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Influence of B4C addition on the tribological properties of bronze matrix brake pad materials

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Published/Copyright: January 22, 2025
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Open Chemistry
From the journal Open Chemistry Volume 23 Issue 1

Abstract

In this study, the effects of B4C particle addition on the microstructural, mechanical, and tribological properties of bronze matrix brake pad materials were analyzed. To this end, bronze matrix brake pads reinforced with different proportions (0, 3, 6, 9, and 12 wt%) of B4C particles were fabricated via hot pressing at 800°C and 40 MPa for 5 min. The hardness, density, and transverse rupture strength (TRS) were measured and friction–wear tests were performed to evaluate the effectiveness of adding B4C. The results revealed that the hardness increased with increasing B4C content, whereas the density slightly decreased. The TRSs of the B4C-reinforced specimens exceeded that of the non-reinforced material. Furthermore, the friction stability, average friction coefficient, and wear resistance, which are crucial quality parameters of the brake pad material, increased upon adding >3 wt% B4C to the specimen compared with those of the non-reinforced specimen. After the wear test, the worn surfaces of the specimens were examined via scanning electron microscopy, X-ray diffraction, and a 3D optical profilometer to identify the wear mechanisms involved during friction.

Keywords: bronze matrix composites; brake pads; boron carbide; friction coefficient; wear mechanisms

1 Introduction

Braking systems primarily transform the kinetic energy of a moving vehicle into mechanical energy using the friction between the brake disk and pad, thereby reducing the vehicle's speed or bringing it to a complete stop [1,2]. Brakes are used an average of 50,000 times per year in a vehicle [3]. Thus, the correct functioning of the braking system is crucial to ensure the safety of individuals and roads [4,5].

Brake pads are crucial components of the braking system and considerably impact its overall performance [6,7]. An ideal brake pad material must have a high and stable friction coefficient, a low wear rate, high thermal stability, and no noise and should not cause vibration or damage to the brake disk under various operating and environmental conditions [8]. These characteristics of brake pads can directly impact a vehicle’s braking and safety performance [9].

The organic polymer matrix materials, which are commonly utilized in brake systems as brake pads, are composed of 20–30 different components [10]. These components can be classified into four principal groups based on their anticipated functions: binder materials, filler and reinforcements, abrasives, and friction modifiers [11]. Although brake pads may appear to be a relatively simple component, they are manufactured after extensive engineering work [3]. Manufacturers typically develop pad materials via the trial-and-error process [12]. Polymer matrix pads are employed in vehicle braking systems with low and medium braking energy densities (energy absorbed per unit contact area during braking). However, these materials experience thermal fading under high friction interface temperature in cases wherein high energy density is generated between the friction pairs of high-performance vehicles such as high-speed trains and high-end racing cars. Thus, these materials cannot fully satisfy the low wear rate as well as the high and constant friction coefficient required for the operation of brake pads in high-performance vehicles under such braking conditions [13], hampering their safety performance. Therefore, metal matrix pads that exhibit high wear resistance, as well as a stable and high friction coefficient over a wide operating temperature range [14], can be used to meet the braking performance requirements of high-powered vehicles.

Metal matrix composite (MMC) brake pads are a composition of a metal matrix, a metallic phase to reinforce the matrix, abrasives, and lubricants [15]. Copper and its alloys are extensively employed as matrices in composite materials used in many industrial applications. These materials have inherent properties such as high thermal conductivity, superior corrosion resistance, high melting point, low cost, excellent fatigue strength, high wear resistance, and simple manufacturing processes [16,17]. Solid lubricants such as graphite and molybdenum disulfide can be introduced into matrix materials for stabilizing friction, regulating wear, and reducing noise generated during braking [18]. Abrasives are critical components of brake pads with an MMC because they primarily increase the friction coefficient and clean the friction films formed on the brake disk [19]. The selection of abrasive components that are crucial for brake pad materials depends on several factors, such as the type of reinforcing element used as an abrasive along with its hardness, shape, size, and distribution [20]. These factors considerably influence the mechanical and frictional wear properties of composites [21,22] as well as various critical indicators of brake performance, including stopping distance, counterface wear, and noise propensity [23]. However, the fundamental principles governing the selection of such abrasive particles are poorly understood. This is largely because the precise role of abrasives on the contact surfaces of multi-phase and inhomogeneous brake pad materials has not been accurately determined [24].

The addition of particles such as ZrO2, TiC, Al2O3, Cr, CrFe, SiO2, B₄C, W, fly ash, and short carbon fibers into copper matrix materials can enhance the mechanical and friction–wear properties of the resulting composite [13,24,25,26,27,28]. Similar improvements can be obtained by adding particles such as Al₂O₃, ZrSiO₂, SiC, fly ash, glass powder, Ni3Al, and red mud into bronze matrix materials [29,30,31,32,33,34,35]. B4C is a notable reinforcement element with exceptional properties such as high hardness (28–37 GPa), high modulus of elasticity (450–470 GPa), low density (∼2.52 g/cm³), high melting point (2,500°C), high wear resistance, high thermal stability, and excellent resistance to chemical substances [36,37,38]. However, the tribological properties of B4C ceramic-powder-reinforced bronze matrix pads have been scarcely investigated.

Powder metallurgy comprising pressing and sintering is traditionally used for MMC fabrication [39,40] as it uniformly distributes reinforcing elements in the matrix to create high-performance materials [41]. This method employs lower processing temperatures than other methods, which is a significant advantage [42]. It can also be used to mass-produce complex materials cost-effectively [43]. However, MMCs manufactured via powder metallurgy can have drawbacks such as high porosity, low density, poor thermal conductivity, and poor strength. Moreover, heavy machinery increases the costs of small-scale production. Due to its high cost per product unit, this method is economically viable only for high-volume production [44]. In contrast, hot pressing uses low temperature and pressure for fabricating products with enhanced properties such as high density compared with traditional cold pressing and sintering [45]. Thus, it is an ideal solid-state manufacturing process for fabricating MMC brake pad materials comprising components of different densities.

Herein, the effects of B4C on the mechanical properties, friction layer formation, and friction performance of bronze matrix pads were investigated based on their friction coefficient, wear rate, hardness, density, and transverse rupture strength (TRS). To this end, friction materials containing 0, 3, 6, 9, and 12 wt% of B4C were produced via hot pressing. The as-obtained results provide information for the fabrication of tailored metal matrix brake pads that can be used in high-performance vehicle braking systems.

2 Materials and methods

2.1 Fabrication of composite materials

The characteristics of raw powders, including their size, shape, density, and distribution, considerably impact the mechanical and tribological properties of composite materials [46]. Figure 1 shows the scanning electron microscopy (SEM) images and particle size distribution of raw powders used for fabricating the brake pads. Bronze, used as the matrix material, had a complex shape; whereas iron, used to strengthen the matrix, had a porous spongy structure [47]. Graphite flakes were used as a lubricant [48]. B4C powder, with sharp-edged and polygonal particles, was used as an abrasive in brake pads. Bronze, iron, graphite, and B4C powders had D50 grain sizes of 63.9, 86.5, 35.3, and 15.1 µm, respectively. The dimensions of these powder particles were measured using the Malvern Mastersizer 3,000 device (Panalytical, UK).

