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Article

Effect of Counterbody Material on the Boundary Lubrication Behavior of Commercially Pure Titanium in a Motor Oil

by
Yizhao Liu
,
Mohammed Al-Shan
,
Richard Bailey
and
Yong Sun
*
School of Engineering and Sustainable Development, Faculty of Computing, Engineering and Media, De Montfort University, Leicester LE1 9BH, UK
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(12), 439; https://doi.org/10.3390/lubricants12120439
Submission received: 21 October 2024 / Revised: 27 November 2024 / Accepted: 6 December 2024 / Published: 10 December 2024
Browse Figures
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Abstract

:
Titanium possesses many useful properties and is a technologically important material in engineering. However, lubrication of titanium has long been a problem that has prevented titanium from being more widely used. This is due to its poor tribological properties, deriving from its high tendency towards adhesive wear, material transfer, and abrasive wear. Lubrication is a system engineering which involves material combinations, material surfaces, lubricants, and operating conditions as a system. In this work, the boundary lubrication behavior of commercially pure titanium (CP-Ti) sliding against various counterbody materials in a motor oil (0W-30) was investigated under ball-on-plate reciprocating sliding conditions. The counterbody materials (balls) include CP-Ti, ceramic (Al2O3), steel (AISI 52100), and polymer (nylon). The results show that depending on material combination, the lubricating behavior can be divided into three categories, i.e., (1) lubrication failure (Ti-Ti), (2) improved lubrication but with friction instability (Ti-Al2O3), and (3) effective lubrication (Ti–steel and Ti–nylon). Lubrication failure of the Ti-Ti pair leads to high and unstable friction and severe wear from both the plate and ball, while friction instability of the Ti-Al2O3 pair leads to friction spikes and high wear rates. Effective lubrication of the Ti–steel pair results in low and smooth friction and much-reduced wear rates of the Ti plate by nearly 10,000 times. However, there is a load-dependence of the lubrication effectiveness of the Ti–steel pair. Although the Ti–nylon pair is effectively lubricated in terms of much-reduced friction, the nylon ball suffers from severe wear. The friction and wear mechanisms of the various sliding pairs are discussed in this paper.

1. Introduction

Titanium is a technologically important engineering metal, owing to its many useful mechanical, corrosion resistant, and biomedical properties. It has the highest specific strength among all engineering metals. The use of titanium and its alloys in engineering can achieve reduced weight and reduced energy consumption, which will contribute to meeting the UN “net-zero” sustainable goal [1,2]. Therefore, titanium and its alloys have drawn increasing attention in various sectors of industry. For example, in the automobile industry, Ti alloys have been used to make engine valves, connection rods, and turbine disks in some high-end cars, which helps to reduce weight and improve fuel efficiency [3]. Technology development in refining and manufacturing has also led to a significant reduction in titanium cost, making it more affordable for general engineering uses [4,5].
However, the use of titanium and its alloys for making power transmission and tribological components faces a serious issue, i.e., their extremely poor tribological properties under dry sliding conditions [6,7,8]. Titanium has a high tendency towards adhesion and sticking onto the counterface, that being either itself or other metallic and ceramic materials [6,9]. Although lubrication is the best way to minimize metallic contacts between two contacting surfaces, and thus to minimize adhesion and reduce friction and wear [10], this does not apply to titanium as effectively as to steels and cast irons because it is difficult to lubricate titanium [11,12,13]. Common lubricants and their additives, such as mineral and synthetic oils, are not compatible with titanium surfaces [14,15,16]. This is due to the high tendency of titanium towards adhesion; asperity contacts in the lubrication system can lead to the sticking and transfer of titanium to the counterface, resulting in severe adhesive wear and destruction of the lubricant film. It has been claimed that lubrication is the last issue to be resolved before titanium can find more wide-spread uses in engineering [5].
Effective lubrication is benchmarked by low and smooth friction, much-reduced wear rates by several orders of magnitude as compared to those under unlubricated conditions, and the absence of adhesion and scoring on the worn surfaces. Attempts have been made during recent decades to develop specific lubricants for titanium, e.g., by using benzene and iodine mixtures [9], adding chloride compounds to base lubricants [17], adding nanoparticles to base oils [11,12,18,19], using ionic liquids as the lubricant or lubricant additives [20,21,22], and incorporating polymer microgels in an aqueous solution [23]. These new developments of lubricants have successfully reduced the coefficient of friction of titanium sliding against steel or itself to below 0.2 but, in most cases, the wear rates of titanium are still quite high. So far, there are no universal lubricants for titanium.
Lubrication is a system engineering which involves materials, material combinations, surfaces, lubricants, and operating conditions as a system [24,25]. Modifying the surface of titanium, either by depositing a surface hard coating [26] or through metallurgical surface treatments [4,27,28,29], has dramatic effect on improving the lubrication effectiveness of titanium sliding against steel in conventional oil lubricants. The counterbody material is also expected to have dramatic effect on the tribological properties and lubrication of titanium. Although the effect of various counterbody materials, including itself, steel, ceramic, and polymer, on the dry sliding wear behavior of titanium alloys has been studied by several investigators [7,8,30,31,32], very few studies have been reported on the effect of counterbody materials on the oil lubrication behavior of titanium, except for sliding against itself and steel [9].
In this work, experiments have been carried out to investigate the boundary lubrication behavior of commercially pure titanium (CP-Ti, grade 2) sliding against itself, steel, ceramic, and polymer under oil-lubricated conditions. The lubricant used is a popular motor oil, 0W-30, which contains additives formulated for ferrous alloys. The objectives of this study are threefold: (1) to clarify the effect of counterbody materials on the boundary lubrication behavior of CP-Ti, (2) to study the lubrication mechanisms, and (3) to investigate the load dependence of the Ti–steel sliding pair. This work would provide a useful reference guide for the selection of lubrication systems using conventional lubricants for potential applications, such as for light-weight engine and load bearing component applications.

