Effect of Micro-Dimple Texture on the Tribological Performance of Brass with Titanium Nitride (TiN) Coating under Oil-Lubricated Conditions

Abstract

Surface texture and titanium nitride (TiN) coating have been established as effective methods for enhancing the tribological property of mechanical friction pairs. This study aims to investigate the tribological performance of dimple-textured surfaces with TiN coating under oil-lubricated conditions using a pin-on-disc wear experiment. Four types of pin samples with various end surfaces were designed, including bare rod samples, TiN-coated samples, textured samples, and TiN-coated/textured samples. The surface texture consists of a series of cylindrical micro-dimples with a diameter of 150 μm and a depth of 40 μm fabricated on the end surface of the pin. TiN coating treatment on the textured surface of the end face was performed by vacuum sputtering coating equipment. The study focuses on measuring and comparatively analyzing the friction coefficient, wear morphology, and binding force of the pin-disc friction pairs among the experiments. Compared with bare rod samples, TiN-coated/textured samples will reduce the friction coefficient (COF) of the pin-on-disc friction pair by at least 20% under oil-lubricated conditions in a 50 N normal contact load. The results indicate that the synergistic effect of dimple surface texture and TiN coating optimizes friction performance and reduces wear, highlighting the novelty of this study. Furthermore, the study identifies the hydrodynamic lubrication effect of the surface morphology formed by the dimple surface texture as a key factor in improving lubrication performance and reducing friction. Additionally, the dimple surface texture enables the mitigation of third body wear due to the wear debris storage function of the micro-dimples. This research provides valuable insights for the design and fabrication of mechanical friction pairs with high wear resistance under oil-lubricated conditions.

1. Introduction

In recent years, the tribological mechanism of mechanical friction pairs has been deeply investigated. Surface coating, surface texture, materials of friction pairs, and lubricant properties have been proven to be effective methods to enhance tribological performance [1,2,3,4,5]. Surface texture can transform the gap-lubricating film of two parallel surfaces into a variable gap-lubricating film, which makes the “convergent wedge” and “divergent wedge” cycles alternately appear, and then changes the contact state and lubrication state of the friction pair. Therefore, most scholars believe that in the case of hydrodynamic lubrication or mixed lubrication, the existence of surface texture can generate additional hydrodynamic pressure on the surfaces of two solids in relative motion, so as to avoid friction and wear caused by direct contact of two solid surfaces [6,7]. Much research on the optimization of surface texture topography parameters, such as surface texture geometry (i.e., round, reticulated, oval, square, triangle), size (i.e., diameter or side length, depth), distribution density, and other parameters [8,9,10,11], has been conducted extensively. Micro-dimple texture is a typical type of surface texture that has been widely studied. The depth and the diameter play an essential role in the tribological performance of micro-dimple textured surfaces [6]. The parameter optimization design should be combined with the specific working condition [12,13,14]. Relevant studies have shown that the application of appropriate surface coating, appropriate surface texture, and the use of low viscosity lubricating oil and additives on the surface of the friction pair can reduce the friction loss by more than 60% during the long-term use of the friction pair (15–20 years) [15]. Therefore, surface texture has been widely used in cutting tools [16], plain bearings [17], internal combustion engines [18], plunger pumps [19], hydraulic cylinders [20], and other fields.

Surface coating, one of the surface modification technologies, can strengthen the surface, effectively improve the surface strength and hardness, and greatly reduce the wear rate of the substrate. Titanium nitride (TiN) coating, one of the typical surface coatings, is widely used in industrial production as a superhard coating material [21].

