Tribological Evaluation of Brake Materials with Silk and Grewia optiva Natural Fibers

Abstract

The growing demand for sustainable, high-performance composite materials has increased the interest in natural fibers as reinforcements for brake friction materials (BFMs). Silk and Grewia optiva fibers, in particular, have emerged as promising candidates for BFMs due to their good mechanical properties, biodegradability, and availability. To evaluate their potential, friction materials were formulated with 6% Grewia (GF), 6% silk (SF), and a hybrid formulation containing 3% of both fibers (SGF), alongside a reference material reinforced with 6% aramid fiber (AF). These composites were then tested on a braking tribometer using an extended SAE J2522 procedure to assess their performance. The AF formulation showed slightly better wear resistance and the GF formulation showed inferior performance during high-temperature cycles, whereas SF and SGF performed close to the reference formulation (AF) in these sections. In terms of friction stability, SF matched the AF formulation, while GF demonstrated significantly poorer stability. The first high-temperature exposure of the BFMs (Fade 1) served as a critical thermal settlement phase, after which they demonstrated both improved friction stability and repeatable performance characteristics. Finally, this study demonstrates that silk fiber represents a viable, sustainable alternative to aramid in BFMs, exhibiting comparable performance in terms of friction stability and thermal resistance.

1. Introduction

In light-duty vehicles, brakes are the main safety and critical system, since this component is responsible for reducing the vehicle’s speed by dissipating kinetic energy into thermal energy. The disc brake is the most common configuration, composed of a gray cast iron disc and a brake friction material. A brake friction material (BFM) is a composite made of a combination of several ingredients, which can have the role of a binder, filler, friction modifier, or reinforcing fiber. The attention to environmental and human health impact has increased in the last years. Carbon emissions during manufacturing and the future release of wear particulates into the environment are among the main concerns. In this scenario, the demand for sustainable and high-performance composite materials has led to the exploration of natural fibers as reinforcements. Natural ingredients are derived from plants and are naturally renewable [1], being mostly composed of cellulose, hemicellulose, and lignin. Their attractive attributes are their non-toxic nature, low cost, and high specific mechanical characteristics [1,2]. Some natural fibers have already been studied as reinforcements in BFMs like jute [3], palm kernel [4], agave [5], hemp [6], banana peel [7], and bamboo [8]. Jute was reported to improve wear resistance, palm kernel improved friction stability, 5% of agave fiber enhanced fade and wear resistance, hemp fiber reduced the specific wear of friction material samples, banana peel power increased the mechanical properties and kept the friction coefficient similar, and bamboo fiber improved the wear resistance.

Silk and Grewia optiva fibers have gained significant attention due to their unique mechanical properties, biodegradability, and widespread availability. Silk fibers are natural proteins derived primarily from silkworms. They mainly consist of 75% fibroin, 23% sericin, and a combination of wax and mineral salts [9]. Silk fiber stands out from other fibers due to the fact that it is the only natural fiber to exist as a continuous filament, and that the sericin present in its composition works as a binder, which results in high stiffness and good adhesion with polymeric matrices [10,11]. Shah et al. [11] analyzed the mechanical properties of epoxy resin composites reinforced with silk, flax, or glass fiber. The authors reported that the composite with silk fiber exhibited better mechanical properties than the one with flax fiber and was comparable to the one with glass fibers. To the best of the author’s knowledge, while several studies have investigated the mechanical properties of silk fibers, there are no published reports examining their tribological characteristics when used as a reinforcement in brake friction materials.

