Analysis of Wear Behavior Between Tire Rubber and Silicone Rubber

Featured Application

This study explores a wheel redesign concept based on a detachable tread made from silicone instead of conventional rubber, while keeping the wheel body unchanged. The proposed configuration is intended to increase durability, reduce abrasive wear, and improve the sustainability of tire components under service-relevant friction conditions.

Abstract

Vulcanized NR-SBR is widely used in vehicle components; however, its irreversible crosslinking limits recyclability and contributes to the large number of tires discarded annually worldwide, and in this context, this work presents an experimental comparative assessment of the tribological behavior of conventional tire rubber and silicone VMQ, motivated by a wheel concept based on a detachable tread aimed at improving durability and sustainability rather than proposing an immediate material substitution. Wear and friction behavior were investigated under abrasive and self-friction conditions using pin-on-disk testing with an abrasive counterpart representative of asphalt, supported by optical and scanning electron microscopy. The results show that NR-SBR undergoes severe abrasive and erosive wear, characterized by deep and irregular wear tracks, pronounced fluctuations in the dynamic friction coefficient, and strong sensitivity to load and sliding speed, particularly during the initial stages of track formation. In contrast, VMQ exhibits mild abrasive wear dominated by viscoelastic deformation, leading to shallow and stable wear tracks, lower friction coefficients, and significantly reduced material loss once the contact track is fully developed. These differences are attributed to the distinct mechanical responses of the elastomers, as the higher hardness and limited strain capacity of rubber promote micro-tearing and unstable material removal, while the high elasticity of silicone enables stress redistribution and stable contact conditions under abrasive loading. UV aging increases stiffness of rubber, resulting in reduced wear and friction, while silicone remains largely unaffected after 750 h due to the stability of its Si–O–Si backbone. Self-friction tests further indicate that smooth silicone sliding against rubber yields the lowest friction values, highlighting a favorable material pairing for detachable tread concepts. Factorial design analysis confirms material type as the dominant factor influencing both wear and friction. Overall, for the specific materials and operating conditions investigated, VMQ demonstrates higher durability, greater tribological stability, and improved aging resistance compared to NR-SBR, providing experimental evidence that supports its potential for long-life, more sustainable detachable tread applications.

1. Introduction

Tire rubber is composed of natural rubber (NR, polyisoprene) and synthetic materials such as styrene-butadiene rubber (SBR) blended in specific proportions. NR-SBR, commonly used in vehicle components, constitutes 5.1% of a vehicle’s weight, with 3.6% attributed to wheels. These rubber compounds, primarily used in the tread and sidewalls, provide durability and performance [1]. To enhance these properties, carbon black and silica are often added as reinforcing fillers, improving wear resistance and tensile strength throughout the tire’s lifecycle [2]. Traditionally, metals such as beads and belts strengthened the tire’s structure, but synthetic fibers like rayon, nylon, and polyester now replace them, reducing both cost and weight [3].

Vulcanization processes introduce sulfur to enhance the rubber’s mechanical and wear properties, facilitating the irreversible crosslinking and resistance, which complicates recycling [4,5,6]. Zinc oxide and stearic acid accelerate this process [7]. Additives (stabilizers, antioxidants, antiozonants, extenders, and waxes) optimize the rubber’s resistance to environmental conditions like UV radiation and high temperatures [8]. Recycling tire rubber requires breaking the sulfur bonds formed during vulcanization [5,6].

The global tire industry consumes about 65% of all natural and synthetic rubbers produced [5,9]. Over 1.5 billion end-of-life tires (ELTs) containing approximately 40% vulcanized rubber are discarded annually [5,10]. Around 1 billion tires become unfit for reuse each year, with projections reaching 1.2 billion by 2030 [11,12]. Discarded tires pose significant environmental and health risks, accumulating water that fosters disease-carrying pests and creating fire hazards that release toxic by-products, as demonstrated by incidents such as the Seseña fire in Spain and the Tire King Recycling fire in Canada [13,14,15].

Legislative efforts promote sustainable ELT management, favoring energy and material recovery over landfilling while encouraging supply chain innovation [16]. However, the key challenge remains the establishment of economically viable recycling systems and markets for ELT-derived products.

Tire recycling is hindered by their chemically and mechanically complex structure, designed for durability and abrasion resistance. Early disposal methods, such as incineration and landfilling, proved unsustainable due to space consumption and non-biodegradability. However, new recycling techniques are continuously emerging [17].

The permanence of vulcanization complicates rubber reprocessing, as it is not a thermoplastic [18]. Chemical and mechanical methods (such as grinding) have been explored to blend vulcanized rubber with thermoplastics [19,20] or thermosetting polymers [21,22], with or without compatibilization, as well as with construction materials or pavements [11,18]. Kazemi et al. [23] analyzed the environmental, health, and economic effects of using ELTs for energy recovery (fuel) and asphalt filler.

Currently, the trend is shifting towards devulcanization methods [24], where crosslinks are broken to improve adhesion between devulcanized rubber particles and the matrix. Various devulcanization techniques exist, including thermomechanical, thermochemical, mechanochemical, physical, supercritical CO₂, and biological techniques [25], as well as biotechnological approaches using microorganisms or enzymes [26]. Additionally, microwave- [18] and ultrasound-based [27,28] methods have been developed.

Another approach involves rubbers with reversible double bonds, or dynamic covalent bonds, which break and reform in response to external stimuli [4,29]. These vitrimeric rubbers hold promise for reprocessable and recyclable materials, reducing waste [30]. However, balancing tensile properties, self-healing ability, and recyclability remains a challenge. The incorporation of dynamic covalent bonds, supramolecular interactions, and nanofillers enhances mechanical strength without compromising recyclability [31,32].