Thus, both surface texture and surface coating effectively improve the friction characteristics of the material surface. Considering the research field of friction, combining the anti-friction characteristics of the surface micro-texture with the lubrication characteristics of the surface coating, and using the synergistic comprehensive action of the two, the friction properties of the material surface can be further optimized to obtain better anti-friction and anti-wear effects. Chen [9] compared the friction and wear properties of an untextured/uncoated steel plate, textured steel plate, coated steel plate, and textured/coated steel plate through the pin-on-disc experiment, and found that the wear volume of the latter three was significantly smaller than that of untextured/uncoated steel plate. Among them, the wear resistance of the textured/coated steel plate was the best, and the wear volume was reduced by more than 60%. Shum [22] also found that the existence of circular texture in the dry friction state can significantly improve the tribological performance of surface coating. This phenomenon is because the texture can store wear chips and reduce the third body wear. At the same time, the solid surface is graphitized by ion implantation technology, and the particles generated by surface wear during the low friction cycle can provide lubrication for the subsequent friction process, thereby reducing friction and wear. Obikawa [23] applied the composite surface to the tool surface and proved through experiments that the surface texture can effectively reduce the tool chip contact area, improve the tool chip contact conditions, improve the tribological and cutting properties of the tool, extend its service life, and obtain the surface texture morphology and geometric size parameters suitable for its working conditions. The study of Wang et al. [10] indicated that the deposition of TiN coating on the textured surface can reduce wear, on the one hand, because the surface texture reduced the contact area between friction pairs; on the other hand, the processing of part of the substrate surface texture is conducive to improving the surface hardness and surface wear resistance of the coating. Substrate texture is beneficial to reduce furrow generation and adhesion and contact fatigue wear of the coating surface.

The present research on the effect of dimple textured surface with TiN coatings on the tribological performance is mostly in the state of dry friction. Its application objects are also concentrated in cutting tools, brake pads, and other industries with little lubricating media or even no lubricating media. However, many mechanical friction pairs work under fully lubricated conditions. Therefore, it is also necessary to further study the friction and wear properties under oil-rich lubrication.

This work aims to study the tribological performance of dimple textured surfaces with TiN coatings under oil-lubricated conditions by pin-on-disc wear experiment. The contributions of this paper are summarized as follows:

(a)

The study presents that surface texture and TiN coating can improve the tribological properties of pin-on-disc friction pairs under oil lubrication due to their synergistic effect. TiN coating itself enhances the surface texture’s anti-wear properties, which make the enhanced surface texture further provide conditions for hydrodynamic lubrication and the migration of abrasive chips and particles.

(b)

The differences of the wear mechanisms of pin-on-disc friction pairs with four types of pin samples, including bare rod samples, TiN-coated samples, textured samples, and TiN-coated/textured samples, are comparatively analyzed based on the wear experiment.

(c)

This study provides valuable insights for the design and fabrication of mechanical friction pairs with high wear resistance under oil-lubricated conditions. In particular, it gives the guidance of optimizing the tribological properties of pin-on-disc friction pairs composed of soft and hard materials under heavy load conditions, such as the design of the slippers of plunger pump.

2. Experimental Design

2.1. Description of Experimental Device

The wear experiment was designed to test the effect of micro-dimples on the tribological performance of brass with TiN coatings under oil-lubricated conditions. The wear situation of the relative frictional motion between the pin-on-disc fiction pair (shown in Figure 1a) is used to reflect the influence of the synergistic effect of micro-dimples, TiN coating, and lubricating oil. The disc was fixed in the oil tank of the friction testing machine through eight clamps. The pin was a rod where the machined end surface would rub against the disc.

The multifunctional friction testing machine (UMT-3, BRUKER, Billerica, MA, USA, shown in Figure 1b) is used in the experiment. The main part of the experimental device with three degrees of freedom of motion consists of a rotational motion device, a horizontal motion device, and a vertical motion device. The rotational motion device allows the disc mounted on the bottom turntable to rotate during the experiment. The position of the pin can also be changed by the horizontal and vertical motion devices to ensure contact between the pin and the disc mounted on the bottom turntable. The friction-measuring device utilized the force sensor to provide feedback on the normal contact between the pin and the disc and tested their real-time friction force during the experiment.

