Tribological Impact of Carbon Black Nanoparticles Obtained From Recycled Waste Tires as Additive to Motor Oil

Tire waste represents a significant environmental and economic issue faced globally. With the increase in the number of used cars worldwide, the number of used tires is also increasing. As a result, tire waste disposal has become a major global environmental and economic problem that will continue to grow without proper recycling practices. According to Gao et al. [1], it was estimated that the annual production of tires exceeds 3 billion, with an expected growth of over 4% in market demand. Consequently, billions of used tires will be discarded, which can cause environmental problems if not disposed of properly. Improper disposal of waste tires can create breeding grounds for insects or provide sites for vermin, and it can also pose a potential fire hazard in large tire dumps [2]. Moreover, Kuwait has the largest end-of-life tire graveyard; around 7 million units are stockpiled in one place [3]. To combat this environmental issue, it is essential to rethink and restructure basic human systems, which include waste management and recycling.

The main component of tires is rubber (around 47 wt%), which is neither a fusible nor soluble chemically cross-linked polymer that cannot be remolded into shapes without severe degradation [4,5]. In addition to rubber, tires are made of a complex mixture of carbon black (CB), steel belts and cord, sulfur, zinc oxide, and other components [6]. Due to this complex mixture, tires do not degrade naturally, and recycling them is not an easy task [7]. There are many different methods for recycling tires, such as retreading, reclaiming, combustion, grinding, and pyrolysis [8–10]. In each recycling method, the chemical composition of the recycled product varies.

The pyrolysis process is a widely known recycling method in the waste tire treatment field [11]. The pyrolysis process occurs by thermally decomposing waste tires in an oxygen-free atmosphere at 400 deg Celsius [8]. Pyrolysis has turned used tires into valuable raw materials through the thermal chemical process. After recycling a tire using the pyrolysis process, the following products are produced: tire pyrolysis oil, gas, char, and steel wire [3,12]. The advantage of using the pyrolysis process is that it helps reduce the economic and environmental impact of used tires. The benefits of pyrolysis over other tire-recycling methods are low amounts of harmful waste, the products formed have higher economic value, help reduce air pollution, and support energy control [13].

Lubricants are primarily used to create a barrier between two surfaces sliding against each other and are the most efficient way to control friction and wear in industrial processes [14]. Lubricants are typically liquids composed of oil and additional chemicals called additives, designed to enhance the oil's performance. During the lubricant operation, the three fundamental lubrication regime classifications are boundary lubrication, mixed lubrication, and hydrodynamic lubrication. A schematic representation of all these transition regimes is typically referred to as the Stribeck curve.

A mixed lubrication regime is where the applied load is supported by part through the fluid film and surface asperities [15]. Many engine components, such as piston rings and cams, operate in the mixed lubrication regime. The engine bearings could also work in a mixed lubrication regime under severe immediate loading [16]. Oil lubricants are mostly utilized to lower energy losses and avoid mechanical failure. However, in boundary and mixed lubrication regimes (particularly, during transit situations), friction and wear arise regardless of the presence of oil lubricants. Friction loss during the start-up of engines (at a temperature around 20 °C) can be 2.5 times higher than when the oil lubricant is entirely warm [17]. Therefore, extensive research has been devoted to improving the lubrication performance of existing oils in the mixed lubrication regime.

The role of additives in the mixed lubrication regime is vital, which is the focus of this study. The oil and solid additive mixture can minimize friction loss and reduce wear damage during transient conditions (in start-up and shut-down), low-speed, and high-load operation of automobile vehicles. Numerous studies have proven that mixing solid additives with oil lubricants helps lower wear and friction within the operating system [18–24]. Reduction of friction and wear depends on the additives' characteristics, such as shape, size, and concentration in the fluid [18]. Studies showed that nanoparticles are more effective than micro-sized particles in reducing friction due to their ability to show a ball-bearing effect, mending effect, creating a protective film, and polishing effect [25–27]. For example, carbon nanofibers were shown to have better friction reduction and antiwear properties than microscale carbon fibers [27].

Due to the increasing demands to minimize energy losses and improve surface protection of mechanical systems, several antiwear and antifriction additives were explored to improve the tribological performance of traditional oils [28–30]. Stearic acid-modified ceramic nanoparticles were shown to have lower wear-rate than zinc dialkyldithiophosphates (ZDDP) [31]. Their improved antiwear performance was attributed to the nanobearing effect of the nanoparticles in addition to the tribosinterization process on the sliding surfaces.