The Grewia optiva fiber comes from a tree originating from the Himalayas and is classified as a bast fiber, which is situated in the bast region of the plants, around the stem [12]. Indigenous populations have used it for the production of many commodities including ropes, mats, bags, boots, and socks [12,13,14]. Some authors have previously studied GFs in different polymeric matrix composites, reporting improvements in the mechanical properties [12,13,14,15,16]. Regarding the application in BFMs, Nain et al. [17] studied eco-friendly friction materials made of white ark shell powder and GFs. The authors compared them to a commercial friction material through a pin-on-disc test and reported a decrease in the coefficient of friction with the addition of GF, however, some improvements in the friction stability and wear resistance were observed. In a previous work [18], the present research team investigated hybrid friction materials, adding GFs and ceramic fibers at varying concentrations (0.0, 2.5, 5.0, 7.5, and 10 wt%). Results showed that the addition of GFs did not deteriorate the friction and wear properties. It also improved the fade resistance, which was attributed to a greater formation of contact plateaus. On the other hand, a higher content of GFs increased the susceptibility to low frequency brake noise. In the above-mentioned study, it was reported that a combination of 7.5% GF and 2.5% ceramic fibers performed best among all of the tested samples. Although previous studies [17,18] have examined the tribological behavior of BFMs reinforced with Grewia optiva, a comprehensive evaluation under real-braking conditions is lacking. For instance, in [18], the authors only employed two high-temperature (fade) test sections, whereas actual automotive brake systems typically experience numerous high-temperature events throughout their service life. Furthermore, the sensitivity of GF-reinforced friction materials to speed and pressure has only been superficially investigated. As established in the literature [19,20], optimal brake performance requires stable friction coefficients over a wide range of operating conditions including varying speeds, pressures, and temperatures. Consequently, speed and pressure sensitivity have emerged as critical parameters for assessing friction stability, highlighting the need for more comprehensive investigation.

Within this context, the present study investigated two natural reinforcements for BFMs: Grewia optiva fiber (previously studied only under limited conditions) and silk fiber (a novel reinforcement in BFM applications). A rigorous evaluation of temperature, speed, and pressure effects was conducted, addressing some gaps observed in prior research. Four formulations with an identical parent composition (94 wt%) were developed, varying only the 6% fiber content: GF: 6% Grewia optiva, SF: 6% silk fiber, SGF: hybrid (3% GF + 3% silk), and AF: reference (6% aramid fiber). The materials were tested using an extended SAE J2522 braking procedure, incorporating additional high-temperature (fade) sections to assess their influence on friction behavior and friction stability. The friction performance, wear behavior, and the implications of natural fiber incorporation are systematically analyzed and discussed throughout the paper.

2. Materials and Methods

2.1. Raw Ingredients

Apex Phenolics, located in Delhi, India, provided the phenolic resin (JA 10 grade) that was utilized as the binder in the composite fabrication process. Bharat Industries (Delhi, India) provided aid with exfoliated vermiculite, while Sky Minerals (Delhi, India) supplied the barium sulfate and synthetic graphite. Steel fibers were obtained from Sarda Industries in Delhi, India, and alumina fibers from Hindalco Industries in Delhi, India. Murugappa Lapinus Intelligent (Delhi, India) supplied the Lapinus fiber. Aramid fibers were sourced from Yancheng Labon Technical Textile Group Co., Ltd., in Jiangsu Yancheng, China. Local farmers in Dehlwin village (Himachal Pradesh, India) provided the Grewia optiva fibers, while tasar silk fibers were sourced from a local supplier in Bhubaneswar, Orissa, India. Both Grewia optiva and silk fibers were utilized in their untreated form, having been trimmed to fiber lengths ranging from 2 to 6 mm. Surface treatments to enhance the adhesion between the natural fibers and phenolic resin were not performed, as the objective was to maintain the lowest possible level of ingredient processing. This approach not only facilitates integration into industrial production processes, but also minimizes energy and resource consumption, contributing to a lower environmental impact.

2.2. Sample Fabrication

A parent composition (94 wt%) of seven elements was used for the fabrication of four friction material composites, as listed in

Table 1. Content, in percentage of weight, of ingredients used in the developed BFMs.

	Main Ingredients (in Weight%)
	Phenolic Resin	Barite	Graphite	Vermiculite	Alumina	Steel Wool	Lapinus Fiber	Aramid	Silk	Grewia
AF	10	50	6	5	3	10	10	6	--	--
SF								--	6	--
GF								--	--	6
SGF								--	3	3

. The alterations in Grewia optiva, silk, and aramid fibers were used to modify the remaining 6%. Formulation AF had no natural fibers and was used as reference formulation, representing a commercial friction material. Formulations SF and GF possessed only silk and Grewia fiber, respectively, whereas the material SGF was a hybrid formulation, containing 3% of silk fiber and 3% of Grewia optiva fiber. The content of each ingredient was defined based on typical amounts commonly used in automotive friction material formulations [21,22,23,24].

