show that slag additions (6–20 weight percent) produce stable coefficients of friction (0.45–0.51), comparable wear resistance, and, in some cases, reduced particulate emissions, relative to alumina-based formulations [60]. Microstructural analyses revealed that slag-derived Ca-Si-Fe phases become embedded in secondary contact plateaus, thereby stabilizing friction and reducing wear particle release. Importantly, SFS with high iron oxide content was identified as particularly effective, creating smoother wear surfaces and lowering disc aggressivity. These findings highlight that metallurgical slags—once considered disposal-intensive wastes—can act as functional abrasives in braking systems while contributing to emission reductions.

Ultimately, these findings confirm that industrial by-products, considered disposal burdens, can be transformed into advanced composite reinforcements. Fly ash, red mud, aluminum dross, and metallurgical slags function as active tribological reinforcements rather than inert fillers. Their oxide-rich, ceramic-like phases enhance load-bearing capacity and surface hardness while promoting the formation of stable contact plateaus and mechanically mixed layers that suppress severe adhesive wear. At optimized loadings, these wastes reduce wear through third-body load sharing and micro-polishing effects, stabilizing friction under increasing load and speed. Coating and composite studies further reveal that synergistic combinations of waste-derived oxides with carbonaceous phases improve splat cohesion, reduce cavitation, and enhance transfer-film integrity. Their ability to enhance wear resistance and frictional stability while reducing costs establishes their growing role in sustainable tribological engineering and provides a foundation for scaling such materials in real-world systems.

3.3. Post-Consumer Materials

Post-consumer waste represents an important category of materials for tribological applications, driven by the push toward circular economic practices, shown in Figure 5, and sustainable resource utilization [43,61,62]. Unlike industrial byproducts—generated during manufacturing—post-consumer wastes originate from end-of-life products such as discarded plastics, textiles, and tires. These materials are produced in vast volumes worldwide and present significant disposal and environmental challenges. However, their intrinsic properties, including high carbon content, residual oils, elastomeric flexibility, and porous morphologies, make them valuable candidates for tribological reinforcements. When processed into fine particulates, fibers, or nanostructures, post-consumer wastes have been shown to enhance wear resistance, stabilize coefficients of friction, and improve thermal dissipation in composites, often rivaling or surpassing conventional fillers. Their reuse not only mitigates ecological burdens but also demonstrates the potential for advanced functional materials to emerge from what would otherwise be landfill or incineration waste.

End-of-life tires (ELTs) are among the most studied post-consumer wastes in tribological research due to their high carbon content and elastic properties. When processed into fine crumb rubber or pyrolyzed carbon black, these materials act as both reinforcing fillers and lubricating agents. For example, brake pad composites with 15 wt.% waste tire dust demonstrated stable coefficients of friction between 0.25 and 0.35, comparable to conventional asbestos-free pads, while maintaining wear rates within industry standards [1]. The pores of the rubber particulates dissipate frictional heat and promote energy absorption during repeated loading cycles, making them highly effective in braking and clutch applications.

Beyond solid fillers, ELTs can also be turned into liquid lubricants. Alazemi et al. performed a detailed tribological investigation in 2024 of pyrolysis oil derived from ELTs and benchmarked it against virgin engine oil across a range of temperatures and speeds [63]. As shown in Figure 6, both oils operated in the mixed lubrication regime, with coefficients of friction decreasing as speed increased. At 25 °C, the COF at 1500 rpm for engine oil was approximately 25 percent lower than that for pyro-oil, while at 50 °C the difference narrowed to 20 percent. At 75 °C, the disparity grew again, with engine oil outperforming pyro-oil by 36 percent at high speeds [63]. Despite these variations, the COFs of both oils remained close, confirming that ELT pyro-oil delivers tribological performance nearly equivalent to that of conventional lubricants under moderate conditions. These results highlight the potential of pyro-oil as a sustainable lubricant for low-load, low-speed applications, providing a productive outlet for stockpiled waste tires that otherwise pose serious environmental hazards.

Beyond their role as fillers, tire-derived particulates are important environmentally because tire wear is a recognized contributor to road-traffic particulate matter with identifiable characteristics. Detailed investigation of tire wear particles demonstrated that morphology, surface chemistry, and residue content vary depending on rubber composition and service conditions [64]. These variations influence both tribological response (e.g., micro-cutting versus compliant third-body lubrication) and the nature of emitted wear debris. When end-of-life tire streams are repurposed into crumb rubber, recovered carbon black, or pyrolysis-derived oils, residual ash content, zinc-containing species, and surface oxidation states must be controlled, because they affect abrasive behavior, interfacial bonding, and potential emission profiles during sliding contact.

