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Article

Tunable Thermal, Mechanical, and Tribological Properties of Polybenzoxazine-Based Composite for Vehicle Applications

by
Yue Guo
1,
Xuhui Chen
2,
Guorong Wu
1,* and
Shiwen Huang
1
1
School of Architecture and Design, Nanchang University, Nanchang 330031, China
2
College of Art and Design, Shaanxi University of Science & Technology, Xi’an 710016, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(7), 1147; https://doi.org/10.3390/coatings13071147
Submission received: 22 May 2023 / Revised: 7 June 2023 / Accepted: 17 June 2023 / Published: 24 June 2023
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Abstract

:
In this study, a series of composites comprising polyether ether ketone (PEEK) and carbon fiber (CF)-reinforced polybenzoxazine for high-temperature friction materials for vehicle brake applications were developed using a high-temperature compression molding technique. The objective of this research was to systematically investigate the thermal, mechanical (tensile and flexural), and tribological performance of friction materials made from polybenzoxazine-based composites by varying the PEEK/CF mass ratio. Our study reveals the substantial improvement effect of the increased content of PEEK fibers on the thermal conductivity, the coefficient of friction, and the friction strength of the polybenzoxazine-based composite materials. Meanwhile, the introduction of carbon fibers was found to have a monotonic positive effect on the mechanical (tensile and flexural) properties and wear performance of the polybenzoxazine-based composites. The polybenzoxazine-based composites exhibit high mechanical strength, with a tensile strength of 50.1–78.6 MPa, Young’s modulus of 10.2–24.3 GPa, a flexural strength of 62.1–88.3 MPa, and a flexural modulus of 13.1–27.4 GPa. In addition, the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 exhibits a high and stable coefficient of friction (0.33) and a specific wear rate (1.79 × 10−7 cm3/Nm at room temperature). Subsequent to the wear test at ambient temperature, the worn surfaces of five polybenzoxazine-based composite samples with various PEEK/CF mass ratios were studied using electron microscopy technology (SEM). The observation of small cracks and tiny grooves on the worn surfaces indicates a combined abrasive and adhesive wear mechanism of the material. Our experimental results clearly reveal superior mechanical properties and excellent tribological characteristics. As a result, these composites show promising potential for the application of friction materials in terms of vehicle braking system applications.