2. Materials and Methods

Commercially pure titanium (CP-Ti) plates in the form of 20 mm × 20 mm square were cut from a titanium grade 2 sheet of 1.2 mm thickness. The composition and hardness of CP-Ti are given in Table 1. The surfaces of the plates were wet ground manually using SiC grinding papers down to the P1200 grade to achieve a surface finish of 0.11 μm (Ra). Four different counterbody materials, all in the form of spherical balls of 8 mm diameter with a surface finish of 0.08 μm (Ra), were used, including CP-Ti, alumina Al2O3 (ceramic), AISI 52100 steel, and nylon 6/6 (polymer). The nominal compositions and measured microhardness of the counterbody materials are listed in Table 1. Thus, four tribopairs were tested in this work, i.e., Ti-Ti, Ti-Al2O3, Ti–52100 steel, and Ti–nylon pairs.
Both dry and oil-lubricated sliding tests were performed using a laboratory scale reciprocating tribometer, where the Ti plate was sliding reciprocally against the stationary ball at a frequency of 1 Hz and a stroke amplitude of 8 mm. Figure 1 shows the test setup in the experiments. The friction force was measured by the attached load cell and recorded continuously by the incorporated computer. The measured friction force was divided by the applied contact load to obtain the coefficient of friction (COF). The first series of experiments were conducted under a constant load of 5 N for a duration of 3600 s, resulting in a total sliding distance of 57.6 m. The initial contact pressures for the four tribopairs under 5 N load were calculated according to the Hertzian theory and are listed in Table 2. The second series of experiments were conducted on the Ti–52100 steel pair at various contact loads under oil-lubricated conditions, ranging from 3 N to 20 N, corresponding to initial contact pressures ranging from 595 MPa to 1121 MPa. The Young’s modulus values used in the contact pressure calculations are as follows: E T i = 105   G P a ,   E A l 2 O 3 = 370   G P a ,   E s t e e l = 205   G P a ,   E n y l o n = 1.4   G P a .
Dry sliding tests were conducted at room temperature (22 °C) and in ambient air with a humidity of 40%. Oil-lubricated tests were conducted by immersing the plate and the ball in the lubricant oil during the test. The lubricant used was Castrol Edge 0W-30, a fully synthetic engine oil. This is a popular motor oil which has been formulated with a proprietary additive containing Zn and S to enhance protection against the friction and wear of engine systems. Table 3 lists the main properties of the 0W-30 oil. At least three tests were conducted under each condition, and the results were reproducible. The test setup and conditions were based on our previous work on tribology of titanium [31] and were similar to those reported by other investigators [4,15,23]. The test sliding and lubrication conditions are commonly found in engineering, such as in internal combustion engines, hydraulic systems, and pneumatic assemblies. Hertzian point contact is common in ball bearings in the gear box assembly and timing assembly of many internal combustion engines.
The initial minimum oil film thicknesses under 5 N load for the four contact pairs were calculated by the Hamrock and Dowson equation [33], considering the ball-on-plate contact configuration:
h m i n = 1.79 G 0.49 U 0.68 W − 0.073 R
where G, U, and W are the dimensionless parameters defined as follows:
G = αE′
U = (η0V)/(E′R)
W = w/(E′R2)
where E′ is the reduced modulus of the contact pair, α is the viscosity pressure coefficient of the oil, η0 is viscosity of the oil at ambient pressure, V is the mean velocity, and R is the radius of the ball. In the calculation, the α and η0 values at room temperature are taken from [34]: η0 = 142.17 mPa·s = 0.14217 Pa·s, α = 2.33 × 10−8 m2/N. The calculated results are listed in Table 2. It can be seen that the initial oil film thickness is around 0.011 μm for the Ti-Ti, Ti-Al2O3, and Ti–steel pairs, which is much smaller than the combined roughness of the contact bodies, i.e., R q 1 2 + R q 2 2 = 0.11 2 + 0.08 2 = 0.136   μ m . This confirms boundary lubrication. For the Ti–nylon pair, due to the much smaller Young’s modulus of nylon, the contact pressure is one order of magnitude smaller than those of the other pairs. This results in the formation of a thicker oil film of about 0.1 μm thickness, which is also smaller than the combined roughness of the two contacting surfaces.
After each sliding test, the plate and ball specimens were cleaned in soup water, running water, and then ethanol. An optical microscope (Nikon LV150N, Tokyo, Japan) was used to examine the morphologies of the worn surfaces and measure the sizes of the wear track on the plate and wear scar on the ball. Three-dimensional images of the wear tracks and scars were constructed by using the extended-depth-of-focus function of the Nikon microscope. The wear volume from the ball was calculated from the measured average diameter of the wear scar, while the wear volume from the plate was obtained by measuring the surface profiles across the wear track at three locations by using a surface profilometer (Taylor-Hobson, Leicester, UK). The wear rate in mm3/Nm was then obtained by dividing the wear volume by the contact load and sliding distance. A scanning electron microscope (SEM) (Carl Zeiss EVO LS 15, Jena, Germany) equipped with EDS facilities was also used to examine the worn surfaces and to perform EDS elemental mapping and spot analysis.