Solid additives are primarily useful in boundary and mixed lubrication regimes, where surface interaction exists despite having a liquid lubricant in between. Various carbon-based particles, such as fullerene, graphene, carbon nanotubes, and carbon nano-onion in the 1–30 nm range, have been examined as solid lubricants or oil additives [21]. In addition, it has been shown that the presence of spherical carbon particles between contacting surfaces in boundary and mixed lubrication regions might lead to rolling motion in the system and may perform as a nanoscale ball bearing [22]. Nowduru et al. [32] showed that 50–100 nm-sized carbon soot particles derived from waste rubber tubes, obtained from isothermal annealing at 900 °C, resulted in friction and wear reduction when added to a base oil lubricant. In their study, a four-ball tester setup was used under constant speed and room temperature conditions. They attributed this friction reduction to the nanobearing action of the carbon soot particles between moving surfaces.

Several studies showed the viable role of tire-recycled material in improving the tribological performance within their systems. Adesina et al. [33] conducted tribological and mechanical studies of epoxy coatings supported with waste tire rubber crumbs (WTRCs). They found that the mechanical and tribological behavior of polymer coatings could be enhanced by employing WTRC fillers. Toughness was found to be 22% greater than the pristine epoxy coating at a loading of 5 wt% of waste tire rubber crumbs. The friction coefficient of the polymer composite coatings decreased significantly after reinforcing with the WTRC for all tested loadings. Correspondingly, the resistance to wear loss was increased by more than 70% for a loading of up to 10 wt%. Nonetheless, the mechanical properties and wear resistance declined at high loading values of WTRC. They concluded that the 5 wt% loading of the WTRC-reinforced polymer coatings is the best loading that showed promising properties.

Due to the continuous efforts by the lubrication industries to improve motor oil lubricants, especially in the mixed lubrication region, while adhering to environmental regulations, this study aims to investigate the usage of carbon black particles obtained from recycled automobile tires via a pyrolysis process at 500 °C to enhance the performance of motor oil lubricants. This aim was achieved through experimental testing of the effect of CB particles on different properties of motor oil, such as friction, wear, and viscosity, when added to reference motor oil in different concentrations.

The CB particle powder sample investigated in this study was obtained from the fixed bed pyrolysis of end-of-life automobile tires reclaimed from municipal sources at an average operating temperature of 500 °C as described in Ref. [34]. An SAE 5W30 motor oil was used as a reference motor oil in this investigation to explore the tribological performance of CB particles in motor oil applications. The reference motor oil has a density of 0.861 g/cm³ at 288 K and a kinematic viscosity of 63 mm²/s and 11 mm²/s at 313 K and 373 K, respectively. This study examines different properties of the motor oil with carbon black (oil–CB) mixture.

The mixture of motor oil with carbon particles was prepared using ultrasonication. Preparing the lubricant mixture involves accurate and precise measurements of the weight of 10 ml of the reference motor oil (SAE 5W30). Then, a specific weight percentage (wt%) of CB particles is added to the motor oil. After that, the oil and CB were mixed with the ultrasonic horn processor; the sonication was applied in 60-s intervals with 30-s rest intervals in between to cool down the mixture. These steps were repeated until the mixture had a uniform distribution of the CB particles as determined by visual inspection (typically achieved in 3 min). The power used in the sonication apparatus was 20% amplitude. The same procedure was repeated for samples that included CB at different concentrations (0.5, 1, 2, 4 wt%).

Visual inspection of the CB particles' stability and suspension in the motor oil revealed that the ultrasonic mixing of CB particles in motor oil results in high stability of the homogeneity of the lubricant mixture for a long duration (up to 3 months).

First, a 1 wt% CB particles that were ultrasonically dispersed in 10 ml distilled water. Then, a drop of this mixture was placed on a steel disk and allowed to dry. Later, the dry sample of carbon black particles was tested to explore the particles' chemical components, size, and morphology using a field emission scanning electron microscope (FESEM) in addition to energy-dispersive spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffractometer (XRD), and wavelength-dispersive X-ray fluorescence (XRF) instruments. Then, the static and dynamic light scattering (SDLS) instrument was used to examine the size and stability of carbon black particles in motor oil without using any dispersant or surfactant.

