Tribological investigations of hemp reinforced NAO brake friction polymer composites with varying percentage of resin loading

TYPE Original Research PUBLISHED 06 February 2024 DOI 10.3389/fmats.2024.1348265 OPEN ACCESS EDITED BY Natrayan L, Saveetha University, India REVIEWED BY Jitendra Kumar Katiyar, SRM Institute of Science and Technology, India Sathish Kumar Palaniappan, King Mongkut’s University of Technology North Bangkok, Thailand Felix Sahayaraj A, Kalaignarkarunanidhi Institute of Technology (KIT), India Chandrasekhara Sastry C, Indian Institute of Information Technology Design and Manufacturing, India *CORRESPONDENCE Mithul Naidu, [email protected] RECEIVED 02 December 2023 ACCEPTED 11 January 2024 PUBLISHED 06 February 2024 CITATION Naidu M, Bhosale A, Gaikwad M, Salunkhe S, Čep R and Abouel Nasr E (2024), Tribological investigations of hemp reinforced NAO brake friction polymer composites with varying percentage of resin loading. Front. Mater. 11:1348265. doi: 10.3389/fmats.2024.1348265 COPYRIGHT © 2024 Naidu, Bhosale, Gaikwad, Salunkhe, Čep and Abouel Nasr. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Tribological investigations of hemp reinforced NAO brake friction polymer composites with varying percentage of resin loading Mithul Naidu1*, Ajit Bhosale2, Mahesh Gaikwad3, Sachin Salunkhe4, Robert Čep5and Emad Abouel Nasr6 1Department of Mechanical Engineering, Shrimati Kashibai Navale College of Engineering, Savitribai Phule Pune University, Pune, India, 2Department of Mechanical Engineering, MKSSS’s Cummins College of Engineering for Women, Pune, India, 3Department of Mechanical Engineering, JSPM’S Jayawantrao Sawant College of Engineering, Pune, India, 4Department of Mechanical Engineering, Gazi University, Ankara, Turkey, 5Department of Machining, Assembly and Engineering Metrology, Faculty of Mechanical Engineering, VSB-Technical University of Ostrava, Ostrava, Czechia, 6Department of Industrial Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia NAO brake friction materials with 4%, 5%, and 6% (w/v) sodium hydroxide treated hemp fiber reinforcement having 25% wt. fiber loading and fixed percentage of phenol formaldehyde resin content (20% wt.) along with other fillers have been studied and reported by the authors earlier. However, the effect of variations in the resin content on the tribological performance has been studied and reported in the present paper. Five variants were prepared with varying percentages of phenol formaldehyde resin from 12% wt. to 22% wt. with incremental steps of 2% wt, along with the optimum of 6% (w/v) sodium hydroxide treated hemp fibers and other fillers. The prepared test variants’ tribological characterization was done using Taguchi’s L25 orthogonal array on a pin-on-disc experimental setup, as per ASTM G99, at room temperature and compared with the best of the earlier studied friction composite. Fade and recovery tests of the best of the earlier studied and present ones were performed on a chase tribology tester per SAE J661 standards. The results revealed moderate coefficient of friction of 0.4496, lower wear rate of 0.57gm, and better fade recovery for the HF25P20 variant compared to its counterparts studied here. KEYWORDS hemp fibers, phenol formaldehyde, fade-recovery, anova, Taguchi, Scanning electron microscopy 1 Introduction Brake pads are part of the brake system components along with the master cylinder, wheel cylinder, and hydraulic control system. Brake pads have attracted much research interest due to their nature and impact on the environment (Naiduetal., 2023). Binders, friction modifiers, fillers, and reinforcements are four categories of materials used in the manufacture of brake pads (Aranganathan and Bijwe, 2016;IbukunOlabisi, 2016; Mutlu, 2009). Biomass from agricultural activities like vegetables, animal excrement, Frontiers in Materials 01 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 agricultural residue, leaves, stems, fruits, seeds, grasses, and reeds are trending materials for producing commercially accepted brake pads that are environmentally friendly. Palm kernel shells, bamboo, corn stalks, sugar cane bagasse, banana, cashew