Further advances in polymer chemistry are needed to integrate vitrimer networks into elastomers while maintaining compatibility with reinforcements like carbon black, which are essential for tire applications [32]. Shuangjian et al. [33] designed a manufacturing network for vitrimers, enabling extrudable reprocessability with sufficient crosslinking. This structure promotes chain orientation, improving strength, toughness, and stretchability while preserving elasticity. Cantamessa et al. [34] further refined vitrimeric rubbers via the Diels–Alder reaction using a melt-blending process as an eco-friendly alternative to solvent-based synthesis.

As research advances in sustainable alternatives to rubber, silicone-based elastomers are emerging as a promising replacement for vulcanized rubber. This is attributed to hybrid chains (organic and inorganic elements), which crosslink effectively compared to traditional organic rubber with alkane chains. Silicone inherently offers greater resistance to environmental degradation, high and low temperatures, and UV radiation [35]. However, exposure to high temperatures around 180 °C leads to gradual oxidation of silicone rubber molecules [36]. Silicone rubber products exhibit long service life, lasting up to five years at a temperature between 90 and 150 °C [37], or remain largely unchanged after 400 h at 180 °C [38]. Conversely, microwave exposure [39], freezing, thermal aging, ultraviolet aging, and salt spray aging accelerate degradation, thereby shortening the product’s lifespan [37].

The mechanical and electrical properties of silicone rubber can be significantly enhanced by incorporating particles such as carbon fillers [40], nano-silica particles [41], or fibers. For instance, the addition of hexagonal boron nitride or carbon fibers demonstrates excellent heat dissipation performance [42], while nanorods derived from clay and modified with silane coupling agents improve the mechanical and triboelectric properties of silicone rubbers [43]. Additionally, mechanical properties can be tailored using a solvothermal polymerization process, achieving elongation limits exceeding 30 times and a tensile modulus below 0.15 MPa, making it suitable for applications like flexible sensors, artificial electronic skin, and oil collection [44].

Although silicone is a durable material with a lower environmental impact during use—helping to reduce greenhouse gas emissions by about 52 million tons annually in regions like Europe, North America, and Japan—it still presents a challenge at the end of its life [45]. Silicone waste continues to grow, and because it is non-biodegradable, it is usually incinerated or sent to landfills. Incineration, while safer than landfilling, results in resource loss and CO₂ emissions. Since silicone elastomers and heat-resistant types cannot be melted like thermoplastics, the most sustainable solution involves mechanical or chemical recycling through closed-loop systems that depolymerize silicone for reuse in new products [46].

Following an analysis of the challenges associated with tire recycling and the properties of VMQ, the ECOTIRE project proposes the replacement of NR-SBR in the tire casing with VMQ, which offers enhanced durability, while the tread remains composed of NR-SBR [47,48]. The development of an intelligent tire design within this project aims to reduce the environmental impact of NR-SBR waste. Furthermore, VMQ systems facilitate easier recycling, as silicone crosslinking is achieved through tri- and tetrafunctional silanes (room-temperature curing) or peroxides via a free radical mechanism (high-temperature curing), resulting in a material that does not contain sulfur [46].

Within the ECOTIRE framework, the tribological behavior of the two selected materials, NR-SBR and VMQ, is therefore assessed under conditions representative of service contact. Accordingly, the aim of this study is to experimentally compare the wear and friction behavior of both materials using a pin-on-disk configuration with an abrasive pin. The use of an abrasive counterpart enables a closer simulation of road asphalt, where material removal and track formation play a dominant role in tribological response. In addition, self-friction tests were conducted on both materials to evaluate their behavior under direct elastomer–elastomer contact conditions.

The novelty of this work lies in the combined use of an abrasive pin (stone wheel type) and a systematic comparison between conventional tire NR-SBR and VMQ under identical testing conditions. In previous studies, metallic pins or cylinders—such as stainless steel in wet conditions with sand particles—were commonly employed to simulate abrasive wear [49]. Steel balls have also been used to investigate the behavior of silicone rubber and the influence of lubricants [50], while the effect of exfoliated graphite addition on silicone rubber wear was studied using a hardened steel disk (EN-32, HRC 65) and silica sand as an abrasive medium [40,51]. In contrast, the present study focuses on unmodified commercial elastomers and employs an abrasive counterpart representative of asphalt to identify wear regimes, friction stability, and material-dependent tribological mechanisms relevant to detachable tread concepts, without aiming to propose a complete tire design.

2. Materials and Methods

This section outlines the experimental procedures employed to investigate the wear resistance of NR-SBR and VMQ. The tests were designed to comprehensively evaluate the tribological behavior of these materials under abrasive conditions and self-friction, as well as their degradation under UV radiation. The stages outlined below describe the methodological approach followed throughout this research:

(1)

Tribological tests at different sliding distances (SDs), speeds, and loads for both materials, enabling the calculation of wear and the coefficient of friction (μ) as a function of SD;

(2)

Self-friction tests aimed at understanding the influence of surface roughness on wear behavior;

(3)

Analysis of the wear track using optical microscopy, including measurements of wear volume, height, and width;

(4)

Evaluation of the effect of UV radiation on wear performance;

(5)

Characterization of material degradation under UV radiation performed using FTIR (Fourier-transform infrared spectroscopy);

(6)

Assessment of the most influential wear factors for each material (NR-SBR and VMQ) using a factorial design approach.

