Tribological Assessment of FFF-Printed TPU Under Dry Sliding Conditions for Sustainable Mobility Components

Abstract

We are witnessing a global commitment to sustainable mobility that requires advanced materials and manufacturing techniques, such as fused filament fabrication (FFF), to create lightweight, durable, and recyclable machine components. Acknowledging that friction and wear significantly contribute to energy loss globally, developing high-performance polymeric materials with customizable properties is essential for greener mechanical systems. FFF inherently drives resource efficiency and offers the geometric freedom necessary to engineer complex internal structures, such as the gyroid pattern, enabling substantial mass reduction. This study evaluates the tribological performance of FFF-printed thermoplastic polyurethane (TPU 82A) specimens fabricated with three distinct gyroid infill densities (10%, 50%, and 100%). Ball-on-disc testing was conducted under dry sliding conditions against a 100Cr6 spherical ball, with a constant normal load of 5 N, resulting in an initial maximum theoretical Hertz contact pressure of 231 MPa, over a total sliding distance of 300 m. Shore A hardness and surface roughness (R_a) were also measured to correlate mechanical and structural characteristics with frictional response. Results reveal a non-monotonic relationship between infill density and friction, with a particular absence of quantifiable mass loss across all samples. The intermediate 50% infill (75.9 ± 1.80 Shore A) exhibited the peak mean friction coefficient of $\overline{\mu }=1.002$ (${\mu }_{\max }=1.057$), which can be attributed to its balanced structural stiffness that promotes localized surface indentation and an increased real contact area during sliding. By contrast, the rigid 100% infill (86.3 ± 1.92 Shore A) yielded the lowest mean friction ($\overline{\mu }$ = 0.465), while the highly compliant 10% infill (44.3 ± 1.94 Shore A) demonstrated viscoelastic energy damping, stabilizing at $\overline{\mu }$ = 0.504. This work highlights the novelty of using FFF gyroid architectures to precisely tune TPU 82A’s tribological behavior, offering design pathways for sustainable mobility. The ability to tailor components for low-friction operations (e.g., μ ≈ 0.465 for bushings) or high-grip requirements (e.g., μ ≈ 1.002 for anti-slip systems) provides eco-efficient solutions for automotive, railway, and micromobility applications, while the exceptional wear resistance supports extended service life and material circularity.

1. Introduction

The growing global imperative for sustainable mobility necessitates profound innovation in material science and manufacturing, focused on maximizing resource efficiency and minimizing the energy footprint across the product lifecycle [1,2]. In domains like automotive, aerospace, and railway, where enhanced safety and reduced environmental impact are driven by increasingly restrictive legislation, there is a substantial demand for lightweight, durable, and recyclable components [1,2,3,4,5]. A major challenge in achieving sustainability goals arises from energy losses associated with friction and wear in machine components, which are estimated to account for over 20% of global energy consumption [1,6,7]. As a result, the development and application of advanced, self-lubricating, and high-performance polymeric materials (PBMs) for load-carrying applications, such as seals, bearings, or bushings, still represent an important area for innovation toward greener mechanical systems [2,8,9,10].

Additive manufacturing (AM), often referred to as three-dimensional printing, has emerged as a disruptive technology perfectly aligned with the principles of sustainable production and complex design freedom [1,8,11,12,13]. Among the diverse AM techniques, fused filament fabrication (FFF), also known as fused deposition modeling (FDM^®), stands out due to its simplicity, cost-effectiveness, and capacity to handle a wide type of thermoplastic materials [1,8,11,12,13]. FFF inherently promotes resource-efficient manufacturing by employing an additive, layer-by-layer material deposition process that significantly reduces raw material waste compared to conventional, subtractive methods [1,12,14]. Essentially, FFF enables the creation of hollowed parts and complex internal lattice structures, offering unparalleled geometrical freedom that translates directly into substantial weight and material savings for mobility components [8,15]. As highlighted by Dhakal, FFF facilitates on-demand, decentralized manufacturing, which curtails the resource depletion associated with traditional logistics and supply chains, supporting established Sustainable Development Goals (SDGs) [1].

Among the thermoplastics widely utilized in FFF, thermoplastic polyurethane (TPU) is a linear segmented block copolymer valued for its unique combination of mechanical properties that bridge the gap between traditional rubbers and plastics [16,17]. TPU is characterized by excellent intrinsic flexibility, high tensile strength, and exceptional abrasion and wear resistance [18,19]. Its resilient elastic behavior, combined with its capacity for impact energy absorption and effective damping characteristics, makes it an attractive material for demanding applications in mobility, such as dynamic seals, engine bushings, vibration isolators, and anti-slip components [16,17,20,21,22]. Moreover, the potential for synthesizing TPU from environmentally friendly bio-based materials further enhances its promise as an eco-friendly and sustainable additive for transportation infrastructure and components [23,24].

The effective application of FFF-printed components in tribological systems requires a rigorous understanding of the complex interplay between the printing parameters, the resulting microstructure, and the functional performance under dynamic contact conditions [2,12,25,26]. Traditional FFF parts are known to exhibit challenges such as inherent anisotropy and surface roughness arising from the layered deposition, which can degrade wear resistance and frictional stability, as mentioned by Subramani et al. [2]. Consequently, abrasive and adhesive wear mechanisms often dominate in FFF polymers [2]. Extensive research has focused on enhancing the tribological performance of common FFF polymers like acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and high-performance polymers such as polyether ether ketone (PEEK), often by incorporating reinforcing fillers or optimizing printing speed and orientation [1,12].