3. Results and Discussion

3.1. Friction

The first set of experiments were conducted at a constant load of 5 N to measure friction and wear of CP-Ti sliding against various balls under both dry and oil-lubricated conditions. Figure 2 compares the friction curves recorded under both conditions. It can be seen that under dry sliding conditions, all four sliding pairs experience relatively high and unstable friction with large fluctuations in COF. However, under oil-lubricated conditions, three categories of lubrication behavior can be identified, as follows.
(1)
Lubrication failure. The 0W-30 motor oil fails to lubricate the Ti-Ti pair (Figure 2a), such that the friction curve measured under oil lubrication is similar to that measured under dry sliding conditions. Both friction curves show large variations during sliding, with average COF values larger than 0.3 (Figure 3). Similar results have been reported by several investigators for such a sliding pair [6,16,31]. Clearly, the motor oil fails to prevent adhesion between the two contacting titanium surfaces. The average COF is actually slightly higher under oil-lubricated conditions than under dry sliding conditions (Figure 3). This could be explained by the oxidation of the titanium surfaces during dry sliding, which helps to slightly reduce friction [35,36].
(2)
Improved lubrication but with friction instability. This is observed for the Ti-Al2O3 pair (Figure 2b), where the COF is reduced from around 0.4 under dry sliding conditions to a base level around 0.14 under oil-lubricated conditions but friction under oil-lubricated conditions is instable, indicated by the many friction spikes. Many of these spikes have COF values larger than 0.3 and occur frequently during sliding. This suggests that an oil film is formed at the contact interface, which is responsible for the reduced base COF values, but the oil film is disrupted frequently during sliding, resulting in the friction spikes. As discussed later, the friction spikes are caused by the transfer of material from the Ti plate to the ball and the subsequent pull-out of grains from the Al2O3 ball, which results in third body plowing and destruction of the oil film. Such a phenomenon has not been reported previously.
(3)
Effective lubrication. This is observed for the Ti–52100 steel and Ti–nylon pairs (Figure 2c,d), where the friction curves recorded under oil-lubricated conditions are low and smooth, as compared to the high and fluctuating curves recorded during dry sliding. The effective lubrication of the Ti–steel pair with an average COF of 0.094 was found with some surprise (Figure 3), considering the reported poor lubrication of a Ti–steel pair in other work [9,12,37,38]. Thus, replicate tests were conducted many times in this work which confirmed the results shown in Figure 2c and Figure 3. Further experiments were also conducted to determine the load dependence of this Ti–steel pair under oil-lubricated conditions, as reported in Section 3.4. For the Ti–nylon pair (Figure 2d), although relatively high friction with an average COF of around 0.36 is observed under the dry sliding condition (Figure 3), the dry friction curve is relatively smooth and does not fluctuate as much as the Ti–Ti and Ti–steel pairs. Since nylon is a self-lubricating material [39,40], lubrication by oil is effective when sliding against Ti, with average COF values of around 0.12. There have been few reports on oil lubrication of Ti sliding against nylon in the literature.