Figure 1 SEM images and particle size distributions of raw materials used for manufacturing the brake pads: (a) bronze, (b) iron, (c) graphite, and (d) B4C powder.
Figure 1

SEM images and particle size distributions of raw materials used for manufacturing the brake pads: (a) bronze, (b) iron, (c) graphite, and (d) B4C powder.

Brake pad specimens were manufactured via hot pressing. The bronze matrix brake pad used as the base material contained 80% bronze (CuSn6Zn6Pb3), 15% iron, and 5% graphite powder. Various proportions (3, 6, 9, and 12 wt%) of B4C were added to this matrix material to obtain a series of new specimens. The powders were first weighed on a digital scale with an accuracy of 0.001 g and transferred to a mixing container. Then, they were mixed for 90 min at 30 rpm in a V-cone-type mixer to ensure homogeneous distribution. The powder mixture was then transferred to graphite molds and sintered in the hot-pressing device at 40 MPa and 800°C in an argon atmosphere for 5 min.

2.2 Experimental apparatus

The density measurements of specimens were performed using the Archimedes method at room temperature. Macrohardness tests were conducted using a Rockwell hardness tester with a steel ball (diameter = 6.35 mm) indenter and a load of 981 N, according to the HRM scale (Digirock-RB model, BMS Bulut Makine Co., Turkey).

The TRS of specimens was determined by conducting three-point bending tests on a universal testing machine (Insron 3369, USA), for which a 10 mm × 10 mm × 40 mm test specimen was used at a cross-head rate of 1 mm per minute. The load determined at the instant of rupture was converted into TRS using equation (1) [49]:

(1) TRS = 3 × P × L 2 × T 2 × W ,

where P is the maximum applied load (N), L is the distance between the center of the support points (mm), W is the width of the specimen (mm), and T is the specimen thickness (mm).

Friction–wear tests were performed on a real brake disk-type tester, similar to those of Sugözü [50,51], who investigated the tribological properties of polymer matrix brake pad materials with different compositions. The friction tester comprised several key components such as a disk–caliper system that ensured pad–disk contact, a hydraulic unit that produced the required force for ensuring pad contact with the disk, an electric motor that generated the power required for the disk movement, a load cell that instantaneously measured the friction force between the friction pair and a computer system that rapidly converted this friction force into a friction coefficient and recorded it. The infrared thermometer that could measure temperatures from 0 to 700°C was used to measure the disk surface temperature caused by friction. During the braking test, a gray cast iron disc with a diameter of 227 mm and hardness of 210 HB was used as the counterface material. Brake pad test specimens of 25 × 25 × 7 mm were also used. The tests were conducted in triplicate for each specimen, and the mean of the three test results was taken as the final result. After each experiment, the surface of the disk material was cleaned with 320-grit sandpaper. The friction–wear tests were conducted under dry sliding conditions at a speed of 6 m/s and a load of 1.05 MPa for 680 s [35].

After at least 95% of the surface between the friction pair maintained contact, the instantaneous friction coefficient (µ) was calculated as the ratio of the frictional force (f) to the normal load applied (F) as follows (equation (2)) [52,53]:

(2) μ = f F ,

The mass of each pad specimen was determined using a precision scale before and after the friction–wear test to determine the mass loss. The specific wear rate (V, cm3/Nm) was calculated using equation (3) according to TSE 555 [54]:

(3) V = Δ m 2 × π × Rd × n × f m × ρ ,

where Δm (g) is the change in the mass of the pad before and after the wear test, ρ is the density of the brake pad (g/cm³), Rd (mm) is the distance between the disk and specimen center, f m is the average friction force (N), and n is the number of disk rotations.

Friction stability coefficient (FSC) is the ratio of the average friction coefficient (µ average) to the maximum friction coefficient (µ max) achieved during the braking test and is expressed as follows (equation (4)) [14,55]:

(4) FSC = μ average μ max ,

SEM (Quanta 650 Field Emission) and energy dispersive X-ray spectroscopy (EDS) were employed for fracture surface characterization and metallographic examinations. A 3D optical profilometer (Profilm 3D) and an X-ray diffraction device (XRD; PANalytical EMPYREAN) were used to analyze the wear mechanisms on the friction surfaces of the brake pad specimens

3 Results and discussion

3.1 Microstructural analysis

The microstructure of a material considerably impacts its mechanical properties, such as wear resistance, durability, and stability; therefore, the microstructure and composition of the brake pad materials will impact the effectiveness of automotive brakes [56]. Figure 2 shows the microstructure of the brake pad specimens with different B4C proportions. The phases forming the microstructures were described via EDS analysis, and the corresponding results are shown in Table 1. Microstructural analysis revealed that iron and graphite, which are the components of the brake pad material, were distributed uniformly in the matrix (Figure 2a–j). B4C particles were also homogeneously distributed up to 6 wt% in the matrix (Figure 2c–f). However, beyond this value, B4C particles formed clusters in certain regions (Figure 2g–j). Furthermore, Pb from the CuSn6Zn6Pb3 matrix alloy separated from the matrix during sintering due to its low melting temperature and formed a Pb phase that was visible as white spots in the structure. Boz and Kurt [29] posited that Pb in the bronze matrix brake pad material transitioned into the liquid phase during sintering owing to its low melting temperature and accumulated within the pores. Graphite particles were also situated between the grains and formed a barrier that impeded grain coalescence. Thus, a weaker bond was formed between the grains, forming micropores (Figure 2b and d). B4C particles added to the matrix material as an abrasive reinforcement generated strong matrix–reinforcement interfacial bonds (Figure 2d) due to hot pressing. At a sintering temperature of 800°C, the matrix structure softened, and B4C particles were embedded in the soft matrix under a pressure of 40 MPa, forming a strong matrix–particle interfacial bond. Moreover, B4C particles did not undergo any deformation and retained their original shapes owing to their high hardness (Figure 2c–j). Karakoç et al. [57] proposed that the interactions occurring during hot rolling caused B4C particle embedding in the aluminum matrix, forming a strong matrix–reinforcement interfacial bond. Contrary to that in hot pressing, B4C particles fractured due to mechanical effects during hot rolling [57]. Despite a strong interfacial bond between B4C particles and the matrix, microgaps were formed between the B4C particles due to clustering (Figure 2g–j). Consequently, the flexural strength decreased with increasing B4C content. Hayajneh et al. reported a decrease in the TRS due to particle clustering within the matrix [46].