2.1. Materials

For experimental purposes, an NR-SBR-type material sourced from Eguía Manufacturas de Goma, S.L. (Madrid, Spain) was utilized, with a hardness rating of 70 Shore A. This material demonstrates superior mechanical characteristics ideal for elastic and cushioning applications. Samples measuring 25 mm × 25 mm were obtained by cutting a sheet with a thickness of 5 mm, a width of 1000 mm, and a length of 10 m (reference 0910.001).

The comparative material is a green silicone rubber (referred to in the paper as VMQ), designated TED2021-129604B-100, with a hardness rating of 60 Shore A, supplied by Advantaria (Alcala de Henares, Spain). This VMQ differs from conventional organic rubbers in that it lacks carbon backbone atoms; instead, it consists exclusively of silicon and oxygen atoms, forming a chemically defined polysiloxane chain (VMQ, Vinyl Methylsiloxane).

Abrasive stone wheels made of aluminum oxide were used as counterbodies in the pin-on-disk wear tests, supplied by Ferreteria Unceta (Elgoibar, Spain). Each wheel is bullet-shaped and mounted on a shank, which was adapted as needed to fit the wear tester. Pin roughness was 84 ± 66 μm, with a pin tip diameter of 2.5 ± 0.1 mm (Figure 1A).

The surfaces of the VMQ do not have the same roughness, so a HOMMELWERKE profilometer with a T5E stylus (Tecnimetal, Madrid, Spain) was used, since differences in roughness can influence wear. The roughness of the rubber was Ra = 2.5 ± 0.5 µm, while for the rough side of the silicone, it was Ra = 17.1 ± 1.7 µm, and the smooth side Ra = 1.5 ± 0.2 µm. Therefore, two distinct surfaces of the VMQ were considered: a rough side and a smooth side.

2.2. Wear Experimental Procedure

Before the experiment tests, the pin and NR-SBR or VMQ samples were cleaned with ethanol solvent. Figure 1B shows pin-on-disk tests, which were carried out with a friction wear tester (Microtest, Madrid, Spain). Tests were performed dry, without removing debris during the test. The temperature was kept constant, at approximately 22 °C, since significant differences in temperature affect the tests when dealing with elastomeric polymers [40]. In addition, no lubricant was used; in this case, this represents the most severe condition, since lubricant-like engine oil or dimethyl silicone oil improve wear resistance [50]. All samples were cut from the same bulk NR-SBR or VMQ sheets. Each condition was repeated three times using new specimens.

The parameters considered for the tests were speeds of 120 and 150 rpm, loads of 5 and 10 N, and SDs of 100, 250, 500, and 1000 m. The rotation radius was kept constant at 8 mm. For VMQ, tests were carried out on both sides in order to evaluate the influence of surface roughness.

Wear tracks (Figure 1C) were evaluated by digital microscopic Olympus DSX1000 (Olympus, Barcelona, Spain) with a DSC10-XLOB lens. PRECiV DSX software (version 2.1.1) was employed to generate two- and three-dimensional (2D and 3D) representations of the wear tracks, as well as to obtain their depth, width, and height profiles (Figure 2A). Additionally, it facilitated the calculation of the track volume and the surface area ratio (surface area/projected area) (Figure 2B,C). For each wear track, four images were analyzed to determine geometrical characteristics with higher accuracy.

Wear was measured according to the ASTM G99-05 standard [52] using (Equation (1)). The volume loss was calculated from 3D optical microscope images (Figure 2C) and compared with the volume calculated from the weight loss and the corresponding material density (Equation (2)). The density of the NR-SBR and VMQ was measured using the Archimedes method (Equation (3)), with an alcohol pycnometer (ethanol density: 0.8 g/cm³). Here, M1 is the weight of the empty and dry pycnometer, M2 is the pycnometer with polymer pieces, M3 is the pycnometer with alcohol and polymer, and M4 is the pycnometer with alcohol only. The densities used in Equation (2) were 1.48 ± 0.02 g/cm³ for NR-SBR and 1.30 ± 0.01 g/cm³ for VMQ.

$$W={\displaystyle \frac{Volume Loss}{Load\times Sliding distance}}$$

(1)

$$Volume Loss={\displaystyle \frac{Weight Loss}{Density}}$$

(2)

$$Density={\displaystyle \frac{\left(M2-M1\right)\times alcohol density}{\left(M4-M3\right)+\left(M2-M1\right)}}$$

(3)

To complete the set of materials and test conditions studied, the NR-SBR and VMQ samples were also exposed to UV radiation for 750 h. After this treatment, the irradiated samples were evaluated using the pin-on-disk wear test in order to assess the influence of UV-induced molecular modifications on the wear behavior of both materials. In addition, infrared spectroscopy was performed to characterize the surface changes caused by UV radiation.

2.3. Friction Coefficient (μ)

The dynamic friction coefficient (μ_d) was measured during the wear test by plotting μ against SD. The reported μ_d value corresponds to the steady-state region observed after the initial peak, which reflects the formation of the wear track.

Self-friction refers to the resistance of a material when sliding against itself, especially relevant in soft elastomers like VMQ or NR-SBR. These materials tend to adhere and resist motion due to strong intermolecular forces and their ability to conform to surface asperities, increasing real contact area. Smooth surfaces (low roughness) maximize adhesion and friction, while moderate roughness reduces effective contact and lowers friction. However, excessive roughness can cause mechanical interlocking and raise friction again. For elastomers, an optimal roughness balances reduced adhesion without hindering sliding. The static friction coefficient (μ_s) quantifies this resistance and can be measured following DIN EN ISO 8295:2004-10 standard [53] using a universal testing machine with an additional test fixture consisting of a flat table and a sled of known mass (Microtest, Madrid, Spain). The first peak of the traction force on the sled is used to calculate the μ_s. At least, three pairs of specimens (approx. 80 mm × 200 mm each) were tested.