The rheology study was achieved using a cylinder-in-cylinder configuration in the rheometer at different temperatures of 25, 50, and 75 °C. The obtained results will demonstrate the impact of carbon black particles on the motor oil's viscosity at various temperatures, shear rates, and shear stresses.

The tribological study was conducted according to the ASTM G99 standard using a tribometer apparatus in a ball-on-disk setup. A schematic and photos of the tribometer apparatus used in this study are presented in Supplemental Figure 1 available in the Supplemental Materials on the ASME Digital Collection for the reader's consideration. The pin specimen compromises a cylindrical holder with a ball at the end. The ball was held stationary, while the disk rotated during the tribotest. A 9.5 mm steel ball and a 50 mm in diameter steel disk were used to check the performance of carbon black additive in motor oil. The conditions of the tribotest were as follows: ball-on-disk setup, 20 N of applied normal load, and different lubricant temperatures of 25, 50, and 75 °C. The investigated temperatures were chosen since most industrial applications use oil lubricants in a temperature range from 40 °C to 80 °C [35]. Extreme temperatures can result in thermal decomposition of the oil lubricant. During each tribotest, the disk's rotational speed was varied between 1 and 1500 rpm, while the ball was held stationary. Variable speed and high-load conditions were used in our study to examine the oil–CB lubricant behavior in the boundary and mixed lubrication regimes. The sliding radius of the contact was about 12.5 mm ± 0.5 mm. To do multiple tests, the sliding radius was moved about 0.15 mm after each test to ensure testing on a new surface on the disk (with no wear scar). This slight change in the sliding radius will have a negligible effect on the obtained tribological results. Each disk was used to perform a maximum of three tribotests only. After each tribotest, an optical profilometer was used to scan the wear scar on the disk and the ball using 10X and 50X objective lenses. Postprocessing wear scar images involves obtaining a line profile across the disk wear scar to estimate the wear volume loss by numerically calculating the area between the horizontal axis and each line profile. The tribotest was performed using 10 ml samples of lubricant samples utilizing a liquid container with antisplash design that ensures that the area of contact is always covered by the examined lubricant sample. Before and after each test, a cleanse of the ball and disk was performed with acetone to capture clear wear scar images using the optical microscope. It should be noted that the tribometer study was conducted after only a few minutes of the sonication process of the samples. The duration of each tribometer test was 74 min, and the total sliding distance in each test was 3864 m. Multiple tests were conducted for each sample to confirm the accuracy and consistency of the experimental results, and the obtained outcomes were within a 5% error margin.

FESEM was utilized to determine the shape and the size of the CB particles, while EDS was used to examine the chemical components of the CB particles. Figures 1(a)–1(c) show different magnifications and scan positions of CB particles that were acquired to explore the carbon particles' structure carefully. These microimages clarify the overall submicron-sized and nearly spherical shape of the CB particles with a visible agglomeration between the CB particles. The chemical composition of the CB was studied using EDS, as shown in Fig. 1(d). It is clear that the CB particles are pure carbon (100 wt% of C), and the presence of gold element (Au) is discarded since the sample was coated with gold particles to visualize the CB particles in the FESEM apparatus.

Further examination of the size and shape of the CB particles was performed using AFM instrument. Several AFM tests were conducted in a tapping mode at different magnification to obtain 3D images of the CB particles as shown in Fig. 2. The AFM results revealed that the CB particles have an average size of about 250 nm with nearly spherical morphology (see Fig. 2(c)).

XRD measures the crystallinity and microstructure of solid samples. XRD was used to identify the main microstructure of the submicron CB particles obtained from the tire pyrolysis process. Figure 1(e) shows that the central element in the CB particles is carbon, with three distinctive peaks at 25 deg, 43 deg, and 78 deg. Similar XRD peaks for carbon samples were observed by Sonibare et al. [36]. Those XRD peaks indicate that the tested carbon sample consists of a proportion of particularly disordered materials in the form of amorphous carbon.

X-ray fluorescence is used to identify the composition of the material's element. XRF analyzes the chemistry of a sample to determine the fluorescent measurements of X-rays released from a sample when it is excited by the main X-ray source. Supplemental Figure 2 shows the chemical compounds of the carbon black particles. Using multiinstruments to examine the components of particles reduces the error and verifies that the main component of the CB particles is carbon.