3.2. Wear

Figure 4 shows the measured wear rates from the Ti plates and from various balls after sliding at 5 N under both dry and oil-lubricated conditions. Since in the Ti–nylon pair, the nylon ball hardly causes any wear to the Ti plate, plate wear of this pair is not included in Figure 4a since it is unmeasurable. From Figure 4a, it can be seen that under dry sliding conditions, all the plates, except for the one sliding against the nylon ball, have wear rates of around 9 × 10−4 mm3/Nm, indicating that the plates suffer from severe wear. Similar wear rates in the order of 10−4 mm3/Nm were reported by Qu et al. [8] for a Ti6Al4V alloy sliding against steel and ceramics under dry sliding conditions. The wear of titanium under dry sliding conditions is dominated by adhesive wear and abrasive wear mechanisms [6,7,8,37]. However, ball wear rates are different for different balls under dry sliding conditions: the CP-Ti ball has the highest wear rate of 1.6 × 10−2 mm3/Nm, while the nylon ball has the lowest wear rate of 2 × 10−3 mm3/Nm. This is obviously due to the different structure, composition, and properties of the balls.
Under oil-lubricated conditions, plate wear and ball wear vary in line with the three categories of lubrication behavior, as follows.
(1)
Lubrication failure. In the Ti-Ti pair, the wear rate of the Ti plate under oil-lubricated conditions is similar to that under dry sliding conditions (Figure 4a), which confirms the results of previous studies [13,16,41,42]. Furthermore, the wear rate of the Ti ball under oil-lubricated conditions is slightly larger than that under dry sliding conditions (Figure 4b). This, again, could be explained by the reduced oxidation of the titanium surfaces under oil-lubricated conditions. Oxidation during dry sliding may help to reduce wear [36]. Clearly, the lubricant fails to lubricate the Ti-Ti pair, resulting in high and unstable friction and severe wear to both the plate and ball.
(2)
Improved lubrication but with friction instability. The improved lubrication effectiveness of the Ti-Al2O3 pair results in reduced wear rates of the Ti plate and the Al2O3 ball by 2.4 times and 7 times, respectively (Figure 4). However, wear rates in the order of 10−4–10−3 mm3/Nm are still very high, within the regime of severe wear [43]. The high wear rates are obviously related to the friction instability, with many spikes (Figure 2b), which originates from material transfer from the plate to the ball and pull-out Al2O3 grains from the alumina ball, as discussed later.
(3)
Effective lubrication. Under oil-lubricated conditions, the low and smooth friction of the Ti–52100 steel pair (Figure 2c) is accompanied with a significant reduction in the wear rates of both the Ti plate and the steel ball (Figure 4). The wear rate of the Ti plate is reduced from the order of 10−3 mm3/Nm under dry sliding to the order of 10−7 mm3/Nm under oil-lubricated conditions, a reduction of four orders of magnitude (Figure 4a). The wear rate of the 52100 steel ball is also reduced significantly by three orders of magnitude under oil-lubricated conditions, as compared to that under dry sliding conditions (Figure 4b). Effective lubrication has successfully changed the wear regime of the tribopair from severe wear to mild wear. In the Ti–nylon pair, wear from the Ti plate is unmeasurable under both dry sliding and oil-lubricated conditions, which could be due to the fact that nylon is much softer than CP-Ti (see Table 1) and thus it cannot cause any measurable wear to Ti. However, under both dry sliding and oil-lubricated conditions, the nylon balls suffer from severe wear, with similar wear rates in the order of 10−3 mm3/Nm. Thus, the improved frictional behavior of the Ti–nylon pair by oil lubrication does not prevent severe wear of the soft nylon ball.