1. Introduction

Advances in the automotive industry, especially electric vehicles, require vehicles with greater power to reach higher acceleration. Consequently, brakes turn out to constitute a critical security module in electric vehicles. In order to effectively control the aggressive acceleration of electric vehicles, advanced brake techniques are highly demanded. Technically, brake systems function successfully in vehicles through friction between the two brake discs and the conversion of the mechanical energy into heat [1]. The general operating temperature for a brake rotor in personal vehicles typically ranges from 200 to 270 °C and even reaches 370 °C in some extreme circumstances [2]. As a result, friction materials for brake systems must possess some specific characteristics, including stable friction coefficients over a wide temperature range, high wear resistance, etc. [3]. These requirements place extremely high mechanical and tribological performance demands on friction materials.
There are four key constituents involved in the fabrication of friction materials, namely, polymeric binders, reinforcing fibers, fillers, and friction modifiers [4]. Among them, the polymeric binder is of paramount importance, as it strongly binds all other components of the friction materials together, thus influencing the fundamental tribological properties, as well as the general structural stability [5]. To date, owing to their superb wetting capacity, mechanical properties, and structural stability, as well as their good affinity with other elements, phenolic resins are frequently used as organic binders [6]. Yet, this type of resin also exhibits some intrinsic drawbacks, including the small-molecule volatiles generated during curing, even at ambient temperature, the release of harmful by-products during processing, the existence of voids resulting from by-product release in the final components of vehicles, and the small coefficients of friction and substantial wear rates at high temperatures [7]. These drawbacks seriously restrict the employment of such types of resin in high-end applications, resulting in failures during braking processes and a short lifespan. To overcome these drawbacks, several polymeric binders have been developed for applications in friction materials, such as thermoplastic polyimide resin [8], silicone/phenolic copolymer [9], cyanate ester [10], epoxy resins [11], etc. Even though these polymeric binders display several benefits over their phenolic counterparts, they have numerous intrinsic disadvantages. The industry still thirsts for the development of high-performance polymeric binders for friction materials.
Recently, polybenzoxazine has emerged as a promising alternative polymeric binder that is potentially capable of replacing conventional phenolic resins. With a similar chemical structure to phenolic resins, polybenzoxazine exhibits fascinating and innovative properties, making it a perfect candidate as a polymeric binder in friction materials [12,13]. The advantages of polybenzoxazine include a low melt viscosity at curing temperatures, excellent interfacial adhesion with organic and inorganic fillers, thermally activated catalyst-free and curing-agent-free ring-opening reactions, volatile-free polymerization, etc. In addition, polybenzoxazine also exhibits other outstanding properties, such as little shrinkage during curing, low water absorption, superb glass-to-rubber transition temperature (Tg), high degradation temperature, and superior technical performance [14]. These properties are highly attractive with regard to the application of friction materials with exceptional tribological performance. According to the recently reported literature, the polybenzoxazine-based polymeric binder effectively enhances the tribological performance and mechanical performance of polybenzoxazine-based composites. Davide Carlevaris et al. studied two benzoxazine resins with the aim of testing their capabilities as polymeric binders for brake pads [15]. Their friction materials exhibit a lower mass loss in the operating temperature range of commercial brake pads and show promising tribological properties. Huanhuan Yao et al. employed a phosphorus-containing benzoxazine-bismaleimide resin as a polymeric resin for friction materials [16]. Their study shows that the proper inclusion of the polybenzoxazine resin can improve mechanical properties and also decrease the coefficient of friction and wear rate. Meng Su also studied the tribological properties and wear mechanisms of two high-performance polymer fibers containing composites using a polybenzoxazine resin binder with a ball-on-disk wear tester [17]. The research shows that the polybenzoxazine-based composite with polyimide fibers exhibits more favorable tribological properties at elevated temperatures. These studies all indicate that the incorporation of a polybenzoxazine binder produces friction materials with significantly enhanced thermal, mechanical, and tribological properties.
In addition to polymeric binders, other strengthening fibers are also critical in preserving the rigidity, endurance, and tribological characteristics of these friction materials [18]. Typically, over 5% of the ingredients in friction materials are strengthening fibers, such as asbestos fibers, carbon fibers (CFs), all-aromatic aramid fibers, inorganic ceramic fibers, glass fibers (GFs), steel fibers (SFs), and other types of natural fibers [19]. Both the mechanical and tribological characteristics of friction materials are greatly improved because of the exceptional qualities of these fiber fillers. Due to their excellent mechanical qualities, better structural integrity, thermal robustness, and flame-retardant characteristics, asbestos fibers have historically been used extensively as reinforcing threads in friction materials [20]. However, this type of fiber has been outlawed in numerous nations around the globe due to the cancer-causing and detrimental dangers that they pose to human health [21]. Consequently, non-asbestos organic friction compounds, particularly those without asbestos, have been progressively created. As one of the promising alternative organic fillers, polyether ether ketone (PEEK) has been employed as reinforcing fibers [22]. As a well-known high-performance polymer, PEEK displays exceptional wear resistance, extreme temperature stability, and a good rigidity-to-mass ratio. Additionally, PEEK also exhibits excellent interoperability with polymeric binders and can substantially improve the coefficient of friction of friction composites. For instance, Zhibin Lin et al. studied PEEK-containing hybrid wear systems using the dual-pins-on-disk tribometer [23]. The authors found that the PEEK fibers have a synergistic effect with polytetrafluoroethylene, thus resulting in the improved tribological performance of the composite. Leyu Lin et al. prepared a carbon-fiber-reinforced PEEK coating material using fused deposition modeling and studied its tribological properties [24]. Wei Wang et al. also prepared PEEK-based friction materials with attapulgite nanofibers [25]. The addition of attapulgite to PEEK significantly reduces the wear of PEEK. Moreover, a low dose of attapulgite nanofibers in carbon-fiber-reinforced PEEK greatly lowers the friction and wear.
Apart from PEEK fibers, carbon fibers (CFs) are being used extensively in friction composites owing to their extraordinary mechanical qualities, e.g., extraordinary tensile strength (3.5 GPa), strong modulus (>345 GPa), and outstanding corrosion resilience [26,27]. With the addition of CFs to the friction materials, the flexural strength can be improved to 48–68 MPa [28], and the storage modulus can be enhanced to 3.3–4.8 GPa [26]. Additionally, carbon fibers have advantageous tribological characteristics. As demonstrated in polyetherimide composites, carbon fibers, for example, can improve the wear resilience of friction composites at elevated temperatures [29]. Excellent friction-fade resilience can also be found in phenolic friction composites with CFs and natural rubber [30]. Yet, it should be noted that a low coefficient of friction is typically detected in carbon-fiber-reinforced friction materials. This is because of the self-lubrication effect of CFs [29,30].
In this study, we developed a series of polybenzoxazine-based friction materials with varying weight ratios of PEEK to CF as reinforcing fibers. We used the D940 benzoxazine resin as the organic binder due to its superior Tg of ~300 °C and excellent thermal stability up to 395 °C [31]. To improve the tribological properties of the polybenzoxazine-based composites at elevated temperatures, we employed a combination of PEEK fibers and CFs as reinforcing fibers. The resulting friction materials possess excellent wear resilience and a steady, high coefficient of friction. We methodically looked into how the mass ratio of PEEK to CF affects the mechanical and tribological characteristics of the polybenzoxazine-based composites. Additionally, we used scanning electron microscopy to examine the worn surfaces of the polybenzoxazine-based composites and suggested a reasonable wear process for the composites. The objective of this work was to systematically reveal the variations in the thermal, mechanical, and tribological properties of PEEK/CF-reinforced polybenzoxazine as a function of the materials’ compositions. In addition, we aimed to develop a practical high-temperature friction material for vehicle braking system applications.