Figure 2 Back-scattered electron (BSE) and secondary electron (SE) micrographs of specimens: (a)–(b) 0 wt% B4C (BSE), (c)–(d) 3 wt% B4C (BSE), (e) 6 wt% B4C (BSE), (f) 6 wt% B4C (SE), (g) 9 wt% B4C (BSE), (h) 9 wt% B4C (SE), (i) 12 wt% B4C (BSE), and (j) 12 wt% B4C (SE).
Figure 2

Back-scattered electron (BSE) and secondary electron (SE) micrographs of specimens: (a)–(b) 0 wt% B4C (BSE), (c)–(d) 3 wt% B4C (BSE), (e) 6 wt% B4C (BSE), (f) 6 wt% B4C (SE), (g) 9 wt% B4C (BSE), (h) 9 wt% B4C (SE), (i) 12 wt% B4C (BSE), and (j) 12 wt% B4C (SE).

Table 1

Chemical compositions of regions 1–6 in Figure 2 (wt%)

Regions Cu Sn Zn Pb Fe C B O Possible phase
1 — — — — 97.6 2.4 — — Iron
2 3.32 — — 86.12 3.6 6.97 — — Lead
3 88.08 3.41 6.47 — — 2.05 — — Bronze
4 — — — — 11.72 86.52 — 1.76 Graphite
5 — — — — — 24.09 75.91 — B4C
6 — — — — — 22.05 77.95 — B4C

3.2 Physical properties of the manufactured materials

3.2.1 Density

Table 2 shows the theoretical, experimental, and relative densities of the specimens. The experimental density decreased with increasing B4C content in the matrix. Upon the addition of B4C into the powder mixture used in brake pads, the internal friction between particles and their resistance to compaction increased. As B4C powder is the third hardest material, the powder particles also exhibit resistance to compression, thereby reducing the density of the specimens [58]. Moreover, the density of B4C particles (2.52 g/cm³) [36] is considerably lower than that of the bronze matrix (8.74 g/cm³) [59], which ultimately reduces the density of specimens.

Table 2

Theoretical, experimental, and relative densities of specimens based on the B4C content

B4C content (wt%) Theoretical density (g/cm3) Experimental density (g/cm3) Relative density (%)
0 8.33 7.22 87
3 8.16 6.95 85
6 8 6.55 82
9 7.85 6.36 81
12 7.70 6.07 79

3.2.2 Hardness

Figure 3 shows the hardness values of specimens, which increased with increasing B4C content.

Figure 3 Hardness values of the specimens.
Figure 3

Hardness values of the specimens.

As shown in Figure 3, the hardness value of the bronze matrix pad material was 91 HRM. The addition of 3, 6, 9, and 12 wt% of B4C to the matrix material progressively increased the hardness of specimens to 96, 100, 103, and 105 HRM, respectively. The hardness value of the specimen reinforced with 12 wt% B4C considerably increased by ∼15% compared with that of the non-reinforced specimen, with the lowest hardness value due to the mixing rule proposed by Islak [60]. Based on this mixing rule, the hardness value of a composite material increases with an increasing proportion of reinforcing elements that have a higher hardness value than the matrix material. Moreover, particle-reinforced composites have higher hardness values than non-reinforced materials [61–63]. The hardness values of specimens gradually enhanced due to the dispersion hardening effect exerted by B4C particles [64].

3.2.3 TRS

The TRSs of specimens reinforced with 0, 3, 6, 9, and 12 wt% of B4C were 114, 192, 187, 169, and 152 MPa, respectively (Figure 4). These values increased for specimens containing 3 wt% of B4C and slightly decreased as the B4C content continued to increase. The B4C-reinforced brake pad specimens exhibited high TRS than non-reinforced specimens, and their flexural strength increased as the applied load transferred from the matrix to rigid B4C particles. This ultimately enhanced the composite’s resistance to the applied load, confirming that B4C particles considerably impacted the TRS. When B4C particles formed clusters in the matrix as their contents increased (particularly at 9 and 12 wt%), the TRS decreased. The uniform distribution of reinforcement is crucial for enhancing the material performance [65]. The notch effect resulting from the presence of B4C particles with sharp edges also decreased the TRS with increasing B4C content [66,67].

Figure 4 TRS of the specimens.
Figure 4

TRS of the specimens.

Figure 5 shows the SEM images of the fracture surfaces of specimens after the transverse rupture test. During the traverse rupture test of the specimen containing 0 wt% B4C, the applied load was transferred from the matrix to the reinforcing elements such as iron and graphite. At the graphite–matrix interface with a weak particle–matrix interfacial bond, localized stresses generated by the applied load caused the particle to dissociate from the matrix when its strength exceeded the matrix interfacial strength threshold (Figure 5a). Graphite particles at the grain boundaries and the relatively weak bond structure formed with the matrix increased the propensity of crack formation and propagation in these regions. These matrix defects resulted in a fracture mechanism, i.e., intergranular separation in the bronze matrix specimen (Figure 5b), and yielded the lowest TRS across all specimens. Upon adding B4C particles into the matrix material, the load-bearing by the matrix is transferred to rigid B4C reinforcing elements. During mechanical tests, a robust matrix–particle bond is essential for the transfer of load from the matrix to the ceramic reinforcement elements. Additionally, ensuring a homogenous distribution of these reinforcement elements within the matrix, in turn, is of significant importance [68]. Thus, the relatively strong interfacial bond formed between the B4C particles and matrix in the 3 wt% B4C-reinforced specimens increased its TRS resistance. Rigid B4C particles in the matrix also prevented crack formation and propagation in the matrix (Figure 5c), thereby increasing the TRS value of the reinforced specimens compared with that of the non-reinforced specimens. High B4C contents caused B4C particles to cluster in the matrix, thereby creating micropores that promoted crack initiation and propagation at the points of contact between the particles. Consequently, the TRS of specimens reinforced with 6, 9, and 12 wt% B4C decreased compared with that of the specimen reinforced with 3 wt% B4C (Figure 5d–f).

Figure 5 Fracture surface of the specimens reinforced with (a)–(b) 0 wt% B4C, (c) 3 wt% B4C, (d) 6 wt% B4C, (e) 9 wt% B4C, and (f) 12 wt% B4C.
Figure 5

Fracture surface of the specimens reinforced with (a)–(b) 0 wt% B4C, (c) 3 wt% B4C, (d) 6 wt% B4C, (e) 9 wt% B4C, and (f) 12 wt% B4C.

3.3 Friction behavior of brake pad specimens

Figure 6a–e shows the changes in the time-dependent friction coefficient and friction interface temperature of the specimens. The load applied to prevent damage to the tested specimens was incrementally elevated at the onset of the friction experiment. As a result, the surface underwent plastic deformation, and the actual contact area between the contact surfaces increased. This increased the friction coefficient of specimens up to ∼40 s.