The SDLS instrument measures the size of dispersed particles from a few nanometers to several micrometers in diameter. The SDLS instrument was used to perform several measurements of the size of CB particles directly after the ultrasonication process. As shown in Fig. 3(a), the average size of the CB particles was about 250 nm. Figure 3(b) shows continuous measurements of the average size of CB particles in oil for 21 days. The measurements in the average size of particles continued to be around 379 nm for the whole test duration. It should be noted that the sample in the container was held still during the entire test. These results demonstrate the stability and the suspension of the CB particles in the motor oil for an extended period of time.

After verifying that the CB particles used in this experiment are mainly made of carbon, and when mixed with motor oil utilizing the sonication process discussed earlier, the mixtures are stable with suspended particles, and rheological and tribological tests are conducted on the oil–CB mixtures, as shown later.

Viscosity is a critical physical characteristic of oil lubricants, which arises from resistance to fluid movement caused by intermolecular and internal shear friction forces. Various factors, including temperature, pressure, and shear rate, can influence the viscosity of an oil lubricant. Typically, the viscosity of oil lubricants declines as temperature rises and increases as pressure increases [37]. Newtonian fluid has a constant viscosity for any rate of shear strain, while the shear stress is linearly related to the shear strain rate. The viscosity of a fluid is significantly influenced by temperature. This study aimed to measure and compare the viscosity of a reference motor oil and oil–CB lubricant mixtures using the rheometer apparatus.

The viscosity measurements were performed at 25, 50, and 75 °C temperatures with a shear rate varying from 1 s⁻¹ to 100 s⁻¹. Figures 4(a)–4(c) demonstrate the viscosity changes versus the shear rate for pure oil and various concentrations of oil–CB lubricant mixtures. The results showed that the viscosity of all examined fluids slightly decreased with the temperature increase in all shear rates. Moreover, Figs. 4(a)–4(c) demonstrate that for pure oil, 0.5 wt% oil–CB mixture, and 1 wt% oil–CB mixture, similar viscosity behavior is seen with almost a constant line versus the shear rate, indicating that these low concentrations of CB in oil will not alter the viscosity behavior of oil lubricants.

In addition, Fig. 4(c) illustrates that at elevated temperatures of 75 °C and low shear rate, both 2 wt% and 4 wt% oil–CB samples showed a shear-thinning (non-Newtonian) behavior in the shear rate range of 1–10 s⁻¹. This non-Newtonian behavior could be attributed to the high concentration of the CB particles in thinner motor oil at high temperature. On the other hand, the carbon black particles at a low concentration (less than 2 wt%) had a minimal effect on the viscosity of the pure oil at 75 °C, as shown in Fig. 4(c). In agreement with these results, Motaher et al. [38] have shown that when TiO₂ nanoparticles were dispersed into n-octadecane at low mass fractions, a Newtonian behavior was found; however, increasing the TiO₂ particle’s mass fraction beyond 2 wt% led to shear-thinning and non-Newtonian behavior.

Supplemental Figure 3 shows the variation of shear stress with the shear rate for various temperatures of the motor oil and different oil–CB mixtures. For the same shear rate for all samples, the shear stress slightly decreases with the temperature rise, as shown in Supplemental Figure 3. Also, the shear stress for all samples increases linearly with the increase of the shear rate for all examined temperatures, except for the 75 °C temperature and at a low shear rate. For the case of 75 °C fluid with the higher concentrations of carbon black of 2 wt% and 4 wt%, samples behaved as a non-Newtonian fluid in the shear rate range starting from 1 s⁻¹ up to 10 s⁻¹. Supplemental Figure 3 indicates that the carbon black particles at low concentrations have a minor impact on the viscous performance of the motor oil and that the shear stress behavior for the motor oil and the 0.5 wt% and 1 wt% oil–CB lubricants are almost the same for all temperatures.

To carefully examine how carbon black particles affect the viscosity of motor oil with respect to temperature, a temperature ramp from 20 to 65 °C was executed, see Fig. 4(d). The sample's viscosity was determined under a constant shear rate of 5 s⁻¹. The viscosity of all samples decreased significantly as the temperature increased. Although adding 1 wt% carbon particles resulted in a slight increase (∼15%) in viscosity compared to pure oil, the pure oil and 0.5 wt% were similar in viscosity values at all temperatures. In contrast, the 2 wt% and 4 wt% oil–CB lubricants demonstrated a significant increase (more than 100%) in viscosity compared to the pure oil.