The ingredients were combined one after the other using a high rotation shear mixer (Fabdecon Engineers, Mumbai, India) with a chopper speed of 3000 rev/min. To create a uniform dispersion, the fibrous components were first mixed with phenolic resin for five minutes, and then the powdered materials were added for five more minutes. Next, an iron mold cavity with a steel plate was filled with 80 g of the prepared mixture. After that, the prepared mold was heated for eight minutes at 155 °C under 150 bar of pressure in a compression molding machine (Fabdecon Engineers, Mumbai, India). During compression molding, four intermittent breathing cycles were used to eliminate the volatiles created during the curing process. After being taken out of the mold, the composites were allowed to cool in ambient temperature before being cured for three hours at 160 °C in an oven. All of the BFMs could be categorized as low-metallic based on their formulation. To evaluate the thermal properties of the fabricated materials, thermogravimetric analysis (TGA) was conducted with a TGA Q50 device (TA Instruments, New Castle, DE, USA) under a synthetic air atmosphere with air flow at 90 mL/min and nitrogen at 10 mL/min, between 40 and 650 °C, and at a heating rate of 10 °C/min.

2.3. Set-Up of the Experiments

For the tribological characterization of the composites, pin-shaped samples with a diameter of 14 mm were machined from the prepared composites. Gray cast iron, which is the most widely used material in brake system rotors, was used in the experiments as the counterface material (brake disc). Figure 1 shows an image of the test ring used, a braking tribometer. This test rig is able to replicate real braking conditions [25]. The adopted braking protocol was SAE J2522 (AK Master), which was performed four times for each formulation. Out of 274 total braking applications, this “standard AK Master” test protocol accounted for a speed and pressure sensitivity (SPS) section from braking 101 to 146 and two fade sections starting at braking 166 and 242, respectively. However, the fourth repetition of the AK Master was performed with additional sections to thoroughly examine the material’s friction behavior and friction stability with multiple temperature and speed cycles. This so-called “extended AK Master” consisted of the standard one with the addition of two extra fade sections, each one followed by a recovery section, and also followed by a SPS section, resulting in 380 braking applications on the extended test protocol. Details of the operating conditions of each section of the AK Master can be found in [26]. The coefficient of friction (µ) measured in the tribometer is given by Equation (1):

$$\mathrm{\mu }={\displaystyle \frac{\mathrm{T}}{{\mathrm{F}}_{\mathrm{N}}\mathrm{R}}}$$

(1)

where T corresponds to the braking torque measured in the machine’s rotating system (in Nm), F_N represents the normal load (force) exerted by the sample of BFM on the disc (in N), and R is the friction track radius (in m). As presented in [25], the precision of the braking tribometer is ±0.013 in terms of the coefficient of friction.

Friction stability is essential for braking systems, as it ensures consistent performance regardless of changes in temperature, speed, or pressure. This consistency is crucial for vehicle control and directly impacts the safety of the driver, passengers, and surrounding traffic. Hence, some metrics were employed to quantify the results of friction. The metrics presented in Equation (2) and Equation (3) were used to evaluate the friction stability performance of the materials in the fade and recovery sections, respectively:

$$\%\mathrm{F}\mathrm{a}\mathrm{d}\mathrm{e} \mathrm{r}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\left(\mathrm{F}\mathrm{R}\right)={\displaystyle \frac{\mathrm{F}\mathrm{a}\mathrm{d}\mathrm{e} \mathrm{\mu }}{\mathrm{P}\mathrm{e}\mathrm{r}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \mathrm{\mu }}}\times 100$$

(2)

$$\%\mathrm{R}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y}={\displaystyle \frac{\mathrm{R}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{y} \mathrm{\mu }}{\mathrm{P}\mathrm{e}\mathrm{r}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e} \mathrm{\mu }}}\times 100$$

(3)

where the lowest coefficient of friction (CoF) measured during the fade section was the Fade µ, the mean friction of the recovery and characteristic value sections was the Performance µ, and the mean CoF of the recovery section was Recovery µ. These metrics have been commonly used in several studies [27,28,29] to evaluate the brake friction materials’ sensitivity to temperature. Higher values for both parameters, %Fade resistance and %Recovery, indicate superior stability performance since these metrics are relative to Performance µ. Higher values of %Fade indicate that the BFM was less affected by thermal degradation compared with its Performance µ. Meanwhile, higher %Recovery values are expected if the BFM is less affected by thermal degradation (i.e., higher %Fade) or if the material exhibits a quick recovery to its Performance µ.

