Tribological Behavior and Cold-Rolling Lubrication Performance of Water-Based Nanolubricants with Varying Concentrations of Nano-TiO₂ Additives

Abstract

This study aimed to investigate the effect of water-based nanolubricants containing varying concentrations (1.0–9.0 wt.%) of TiO₂ nanoparticles on the friction and wear of titanium foil surfaces. Water-based nanolubricants containing TiO₂ nanoparticles of varying concentrations were prepared and applied in friction and wear experiments and micro-rolling experiments to evaluate their performance regarding friction and wear properties. The findings indicated that the best results were achieved with a 3.0 wt.% TiO₂ nano-additive lubricant that significantly improved the tribological properties, with reductions in the COF and wear of 82.9% and 42.7%, respectively, compared to the dry conditions without any lubricant. In addition, nanolubricants contribute to a reduction in rolling forces and an improvement in the surface quality of titanium foils after rolling. In conclusion, nanolubricants exhibit superior lubricating properties compared to conventional O/W lubricants, which is attributed to the combined effect of the rolling effect, polishing effect, mending effect and tribo-film effect of the nanoparticles.

1. Introduction

Owing to their high specific strength, toughness and corrosion resistance, titanium foils (TFs) are used in many fields such as medical science and aerospace engineering [1,2,3]. Numerous advanced rolling methods have been developed to produce high-quality TF, but friction and wear are unavoidable during rolling, impacting the quality of the formed product and resulting in energy loss [4,5]. To minimize the effects of friction and wear, lubricants are commonly used to form a protective layer between contact pairs during the forming process [6]. However, traditional oil-based lubricants cause considerable environmental damage during the rolling process, which limits their application in industry [7]. In order to minimize the environmental pollution of lubricants, environmentally friendly lubricants have been developed to replace traditional oil-based lubricants. With the application of nanomaterials in lubrication technology and the growing insights into the special characteristics of nano-functional materials, nanolubricants exhibit excellent tribological behavior during the forming process, which leads to a decrease in energy loss caused by friction and an improvement in the surface finish of the formed products [8].

As a result of high stability and excellent tribological properties, nanoscale metallic oxide is frequently added to water-based lubricants to improve their overall tribological performance [9]. Thapliyal et al. [10] examined the service life of AISI E-52100 steel balls using copper nanofluid lubrication via rolling contact fatigue tests. The results indicated a significant enhancement in the fatigue life of the bearing balls with the use of nanofluids. Bao et al. [11] conducted a series of hot-rolling experiments based on a water-based lubricant containing SiO₂ nanoparticles and found that more refined grains were formed within the superficial layer of the rolling strips. Kong et al. [12] studied the frictional characteristics of a graphene-based hybrid lubricant using a ball-and-disk tester, demonstrating its effectiveness in reducing the coefficient of friction (COF) between the contact surfaces. Du et al. [13] examined the tribological performances of GO-TiO₂ nanofluids using a four-ball tribometer. The enhanced lubricating characteristics of GO-TiO₂ nanocomposites are attributed to the formation of absorption films, carbonaceous protective films and transfer films. Srivyas et al. [14] investigated the tribological performance of graphite, graphene nanoplatelets and h-BN. The results showed that the hybrid graphene nanoplatelets plus h-BN-based lubricating oil exhibited superior friction and wear properties with minimized COF. Nassef et al. [15] investigated the tribological and chemical–physical behavior of palm oil grease, which was improved by reduced graphene oxide (rGO) and zinc oxide (ZnO) nano-additives at different concentrations, and found that the addition of ZnO and rGO to palm oil increased the load-bearing capacity by 30% and 60%, respectively. In addition, the coefficient of friction was lowered by as much as 60%, which can be attributed to an absorbing layer formed by the 2D graphene nanoparticles that increased the load capacity, and ZnO chemically reacted with the metal surface layer to form zinc compounds that formed a lubrication film that served as a protective boundary.