3.3. Friction and Wear Mechanisms

The worn surfaces of the Ti plates and various balls were examined microscopically in order to shed light on the principal wear mechanisms. Since oil lubrication is the main focus of this work, the worn surfaces resulting from lubrication testing are presented in this section. The friction and wear mechanisms are discussed in terms of the three categories of lubrication behavior, as follows.
(1) Lubrication failure. Figure 5 shows the microscopic images of the wear track on the Ti plate sliding against a Ti ball (Ti-Ti pair). The wear track on the plate is wide, deep, and rough, with many parallel abrasion marks (Figure 5a). The wear track is more than 1.5 mm wide and 50 μm deep. There are clear signs of adhesion, abrasion, and crack formation (Figure 5b). Thus, the principal wear mechanisms under oil-lubricated conditions are adhesive wear and abrasive wear, which are similar to those observed under dry sliding conditions [31]. Similar wear mechanisms are also observed on the corresponding Ti ball (Figure 6a). Clearly, the oil film fails to prevent severe adhesion between the two Ti surfaces. Adhesive wear leads to material transfer and prow formation, which in turn can lead to accelerated abrasive wear. The severe wear can then disrupt and squeeze out the oil film [4], leading to total failure of lubrication, as shown in Figure 2a and Figure 3.
EDS composition analysis reveals that the worn surface is mainly composed of Ti (Figure 6b), with traces of C (Figure 6c) and O (Figure 6d). The C and O, which are believed to be the residuals of the lubricant, are mainly accumulated at the adhesive wear areas (Figure 6). No other elements relating to oil additives, such as Zn and S, are detected on the worn surfaces of the plate and ball, suggesting that no tribofilm is formed or sustained on the sliding surfaces. This is not surprising because the oil and its additives are formulated for ferrous materials and are not compatible with Ti surfaces [14,15,16]. A tribofilm would have prevented direct metallic contact and helped to reduce friction and wear [25,29]. But this does not happen to the Ti-Ti sliding pair.
(2) Improved lubrication but with friction instability. Figure 7 shows the wear track on the Ti plate sliding against an Al2O3 ball (Ti-Al2O3 pair). Again, the wear track is very rough, with many parallel abrasion marks, and has clear signs of adhesion, suggesting that the principal wear mechanisms are adhesive wear and abrasive wear. Prow formation is also evident on the worn surface. Some of the prows are flattened and have smooth surfaces (areas marked A in Figure 7b), indicating that they become the real areas of contact. The reduced real contact area may help to sustain an oil film at the non-contact areas and to reduce friction, thus achieving a relatively low base level of COF, shown in Figure 2b.
On the other hand, the worn surface on the Al2O3 ball has different morphologies (Figure 8). Two distinct regions are observed on the wear scar on the ball: relatively smooth regions with patches of metallic colored materials, and dark regions with pits and craters. The metallic colored patches are the material transferred from the Ti plate, as confirmed by EDS analysis, shown in Figure 9. EDS elemental mapping (Figure 9b) clearly shows the existence of Ti on the wear scar on the alumina ball, and EDS spot analysis shows that the amount of Ti in the metallic patches (spot 2 in Figure 9a,d) is much larger than that in the pitted area (spot 1 in Figure 9a,c). Clearly, material transfer from the Ti plate to the alumina ball occurs during sliding. However, EDS analysis of the worn surface on the Ti plate could not detect any Al, suggesting that material transfer is a one-way process. It can thus be postulated that the transferred Ti forms patches of material on the ball, which then cause the accelerated abrasion of the plate. The subsequent removal of the transferred material from the ball during repeated sliding leads to the pull-out of Al2O3 grains from the ball. The pull-out grains are trapped at the contact interface and lead to the formation friction spikes, shown in Figure 2b.
Figure 8b shows the grain pull-out areas on the Al2O3 ball, which have various sizes and can be as deep as several microns (Figure 8c). Figure 9c clearly shows that the pull-out areas are almost Ti-free, suggesting that it is the removal of the transferred material that leads to grain pull-out from the ball. If particles of such sizes are trapped at the contact interface, it can be conceivable that friction would be spiked due to the increased plowing contribution to friction [37,44,45]. From the EDS spectra shown in Figure 9, it is also evident that no elements related to oil additives, such as Zn and S, are detected on the ball. Similar analysis was also performed on the Ti plate which detected Ti, C, and O only. Clearly, a tribofilm based on oil additives does not form or sustain at the worn surfaces. This explains why material transfer from the Ti plate to the alumina ball could not be completely prevented under the present oil-lubricated conditions.
(3) Effective lubrication. For the Ti–52100 steel pair, where effective lubrication is achieved, different worn surface morphologies are observed, see Figure 10. The wear track on the Ti plate is narrow and shallow and has no signs of scoring and adhesion (Figure 10a). On the corresponding steel ball, the wear scar has two distinct regions, i.e., the outer region which has a shiny and polished appearance and the central region which has a gray color and contains some shallow scratch marks (Figure 10b). There is no evidence to suggest adhesive wear in this sliding pair. Thus, the lubricant successfully prevents adhesion between the two sliding surfaces, and effective lubrication is achieved. It is believed that this is due to the formation of a tribofilm in the central region of the wear scar on the steel ball (Figure 10b).
Indeed, EDS analysis detects Zn and S on the wear scar on the ball, but Ti is not detected, see Figure 11. The existence of Zn and S on the ball wear scar is further confirmed by EDS spot analysis at several locations. An example spectrum is given in Figure 11d. This suggests that a tribofilm based on the oil additives is formed on the steel ball and there is no material transfer from the Ti plate to the steel ball, i.e., there is no adhesive wear on the Ti plate.
SEM examination shows that there are only asperity contacts on the plate and the worn surface on the plate has a smooth and polished appearance (Figure 12a). EDS analysis could only detect Ti, C, and O on the plate and no other elements such as Zn, S, and Fe are detected (Figure 12), suggesting no tribofilm formation on the Ti plate and no material transfer from the steel ball to the plate. Clearly, a tribofilm is formed only on one of the contacting surfaces (i.e., the steel ball surface) and this is sufficient to prevent adhesive wear and thus to achieve effective lubrication. Since the lubricant oil and its additives are formulated for ferrous alloys, it is reasonable that a tribofilm can be formed on the steel ball surface but not on the Ti plate surface. However, to achieve effective lubrication, the tribofilm should be formed at the very early stage of sliding to prevent the adhesion of Ti to the steel ball. Any Ti transferred to the steel ball would inhibit the formation of a tribofilm, e.g., under heavy loading conditions, as further discussed in Section 3.4.
For the Ti–nylon ball pair, lubrication is effective in terms of much-reduced friction (Figure 2d). However, in terms of wear, although wear from the Ti plate is unmeasurable, the nylon ball suffers from severe wear (Figure 4b). Figure 13 shows the wear scar on the nylon ball. The wear scar is large, with many micro cracks and a few scratch marks. When sliding against metal, nylon normally suffers from adhesive wear through material transfer to the metal surface [39,40]. Micro crack formation on the worn surface under the present oil-lubricated conditions also suggests fatigue wear arising from the repeated contact stress cycles. It is known that liquid lubricants may cause the premature decay of nylon materials, which may explain the high wear rates under oil-lubricated conditions.