Besides temperature, the developed formulations must demonstrate stable friction behavior under varying speed and pressure conditions. This stability is quantitatively assessed through two key metrics: pressure sensitivity (ΔμPS) and speed sensitivity (ΔμSS), obtained from the speed and pressure sensitivity (SPS) sections of the AK Master test procedure. The SPS sections consist of five blocks (b1 to b5) of braking applications in increasing steps of speed: 40–5 km/h, 80–40 km/h, 120–80 km/h, 160–130 km/h, and 200–170 km/h. In each block, eight braking applications with varying pressure between 10 bar and 80 bar) are performed. These parameters can be calculated using Equations (4) and (5), respectively.

$$∆\mathrm{\mu }\mathrm{P}\mathrm{S}={\displaystyle \frac{1}{5}} \sum _{b=1}^5{\displaystyle \frac{{{\mathrm{\mu }}_{min}}_b}{{{\mathrm{\mu }}_{max}}_b}}$$

(4)

where ∆µPS is the sensitivity to pressure, and ${\mathrm{\mu }}_{min}$ and ${\mathrm{\mu }}_{max}$ are the lowest CoF and the highest CoF, respectively, in each of the 5 blocks ($b$) of pressure-varying steps of the AK Master test procedure. The value of ∆µPS ranges from 0 to 1, and the closer it is to 1, the less sensitive the material is to pressure variations. In other words, a value approaching unity (1) indicates a lower pressure sensitivity (i.e., a very stable composite material).

$$∆\mathrm{\mu }\mathrm{S}\mathrm{S}={\displaystyle \frac{min(\overline{{\mathrm{\mu }}_1},\overline{{\mathrm{\mu }}_2},\overline{{\mathrm{\mu }}_3},\overline{{\mathrm{\mu }}_4},\overline{{\mathrm{\mu }}_5})}{max (\overline{{\mathrm{\mu }}_1},\overline{{\mathrm{\mu }}_2},\overline{{\mathrm{\mu }}_3},\overline{{\mathrm{\mu }}_4},\overline{{\mathrm{\mu }}_5})}}$$

(5)

where ∆µSS is the sensitivity to speed; $\overline{{\mathrm{\mu }}_1}$, $\overline{{\mathrm{\mu }}_2}$, $\overline{{\mathrm{\mu }}_3}$, $\overline{{\mathrm{\mu }}_4}$, $\overline{{\mathrm{\mu }}_5}$ are the average CoF calculated from eight consecutive brake applications with varying pressure at each of the five speed blocks in the SPS section; $min$ and $max$ respectively represent the lowest and the highest average CoF determined among $\overline{{\mathrm{\mu }}_1}$ to $\overline{{\mathrm{\mu }}_5}$. The value of ∆µSS varies from 0 to 1. The closer it is to 1, the less sensitive the material is to speed variations. In other words, a value approaching unity (1) indicates a composite material with very stable friction under different speeds.

Wear of the BFMs was measured through mass loss by subtracting the mass of each sample of BFM before and after the following sections of the AK Master procedure: block 1 (green, burnish, speed and pressure sensitivity, cold and characteristic value), fades, recoveries, speed, and pressure sensitivity. An electronic balance with a precision of ±0.1 mg was used for weighing the specimens. Specific wear rate (cm³/N-m) of the friction materials was calculated according to Equation (6):

$$\mathrm{S}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c} \mathrm{w}\mathrm{e}\mathrm{a}\mathrm{r} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}={\displaystyle \frac{∆\mathrm{W}}{\mathrm{F}·\mathrm{S}}}$$

(6)

where ∆W is the mass loss (g), F is the friction force (N), and S the sliding distance (m). Wherever possible, an analysis of variance (ANOVA) was performed to check the significance of the results. Only significant differences (C.I. = 95%) between the results are reported in this paper.

3. Results and Discussions

3.1. Thermogravimetric Analysis (TGA)

The results of the TGA and DTG performed in an air atmosphere for all samples are shown in Figure 2. This procedure was performed up to 650 °C, which is slightly above the limit of 550 °C described in the braking standard SAE J2522. All BFMs presented a decrease in mass around 300 °C, which was linked to the degradation of the phenolic resin [30]. Materials with natural fibers (GFs, SFs, and SGFs) exhibited a peak in mass loss around 450 °C, as can be seen in Figure 2b. The higher degradation in this range was associated with the breakage of the cellulose and hemicellulose fibers in the GFs and SFs. For the material with only aramid fibers (AFs), the degradation peak was higher (around 560 °C) in the range where aramid fibers commonly decompose [31]. Considering the whole range of temperature, BFMs with natural fibers demonstrated a slightly enhanced thermal stability compared with the AF formulation, retaining over 85% of their initial mass at 650 °C (the AF samples retained approximately 81%). This suggests that the GFs and SFs exhibit a synergistic interaction with the other components in formulation, enhancing their thermal stability and making them more resistant to decomposition than aramid fibers under high-temperature conditions.