Of the different types of nanoparticles used in nanolubricants, TiO₂ nanoparticles have shown great potential owing to their unique properties such as high hardness, excellent antioxidant capabilities and thermal stability, making them ideal candidates for enhancing the lubricating properties of water-based lubricants [16,17]. By adding TiO₂ nanoparticles to water-based lubricants, the lubricating efficiency and reductions in friction and wear resistance can be improved in mechanical systems [18]. Beel et al. [19] investigated the tribological effects of TiO₂ nanostructured particle dispersions in automotive engine base oils using a disk–disk tribometer. Their findings demonstrated a notable reduction in frictional force between the bearing steel and carbon steel surfaces. Wu et al. [20] studied the tribological behavior of innovative nano-TiO₂ additive lubricants on ferritic stainless steel 445 using a ball-on-disk tribometer. They found that the proposed lubricants exhibited excellent frictional characteristics due to the formation of a tribofilm of the TiO₂ nanoparticles. Sharma et al. [21] explored the TiO₂ surface modifications and found that the introduction of boron atoms promotes the formation of protective anti-wear films at the surface. Wang et al. [22] investigated the lubrication effect of a mixture of sodium polyphosphate and nano-TiO₂ as a water-based lubricant in hot rolling. It was found that the rotational and protective action of TiO₂ particles provided effective lubrication. Xia et al. [23] detected the contact angle to investigate the role of a nano-TiO₂ additive in O/W lubricant. The results showed that the minimum contact angle was obtained with the addition of 4.0 wt.% of the nano-TiO₂ additive to the O/W lubricant.

In this study, tribological tests were conducted to explore the performance of nano-TiO₂ additives as nanolubricants in terms of friction and wear characteristics. Additionally, the tribological performance of nano-TiO₂ additive lubricants was studied in the micro rolling of TFs. This study aims to clarify the friction reduction and anti-wear capabilities of water-based nanolubricants on TF surfaces compared to conventional oil-in-water (O/W) lubricants. It also aims to illustrate the lubrication mechanisms of water-based lubricants that incorporate TiO₂ nanoparticles.

2. Experimental Procedures

2.1. Materials

In this study, GCr15 steel balls and disks fabricated from pure TF were employed, and the detailed chemical compositions are shown in

Table 1. Chemical compositions of GCr15 steel (wt.%).

Element	C	Mn	Si	P	Cr	S	Ni
Content	0.96	0.31	0.22	0.014	1.52	0.002	0.004

and

Table 2. Chemical compositions of TF disks (wt.%).

Element	Ti	Fe	N	H	O	C
Content	≥99.6	≤0.014	≤0.008	≤0.0013	≤0.046	≤0.010

, respectively. The disks were machined to the dimensions of 100 × 25 × 0.1 mm³, and Figure 1 illustrates the surface morphologies and 3D profiles of the contact pair. Before the experiment, both balls and TF disks were cleaned with alcohol to ensure consistent surface conditions.

In addition, pure TFs were machined to dimensions of 150 × 5 × 0.1 mm³ for micro-rolling tests. Before conducting rolling experiments, the specimens were cleaned with alcohol to avoid residues left from the machining processes.

The detailed procedure for the preparation of the lubricant is presented as follows. First, for the nano-TiO₂ additive lubricants, sodium dodecylbenzene sulfonate (SDBS) was uniformly dissolved in water and stirred in a disperser at 8000 rpm for 10 min. Subsequently, polyacrylic acid sodium salt (PAAS) was added and dissolved in the suspension using a disperser under the same conditions. Then, the nano-TiO₂ particles were added to the suspension and stirred with the disperser at 8000 rpm for 10 min to make a homogeneous dispersion. Finally, the suspension was ultrasonically stirred for 10 min to disperse any remaining agglomerates. SDBS is a potent anionic surfactant that electrostatically prevents nanoparticle aggregation [24,25]. PAAS is a new functional polymeric material that facilitates the enhancement of lubricant viscosity [26]. The TiO₂ nanoparticles are rutile and are approximately 30 nm in diameter. For the O/W lubricants, 1.0 wt.% of oil was dispersed in water and stirred with the disperser at 8000 rpm for 10 min, followed by 10 min of ultrasonic vibration. The lubrication conditions for the tribological and micro-rolling tests are listed in

Table 3. Varying compositions of lubricants.