2. Experimental Procedures

2.1. Materials

Benzoxazine (D940) was purchased from Sichuan EM Technology Co., Ltd. (Mianyang, China). Polyetheretherketone (PEEK) fibers were commercially supplied by Victrex (Lancashire, UK). Carbon fibers (CFs) (PUT C30 S003/6) were purchased from Sigrafil (Wiesbaden, Germany). Glass fibers (GFs) (552 b) were purchased from Jushi Co., Ltd., Tongxiang, China. Other materials, including steel fibers (SFs), graphite, zirconium silicate, and barium sulfate, were all purchased from Aladdin Co., Ltd. (Ontario, CA, USA) and used as received. Carboxylated nitrile rubber was purchased from Rahco Rubber Inc. (Plaines, IL, USA). The rubber dust was acquired as the bottom product of the cyclone unit using carboxylated nitrile rubber.

2.2. Preparation of Polybenzoxazine-Based Composites

Polybenzoxazine-based composites have been developed for use in friction materials using a classical approach [5]. These composite materials, which are commonly employed in braking applications, consist of four primary elements: polymeric binders, reinforcing fibers, organic and inorganic fillers, and others. To reinforce the composites, PEEK fibers, carbon fibers, iron fibers, and glass fibers are comprehensively used, while the polymeric binder selected in this study is the polybenzoxazine resin D940 due to its very high Tg and high thermal stability. A wide range of polybenzoxazine-based composites were formulated using various weight ratios of PEEK to carbon fibers, i.e., 100:0, 75:25, 50:50, 25:75, and 0:100. To make the polybenzoxazine-based composites comparable, the total mass contents of PEEK and CF in composites remained constant at 5%, a typical value for commercial brake pads. The detailed formulations of the polybenzoxazine-based composites can be found in Table 1.
In the fabrication process for the polybenzoxazine-based composites, all ingredients were carefully weighed and mixed using an internal mixer from Shanghai Baihong Machinery Co., Ltd. (Shanghai, China) at a temperature of 100 °C for a duration of 30 min. The total mass of the ingredients used was approximately 100 g. Once the ingredients were thoroughly mixed, the resulting homogeneous mixture was cooled naturally to room temperature before being transferred into a steel mold with specific dimensions. The mixture was then molded in a compression molder from Horb am Neckar, Lauffer, Germany, at a molding temperature of 280 °C and a hydraulic pressure of 20 MPa for a duration of 2 h. After the complete reaction of the benzoxazine precursor, the final cured polybenzoxazine-based composites were obtained after cooling to room temperature.