3.2. Standard AK Master Results

Figure 3 presents the specific wear rate of all BFMs measured at each section (stage) of the AK Master procedure. The initial sections conducted before the first high-temperature brakings (fade 1) were collectively labeled as “Block 1”. The recovery sections were abbreviated as “Reco”, while the temperature and pressure sensitivity section was denoted as “TPS”. It is possible to observe higher wear at the sections performed under high temperatures (i.e., Fade 1, TPS, and Fade 2). This suggests that temperature significantly influences the wear behavior. Comparing the specific wear rates of the BFMs across different sections, the ANOVA results indicated no statistical differences among the formulations. This can be attributed to the high variability in the results, which is an expected characteristic of standard procedures involving consecutive changes in operating conditions (e.g., pressure, speed, and temperature) between sections. The ANOVA analysis was performed comparing two formulations at time, for each section of the procedure.

Figure 4 presents the cumulative specific wear rate measured over the entire AK Master procedure. The single factor ANOVA results indicate that all natural fiber-reinforced formulations (SF, GF, and SGF) exhibited statistically similar specific wear rates. A significant difference was only observed between SF and AF, where the latter (AF) proved to have to lower wear than the formulation with silk fiber. The ANOVA results performed among SGF, GF, and AF were statistically inconclusive (confidence interval: 95%), with insufficient evidence to differentiate them in terms of specific wear rate.

Figure 5 illustrates the average CoF for both high-temperature braking sections (fades and TPS) and their respective recoveries. All BFMs exhibited similar trends during braking. The GF formulation exhibited the most pronounced fluctuations in CoF, reaching the lowest and the highest values during high-temperature sections (fades and TPS) and the lowest during recoveries. The level of friction in the recoveries ranged between 0.3 and 0.5, which can be considered as acceptable. Another interesting result is that Fade 1 differed from Fade 2, despite identical operating conditions. This can be attributed to the thermal degradation of phenolic resin [26,30,32] and natural fibers, this latter evidenced by TGA in Section 3.1. The first fade (Fade 1) marks the first braking application in the AK Master procedure where the braking temperature exceeded 200 °C, ultimately reaching 550 °C. Thus, this degradation triggered a sharp decline in CoF around 300 °C, followed by stabilization beyond 400 °C. In contrast, Fade 2 showed a brief increase in CoF before decreasing after 450 °C. This phenomenon may have resulted from variations in the operating parameters (hydraulic pressure and sliding speed) from the previous section (recovery), which induced transient morphological changes in the contact materials, either through the alteration in the contact plateaus on the brake composite or in the tribofilm on the disc surface.

Figure 6 exhibits the results of fade resistance, according to the metric previously described in Equation (2). BFMs exhibited more pronounced differences in fade resistance (FR) in Fade 1, with GF showing the lowest FR, while silk fiber formulations (SF/SGF) performed closer to the reference material (AF formulation). This trend persisted in Fade 2 but at higher FR magnitudes. The significantly lower FR observed at Fade 1 compared with Fade 2 suggests that thermal settling occurs after the composites are subjected to Fade 1. This difference in FR between Fade 1 and Fade 2 motivated further testing to determine whether Fade 1 could be considered as a thermal settling, indeed, since the AK Master standard limits the procedure to only two fade sections. The results of the additional sections are seen in Section 3.3.

Figure 7 presents the results of %Recovery, determined according to the metric previously described in Equation (3). Figure 7 shows that GF exhibited the lowest friction recovery among the BFMs. This is in agreement with the lower fade resistance presented previously, which indicates that the addition of Grewia optiva led to higher thermal damage on the composite material. Considering 85% recovery as a threshold criterion, the GF formulation failed and all other BFMs passed. Interestingly, SF demonstrated better recovery performance (higher %Recovery) as the test proceeded. This suggests that the high-temperature sections employed prior to each recovery progressively enhanced its stability.