nut shell, coir (coconut shell), rice straw, pineapple, rice husks, hemp, etc. form the reinforcing agents. Friction modifiers or fillers in natural fibre friction materials (Joshietal., 2023). The present authors prepared and investigated the properties of brake friction material with 4,5% and 6% (w/v) NaOH-treated hemp fiber reinforcements having 25% wt. fiber loading and with 20% wt. of phenol formaldehyde resin content, along with other fillers, in order to understand the effect of the variation in the concentration of sodium hydroxide (NaOH) on the tribological performance of 20% wt. phenol formaldehyde binder contained friction material (Naiduetal., 2023). The results revealed better properties for the friction material with 6% (w/v) NaOH-treated hemp fibers. However, to further understand the influence of gradual increment of the resin content between 10% and 20% wt., wherein results with 10% wt. were already available with the authors, the present paper focuses on studying the effect of phenol formaldehyde resin content variation on the 6% (w/v) NaOHtreated hemp fibers reinforced friction composite with similar filler contents. Five variants were prepared with varying percentages of phenol formaldehyde resin from 12% wt. to 22% wt. with incremental steps of 2% wt and 6% (w/v) NaOH-treated hemp fibers and other fillers. Tribological characterization of properties, namely, Specific Wear Rate (SWR) and Coefficient of Friction (COF) of the prepared test variants was done using a pin-on-disc experimental setup, as per ASTM G99, at room temperature and compared with the best of the earlier studied friction composite, namely, HF25P20 (earlier named HF6P20) having SWR of 3.5417 × 10−5mm3/Nm and COF of 0.4496. Fade and recovery tests of the best of the earlier studied friction composite, namely, HF25P20 and the present one, were performed on a chase tribology tester as per SAE J661 standards. The results revealed moderate COF (within acceptable standard limits), lower wear rate, and better fade recovery for the HF25P20 variant compared to its counterparts studied here. 2 Materials and methods 2.1 Fabrication of composites Hemp fibers were sourced from Hemp Affair Pvt. Ltd. in Varanasi and underwent a chemical treatment for 24h using sodium hydroxide (NaOH) aqueous solution with a concentration of 6% (w/v). Following this treatment, they were thoroughly rinsed with distilled water and dried under sunlight for 10h. The hemp fibers were cut into lengths ranging from 3 to 5mm Table1 displays the components’ proportions used in creating the test composites. These components include barium sulfate as a non-functional filler, hemp fibers for reinforcement, phenol-formaldehyde serving as a binder, graphite acting as a dry lubricant, and vermiculite and alumina serving as property modifiers (Naiduetal., 2023; Rajaketal., 2019;Singhetal., 2019). However, to understand the influence of gradual increment of the resin content between 10% and 20% wt., wherein 10% wt. results were already available with the authors; the increment of resin was done from 12% wt. onwards. Increment steps of 2% wt. was selected as it was understood that at least 2% wt. step was required to observe the influence of the resin variations on the tribological performance of the friction composites. Five compositions (variants) were developed using the ingredients shown in Table1, HF25P12, with 12%wt. phenol formaldehyde (PF) resin, HF25P14–with 14%wt. PF resin, HF25P16–with 16%wt. PF resin, HF25P18–with 18%wt. PF resin and HF25P22–with 22%wt. PF resin. 