2.3. Characterization

The linear coefficients of thermal expansion (LCTE) of the polybenzoxazine-based composites were all measured on linear thermal expansion testing equipment (IDM-C0007-M1, IDM instruments, Victoria, Australia). The average LCTE values are calculated using the following Equation (1):
α = ∆ l / l 0 ∆ T
where  l 0  is the initial length,  ∆ l  and  ∆ T  are the length and temperature variations before and after the test, respectively.
The tensile and flexural experiments on the polybenzoxazine-based composites were all carried out on a universal testing machine (Instron 5882, Boston, MA, USA). The loading rate was set to 2 mm/min for all tests to ascertain the mechanical characteristics of the polybenzoxazine composites. For each sample, 5 specimens were tested. The mechanical performance was evaluated using the mean values and standard deviations of the 5 specimens. Using a pin-on-disk tribometer (KaiHua HT-1000, Jiangyin, China), the tribological properties of the polybenzoxazine-based composites at room temperature were assessed. The normal load was 800 g, while the sliding speed was 364 rpm over a period of 5 h. In the test, a 45# steel disc (Wuyang steel Co., Ltd., Pingdingshan, China) was used. The surface hardness was measured to be HRC44-55. Both the specimens and the friction pairs were polished with SiC sandpaper (2000#), followed by cleaning with ethanol in an ultrasonication bath for 10 min. Prior to the test, all the samples were dried at 80 °C for 2 h. The coefficient of friction was determined on the basis of data collection over a 5 h testing period, with a data interval of 2 s. The specific wear rate (w) is determined by Equation (2):
w = ∆ m ρ t F N L
where Δm is the weight loss of the sample after the tribological test;  ρ t  is the density of the polybenzoxazine-based composite; FN is the normal load; and L is the sliding distance.
The worn surfaces of the polybenzoxazine-based composites after the tribological test were meticulously observed by employing a scanning electron microscope (SEM, Hitachi S4300, Hitachi, Japan). The acceleration voltage was set to 5 kV. To ensure optimal imaging results, a thin gold layer was delicately deposited on all samples through a sputter coater (Ted Pella 108, Redding, CA, USA)

3. Results and Discussion

The mechanical (tensile and flexural) and tribological performance of the polybenzoxazine-based composite friction materials is strongly correlated with the reinforcing fibers utilized. Theoretically, these fibers directly determine rigidity, durability, and other frictional characteristics. To explore the effects of the PEEK and CF contents on these properties, a series of polybenzoxazine-based composite-based friction materials were developed by varying the PEEK/CF weight ratio as follows: 100:0, 75:25, 50:50, 25:75, and 0:100. In this formulation, the benzoxazine resin (D940) acts as the organic binder, while the PEEK fibers, CFs, GFs, and SFs serve as reinforcing fibers. BaSO4 and rubber dust are utilized as fillers, while graphite and ZrSiO4 are used as friction modifiers. All the ingredients were meticulously blended in an internal mixer to ensure thorough and complete mixing, after which the benzoxazine resin polymerized into a polybenzoxazine-based composite at 220 °C using the compression molding technique.

3.1. Thermal Conductivity

The thermal conductivity of friction material is critical in terms of the application of the braking system. As illustrated in Figure 1a, the polybenzoxazine-based composite without CFs (PEEK/CF mass ratio of 100:0) displays a low thermal conductivity of 0.8 W/(m·K). With the introduction of CFs into the composites, the thermal conductivity is significantly enhanced. In detail, the polybenzoxazine-based composites with PEEK/CF mass ratios of 75:25, 50:50, 25:75, and 0:100 exhibit thermal conductivity values of 1.3, 1.5, 2.3, and 2.6 W/(m·K), respectively. This confirms the effectiveness of using CFs to enhance the thermal conductivity of friction materials, which is consistent with a previous study on the enhancing effect of CFs on the thermal conductivity of epoxy [32]. Evidently, the enhanced thermal conductivity of the polybenzoxazine-based composite is attributed to the high thermal conductivity value of CFs. In addition, the linear coefficients of thermal expansion (LCTE) of the polybenzoxazine-based composites are also plotted as a function of the PEEK/CF mass ratio in Figure 1b. The polybenzoxazine-based composite without CFs, i.e., a PEEK/CF mass ratio of 100:0, displays an LCTE value of 38 × 10−6/°C. In contrast, the inclusion of CFs significantly reduces the LCTE values of the polybenzoxazine-based composites. Specifically, the polybenzoxazine-based composites with PEEK/CF mass ratios of 75:25, 50:50, 25:75, and 0:100 exhibit LCTE values of 25, 23, 21, and 22 × 10−6/°C, respectively, clearly demonstrating effectiveness in reducing the LCTE of friction materials using CF. The polybenzoxazine-based friction materials with lower LCTR values exhibit enhanced material dimensional stability and thus release internal thermal stresses during servicing in braking systems.