Figure 6 Variation in time-dependent friction coefficient and friction interface temperature of specimens reinforced with (a) 0 wt% B4C, (b) 3 wt% B4C, (c) 6 wt% B4C, (d) 9 wt% B4C, and (e) 12 wt% B4C.
Figure 6

Variation in time-dependent friction coefficient and friction interface temperature of specimens reinforced with (a) 0 wt% B4C, (b) 3 wt% B4C, (c) 6 wt% B4C, (d) 9 wt% B4C, and (e) 12 wt% B4C.

Figure 6a shows the time-dependent friction coefficient of the bronze matrix specimen, which increased and decreased between ∼40 and 220 s. As the bronze matrix and gray cast iron disk established contact, adhesion points were formed between their hard micro-protrusions. These points initially underwent plastic deformation and subsequently fractured due to frictional shear stress between the friction pair. Then, an adhesive bond was formed between the underlying and counterface, and the friction coefficient continued to increase and decrease for ∼220 s. The bronze matrix exhibited a relatively stable and higher friction coefficient between 220 and 480 s, possibly due to a high friction interface temperature that reinforced the adhesive bond at the contact points. Increasing temperatures increase the actual contact area and adhesion [69], thereby increasing the friction coefficient. The bronze matrix exhibited a stable but lower friction coefficient from the 480 s to the end of the test. This was caused by the formation of a lubricating film layer at the friction interface due to the partial melting of the matrix from increasing friction interface temperature. This lubricating film layer stabilized and decreased the friction coefficient of the specimens. Peng et al. [70] reported that copper matrix melted due to a high friction interface temperature from braking and formed lubricant film, thereby reducing the friction coefficient.

The specimen reinforced with 3 wt% B4C exhibited a remarkably stable friction coefficient until ∼480 s and increased thereafter (Figure 6b). Temperature fluctuations resulting from the friction interaction between the specimen reinforced with 3 wt% B4C and the counterface formed cracks beneath the surface. These cracks also manifested at the B4C–matrix interface and intensified as the friction continued, thereby weakening the particle–matrix interfacial bond. Consequently, B4C particles detached from the matrix due to shear stress generated by friction and disintegrated at the contact interface and trapped between the matrix and counterface. These particles accumulated at the friction interface, further promoting scratching and material removal via cutting and ploughing-type abrasion friction from the brake pad specimen [71]. The friction coefficient increased at the end of the friction test. The time-dependent friction coefficient curves of the specimens reinforced with 6 and 9 wt% B4C showed two distinct friction regimes (Figure 6c and d). Their friction coefficient was consistent and high until a critical duration (∼300 s), as observed in the initial friction regime. Then, due to their low toughness (<4 MPa m1/2) [72], some B4C particles fractured without removing from the matrix by tangential stress formed between the friction pair at the end of critical time. This, in turn, reduced the abrasive effect of hard B4C particles on the counterface after the critical time, thereby considerably decreasing the friction coefficient. Abrasive particles with low toughness may exhibit poor friction performance and wear resistance [73]. Moreover, some asperities that were formed by B4C particles within the matrix gradually smoothened on contact with the counterface. Due to an increased friction interface temperature and shear stress, materials from the counterface smeared on B4C particles during friction, including their sharp edges. This process reduced the abrasive effect of hard B4C particles on the counterface after the critical time, thereby considerably decreasing the friction coefficient. These phenomena are discussed in detail in Section 3.4, taking into consideration the SEM images of the worn specimen surface. Kim et al. [23] reported that the friction coefficient decreased with increasing sliding time for different abrasives. This decline was attributed to the blunting of abrasive particles and the filling of metallic wear debris between the gaps [23]. In the second friction regime, the wear particles accumulated around the wear-resistant component of the pad material, i.e., the primary contact plateau. The primary contact plateaus contained B4C particles. Under the influence of the friction interface temperature and contact pressure, these wear particles sintered around the primary contact plateaus and formed secondary contact plateaus. The formation of such contact plateaus has been extensively studied [74,75]. These secondary contact plateaus also prevent direct contact between the brake pad and disk, thereby decreasing ploughing and eliminating adhesive wear [52]. As a result, the abrasive effect of brake pad specimens reinforced with 6 and 9 wt% B4C on the counterface reduced, which decreased the friction coefficient compared with that observed in the first friction regime. The friction coefficient of the brake pad reinforced with 12 wt% B4C did not decrease considerably after a certain critical time, contrary to the specimens reinforced with 6 and 9 wt% B4C (Figure 6e). This can be attributed to the reduced tendency for fracture and blunting due to the contact of abundant B4C particles with the counterface.

The mean friction coefficient of the bronze matrix specimen (0.366) was higher than that of the specimen reinforced with 3 wt% B4C (0.350) because they predominantly underwent adhesive wear. Adhesive wear increases the friction force by forming shear friction junctions between the contact surfaces [76]. The addition of 3 wt% B4C impeded the asperities of the counter disk from reaching the matrix material and decreased the strength of the adhesive bond formed in certain contact areas. Consequently, the specimen reinforced with 3 wt% B4C exhibited a slightly lower mean friction coefficient than the unreinforced specimen. The mean friction coefficients of specimens reinforced with 6 and 9 wt% B4C were 0.422 and 0.462, respectively. This phenomenon could be attributed to the formation of additional micro-protrusions originating from a large number of B4C particles on the wear surface. These microprotrusions exhibited high interfacial strength (Section 3.1), low density, high hardness, and high thermal stability [77]. These properties led to an increase in the mean coefficient of friction due to the enhanced abrasive effect applied to the counterface, up to 12 wt% B4C content. Cai et al. [78] posited that the friction coefficient increased with increasing mullite content in brake pad materials due to the scraping effect exerted by mullite. However, the 12 wt% B4C-reinforced specimen had lower mean friction coefficients than the 9 wt% B4C-reinforced specimen because the B4C particles clustered in the matrix as the B4C content increased. The particles within the clustered regions were unable to be adequately surrounded by the matrix, resulting in a weakening of the particle–matrix interface bond. This negatively impacted the matrix’s capacity to retain abrasive components, thereby limiting the effectiveness of abrasives in increasing the friction coefficient. Furthermore, the shedding of the soft bronze matrix surrounding the B4C particles that did not detach from the matrix during the friction process resulted in an increase in the contact area of the B4C particles with oxygen. This led to the oxidation of the B4C particles due to the effect of the temperature formed at the friction interface, which formed a B2O3 tribofilm on the worn surface [79]. This tribofilm functioned as a lubricating film [80] and reduced the average friction coefficient of the 12 wt% B4C-reinforced specimen relative to the 9 wt% B4C-reinforced specimen. The mean friction coefficients of all brake pads were between 0.3 and 0.6, within the acceptable application range [11].