Moreover, in light of the potential frictional interaction between the VMQ and the NR-SBR induced by the wheel assembly, the frictional behavior of these two materials was systematically evaluated using the same experimental protocol applied in the self-friction assessments.

The μ_s was calculated using Equation (4), f_N is the normal load, and f_R is the friction force at the onset of sliding. Due to surface roughness differences, it is necessary to study self-friction on both sides of the silicone, on the rubber, and between silicone and rubber. Friction is measured under incremental normal loads (11 N, 21 N, and 31 N).

$${\mu }_s={\displaystyle \frac{f_R}{f_N}}$$

(4)

2.4. UV Radiation and Infrared Spectroscopy

The accelerated aging test was performed in a Solarbox 3000E chamber (Neutrek, Guipúzcoa, Spain) equipped with a 2500 W xenon arc lamp, in accordance with UNE-EN ISO 4892-3 [54]. The black standard temperature (BST) was maintained at 65 °C, with an irradiance of 550 W/m². Samples were exposed in the chamber for up to 750 h, which corresponds to approximately 187 days of natural outdoor exposure, based on the equivalence of 20 min in the chamber to two hours of sunlight [55].

Fourier-transform infrared (FTIR) spectra of the NR-SBR samples, before and after UV aging, were acquired using a Bruker Optik GmbH (Ettlingen, Germany) infrared spectrometer equipped with an attenuated total reflection (ATR) accessory in order to assess surface chemical changes. The spectra were recorded on a Bruker Tensor 27 spectrometer with a resolution of 4 cm⁻¹, averaging 32 scans at an incident angle of 45°. For each NR-SBR sample, three spectra were collected to ensure reproducibility.

Wear tracks were analyzed by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDAX) using a Philips X-30 model (Philips Electronic Instruments, Mahwah, NJ, USA) to determine the mechanism of wear [56] and elemental composition.

2.5. Factorial Design

Factorial experimental design is based on the analysis of different factors that may influence the outcome of an experiment. This tool makes it possible to identify the optimal combination of factor levels by testing the appropriate hypotheses for each factor and estimating their effects on the test results, as well as the interactions between factors.

In this study, the factorial design was implemented with four variables: type of material (m), sliding distance (sd), speed (s), and load (l) in the wear test, yielding 16 possible combinations (2⁴). Given the number of variables involved, only two SDs were considered instead of the four used in the initial tests; the two shortest distances were selected, as they exhibited greater wear. Additionally, the two surfaces of the VMQ were not distinguished in this analysis: the rough surface was chosen arbitrarily, since both sides were ultimately observed to behave similarly.

Once the factors were selected and their high and low levels defined, the test plan was developed.

Table 1. Factorial design matrix (2⁴) showing the combinations of material type, SD, speed, and applied load used in the wear and friction analysis.

Combinations	Material	Distance	Speed	Load
1	−1	−1	−1	−1
m	1	−1	−1	−1
sd	−1	1	−1	−1
s	−1	−1	1	−1
l	−1	−1	−1	1
m-sd	1	1	−1	−1
m-s	1	−1	1	−1
m-l	1	−1	−1	1
sd-s	−1	1	1	−1
sd-l	−1	1	−1	1
s-l	−1	−1	1	1
m-sd-s	1	1	1	−1
m-sd-l	1	1	−1	1
sd-s-l	−1	1	1	1
m-s-l	1	−1	1	1
m-sd-s-l	1	1	1	1

presents the test plan, listing all combinations of factors and the corresponding level for each. In this case, the high level (+) corresponds to VMQ, 250 m, 150 rpm, and 10 N, while the low level (−) corresponds to NR-SBR, 100 m, 120 rpm, and 5 N.

From the experimental tests, two response variables were obtained: wear and μ_d. These two responses were used to evaluate the main effects and the interactions between the factors, as well as to study the relationship between wear and friction under the different test conditions. Using Yates’ algorithm [57,58,59], the influence of each factor and interaction was determined.

3. Results

3.1. Wear

Figure 3 shows the wear values calculated from mass loss as a function of SD, where both NR-SBR and VMQ exhibit a decrease in wear with increasing SD. For NR-SBR (Figure 3A), the most pronounced reduction occurs between 100 m and 250 m (from 0.45 to 0.28 mm³/Nm). After 750 h of UV aging, the wear at 500 m decreases substantially (from 0.24 to 0.11 mm³/Nm), likely as a result of increased material stiffness. For VMQ (Figure 3B), wear values are approximately one order of magnitude lower than those of NE-SBR and follow the same decreasing trend. Differences between rough and smooth surfaces are observed at short distances but converge at 500 m and continue decreasing at 1000 m. UV exposure produces no significant change in this material.

Because elastomer deformation can bias volume estimates, wear volume was also calculated using Equation (1), where volume is obtained from digital microscope (Figure 2B).

Table 2. Geometrical parameters obtained from digital microscopy analysis of wear tracks on NR-SBR at different SDs, before and after UV aging. Increasing SD leads to larger and deeper tracks, while UV aging results in reduced track dimensions due to increased material stiffness.