3.2. Mechanical Properties

Tensile properties
The mechanical performance, including tensile strength and Young’s modulus, of the polybenzoxazine-based composite friction materials is of utmost importance and depends mainly on the organic binder, fiber content, and fiber properties. The influence of PEEK fiber and CF fractions on the tensile properties of polybenzoxazine-based composites is depicted in Figure 2. It is apparent from the figure that the tensile properties of polybenzoxazine-based composites are significantly enhanced with an increase in the CF mass ratio. As demonstrated by Figure 2a, with the increase in CF contents in the friction materials, the polybenzoxazine-based composites exhibit a monotonic increase in tensile strength. The polybenzoxazine-based composite devoid of CFs (at a PEEK/CF ratio of 100:0) exhibits a tensile strength of 50.1 MPa. The polybenzoxazine-based composites containing CFs exhibit tensile strength values of 58.9, 64.8, 70.2, and 78.6 MPa, respectively, at PEEK/CF mass ratios of 75:25, 50:50, 25:75, and 0:100. Similarly, the Young’s moduli of the polybenzoxazine-based composites also exhibit a monotonic increase with the increasing PEEK/CF mass ratio, as shown in Figure 2b. The polybenzoxazine-based composite without CFs (at a PEEK/CF ratio of 100:0) displays a Young’s modulus of 10.2 GPa. The polybenzoxazine-based composites containing CFs exhibit Young’s modulus values of 14.5, 16.7, 19.8, and 24.3 GPa, respectively, at PEEK/CF mass ratios of 75:25, 50:50, 25:75, and 0:100. Such extraordinary tensile strength and Young’s modulus values are greatly contributed to by the exceptional mechanical characteristics of CFs. It is notable that the CFs exhibit an extremely high strength of >345 GPa and a strong modulus of >3.5 GPa [26]. The superb mechanical characteristics of polybenzoxazine-based composites are realized by the tension transference from the polybenzoxazine matrix to the CF fillers [33].
Flexural properties
Flexural characteristics, including flexural strength and the flexural modulus, play a significant role in how polybenzoxazine-based composite friction materials behave in terms of tribology. In Figure 3, the flexural characteristics of the PEEK/CF-reinforced polybenzoxazine-based composite friction materials are plotted as a function of the PEEK/CF mass ratio. Clearly, the trends in flexural strength and flexural moduli are similar to those observed for the tensile properties with the increasing PEEK/CF mass ratio. Briefly, the flexural properties of the polybenzoxazine-based composite are greatly enhanced with the increasing CF mass ratio. With the rising CF content, the flexural strength of the polybenzoxazine-based composite friction materials increases monotonically. The polybenzoxazine-based composite without CFs, i.e., a PEEK/CF ratio of 100:0, exhibits a flexural strength of 62.1 MPa. The flexural strength of the polybenzoxazine-based composites with increasing CF contents at PEEK/CF mass ratios of 75:25, 50:50, 25:75, and 0:100 is 71.5, 75.6, 83.2, and 88.3 MPa, respectively. Similarly, the flexural moduli of the polybenzoxazine-based composite friction materials are also enhanced by the rising CF concentrations. In detail, the polybenzoxazine-based composites with PEEK/CF ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 display flexural moduli values of 13.1, 17.2, 21.6, 24.0, and 27.4 GPa, respectively. The improvement in the flexural properties of the PEEK/CF-reinforced polybenzoxazine-based composites is attributed to the reinforcement of carbon fibers, which is similar to the reinforcing effect on tensile properties, as described above. The outstanding modulus, extraordinary strength, and high aspect ratio of CFs produce polybenzoxazine resins with exceptional mechanical properties [26]. The mechanical characteristics of the polybenzoxazine-based composites are enhanced by the CFs’ stress transference from the polybenzoxazine matrix to the fibers [33]. Notably, our PEEK/CF-reinforced polybenzoxazine-based composites exhibit significantly higher values of flexural strength over those of phenolic composites with Kevlar and nano-clay, i.e., flexural strength values from 29.5 to 46.5 MPa [34].