6% (w/v) alkali pre-treated hemp fibers with 25%wt. fiber loading was used for all these compositions. The PF content compensated for the barium sulphate content. HF25P20 was already fabricated and tested in the authors’ earlier work (Naiduetal., 2023), which was then termed HF6P20; hence, it was not refabricated here. All the components were meticulously measured using a digital scale (Wensar®etal.: 0–220g, Least Count: 0.01g). The chopped fibers and the phenolic powder formulations were subjected to a dry mixing process for 15minat a rotational speed ranging from 250 to 500 revolutions per minute (rpm) using a mechanical stirrer to attain a uniform blend. Figure1A illustrates that the mixtures were then subjected to compression in a compression molding machine (Manufacturer: Santec). The mixes were cured for 10min, with four breathings of applied pressure at 15MPa and a temperature of 155°C. For 3h, a post-curing process was carried out in a hot air oven (Manufacturer: Athena Technology), as depicted in Figure1B, at 170°C. This step was performed to eliminate moisture and release any trapped gases that may have formed during the polymerization process of the matrix constituents, thereby relieving induced compressive stresses (Singhetal., 2019). Plates of five different compositions, identified as HF25P12, HF25P14, HF25P16, HF25P18, and HF25P22, with dimensions measuring 100 × 100 × 10mm in thickness, were fabricated utilizing the compression molding method. From each type of composition, specimens for conducting a pin-on-disc test by ASTM G 99 and a fade and recovery test as outlined in SAE J661 were extracted from these 100 × 100 × 10mm plates. Three test specimens of each type were extracted. A sample of this is illustrated in Figure2. 2.2 Tribological characterization 2.2.1 Design of experiments using orthogonal array Tribological properties, particularly specific wear rate (SWR) and coefficient of friction (COF) were analyzed using the Taguchi experiment design. Taguchi’s experiment design considers factors it terms as “signals,” which control the process response evaluated at different levels. Composition, normal (braking) load, and sliding velocity influenced SWR and COF. The values of the sliding velocities and the normal loads were decided from earlier literature (Rashidetal., 2017a;Shanmughasundaram, 2017). Table2 shows the factors and levels selected for the friction composites. 2.2.2 Experimental procedure To assess the tribological performance of SWR and COF of the prepared hemp/PF friction bio-composites at room temperature, they were tested on a pin-on-disc experimental setup (DUCOM™ TR-20LE)asperASTMG99standards, asshown in Figure3.Atrack diameter of 100mm for a 5,000m sliding distance was selected. Frontiers in Materials 02 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 TABLE 1 Material composition. Materials Weight contribution (%) HF25P12 HF25P14 HF25P16 HF25P18 HF25P22 Hemp fibers 25 25 25 25 25 Phenol formaldehyde 12 14 16 18 22 Graphite powder 5 5 5 5 5 Vermiculite 5 5 5 5 5 Alumina 5 5 5 5 5 Barium sulphate 48 46 44 42 38 Total 100 100 100 100 100 FIGURE 1 (A) Compression moulding machine (B) Hot air oven. TABLE 2 Factors and levels. Factors Units Level 1 Level 2 Level 3 Level 4 Level 5 Composition - HF25P12 HF25P14 HF25P16 HF25P18 HF25P22 Load N 30 50 70 90 110 Sliding Velocity m/s 2.6 3.9 5.2 6.5 7.8 The test trials were conducted per Taguchi L25 orthogonal array combinations, wherein three factors were evaluated for five levels, as shown in Table2. Specifications of the pin-on disc setup used. Make: DUCOM™ TR-20LE. ➢ Disc speed: 200rpm–2000rpm (in step of 1rpm) ➢ Disc size: Dia.165 × 8mm thick ➢ Disc material: J431 (22–24 HRC, ground to 1.6 Ra surface roughness) ➢ Normal load: 2kg–20kg (step of 0.5kg by dead weights) ➢ Friction force: 0–200N (least count: 0.1N) ➢ Wear: 0–2000 microns (least count: 1 micron) Frontiers in Materials 03 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 FIGURE 2 Sample specimen for pin on disc test. FIGURE 3 Pin-on-Disc Experimental setup. Mass loss arising from rubbing of the test specimens against the rotor of the pin-on disc setup was measured using an analytical weighing balance (Wenser make, model MAB 201) with an accuracy of 0.1mg. The mass loss so obtained for three different test specimens of each type of composition were used to calculate their respective SWR using the relation (a). Fade and recovery analysis of the composites performing better at room temperature amongst those investigated were done on a Chase dynamometer following SAE J661 standards at Indian Friction Material Engineering Company, Noida, India. A Chase dynamometer comprises a spinning drum with a 25.4-mm-square friction pad pressed against its inner circumference using an air pressure mechanism. A minor portion of the friction material contacts the drum to collect information regarding friction and wear (Akincioğluetal., 2021). 