Parameters	Sliding Distance (SD)
	100 m		250 m		500 m		500 m UV 750 h
Width (µm)	2161.19	±603.27	2199.66	±112.60	2245.33	±175.03	2012.41	±45.36
Height (µm)	1630.25	±336.50	2670.30	±144.10	3937.67	±360.41	2218.11	±100.04
Length (µm)	2756.23	±393.94	3460.37	±165.59	4532.99	±398.86	2995.14	±103.46
Angle (°)	38.17	±10.98	50.51	±1.26	60.28	±0.48	47.77	±0.70
Area (mm2)	25.32	±0.47	29.19	±0.63	34.42	±0.71	28.37	±0.46
Surface area (mm2)	41.77	±1.14	49.97	±1.63	71.64	±0.95	47.00	±1.03
Volume (mm3)	30.92	±2.10	47.36	±2.44	84.51	±1.80	40.63	±1.17
Sratio	1.65	±0.02	1.71	±0.03	2.08	±0.02	1.66	±0.03

lists the parameters derived from the wear tracks for NR-SBR.

The 2D track parameters (width, depth, and angle) increase with SD, consistent with increased erosion (Table 2), although not necessarily with higher wear rates, since wear is normalized by SD (Figure 3A). After UV exposure, these parameters decrease, yielding values closer to those measured at 250 m than at 500 m, reflecting the effect of aging.

The 3D parameters (area, surface area, and volume) also increase with SD, although not proportionally. The surface roughness ratios (Sratio) quantifies the difference between surface area and projected area. Surface area represents the actual 3D area of the wear track, whereas projected area corresponds to its planar projection. Consequently, surface area provides a more representative description of track roughness. Table 2 therefore shows larger surface area values than projected area, reflecting increased curvature or roughness induced by abrasion.

For VMQ (

Table 3. Geometrical parameters obtained from digital microscopy analysis of wear tracks on rough VMQ at different SDs, before and after UV aging. All parameters are significantly smaller than those of NR-SBR, confirming a milder wear regime dominated by elastic deformation.

Parameters	Sliding Distance (SD)
	100 m		250 m		500 m		1000 m		500 m UV 750 h
Width (µm)	1200.96	±428.07	1140.91	±227.50	1284.80	±216.07	1479.30	±181.43	1320.91	±164.87
Height (µm)	469.22	±225.81	602.71	±139.52	944.61	±152.53	1214.21	±207.09	1011.31	±158.80
Length (µm)	1291.90	±476.20	1291.23	±261.80	1597.81	±241.93	1916.09	±256.51	1665.01	±214.74
Angle (°)	20.35	±4.15	27.77	±2.39	36.39	±4.06	39.25	±2.98	37.38	±2.67
Area (mm2)	11.06	±1.50	12.65	±0.67	13.35	±0.54	15.89	±0.32	16.04	±0.98
Surface area (mm2)	13.24	±2.00	15.34	±1.15	18.39	±0.92	22.01	±0.90	23.13	±1.82
Volume (mm3)	2.19	±0.89	3.43	±0.71	5.84	±0.74	7.50	±0.93	9.35	±1.30
Sratio	2.08	±0.02	2.08	±0.02	2.08	±0.03	2.16	±0.02	1.44	±0.03

), all wear-track parameters are considerably smaller than those of NR-SBR. Track width, depth, and length are reduced, and the resulting wear grooves are flatter due to smaller angles. Although the 3D parameters are also lower, the Sratio is slightly higher, consistent with more pronounced abrasion lines. UV aging reduces this Sratio. These changes, for both NR-SBR and VMQ, are examined in detail in the Discussion section.

Figure 4 presents the wear calculated from the wear track volume obtained by digital microscopy (Table 2 and Table 3). Although the absolute values are lower than those derived from mass loss, the trend is identical: wear progressively decreases with SD. For NR-SBR (Figure 4A), UV aging leads to a reduction of up to 70%, whereas for VMQ (Figure 4B), the decrease ranges from 51% to 65% depending on surface finish and SD, with a smaller effect after UV exposure.

Both measurement methods therefore confirm the same overall trend, although they differ in magnitude due to elastic deformation and the inherent limitations of each technique. The volumetric method provides a more conservative estimate of wear, whereas mass loss is more sensitive to differences between materials and testing conditions.

Figure 5 shows 3D digital microscope images of the wear tracks. These images illustrate the measured parameters and the high wear levels observed for NR-SBR, as well as their variation with SD. According to the z-axis color scale (Figure 5E), darker blue tones, indicating greater depth, increase from Figure 5A to Figure 5C. From 250 m (Figure 5B) to 500 m (Figure 5C), the track widens in its upper region while becoming narrower and deeper at the lower region. In contrast, the track obtained after UV aging at 500 m (Figure 5D) shows reduced depth, with values between those observed at 100 m and 250 m.

VMQ exhibits shallower and flatter wear tracks, requiring a reduction in the z-axis scale from 4500 mm (Figure 5E) for NR-SBR to 2000 mm (Figure 6F) for VMQ. As SD increases from Figure 6A (100 m) to Figure 6E (1000 m), darker blue tones progressively appear, indicating increased track depth.

Abrasion lines are more clearly observed in the 2D images. Figure 7A (NR-SBR, 100 m) shows abrasion predominantly along the track edges, with a smoother bottom, similar to the wider track observed at 500 m (Figure 7B). In all cases, light-colored particles—likely originating from the pin or some NR-SBR additives—are visible, although adhesion is not observed. For VMQ at 250 m, abrasion lines extend across the track and are accompanied by small VMQ filaments that remain attached. At 1000 m (Figure 7D), these lines resemble cuts in the material and are more frequent than in NR-SBR.