A critical, yet often ambiguous, parameter influencing both the mechanical and tribological response of FFF parts is the infill density [2,12,24]. Infill density governs the overall material usage, component weight, and internal porosity, which is vital for lightweight applications and, in some cases, for enhancing self-lubrication capabilities [2,8,12]. While studies on FFF polymers generally suggest that lower infill densities can lead to material savings, the resulting internal microstructure directly impacts the strength and failure modes [2,8,12]. Research on flexible FFF materials, particularly TPU, has indicated that infill percentage significantly modifies the performance, revealing an important trade-off: higher infill (e.g., 100% TPU 60A in Stoica et al. study) can yield increased friction coefficients necessary for grip but often leads to accelerated wear due to enhanced rigidity [24]. Conversely, materials like TPU 95A demonstrate superior wear resistance but potentially lower friction coefficients, Stoica et al. [24]. Additionally, the introduction of complex infill patterns (such as grid or honeycomb) is recognized for modulating mechanical strength, wear resistance, and the component’s energy absorption capacity [2,22]. The gyroid pattern, specifically, is highly favored in lightweight design due to its high strength-to-weight ratio and self-supporting structure.

Despite these foundational insights, a significant research gap remains regarding the systemic influence of intricate internal geometries and density variations on the tribological behavior and surface properties of flexible FFF-printed TPU, particularly under dry sliding conditions relevant to unlubricated mobility applications. There is a lack of comprehensive data correlating specific infill patterns and varying densities with the resulting friction characteristics and wear mechanisms of FFF-TPU elastomers [2,27,28]. Addressing this gap is paramount for reliably utilizing FFF technology to manufacture bespoke, lightweight, and durable TPU components for sustainable transportation systems.

The main objective of this study is to systematically assess how gyroid infill density (10%, 50%, and 100%) governs the dry-sliding tribological response of FFF-printed TPU 82A by quantifying its influence on friction evolution, wear behavior, and associated surface and hardness characteristics. By establishing these structure–property–performance relationships under a controlled ball-on-disc configuration, this work aims to provide design-relevant guidelines for lightweight, durable, and recyclable TPU components in sustainable mobility applications such as bushings, seals, and anti-slip elements.

2. Materials and Methods

2.1. Material and Specimen Preparation

The material utilized throughout this tribological investigation was Filaflex 82A Original (Recreus Industries S.L., Alicante, Spain) thermoplastic polyurethane elastomer, specifically characterized by a Shore A hardness of 82A. This elastomer belongs to the class of thermoplastic polyether-polyurethane materials, containing specific additives engineered to ensure high-printability fused filament fabrication (FFF) technology. The chemical composition confirms the polymer is derived from methylenediphenyl diisocyanate, glycols, polyether polyol, and various additives. Key properties of the filament include high flexibility, demonstrating an elongation at break capability of up to 650%, alongside noteworthy resistance to hydrolysis, bacteria, and certain solvents and fuels. The material exhibits a relative density of 1.12 g/cm³ and is characterized by a softening point of 105 °C and a recommended melting point range between 220 °C and 240 °C [29,30].

The specimens were produced using an FFF additive manufacturing process, using Creality K1C FDM printer (Creality Co., Ltd., Shenzhen, China), employing parameters optimized for flexible filaments. Critical printing parameters were maintained to ensure consistency across the tested samples. The nominal layer height was set at 0.2 mm. The recommended printing temperature range for the material is 215 °C to 250 °C [29], applied at the extrusion nozzle. Printing adhesion was considered high, eliminating the necessity for a heated bed or specialized adhesive sprays. The recommended printing speed was maintained within the typical range of 20–60 mm/s [29]. In this study the printing temperature was set at 230 °C and the printing speed at 20 mm/s.

All TPU disc specimens (three specimens for each infill density, with a total of nine specimens studied) were fabricated with a diameter of 40 mm and a height of 10 mm, printed flat on the build plate with the layers deposited in the XY-plane (build direction along the Z-axis) and with the top surface generated using an Archimedean chord toolpath. The internal structure was controlled by varying the density using a gyroid infill pattern with Archimedean external pattern surface for top and bottom. Three primary infill density configurations were prepared for testing:

• 10% Infill (G10);
• 50% Infill (G50);
• 100% Infill (G100).

2.2. Tribological Testing (Ball-on-Disc)

Test setup and parameter tribological testing was conducted under dry sliding conditions using a specialized Ball-on-Disc TRIBOtester apparatus (Tribotechnic US LLC, San Francisco, CA, USA). The printed TPU disc sample, serving as the disc, was tested against a stationary counterbody, 100Cr6 spherical pin (Tribotechnic US LLC, San Francisco, CA, USA).

Constant parameters for all tribological tests were set as follows:

• Counterbody (100Cr6 spherical pin): The spherical pin had a nominal dimension diameter of Ø6 mm. Its mechanical properties include a Young modulus of 205,000 MPa and a Poisson’s ratio of 0.33.
• Loading and Motion: A constant normal load of 5 N was applied. The corresponding initial maximum theoretical Hertz contact pressure generated was 231 MPa. The tangential sliding speed was maintained at 150 mm/s.
• Test geometry and duration: Each test run involved a total sliding distance of 300 m (specifically, 300.14 m recorded). This duration corresponded to approximately 5308 total cycles and a total test duration of roughly 2001 s (at an acquisition interval of 1.00 s).
• Environment: Testing was strictly conducted under dry sliding conditions, with no lubricant used. The ambient testing environment was characterized by a stable temperature of ±25 °C and a humidity of 50% r.H.

The primary variable investigated in the tribological assessment was the internal structure of the specimens, represented by the infill density (10%, 50%, and 100%). Tribological testing was performed on three independent specimens for each infill density.