At the end of the friction test, a temperature of 124°C was recorded for the non-reinforced specimen. Upon adding 3, 6, 9, and 12 wt% B4C into this specimen, the temperatures increased to 130, 160, 177, and 165°C, respectively (Figure 6a–e). The highest temperature was observed in the specimen reinforced with 9 wt% B4C, which had the highest average friction coefficient. This indicated that one of the most critical factors determining the temperature at the friction interface is the coefficient of friction, as also reported by Yavuz and Bayrakçeken [81].

During continuous braking, as seen in Figure 6a–e, the friction coefficient fluctuated across all specimens because of the dynamic structure of the tribofilm that formed and degraded on the friction surface [26]. These instabilities and variations in the friction coefficient caused a squeaking noise at the interface, which is undesirable and disturbing for braking systems [82].

An ideal brake pad material must have an FSC of 1 [83], but this value cannot be obtained in practice. Thus, the closer the FSC is to 1, the more favorable the brake pad performance [81]. Figure 7 shows the FSCs of specimens.

Figure 7 FSC of the specimens.
Figure 7

FSC of the specimens.

The specimen reinforced with 9 wt% B4C exhibited the highest FSC of 0.794, whereas that reinforced with 3 wt% B4C exhibited the lowest FSC of 0.698. The FSC of all fabricated specimens was considerably higher than 0.5, which is the lowest value, indicating stable friction properties [79]. Except for the 3 wt% B4C-reinforced specimen, all other specimens exhibited a higher FSC than the unreinforced specimen due to the formation of secondary contact plateaus on the wear surface [84,85].

To improve the wear resistance of specimens, the ease with which the brake pad components separate from the matrix during braking must be minimized [81]. A high wear resistance of the brake pad material will extend the service life, i.e., the interval for replacing pads. This will reduce the associated replacement costs. The wear behavior of brake pad materials is influenced by various factors such as pad composition, manufacturing parameters, and braking conditions [86]. Figure 8 shows the specific wear rates of specimens.

Figure 8 Specific wear rates of the specimens reinforced with different B4C contents.
Figure 8

Specific wear rates of the specimens reinforced with different B4C contents.

The specimen reinforced with 3 wt% B4C indicated the highest specific wear rate due to intense abrasive and delamination wear. Moreover, all B4C-reinforced specimens, except the 3 wt% B4C-reinforced specimens, exhibited lower specific wear rates than the non-reinforced specimens. The specific wear rate decreased with increasing B4C content because the secondary contact plateaus expanded. This considerably enhanced the wear resistance as the contact between the disk and pad changed from metal–metal to film–film [70]. Yanar et al. [87] found that secondary contact plateaus covering a worn surface reduced the wear considerably. To gain insights into the wear mechanisms that cause differences in specific wear rate and friction behavior, the wear surface morphologies of specimens reinforced with different B4C contents were analyzed via SEM, 3D optic profilometer, and wear debris analysis (Section 3.4).

3.4 Characterization of frictional surfaces and wear debris

Figure 9 shows the surface topography of specimens reinforced with different B4C contents subjected to a wear test. The prevailing wear mechanism on the wear surface of the bronze matrix specimen was adhesive wear. In this wear mechanism, the adhesion points formed between the specimen and disk first underwent plastic deformation and were subsequently broken under the influence of the shear stress generated between the friction pair as a result of relative motion. The flake-shaped material transferred from the bronze matrix, which was softer than the brake disk, to the brake disk and formed pits (Figure 9a and b). A comparable wear mechanism was identified in the low-temperature friction test of the Cu MMC material conducted by Xiao et al. [88]. Moreover, small particles formed by adhesive bond breakage acted as abrasive components and scratched the specimen surface (Figure 9b). Due to this friction–wear mechanism, the bronze matrix brake pad specimen exhibited a relatively high wear rate.

Figure 9 SEM micrographs of the worn specimen surface: (a)–(b) 0 wt%, (c)–(d) 3 wt%, (e)–(f) 6 wt%, (g)–(h) 9 wt%, and (i)–(j) 12 wt% B4C.
Figure 9

SEM micrographs of the worn specimen surface: (a)–(b) 0 wt%, (c)–(d) 3 wt%, (e)–(f) 6 wt%, (g)–(h) 9 wt%, and (i)–(j) 12 wt% B4C.

Moreover, cutting-type wear formed ploughing grooves on the wear surface of the specimen reinforced with 3 wt% B4C (Figure 9c and d). This process resulted due to the detachment of B4C particles from the matrix material on the wear surface. The voids formed by B4C particles provided suitable sites for crack initiation in the matrix due to local stress accumulation [45]. The weak bond structure formed by the graphite added to the specimen as a solid lubricant created another defect point for crack initiation [89]. Temperature fluctuations and tangential stresses during braking further promoted the formation and growth of cracks at these defect points. After the cracks reached a certain size, the thin plate-like wear particles between the surface and cracks detached and formed pits on the wear surface (Figure 9c and d). Thus, the wear rate increased [90]. Although some wear particles formed at the friction interface were expelled, some disintegrated into smaller particles. These particles accumulated around the B4C particles, which served as the primary contact plateau on the wear surface, and were transformed into secondary contact plateaus due to the combined effects of the contact pressure and temperature at the friction interface (Figure 9c and d). However, as B4C particles detached from the matrix, secondary contact plateaus that lacked the required primary contact support could not maintain their structural integrity and rapidly deteriorated [75]. This ultimately increased the wear rate.

Upon adding 6 wt% B4C to the matrix, additional nucleation points were formed that promoted the formation of secondary contact plateaus on the wear surface (Figure 9e and f). Secondary contact plateaus acting as a protective layer enhance the wear resistance by hindering the direct metallic contact between the friction pair during braking [91,92]. However, these plateaus were destroyed under repeated thermal stress because the contact plateau and substrate had different physical properties [25] (Figure 9f). The contact plateaus degraded to form wear debris that created abrasive wear lines on the wear surface.