Post-consumer waste can rival or surpass virgin reinforcements in tribological composites. End-of-life tire derivatives, including crumb rubber, pyrolyzed carbon black, and pyro-oils, reduce wear and stabilize friction through a combination of third-body lubrication, energy dissipation, and thermal buffering at the sliding interface. Solid tire-derived fillers promote the formation of compliant contact plateaus that absorb load fluctuations and suppress severe adhesive wear, while their internal porosity enhances heat dissipation during repeated sliding. In liquid form, tire-derived pyro-oils operate within mixed lubrication regimes, where residual hydrocarbons and polar species contribute to boundary film formation comparable to conventional oils under moderate conditions. Their reuse not only reduces the environmental footprint of material production but also supports the development of circular economic models in high-performance engineering. Instead of being limited to downcycled or low-value applications, post-consumer wastes demonstrate real potential for upcycling into advanced tribomaterials, opening new design pathways for sustainable mechanical systems.

3.4. Recycled Polymers

Unlike agricultural residues or industrial by-products that primarily act as fillers, recycled polymers often serve as matrix materials, directly defining the structural behavior of composites. Commonly recovered from post-consumer and industrial streams, polymers such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) can be reprocessed. Despite concerns regarding contamination, degradation, and performance loss during multiple recycling cycles, recent studies show that reprocessed polymers can maintain competitive wear resistance and low coefficients of friction compared to virgin grades when reinforced with nanocarbon or fiber additives. This shift transforms recycled polymers from low-value products into functional engineering materials aligned with circular economy principles.

Raghuram et al. (2023) provided direct evidence that reprocessed high-density polyethylene (HDPE) can achieve performance on par with virgin materials [22]. Figure 7 illustrates the comparative wear rates and coefficients of friction for virgin and reprocessed HDPE against ultra-high molecular weight polyethylene (UHMWPE), emphasizing the feasibility of designing systems around recycled polymers. Despite showing up to twenty percent variation in mechanical properties (mass flow rate, modulus, elongation at break) after multiple reprocessing cycles, reprocessed HDPE maintained comparable wear mechanisms and coefficients of friction (µ < 0.1) to virgin HDPE and even UHMWPE. Ball-on-plate sliding tests under 30 N load confirmed that repeated recycling did not degrade tribological performance, while scratch resistance tests indicated that surface durability remained intact even after ten reprocessing cycles. These results suggest that recycled PE-HD can be upcycled into higher-value tribological applications such as bearings and liners, rather than being downcycled into low-value goods.

In 2022, Volpe et al. expanded on this concept by examining recycled polyethylene terephthalate (R-PET) derived from post-consumer bottles, compounded with virgin PET reinforced with glass fibers [63]. The resulting blend contained sixty weight percent R-PET and forty weight percent glass fiber-reinforced virgin PET, which exhibited a marked improvement in crystallinity, flexural modulus, and thermal resistance compared to neat R-PET. Mold temperature strongly influenced the balance between stiffness and ductility: lower mold temperatures promoted toughness and strain tolerance, while higher mold temperatures enhanced crystallinity and stiffness, enabling performance comparable to engineering-grade polymers. These results show that recycled PET can be elevated to applications in the automotive sector, such as cargo trunk floors and aerodynamic shields, where mechanical strength and tribological stability are essential

Beyond solid reinforcements, recent advances have demonstrated that post-consumer plastics can be directly upcycled into liquid lubricants. This opens new opportunities for high-value circular economic pathways. Hackler et al. reported the catalytic hydrogenolysis of polyolefin waste, including HDPE, LLDPE, and even post-consumer bubble wrap, to produce high-quality liquid (HQL) lubricants [65]. These waste-derived lubricants exhibited wear scar volumes as low as 7.5 × 10⁻⁵ mm³, which is comparable to premium synthetic poly-α-olefins (PAOs) and represented a 44 percent reduction in wear compared to Group III mineral oils. Coefficient of friction (COF) values also matched PAO performance, and synergistic blends of HQLs with PAO10 reduced wear by up to 34 percent and friction by 9 percent relative to virgin PAO10. Life cycle and techno-economic analyses further confirmed that the process is not only technically viable but also environmentally and financially attractive. With a production cost of approximately $4 per gallon and a selling price of $9 per gallon, the upcycled lubricants offer both economic competitiveness and environmental benefits, including drastic reductions in CO₂ emissions compared to conventional lubricant production.

This proves that recycled polymers can move beyond the traditional downcycling into low-value goods. Both HDPE and PET, two of the most widely used and discarded plastics, have been shown to retain or even enhance their tribological performance through careful processing and reinforcement strategies. Recycled polymers exhibit low coefficients of friction and comparable wear mechanisms to virgin grades due to the formation of smooth transfer films and stable contact plateaus, particularly when reinforced with fibers or nanofillers that suppress plastic deformation and debris generation. Reprocessed polymers demonstrate viability in bearings, liners, and automotive components, which were previously reserved for virgin engineering materials. Thus, recycled polymers embody the principles of upcycling within the circular economy, transforming plastic waste into advanced functional materials that both reduce ecological burden and deliver high-performance solutions.