The previous findings indicate that carbon black particles can be used at low concentrations (0.5 wt% and 1 wt%) with a minimal change in the rheological characteristics of motor oil at various shear rates and temperatures. Therefore, the tribological performance in the hydrodynamic lubrication regime of motor oil and oil–CB mixtures with low concentrations of carbon black will not be impacted.

After examining the effects of CB particles on the viscosity of the motor oil, the tribological performance of the motor oil and oil–CB lubricants is investigated at different speeds and temperatures in this section.

Tribotests were performed on pure motor oil and oil with different weight percent concentrations of CB particles (0, 0.5, 1, 2, 4 wt%). The tribometer was used in a ball-on-disk setup with a 9.5 mm steel ball sliding on a 50 mm diameter steel disk in each tribotest. The applied normal load was set in the tribometer test to be constant at 20 N, resulting in a maximum Hertzian contact pressure of 1.3 GPa. This high contact pressure of the ball-on-disk setup will ensure that the lubricant is examined at the extreme conditions in the boundary and mixed lubrication regimes. The first step was to set up the tribometer to the desired testing temperature, which takes one minute to reach. Then, the rotational speed was increased from 0 to 1500 rpm in 2 min. Afterward, the speed was reduced in multiple steps from high to low in order to study the lubricant performance in the boundary and mixed lubrication regimes. Each step lasts 2 min, leading to a total tribotest duration of 74 min. The load and oil viscosity (η) were constant during each tribotest conducted in this study. While the speed varied from 0 to 1500 rpm and, therefore, the coefficient of friction (COF) was plotted against the (η × velocity)/load parameter.

The coefficient of friction of all samples varies as a function of (η × velocity)/load, as depicted in Fig. 5(a). The boundary lubrication regime is known to have almost a constant COF and occurs at low speeds [39]. The inset in Fig. 5(a), for a zoomed region of (η × velocity) per load between 0 and 10⁻³, shows that the COF is almost constant at low speeds for all oil samples. Also, the transition from boundary lubrication regime to mixed lubrication is clearly observed from the noticeable and rapid decline in COF values with the increasing speed as shown in Fig. 5(a). In Fig. 5(a), it is clear that the motor oil has the highest COF compared to all other oil–CB samples at all speeds. The obtained results demonstrate that the 2 wt% oil–CB has the lowest value of COF compared to the other mixtures at nearly all speeds. It can also be noticed that the 4 wt% oil–CB sample has very close COF values to the 2 wt% oil–CB sample; however, additional CB particles beyond 2 wt% led to increased friction. This rise in COF could be attributed to the interlayer shearing due to increasing the concentration of CB particles [40,41].

The percentage of friction reduction of all oil–CB samples compared to the pure motor oil sample at room temperature is presented in Fig. 5(b). In Fig. 5(b), the 2 wt% possesses the highest friction reduction at nearly all speeds, while the order from highest to the lowest friction reduction for the other samples is 4 wt%, then 1 wt%, and finally 0.5 wt%. Figure 5(b) illustrates that the friction reduction in 2 wt% is the highest, and any additional CB particles will not improve the friction reduction. Furthermore, the maximum friction reduction (10% to 25%) for all samples is in the speed range of 500–1000 rpm, which indicates that in this speed range, the lubricant is operating in the mixed lubrication regime. Therefore, the role of CB particles is vital in this mixed lubrication region by supporting some of the applied load and possibly functioning as freely rolling submicron particles. This role of CB particles helps reduce friction and minimize wear.

After each tribotest, the ball and disk specimens were cleaned and scanned using the surface profilometer and the optical microscope to obtain a 3D image and examine the wear scar. Wear analysis helps identify the capability of the lubricant to protect the sliding surfaces from wear losses, which eventually affects the functionality of mechanical components. Supplemental Figure 4 depicts the profilometer scan of the steel ball after the tribotest at room temperature for all samples. The ball specimen suffered from minor wear loss during the tribotests for all lubricant samples. Furthermore, surface roughness measurements were performed for a line profile across the ball wear scar for all lubricant samples (see Supplemental Table 1). The pure motor oil resulted in the highest surface roughness on the ball wear scar compared to other samples. While 2 wt% and 4 wt% oil–CB samples have the lowest surface roughness parameters.

An optical microscope was used to acquire images of the disk specimens after the tribotest for all lubricant samples. Figure 6 shows the disk wear scar after tribotest using pure motor oil lubricant compared to oil–CB lubricant mixtures. The microscopic images clearly show the significant wear scar reduction when 2 wt% of CB particles were added to the oil. Wear volume loss calculations revealed that the 2 wt% oil–CB reduced wear by about 88% compared to pure oil. This result demonstrates the role of CB particles in protecting and minimizing direct contact between sliding surfaces.