To evaluate the influence of load and sliding speed, SD was kept at 100 m and 250 m, while load (5 and 10 N) and rotational speed (120 and 150 rpm) were varied. Figure 8A summarizes the results obtained for NR-SBR. All comparisons are made relative to the reference condition of 5 N and 120 rpm, which corresponds to the most favorable case. increasing the load leads to higher wear at both distances, with a stronger effect at 250 m (122%) than at 100 m (15%). Sliding speed has negligible effect at 100 m (4%) and increases wear at 250 m (18%). The combined increase in load and speed produces the largest effect, resulting in wear increases of up to 122–136%.

For VMQ (Figure 8B), a different behavior is observed. Increasing the load to 10 N or speed to 150 rpm reduces wear (up to 18–40%), particularly longer at SD. When both load and speed are increased simultaneously, wear increases slightly (6%) at 100 m but decreases by 9% at 250 m.

The contrasting wear behavior between NR-SBR and VMQ remains evident regardless of whether wear is calculated from mass loss or from wear track volume obtained by digital microscopy.

Under these conditions, the digital microscope parameters for NR-SBR also increase. At 100 m with 10 N and 120 rpm (Figure 9A), as well as at 250 m with 10 N and 150 rpm (Figure 9B), the wear track reaches a volume of approximately 82 mm³ and an Sratio of 2, indicating more severe abrasion. In contrast, when the load remains at 5 N and only speed increases, the volume increases by only 5% and Sratio by 3%. The track profile parameters (width, height, length, and angle) at 100 m under higher load and speed resemble those obtained at 500 m under the most favorable conditions (Table 2).

For VMQ at 100 m, 10 N, and 120 rpm (Figure 9C), the track profile parameters are comparable to those measured at 250 m, 5 N, and 120 rpm (

Table 4. Wear-track geometrical parameters for rough VMQ at an SD of 250 m under different combinations of load and sliding speed. Variations in load and speed modify track geometry, although overall wear remains low compared to NR-SBR.

Parameters	Sliding Distance—250 m
	5 N-120 rpm		5 N-150 rpm		10 N-120 rpm		10 N-150 rpm
Width (µm)	1140.91	±227.50	1143.85	±149.53	1366.45	±160.35	1258.79	±139.47
Height (µm)	602.71	±139.52	203.34	±130.09	667.25	±145.89	1368.83	±129.85
Length (µm)	1291.23	±261.80	1160.80	±225.51	1520.66	±230.83	1859.64	±245.83
Angle (°)	27.77	±2.39	10.09	±1.99	26.03	±3.15	47.40	±2.95
Area (mm2)	12.65	±0.67	10.38	±1.45	14.23	±1.60	16.01	±1.39
Surface area (mm2)	15.34	±1.15	10.65	±1.36	17.94	±1.45	26.96	±1.62
Volume (mm3)	3.43	±0.71	0.86	±0.22	5.04	±0.15	12.07	±0.59
Sratio	2.08	±0.02	1.26	±0.01	1.68	±0.03	1.68	±0.05

), although the wear volume is lower (3.06 mm³) and Sratio is 1.32. In general, at 100 m, increasing speed, load, or both reduces the Sratio, a trend also observed at 250 m (Table 4).

At 250 m under maximum load and speed, VMQ exhibits increased track depth, length, and angle, as well as higher surface area and volume (Table 4). However, Sratio decreases relative to the most favorable condition. Although abrasion lines are clearly visible and appear deeper, they are fewer in number, which may contribute to a lower overall surface roughness (Figure 9D).

3.2. Friction Coefficients (μ_d and μ_s)

The plots of μ_d as a function of SD show markedly different behaviors for NR-SBR and VMQ (Figure 10). For NR-SBR (Figure 10A), strong fluctuations in μ_d are observed as a consequence of continuous abrasion wear. Material removal by the pin is not uniform across the entire area, leading to the formation of small steps on the wear track. These irregularities cause intermittent pin displacement, which results in pronounced fluctuations in the force signal recorded by the equipment. As a result, μ_d for NR-SBR varies from values close to zero to up to values of three or higher, as observed at an SD of 500 m (Figure 10).

The μ_d values reported in Figure 11 correspond to average values between the maximum and the minimum values recorded during each test. For the three SDs investigated, μ_d for NR-SBR is equal to or greater than 1 at 100 m and 250 m and reaches values of approximately 1.5 at 500 m (Figure 11A). The effect of UV aging on NR-SBR is clearly reflected in the reduction in μ_d (Figure 11A), which decreases from 1.5 for non-aged NR-SBR to approximately 0.8 after aging.

In contrast, VMQ exhibits a more uniform μ_d along the wear track, as illustrated in Figure 10B, for a sample tested at an SD of 1000 m, with an average μ_d around 0.7 (Figure 11B). Under none of the tested conditions does VMQ reach a μ_d values equal to or higher than 1, indicating a more homogeneous frictional response with few fluctuations. Consistent with the wear results (Figure 4B), UV aging has a negligible effect on the μ_d of VMQ.

Regarding the effect of load, increasing the normal load to 10 N leads to an 18% increase in μ_d for NR-SBR at 100 m of SD and a 44% increase at 250 m (Figure 12A). When the sliding speed is increased to 150 rpm while maintaining the load at 5 N, μ_d decreases by 23% at 100 m and increases by 29% at 250 m. When both load and speed are increased, μ_d rises by 19% at 100 m and by 46% at 500 m (Figure 12A).