3 Results and discussion Taguchi Design of Experiments (D.O.E.) was conducted with three factors - composition, load and sliding velocity, and each factor with five varying levels as shown in Table2. L25 orthogonal array was suggested by Taguchi design of experiment method for three factors and five levels. The derived experiments were used for tribological testing on a Pin-on-Disc tribo-machine (as per ASTM G99) at room temperature. Subsequently, Specific Wear Rate (SWR) and Coefficient of Friction (COF) were obtained using the Eqs1,2as shown below and analyzed using S/N ratios to decide the optimum parameters. SWR =∆m ρLD (1) COF =F L(2) Where, ∆m=massloss ρ=density L= applied load. D= sliding distance. F= frictional force. 3.1 Signal-to-noise ratio The SWR and COF values were transformed into Signal-toNoise (S/N) ratios, with the goal of minimizing both of them, making use of the “Smaller is Better” quality characteristic. The formula for computing the corresponding S/N ratio is represented by Eq.3as shown. S Nratio =−10log10 {1 n∑n i=1y2 i}(3) Where, y = SWR or COF n= Number of trials. The purpose is to compute the highest signal-to-noise ratio which means there are minimum random factors (noise) affecting the required parameters. The values of S/N ratios are tabulated in Table3. 3.2 Variation of friction force (F), SWR and COF with respect to normal load Variation of friction force with respect to time were recorded for all five compositions at 30N, 50N, 70N, 90N, and 110N normal loads as shown in Figures4A–E for HF25P12, HF25P14, HF25P16, HF25P18 and HF25P22 respectively. The results typically showed two friction regimes, initially a running-in period followed by a steady-state period similar to those reported in earlier literature for sliding friction cases (Chand and Fahim, 2008). These regimes were due to the higher Frontiers in Materials 04 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 TABLE 3 Specific wear rate (SWR) and coefficient of friction (COF) using L25 orthogonal array with 3 replicates. Load (N) Sliding velocity (m/s) Composition SWR (mm3/Nm) × 10–5 COF S/N for SWR S/N for COF 30 2.6 H25P12 9.5645 0.65 −20.20302982 3.522693814 30 2.6 H25P12 10.2365 0.67 ∗ ∗ 30 2.6 H25P12 11.3658 0.7 ∗ ∗ 50 3.9 H25P12 7.3265 0.58 −18.76558992 4.436974992 50 3.9 H25P12 8.6752 0.6 ∗ ∗ 50 3.9 H25P12 9.3658 0.62 ∗ ∗ 70 5.2 H25P12 7.2956 0.55 −18.3139049 4.753932734 70 5.2 H25P12 8.2356 0.58 ∗ ∗ 70 5.2 H25P12 9.3658 0.62 ∗ ∗ 90 6.5 H25P12 6.3545 0.53 −17.89285537 5.104755259 90 6.5 H25P12 7.8459 0.56 ∗ ∗ 90 6.5 H25P12 8.3568 0.52 ∗ ∗ 110 7.8 H25P12 6.9568 0.51 −17.05642879 5.39727992 110 7.8 H25P12 7.1256 0.54 ∗ ∗ 110 7.8 H25P12 8.3654 0.58 ∗ ∗ 30 2.6 H25P14 7.3548 0.57 −18.61531406 4.436974992 30 2.6 H25P14 8.5264 0.6 ∗ ∗ 30 2.6 H25P14 9.3265 0.63 ∗ ∗ 50 3.9 H25P14 6.6598 0.52 −17.22291882 5.03623946 50 3.9 H25P14 7.2635 0.56 ∗ ∗ 50 3.9 H25P14 8.6589 0.59 ∗ ∗ 70 5.2 H25P14 5.1415 0.51 −16.32296928 5.30720322 70 5.2 H25P14 6.5486 0.54 ∗ ∗ 70 5.2 H25P14 7.3568 0.57 ∗ ∗ 90 6.5 H25P14 4.6959 0.49 −15.50244732 5.661578537 90 6.5 H25P14 5.9583 0.52 ∗ ∗ 90 6.5 H25P14 6.3659 0.54 ∗ ∗ 110 7.8 H25P14 4.4897 0.48 −15.41026554 5.863938059 110 7.8 H25P14 5.8954 0.51 ∗ ∗ 110 7.8 H25P14 6.3569 0.54 ∗ ∗ 30 2.6 H25P16 6.3698 0.55 −17.8691319 4.779494966 30 2.6 H25P16 7.8245 0.58 ∗ ∗ 30 2.6 H25P16 8.3365 0.62 ∗ ∗ (Continued on the following page) Frontiers in Materials 05 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 TABLE 3 (Continued) Specific wear rate (SWR) and coefficient of friction (COF) using L25 orthogonal array with 3 replicates. Load (N) Sliding velocity (m/s) Composition SWR (mm3/Nm) × 10–5 COF S/N for SWR S/N for COF 50 3.9 H25P16 5.1256 0.51 −15.74339806 5.352124804 50 3.9 H25P16 6.1259 0.54 ∗ ∗ 50 3.9 H25P16 7.6959 0.59 ∗ ∗ 70 5.2 H25P16 4.1256 0.48 −14.52769707 5.644926136 70 5.2 H25P16 5.3258 0.52 ∗ ∗ 70 5.2 H25P16 5.6895 0.55 ∗ ∗ 90 6.5 H25P16 4.2635 0.46 −13.15812779 6.018862909 90 6.5 H25P16 4.5489 0.5 ∗ ∗ 90 6.5 H25P16 5.6329 0.54 ∗ ∗ 110 7.8 H25P16 3.2639 