3.4. Effect of Load on the Ti–52100 Steel Pair

The previous sections demonstrate that the Ti–52100 steel pair can be effectively lubricated under a contact load of 5 N. This is due to the formation of a tribofilm on the steel ball surface, which prevents the adhesion of Ti from the plate to the steel ball. According to the theory [16,25,46], the formation of a tribofilm requires the decomposition of the additive molecules and chemical reactions between the decomposed products and the steel sliding surface. The tribofilm then prevents further metallic contacts and adhesion between the contacting asperities. Thus, the formation and sustainability of the tribofilm play an important role in maintaining lubrication effectiveness. Therefore, further tests were conducted in this work to investigate the effect of load on the lubrication effectiveness of the Ti–52100 steel pair.
Figure 14a shows the friction curves recorded at various contact loads ranging from 3 N to 20 N. It can be seen that lubrication is effective at loads from 3 N to 10 N, while lubrication is ineffective at 15 N and 20 N. In the effective lubrication regime, the friction curves are low and smooth, and the COF decreases slightly with increasing load. Correspondingly, the wear of the Ti plate and steel ball are mild: the wear rate of the Ti plate is in the order of 10−8 mm3/Nm (Figure 14b). However, in the ineffective lubrication regime, the friction curves are high and fluctuating largely, and wear of the Ti plate and steel ball is severe: the wear rate of the Ti plate is in the order of 10−3 mm3/Nm, an increase by four to five orders of magnitude, as compared to effective lubrication (Figure 14b).
Figure 15 shows the SEM images and EDS elemental mappings of the worn surfaces of the Ti plate and steel ball tested at 15 N. The Ti plate suffers from adhesive wear and abrasive wear (Figure 15(a1)). Only Ti, C, and O are detected on the worn surface on the plate and Fe is not detected (Figure 15(a2)), suggesting no transfer of steel to the plate. However, on the wear scar on the steel ball (Figure 15(b1)), Ti is detected (Figure 15(b2)), suggesting the transfer of Ti from the plate to the ball. No other elements relating to oil additive (Zn and S) are detected on the plate and on the ball. Clearly, a tribofilm fails to form or sustain under relatively heavy loading conditions. Without a tribofilm, adhesion between the contacting surfaces occurs from the beginning of sliding, leading to ineffective lubrication. There is clearly a limit in the load-bearing capacity of the lubricant film for the Ti–52100 steel pair to achieve effective lubrication.

4. Conclusions

The friction and wear behavior of four tribopairs involving CP-Ti sliding against various balls has been investigated under boundary lubrication conditions using a popular motor oil (0W30) as the lubricant. The friction and wear mechanisms and the load dependence of the Ti–steel tribopair have been studied. The conclusions that can be drawn are as follows.
(1)
Three categories of lubrication behavior are identified, including lubrication failure (Ti-Ti pair), improved lubrication with friction instability (Ti-Al2O3 pair), and effective lubrication (Ti–steel and Ti–nylon pairs).
(2)
Lubrication failure of the Ti-Ti pair leads to high and unstable friction and severe wear from both the plate and ball. The lubricant fails to prevent adhesion between the contacting Ti surfaces and a tribofilm fails to form on the sliding surfaces, such that severe adhesive wear and abrasive wear are dominant during sliding.
(3)
The Ti-Al2O3 tribopair experiences reduced baseline friction compared to the Ti-Ti pair. However, friction is unstable, characterized by frequent spikes. The lubricant could not completely prevent material transfer from the Ti plate to the Al2O3 ball. These spikes are attributed to the removal of transferred material and grain pull-out from the ceramic ball, which disrupts the sliding process.
(4)
The effective lubrication of the Ti–52100 steel pair results in low and smooth friction and much-reduced wear rates of the Ti plate and the steel ball by three to four orders of magnitude, as compared to the wear rates measured under dry sliding conditions. This is due to the formation of a tribofilm, based on the additives in the oil (Zn and S), on the steel ball surface. The tribofilm prevents the adhesion of Ti to steel; thus, severe wear is avoided.
(5)
The Ti–nylon pair is effectively lubricated in terms of much-reduced friction. However, although wear from the Ti plate is unmeasurable, the nylon ball suffers from severe wear.
(6)
There is a limit in the load-bearing capacity of the Ti–52100 steel pair lubricated by the oil. Under heavy loading conditions, the lubricant fails to prevent the adhesion of Ti to the steel surface, such that a tribofilm fails to form on the steel ball surface, leading to high and unstable friction and severe wear.
One of limitations of the present study lies in the fact that the effect of temperature was not studied. Temperature at the contact interface plays a crucial role in boundary film formation. During sliding in the oil, temperature is expected to increase on the stationary balls and the temperature increase would be different for different balls because of their different thermal conductivity. This temperature effect remains to be studied in the future.

Author Contributions

Conceptualization, Y.S. and R.B.; methodology, Y.S. and Y.L.; software, M.A.-S.; validation, Y.L., Y.S. and M.A.-S.; formal analysis, Y.S. and Y.L.; investigation, Y.L. and M.A.-S.; resources, Y.S.; writing—original draft preparation, Y.S., Y.L. and M.A.-S.; writing—review and editing, R.B.; visualization, Y.S. and Y.L.; supervision, Y.S. and R.B.; project administration, Y.S. and R.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original data of this work is available from the authors upon reasonable request.

