Experimental Investigation of Lubrication Performance of Rhombic-Textured TiN-Coated Surfaces Under Lubricated Conditions

Abstract

Surface texture and titanium nitride (TiN) coating have been established as effective methods for enhancing the tribological properties of mechanical friction pairs. This work aims to investigate the lubrication performance of rhombic-textured TiN-coated surfaces under oil-lubricated conditions using a pin-on-disk test mode. A total of 17 sets of samples were designed, including a control sample (with no rhombic texture and no TiN coating), a TiN-coated sample and rhombic-textured TiN-coated samples. The rhombic surface texture was fabricated using the end surface of a brass bar. TiN coating deposited TiN on the textured surface. This study focuses on measuring and comparatively analyzing the lubrication load capacity, friction coefficient (COF) and binding force of TiN coatings/substrates in the pin-on-disk friction test mode. Compared with the bare control sample, a rhombic texture can enhance lubrication load-carrying capacity by generating hydrodynamic lubrication effects, thereby reducing friction. Additionally, a rhombic texture enables the mitigation of third-body wear due to wear debris. This research provides valuable insights into the design and fabrication of mechanical friction pairs with high wear resistance under oil-lubricated conditions. For lubrication property enhancement, the influence of groove depth was larger than that of the length of the rhombic side.

1. Introduction

In recent decades, with the deepening understanding of surface engineering and the advancement of manufacturing technologies, surface engineering technology has demonstrated broad application prospects in various fields, including friction reduction and wear resistance [1,2], friction enhancement [3], vibration damping [4], anti-adhesion [5] and anti-creep [6]. This has become an effective approach to achieving high efficiency, miniaturization and high reliability in mechanical equipment. Surface-texturing technology and surface-coating technology, as typical surface modification methods, are commonly used to enhance the tribological performance of friction pairs [7]. By designing the micro-topography of the friction pair surfaces, surface-texturing technology can artificially create wedge-shaped structures to generate hydrodynamic lubrication effects, which periodically produce hydrodynamic lift force and reduce the solid contact between the friction pairs [8,9]. Many scientists have conducted research on texture parameter optimization and lubrication and drag reduction mechanisms under specific operating conditions, including different geometric parameters of surface texture, such as shape (i.e., circular, rhombic, triangle, elliptic, square) [10,11], size (i.e., depth, width, depth) [11], orientation (i.e., horizontal, at a certain angle) [12] and cross-sectional morphology (i.e., rectangular, curved), as well as the arrangement pattern of surface textures. Rhombic textures, as commonly used texture morphologies, have been extensively studied.

The present research on rhombic texture is also focused on the optimal design of its geometric parameters [11,12,13,14,15,16,17,18]: the optimal structural parameters for specific operating conditions [11,15,17,18] are explored through fluid simulations or experiments [13,14,15,19]. Wang [11] investigated the influence of angle and depth on the oil-film load-bearing capacity of textured surfaces under oil lubrication conditions and confirmed that reticular-texture, which is similar to rhombic texture, positively enhances lubrication performance. Guo [13] discovered that rhombic-like textures can regulate the balance between fluid flow and hydrodynamic effects through numerical simulation. And Sun [17] found that textured surface friction pairs exhibit significantly lower grinding forces during machining, with markedly reduced clogging and adhesive wear phenomena. Particularly during the rapid wear stage, surface roughness shows a notable improvement. Although a rhombic texture can effectively improve lubrication performance, it is difficult to avoid localized damage to the processed textures under heavy loads or transient impact loads, causing them to fail to generate local hydrodynamic lift force. As one of the most widely used surface modification technologies, TiN coating, as an ultra-hard coating material, has seen extensive research and application in the world [19]. Therefore, TiN coating should also be considered as a method to enhance the wear resistance of textured surfaces.

The state of the research in the field on the combination of TiN coating and rhombic texture is limited. This work aims to study the lubrication performance of rhombic-textured TiN-coated surfaces under oil-lubricated conditions by a pin-on-disk lubrication test and friction test.

2. Experimental Details

2.1. Description of Experimental Device

The lubrication test apparatus was designed to measure the lubrication capacity performance of textured surfaces with TiN coatings under oil-lubricated conditions. The relative moving surfaces consisted of a geometric probe (rotating surface) and a brass rod (fixed surface), shown in Figure 1. The sample to be tested was fixed on the lubricant container. The distance between the center of the brass rod and the center of the geometric probe was 15 mm. The geometric probe was fixed in the draw rod of the rheometer head. The rheometer head could move along the vertical direction to adjust the gap clearance between the geometric probe and the sample. The AR2000ex rotational rheometer instrument, TA Instruments, New Castle, DE, USA, shown in Figure 1, was adopted in these tests.

