Wear and Lubrication Performance of Different Reticular-Textured TiN-Coated Surfaces under Lubricated Conditions

Abstract

Texture and coating technology can significantly improve the tribological properties of mechanical components. In this study, the lubricating and wear properties of the reticular surface texture under the action of TiN were studied. Lubrication and wear experiments at different speeds were conducted using a UMT-3 wear and friction testing machine. Using Fluent fluid simulation, the bearing capacity of the oil film was obtained, and the lubrication performance of the texture was verified. The results showed that the simulation experiment and the lubrication experiment were consistent to a certain extent. For the groove width and angle parameters studied in this paper, optimal parameters existed to achieve the maximum bearing capacity, 1.27 N. Due to the high hardness and low elastic modulus of the TiN coating, the coated reticular texture was more wear-resistant, and it achieved the minimum wear volume 1.148 × 10⁻⁶ mm³ from the grinding stainless-steel matrix. The effect of the fluid dynamic pressure, wear debris collection, and lubricating oil storage were the main reasons for the increase in load-carrying capacity and the decrease in wear with the coated reticular texture.

1. Introduction

Surface texture is achieved by machining various shapes on the material surface. This can enable storage of lubricating fluid, increase the bearing capacity of the lubricating film, and improve the lubrication effect. At present, surface texture types mainly include circles, ellipses, squares, triangles, and chevrons [1,2,3,4,5]. Surface texturing has been successfully used in many applications to improve the performance of surfaces, such as in thrust bearings, journal bearings, cylinder liners, piston rings, mechanical seals, piston pins, and drill bits [6,7]. In recent years, surface texturing has emerged as a viable option for surface engineering. Thus, the load capacity and wear resistance coefficient of tribological mechanical components has been significantly improved [8]. A reticular surface texture can be used to store wear particles and lubricating oil, making it easier to form a continuous lubricating film on the surface of the friction pair [9]. At present, hydraulic components are widely used. In order to improve the properties of friction pair of hydraulic components, many studies have been conducted to verify the excellent lubrication performance of textured surfaces.

In addition, surface coating is another effective way to improve the tribological performance of components. TiN coatings have been widely used in industry as wear-resistant coatings, and especially for forming tools due to their high hardness, favorable corrosion resistance, and good chemical stability [10,11]. In order to further improve the friction performance, combining the coating and surface texture can improve the tribological properties of a hard coating. On the one hand, the protection of the surface texture plays a role in preventing wear to some extent; on the other hand, it can increase the load-carrying capacity of the surface, improving the surface’s wear. Due to the high hardness and wear resistance of the coating, the coating can protect the texture samples from excessive wear and help prevent the texture from losing the function of storing oil debris. Chen et al. [3] studied the effect of a triangular surface texture on the tribological and wear properties of a TiN-coated die-steel matrix under oil-lubricated conditions. The combination of the surface texture process and surface coating process exhibited excellent tribological properties (the lowest friction coefficient and wear volume). Arslan et al. [12] investigated the effect of changing the surface texture parameters under oil lubrication conditions on the tribological properties of DLC hard coatings. Xing et al. [13] studied the synergistic effect of a DLC hard coating; a circular surface texture had the best effect in reducing the friction and wear, due to the formation of a oil film and secondary lubrication, and the texture captured the wear debris, resulting in excellent tribological performance.

In addition to the effect of the TiN coating, which can improve the tribological properties and protect the sample, the reticular texture parameters also have extremely complex and important effects on improving tribological properties. There have been several simulation analyses in respect of texture theory, such as that of Papadopoulos et al. [14], who studied the Navier–Stokes and energy equations, which were solved using computational fluid dynamics software to determine the effect of rectangular dimples on the bearing capacity of thrust bearings with part textures. Ma et al. [15] numerically analyzed the bearing capacity of an oil film with cross-hatched grooves, and the bearing capacity of the oil film was maximized by the prediction of the texture parameters. Ji et al. [16] used an analytical model of the Reynolds equation on a cross-hatched texture, where the fluid mechanics were established using theoretical solution methods. Multigrid methods were used to obtain the cross-hatched texture of geometric parameters, which had a significant effect on the hydrodynamic pressure distribution and load-carrying capacity. Scaraggi et al. [17] experimentally investigated the frictional behavior of surface texture under lubricated conditions using the method of pin-on-disk grinding; they found that the friction coefficient decreased to the minimum under fluid lubrication conditions. Suh et al. [18] conducted tribology experiments with sliding in paraffin oil and micro-grooved a crosshatch-pattern texture with different angles and widths; they concluded that the width and angle of the micro-grooved crosshatch-pattern texture were important parameters, and had a strong influence on the tribological properties.