0.46 −12.33077053 6.180139358 110 7.8 H25P16 4.1356 0.49 ∗ ∗ 110 7.8 H25P16 5.2635 0.52 ∗ ∗ 30 2.6 H25P18 6.1254 0.49 −17.09765387 5.460568326 30 2.6 H25P18 7.1595 0.53 ∗ ∗ 30 2.6 H25P18 8.1265 0.57 ∗ ∗ 50 3.9 H25P18 4.1256 0.48 −14.5920421 5.679933127 50 3.9 H25P18 5.3654 0.52 ∗ ∗ 50 3.9 H25P18 6.6359 0.57 ∗ ∗ 70 5.2 H25P18 3.9256 0.47 −12.28781894 6.020599913 70 5.2 H25P18 4.1152 0.5 ∗ ∗ 70 5.2 H25P18 4.5364 0.54 ∗ ∗ 90 6.5 H25P18 2.1154 0.43 −10.43740023 6.460643714 90 6.5 H25P18 3.3256 0.48 ∗ ∗ 90 6.5 H25P18 4.3679 0.53 ∗ ∗ 110 7.8 H25P18 2.5556 0.43 −9.416438991 6.697765258 110 7.8 H25P18 2.9568 0.46 ∗ ∗ 110 7.8 H25P18 3.9246 0.5 ∗ ∗ 30 2.6 H25P22 5.3249 0.47 −15.74297268 5.980736437 30 2.6 H25P22 6.1256 0.5 ∗ ∗ 30 2.6 H25P22 7.3659 0.53 ∗ ∗ 50 3.9 H25P22 3.1258 0.46 −13.15755494 6.158932699 50 3.9 H25P22 4.5486 0.49 ∗ ∗ 50 3.9 H25P22 5.2359 0.52 ∗ ∗ (Continued on the following page) Frontiers in Materials 06 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 TABLE 3 (Continued) Specific wear rate (SWR) and coefficient of friction (COF) using L25 orthogonal array with 3 replicates. Load (N) Sliding velocity (m/s) Composition SWR (mm3/Nm) × 10–5 COF S/N for SWR S/N for COF 70 5.2 H25P22 2.2369 0.46 −9.892830206 6.532208436 70 5.2 H25P22 3.1235 0.47 ∗ ∗ 70 5.2 H25P22 4.3215 0.51 ∗ ∗ 90 6.5 H25P22 2.052 0.43 −6.550039451 7.068004966 90 6.5 H25P22 2.1257 0.44 ∗ ∗ 90 6.5 H25P22 3.3469 0.49 ∗ ∗ 110 7.8 H25P22 2.0007 0.41 −7.792534479 5.560491968 110 7.8 H25P22 2.4526 0.46 ∗ ∗ 110 7.8 H25P22 3.3215 0.49 ∗ ∗ TABLE 4 ANOVA for S/N ratio of specific wear rate. Source DOF Seq. SS Adj. SS Adj. MS F Value p-Value Contribution (%) Load 4 105.619 105.619 26.405 23.27 0.000 33.15 Sliding velocity 4 9.240 9.240 2.310 2.04 0.153 2.90 Composition 4 190.045 190.045 47.511 41.86 0.000 59.66 Error 12 13.619 13.619 1.135 - - Total 24 318.523 - - - - Seq. SS, sequential sum of squares, Adj; SS , adjusted sum of squares, Adj; MS , adjusted mean square. initial adhesive forces between the test materials and the metal rotor, followed by a steady state showing consistent friction forces concerning time for all three normal load conditions observed in HF25P12, HF25P14, HF25P16, HF25P18, and HF25P22 as shown in Figures4A–E respectively. Friction force trends show a direct relation to the applied normal loads. As evident from these figures, friction forces increased with the increase in normal loads. Mean values of SWR and COF were plotted at five different load values: 30N, 50N, 70N, 90N, and 110N, as shown in Figure5,6, respectively. The sliding velocity was not considered in this case becauseofitsnegligibleinfluenceonthe responsebehavior,asshown in Tables4,5. Figure5 shows higher Specific Wear Rate (SWR) values for HF25P12 followed successively by HF25P14, HF25P16, and HF25P18, and lowest for HF25P22. This might be due to an improved percentage in the phenol formaldehyde resin, as a result of which improved interfacial bonding between fibres and matrix might have formed (Chand and Fahim, 2008). The drop in the SWR with the rise in the normal load from 30N to 110N is expected, as seen for most of the compositions here. This is because, as per equation (a), SWR is inversely proportional to the normal load, and also, the wear type at the initial stage is likely to be adhesive type, which gradually changes to abrasive type with an increase in the asperity contact temperatures at the interface of the friction composites and the metal counter face (Karthikeyanetal., 2017). Further, it is observed that beyond 90N load, SWR increases for HF25P22. This is attributed to the overloading of the phenol-formaldehyde resin content beyond the suggested range of 20–25%vol, which is approximately 20–21%wt. for phenolic resins, as suggested in the early reported literature (Blau, 2001). In Figure6, COF values for HF25P12 are higher, followed successively by HF25P14, HF25P16, HF25P18, and lowest for HF25P22. Also, all five compositions show a decreasing trend with the increase in the normal load from 30N to 110N, following the mathematical relation shown in equation (b). This might be due to worn surface modification due to a transfer layer that might have