These abrasive wear marks decreased, and the number of secondary contact plateaus increased as the number of primary contact plateaus increased on the worn surfaces of specimens reinforced with 9% B4C (Figure 9g and h). Higher B4C contents increased the abrasive effect of the specimen on the counterface, and iron particles transferred from the counterface material to the specimen [33]. The EDS analysis of region 1 (Table 3) revealed that the secondary contact plateau formed on the worn surface of the 9 wt% B4C-reinforced specimen contained ∼31.35% iron by atomic. This confirmed the transfer of iron particles from the counterface to the specimen surface. Due to thermomechanical interaction during braking, the surfaces of B4C particles that exerted an abrasive effect on the counterface were coated (Figure 9h) with elements such as Fe, Cu, C, Sn, Zn, Pb, Si, and O; this was confirmed via the EDS analysis of region 2 (Table 3). This coating considerably reduced the abrasive effect, causing the blunting of B4C particles. This finding was supported by the rapidly decreasing friction coefficients of specimens after the critical time required for abrasive particle blunting during braking. The B4C particles fractured due to tangential stress at the friction interface after the critical time period (Figure 9h), thereby the friction coefficient reduced. The wear surface of the 12 wt% B4C-reinforced specimens was characterized by the presence of wider and more continuous secondary contact plateaus with fewer defects, such as pits (Figure 9i). These plateaus contained lower amounts of iron compared with that in the secondary contact plateau of the 9 wt% B4C-reinforced specimen; this finding was confirmed via the EDS analysis of region 3 (Table 3). Some B4C particles oxidized and transformed into a B2O3 tribofilm that decreased the friction coefficient and specific wear rate by acting as a lubricant between the friction pair. Consequently, the transfer of iron particles from the counterface to the brake pad material was diminished. The dispersion strengthening effect was observed due to the detachment of hard ceramic B4C particles from the matrix during the friction process and their disintegration into small particles at the friction interface to participate in the formation of the secondary contact plateaus (Figure 9j). This effect also decreased the specific wear rate [25,90]. The presence of 74.15 and 72.86 at% of Fe and O, respectively, in the secondary contact plateaus of 9 and 12 wt% B4C-reinforced specimens indicated that oxidative wear was a significant phenomenon therein (regions 1 and 3, respectively). Zhang et al. [14] also reported via EDS analysis that the dominant oxidative wear mechanism was confirmed by the presence of 67 at% Fe and O in the film covering the wear surface of the Cu matrix brake pad.

Table 3

EDS analysis results of regions 1–4 in Figure 9g, h, i, and j (at%)

Regions Cu Sn Zn Pb Si Mn Fe C B O Possible phase
1 13.24 0.24 1.31 0.18 1.24 0.29 31.35 9.35 — 42.8 Iron-rich phase
2 7.59 0.09 0.73 0.2 1.6 — 27.84 11.81 — 50.13 Iron-rich phase
3 13.76 0.23 1.41 0.19 1.18 — 29.54 10.37 — 43.32 Iron-rich phase
4 0.17 — — — — — 0.67 25.28 70.37 3.51 B4C

Figure 10 shows the XRD analysis results, which offer insights into the phases formed due to thermomechanical interactions on the friction surfaces of specimens. The XRD spectra showed oxide peaks corresponding to Cu2O, CuO, FeO, Fe2O3, Fe3O4, ZnO, and B2O3 on the worn surfaces of specimens with different B4C contents. Xiao et al. [88] observed the formation of numerous analogous phases on the wear surface of copper matrix brake pad material due to tribochemical reactions occurring during friction. CuO₂, CuO, and ZnO were the predominant phases formed on the wear surface of specimens reinforced with B4C up to 3 wt%. When B4C content exceeded 3 wt%, the intensity of iron oxide peaks increased due to the abrasive effect exerted by B4C particles (primary contact points) on the counterface. Consequently, secondary contact plateaus were formed on the worn surface that were rich in iron oxide. XRD analysis also revealed that the B2O3 lubricating film formed on the wear surface of 12 wt% B4C-reinforced specimen due to the oxidation of B4C particles. This contributed to a reduction in the friction coefficient and specific wear rate.

Figure 10 XRD spectra of the worn surfaces of the specimens.
Figure 10

XRD spectra of the worn surfaces of the specimens.

A 3D optical profilometer was used to analyze the topographic features of the worn surfaces of specimens, and the corresponding images are shown in Figure 11. The surface roughness values of specimens reinforced with different B4C contents were ranked in the order of magnitude from high to low as follows: 3 wt% > 0 wt% > 6 wt% > 9 wt% > 12 wt% B4C. The participation of various wear mechanisms, including adhesive, abrasive, delamination, and oxidation, in the friction process that occurs during braking, has been identified as a contributing factor to the increasing surface roughness of brake pads [93]. The worn surface of the bronze matrix specimen contained pits of various dimensions and shallow abrasive wear marks due to adhesive wear, indicating moderate wear loss (Figure 11a). The 3 wt% B4C-reinforced specimen surface exhibited severe wear, forming deeper pits as well as wider and deeper grooves than those formed on the non-reinforced specimen; this indicated severe wear loss. These deep pits were formed due to delamination wear caused by cracks propagating from the subsurface region to the specimen surface. The deep and wide abrasive wear marks were formed because B4C particles detached from the matrix and caused cutting-type abrasive wear on the worn surface. These delamination and abrasive wear mechanisms resulted in the highest surface roughness of the 3 wt% B4C-reinforced specimen (Figure 11b). Thus, a higher B4C content facilitated the formation of secondary contact plateaus on the worn surface and reduced pits and abrasive wear marks. Consequently, the undulations on the worn surface decreased, ultimately reducing the surface roughness (Figure 11c–e). Similar observations were made by Yanar et al. [87].

Figure 11 3D morphologies of the worn surfaces of the specimens: (a) 0 wt% B4C, (b) 3 wt% B4C, (c) 3 wt% B4C, (d) 9 wt% B4C, and (e) 12 wt% B4C.
Figure 11

3D morphologies of the worn surfaces of the specimens: (a) 0 wt% B4C, (b) 3 wt% B4C, (c) 3 wt% B4C, (d) 9 wt% B4C, and (e) 12 wt% B4C.

Figure 12 shows the SEM images of the wear debris obtained after the wear test of specimens. Table 4 shows the EDS analysis results of regions A–F marked on the wear debris in this figure. With increasing B4C content, the size and shape of wear debris changed considerably. A large portion of wear debris from non-reinforced specimens and the 3 wt% B4C-reinforced specimens appeared as flakes (Figure 12a and b). This was caused by the breaking of adhesive bonds at the specimen–brake disk contact points in the non-reinforced specimen and delamination-type wear in the 3 wt% B4C-reinforced specimen. In both wear mechanisms, flake-shaped wear particles detached from the specimen surface undergoing wear. In addition, the 3 wt% B4C particle-reinforced specimen contained higher amounts of flake-shaped debris, indicating a high wear rate compared with the non-reinforced specimen (Figure 12b). A comparison of the wear debris composition on these two specimens revealed iron content as the distinguishing factor. The iron content in the wear debris of the non-reinforced specimen could not be determined, whereas an iron content of 14.21 wt% was detected in the wear debris of the 3 wt% B4C-reinforced specimen (regions A and B). The iron content resulted from the abrasive effect of the specimen on the counterface that caused iron transfer from the disk material to the specimen surface. As the B4C content increased beyond 3 wt%, the size of the wear debris particles decreased (Figure 12c–e) due to subsurface strengthening. The reinforced subsurface impeded the counterface asperities from penetrating deeply and cutting the specimen surface. Consequently, with each rotation of the brake disk, smaller wear debris particles were removed from the brake pad material; this improved the wear resistance [27]. Furthermore, fiber-shaped wear particles were formed when the B4C content exceeded 3 wt% (Figure 12c–e) due to the enhanced abrasive effect of B4C-reinforced specimens on the counterface. These fiber-shaped wear particles detached from the disk material, which was confirmed by the high iron content in the wear debris (region D). The EDS analyses of regions C and E revealed that 6 and 9 wt% B4C-reinforced specimens contained higher iron content in their wear debris than that found in the 3 wt% B4C-reinforced specimen. This clearly indicated the enhanced abrasive effect observed in specimens containing high B4C contents. The presence of B4C particles in the wear debris of the 12 wt% B4C-reinforced specimen was determined via EDS analysis of region F (Figure 12e). This was due to an increase in the tendency of the particles to cluster, which can be attributed to the elevated content of B4C. As a result of the cluster, microvoids were formed between the B4C particles in contact with each other. These microvoids weakened the B4C–matrix interfacial bond and caused B4C particles to detach from the matrix due to shear stress generated during friction.