Lubricant Conditions	Description
DR	Dry
O/W	1.0 wt.% oil + balance water
L1	1.0 wt.% TiO2 + 0.2 wt.% SDBS + 0.3 wt.% PAAS + balance water
L2	3.0 wt.% TiO2 + 0.6 wt.% SDBS + 0.3 wt.% PAAS + balance water
L3	5.0 wt.% TiO2 + 1.0 wt.% SDBS + 0.3 wt.% PAAS + balance water
L4	7.0 wt.% TiO2 + 1.4 wt.% SDBS + 0.3 wt.% PAAS + balance water
L5	9.0 wt.% TiO2 + 1.8 wt.% SDBS + 0.3 wt.% PAAS + balance water

. The concentrations of the nano-TiO₂ additive lubricants varied from 1.0 to 9.0 wt.%. To assess the lubrication performance of the nano-TiO₂ additive lubricants, dry (DR) and O/W conditions were used as benchmarks.

2.2. Tribological and Micro-Rolling Tests

The tribological properties of the prepared nanolubricants were assessed with the reciprocating motion module using the MFT-5000 Multi-Environment Lubrication Test System (as shown in Figure 2). Before the tribological tests, the balls and TF disks were wiped with alcohol to standardize the surface condition. The tribological tests were conducted at room temperature, and the experimental conditions that were employed are listed in

Table 4. Tribological test conditions.

Sliding Distance/mm	Load/N	Frequency/Hz	Time/s
10	10	1	600

. Each lubrication condition was tested using three specimens to ensure accurate results through averaging.

Micro-rolling tests were conducted using a precise four-high micro-rolling mill with work rolls measuring 22 mm in diameter and 44 mm in barrel length. The micro mill is installed with high-precision piezoelectric force sensors, and the roll gap adjustment is accurately controlled by a self-designed PLC program. The tests were performed at a rate of 1 m/min with a 5% reduction under the various lubrication conditions detailed in Table 2. Before each test, the work roll surface was cleaned and uniformly sprayed with lubricant to ensure a consistent liquid film. Each micro-rolling test was repeated three times to minimize fluctuations in rolling force data and to derive average values for subsequent analysis.

2.3. Analytical Techniques

The surface morphologies of steel balls and rolled TFs were examined using a KEYENCE VK-X1000 3D laser scanning microscope (KEYENCE, Osaka, Japan). In addition, elemental distributions on the surfaces of the TF disks and rolled TFs were obtained using a JEOL-IT500 (Oxford, British) scanning electron microscope (SEM) that includes an energy-dispersive spectroscopy (EDS) detector. Nanoparticle distribution and micromorphology of TFs were also observed using SEM.

3. Results and Discussion

3.1. Tribological Behavior of Nanolubricants

Figure 3 shows the average COF values of rolled TFs under varying lubrication conditions. The highest COF value (0.473) was obtained under the DR condition. The average COF values of rolled TFs were significantly lower when using O/W or nano-TiO₂ additive lubricants compared to DR conditions. The minimum COF value of 0.081 was achieved when using nanolubricants containing 3.0 wt.% nanoparticles. Nevertheless, a remarkable increase in COF values were observed when the nanoparticle concentration increased from 3.0 wt.% to 9.0 wt.%. Compared to the DR and O/W conditions, a maximum reduction of 82.9% and 22.4% can be achieved by nanolubricants, indicating an outstanding reduction in the frictional behavior of nanolubricants.