In recycled polymer systems, tribological performance is governed by the interplay between molecular structure, crystallinity, and transfer film stability. Reprocessing can enhance wear resistance when chain scission and oxidation are limited, allowing recycled polymers to form transfer films comparable to those of virgin materials. Increased crystallinity and molecular orientation improve load-bearing capacity, while surface smoothing during sliding mitigates abrasive wear. Excessive degradation disrupts transfer film formation and accelerates fatigue wear, highlighting the sensitivity of tribological mechanisms to their processing history.

Across the waste discussed in this section, certain tribological characteristics can be examined beyond material class alone. Agricultural residues and bio-derived fillers—those rich in silica or carbonaceous phases—tend to enhance wear resistance through abrasive strengthening and micro-load-bearing effects when appropriately dispersed. Industrial by-products such as fly ash, red mud, and metallurgical slags primarily contribute through hardness enhancement and improved thermal stability, suppressing adhesive and fatigue wear under moderate-to-high contact loads. Contrastingly, post-consumer polymers and rubber-derived wastes often promote transfer film formation and viscoelastic energy dissipation, leading to reduced friction fluctuations, vibration, and noise, albeit with increased sensitivity to temperature and sliding speed.

These trends indicate that waste-derived materials do not act as generic fillers but as mechanism-specific modifiers, with their performance governed by morphology, interfacial bonding, and operating conditions. Using these modifiers provides a framework for tailoring waste-derived systems toward application-specific requirements and informs the system-level design strategies discussed in the following section.

4. System-Level Enhancements for Waste Material Tribology

System-level enhancements ranging from interface modification to advanced processing and hybrid reinforcement strategies play a decisive role in translating recycled or waste-derived fillers into tribomaterials. This section focuses on the design strategies that enable waste-based fillers and matrices to function reliably, from interfacial engineering to multifunctional hybridization. By addressing filler–matrix interactions, dispersion, morphology, and multi-functionality, researchers have discovered that system design often dictates whether waste-based composites achieve parity or superiority over conventional formulations. As illustrated in Figure 8, waste sources undergo processing strategies such as pyrolysis, ball milling, and compatibilization, which lead to modifications in key microstructural properties, including porosity, dispersion, morphology, and size distribution. These structural changes directly govern tribological outcomes, such as lowering the friction coefficient and wear rate while enhancing thermal stability and vibration damping, highlighting how system-level design bridges waste resources and high-performance tribological applications.

4.1. Interface Engineering and Compatibility

A constant challenge in incorporating waste-derived fillers into polymer systems lies in their compatibility and interfacial adhesion with matrices. Surface treatments and chemical modifications can significantly enhance filler–matrix interaction, leading to improved wear and tribological performance. For instance, Alshahrani et al. investigated the incorporation of rice husk biomass-derived bio-silica into epoxy coatings [28]. They demonstrated that these biogenic silica particles improved mechanical strength, thermal stability, and barrier performance, which showed how such agricultural waste fillers can deliver functionally robust surface coatings [28].

4.2. Hybridization and Multi-Scale Reinforcement

Single-source waste fillers may provide incremental property gains, but hybridization with nanomaterials often produces more effects. Fly ash combined with carbon nanotubes (CNTs) has been shown to simultaneously increase hardness and reduce wear rates by over 40 percent compared to fly ash alone, due to CNTs bridging microcracks and reducing stress concentrations [36]. Similarly, Wang et al. investigated graphene oxide-reinforced PPTA/PTFE composites and found that the introduction of graphene oxide markedly enhanced the formation and stability of the transfer film, resulting in significantly improved wear resistance and reduced friction compared to neat PTFE composites [66]. These hybrid strategies enable the simultaneous utilization of large-volume waste fillers for cost efficiency and nanoscale additives for fine-tuned tribological performance.

4.3. Advanced Processing and Morphological Control

The processing methods strongly influence the morphology and dispersion of waste-based fillers, which in turn affects their tribological performance. Techniques such as stir casting, melt compounding, and high-energy ball milling have been widely adopted to refine particle size and promote uniform filler distribution. For instance, Jayaprakash et al. reported that red mud nanoparticles subjected to ball milling and incorporated into ZA-27 alloys reduced abrasive wear by improving surface integrity and particle–matrix bonding [37]. Similarly, Jiang et al. showed that dual acid treatment of pyrolytic carbon black recovered from end-of-life tires enhanced surface chemistry and lubricity, allowing the material to function as an effective reinforcing agent in polymer matrices [45]. Beyond conventional recycling, additive manufacturing has emerged as a powerful method to control filler orientation and microstructure. Balla et al. demonstrated that natural fiber-reinforced composites fabricated through additive manufacturing achieved tunable friction and wear properties due to precisely aligned reinforcements and porosity, suggesting that similar strategies can be extended to waste-derived fillers [67].

4.4. Functionalization for Multi-Performance Targets

Beyond simple wear and friction reduction, system-level enhancements enable multifunctionality such as thermal dissipation, vibration damping, and corrosion resistance. Adsoy et al. demonstrated that composite brake linings reinforced with waste tire rubber particles and fly ash not only stabilized friction coefficients but also enhanced damping capacity, reducing vibration amplitudes during braking while maintaining acceptable wear performance [34].