3.3. Tribological Properties

For friction materials, essential parts of vehicle braking applications, the tribological properties are critically important for polybenzoxazine-based composites [35]. The coefficient of friction (COF) and specific wear rate are the two key metrics that were utilized to assess the tribological performance of the polybenzoxazine-based composites. As shown in Figure 4a, the coefficient of friction is plotted as a function of the sliding distance of the PEEK/CF-reinforced polybenzoxazine-based composites. In general, there is a dramatic increase in the COF values when the initial sliding distance is lower than 100 m for all five polybenzoxazine-based composite samples. The polybenzoxazine-based composite with a PEEK/CF mass ratio of 100:0 exhibits the largest upsurge in the COF value with the initial sliding distance. With the increase in the CF mass concentration, the polybenzoxazine-based composites thereof display lower increasing rates of COF values as a function of the sliding distance. For friction materials, the real contact area is typically described as the momentary contact spots within a contact plateau. In general, the COF value of a friction material is inversely proportional to this real contact area [36]. In our polybenzoxazine-based composites, the PEEK fibers are embedded in the polybenzoxazine matrix, but they are not involved in the formation of primary contact plateaus. Actually, the contact surfaces become rougher with increased PEEK fiber loadings (i.e., decreased PEEK/CF mass ratios) in the polybenzoxazine matrix [37]. When performing a tribological test, the ball asperity wears the coarse contact surfaces of the composites. As a result, the major contact phases are thus generated. It also greatly enhances the actual contact field between the ball asperity and the test sample of the polybenzoxazine-based composites. Consequently, the mass concentration of PEEK fibers in the polybenzoxazine-based composites is inversely correlated with the increased rate of COF as a function of the sliding distance.
The polybenzoxazine-based composite with a PEEK/CF mass ratio of 100:0 manifests a COF plateau at ~0.39, whereas the corresponding composite with a PEEK/CF mass ratio of 0:100 presents a COF plateau at ~0.27. When the PEEK/CF mass ratios of the polybenzoxazine-based composites are in the moderate regions of 75:25, 50:50, and 25:75, the COF values are measured as 0.33, 0.30, and 0.28, respectively. It is worth noting that the COF values of our polybenzoxazine-based composites are significantly higher than those of the phenolic composites with CFs (~0.15) [38]. Notably, the COF values demonstrate a consistent declining trend with rising CF amounts in the polybenzoxazine-based composites. In contrast, the polybenzoxazine-based composite strengthened exclusively with CFs exhibits the lowest friction coefficient (~0.27), while that of the polybenzoxazine-based composite with only PEEK exhibits the greatest COF value of ~0.39. The reported phenolic composite with CF reinforcement also generates a similar trend in the COF reduction versus the rising CF concentration [39]. Since the CFs have a self-lubrication effect, augmenting the CF concentrations in polybenzoxazine-based composites will result in a reduction in COF values [40,41].
In Figure 4b, the specific wear rates of the PEEK/CF-reinforced polybenzoxazine-based composites are plotted. As observed in the trend of the COF variation, with high CF concentrations, the specific wear rates of the test samples of the polybenzoxazine-based composites decline accordingly. When the PEEK/CF mass ratios are 100:0, 75:25, 50:50, 25:75, and 0:100, the specific wear rates of the polybenzoxazine-based composites are measured as 2.13 × 10−7, 1.79 × 10−7, 1.32 × 10−7, 1.28 × 10−7, and 1.23 × 10−7 cm3/Nm, respectively. A similar improvement in the wear resistance through the addition of CFs has also been reported for other phenolic composites [39]. The higher specific wear rates observed in the PEEK/CF-reinforced polybenzoxazine-based composites can be attributed to the enhanced storage moduli and stiffness of the composites with higher CF contents (Figure 2). As a result, the structural stability of the polybenzoxazine-based composites is enhanced [28]. Notably, the specific wear rate values of the polybenzoxazine-based composites in this study are significantly lower than those of the CF-reinforced phenolic composites (2.5 × 10−7 cm3/Nm) [38].
Given the fact that the typical operating temperature range for a brake disc system is between 200 and 270 °C and can reach as high as 370 °C under extreme conditions, the COF and specific wear rate variations in the polybenzoxazine-based composite were investigated in the temperature range of 100–400 °C, slightly above the operational temperature. Notably, the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 exhibited a COF value of 0.33 in our study, which falls into the recommended COF range of commercial brake pads. Thus, the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 was chosen for further testing at elevated temperatures. In addition, the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 displays excellent mechanical and wear properties, which is a benefit for brake pad applications. The COF variations, as well as the specific wear rate as a function of the temperature of the chosen polybenzoxazine-based composite, are plotted in Figure 5. As shown in Figure 5a, the COF values of the PEEK/CF composite with a mass ratio of 75:25 are 0.35, 0.36, 0.39, 0.40, 0.44, 0.46, and 0.40 at 100, 150, 200, 250, 300, 350, and 400 °C, respectively. With the rise in temperature, the polybenzoxazine-based composite shows higher COF values. At temperatures exceeding Tg, the COF values increase within the temperature range of 300–400 °C, owing to the enhanced contact area between the friction pairs. Clearly, the polybenzoxazine-based composite is in a rubbery state when the temperature surpasses Tg, leading to a dramatically reduced storage modulus and stiffness. The composite is thus easy to deform, resulting in an increased real contact area [42]. When the temperature reaches 400 °C, a portion of the polybenzoxazine resin degrades, causing a small decline in the COF value of the polybenzoxazine-based composite at 400 °C [31]. Most importantly, the COF value of our PEEK/CF-reinforced polybenzoxazine-based composite at 350 °C is 0.46, a considerably greater value compared with that of friction materials based on ceramics, mullite fibers, and SFs (0.37) [43], as well as phenolic friction materials based on nanocomposites (0.34) at the same testing temperature [44].
The specific wear rate variation in the temperature range of 100–400 °C of the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 is depicted in Figure 5b. The specific wear rate values measured at temperatures of 100, 150, 200, 250, 300, 350, and 400 °C are 0.16 × 10−7, 0.24 × 10−7, 0.52 × 10−7, 0.59 × 10−7, 1.02 × 10−7, 1.10 × 10−7, and 1.21 × 10−7 cm3/Nm, respectively. The general trend of the specific wear rates is a monotonic increase with temperature, which has also been observed in other friction material systems [45]. Notably, a dramatic increase in the specific wear rate at 300 °C was detected for the sample of the polybenzoxazine-based composite, which is clearly due to the glass-to-rubber transition [31]. In the rubbery state, the composite is easily deformed, increasing the connection between the test samples and leading to an increase in the specific wear rate [42]. The substantial increase in the specific wear rate of the polybenzoxazine-based composite at 400 °C is due to the heating decomposition of the polybenzoxazine binder, causing the weakened binding between the polybenzoxazine resin and other parts of the composites [46]. Most importantly, our polybenzoxazine-based composites with PEEK and CFs exhibit much lower specific wear rate values compared with those of ceramic-based composite friction materials with mullite fibers and SFs (1.12 × 10−7 cm3/Nm) [43] and phenolic friction materials based on nanocomposites (1.12 × 10−7 cm3/Nm) at the same temperature [46].