Figure 4 presents the surface profiles of the worn balls after the tribological tests at varying lubrication conditions. Under DR conditions, obvious scratches were induced on the surface of the worn ball. Compared to the DR condition, the wear scar under the action of the nanolubricants was smaller and smoother with clean boundaries, indicating enhanced wear resistance. The smallest wear scar diameter (361.318 µm) was observed on the balls when using nanolubricants containing 3.0 wt.% nanoparticles, which was reduced by approximately 42.7% compared to the wear diameter of the worn ball under DR conditions (630.514 µm). However, wear became more severe as the concentration of TiO₂ nanoparticles increased to 9.0 wt.%, indicating a poor tribological performance of the nanolubricants at higher nanoparticle concentrations.

The SEM images and EDS mappings of the center zone of the wear tracks generated after tribological testing under varying lubrication conditions are illustrated in Figure 5. From the distribution of O elements within the EDS images, the distribution of nanoparticles can be analyzed. During the rolling processes, the nanoparticles with a spherical shape rolled at the contact area, reducing friction and polishing the surface, thereby exhibiting excellent anti-friction and anti-wear performance [27,28]. In Figure 5a, nanolubricants with 1.0 wt.% TiO₂ particles reveal that only a small number of particles entered the contact zone, which limited their ability to decrease the COF and ball wear. With an increase in the concentration of nanoparticles to 3.0 wt.%, more TiO₂ nanoparticles remained within the contact area, leading to an improved lubricating performance of the nanolubricants. In addition, the formation of a thin TiO₂ film on the worn surfaces of the disk was also clearly observed, as shown in Figure 5b. As shown in Figure 5c–e, as the concentration of TiO₂ nanoparticles further increased from 3.0 wt.% to 9.0 wt.%, a film formed by the deposition of TiO₂ nanoparticles was observed over almost the entire worn surfaces of the disk. The nanoparticles aggregated around the contact zone, forming a barrier that limited the supply of external nanoparticles [29]. Furthermore, nanoparticles that were in excess tended to aggregate and formed a dense film that resulted in an increased COF and more pronounced wear on the surface of the ball [30].

3.2. Tribological Performance of Nanolubricants in Micro Rolling

Owing to the superior tribological performance of nanolubricants containing 3.0 wt.% nanoparticles, their behavior in micro rolling was evaluated in comparison to their behavior in DR and O/W conditions. Figure 6 illustrates the rolling forces during TF micro rolling under varying lubrication conditions. In contrast to the DR condition, both O/W lubricants and nano-TiO₂ additive lubricants were effective in reducing the rolling force during micro rolling. A minimum rolling force (3.914 kN) was obtained when nanolubricants containing 3.0 wt.% nanoparticles were used. However, as the concentration of nano-TiO₂ additives increased from 3.0 wt.% to 9.0 wt.%, the rolling force increased from 3.914 kN to 5.382 kN. This phenomenon can be attributed to nanoparticle aggregation [5]. The surface roughness of the rolled TFs under varying lubrication conditions is depicted in Figure 7. The application of O/W lubricant reduced the surface roughness of the rolled TF, and a further reduction in the surface roughness of the rolled TF was observed when the same concentration of TiO₂ nanolubricant was applied. With a further increase in the concentration of TiO₂ additives to 3.0 wt.%, the surface roughness of the TF reached a minimum of 0.107 µm at this point. Therefore, a further increase in TiO₂ concentration led to an increase in TF surface roughness, which may have been caused by nanoparticle aggregation [23]. Overall, the optimal nanoparticle concentration that led to an enhanced lubricating behavior of the nanolubricants, i.e., the lowest rolling force and surface roughness, was 3.0 wt.%.

3.3. SEM–EDS Analysis

Figure 8 shows the surface profile and 3D morphologies of the rolled TFs. Characteristic undulations formed at the surface of rolled specimens under the DR condition (as shown in Figure 8a). When lubricated with nanolubricant containing 3.0 wt.% TiO₂ nanoparticles, the surface quality was improved (as shown in Figure 8d), indicating that the use of these nanoparticles can positively affect the surface quality of TFs. However, increasing the nanoparticle concentration to 9.0 wt.% resulted in a significant decrease in surface quality, which exactly matches the results presented in Figure 7.