4.5. Integration into Circular Economy Frameworks

The ultimate success of waste-derived tribomaterials lies not only in laboratory performance but also in their integration into sustainable material cycles. Life-cycle assessments (LCAs) of recycled PET composites have shown up to 45 percent reductions in CO₂ emissions compared to virgin counterparts, even when hybridized with glass fibers [49]. Similarly, valorization of fly ash and red mud into tribological systems offsets the environmental costs of hazardous waste disposal while providing industry with low-cost reinforcement alternatives.

However, recent LCA-focused literature highlights that sustainability claims for recycled materials are highly sensitive to methodological choices, including system boundary definition, functional unit selection, allocation methods, and end-of-life modeling. Many LCAs remain limited to cradle-to-gate assessments and mass-based functional units. This can obscure the true environmental implications of material reuse, extended service lifetimes, and multiple recycling loops. Allocation approaches such as cut-off, substitution, or system expansion have been shown to substantially alter outcomes and, in some cases, reverse conclusions regarding the environmental superiority of recycled materials. Moreover, reliance on a narrow set of midpoint indicators, such as CO₂ emissions or energy demand, may overlook broader trade-offs related to resource depletion, ecosystem impacts, and long-term sustainability.

In this context, system-level design approaches, including tailoring filler morphology, hybridizing reinforcements, ensuring interfacial compatibility, and optimizing processing routes, play a critical role not only in enhancing performance but also in enabling more robust and meaningful LCA outcomes. System-level enhancements thus represent the bridge between the intrinsic properties of waste resources and their adoption in practical engineering systems. By leveraging interface engineering, hybrid reinforcement, advanced processing, and multifunctional design, researchers are moving beyond proof-of-concept demonstrations toward scalable, high-value applications. These strategies place waste-derived tribological materials not as compromises, but as optimized solutions that achieve both technical performance and sustainability targets.

5. Optimization and Smart Design of Waste-Based Tribomaterials

Processing optimization plays a central role in ensuring consistent microstructures and predictable wear behavior. Whereas system-level strategies define how waste fillers interact in tribological systems, optimization methods ensure these strategies can be realized consistently through controlled processing, predictive tools, and smart material design. Veeranaath et al. reported that blending duration in aluminum matrix composites directly influences particle dispersion, hardness, and wear stability, demonstrating the sensitivity of tribological outcomes to processing conditions [68]. Dhakal et al. similarly showed that processing defects in 3D-printed polymers lead to increased roughness and accelerated wear, emphasizing the importance of manufacturing quality when using recycled or waste-based materials [69]. For composites reinforced with waste fillers, homogenization through techniques such as melt compounding or controlled mixing is often decisive in whether properties are enhanced or degraded.

Furthermore, machine learning and artificial intelligence are increasingly used to predict and optimize tribological behavior. Yan et al. developed predictive models for epoxy composite coatings that could accurately estimate friction coefficients and wear resistance based on input parameters, reducing the need for exhaustive physical testing [70]. Kumar et al. applied machine learning to optimize machining of squeeze-cast TiB₂/AA6061 composites, correlating microstructural wear features with optimized process settings [71]. These approaches are especially relevant to waste-based tribomaterials, where feedstock variability makes predictive modeling valuable for designing reliable systems.

Additionally, smart design choices enable recycled and waste-based polymers to be used as functional materials rather than low-value fillers. Jan et al. examined wood-based polypropylene composites and found that recycled PP grades exhibited distinct wear mechanisms compared to unrecycled counterparts, showing that recycling history must be considered in application-specific designs [52]. Supriyanto et al. combined simulation and experimental validation for recycled PVC composites, confirming that computational modeling can accurately anticipate wear trends and load transfer behavior [72]. Raghuram et al. further demonstrated that recycled polypropylene can be re-engineered for tribological systems by tailoring its formulation environment, effectively improving its performance up to that of conventional engineering polymers [52].

These modern optimization strategies build on earlier findings, such as those of Volpe et al., who showed that blending recycled PET with glass fiber-reinforced virgin PET restored crystallinity and mechanical resilience for automotive components, and those of Adsoy et al., who showed that hybrid brake pads made from waste tire rubber and fly ash maintained stable friction while also enhancing vibration damping. These studies demonstrate that optimization frameworks allow waste-derived materials to substitute for virgin resources while also delivering added functionality, including improved wear resistance, noise suppression, and thermal stability.

Integrating processing science, predictive modeling, and system-aware material design allows waste-based tribomaterials to transition from simple substitutions to sustainable replacements. This shift makes recycled and waste-derived materials reliable, multifunctional components for advanced mechanical systems while progressing toward a circular economy.