Acknowledgments

The authors would like to express their gratitude to DMU for providing the time and resources necessary to carry out this work. Additionally, they wish to thank Shamima Choudhury for her valuable assistance with SEM imaging and EDX mapping.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. The test setup in the experiments.
Figure 1. The test setup in the experiments.
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Figure 2. Recorded friction curves of various balls sliding against CP-Ti plates under both dry and oil-lubricated conditions at 5 N load. (a) CP-Ti ball, (b) Al2O3 ball, (c) AISI 52100 steel ball, and (d) nylon ball.
Figure 2. Recorded friction curves of various balls sliding against CP-Ti plates under both dry and oil-lubricated conditions at 5 N load. (a) CP-Ti ball, (b) Al2O3 ball, (c) AISI 52100 steel ball, and (d) nylon ball.
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Figure 3. Average coefficient of friction of various balls sliding against CP-Ti plates under both dry and oil-lubricated conditions at 5 N load.
Figure 3. Average coefficient of friction of various balls sliding against CP-Ti plates under both dry and oil-lubricated conditions at 5 N load.
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Figure 4. Plate wear rate (a) and ball wear rate (b) after sliding of the Ti plate against various balls under both dry and oil-lubricated conditions at 5 N.
Figure 4. Plate wear rate (a) and ball wear rate (b) after sliding of the Ti plate against various balls under both dry and oil-lubricated conditions at 5 N.
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Figure 5. Ti–Ti pair. Three-dimensional images of the worn surface of the Ti plate: (a) overall view and (b) enlarged view after sliding against a CP-Ti ball at 5 N under oil-lubricated condition.
Figure 5. Ti–Ti pair. Three-dimensional images of the worn surface of the Ti plate: (a) overall view and (b) enlarged view after sliding against a CP-Ti ball at 5 N under oil-lubricated condition.
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Figure 6. Ti-Ti pair. SEM image of the worn surface of the CP-Ti ball (a) and EDS elemental mapping of Ti (b), C (c), and O (d) after sliding against a CP-Ti plate at 5 N under oil-lubricated condition.
Figure 6. Ti-Ti pair. SEM image of the worn surface of the CP-Ti ball (a) and EDS elemental mapping of Ti (b), C (c), and O (d) after sliding against a CP-Ti plate at 5 N under oil-lubricated condition.
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Figure 7. Ti-Al2O3 pair. Three-dimensional images of the worn surface of the Ti plate after sliding against an alumina ball at 5 N under oil-lubricated condition. (a) Overall view of the wear track and (b) enlarged view of the wear track, showing abrasive and adhesive wear and prow formation (marked A).
Figure 7. Ti-Al2O3 pair. Three-dimensional images of the worn surface of the Ti plate after sliding against an alumina ball at 5 N under oil-lubricated condition. (a) Overall view of the wear track and (b) enlarged view of the wear track, showing abrasive and adhesive wear and prow formation (marked A).
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Figure 8. Ti-Al2O3 pair. Two-dimensional and 3D images of the wear scar on the Al2O3 ball after sliding against a CP-Ti plate at 5 N under oil-lubricated condition. (a) Overall view of the wear scar on the alumina ball, (b) enlarged 3D image of the worn surface on the alumina ball, showing grain pull-out, and (c) a surface profile measured across the pull-out area, indicated by the dashed line in (b).
Figure 8. Ti-Al2O3 pair. Two-dimensional and 3D images of the wear scar on the Al2O3 ball after sliding against a CP-Ti plate at 5 N under oil-lubricated condition. (a) Overall view of the wear scar on the alumina ball, (b) enlarged 3D image of the worn surface on the alumina ball, showing grain pull-out, and (c) a surface profile measured across the pull-out area, indicated by the dashed line in (b).
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Figure 9. Ti-Al2O3 pair. SEM image of the worn surface on the alumina ball after sliding against a CP-Ti plate at 5 N under oil-lubricated condition (a), EDS Ti element mapping (b), and EDS spot analysis spectra measured at location 1 (c) and 2 (d), shown in (a). Element Au originates from the Au plating on the alumina ball to enhance electric conductivity for SEM examination.
Figure 9. Ti-Al2O3 pair. SEM image of the worn surface on the alumina ball after sliding against a CP-Ti plate at 5 N under oil-lubricated condition (a), EDS Ti element mapping (b), and EDS spot analysis spectra measured at location 1 (c) and 2 (d), shown in (a). Element Au originates from the Au plating on the alumina ball to enhance electric conductivity for SEM examination.
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Figure 10. Ti–52100 steel pair. Two-dimensional images of the wear track on the CP-Ti plate (a) and the wear scar on the steel ball (b) after sliding at 5 N under oil-lubricated condition, showing mild wear of both the disk and ball and the formation of a tribofilm on the ball.