Figure 12 SEM micrographs of the wear debris of the specimen with reinforced (a) 0 wt% B4C, (b) 3 wt% B4C, (c) 6 wt% B4C, (d) 9 wt% B4C, and (e) 12 wt% B4C.
Figure 12

SEM micrographs of the wear debris of the specimen with reinforced (a) 0 wt% B4C, (b) 3 wt% B4C, (c) 6 wt% B4C, (d) 9 wt% B4C, and (e) 12 wt% B4C.

Table 4

EDS analysis result of regions A–F in Figure 12 (wt%)

Regions Cu Sn Zn Pb Mg Si S Co Fe C B O
A 77.35 2.78 8.47 — — — — — — 8.82 — 2.57
B 51.99 1.66 5.6 — 1.97 0.89 1.55 1.01 14.21 12.25 — 8.86
C 56.2 1.86 5.94 1.75 — — — 1.16 23.84 6.53 — 2.62
D 5.07 — — — — 1.85 — 1.59 84.82 6.67 — —
E 43.64 1.41 4.79 — — 0.39 — — 39.17 6.33 — 4.18
F — — — — — — — — 1.93 29.28 66.54 2.25

4 Conclusions

Herein, bronze matrix brake pad materials reinforced with various proportions of B4C (0, 3, 6, 9, and 12 wt%) were fabricated via hot pressing. Their mechanical and friction–wear properties were analyzed in detail, and the following results were obtained:

  • The non-reinforced specimen had the highest density (7.22 g/cm³), whereas the 12 wt% B4C-reinforced specimen had the lowest density (6.07 g/cm³). The density decreased with increasing B4C content. The addition of B4C into the matrix considerably enhanced the hardness of the resulting specimens. The hardness value increased with increasing B4C content. The 12 wt% B4C-reinforced specimen exhibited the highest hardness value of 105 HRM, which is ∼15% higher than that of the non-reinforced specimen with the lowest hardness value.

  • Microstructural analysis revealed that when the B4C content exceeded 6 wt%, they formed partial clusters within the microstructure and decreased the TRS correspondingly.

  • The 9 wt% B4C-reinforced specimen exhibited the highest average friction coefficient (0.462) among all specimens. In contrast, a 3 wt% B4C-reinforced specimen exhibited the lowest friction coefficient (0.350). These values were within the desired range of 0.3–0.6 for automobile brake pads.

  • Except for the 3 wt% B4C-reinforced specimen, all other specimens exhibited lower specific wear rates than the non-reinforced specimen. This indicated that a B4C content of >3 wt% can enhance the wear resistance, which reduces the size of the wear debris particles. The specimen with the highest wear resistance generated smaller wear particles, whereas that with the lowest wear resistance produced larger plate-type wear particles. Various wear modes – abrasive, adhesive, delamination, and oxidative – were identified in the wear process of specimens.

  • The 9 wt% B4C-reinforced specimen exhibited the highest FSC (0.794) among all the specimens. Except for the 3 wt% B4C-reinforced specimen, all the other specimens exhibited higher FSC than the non-reinforced specimen due to the formation of highly stable secondary contact plateaus on the worn surface.

The addition of B4C (>3 wt%) improved the friction-wear properties of bronze matrix brake pad materials. The 9 wt% B4C-reinforced specimen exhibited the most favorable outcomes in terms of friction coefficient and FSC. In contrast, the 12 wt% B4C-reinforced specimens exhibited higher wear resistance. When the friction coefficient, wear resistance, and FSC values were evaluated together, specimens reinforced with 6, 9, and 12 wt% B4C were deemed the most suitable pad materials that met the brake performance requirements of high-performance vehicles. Although these materials met the necessary performance requirements as brake pads, their production costs must be reduced without posing any additional burden on the environment to improve their competitiveness. This can be achieved using diverse agricultural or industrial waste materials as reinforcement materials during the fabrication of brake pad materials without compromising their braking performance. This strategy will reduce the fabrication costs and mitigate the environmental damage caused by waste materials.

  1. Funding information: This research was supported by Tarsus University Scientific Research Coordination Unit (Project Number OSB.23.003).

  2. Author contributions: The author confirms sole responsibility for the following: study conception and design, data collection, analysis and interpretation of results, and manuscript preparation.

  3. Conflict of interest: Author states that there is no conflict of interest in the study.

  4. Ethical approval: The conducted research is not related to either human or animal use.

  5. Data availability statement: All data generated or analyzed during this study are included in this published article.

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Received: 2024-10-11
Revised: 2024-12-24
Accepted: 2025-01-03
Published Online: 2025-01-22