6. Applications

The adoption of waste-derived fillers and matrices in tribology has progressed from laboratory evaluations to functional applications across automotive, marine, industrial, and structural systems. This section highlights the most impactful application areas, emphasizing how industrial, agricultural, and post-consumer wastes are being transformed into viable tribological solutions. Figure 9 provides an overview of these connections, illustrating how different waste categories, including agricultural residues, industrial by-products, post-rubber wastes, and recycled polymers, are utilized in tribological applications such as braking systems, carbon substitution, acoustic damping, and protective coating. It is worth noting that agricultural “waste” materials generally require several pre-processing steps—such as drying, size reduction, surface modification, or thermal treatment—before they can be effectively incorporated into tribological systems.

6.1. Automotive Braking Systems

Friction materials for braking applications are one of the most researched areas of waste utilization [59]. Dirisu et al. developed eco-friendly brake pads using a hybrid combination of industrial and agricultural wastes, reporting stable coefficients of friction and reduced wear rates compared to conventional commercial pads [73]. These composites also demonstrated reduced noise and vibration, underlining their potential for sustainable integration into large-scale automotive systems. Complementing this, Singh et al. employed a multi-criteria decision-making approach to rank agro-waste and natural fiber reinforcements for brake pads, concluding that rice husk and coconut shell fillers offer the best balance of wear resistance, thermal stability, and manufacturability [74], as well as confirming that agro- and industrial waste can reliably meet safety-critical demands in braking systems.

Recent studies extend this progress by highlighting the tribological benefits of industrial by-products in braking systems. Rajan et al. formulated slag waste-filled phenolic composites and showed that slag fractions of 60–65 wt.% not only stabilized the coefficient of friction (~0.40) but also minimized fade (~20 percent) and frictional fluctuations compared to resin-rich formulations [75]. Although wear mass loss increased at the highest slag loading, slag-derived composites offered performance within ECE R90 automotive braking standards, confirming their industrial scalability. Similarly, Kuş et al. demonstrated that combining fly ash (12 weight percent) with varying levels of red mud in bronze matrix brake pads improved hardness and reduced wear loss by nearly 47 percent, with 8 weight percent red mud composites delivering the most stable friction coefficient (0.38–0.44) during long braking cycles [32].

In addition to friction stability and wear resistance, brake formulations are evaluated in terms of particulate emissions. This is because braking generates airborne debris whose morphology reflects both composition and wear mechanisms. Analysis of brake particles shows that debris usually contains angular fragments and agglomerates that contain metallic and oxide phases. Particle size distribution and phase composition mostly depend on dominant wear modes such as abrasion, adhesion, and fatigue-driven delamination [76]. This is connected to waste-derived friction materials: industrial by-products (e.g., slag, fly ash, red mud) and tire-derived carbon fillers modify contact plateau formation, which influences whether debris remains as stable load plateaus or fragments. Furthermore, tire particulate matter possesses distinct signatures influenced by rubber formulation and additive chemistry, emphasizing that recycled tires are not compositionally uniform [64]. Therefore, waste-based brake pads should be benchmarked not only by COF and wear loss but also by emitted particle metrics, including morphology, composition, and size under standardized braking cycles.

From the agricultural side, Kılıç reported that rice husk-based composites outperformed rice stalk formulations, with a 5 wt.% rice husk brake pad achieving a high-performance coefficient of friction (~0.41), good fade resistance, and excellent recovery behavior, ranking first in a hybrid AHP-VIKOR evaluation of multiple braking criteria [77]. Together, these findings establish that industrial (slag, fly ash, red mud) and agro-residues (rice husk, coconut shell) can meet stringent safety and performance standards, replacing copper- or asbestos-based friction materials with sustainable alternatives.

6.2. Carbon Black Substitution from Tire Recycling

Carbon black is widely used in friction materials, but its high production energy and cost drive the search for recycled substitutes. Studies on EPDM composites showed that increasing CB content (0–60 phr) steadily reduced specific wear and stabilized the coefficient of friction [78]. A complementary study showed that increasing CB up to 60 phr reduced wear loss by over two orders of magnitude compared to unfilled rubber while stabilizing friction through the development of Schallamach waves and smoother wear surfaces [79]. In PTFE, even 1 wt.% CB improved wear resistance by 40 times with a low-activity CB and by 700 times with a high-surface-area CB, underscoring the critical role of CB surface chemistry in tribological performance [80].

Beyond conventional CB, recycled sources provide a sustainable alternative. Pyrolysis of end-of-life tires yields ~45–50 weight percent recovered carbon black (rCB), with morphology and particle size distributions comparable to commercial N330, though with higher ash and ZnO residues [81]. Similarly, autoclave pyrolysis of waste truck tires yielded ~48 wt.% pyrolytic carbon black (CBp), with morphology and functional groups broadly comparable to commercial N330 [82]. However, CBp contained higher ash (23.8 wt.%) and sulfur, along with ZnO and ZnS residues from tire additives, indicating that while reinforcement potential exists, surface modification or blending may be needed for high-duty applications.