3.4. Morphological Study

By analyzing the microscale morphology of the worn surfaces, the wear mechanism of the friction materials can be deduced. The microscale morphology of the worn surfaces is highly correlated with the coefficient of friction and the specific wear rate [47,48]. In this study, scanning electron microscopy (SEM, Hitachi S4300, Hitachi, Japan) was employed to study the worn surfaces of the polybenzoxazine-based composite friction materials. In Figure 6, the worn surface morphology of the polybenzoxazine-based composite samples with various PEEK/CF mass ratios is illustrated. The SEM images were recorded to observe the worn surfaces of the samples after room-temperature tribological experiments. As shown in Figure 6a, the presence of small cracks and tiny grooves can be detected on the worn surfaces of polybenzoxazine-based composite samples with a PEEK/CF mass ratio of 100:0. This is strongly associated with the abrasive wear mechanism [49]. Moreover, after the addition of CFs, tiny fractures perpendicular to the rolling directions can be detected on the worn surfaces (Figure 6b–e). This is due to the self-lubrication effect of CFs, which facilitates the formation and stabilization of the friction film [41]. The adhesive wear mechanism is strongly suggested by the observation of tiny pieces of friction films on the worn surfaces of the polybenzoxazine-based composites [50]. In general, the increase in the CF concentration reduces the tiny fractures and grooves on the worn surfaces; thus, the wear resistance is greatly enhanced. The self-lubrication characteristics of CFs also contribute to maintaining the contact tension within a reasonable range, resulting in a lower COF value and enhanced wear resilience [51].

4. Conclusions

To conclude, we have successfully developed a series of polybenzoxazine-based composites reinforced with various mass ratio combinations of PEEK/CF, which exhibit remarkable thermal conductivities and mechanical and tribological properties. The incorporation of CFs into the polybenzoxazine-based composites induces the significant enhancement of thermal conductivity and a decrease in LCTE. The polybenzoxazine-based composites boast high mechanical strength, as evidenced by a tensile strength of 50.1–78.6 MPa, Young’s modulus of 10.2–24.3 GPa, a flexural strength of 62.1–88.3 MPa, and a flexural modulus of 13.1–27.4 GPa. In addition, they also display high and stable COF values in a wide temperature range. The polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 exhibits a COF value of 0.33 and a specific wear rate value of 1.79 × 10−7 cm3/Nm at room temperature, while these values are 0.46 and 1.10 × 10−7 cm3/Nm at 350 °C. The incorporation of PEEK into the polybenzoxazine-based composites led to a large improvement in both the COF and friction stability, while the addition of CFs enhanced the wear resistance. In our study, the optimal PEEK-to-CF mass ratio for reinforcing the polybenzoxazine-based composites was 75:25. To further investigate the tribological mechanism of the polybenzoxazine-based composites, the worn surfaces of all the samples after room-temperature wear tests were analyzed by SEM. The appearance of small cracks and tiny grooves on the worn surfaces strongly suggests a combination of abrasive and adhesive wear mechanisms. Overall, our experiment suggests that PEEK/CF-reinforced polybenzoxazine-based composites hold great potential as high-performance friction materials for personal vehicles.