Figure 10. Ti–52100 steel pair. Two-dimensional images of the wear track on the CP-Ti plate (a) and the wear scar on the steel ball (b) after sliding at 5 N under oil-lubricated condition, showing mild wear of both the disk and ball and the formation of a tribofilm on the ball.
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Figure 11. Ti–52100 steel pair. (a) SEM image of worn surface on the 52100 steel ball after sliding against a CP-Ti plate at 5 N under oil-lubricated condition, EDS element mappings of Zn (b) and S (c), and EDS spot analysis spectrum (d) measured at location 5, shown in (a).
Figure 11. Ti–52100 steel pair. (a) SEM image of worn surface on the 52100 steel ball after sliding against a CP-Ti plate at 5 N under oil-lubricated condition, EDS element mappings of Zn (b) and S (c), and EDS spot analysis spectrum (d) measured at location 5, shown in (a).
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Figure 12. Ti–52100 steel pair. (a) SEM image of the worn surface on the Ti plate after sliding against a 52100 steel ball at 5 N under oil-lubricated condition, and EDS element mappings of Ti (b), C (c), and O (d).
Figure 12. Ti–52100 steel pair. (a) SEM image of the worn surface on the Ti plate after sliding against a 52100 steel ball at 5 N under oil-lubricated condition, and EDS element mappings of Ti (b), C (c), and O (d).
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Figure 13. Ti–nylon pair. Two-dimensional images of the wear scar on the nylon ball after sliding against Ti at 5 N under oil-lubricated condition. (a) Overall view of the wear scar on the ball and (b) enlarged view of the wear scar, showing fatigue wear. The corresponding wear track on the Ti plate is hardly visible.
Figure 13. Ti–nylon pair. Two-dimensional images of the wear scar on the nylon ball after sliding against Ti at 5 N under oil-lubricated condition. (a) Overall view of the wear scar on the ball and (b) enlarged view of the wear scar, showing fatigue wear. The corresponding wear track on the Ti plate is hardly visible.
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Figure 14. Ti–AISI 52100 steel pair. Recorded friction curves of AISI 52100 steel ball sliding against CP-Ti plate under oil-lubricated conditions at various loads (a) and plate wear rate as a function of initial contact pressure (b).
Figure 14. Ti–AISI 52100 steel pair. Recorded friction curves of AISI 52100 steel ball sliding against CP-Ti plate under oil-lubricated conditions at various loads (a) and plate wear rate as a function of initial contact pressure (b).
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Figure 15. Ti–52100 steel pair. SEM image of the Ti plate (a1), EDS Ti element mapping (a2), SEM image of the 52100 steel ball (b1), and EDS Ti element mapping (b2) after sliding at 15 N under oil-lubricated condition.
Figure 15. Ti–52100 steel pair. SEM image of the Ti plate (a1), EDS Ti element mapping (a2), SEM image of the 52100 steel ball (b1), and EDS Ti element mapping (b2) after sliding at 15 N under oil-lubricated condition.
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Table 1. Material composition and hardness.
Table 1. Material composition and hardness.
MaterialComposition (wt%, or Chemical Formula)Hardness (HV0.1)
CP-Ti0.08C, 0.024Fe, 0.03N, 0.21O, 0.014H, Ti (balance)207 ± 14
AluminaAl2O32116 ± 260
AISI 521001.01C, 1.48Cr, 0.33Mn, 0.23Si, 0.25Ni, 0.10 Mo, Fe (balance)801 ± 19
Nylon(C12H22N2O2)n4 ± 0.5
Table 2. Calculated maximum contact pressures (Pmax) and minimum oil film thickness (hmin) for various contact pairs at 5 N load.
Table 2. Calculated maximum contact pressures (Pmax) and minimum oil film thickness (hmin) for various contact pairs at 5 N load.
Contact PairPmax (MPa)hmin (μm)
Ti–Ti5940.012
Ti–Al2O37920.011
Ti–52100 steel7060.011
Ti–Nylon530.105
Table 3. Typical properties of 0W-30 motor oil.
Table 3. Typical properties of 0W-30 motor oil.
Density @ 15 °CViscosity
(Kinematic 40 °C)		Viscosity
(Kinematic 100 °C)		Viscosity IndexFlash Temperature
0.842 g/mL72 cSt12.3 cSt169200 °C
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Liu, Y.; Al-Shan, M.; Bailey, R.; Sun, Y. Effect of Counterbody Material on the Boundary Lubrication Behavior of Commercially Pure Titanium in a Motor Oil. Lubricants 2024, 12, 439. https://doi.org/10.3390/lubricants12120439

AMA Style

Liu Y, Al-Shan M, Bailey R, Sun Y. Effect of Counterbody Material on the Boundary Lubrication Behavior of Commercially Pure Titanium in a Motor Oil. Lubricants. 2024; 12(12):439. https://doi.org/10.3390/lubricants12120439

Chicago/Turabian Style

Liu, Yizhao, Mohammed Al-Shan, Richard Bailey, and Yong Sun. 2024. "Effect of Counterbody Material on the Boundary Lubrication Behavior of Commercially Pure Titanium in a Motor Oil" Lubricants 12, no. 12: 439. https://doi.org/10.3390/lubricants12120439

APA Style

Liu, Y., Al-Shan, M., Bailey, R., & Sun, Y. (2024). Effect of Counterbody Material on the Boundary Lubrication Behavior of Commercially Pure Titanium in a Motor Oil. Lubricants, 12(12), 439. https://doi.org/10.3390/lubricants12120439

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