Laithong et al. advanced this field by improving the quality of recycled carbon black obtained from tire pyrolysis, achieving performance characteristics comparable to the widely used commercial N330 grade [47]. When incorporated into composites, this recycled carbon black provided reinforcement and wear protection similar to virgin fillers, introducing cost-effective and environmentally friendly brake linings, clutches, and damping components. Commercial-scale trials demonstrated that CBp can safely replace up to 50 percent of furnace carbon black (N234) in NR/BR rubber formulations without compromising tensile strength or vulcanization stability [42]. At these levels, scorch safety improved, and production costs decreased, confirming that partial substitution is both technically feasible and economically beneficial.

6.3. Acoustic and Vibration Damping

Beyond wear reduction, tribology also intersects with noise and vibration control through friction-induced excitation, interfacial energy dissipation, and contact damping mechanisms. In many tribological systems, such as brakes, clutches, bearings, and sliding interfaces, frictional interactions are a primary source of noise and vibration, making acoustic performance an indirect but relevant outcome of tribological design. Sharma et al. surveyed fibrous composites for acoustic performance, emphasizing the role of natural fibers and recycled textiles in improving sound absorption while maintaining structural stability [83]. Such composites offer lightweight, sustainable solutions for vibration damping in automotive interiors, railway systems, and building materials.

Borges et al. showed that cementitious subfloors modified with EVA waste and rice husks achieved up to 24 dB of impact noise reduction (DLw) and reduced dynamic stiffness by nearly 90 percent compared to reference mortar, confirming the effectiveness of agricultural and polymer wastes in impact sound insulation [30]. Marques et al. extended this by designing rice husk–rubber composite boards for building applications, reaching >90 percent vibration isolation efficiency and tailoring high- vs. low-frequency damping via husk–rubber ratios [29].

Recently, Buddhacosa et al. demonstrated that syntactic foams incorporating 5–23 weight percent rubber particles from end-of-life tires provide substantial vibration and acoustic improvements [84]. The vibration damping ratio increased by 38–75 percent, with optimal performance at 250–425 µm particle size. Similarly, Nguyen et al. demonstrated that ABS composites reinforced with 50 weight percent ground tire rubber (GTR) produced up to a 260 percent improvement in damping ratio, confirming that recycled elastomers can be engineered into vibration isolation components for automotive and structural parts [46]. These effects stemmed from the viscoelastic nature of rubber fillers, which dissipated vibrational energy and suppressed noise radiation pathways.

Expanding into hybrid formulations, Dobrotă et al. reported that combining scrap rubber with fly ash (FA) and recycled PVC enhanced damping efficiency by balancing stiffness reduction and energy dissipation. Composites with ~30 phr FA and PVC exhibited the lowest vibration amplitudes and fastest modal attenuation, highlighting the multifunctional benefits of integrating multiple recycled fillers [35]. Collectively, these studies confirm that recycled EVA, rice husks, waste tire rubber, FA, and PVC can be engineered into broadband vibration and noise-damping materials. Such systems are scalable for automotive interiors and construction applications, advancing both sustainability and performance.

6.4. Advanced Coatings and Surface Engineering

Coating represents another major sector of application for recycled and waste materials. Yao and Lian presented nanoscale insights into composite coatings, showing how nanofiller dispersion directly enhances hardness, reduces wear, and improves load-carrying capacity [85]. Although their study used model nanocomposites, the principles can be translated to recycled fillers, where nanoscale reinforcements recovered from wastes can be engineered for similar tribological improvements.

Recent work has demonstrated that industrial slags can function as effective fillers in epoxy-based coatings. Erdoğan et al. developed epoxy composites filled with 30 weight percent slag wastes (blast furnace, ferrochromium, and converter slags) and reported that all slag-filled coatings exhibited superior wear resistance compared to alumina-reinforced controls [86]. Blast furnace slag offered the best tribological response, yielding the lowest wear volume under 10–20 N loads. SEM revealed that SiO₂- and CaO-rich phases increased load-bearing capacity, while fatigue wear and plastic deformation dominated the wear mechanism.

In parallel, Karabork investigated epoxy coatings modified with recycled tire products, including ground tire rubber (GTR), devulcanized GTR (DGTR), pyrolytic carbon black (PCB), and virgin CB [48]. The DGTR-filled coating reduced wear rate by 77.7 percent, while PCB-filled coatings provided the highest corrosion resistance, showing no white rust formation after 1000 h of salt spray exposure. These results highlight how different tire-derived fillers can be tailored for either tribological reinforcement or corrosion protection.

Bendikienė et al. extended the scope to industrial metal waste coatings, fabricating hard-facings via submerged arc welding (SAW) with reinforcements from HSS chips, chromium steels, boron carbide waste, WC-Co hard metal debris, and cast iron grindings [62]. Their results showed hardness up to 63 HRC after tempering and wear resistance 6 times higher than standard tool steels and commercial wear plates (EIPA500). Waste-derived carbides (WC, TiC, VC) and oxides provided high hardness and wear stability, while SiC/glass flux improved weld integrity.