Author Contributions

Conceptualization and resources, G.W.; validation, S.H.; methodology, formal analysis, and investigation, Y.G.; data curation, X.C.; writing—original draft preparation, review, and editing, Y.G.; visualization and supervision, G.W.; project administration and funding acquisition, G.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Nanchang University Grant No. 220604023.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) The linear coefficients of thermal expansion of the polybenzoxazine-based composites as a function of the PEEK/CF mass ratio. (b) The thermal conductivity of the polybenzoxazine-based composites as a function of the PEEK/CF mass ratio.
Figure 1. (a) The linear coefficients of thermal expansion of the polybenzoxazine-based composites as a function of the PEEK/CF mass ratio. (b) The thermal conductivity of the polybenzoxazine-based composites as a function of the PEEK/CF mass ratio.
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Figure 2. (a) The tensile strength of the polybenzoxazine-based composites with various PEEK/CF mass ratios. (b) The Young’s moduli of the polybenzoxazine-based composites with various PEEK/CF mass ratios.
Figure 2. (a) The tensile strength of the polybenzoxazine-based composites with various PEEK/CF mass ratios. (b) The Young’s moduli of the polybenzoxazine-based composites with various PEEK/CF mass ratios.
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Figure 3. (a) The flexural strength of polybenzoxazine-based composites with various PEEK/CF mass ratios. (b) The flexural moduli of polybenzoxazine-based composites with various PEEK/CF mass ratios.
Figure 3. (a) The flexural strength of polybenzoxazine-based composites with various PEEK/CF mass ratios. (b) The flexural moduli of polybenzoxazine-based composites with various PEEK/CF mass ratios.
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Figure 4. (a) The COFs of the polybenzoxazine-based composites with various PEEK/CF mass ratios. (b) The specific wear rates of the polybenzoxazine-based composites with various PEEK/CF mass ratios. Both values were obtained at 25 °C.
Figure 4. (a) The COFs of the polybenzoxazine-based composites with various PEEK/CF mass ratios. (b) The specific wear rates of the polybenzoxazine-based composites with various PEEK/CF mass ratios. Both values were obtained at 25 °C.
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Figure 5. (a) COF value variation in the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 in the temperature range of 100–400 °C. (b) The specific wear rates of the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 in the temperature range of 100–400 °C.
Figure 5. (a) COF value variation in the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 in the temperature range of 100–400 °C. (b) The specific wear rates of the polybenzoxazine-based composite with a PEEK/CF mass ratio of 75:25 in the temperature range of 100–400 °C.
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Figure 6. SEM micrographs of worn surfaces of the polybenzoxazine-based composites with various PEEK/CF mass ratios. (a) PEEK/CF = 100:0, (b) PEEK/CF = 75:25, (c) PEEK/CF = 50:50, (d) PEEK/CF = 25:75, (e) PEEK/CF = 0:100.
Figure 6. SEM micrographs of worn surfaces of the polybenzoxazine-based composites with various PEEK/CF mass ratios. (a) PEEK/CF = 100:0, (b) PEEK/CF = 75:25, (c) PEEK/CF = 50:50, (d) PEEK/CF = 25:75, (e) PEEK/CF = 0:100.
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Table
Table 1. The ingredients and their concentrations in the polybenzoxazine-based composites.
Table 1. The ingredients and their concentrations in the polybenzoxazine-based composites.
Ingredients (wt%)Ingredient Compositions
100:075:2550:50250:750:100
Benzoxazine resin (D940)1515151515
PEEK fiber53.752.51.250
CF01.252.53.755
GF1010101010
SF2525252525
Graphite1515151515
ZrSiO466666
BaSO42020202020
Rubber dust44444
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Guo, Y.; Chen, X.; Wu, G.; Huang, S. Tunable Thermal, Mechanical, and Tribological Properties of Polybenzoxazine-Based Composite for Vehicle Applications. Coatings 2023, 13, 1147. https://doi.org/10.3390/coatings13071147

AMA Style

Guo Y, Chen X, Wu G, Huang S. Tunable Thermal, Mechanical, and Tribological Properties of Polybenzoxazine-Based Composite for Vehicle Applications. Coatings. 2023; 13(7):1147. https://doi.org/10.3390/coatings13071147

Chicago/Turabian Style

Guo, Yue, Xuhui Chen, Guorong Wu, and Shiwen Huang. 2023. "Tunable Thermal, Mechanical, and Tribological Properties of Polybenzoxazine-Based Composite for Vehicle Applications" Coatings 13, no. 7: 1147. https://doi.org/10.3390/coatings13071147

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