Since the 2 wt% of oil–CB lubricant demonstrated the best lubrication performance in the previous tribotest at room temperature, additional tribological analysis is conducted in this section to examine the performance of the 2 wt% of oil–CB lubricant at elevated temperatures compared to the pure motor oil. Given that fluid lubricants in most industrial systems operate at a temperature ranging between 40 °C and 80 °C [35], additional tribotesting was performed at 50 °C and 75 °C. Figure 7(a) depicts the Stribeck curve for pure motor oil and 2 wt% oil–CB samples at 50 °C. It can be seen that the CB particles slightly reduce the COF when compared to pure motor oil for (η × velocity) per load of 2.7 × 10⁻³ or higher. The friction reduction increases at higher speeds, as shown in Fig. 7(b), with a maximum value of 25% reduction at nearly 950 rpm.

The lubrication process of the CB particles is attributed to their submicron size, which makes them suitable for filling the void between asperities within the sliding surfaces, which helps minimize friction. However, at high speeds, the lubrication film thickness is greater than a micrometer in the hydrodynamic lubrication regime, which diminishes the role of CB particles.

Figure 8 depicts 3D profilometer scans of the steel ball after the tribotest at 50 °C for pure motor oil and 2 wt% oil–CB sample. The ball specimen suffered from minor wear loss during the tribotests for both samples. However, the pure motor oil produced a rougher wear scar with several wear pits on the ball specimen after tribotesting at 50 °C. In contrast, the 2 wt% oil–CB sample demonstrated a wear scar with a protective film adhered to the ball specimen after tribotesting at 50 °C.

The microscopic images of the steel disk after tribotests at 50 °C for the pure motor oil and the 2 wt% oil–CB lubricant, along with the line profile across the wear scar, are presented in Fig. 9. It is clear that the wear scar after the tribotest using the pure motor oil lubricant is wider, deeper, and higher in surface roughness than that of the 2 wt% oil–CB lubricant. The maximum wear scar depth after tribotest using the pure motor oil and 2 wt% oil–CB lubricant is about 0.6 μm and 0.4 μm, respectively. The microscopic images clearly show the significant wear scar reduction when 2 wt% of CB particles were added to the oil. Wear volume loss calculations revealed that the 2 wt% oil–CB was able to reduce wear by about 53% compared to pure motor oil at 50 °C. This result reveals the CB particles' role in protecting and decreasing direct contact between sliding surfaces, even at high temperatures.

Figure 10(a) depicts the Stribeck curve for the pure motor oil and the 2 wt% oil–CB sample at 75 °C. It can be seen that the 2 wt% oil–CB sample slightly reduced the COF compared to the pure motor oil for (η ×velocity) per load of more than 1.4 × 10⁻³. The friction reduction increases when reaching high speed; as shown in Fig. 10(b), it reaches a maximum of almost 30% reduction at nearly 1100 rpm. This behavior of the CB particles at high speeds was explained previously.

Figure 11 represents 3D profilometer scans of the steel ball after the tribotest at 75 °C for pure motor oil and 2 wt% oil–CB sample. The ball specimen experienced minor wear loss during the tribotests for both samples. Nevertheless, the pure motor oil resulted in a rougher wear scar with multiple wear pits on the ball specimen after tribotesting at 75 °C. On the other hand, the 2 wt% oil–CB sample produced a small wear scar with a protective film adhered to the ball specimen after tribotesting at 75 °C.

The microscopic images of the steel disk after tribotests at 75 °C for the pure motor oil and the 2 wt% oil–CB samples are presented in Fig. 12. The wear scar after the tribotest using the pure oil lubricant is larger with more depth than the 2 wt% oil–CB sample. The microscopic images clearly show the significant wear scar reduction when 2 wt% of CB particles were added to the oil. Wear volume loss calculations revealed that the 2 wt% oil–CB reduced wear by about 43% compared to pure motor oil at 75 °C. This result proves the role of CB particles in protecting and minimizing direct contact between moving surfaces.