2.2. Design and Fabrication of Experimental Samples

The four sets of samples of the pin-on-disc pair are designed and manufactured. The aluminum alloy discs in the four sets have the same structural dimensions, material properties, and surface roughness. The discs are a series of aluminum alloy square plates with a side length of 40 mm and a thickness of 5 mm. The hardness of the discs is 120 HBS. Their surfaces are all polished by high-precision polishing paste to make the surface roughness reach about 10 nm. The brass pins with a diameter of 6.3 mm and a hardness of 165 HBS are used in the four sets. The end surfaces of the pins are different in the four sets. There are four types of pin samples, namely bare rod sample (Pin sample 1), TiN-coated sample (Pin sample 2), textured sample (Pin sample 3), and TiN-coated/textured sample (Pin sample 4). Pin sample 1, whose end surface roughness is machined to 5 μm, is a reference for the other three samples. Pin samples 2–4 are fabricated based on Pin sample 1. The fabrication process of Pin sample 4 is shown in Figure 2. TiN-coated/textured end surface of Pin sample 4 are fabricated by two steps: surface milling and TiN coating deposition. A series of the cylindrical micro-dimples with a diameter of 150 μm and a depth of 40 μm were fabricated on the end surface of the pin by DMG five-axis CNC machining center (CMX 70U, DMG MORI, Nagogy, Aichi, Japan). TiN coating treatment on the textured surface of the end face were performed by vacuum sputtering coating equipment (MC-hybrid, SKY TECHNOLOGY DEVELOPMENT CO., LTD. CHINESE ACADEMY OF SCIENCES, Shengyang, China). In the vacuum chamber, the pins fixed to the sample holder would be deposited with a 4~5 μm thick TiN layer using the Ti targets in the N₂ environment. The process parameters of the TiN coating deposition were given in

Table 1. The 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

. Compared with Pin sample 4, Pin samples 2–3 were comparative samples obtained by omitting the processing steps of surface milling and TiN coating deposition, respectively. The experiment grouping was listed in

Table 2. The experiment grouping.

Group No.	Pin Sample	Disc Sample
1	Pin sample 1 (Bare rod)	Aluminum disc
2	Pin sample 2 (TiN-coated)	Aluminum disc
3	Pin sample 3 (Textured)	Aluminum disc
4	Pin sample 4 (TiN-coated/textured)	Aluminum disc

.

2.3. Description of Experimental Process

In this study, four sets of wear experiments in an open environment with an indoor temperature of 25 °C and a relative humidity of 45% were conducted using Pin samples 1–4, respectively. Before and after each experiment, the tested pin sample was cleaned with absolute alcohol, acetone solution, absolute alcohol, and distilled water in an ultrasonic cleaning instrument (KQ-300DV, KUNSHAN ULTRASONIC INSTRUMENTS CO., LTD., Suzhou, China), and then dried naturally. After the pin sample was completely dry, it was weighed. The weight change of the pin sample before and after the experiment would be recorded for analysis.

Before the experiment, the friction testing machine was initialized and its sensor measurements were cleared. The pin sample fixed to the force sensor of the friction testing machine would contact with the disc sample fixed in the oil tank through the clamps, maintaining a constant normal contact force of 50 N. During the experiment, the corresponding mating surfaces of the pin-on-disc friction pair were immersed by the lubricating medium in the oil tank. The lubricant used in the experiments was Mobil CI-4 oil, and its relevant performance parameters are given in

Table 3. Properties of the lubricant.

Parameter	Value
Density at 15 °C (kg/L)	0.8747
Dynamic viscosity at −20 °C (mPa·s)	5418
Kinematic viscosity at 100 °C (mm2/s)	14.4

. The disc sample would rotate counterclockwise with the oil tank at an angular speed of 300 rpm. The central axis of the pin sample was kept 15 mm offset from the center of rotation of the disc sample. Each wear experiment would last 20 min.

2.4. Scratch Test

To obtain the interfacial adhesion and bond strength between the TiN coating and the brass substrate, a scratch test was conducted on a tribometer (UMT-3, BRUKER, Billerica, MA, USA). Before the tests, the samples were cleaned with alcohol in an ultrasonic bath (KQ-300DV, KUNSHAN ULTRASONIC INSTRUMENTS CO., LTD., Suzhou, China) for about 10 min. Then, a standard diamond indenter (UMT-3, BRUKER, Billerica, MA, USA), a cono-spherical tip with a radius of 200 μm and an angle of 120° was used for scratching, as shown in Figure 3. The normal force F_N controlled by the upper carriage was increased linearly from 0 N to 6 N in 5 min over a distance of 5 mm.

3. Results and Discussions

3.1. Surface Characteristics of Samples

Figure 4 showed the surface profiles of textured sample, which could obviously display the dimple diameter (150 μm), dimple depth (40 μm) and interval between the adjacent dimples. Figure 5 revealed morphology profile and EDS composition of scratch score. By analyzing the elementary composition of scratch score, the position of TiN coating failure could be confirmed. The failure position was also used to compare with the result of scratch test, which could help to further confirm the value of applied force making the TiN coating failure. In this study, the binding force between the TiN coating and the substrate measured by the scratch test was 55 N. And the coating thickness was the scratch depth of the coating failure point, which was 5 μm.