The friction coefficient (μ) was measured continuously throughout the duration of the test by monitoring the tangential friction force generated during sliding against the constant normal load (5 N). The coefficient of friction at any point in time was calculated directly as the ratio of the measured instantaneous friction force to the constant normal load. Results were recorded and analyzed based on the coefficient variation over test duration/sliding distance, yielding parameters such as the starting, minimum, maximum, and average steady-state friction coefficients. Figure 1 illustrates the main experimental procedures applied to the FFF-printed TPU 82A samples.

2.3. Surface Roughness and Hardness

The surface texture of the printed specimens was characterized using contact surface profilometry equipment (Tribotechnic US LLC, San Francisco, CA, USA). Profilometric measurements were performed on both the top and bottom surfaces of the disc samples using a measurement length of 4 mm. The analysis employed a Gaussian filter with a cut-off length of 0.8 mm to separate roughness and waviness components, allowing for the quantification of standard roughness parameters defined by the ISO 4287 standard [31], including arithmetic mean roughness (Ra), root mean square roughness (R_q), and maximum height of the profile (R_z or R_t).

The methodology explicitly addressed the unique surface texture resulting from the FFF manufacturing process. The top and bottom surfaces exhibited an Archimedean spiral texture (concentric circular ridges), characteristic of fused filament layer deposition. This inherent surface topography on the printed face is critical as it dictates the initial contact conditions and the real contact area during tribological testing. Surface roughness was evaluated directly within the wear track, along the region where contact with the counterbody sphere occurred, using a 4 mm profilometric sampling length from which roughness parameters were extracted, while complementary 2D profilometry was performed over the same 4 mm trace. All measurements were repeated three times per specimen to ensure statistical accuracy and reproducibility and all reported error bars represent the standard deviation (SD) of these measurements.

Microscopic imaging techniques were employed to visually examine the specimens with an optical microscope (NOVEX RZT-SF, Euromex, Duiven, The Netherlands), focusing on the layer morphology and variations in density induced by the infill percentage. This visual analysis assists in correlating the macro-scale mechanical properties and surface finish with the internal structure and quality of the printed layers.

Shore A hardness measurements were performed on the FFF-printed TPU samples by using a Shore A durometer (Shenzhen Rongbo Jiachuang Technology Co., Ltd., Shenzhen, China). The hardness values were recorded across multiple positions for each infill density. Shore A hardness was assessed through six measurements taken on each specimen, performed at different surface locations to ensure statistical reliability and measurement repeatability, with all reported error bars representing the standard deviation (SD) of these measurements. The final material hardness was reported as the mean of these measurements, typically presented as the mean value followed by an implicit standard deviation.

Wear quantification was primarily conducted through analysis of the resulting wear track dimensions on the TPU sample. Although the TPU specimens did not develop a pronounced or easily visible wear track, the 2D profilometric profiles captured within the contact zone showed localized surface smoothing and shallow depressions, enabling quantitative characterization of the wear-induced deformation. Therefore, wear assessment involved analyzing profile changes such as eventual track depth and profile amplitudes across the wear path, measured using the profilometry equipment. These detailed profile analyses provided essential data for assessing volumetric material removal or eventual track depth development.

3. Results

3.1. Tribological Performance

Analysis of the friction coefficient (μ) revealed a strong dependence on the internal infill density, particularly concerning steady-state friction magnitude and transitional stability (

Table 1. Summary of tribological results (μ).

Sample Designation	Infill [%]	μ 1 at Start	$\overline{\mathit{\mu }}$ 2 ± SD 3	μmax 4
G100	100	0.472	0.465 ± 0.010	0.498
G50	50	0.981	1.002 ± 0.023	1.057
G10	10	0.614	0.504 ± 0.015	0.614

¹ Friction coefficient at start (μ). ² Mean value of friction coefficient ($\overline{\mu }$). ³ SD denotes the standard deviation, computed using $SD=\sqrt{{\displaystyle \frac{1}{n-1}}{\sum }_{i=1}^n{\left(x_i-\overline{x}\right)}^2}$, where in this study $n=3$ measurements were performed per specimen and $\overline{x}$ is the arithmetic mean. ⁴ Maximum value of friction coefficient (μ_max).

).

Across all densities (10%, 50%, and 100%), the tribological tests displayed a classic two-stage frictional behavior. Initially, a brief running-in phase occurred, followed by a relatively stable steady-state friction regime that persisted over the full sliding distance of 300 m. Figure 2 shows that the TPU 82A G10 sample with 10% infill exhibits a stable friction coefficient around 0.5 after the running-in phase, with a slight increase toward the end of the sliding distance.

The G50 (50% infill) specimen exhibited the highest friction level, achieving a mean steady-state $\overline{\mu }$ of 1.002, peaking at 1.057. This high value suggests severe interfacial resistance, primarily attributed to strong adhesion between the TPU and the steel counterbody, facilitated by significant instantaneous plastic deformation that maximized the real contact area. The G50 sample displayed minimal transient behavior after a brief initial drop from 0.981 at the start, as we can observe in Figure 3.

In contradiction, the G100 (100% infill) sample demonstrated the lowest friction, reaching a mean $\overline{\mu }$ of just 0.465. This notably lower friction indicates that the fully dense material structure restricted lateral motion and minimized the area available for viscoelastic adhesion, promoting a smoother shear interface. Figure 4 shows that the TPU 82A G100 sample with 100% infill maintains a lower and more stable friction coefficient of about 0.45–0.48 throughout the sliding distance, indicating improved tribological stability due to the higher infill density.

The G10 (10% infill) sample started with a high μ of 0.614 but quickly stabilized to an intermediate mean value of 0.504. The initial friction drop suggests rapid consolidation or smoothing of the highly compliant surface texture during running-in.