The frictional tests used the UMT-3 multifunctional friction testing machine—BRUKER, Billerica, MA, USA. In this study, the pin-on-disk mode was adopted to measure the coefficient of friction (COF) of the frictional pairs.

2.2. Design and Preparation of Samples

The samples were brass rods with a diameter of 6.3 mm. The rhombic textures and TiN coatings were designed and fabricated on the end-surface face of the brass rods. The manufacturing process can be seen in Figure 2. The specific manufacturing process was as follows: firstly, surface textures were fabricated on the end surface of the brass rods by surface milling in the DMG five-axis CNC machining center (CMX 70U, DMG MORI, Nagogy, Aichi, Japan); secondly, TiN coatings were deposition on the textured surfaces by PVD (physical vapor deposition) with vacuum-sputtering coating equipment (MC-hybrid, Sky Technology Development Co., Ltd. Chinese Academy Of Sciences, Shengyang, China).

The rhombic texture consisted of raised rhombic parts and grooves, as shown in Figure 3. The acute angles of the rhombi α for all the samples were 60°. As the length sum of rhombic side length b and groove width w was the constant value 0.2 mm, the two variable parameters were rhombic side length b and groove depth h, which are listed in

Table 1. Geometric parameters of samples.

Sample No.	Length of Rhombic Side b (mm)	Groove Depth h (mm)
1	0.14	0.01
2	0.14	0.02
3	0.14	0.03
4	0.14	0.05
5	0.14	0.06
6	0.14	0.07
7	0.14	0.08
8	0.14	0.04
9	0.11	0.04
10	0.12	0.04
11	0.13	0.04
12	0.15	0.04
13	0.16	0.04
14	0.17	0.04
15	0.18	0.04
16	Control sample (untextured and uncoated brass bar)
17	TiN-coated sample with no texture

. All the samples could be divided into two groups: one group (samples No. 1–8) was designed with the given rhombic side length of 0.14 mm (b = 0.14 mm), and the other group (samples No. 8–15) was designed with the given groove depth of 0.04 mm (h = 0.04 mm).

TiN coating treatment on the textured surface was deposited by vacuum-sputtering coating equipment (MC-hybrid, Sky Technology Development Co., Ltd. Chinese Academy of Sciences, Shengyang, China). The deposition parameters are listed in

Table 2. Process parameters of TiN coating deposition.

Parameter	Value
Metal target	Ti
Sputtering gas	Ar
Reaction gas	N2
Ar rate (sccm)	57
N2 rate (sccm)	152
Vacuum chamber temperature (°C)	163
Deposition time (min)	38
Workpiece speed (rpm)	4
Working pressure (Pa)	1.0
Effective current (A)	101
Peak current (A)	115
Distance between target and sample (cm)	23

. The thickness of the TiN coating was about 4–5 μm.

2.3. Design of Experimental Procedure

The interface bonding force of the substrate/coating system was tested by a scratch test [20]. The normal force F_N was increased linearly from 0 N to 6 N over 5 min and a distance of 5 mm. The scratch test was conducted with an untextured sample with TiN coating.

The contact angles were measured by a video optical contact angle/surface tension measuring instrument (DropMeter Experience A-300, MAIST, Ningbo, China). All the measurements were conducted by the seat drop method. Each measurement was repeated three times to ensure reliability. The surface roughness for the uncoated sample and TiN-coated sample were 484.10 nm and 520.55 nm, respectively. The measurement was carried out under constant temperature and humidity conditions (the indoor temperature was 25 °C and relative humidity was 55%).

The lubrication tests adopted a fixed gap clearance under rich lubricated conditions. The gap clearance was set at 10 μm in the test. Before each test, the rotational rheometer was zero set. And then, the gap clearance between the two surfaces was adjusted by the geometric probe. During each test, the gap clearance remained unchanged. The rotation speeds of the geometric probe were 30, 45, 60, 75 and 90 rad/s, and the duration of each group of experiments was 300 s. During the tests, the force sensor at the bottom of the rheometer measured the normal force every 15 s. As the rotational rheometer was zero set before testing, the measured normal force was the oil-film bearing capacity between the two surfaces. The lubricant used in all the lubrication tests was Mobil CI-4 oil (Exxon Mobil Corporation, Irving, TX, USA), which has a density of 0.8747 kg/L and a kinematic viscosity of 14.4 mm²/s.