3.2. Tribological Behavior Analysis

Through comparative analysis of four sets of wear samples, the effects of surface texture and surface coating on surface tribological performance were analyzed and discussed. Figure 6 shows that the friction coefficient (COF) of the four experiment groups changed over test time. It was found that the existence of micro-dimples and TiN coating necessarily had an excellent lubrication effect on the friction properties of the pin-on-disc friction pairs by comparing the friction coefficient curves of the four experiment groups. The measured values of the friction coefficient at the start and end stages of the wear experiment are not included in Figure 6. The experimental results show that the friction coefficient of the pin-on-disc friction pair from the Group No. 1 experiment changes significantly during the entire experiment. More specifically, the friction coefficient is always greater than 0.2. As time increases, it increases to about 0.8, then decreases to about 0.5 at about 600 s, and finally shows an upward trend. The COF curve of Group No. 1 with Pin sample 1 (Bare rod) shows a typical wear performance with the maximum COF: there was much shaking at the beginning, which then leveled off with a gradual rise. The cause of the change at around 500 s might be the generation and the remains of wear debris. Moreover, the COF of Group No. 1 exceeded 0.5 most of the test time. The COF curves of the remaining three groups were much lower than those of Group No. 1. The COF curves of Group No. 2 with Pin sample 4 (TiN-coated/textured) were the lowest and the most stable. This phenomenon was also found in Chen’s [9] and Wang’s [10] research. The synergistic effect of surface texture and coating helped to reduce friction between pin and disc. On the one hand, the existence of the micro-dimple was one factor with a positive role in generating hydrodynamic lifting force and effectively reducing the third body wear. On the other hand, TiN coating could protect the textured surface, guaranteeing that the positive effect caused by surface texture is a long-term phenomenon.

The wear mechanisms of the four groups were identified according to the value range of the friction coefficient under different lubrication states. The pin-on-disc friction pair from the Group No. 1 experiment was in a dry friction state, while that from the Group No. 2 experiment was in a boundary lubrication state considering its friction coefficient fluctuating between 0.1 and 0.2. In Group No. 3 experiments, the friction coefficient of the sample fluctuated in the range of 0.06~0.15, indicating that the sample was in the state of boundary lubrication and fluid lubrication during the experiment. The sample in the Group No. 4 experiment presented fluid lubrication as the sample’s friction coefficient constantly fluctuated around 0.05. Compared with bare rod samples, TiN-coated/textured samples will reduce the friction coefficient (COF) of the pin-on-disc friction pair by about 40% in the first 250 s and about 60% in the last 600 s under oil-lubricated conditions in 50 N normal contact load.

According to the analysis above, it can be verified that the existence of surface texture is conducive to improving the lubrication state of the pin-on-disc friction pair surface. Furthermore, after the TiN coating is deposited on the textured surface, the surface damage caused by abrasive particles can be avoided, making the friction coefficient of the pin-on-disc friction pair stable within a certain range during the grinding process.

3.3. Wear Behavior Analysis

Figure 7 shows the two-dimensional contour map of the surface wear scar morphology of the aluminum disc worn by the TiN-coated pin in the Group No. 2 experiment. In Figure 7a, it is indicated that the annular wear scars formed on the grinding surface of the disc sample during the entire wear experiment are not entirely consistent at each position. The main reason for this phenomenon is that the two mating surfaces are not ideal parallel surfaces at the micro-nano scale. Surface flatness, surface roughness, and other surface parameters will cause asperity contact between the two surfaces. Furthermore, the wear debris generated during the wear process will aggravate local wear. The two-dimensional morphology corresponding to A, B, C, and D is shown in Figure 7b. The four morphology curves are not completely consistent, especially in the internal contact area, because there are certain differences in local wear due to the presence of wear debris. Therefore, the wear scar depth curve used in the analysis in this study is obtained by fitting the average of four wear scar depth curves.