Collectively, these studies confirm that recycled fillers from industrial slags and waste tires can not only replace conventional reinforcements but also impart multifunctionality—combining wear resistance, hardness, and corrosion protection in protective coatings.

6.5. Circular Economy in Polymer Systems

Recycled fibers are also increasingly integrated into polymer-based tribological applications. Ventura et al. demonstrated that recycled carbon fibers can successfully reinforce ultra-high-molecular-weight polyethylene composites, leading to improved wear resistance and stable frictional behavior while simultaneously preventing significant textile waste from landfills [87].

Expanding this concept, Lin and Schlarb showed that PEEK composites reinforced with recycled carbon fibers (rCF) exhibited tribological performance nearly identical to virgin CF composites, even under severe pv conditions. Both rCF- and vCF-filled PEEK reached coefficients of friction as low as 0.115–0.117 and wear rates in the 10⁻⁶ mm³/Nm range [88]. The rCF composites promoted more homogeneous tribofilms and effective load transfer, confirming that recycled fibers can substitute for virgin reinforcements in demanding tribosystems.

Beyond fiber reinforcements, polymers themselves are also being recycled into tribological components. Raghuram et al. reported that reprocessed HDPE retained low friction coefficients (<0.1) and wear resistance comparable to UHMWPE, enabling its upcycling from packaging into higher-value applications such as bearings and liners [22]. Importantly, HDPE’s lower embodied energy and CO₂ footprint make this substitution environmentally impactful, aligning recycling with climate targets [45].

Khattab and Ali provided further evidence from polymeric coatings, where recycled HDPE and PVC coatings outperformed virgin counterparts in wear resistance and friction, respectively [89]. Recycled PVC achieved friction values nearly 40 percent lower than virgin PVC (μ ≈ 0.5 vs. 0.8), while recycled HDPE coatings resisted abrasion more effectively. These findings highlight how contaminants and altered crystallinity in recycled polymers can fortuitously enhance tribological performance.

Their study emphasized that such reinforcements not only reduce reliance on virgin carbon fibers but also establish scalable circular economy practices within high-performance polymer tribology. Overall, the reviewed studies demonstrate that waste-derived fillers and matrices can be tailored for diverse tribological systems ranging from braking and lubrication to coatings and damping.

Table 2. Summary of waste-derived and recycled material for various applications.

Application Sector	Agricultural Waste	Industrial Waste	Post-Consumer Waste	Recycled Polymers
Brake Pads	High-wear resistance, low friction [31,90,91,92,93,94,95,96,97,98,99,100,101]	Anti-wear behavior, low cost [32,102,103,104,105,106,107,108,109]	Brake lining, recycled tire particles [110,111,112]	Hybrid composites, Resin-based composite [91,93,113,114]
Coatings	Bio-derived coating, thermal barrier, flame retardant, antibacterial, anticorrosion behavior [28,115,116,117,118,119,120,121,122,123,124,125]	Anti-corrosion, hydrophobic, geopolymer, oil-in-water emulsion, humidity controller [126,127,128,129,130,131,132,133,134,135,136,137,138]	Tire-derived particles in epoxy coatings, sustainable engineering, hydrophobic [139,140,141,142]	Resin coating, oil spill cleaning, Hydrophobic [143,144,145,146,147,148]
Lubricants	Frictional additives, composites, Grease, water lubricant [31,56,149,150,151,152,153]	Additives, carbon-based nano-additives, viscosity modification [154,155,156,157,158,159,160]	Pyrolysis oil from ELTs as an engine oil and motor oil substitute [63,161,162]	High-quality base liquid lubricants [163,164,165]
Vibration Damping	Damping capacity, engine mount [166,167,168,169]	Dynamic properties, road construction [35,170,171,172,173,174,175,176,177,178]	Ground tire rubber, EVA waste composites [46,179]	Elastomer-based composite, noise control [180,181,182,183,184]

summarizes the key waste categories and their associated applications, along with representative studies that confirm their feasibility.

Across the reviewed studies, waste-derived tribomaterials repeatedly demonstrate that performance is mechanism-driven rather than waste-class driven. Silica- and carbon-rich agricultural fillers most often improve wear resistance through micro-load-bearing and abrasion strengthening when dispersion is controlled. These trends support the conclusion that waste-derived systems can meet application-level tribological requirements when interface compatibility, particle morphology, and processing repeatability are engineered alongside performance. While these materials demonstrate clear functional feasibility and performance parity under controlled conditions, their translation to industrial deployment depends on meeting durability, standardization, and regulatory requirements discussed in Section 7.