3.3. Results of the Extended AK Master

Motivated by the distinct behavior observed in Fade 1 and Fade 2, two additional fade sections were added (Fade 3 and Fade 4). Furthermore, in order to check the friction stability of the BFMs, an additional speed and pressure sensitivity (SPS) was included in the analysis. Figure 8 shows the average coefficient of friction (CoF) results for each temperature across the four fade sections—the first two from the AK Master procedure and the last two from the additional sections. At Fade 1, GF exhibited the sharpest drop in CoF followed by the highest increase in CoF. This higher oscillation can be linked to the weak thermal resistance of the Grewia optiva fiber, which degrades and creates instability in the interface. SF and SGF showed similar behavior, but on a smaller scale. In this scenario, the combination of Grewia optiva and silk (SGF) worked better, since the higher friction stability of silk fiber was transmitted to the hybrid material. Meanwhile, AF exhibited a slower but more stable decline in CoF, demonstrating the best friction performance at Fade 1. During Fade 2, all BFMs displayed more stable friction behavior, with AF performing slightly better. Fade 3 and Fade 4 exhibited similar trends to Fade 2, suggesting that Fade 1 acts as a thermal settlement phase for the composite materials. After Fade 1, the friction behavior became more consistent and repeatable.

Figure 9 presents the fade resistance (see Equation (2)) for all fade sections including the two additional fade cycles. Since Fade 3 and Fade 4 were conducted only once per formulation, no standard deviation bars are shown for these sections. AF demonstrated superior performance in Fade 1. In contrast, GF exhibited poor performance across all fade cycles, indicating that Grewia optiva fiber is unsuitable for high-temperature applications. Moreover, the results confirm that Fade 1 serves as a thermal settlement indeed, as the fade resistance stabilized and became more repeatable afterward. This suggests that only the Fade 2 results reliably characterized the high-temperature friction behavior of the BFMs. A possible explanation for the thermal settlement is the combined effect of phenolic resin degradation and the formation and stabilization of a transfer layer on both contact surfaces, the disc and the friction material. Elevated temperatures promote increased wear, resulting in the greater generation of wear debris. These debris particles soften under heat, and through the sliding action during braking, become compacted. This compaction leads to the formation of secondary plateaus on the sample surface and a tribofilm on the disc, contributing to the stabilization of the friction behavior.

Figure 10 presents the speed and pressure sensitivity (SPS) results before Fade 1 and after Fade 4. Conducting SPS tests at these two stages allowed for an evaluation of the BFMs’ stability and whether thermal settling occurred after Fade 1. Before Fade 1, all BFMs exhibited a similar trend in friction: a decrease in CoF with rising pressure and a slight increase in friction at higher speed. This friction–pressure relationship has also been reported in previous studies [4,26,33]. Furthermore, all BFMs showed a friction variation of approximately 0.2–0.3, with GF demonstrating the poorest stability (the highest friction variation) in the SPS performed before Fade 1. On the other hand, the results of the SPS performed after Fade 4 were quite different. In this case, the variation in the mean values of CoF throughout the braking operations was lower than 0.2. In other words, the BFMs exhibited a more stable friction behavior in the SPS performed after all high-temperature cycles. The decrease in CoF with rising pressure was more evident at a braking speed of 120–80 km/h and higher, although the GF formulation remained with an erratic (not clear) friction behavior.

Pressure sensitivity (Equation (4)) and speed sensitivity (Equation (5)) were used to quantify the friction stability (CoF variation) under varying operating conditions. This was performed to investigate the thermal settlement’s influence on the friction stability of the BFMs. These metrics, plotted in Figure 11, represent stability inversely: values closer to unity (1) indicate lower sensitivity to pressure or speed changes, corresponding to better friction stability.

Regarding pressure sensitivity (Figure 11a), it was possible to see an enhanced performance for all BFMs when tested after Fade 1 (indicated in red). The highest improvement was for the GF formulation, which increased 42.2% its pressure sensitivity after Fade 1. The sensitivity to speed (Figure 11b) showed smaller changes in the magnitude compared with the sensitivity to pressure. Materials SF, SGF, and AF exhibited a small improvement in sensitivity to speed, whereas GF was the only one to decrease its value in this metric (the poorest performance). The SF formulation demonstrated stability comparable to the reference AF material in both sensitivity metrics. Notably, silk’s continuous filament structure and sericin binder enhanced interface adhesion, mitigating GF’s drawbacks in SGF.