The friction test adopted the pin-on-disk rotational mode. The distance between the central axes of the end of the brass rod and the lower disk was 15 mm. The lower disk was a 304 stainless steel square plate with a side length of 40 mm and a thickness of 5 mm. The experimental normal loads were 2.5, 5, 7.5, 10, 12.5, 15 and 17.5 N, respectively. Each test lasted 20 min. The samples were tested continuously over a 140 min interval. The experimental rotation speed was set at 600 rev/min, so for each sample the total test time was 140 min. Mobil CI-4 oil was also used as the lubricant in the friction tests.

3. Results and Discussions

3.1. Surface Characteristics

Figure 4 shows the trend diagram of the tangential she ar force F_X with the increase in the normal load F_N, where F_N is the external normal load applied to the diamond indenter in the scratch test and F_X is the shear stress measured by the force sensor. It can be seen from Figure 4 that F_X changes sharply when the normal load F_N is 4.6 Kg. Therefore, it can be preliminarily concluded that the critical load for coating failure is 46 N (where the gravity constant g = 10 N/Kg). In order to verify the correctness of this conclusion, it is necessary to analyze the chemical elements inside the scratch trace. When comparing the EDS compositions of the original surface and the coating failure point, the constituent elements of the substrates “Cu” and “Zn” have already appeared at the coating failure point, while the constituent elements of the TiN coating, “Ti” and “N”, have almost disappeared. So, the bonding force is 46 N.

3.2. Lubrication Performance Analysis

Figure 5 shows the average oil-bearing capacity of each sample under the different tested rotation speeds. Figure 5a shows the average oil-bearing capacities of the samples with different rhombic side lengths and Figure 5b shows the average oil-bearing capacities of the samples with different groove depths.

By the comparative analysis of the control sample (untextured and uncoated brass bar), TiN-coated sample and rhombic-textured TiN-coated samples, it was found that the rhombic-textured TiN-coated samples could indeed generate hydrodynamic lubrication. The oil-film-supporting forces produced between two lubricated surfaces with textures were more than twice those between smooth surfaces without textures. Rhombic textures could improve lubrication characteristics by creating larger supporting forces to separate the two friction surfaces. The bearing capacity of the TiN-coated sample was slightly increased compared with the control sample. The reason for this phenomenon might be that the existence of TiN coating changed the wettability of the surface to a certain extent. In order to verify the correctness of this conjecture, the surface contact angles of the two samples were measured. The measurement results showed that the surface contact angle of the latter was slightly larger than that of the former, which was consistent with the experimental results for lubricating support force.

By the comparative analysis of the samples with different groove depths, the groove depth was found to have a significant effect on the average oil-bearing capacity in all cases and different rotation speeds. Comparing the supporting forces of the eight samples under different rotation speeds, the results showed that, when the rotation radius was set at a constant value, the average oil-bearing capacities of all the samples increased with the increasing rotation speeds, which indicates that all the tested friction pairs were in a hydrodynamic lubrication state. The No. 8 sample (b = 0.14 mm, h = 0.04 mm) displayed the best lubrication-bearing properties. For the No. 8 sample, when the rotation speed increased from 30 rad/s to 90 rad/s, the supporting force increased from 0.372 N to 0.98 N; that is, the increase amplitude exceeded 100%. But in the cases of the No. 1 and No. 2 samples, the average oil-bearing capacities were less than 0.2 N from 30 rad/s to 90 rad/s, which indicated that the hydrodynamic lubrication effect was not strong. So, oil-bearing capacity can be increased by changing the groove depth or rotation speed.

Similar experimental results were found when the length of the rhombic side was investigated. The No. 8 and No. 12 samples showed better lubrication performances. For these two samples, the support forces were 0.372 N and 0.368 N when the rotation speed was 30 rad/s. As for the other six samples, most of the support forces were less than 0.2 N under all rotation speeds.

So, by designing the geometric parameters of rhombic surface textures, the support force can be changed. Under the same experimental conditions, the influence of groove depth was larger than that of the length of the rhombic side.

3.3. Friction Behavior Analysis

Figure 6 shows the variation in the COF for each sample with different tested normal forces. Figure 6a shows the COF variation in samples with different rhombic side lengths and Figure 6b shows the COF variation in samples with different groove depths.