7. Future Challenges and Directions

While significant progress has been made in utilizing agricultural residues, industrial by-products, and post-consumer wastes in tribological applications, several challenges still prevent large-scale adoption. A recurring issue is the variability of waste feedstock quality. Unlike virgin materials, wastes often carry contamination, inconsistent morphology, and unpredictable thermal stability, which complicates processing and performance prediction. For example, recycled polymers can suffer from molecular degradation across repeated cycles, while waste-derived fillers such as crumb rubber or biochar display morphology-dependent wear responses that make consistency difficult to achieve.

Another issue lies in scaling processing strategies. Techniques such as ball milling, hybrid reinforcement with nanomaterials, and additive manufacturing have shown promise at the laboratory scale, but transitioning these to industrial volumes requires careful balancing of cost, energy inputs, and environmental impact. Similarly, pyrolysis-based recovery of oils or carbon black from end-of-life tires offers viable routes, but thermal stability and long-term durability of the resulting lubricants remain limited compared to commercial requirements.

Beyond technical challenges, industrial commercialization is strongly constrained by the lack of standardized testing protocols, regulatory alignment, and quality control frameworks. Tribological performance is often reported under non-uniform testing conditions, with variations in load, speed, temperature, and counterface materials. This makes direct comparison across studies and benchmarking against commercial products difficult. In safety-critical applications such as braking systems and bearings, regulatory certification requires reproducible performance, long-term durability, and contamination tolerance. These criteria are rarely addressed in laboratory-scale studies of waste-derived materials. Moreover, the inherent variability of waste feedstocks complicates quality assurance, necessitating tighter control over preprocessing, classification, and batch-to-batch consistency before industrial adoption is feasible.

Beyond durability, increasing regulatory examination of vehicle-derived particulate emissions highlights the importance of combining tribological evaluation with particle characterization. Microstructural analysis has proven critical for environmental and regulatory assessment [183]. For waste-derived tribomaterials to achieve widespread industrial acceptance in transportation applications, standardized testing protocols should integrate friction and wear metrics with emission-relevant particle analysis. This includes characterization of wear debris size distribution, morphology, and elemental composition to ensure that sustainability gains do not introduce unintended environmental consequences.

An additional barrier to large-scale adoption lies in market acceptance and supply chain integration. Even when technical performance is demonstrated, industries remain hesitant to adopt waste-derived tribomaterials due to uncertainties in long-term supply reliability, cost volatility, and compatibility with existing manufacturing infrastructure. Established industries often rely on tightly controlled supply chains and certified material specifications, whereas waste-derived resources may originate from geographically dispersed and seasonally variable streams. The lack of techno-economic assessments that account for feedstock logistics, preprocessing costs, and scale-dependent manufacturing efficiency further limits confidence in industrial deployment. Addressing these challenges will require closer collaboration between material developers, waste processors, and end-users to align material design with industrial workflows and economic constraints.

For the future, several opportunities can advance this field. Integration of machine learning and predictive modeling can help mitigate feedstock variability by forecasting tribological behavior based on input composition and processing history. Hybrid systems combining large-scale waste fillers with nanoscale reinforcements offer a pathway to balance cost efficiency with high performance. Additive manufacturing could enable tailored microstructures and porosities for application-specific needs while also accommodating diverse filler matrices. Importantly, lifecycle assessment and standardized qualification protocols should be incorporated early in material development, ensuring that performance gains translate into reliable, certifiable, and environmentally credible solutions.

Despite increasing emphasis on sustainability, quantitative life-cycle assessment and carbon-footprint data remain sparse and fragmented across waste-derived tribomaterial studies. Many reports infer environmental benefits from material substitution alone, without integrating durability-based functional units or end-of-life scenarios. Addressing these gaps through standardized LCA and techno-economic frameworks will be essential to validate sustainability claims and guide industrial adoption.

In essence, the future of waste-derived tribology depends not only on advancing laboratory performance but also on overcoming fundamental barriers to industrial commercialization. Addressing challenges related to standardization, regulatory acceptance, quality assurance, and supply chain integration will be critical for enabling reliable certification and industrial confidence. With better coordination in processing science, predictive modeling, and circular economy frameworks, these materials can go from experimental substitutes to mainstream solutions that deliver both technical and environmental benefits.

8. Conclusions

This review has shown how agricultural residues, industrial by-products, post-consumer waste, and recycled polymers can be developed into useful tribological materials. Instead of being seen only as low-value substitutes, these waste sources have demonstrated the ability to provide wear resistance, stable friction, and added functions that meet the needs of engineering systems.

Progress in areas such as interfacial treatments, hybrid reinforcement, and improved processing has helped overcome the natural variability of waste feedstocks. In addition, new modeling and design tools are making it easier to predict and optimize performance, which is important for moving toward large-scale use.

Practical applications already include brake pads, recycled carbon black composites, damping structures, and reinforced polymers. These examples confirm that waste-based materials can meet performance requirements in real-world tribological systems.

While challenges remain around material consistency and long-term validation, ongoing research continues to push these materials from laboratory trials toward reliable industrial adoption. The development of tribological materials from recycled and waste resources not only addresses environmental concerns but also opens new pathways for sustainable engineering design.