In this study, we used a reticular texture to explore the effect of hydrodynamic lubrication and wear performance. Through a tribology experiment with a pin plate and the method of software simulation, we verified the optimal reticular size parameters of the texture (groove width and angle of groove), determined the friction performance of the different widths of the surface of the reticular texture with the measurement of the load-carrying capacity, and analyzed the reticular texture’s surface-lubrication characteristics. In addition, with the application of the TiN coating, we used the synergistic effect of the coating and texture to measure the wear volume, in order to determine the tribological performance.

2. Experimental Details

2.1. Surface Texture and Coating

Brass and 304 stainless steel (40 mm × 40 mm), extensively used in friction pairs of hydraulic elements, were used as the test materials in this study. Surface textures were fabricated on the end faces of the pins. The brass nominal hardness was 105–175 HV, and the 304 stainless steel nominal hardness was 195–210 HV. In this study, brass consisting of copper (nominal composition: 0.68 wt%) and zinc (nominal composition: 0.32 wt%) was selected for the pins. The 304 stainless steel consisted of Fe (67–71.5%), Cr (17.5–19.5%), and Ni (8–10.5%).

The reticular texture was processed on the brass surface using a five-axis CNC machine tool to prepare the different parameters of the reticular texture. The schematic diagram of the reticular texture morphology is shown in Figure 1. The reticular texture’s size parameters are shown in

Table 1. Reticular texture size parameters.

Sample	Groove Width	Sample	Groove Angle
TiN-Wid-0.2	C = 0.2 mm	TiN-Ang-150°	(150°)
TiN-Wid-0.25	C = 0.25 mm	TiN-Ang-135°	(135°)
TiN-Wid-0.3	C = 0.3 mm	TiN-Ang-120°	(120°)
TiN-Wid-0.4	C = 0.4 mm	TiN-Ang-90°	(90°)

.

The TiN coating was deposited on the texture surface of the samples. The TiN coating was prepared using the PVD (physical vapor deposition) method to form a composite surface and enhance the friction reduction and resistance of the surface texture. The coating thickness was 6μm. The compositions of the TiN coating are shown in

Table 2. Compositions of TiN coating.

Element	Ti	N
Atomic (%)	43.57	56.43

.

The surface morphology of the micro-dimple textured sample is shown in Figure 2, and was detected using a hyper-depth three-dimensional microscope (Olympus, DSX510). Figure 2 shows the three-dimensional and two-dimensional surface morphology of the reticular texture.

2.2. Tribological Test

A tribology test was conducted using a friction test machine (UMT-3, BRUKER, Rheinstetten, Germany), under lubricated conditions with specific lubricants and the chosen pin-on-disk test mode.

Figure 3 shows the scheme of the experimental apparatus. The upper sample was a cylindrical copper stick; the length was 50 mm and the diameter was 6 mm. The lower sample was a square stainless-steel plate; the size was 40 × 40 mm and the thickness was 4 mm. The direction of rotation is shown in Figure 1. All the wear tests were conducted in an ambient environment, at a room temperature of 25 °C and a relative humidity of 45%. The pin-on-disk tests were performed under adequate lubrication. The basic properties of the lubricant used in the tests were a dynamic viscosity of 0.0144 Pa⋅s and a density of 860 kg/m³; shown in Figure 3.

The velocity of the rotation disk was set to 200 rev/min, 400 rev/min, 600 rev/min, and 800 rev/min. In the lubrication experiment, the method adopted in [2,19,20] was used to ensure the two mating surfaces were parallel; the gap between the upper sample and the lower sample was 5 μm. The accuracy of the position sensor was 0.01 μm. In the wear experiment, the load was applied on the upper pins. Each experimental condition lasted for 5 min. Then, the load-carrying capacity was measured using a stress sensor, and the data were collected by a friction test machine. Before and after the tests, all samples were washed with alcohol and acetone using ultrasonic cleaning.

3. Analytical Study

3.1. Lubrication Experiments

The load-carrying capacity under different parameters is shown in Figure 4. Figure 4a shows the changing trend of the load-carrying capacity of the reticulated-texture groove with the change in groove width when the angle of the reticulated groove was 135°. Figure 4b shows the changing trend of the load-carrying capacity of the reticular texture with the change in the angle when the groove width was 0.2 mm. The load-carrying capacity increased with the increase in the rotation velocity; because the lubrication was insufficient at low speed, the lubricating oil trapped in the groove of the reticulated fabric could not form a complete lubricating-oil film, and the hydrodynamic lubrication pressure was more obvious as the speed increased. The anti-wear ability increased as load-carrying capacity increased.

Figure 4a demonstrates the load-carrying capacities in respect of the increase in the groove widths. Comparing the five groups of samples with different load-carrying capacities, under different working conditions, the existence of the reticulation texture was beneficial to the generation of dynamic pressure lubrication. This is because the fluid dynamic pressure is generated in the surface gap when the two mating surfaces immersed in the oil are moving in relative rotation. Research has shown that hydrodynamic pressure is caused by local cavitation in the network grooves [21], and the existence of the network grooves leads to the convergence wedge of the gap oil film, which can provide additional film-lifting force and form an oil film with sufficient pressure, which then produces hydrodynamic lubrication.