The wear rates recorded for all samples (G100, G50, and G10) were negligibly small; the external surface did not present wear tracks. The absence of a pronounced or measurable wear groove on the TPU surface, together with the shallow surface alterations revealed by profilometry, suggests that wear in these highly elastic FFF-printed TPU samples is dominated by viscoelastic energy dissipation and localized plastic deformation, rather than by conventional abrasive micro-cutting or particle detachment. The low modulus of the polymer (Young modulus estimated at 10,000 MPa) under a high calculated Hertz pressure (231 MPa) ensures that material deformation, not removal, is the primary mechanism for accommodation during sliding. As shown in Figure 5, the friction coefficient decreases with increasing infill density, with the G100 sample exhibiting the lowest and most stable μ values, while the G50 sample shows significantly higher friction throughout the test.

3.2. Surface Characterization

The original FFF printing process imparts a characteristic topography to the specimen surfaces (

Table 2. Summary of surface roughness parameters.

Infill [%]	Rp Avg. 1 ± SD 2 [µm]	Rv Avg. ± SD [µm]	Rz Avg. ± SD [µm]	Rc Avg. ± SD [µm]	Rt Avg. ± SD [µm]	Ra Avg. ± SD [µm]	Rq Avg. ± SD [µm]	Rdc Avg. ± SD [µm]	Rsk Avg. ± SD	Rku Avg. ± SD	Rmr Avg. ± SD [%]
10	4.61 ± 0.45	18.2 ± 0.88	22.8 ± 0.57	13.6 ± 1.7	28.1 ± 1.50	2.74 ± 0.34	4.39 ± 0.29	2.99 ± 0.90	−2.38 ± 0.57	9.44 ± 2.31	0.62 ± 0.43
50	7.64 ± 0.88	27.4 ± 2.94	35.1 ± 2.28	12.8 ± 3.56	49.3 ± 2.70	4.22 ± 0.71	6.82 ± 0.66	5.35 ± 0.98	−1.96 ± 0.29	9.18 ± 0.88	0.83 ± 0.82
100	7.84 ±1.61	34.97 ± 5.17	42.8 ± 6.59	37.1 ± 7.83	48.7 ± 8.16	7.6 ± 1.40	10.56 ± 1.73	8.31 ± 2.79	−2.03 ± 0.17	6.19 ± 0.80	3.56 ± 1.23

¹ Avg. denotes the average of the measured values. ² SD denotes the standard deviation, computed using $SD=\sqrt{{\displaystyle \frac{1}{n-1}}{\sum }_{i=1}^n{\left(x_i-\overline{x}\right)}^2}$, where in this study $n=3$ roughness measurements were performed per specimen and $\overline{x}$ is the arithmetic mean.

), primarily defined by the Archimedean spiral texture of the fused filaments. This texture fundamentally dictates the initial real contact geometry. The roughness values presented in Table 2 are averaged over three measured profiles per specimen.

In this study, surface roughness was measured directly within the wear track, precisely in the region where the TPU surface was in contact with the spherical counterbody. All profilometric traces were recorded over a 4 mm sampling length inside the worn zone, ensuring that the extracted roughness parameters reflect the actual surface condition resulting from sliding.

Comparison of Roughness and Contact Implications

The roughness analysis revealed a counterintuitive trend: the densest sample (G100) exhibited the highest average R_a (∼7.6 μm), approximately three times rougher than the most compliant sample, G10 (∼2.74 μm). This phenomenon likely stems from the interplay between material stiffness and measurement technique. The highly porous G10 sample readily collapses or deforms elastically under the light load of the profilometer stylus, leading to apparent smoothing (lower measured R_a). By contrast, the rigid G100 structure resists local indentation, maintaining the full height variation produced by the FFF process. To assess the local surface deformation generated by sliding contact, 2D profilometric traces were recorded within the wear track for all infill densities, as shown in Figure 6. The parameter Pt reported in each profile denotes the total peak-to-valley height of the measured section, representing the maximum vertical distance between the highest asperity and the deepest valley along the 4 mm trace.

Microscopic analysis (e.g., G10 shown in micrographs) confirms the integrity of the Archimedean structure and demonstrates that surface wear was negligible. The high roughness observed in stiff samples (G100) suggests a limited initial real contact area, potentially concentrating the 5 N normal load onto fewer asperities. However, the high compliance of the TPU material means that even this concentrated stress leads to rapid flattening and local plastic deformation of the polymeric asperities into the valleys, leading to a dynamic increase in the real contact area once sliding commences. Figure 7 shows the Archimedean spiral surface texture of the FFF-printed TPU 82A and the fine scratches observed on the steel counterbody after sliding contact.

The relationship between infill density and surface roughness parameters for TPU 82A is presented in Figure 8.

3.3. Hardness Evaluation

Hardness is a direct measure of material resistance to local plastic deformation, correlated strongly with the bulk material content defined by the infill density.

Table 3. Shore A hardness of TPU 82A samples.

Sample Designation	Infill [%]	Mean Shore A Hardness ± SD 1
G100	100	44.3 ± 1.94
G50	50	75.9 ± 1.80
G10	10	86.3 ± 1.92

¹ SD denotes the standard deviation, computed using $SD=\sqrt{{\displaystyle \frac{1}{n-1}}{\sum }_{i=1}^n{\left(x_i-\overline{x}\right)}^2}$, where in this study $n=6$, hardness measurements were performed per specimen and $\overline{x}$ is the arithmetic mean.

presents the Shore A hardness values as a function of infill density for each sample designation.