By the comparative analysis of the control sample (untextured and uncoated brass bar), TiN-coated sample and rhombic-textured TiN-coated samples, it was found that the existence of TiN coating and a rhombic texture could reduce surface friction and synchronously enhance surface wear resistance. The COF of the control sample was greater than that of the TiN-coated sample, especially in heavy normal-load cases. There might be several reasons for this phenomenon. Firstly, similarly to the analysis of the lubrication test results, the presence of the TiN coating improved surface wettability, which could help to reduce friction. Secondly, the friction of the TiN-coated sample was smaller, resulting in less wear during the friction test. Consequently, the “third-body” wear caused by debris might also be reduced. As the hardness of brass is lower than that of stainless steel, the lower disk in the control sample did not exhibit wear. In order to compare the degree of wear on the upper sample, weight measurement was conducted. It was found that the weight of the control sample decreased by 110 g after the friction test. In contrast, in the other 16 sets of tests, the weight of the upper samples remained virtually unchanged before and after the tests. For these 16 sets of coated samples, it was evident that the rhombic-textured TiN-coated samples exhibited a significantly lower COF than the TiN-coated sample, particularly in the latter stages of the tests. The main reason for this was that the hydrodynamic lubrication effect caused by the rhombic textures helped to reduce surface friction, and this effect became more pronounced as the normal load increased [21,22]. Meanwhile, surface texture could mitigate third-body wear caused by debris, which also optimized friction performance to some extent. This has been investigated in a previous work [23].

In the vast majority of cases, the COF gradually increased when the normal load increased. According to the description of lubrication states in the classical Stribeck curve, under conditions where the rotation speed and lubricant viscosity remain unchanged, the COF between the pin and disk in the hydrodynamic lubrication regime stage exhibits a negative correlation with normal pressure. In contrast, in the mixed lubrication regime stage, the COF between the friction pairs showed a positive correlation with the normal pressure. So, this indicates that, in the vast majority of experiments, the pin-on-disk friction pairs operated in the hydrodynamic lubrication regime stage.

Through the further comparative analysis of the influence of groove depth and the length of the rhombic side on the COF, it was found that the two factors had different degrees of impact on the trend of the COF. Generally, as the groove depth increased, the COF initially decreased, as shown in Figure 6a. In the case of the NO. 8 sample, the COF remained the lowest throughout, and it consistently operated in the hydrodynamic lubrication regime stage. The lowest COF of 0.0908 occurred when the normal load was 2.5 N. And the COF changed within the range of 0.0908~0.106. The COFs of the NO. 1 and NO. 6 samples were nearly equal under higher normal loads, while under lower normal loads, the COF of the NO. 6 sample was smaller. But, for the NO. 7 sample, as the normal load increased, the COF initially decreased, reaching its minimum value (0.0975) at a normal load of 12.5 N and then gradually increasing. This behavior aligned with the characteristics of the mixed lubrication regime, indicating that the lubrication state of the NO. 7 sample turned from the hydrodynamic lubrication regime to the mixed lubrication regime. Thus, it can be seen that, under higher normal loads, appropriately increasing the groove depth helped reduce friction. However, when the groove depth exceeded a certain threshold, the textured surface became less conducive to generating a hydrodynamic effect.

Similar conclusions were found in the different groove depths cases; the COF decreased as the length of the rhombic side increased, as shown in Figure 6b. After reaching the minimum value (NO. 12 sample), the COF gradually increased. For the NO. 12 sample, the COF changed within the range of 0.088~0.103. However, different from the above case, changes in the length of the rhombic side had a smaller impact on the COF. When the textured samples operated in the hydrodynamic lubrication regime, the COF of the textured surface increased as the normal load decreased, eventually stabilizing at a certain level (corresponding to the horizontal segment on the right side of the Stribeck curve). Thus, this was the reason that the COF remained almost unchanged as the length of the rhombic side increased. This also implied that the samples with different lengths of their rhombic sides designed for this study might have been operating at the limiting state of hydrodynamic lubrication under these conditions.

4. Summary and Conclusions

An experimental investigation was conducted on the effect of rhombic texture and TiN coating on lubrication performance by a series of lubrication tests and friction tests. All the tests were conducted in the pin-on-disk mode. Eight different groove depths and eight different lengths of rhombic side were designed to study lubrication load-carrying capacity and friction force. The following results were found:

1. Under oil-rich lubrication conditions, rhombic texture can enhance lubrication load-carrying capacity by generating hydrodynamic lubrication effects, thereby reducing friction. TiN coatings can significantly improve wear resistance, while the deposition of TiN coatings can slightly enhance surface wettability, leading to a minor reduction in friction.

2. By designing rhombic textures with different geometric sizes, the optimal texture dimension scheme for a specific operating condition can be obtained. In this case, where clearance was set at 10 μm, the influence of groove depth was larger than that of rhombic side length. It is recommended to prioritize the design of groove depth.

3. When the rotation speed is 600 rad/s, the COF is more sensitive to changes in groove depth. Considering the influence of normal load on friction performance, a rhombic texture with a groove depth of 0.04 mm and rhombic side length of 0.14 mm is recommend.