Table 2. Improvement in friction performance observed when replacing aramid fibers with the proposed natural fibers, expressed in terms of percentage.

	SF	SGF	GF
Fade 1	−29.6	−23.6	−23.6
Fade 2	−3.15	−6.2	−16.8
Fade 3	−20.9	−5.6	−28.3
Fade 4	3.13	7.5	−28.9
PS1	−4.2	−0.7	−8.5
SS1	4.3	1.9	4.2
PS2	−1.1	−0.7	11.4
SS2	1.4	−4.4	−20.5

summarizes the difference in friction performance when aramid fibers were replaced by Grewia optiva (GF) or silk fibers (SFs), highlighting the friction coefficient stability under temperature change (fade sections), pressure (PS), and speed (SS). PS1 and SS1 refer to the pressure and speed sensitivity before high-temperature braking (Fade 1), and PS2 and SS2 refer to the same measurements after this phase. Overall, the incorporation of natural fibers led to a decrease in fade resistance, especially in the formulation with only Grewia optiva. However, the hybrid formulation (SGF) combining silk and Grewia optiva showed the smallest drop in fade performance among all samples, with a maximum deterioration of approximately 6%, indicating a more stable behavior under high-temperature conditions.

Concerning the pressure sensitivity and speed, the natural fiber-based formulations experienced both slight improvements and deteriorations under all of the conditions tested. The hybrid formulation remained the most stable, with very little fluctuation between PS1, PS2, and SS1, and a moderate decline in SS2 alone. In contrast, the GF formulation registered a sharp rise in pressure sensitivity after Fade 1 (PS2 = +11.4) and an abrupt drop in speed stability (SS2 = −20.5). In general, replacement of the aramid fibers by natural fibers reduced the fade resistance but led to small changes in PS and SS. Practically, minor variations in PS and SS are usually acceptable in industrial brake systems. Major post fade instabilities like those in the GF formulation may compromise the safety and reliability of the braking response under real driving conditions.

4. Conclusions

The present study systematically evaluated the tribological performance of brake friction materials (BFMs) reinforced with natural fibers (Grewia optiva and silk) under an extended SAE J2522 braking procedure. The main conclusions drawn from the results are listed below:

• The specific wear rate measured for the whole AK Master procedure revealed that natural fiber-reinforced formulations (SF, GF, and SGF) exhibited statistically similar wear resistance. The aramid-fiber reference material (AF) demonstrated significantly lower wear compared with the silk fiber formulation (SF).
• Grewia optiva fiber (GF) showed inferior fade resistance, with significant CoF fluctuations during high-temperature sections (fades and TPS). SF and SGF (3% GF + 3% silk) performed close to the reference formulation (AF) in these sections.
• All brake friction materials (BFMs) met the 85% recovery threshold criterion, with the exception of the Grewia optiva fiber (GF) formulation, which failed to achieve adequate friction recovery after high-temperature braking sections.
• The analysis of friction stability through speed and pressure sensitivity (SPS) revealed that SF achieved stability comparable to the reference AF formulation, while GF demonstrated significantly poorer performance across these metrics.
• Fade 1 acted as a critical thermal settlement phase, after which the BFMs improved their friction stability. Furthermore, subsequent fade sections (Fades 2–4) yielded repeatable results, suggesting that Fade 1 indeed serves as thermal settlement.

To conclude, this study contributes to the development of more sustainable alternatives for friction materials by demonstrating the technical feasibility of partially replacing aramid fibers with natural fibers such as silk and Grewia optiva. The results revealed that, although the use of natural fibers led to a modest decline in fade performance, the hybrid formulation combining both natural fibers showed a balanced behavior with minimal sensitivity to pressure and speed variations. In addition, the effect of the thermal settlement on the tribological behavior is relevant for the automotive industry, as they provide insights into how high-temperature braking influences the tribological stability of materials, which can guide decisions on formulation and processing strategies. The proposed ingredients are not yet ready for commercial application. Further testing, including mechanical strength, wear resistance, and noise generation, is necessary, especially in a more realistic formulation. A comprehensive life cycle assessment (LCA) will also valuable to validate the environmental benefits of these natural fibers and ensure that the sustainability gains are not offset by other stages of the production chain.