The Shore A hardness measurement clearly demonstrated that the mechanical properties of the printed part are dictated not solely by the bulk filament hardness (82A Shore A nominal) but fundamentally by the internal FFF structure. The 100% infill (G100) achieved a mean Shore A hardness of 86.3, closely matching the nominal value for the material, indicating maximum dimensional stability and load-bearing capability. In contrast, the 10% infill (G10) exhibited extremely high compliance, resulting in a significantly lower hardness value of 44.3. Figure 9 shows the variation in Shore A hardness of TPU 82A samples.

This variation confirms that increasing the infill density substantially increases the effective stiffness of the FFF structure, moving the material behavior from a spongy, highly compliant foam (G10) toward a nearly solid elastomer (G100).

Interpretation in Context of Tribological Behavior

Hardness is inversely linked to the size of the real contact area under fixed load; thus, softer materials generally yield larger contact areas.

• The high stiffness of G100 minimized large-scale contact flattening, resulting in low, stable adhesion and low $\overline{\mu }$ (0.456).
• The extreme compliance of G10 allowed the material to accommodate stress through large bulk elastic and viscoelastic deformation. While soft, porous structure limits the capacity for strong adhesion across the entire nominal contact zone, contributing to a moderate $\overline{\mu }$ (0.504) and effective energy dissipation via damping.
• The G50 sample sits at an intermediate stiffness point (75.9 Shore A) where it is rigid enough to transmit the localized Hertzian pressure effectively but still compliant enough to fully plastically yield and flatten the surface asperities within the wear track. This optimal level of deformation maximized the true area of contact (real contact area saturation), leading directly to the observed peak adhesion and highest friction coefficient $\overline{\mu }$ = 1.002).

3.4. Correlation and Mechanism Discussion

The experimental data established a complex, non-linear correlation between infill density, surface roughness, effective hardness, and tribological response (μ).

Key correlation synthesis:

• Hardness → μ: The relationship between increasing bulk hardness (G10–G100) and μ was non-monotonic. Instead of consistently decreasing (as usually seen in softer polymers due to rising real contact area), the μ peaked at the intermediate density (G50).
• Roughness → μ: The nominally roughest surface (G100, high R_a) yielded the lowest friction, whereas the specimen that experienced the highest friction (G50) possessed intermediate roughness. This indicates that compliance (driven by infill structure), rather than initial measured R_a, is the decisive factor governing the effective real contact area under load for this hyper-elastic material.

Proposed Wear and Contact Mechanism

Since volumetric material loss was minimal, the observed friction variations primarily relate to the viscoelastic energy required to deform the TPU as the steel ball traverses the surface, modulated by the resulting adhesive forces.

• G10 sample (energy damping dominance): The low infill provides an internal air-cushioned structure. Under the 231 MPa pressure, the material undergoes substantial elastic deformation well beneath the immediate contact surface. This internal rearrangement and viscoelastic damping effectively dissipate sliding energy, thus limiting the shear stress transfer required for high adhesion, resulting in a lower μ (∼0.504).
• G50 sample (adhesion maximization): The higher stiffness ensures that the contact stress is contained closer to the surface. The 50% infill structure likely prevents the deep bulk relaxation/deformation seen in G10 sample. Simultaneously, the intermediate compliance permits optimal flattening of the FFF surface topography, maximizing molecular adhesion between the TPU and the steel counterbody. This maximization of the adhesive component of friction yields the high μ peak (∼1.002).
• G100 sample (rigidity and reduced contact): The stiff, dense material minimizes both bulk deformation and local asperity flattening. The energy dissipation pathway relies more heavily on localized elastic rebound and limited adhesive shearing due to the smaller, more transient real contact area maintained under sliding conditions, leading to the lowest observed μ (∼0.465).

In summary, the tribological behavior of FFF-printed TPU is governed by a shift in how the material accommodates the applied load: low infill structures dissipate energy mainly through bulk viscoelastic deformation, high infill structures limit deformation through increased rigidity, while the intermediate 50% infill exhibits a balanced mechanical response that promotes localized surface indentation and enhanced real contact area, leading to higher friction.

4. Implications for Sustainable Mobility

The intersection of advanced manufacturing techniques and high-performance, recyclable materials is crucial for realizing the sustainable development goals (SDGs) mandated by future transportation systems, particularly concerning resource efficiency and reduced environmental footprint [1]. The experimental outcomes demonstrate that FFF-printed thermoplastic polyurethane (TPU) specimens, particularly those utilizing internal gyroid architectures, offer a viable and eco-efficient pathway for designing next-generation components for the automotive, railway, and micromobility sectors.

4.1. Promoting Eco-Efficient Design Through FFF-TPU

Fused filament fabrication inherently promotes material and resource efficiency by employing an additive, layer-by-layer deposition process that significantly reduces raw material waste compared to conventional subtractive methods [1,32]. This study specifically leveraged the geometrical freedom of FFF to engineer internal gyroid lattice structures, which directly translate into substantial mass reduction without compromising essential mechanical function [14,32]. By tailoring the infill density, components can be optimized for a superior strength-to-weight ratio. Furthermore, the FFF process facilitates on-demand, decentralized manufacturing, circumventing the intensive resource depletion and complex logistics associated with traditional global supply chains, thereby supporting established Sustainable Development Goals (SDGs) [1,33].

From a material perspective, TPU is recognized for its unique blend of mechanical properties, bridging the gap between traditional rubbers and plastics, alongside its inherent sustainability potential [16]. TPU materials are thermoplastic, allowing them to be recycled, reprocessed, and remolded, facilitating integration into a truly circular economy [1,19,24]. This recyclability, combined with the growing feasibility of synthesizing TPU from environmentally friendly bio-based materials, positions FFF-TPU as a relevant component in the movement toward greener AM feedstocks [9,23].