© 2025 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  4. Discovery of the bacterial HslV protease activators as lead molecules with novel mode of action
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https://doi.org/10.1515/chem-2024-0126
Keywords for this article
bronze matrix composites; brake pads; boron carbide; friction coefficient; wear mechanisms
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. Phytochemical investigation and evaluation of antioxidant and antidiabetic activities in aqueous extracts of Cedrus atlantica
  3. Influence of B4C addition on the tribological properties of bronze matrix brake pad materials
  4. Discovery of the bacterial HslV protease activators as lead molecules with novel mode of action
  5. Characterization of volatile flavor compounds of cigar with different aging conditions by headspace–gas chromatography–ion mobility spectrometry
  6. Effective remediation of organic pollutant using Musa acuminata peel extract-assisted iron oxide nanoparticles
  7. Analysis and health risk assessment of toxic elements in traditional herbal tea infusions
  8. Cadmium exposure in marine crabs from Jiaxing City, China: Insights into health risk assessment
  9. Green-synthesized silver nanoparticles of Cinnamomum zeylanicum and their biological activities
  10. Tetraclinis articulata (Vahl) Mast., Mentha pulegium L., and Thymus zygis L. essential oils: Chemical composition, antioxidant and antifungal properties against postharvest fungal diseases of apple, and in vitro, in vivo, and in silico investigation
  11. Exploration of plant alkaloids as potential inhibitors of HIV–CD4 binding: Insight into comprehensive in silico approaches
  12. Recovery of phenylethyl alcohol from aqueous solution by batch adsorption
  13. Electrochemical approach for monitoring the catalytic action of immobilized catalase
  14. Green synthesis of ZIF-8 for selective adsorption of dyes in water purification
  15. Optimization of the conditions for the preparation of povidone iodine using the response surface methodology
  16. A case study on the influence of soil amendment on ginger oil’s physicochemical properties, mineral contents, microbial load, and HPLC determination of its vitamin level
  17. Removal of antiviral favipiravir from wastewater using biochar produced from hazelnut shells
  18. Effect of biochar and soil amendment on bacterial community composition in the root soil and fruit of tomato under greenhouse conditions
  19. Bioremediation of malachite green dye using Sargassum wightii seaweed and its biological and physicochemical characterization
  20. Evaluation of natural compounds as folate biosynthesis inhibitors in Mycobacterium leprae using docking, ADMET analysis, and molecular dynamics simulation
  21. Novel insecticidal properties of bioactive zoochemicals extracted from sea urchin Salmacis virgulata
  22. Elevational gradients shape total phenolic content and bioactive potential of sweet marjoram (Origanum majorana L.): A comparative study across altitudinal zones
  23. Study on the CO2 absorption performance of deep eutectic solvents formed by superbase DBN and weak acid diethylene glycol
  24. Preparation and wastewater treatment performance of zeolite-modified ecological concrete
  25. Multifunctional chitosan nanoparticles: Zn2+ adsorption, antimicrobial activity, and promotion of aquatic health
  26. Comparative analysis of nutritional composition and bioactive properties of Chlorella vulgaris and Arthrospira platensis: Implications for functional foods and dietary supplements
  27. Growth kinetics and mechanical characterization of boride layers formed on Ti6Al4V
  28. Enhancement of water absorption properties of potassium polyacrylate-based hydrogels in CaCl2-rich soils using potassium di- and tri-carboxylate salts
  29. Electrochemical and microbiological effects of dumpsite leachates on soil and air quality
  30. Modeling benzene physicochemical properties using Zagreb upsilon indices
  31. Characterization and ecological risk assessment of toxic metals in mangrove sediments near Langen Village in Tieshan Bay of Beibu Gulf, China
  32. Protective effect of Helicteres isora, an efficient candidate on hepatorenal toxicity and management of diabetes in animal models
  33. Valorization of Juglans regia L. (Walnut) green husk from Jordan: Analysis of fatty acids, phenolics, antioxidant, and cytotoxic activities
  34. Molecular docking and dynamics simulations of bioactive terpenes from Catharanthus roseus essential oil targeting breast cancer
  35. Selection of a dam site by using AHP and VIKOR: The Sakarya Basin
  36. Characterization and modeling of kidney bean shell biochar as adsorbent for caffeine removal from aquatic environments
  37. The effects of short-term and long-term 2100 MHz radiofrequency radiation on adult rat auditory brainstem response
  38. Biochemical insights into the anthelmintic and anti-inflammatory potential of sea cucumber extract: In vitro and in silico approaches
  39. Resveratrol-derived MDM2 inhibitors: Synthesis, characterization, and biological evaluation against MDM2 and HCT-116 cells
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  41. Chemical composition of essential oils reviewed from the height of Cajuput (Melaleuca leucadendron) plantations in Buru Island and Seram Island, Maluku, Indonesia
  42. Phytochemical analysis and antioxidant activity of Azadirachta indica A. Juss from the Republic of Chad: in vitro and in silico studies
  43. Stability studies of titanium–carboxylate complexes: A multi-method computational approach
  44. Efficient adsorption performance of an alginate-based dental material for uranium(vi) removal
  45. Synthesis and characterization of the Co(ii), Ni(ii), and Cu(ii) complexes with a 1,2,4-triazine derivative ligand
  46. Evaluation of the impact of music on antioxidant mechanisms and survival in salt-stressed goldfish
  47. Optimization and validation of UPLC method for dapagliflozin and candesartan cilexetil in an on-demand formulation: Analytical quality by design approach
  48. Biomass-based cellulose hydroxyapatite nanocomposites for the efficient sequestration of dyes: Kinetics, response surface methodology optimization, and reusability
  49. Multifunctional nitrogen and boron co-doped carbon dots: A fluorescent probe for Hg2+ and biothiol detection with bioimaging and antifungal applications
  50. Separation of sulphonamides on a C12-diol mixed-mode HPLC column and investigation of their retention mechanism
  51. Characterization and antioxidant activity of pectin from lemon peels
  52. Fast PFAS determination in honey by direct probe electrospray ionization tandem mass spectrometry: A health risk assessment insight
  53. Correlation study between GC–MS analysis of cigarette aroma compounds and sensory evaluation
  54. Synthesis, biological evaluation, and molecular docking studies of substituted chromone-2-carboxamide derivatives as anti-breast cancer agents
  55. The influence of feed space velocity and pressure on the cold flow properties of diesel fuel
  56. Acid etching behavior and mechanism in acid solution of iron components in basalt fibers
  57. Protective effect of green synthesized nanoceria on retinal oxidative stress and inflammation in streptozotocin-induced diabetic rat
  58. Evaluation of the antianxiety activity of green zinc nanoparticles mediated by Boswellia thurifera in albino mice by following the plus maze and light and dark exploration tests
  59. Yeast as an efficient and eco-friendly bifunctional porogen for biomass-derived nitrogen-doped carbon catalysts in the oxygen reduction reaction
  60. Novel descriptors for the prediction of molecular properties
  61. Special Issue on Advancing Sustainable Chemistry for a Greener Future
  62. One-pot fabrication of highly porous morphology of ferric oxide-ferric oxychloride/poly-O-chloroaniline nanocomposite seeded on poly-1H pyrrole: Photocathode for green hydrogen generation from natural and artificial seawater
  63. High-efficiency photocathode for green hydrogen generation from sanitation water using bismuthyl chloride/poly-o-chlorobenzeneamine nanocomposite
  64. Special Issue on Phytochemicals, Biological and Toxicological Analysis of Plants
  65. Comparative analysis of fruit quality parameters and volatile compounds in commercially grown citrus cultivars
  66. Total phenolic, flavonoid, flavonol, and tannin contents as well as antioxidant and antiparasitic activities of aqueous methanol extract of Alhagi graecorum plant used in traditional medicine: Collected in Riyadh, Saudi Arabia
  67. Study on the pharmacological effects and active compounds of Apocynum venetum L.
  68. Chemical profile of Senna italica and Senna velutina seed and their pharmacological properties
  69. Essential oils from Brazilian plants: A literature analysis of anti-inflammatory and antimalarial properties and in silico validation
  70. Toxicological effects of green tea catechin extract on rat liver: Delineating safe and harmful doses
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