Further, FESEM and chemical analyses of the wear track were performed to understand the lubrication mechanism of the CB particles. Figures 13(a) and 14(a) depict microimages of the disk wear scar after the tribometer test at 50 °C using pure motor oil and 2 wt% oil–CB sample, respectively. It can be seen that the width of the wear scar is smaller for the 2 wt% oil–CB sample compared to the pure motor oil. Furthermore, a dark film is visible on the disk wear scar for the 2 wt% oil–CB sample as shown in Fig. 14(a). Figures 13(b) and 14(b) present EDS chemical element analysis for the disk wear scar after the tribometer test at 50 °C using pure motor oil and 2 wt% oil–CB sample, respectively. It is clear that the wear scar for the 2 wt% oil–CB sample has a higher carbon content than the wear scar for the pure motor oil. Moreover, the chemical analysis revealed that the wear scar for the 2 wt% oil–CB sample has some sulfur content that is typically found in tires. Therefore, the previous analyses suggest that the CB particles created a tribo-film on the disk specimen during tribotest that helped minimize friction and protect the surfaces from wear losses.

To sum up, the tribometer testing results show that the best tribological properties were achieved using the 2 wt% concentration of CB in oil compared to other samples. The addition of CB particles was proven to lower friction by up to 25% and wear loss by more than 40% compared to pure motor oil at different temperatures. The lubrication mechanism of the CB particles is attributed to their submicron size, which makes them suitable for filling the void within the roughness of sliding surfaces, creating a protective film, and enhancing the area of contact, which helps minimize wear. Also, the spherical nature of the CB particles postulates their rolling movement between solid surfaces, which leads to lower friction.

As discussed in the previous section, the disk's rotational speed (in rpm) was varied from a minimum of 0 to a maximum of 1500, while the concentration has been varied from 0 to 0.5, 1, 2, and 4 wt%, while the temperature has been varied from 25 °C to 50 °C to 75 °C.

The R² value that expresses the proportion of variation explained by independent variables is 0.935, which implies that the model is a good fit for the experimental data. Figure 15 presents a 3D surface of the model where the COF is shown as a function of rpm and concentration for the three testing temperatures. It can be concluded from Fig. 15 that as the concentration increases from 0 wt% (i.e., pure oil) toward 2 wt%, the coefficient of friction decreases, then it slightly increases as the concentration rises beyond 2 wt%. This result agrees with the experimental data discussed earlier, further proving that adding 2 wt% of CB particles to motor oil provides the least friction in the mixtures. Moreover, this mathematical model (Eq. (1)) presents a helpful way to predict the friction at any given operating speed and temperature and for any concentration of the CB particles in oil.

This study investigated a potential application of carbon black extracted from waste tires in the lubrication industry. The morphology and the chemical composition analyses demonstrated that the carbon black material consists of submicron-sized and near round-shaped pure carbon particles that exist as agglomerated large particles with a size of less than 1 µm. The study explored mixing motor oil with carbon black particles at concentrations of 0.5, 1, 2, and 4 wt% to advance its lubricating properties. The oil and CB mixtures showed a homogenous nature with stable and suspended CB particles for an extended period.

The tribological behavior of pure motor oil and oil–CB lubricants at different speeds and temperatures was examined to recognize the role of CB particles in improving the oil's lubrication behavior. The obtained results demonstrate that the 2 wt% oil–CB has the lowest value of COF compared to other samples at nearly all speeds and with an operating temperature of 25 °C. Further tribological studies were then performed at elevated temperatures (50 °C and 75 °C), and the results proved the enhanced tribological performance of the 2 wt% oil–CB compared to the pure motor oil. The microscopic images clearly show the significant wear scar reduction when 2 wt% of CB particles were added to the oil compared to pure motor oil. Also, adding 2 wt% of CB particles was proven to lower friction by up to 25% and wear by about 40% relative to pure motor oil at different temperatures. The lubrication mechanism of the CB particles is attributed to their submicron size, which makes them suitable for filling the void within the roughness of sliding surfaces and increasing the area of contact, which helps minimize wear. Also, the spherical shape of the CB particles can make them perform as submicron ball bearings, which leads to lower friction.

The current study presented a feasible application of carbon black extracted from the tire pyrolysis process (recycled tires) as a solid additive to commercial motor oil. The obtained results showed that using a low concentration of CB in oil will improve the oil's tribological performance without affecting its rheological properties. Consequently, this study offers a viable application of recycled tire material that can help reduce waste tires' environmental and economic impact.

• The Kuwait University Nanotechnology Research Facility (KUNRF) (Grant No. GE01/07).

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.