Figure 9 presents SEM images and EDS mappings of the rolled TFs under various lubrication scenarios. In the case of samples rolled with lubricants that included 1.0 wt.% TiO₂ nanoparticles, Figure 9a illustrates a surface with more deformation and fewer scattered nanoparticles. As for the rolled specimens with lubricant containing 3.0 wt.% TiO₂ nanoparticles, more O elements are uniformly distributed on the rolled foil surface (as shown in Figure 9b), indicating an improved lubricating effect and leading to less rolling energy consumption and reduced surface roughness of foils. Nevertheless, more aggregated nanoparticles can be found on the surface as TiO₂ nanoparticles concentration increases to 9.0 wt.% (as shown in Figure 9e), resulting in significant contamination of the rolled foil surface [14].

3.4. Lubrication Mechanisms

To reveal the effects of the nanoparticles on the surface of TFs, a comparison of the lubricating mechanism in the DR condition and the nanolubricant condition is illustrated in Figure 10. Under the DR condition, severe abrasive wear occurred, and metal debris worn from the ball and disk surfaces remained in the contact area, as shown in Figure 10a. When using nanolubricants with 1.0 wt.% TiO₂ nanoparticles, few nanoparticles entered the frictional contact zone. The spherical TiO₂ nanoparticles suspended in the lubricant functioned akin to ball bearings within this zone. Additionally, the rolling and polishing effects of the nanoparticles contributed to a decrease in the COF between the frictional surfaces, resulting in a reduction in wear of the steel balls (as shown in Figure 10b). As the TiO₂ nanoparticle concentration rose from 1.0 wt.% to 3.0 wt.%, a protective frictional film was created by the nanoparticles between the contacting surfaces (as shown in Figure 10c), which prevented direct contact between the balls and the disk and thus enhanced the tribological behavior of the lubricant. Additionally, the scratches formed due to intense wear on the surface of the TF disk, allowing TiO₂ nanoparticles to fill the existing grooves. These grooves, together with the nanoparticles supplemented in the external area, filled the surface valleys through the so-called “mending” effect and kept the COF at a relatively low level, thus effectively reducing the wear of the disk after the running-in stage [31]. However, further increasing the concentration of TiO₂ nanoparticles to 9.0 wt.% accelerated the agglomeration of TiO₂ nanoparticles, as shown in Figure 10d. When the steel ball slid on the relatively rough TF disk, a number of nanoparticles agglomerated around the friction region to form a barrier, preventing a continuous supply of nanoparticles to the friction region and leading to an elevation in the COF and wear of the steel ball [32]. In addition, the agglomerated nanoparticles rolled between the roll and TF during micro rolling, which deteriorated the surface quality of the rolled TFs [33]. Overall, the optimal concentration of nano-TiO₂ additives in the lubricant, which was 3.0 wt.%, exhibited improved friction reduction and anti-wear effects on the surface of the TFs, which can be attributed to the combination of the rolling effect, repairing effect, polishing effect and tribo-film effect.

4. Conclusions

In this manuscript, the tribological performance of water-based nanolubricants was investigated. The main conclusions are as follows:

• Nanolubricants with an optimal concentration of TiO₂ nanolubricant additives, which was 3.0 wt.%, exhibited excellent tribological performance, reducing the COF and ball wear by 82.9% and 42.7%, respectively, compared to the DR conditions. This phenomenon can be attributed to the formation of a nano-TiO₂ friction film between the contact pairs.
• Compared with conventional O/W lubricants, the nanolubricants exhibited an improved lubrication performance during the micro-rolling processes, which confirms the superior lubricating effect of the nanolubricants.
• Applying nano-TiO₂ nanolubricants at a concentration of 3.0 wt.% led to a decrease in the rolling force and an enhancement in the surface quality of the TFs, which is primarily ascribed to the combined effects of the rolling effect, repairing effect, polishing effect and tribo-film effect.