Common wear mechanisms led by the changes in surface layer forces and damage forms during friction and wear have been analyzed in the previous related research. The main wear mechanisms include scratches, abrasive wear, peeling, and gluing. To further study the surface wear mechanism in this study, a comparative analysis was conducted on the four positions marked A, B, C, and D on the wear scar ring, as shown in Figure 8. The three-dimensional morphology of the wear scars on the four positions (A, B, C, and D) were provided in Figure 8, respectively.

Since a large amount of wear debris is generated at the pin-on-disc contact interface during the wear process in Group No. 2, the wear debris at the edge is easy to overflow, thereby reducing the secondary wear of the contact interface by the wear debris. Distinct localized circular depressed areas are shown in Figure 8b. In addition, the material of the disc sample is aluminum alloy, a low-hardness metal material. As the material of the disc sample is a soft metal material, there is no damage in the form of peeling on the surface. Regardless, the experimental time was not long enough for metal fatigue damage to occur on the contact surface of the disc sample. Therefore, abrasive wear did not occur in the disc samples of the experiments.

The wear behaviors of the four experimental groups of disc samples were not the same. Figure 9 shows the comparison of the wear depth of the four disc samples. The samples under the three groups of coating pattern, texture sample, and coating sample have slight local wear, while the samples with the coating sample have severe wear. Figure 8a shows the wear performance of Disc 1 when Pin 1′s hardness was lower than its hardness. Because of the high level of hardness, Disc 1 was slightly scratched, which might have been caused by wear debris. The wear depth morphology displayed in Figure 9 also declared that there were slight scratches on the surface of Disc 1. On the contrary, as the mating surface had a low level of hardness in the pin-on-disc friction pair, Disc 2 was severely worn in the test, as shown in Figure 8b and Figure 9. Moreover, the generation of wear debris will also lead to local stress concentration. Even if the normal load between the pin-on-disc friction pair is constant, a certain impact load will still occur where the wear debris exists. The wear morphology also presents a certain fluctuation, as shown in Figure 8b. By comparing the weight of pins’ before and after tests, Pin 1 lost 100 g while Pin 2 had little in weight change. Disc 3 and 4 had slight abrasion marks. And the wear of Disc 3 was more severe than that of Disc 4. Meanwhile, there was also no weight loss of two pins. It demonstrated that TiN coating could optimize friction performance and further reduce the wear of the whole friction pair. What is more, both of the two discs had no apparent third body wear. On the one hand, this was due to the decrease in wear debris. On the other hand, micro-dimples were able to store wear debris.

3.4. Strengths and Limitations

Novel findings on the synergistic effect of dimple surface texture and TiN coating in reducing wear and improving friction performance contribute to the advancement of tribological research. First, the study investigated the tribological performance of dimple-textured surfaces with TiN coating under oil-lubricated conditions, providing valuable insights into the performance of mechanical friction pairs in real-world operating conditions. Second, the study identified a significant reduction in the friction coefficient for TiN-coated/textured samples compared to bare rod samples under oil-lubricated conditions, highlighting the effectiveness of the combined dimple surface texture and TiN coating in optimizing friction performance. Third, insights into the hydrodynamic lubrication effect and the mitigation of third body wear due to the dimple surface texture provided a comprehensive understanding of the wear mechanisms involved in improving lubrication performance and reducing friction.

The study’s limitations include the lack of investigation into the quantitative influence of specific morphological types, geometric dimensions of the surface texture, the magnitude of the normal load, and different friction pair materials on the wear of the pin-on-disc friction pair. Future research requires more in-depth analysis and quantification to provide a comprehensive understanding of the tribological performance under various operating conditions.

4. Summary and Conclusions

An experimental study was conducted on the influence of surface texture and TiN coating on the lubrication and wear performance of the pin-on-disc friction pair under oil-lubricated conditions. The wear mechanism could be summarized in Figure 10. Surface texture could significantly improve the surface lubrication performance and avoid severe wear on the surface. On the one hand, surface coating could promote the improvement of lubrication performance to a certain extent, and on the other hand, it could enhance the surface wear resistance. Compared with the smooth surface, there is a certain space in the surface texture to store abrasive chips and particles, which can reduce surface wear to a certain extent. TiN coating could further reduce friction between the pin-on-disc friction pair and provide a hard, protective outer layer for the textured surface. Therefore, the synergistic effect of dimple surface texture and TiN coating made the surface have excellent tribological performance under oil-lubricated conditions.