For VMQ, the influence of load and speed on μ_d is less systematic (Figure 12B). Increasing load results in a 50% increase at 100 m and a 10% increase at 250 m. Increasing the sliding speed leads to a decrease in μ_d of 33% and 14% at 100 m and 250 m, respectively (Figure 12B). When both load and speed increased simultaneously, μ_d increases by 14% at 100 m but decreases by 49% at 250 m (Figure 12B). In all cases, μ_d remains below 1, with average values ranging between 0.40 and 0.85.

Self-friction (μ_s) was calculated according to Equation (4) (Figure 13). The highest μ_s corresponds to NR-SBR, (μ_s = 1.52). Significant differences are observed between the two surfaces of VMQ,: the smooth surface exhibits a μ_s approximately twice that of the rough surface (0.91 and 0.42, respectively). The lowest μ_s among all tested configurations is obtained for the pairing between the smooth VMQ and NR-SBR, with a value of 0.37. This result is discussed in detail in the following section.

3.3. Factorial Design

To perform the factorial design analysis, the mean values of wear and μ_d obtained for each tested condition (

Table 5. Results of the 2⁴ factorial design showing the influence of material type, SD, speed, and load on wear and µ_d. Material type is the dominant factor affecting both wear and friction.

Combination	Parameters				Answer
	Material (m)	Distance (sd)	Speed (rpm)	Load (N)	μd	Wear
1	Rubber	100	120	5	1.040	0.448
m	Silicone	100	120	5	0.559	0.041
sd	Rubber	250	120	5	0.959	0.281
s	Rubber	100	150	5	0.802	0.461
l	Rubber	100	120	10	1.214	0.509
m-sd	Silicone	250	120	5	0.790	0.281
m-s	Silicone	100	150	5	0.372	0.038
m-l	Silicone	100	120	10	0.840	0.037
sd-s	Rubber	250	150	5	1.233	0.333
sd-l	Rubber	250	120	10	1.377	0.626
s-l	Rubber	100	150	10	1.226	0.576
m-sd-s	Silicone	250	150	5	0.678	0.038
m-sd-l	Silicone	250	120	10	0.796	0.019
sd-s-l	Rubber	250	150	10	1.401	0.665
m-s-l	Silicone	100	150	10	0.635	0.055
m-sd-s-l	Silicone	250	150	10	0.405	0.029
Average					0.895	0.277

) were used. The overall average value of these values was then calculated and employed to determine the percentage contribution of each parameter combination. The combinations of parameters are described in Table 1.

After applying Yates’ algorithm, material type was identified as the factor with the greatest influence on both responses. NR-SBR exhibits the highest values of both μ_d and wear, showing the largest absolute deviation from the overall average in both variables.

For μ_d, the second most relevant effect corresponds to the combination of NR-SBR tested at high sliding speed (150 rpm). In contrast, for wear, the second most influential combination is NR-SBR subjected to a high applied load (10 N).

Since the material is the dominant factor in these experiments, a separate 2³ factorial design was applied to each material. The results indicate that, for NR-SBR, high load (10 N) is the most influential factor, affecting both μ_d and wear.

For VMQ, the factors with the greatest influence on μ_d are sliding speed (150 rpm) and the interaction between SD (250 m) and the applied load (10 N). However, in terms of wear, the applied load (10 N) remains the most influential factor.

4. Discussion

The tribological behavior of NR-SBR and VMQ is governed by intrinsic material properties, degradation mechanisms, and their response to mechanical and environmental conditions. By integrating wear data, μ_d, microscopy observations, and factorial design analysis, a consistent mechanistic interpretation can be established.

4.1. Wear Mechanisms and Material Differences

NR-SBR exhibits significantly higher wear than VMQ at all SDs (Figure 3A and Figure 4A). In NR-SBR, the deep, angular wear tracks observed under digital microscopy (Figure 5 and Figure 7A,B), together with the pronounced fluctuations in μ_d (Figure 10A and Figure 11A), indicate a severe abrasive/erosive wear regime. SEM confirms an abrasive mechanism characterized by abrasion lines along the entire track (Figure 14A), as well as surface deformation caused by material tearing and removal (Figure 14B). These mechanisms amplify both material loss and friction instability.

Furthermore, EDAX confirms the presence of inorganic particles within the wear track, containing silicon, magnesium, calcium, and sodium of varying particles sizes. This observation can be attributed to the incorporation of silicates, which are commonly added to rubber formulations to improve mechanical properties and thermal stability [60].

In contrast, VMQ exhibits significantly lower wear and smoother friction (Figure 3B, Figure 4B, Figure 10B, and Figure 11B). The shallow wear tracks, fine abrasion lines, and absence of large debris fragments indicate a mild abrasive wear (Figure 6 and Figure 7C,D), dominated by viscoelastic deformation rather than material removal, as evidenced by material accumulation at the bottom of the wear tracks (Figure 15A). The presence of VMQ filaments at long SD suggests localized deformation instead of fracture (Figure 15B). These features explain lower wear rates and reduced μ_d observed for VMQ.

The differences between NR-SBR and VMQ can be directly related to their distinct mechanical properties. NR-SBR presents an SHA hardness of 70 compared to 60 for VMQ. An additional key difference is their deformation behavior: at a tensile stress of 2 MPa, NR-SBR exhibits strain of approximately 50–75%, while VMQ reaches strain of 140–180%, as determined from tensile tests on bulk samples.

The lower hardness of VMQ allows for greater deformation during contact, promoting filament formation during the wear test. Moreover, as a more elastic material, VMQ recovers more rapidly once the normal load is released by the pin. Together, these factors contribute to its reduced wear and improved frictional stability.