formed on the friction surface. Also, the rise in asperity contact temperature at higher loads could be another possible reason for the drop in COF (Chand and Fahim, 2008). Amongst these compositions, HF25P22 shows lower and more stable values of COF up to 90N, which might be due to a higher percentage of phenol formaldehyde resin making better fiber–matrix bonding compared to its counterparts studied here. However, beyond 90N, Frontiers in Materials 07 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 FIGURE 4 (A) Friction force v/s time for HF25P12. (B) Friction force v/s time for HF25P14. (C) Friction force v/s time for HF25P16. (D) Friction force v/s time for HF25P18. (E) Friction force v/s time for HF25P22. TABLE 5 ANOVA for S/N ratio of coefficient of friction. Source DOF Seq. SS Adj. SS Adj. MS F Value p-Value Contribution (%) Load 4 4.9054 4.9054 1.22634 8.56 0.002 31.95 Sliding velocity 4 0.3479 0.3479 0.08698 0.61 0.665 2.27 Composition 4 8.3755 8.3755 2.09386 14.61 0.000 54.56 Error 12 1.7199 1.7199 0.14333 - - Total 24 15.3487 - - - - Seq. SS, sequential sum of squares, Adj; SS, adjusted sum of squares, Adj; MS, adjusted mean square. Frontiers in Materials 08 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 FIGURE 5 Variation of specific wear rate (SWR) with respect to normal load. FIGURE 6 Variation of coefficient of friction (COF) with respect to normal load. a rise in the COF value is seen, which might be due to the overloading of the PF content beyond 20 %wt. as reported by earlier literature (Blau, 2001). 3.3 Analysis of variance Analysis of variance (ANOVA) is a statistical process to acquire the contribution of composition, load, and velocity in the performance characteristics viz SWR and COF. Tables4,5 show the SWR and COF’s ANOVA results, respectively. The percentage contribution of factors is the ratio of the Sum of Squares of the factor to the total Sum of Squares. For SWR, composition plays the most significant role (59.66%), followed by normal load (33.15%) and finally by sliding velocity of the disc (2.90%). Whereas for COF, composition has highest contribution (54.56%),followed bythe normal load (31.95%) andlastly the sliding velocity (2.27%). TABLE 6 Optimum factors as per main effect plots for S/N ratio. Parameter Load (N) Velocity (m/s) Composition SWR 110 5.2 HF25P22 COF 90 7.8 HF25P22 3.4 Optimization of factors The optimum levels suggested by Taguchi’s optimum design for Specific Wear Rate and Coefficient of Friction factors were obtained from respective main effects plots for S/N ratios. Figure7, 8represent optimum factor levels for SWR and COF, respectively. Figure7 shows the composition of HF25P22, an applied load of 110N, and a sliding velocity of 5.2m/s as the combination for optimum SWR. In contrast, Figure8 shows the composition of HF25P22, an applied load of 90N, and a sliding velocity of Frontiers in Materials 09 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 FIGURE 16 (A–G) Fade recovery performance of HF25P22 friction composite. is essential for ensuring the brake pad can regain its previous frictional performance. During this stage, HF25P20 successfully recovered its previous friction coefficient, which remained within the standard range, as shown in Figure15C. In contrast, it was the first recovery performance of HF25P22, which showed a decline in the recovery curve, as evident in Figure16C. Following the initial three stages, the wear behavior was assessed over 100 pedaling cycles at a relatively constant temperature. As depicted in Figure15G and 16G, the friction coefficient during the wear stage remained within the standard definition range, similar to the other stages. The first three stages were repeated to confirm both brake pads’ friction and wear properties: a second baseline, a second fade, and a second recovery. Subsequently, the sample was weighed again, resulting in a loss of mass of 0.57gm. out of an initial 4.710gm. for HF25P20 and 0.66gm. out of an initial 4.810gm for HF25P22. Thus, the wear rate of HF25P22 was greater than HF25P20 by 1.62%. According to SAE J661 standards, a brake pad’s mass loss after undergoing seven wear stages should be below 1.230g, which both HF25P20 and HF25P22 have obeyed. HF25P20 received a classification of “GG” because the recorded normal and hot friction coefficient values were 0.474 and 0.451, respectively, and HF25P22 received a classification of “GG,” as the recorded normal and hot friction coefficient values were 0.493 and 0.452, as automatically recorded by the friction machine. The brake friction pads for commercialvehicleswithgood friction athigh andlowtemperatures usually belong to the “FF” classification and above (Akramifard and Ghasemi, 2016). 