4.2. Functional Advantages and Replacement Potential

The combination of TPU’s intrinsic properties, high flexibility, exceptional abrasion resistance, and relevant damping characteristics [17], with the geometric control offered by FFF provides a compelling alternative to conventionally manufactured thermoset rubber (such as natural rubber—NR, butadiene rubber—BR, or styrene-butadiene rubber—SBR) or traditional thermoplastic elastomers (TPEs) in dynamic transport applications. The experimental results reveal several functional advantages supporting this transition:

• Tailored tribological performance: This study confirmed that the tribological behavior of FFF-TPU 82A is intrinsically linked to its internal architecture. By modulating the gyroid infill density, the component’s functional surface characteristics can be precisely tuned:
  • The G100 (100% infill) material yielded the lowest and most stable mean friction coefficient ($\overline{\mu }$ ≈ 0.465). This dense, rigid structure minimizes bulk deformation, relying on localized elastic rebound to restrict the real contact area. This low and stable friction makes it ideal for dry sliding elements where energy loss must be minimized, such as damping bushings or guiding components.
  • The G50 (50% infill) material exhibited the highest friction ($\overline{\mu }$ ≈ 1.002) due to its intermediate stiffness, which provides an optimal balance of compliance and rigidity, promoting localized surface indentation and an increased real contact area during sliding. This capacity for stable, high-grip performance can be important for anti-slip components or components requiring high interfacial shear resistance.
• Exceptional wear resistance and surface adaptability: TPU is characterized by its superior wear resistance compared to many traditional rubber formulations (e.g., NR, BR, and SBR) [19]. The minimal volumetric material loss observed under dry sliding conditions for all infill densities, with friction primarily relating to viscoelastic energy dissipation rather than material removal, underscores the intrinsic durability of the FFF-printed TPU. This high durability contributes directly to reduced component maintenance and extended service life, a key element of sustainable mechanical systems [1]. Also, the capacity for local plastic deformation and viscoelastic energy damping exhibited by the FFF-TPU confirms its suitability for applications requiring significant impact energy absorption [22].
• Dimensional control and repeatability: Although FFF introduces characteristic surface roughness, the controlled printing process allows the creation of components with consistent material hardness and defined internal features, a level of geometric complexity and functional tailoring difficult to achieve economically with conventional molding techniques [1,15].

4.3. Practical Applications for Transportation

The demonstrated ability to custom design FFF-TPU components with specific friction, damping, and lightweight properties positions this technology for transformative applications across future mobility platforms:

• Railway and heavy transport: TPU is already critical in the railway industry for enhancing impact resistance, particularly against flying ballast, when used as a coating on carbon-fiber-reinforced polymer (CFRP) laminates [20,21]. FFF-printed TPU 82A, leveraging its robust wear resistance and hardness consistency, is highly suitable for manufacturing damping bushings, vibration isolators, and suspension pads [17]. Additionally, TPU/waste rubber powder blends have been successfully evaluated as durable, waterproof seal layers for high-speed railway subgrades [32].
• Automotive and electric vehicles (EVs): The ability to create lightweight parts using gyroid infill structures directly supports the EV trend towards mass reduction for improved range and energy efficiency. FFF-TPU is proposed for manufacturing specialized flexible couplings and adaptive mounts in EVs. Composites including FFF-printed TPU components can replace conventional stiff metal–rubber bushings in suspension systems, allowing custom tuning of vehicle handling properties and trajectory control with minimal energy loss, as verified by preliminary simulations [17].
• Micromobility and anti-slip systems: In domains such as pedestrian infrastructure and micromobility, the FFF-TPU system can produce highly durable anti-slip components. The high friction coefficient achievable with the G50 infill ($\overline{\mu }$ ≈ 1.002) supports developing highly effective surface-textured composite outsoles for footwear or tire treads, surpassing the performance of leading composite materials on surfaces like ice and demonstrating superior abrasion resistance compared to injection-molded counterparts [34]. This approach facilitates the creation of reliable non-pneumatic tires, replacing traditional environmentally challenging rubber materials entirely [19].

Therefore, the investigation into FFF-printed TPU 82A demonstrates that advanced manufacturing, coupled with intelligent material and structural design (such as the gyroid infill), offers compelling solutions for next-generation mobility components [1]. By enabling the precise tailoring of tribological characteristics, achieving substantial mass savings and promoting material circularity, FFF-TPU technology aligns fundamentally with the mission of future transportation to foster sustainable, innovative materials and fabrication strategies [2,15].

4.4. Engineering Considerations for Sustainable Mobility Systems

The transition toward sustainable mobility necessitates fundamental advancements in materials science, manufacturing efficiency, and component durability to mitigate global environmental impact. Tribology sits at the core of this challenge, given that friction and wear phenomena are responsible for a substantial portion of global energy consumption and material waste, resulting in high maintenance costs and material obsolescence across all transportation sectors. This research, investigating the mechanical and tribological performance of FFF-printed TPU structures, is directly relevant to realizing energy-efficient and resource-minimal mobile systems.

The utilization of FFF technology combined with TPU, specifically Filaflex 82A, addresses several pillars of sustainable transportation. Firstly, FFF is an inherently resource-efficient, low-waste manufacturing method. Secondly, TPU polymers, categorized as thermoplastic polyether-polyurethane elastomers, exhibit exceptional properties mandatory for durable applications, including high elasticity (up to 650% elongation at break), flexibility at low temperatures, and resistance to hydrolysis, solvents, and fuels [35]. Critically, TPU is noted as being suitable for mechanical recycling and reprocessing into new molded articles, aligning perfectly with the closed-loop protocols of a circular economy.