4.2. Influence of Load and Speed

NR-SBR is highly sensitive to load and to the combined effect of load and sliding speed. Increasing normal load enhances the penetration of abrasive asperities and promotes micro-tearing, explaining the steep increase in wear (Figure 8A) and μ_d (Figure 12A) under severe conditions. The instability of μ_d at long SDs reflects repeated particle formation and removal processes. Jensen et al. [49] reported an exponential increase in wear with sliding speed and a weak dependence on load, attributed to third-body effects. Since no third-body mechanism is present in the current tests, the load dependence observed here follows a different trend.

VMQ exhibits a markedly different response: increasing load or speed often leads to reduced wear (Figure 8B), as the material redistributes stress through viscoelastic deformation rather than fracture. This behavior results in smoother tracks with lower Sratio (Table 4) and reduced roughness values. These contrasting trends highlight the fundamentally different mechanical responses of the two elastomers.

The abrasive pin used in this study also plays a critical role. Unlike the elastomeric samples, the abrasive pin does not undergo measurable wear. According to Sarath et al. [51], increasing load generally leads to a decrease in μ_d and an increase in wear (Figure 12A). However, the VMQ tested in this work shows the opposite behavior: μ_d increases with load (Figure 12B), while wear decreases (Figure 8B). This response can be attributed to the high elasticity of VMQ, which enables efficient stress redistribution under increasing contact pressures.

4.3. Effect of UV Aging

UV exposure significantly affects NR-SBR but has a limited influence on VMQ. For NR-SBR, the pronounced reduction in wear and μ_d at 500 m indicates UV-induced stiffening (Figure 3A and Figure 11A and Table 2). FTIR analysis reveals a decrease in methylene group intensity (2800–3000 cm⁻¹) and an increase in the absorption area between 1500 and 1650 cm⁻¹, associated with vinyl and butadiene and/or carbonyl groups. These changes indicate increased stiffness and reduced elasticity in the material [61], explaining the shallower wear tracks and the lower μ_d observed after aging.

In contrast, VMQ, whose structure is dominated by Si–O–Si bonds, shows minimal sensitivity to UV degradation after 750 h of exposure (Figure 3B and Figure 11B and Table 3). The minor changes in wear and μ_d are consistent with FTIR results, which show only slight variations in CH₃ (methylene) peaks. However, in previous work [61], where UV aging was extended to 1500 h, a substantial reduction or disappearance of methylene peaks was observed, indicating crosslinking reactions and the onset of increased stiffness.

4.4. Self-Friction Behavior

The self-friction results provide important insight into material compatibility. NR-SBR exhibits the highest µ_s among all tested configuration (Figure 13). In contrast, VMQ shows a strong dependence on surface finish, with the smooth side exhibiting more than twice the µ_s of the rough side (Figure 13). This behavior mirrors the trend observed for µ_d as a function of SD (Figure 11B). Notably, the lowest μ_s of all material combinations is obtained between smooth VMQ and NR-SBR, which may be due to limited interfacial adhesion between the two materials. This result identifies a potentially favorable material pairing for applications requiring controlled sliding and reduces wear. Consequently, the proposed smart tire design becomes more viable, as both materials can remain in contact without any mechanical interlocking, contributing to a longer service life.

4.5. Integrated Interpretation with Factorial Design

The factorial design (Table 5) confirms that material type is the dominant factor governing tribological performance. NR-SBR consistently exhibits the highest wear and μ_d values, with normal load emerging as the most influential mechanical parameter. In contrast, VMQ shows weaker interactions between load, speed, and SD, consistent with its elastic response and chemical stability. The factorial design provides a useful framework for interpolating material behavior within the investigated range of loads and speeds.

Overall, these statistical results reinforce the mechanistic interpretation derived from experimental observations: NR-SBR undergoes aggressive wear processes that intensify with increasing load, whereas VMQ dissipates contact stress through viscoelastic deformation in enhanced tribological stability.

5. Conclusions

NR-SBR and VMQ exhibit clearly distinct tribological behaviors under the abrasive and self-friction conditions investigated in this study. Conventional NR-SBR undergoes severe abrasive wear characterized by deep wear tracks, pronounced fluctuations in μ_d, and strong sensitivity to load and sliding speed. In contrast, VMQ shows a mild abrasive wear regime dominated by viscoelastic deformation, resulting in shallow and more stable wear tracks, lower friction levels, and reduced material loss.

These differences are primarily associated with the distinct mechanical responses of the elastomers. The higher hardness and limited strain accommodation of NR-SBR promote micro-tearing and unstable material removal, whereas the high elasticity of VMQ enables stress redistribution at the contact interface and more stable tribological behavior once the wear track is formed.

UV aging further differentiates the materials, as NR-SBR becomes stiffer and exhibits reduced wear and μ_d after exposure, while VMQ remains largely unaffected after 750 h due to the inherent stability of its Si–O–Si backbone.

Self-friction tests additionally show that smooth VMQ sliding against NR-SBR yields the lowest μ_s values, indicating a favorable material pairing for detachable tread concepts.

Finally, the factorial design analysis identifies material type as the dominant factor governing wear and friction behavior under the specific experimental conditions examined. Overall, the results provide a consistent experimental basis for assessing the relative durability, tribological stability, and aging resistance of VMQ compared to conventional tire NR-SBR, supporting its potential as an alternative elastomer for long-life, more sustainable tread configurations, without aiming to extrapolate beyond the investigated materials and operating conditions.