4 Conclusion Five different formulations were created, each containing varying percentages of phenol formaldehyde resin, ranging from 12% wt. to 22% wt. in 2% wt. Increments. The friction, wear, and fade-recovery performance of the best of the presently formulated friction composite, namely, HF25P22, in comparison to the author’s Frontiers in Materials 16 frontiersin.org Naidu etal. 10.3389/fmats.2024.1348265 earlier best-reported HF25P20 (HF6P20) friction composite, reveals the following: • The SWR at room temperature of HF25P20 is less than that of HF25P22 by nearly 6%. A lower SWR is usually desired for any good friction material. • The actual (optimized) COF of HF25P20 and HF25P22 at room temperature are 0.4496 and 0.4650, respectively, which are in line with those during the fade and recovery stages of the same, respectively and also within the recommended limits of 0.3–0.5 for commercial vehicles as per SAE J661 standards. • The fade recovery of HF25P20 is better than that of HF25P22. • HF25P20 and HF25P22 have been rated with the “GG” friction material by SAE J661 standards, which are suitable for commercial automotive applications. • Thus, the brake friction composite, namely, HF25P20, which was investigated and reported by the authors earlier as HF6P20, still proved to be a better-performing brake friction material than all its counterparts, which are studied and reported in the present exploration. This study thus shows confirmation to earlier reported percentage of phenol formaldehyde resin content (Blau, 2001) (20%–21% wt.) which is also true for hemp (natural) fibre reinforced brake friction materials. However, future explorations on the use of natural resins like starch, proteins, natural gums, etc. as binders in friction materials needs attention, in order to provide still better greener alternatives to existing commercial brake friction materials. Data availability statement The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors. Author contributions MN: Conceptualization, Data curation, Methodology, Project administration, Resources, Writing–original draft, Writing–review and editing. AB: Formal Analysis, Investigation, Methodology, Project administration, Resources, Supervision, Writing–review and editing. MG: Data curation, Formal Analysis, Investigation, Project administration, Resources, Writing–review and editing. SS: Investigation, Project administration, Supervision, Visualization, Writing–review and editing. RC: Investigation, Methodology, Project administration, Resources, Writing–review and editing. EA: Data curation, Funding acquisition, Investigation, Project administration, Resources, Writing–review and editing. Funding The author(s) declare financial support was received for the research, authorship, and/or publication of this article. The authors also thank King Saud University for funding this work through Researchers Supporting Project number (RSP2024R164), King Saud University, Riyadh, Saudi Arabia. Acknowledgments The authors express their thanks to MKSSS’s Cummins College of Engineering for Women, Pune, for providing their composite material processing facility of Composite Material Laboratory Sponsored by AICTE MODROB Scheme (9166/RFID/MODROB/Policy-1/2017-18) for this work. The authors thank King Saud University for funding this work through the Researchers Supporting Project number (RSP2024R164), King Saud University, Riyadh, Saudi Arabia. Conflict of interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Publisher’s note All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. 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