The experimental findings demonstrate how tailoring internal architecture through gyroid infill structures enables performance optimization vital for mobility applications. The significant variation in Shore hardness (from 44.3 to 86.3) and the resulting non-monotonic friction response indicates a highly tunable material system ideal for custom components requiring specific damping or grip characteristics. For real-world applications (e.g., in automotive damping elements, railway bushings, or micromobility grips), the ability to adjust stiffness independently of chemistry minimizes energy dissipation primarily through the viscoelastic deformation component of friction. Additionally, an important finding supporting enhanced durability and reduced maintenance is the absence of measurable wear profile across all tested infill conditions, indicated by a recorded sample wear. This exceptional wear resistance translates directly into a longer service life, reducing material consumption and lowering replacement frequencies, which are key requirements for fleet operational efficiency.

To conclude, the engineered gyroid structures directly contribute to lightweighting initiatives in transportation. By systematically controlling the infill density (e.g., comparing G10 to G100), FFF printing allows the creation of components with minimal mass but sufficient mechanical integrity (hardness) and predictable tribological behavior. This design-for-performance paradigm, facilitated by lattice engineering, supports the rapid prototyping and customized production of resilient, energy-saving parts, accelerating the adoption of new, greener component designs in the automotive, railway, and micromobility sectors, thereby strongly underpinning the requirements for a sustainable, forward-looking transportation infrastructure.

5. Discussion and Future Research Directions

The tribological assessment of FFF-printed thermoplastic polyurethane (TPU 82A) specimens, defined by tailored gyroid infill structures, has elucidated a complex, non-monotonic relationship between internal architecture, effective mechanical properties, and dry sliding performance. This study confirms the critical role of customizable FFF parameters in tuning component function for sustainable mobility applications.

5.1. Critical Synthesis and Contribution to FFF Tribology Literature

Our findings critically reflect upon and, in some aspects, challenge conventional interpretations of polymer tribology and FFF structural mechanics.

5.1.1. Correlation Between Structural Compliance and Friction

The experimental data demonstrated that the mean steady-state coefficient of friction (μ) did not decrease or increase monotonically with infill density but instead exhibited a pronounced peak at the intermediate 50% gyroid infill (G50, $\overline{\mu }$ ≈ 1.002), significantly surpassing the friction of the 100% dense structure (G100, $\overline{\mu }$ ≈ 0.465).

This non-linear result provides an essential insight into the tribo-mechanisms of hyper-elastic FFF elastomers. While existing research on FFF polymers suggests that higher density (closer to 100%) might increase friction due to greater effective contact area and rigidity, our results demonstrate that the intermediate compliance of the G50 sample achieved an optimal balance. This structure was rigid enough to effectively transmit the calculated Hertzian pressure (231 MPa) yet compliant enough to induce optimal plastic yielding and flattening of the FFF-induced asperities, leading to real contact area saturation and maximization of the adhesive friction component. Inversely, the G100 structure, with its high effective stiffness (86.3 Shore A), minimized both bulk deformation and local flattening, resulting in a smaller, more transient real contact area and the lowest observed friction. The G10 sample, characterized by extreme compliance (44.3 Shore A), relied heavily on bulk viscoelastic damping and internal energy dissipation pathways, which also resulted in a moderate, stable friction coefficient ($\overline{\mu }$≈ 0.504). This indicates a definitive switch in the dominant energy dissipation mechanism driven by infill density.

5.1.2. Hardness and Surface Morphology Paradox

This study utilized Shore A hardness measurements to confirm that the mechanical properties of the printed part are fundamentally dictated by the FFF internal structure, not solely the nominal filament hardness (82A), confirming general observations in FFF research.

A particular contribution of this work lies in the critical interpretation of surface roughness data using profilometry. We observed a counterintuitive trend where the stiffest sample (G100) exhibited the highest arithmetic mean roughness (R_a ≈ 7.6 μm), while the most compliant (G10) showed the lowest (R_a ≈ 2.74 μm). This roughness paradox confirms that the measured topography of highly compliant FFF elastomers is strongly influenced by the contact mechanics of the measurement stylus itself, where the compliant G10 structure deforms under light load, leading to an artificially smoothed measurement. This highlights that, for hyper-elastic FFF materials, compliance driven by internal infill geometry, rather than initial measured surface roughness (R_a), is the decisive factor governing the effective real contact area and subsequent tribological response under applied load.

5.1.3. Novelty and Sustainability Implications

The novelty of this work resides in the integrated approach correlating tribological performance, Shore A hardness, and surface profilometry specifically for FFF-printed TPU 82A specimens incorporating the gyroid infill pattern. By identifying the non-linear relationship between gyroid infill density and dry sliding friction stability, we established a reliable pathway for designing lightweight components with precisely tailored frictional characteristics, supporting the development of specialized seals, anti-slip systems, and damping bushings for transport applications. In addition, the profound absence of quantifiable mass loss across all infill densities demonstrates the exceptional intrinsic wear resistance of FFF-TPU 82A under the tested dry conditions, with wear dominated by viscoelastic deformation rather than conventional abrasive material removal. This high durability directly contributes to extended component service life and reduced maintenance, aligning with the core tenets of sustainable mechanical systems.

5.2. Identification of Current Research Gaps

Despite the significant findings, several fundamental gaps remain in the understanding and application of FFF-printed elastomers in tribological systems, particularly those relevant to sustainable mobility:

• Limited understanding of combined geometric and tribological effects: There is an insufficient systemic understanding of the combined effects of complex infill geometries (e.g., gyroid, honeycomb, and lattice structures), FFF-inherent surface topography (anisotropy and spiral texture), and the resulting tribological mechanisms (adhesion vs. deformation vs. damping) in elastomeric FFF materials. Comprehensive models are needed to predict the transition point where bulk viscoelastic damping (G10) gives way to adhesion maximization (G50).
• Lack of long-term wear and fatigue data: The current study utilized a standardized test duration (300 m sliding distance) under stable conditions. For real-world mobility components (such as suspension bushings or seals in railway/automotive applications), data is critically lacking regarding the long-term wear rates, frictional stability, and fatigue performance under more realistic dynamic loads, variable speeds, and elevated temperature conditions, which influence the viscoelastic properties of TPU.
• Insufficient contact mechanics modeling for compliant structures: The application of the initial theoretical Hertzian contact pressure (231 MPa) dramatically changes once the compliant, internally structured material deforms. Current analytical models are inadequate for accurately describing the non-linear contact mechanics of compliant gyroid structures and predicting the true area of contact saturation observed at intermediate infill densities (G50).

5.3. Future Research Directions

Future research should focus on extending these fundamental insights into applied engineering studies, directly supporting transportation and the global drive toward SDGs.

• Future investigations on this topic should incorporate SEM and confocal microscopy analyses to enable high-resolution characterization of wear-track morphology. Such techniques are essential for identifying potential micro-cracking, material transfer, and third-body effects, as well as for verifying deformation features at the micro-scale. Integrating these advanced imaging methods would provide a more comprehensive understanding of the mechanisms governing deformation-dominated wear in FFF-printed TPU components.
• Tribological testing under lubricated or mixed conditions: To increase the applicability of these findings to industrial mobility systems, future work should transition from dry sliding to tribological testing under lubricated or mixed friction regimes (e.g., oil or water-based lubricants). This will help in understanding how the gyroid architecture influences hydrodynamic stability and the formation of films transferred in wet environments.
• Finite element analysis (FEA) of infill deformation: Advanced FEA should be employed to simulate the complex stress distribution and large strain deformation within the gyroid lattice under high localized pressure. Such modeling would accurately predict the correlation between infill percentage, effective stiffness, and the onset of plastic yielding, allowing for optimized structural design that minimizes energy loss through friction.
• Exploring bio-based and recycled TPU filaments: Aligning with the principles of the circular economy, investigations must be initiated into the tribological performance and printability of bio-based or recycled TPU filaments. Assessing how sustainable feedstocks influence mechanical consistency, surface roughness, and friction coefficient is vital for validating FFF-TPU technology as a truly eco-efficient replacement for conventional materials.
• Dynamic mechanical and vibration testing: Given the TPU’s recognized function in vibration isolation and damping in railway and automotive applications, future studies should include Dynamic Mechanical Analysis (DMA) and vibration attenuation testing. Correlating the tailored infill density (and thus the viscoelastic properties) with effective damping ratios under relevant transport frequencies will be important for the adoption of FFF-TPU parts as lightweight structural components.

6. Conclusions

This investigation successfully evaluated the relationship between customizable internal architecture, effective mechanical properties, and dry sliding tribological performance of FFF-printed TPU 82A specimens utilizing a gyroid infill pattern. Analysis of the results demonstrated a complex, non-monotonic correlation between infill density (G10–G100) and the steady-state coefficient of friction (μ), providing important mechanistic insights. The highest infill configuration (G100, 100% density), possessing the greatest effective stiffness (86.3 Shore A) and highest initial surface roughness (R_a ≈ 7.6 μm), yielded the lowest and most stable mean friction coefficient ($\overline{\mu }$ ≈ 0.465) due to structural rigidity minimizing the real contact area. Also, the intermediate G50 sample (75.9 Shore A) exhibited the friction peak ($\overline{\mu }\approx 1.002$), as its intermediate compliance created an optimal balance between deformation and rigidity, promoting localized surface indentation and an increased real contact area during sliding. The most compliant G10 structure (44.3 Shore A) resulted in a moderate, stable friction coefficient ($\overline{\mu }$ ≈ 0.504), with energy dissipation dominated by bulk viscoelastic damping rather than surface adhesion. Significantly, all configurations demonstrated exceptional intrinsic wear resistance under the test conditions, with wear activity dominated by viscoelastic deformation and localized plastic yielding and a profound absence of quantifiable mass loss.

The capacity to precisely tune component function via gyroid infill density offers significant design implications for sustainable mobility components. For lightweighting applications such as damping bushings and guiding components, the G100 configuration is application-ready, providing minimal energy loss through low, stable friction, combined with the material efficiency inherent to FFF. Therefore, the G50 structure delivers stable, high-grip performance suitable for anti-slip components or specialized flexible couplings requiring high interfacial shear resistance. From a sustainability perspective, leveraging FFF’s geometric freedom allows for substantial mass reduction and optimal strength-to-weight ratios. Furthermore, TPU’s thermoplastic nature ensures its potential integration into a circular economy through recyclability and the growing viability of bio-based feedstocks, positioning FFF-TPU as an eco-efficient alternative to conventional thermoset elastomers in automotive and railway systems.

Despite these advances, the current study was limited to dry sliding conditions over a standardized distance (300 m), meaning conclusions regarding long-term durability, frictional stability under thermal effects, and fatigue performance remain pending. Future work must address these gaps by transitioning to tribological testing under lubricated or mixed friction regimes to enhance industrial applicability. Also, the complexity of contact mechanics necessitates the employment of advanced FEM to accurately simulate the stress distribution and large strain deformation within the gyroid lattice, which will define the effective stiffness and onset of plastic yielding. Finally, aligning with resource efficiency goals, investigations into the printability and performance of bio-based or recycled TPU filaments are vital for validating FFF-TPU as a truly sustainable material solution for next-generation transportation systems.