For most samples, the bearing capacity of the oil film tended to increase with the increase in rotation velocity. This is because the oil film was unstable at a low velocity, and the bearing capacity of the oil film increased with the increase in rotation velocity. Figure 4a shows that the bearing capacity of the sample TiN-Wid-0.2 was low, and the load-carrying capacity was not obvious with the increase in rotation velocity. This may be due to the smaller width of the reticular grooves in this sample, which resulted in less lifting force. The carrying capacity of the sample TiN-Wid-0.25 increased with the increase in the rotation velocity, indicating that the sample had a better hydrodynamic lubrication effect at a high rotation velocity. The carrying capacity of the sample TiN-Wid-0.3 changed when the rotation velocity was 600 r/min; the carrying capacity began to increase, indicating that sample TiN-Wid-0.3 entered complete hydrodynamic lubrication at 600 r/min, and the bearing capacity was higher. The TiN-Wid-0.4 groove width of the sample was 0.4 mm. The oil film had the maximum bearing capacity and was more stable. More oil was stored when the groove width was larger.

Figure 4b demonstrates the load-carrying capacity with the increase in the groove angles. At the groove angle of 150°, load-carrying capacity was the lowest. The load-carrying capacity of the texture sample with a groove angle of 120° was the highest. The load-carrying capacity of the groove angle of 120° was the highest. On the one hand, the reason was the effect of velocity gradients [22]. The change of groove angle makes the flow of fluid different, and at a certain angle with the direction of motion velocity, it had better flow when the angle was smaller. On the other hand, the increase of groove angle had a wireless relationship with the increase of bearing capacity. The maximum bearing capacity existed only when the groove angle was 120°. The reason for this phenomenon was that a larger groove angle does not lead to better lubrication properties. When the groove angle increases past a certain extent, negative effects occur; there is an optimal groove angle with which to achieve the best lubrication properties. The lubrication properties did not seem to have a clear dependence on the increase in angle [22]. The convergence wedge and cavitation were the main reasons for the improvement of load carrying capacity. All improvements were purely observed under laboratory conditions.

3.2. Fluid Simulation Investigation of the Reticular Texture

The fluid simulation analysis of the reticular texture was performed using Fluent2020R1 software. Figure 5 shows the reticular-texture fluid model, which was created using 3D drafting software and then imported into ICEM CFD to “automatically divide” the grid. The model was introduced into Fluent, and a new fluid was defined in the material property settings. The fluid density and viscosity were set to 860 kg/m³ and 0.014 Pa·s, respectively. The fluid type was set to turbulent, and the k-epsilon model was selected. The velocity inlet boundary and the pressure outlet boundary conditions were selected; the fluid operation velocity was 1 m/s. The pressure at the pressure outlet boundary was set to atmospheric pressure (101,325 Pa). In order to study the internal mechanisms of the reticular texture, we analyzed a section of the reticular texture, as shown in Figure 5b. The following simulation results used the same section.

Figure 6 and Figure 7 show the static pressure distribution and velocity contour distribution on different reticular-textured surfaces. Along the velocity direction, the negative pressure appeared first and then the pressure peak appeared, because the existence of the reticular texture caused convergence and divergence zones to develop. When the fluid flowed to the texture along the velocity direction, a divergent zone formed negative pressure. When the fluid flowed out of the texture, a convergence gap formed, and a pressure peak appeared [1,5]. Figure 6 shows that the pressure tended to increase with the increase in the groove width, and the distribution range of the positive pressure also continuously increased. The velocity contour diagram shows that the vortex phenomenon [23,24] appeared in the reticular texture. This also confirmed the mechanism of the increased oil-film-bearing capacity during the lubrication experiment. Figure 6 and Figure 7 show that the flow characteristics within the grooves of the velocity contour weaves were complex. The velocity of the internal flow field was relatively small compared to the flow field in the outer fluid domain, and there were vortices in the position where the flow velocity was the lowest in the groove [25,26]. As shown in Figure 6, with the increase in the width, the range of the area with low velocity increased in the transverse range; there was obvious stratification, and the flow velocity increased from low to high from the bottom of the texture. The position with the lowest velocity inside the texture groove can be ignored as 0, at the bottom of the texture and in the middle area of the groove, where the vortex area with the lowest speed affected the pressure on both sides of the texture and the load-carrying capacity. Figure 7 shows a similar phenomenon.

Figure 8 presents the load-carrying capacity versus the constant gap and the rotating speed for the friction pairs. Figure 8a shows the influence of different groove widths on the load-carrying capacity. The bearing capacity increased with the increase in the rotation velocity and groove width. When the groove widths were 0.3 mm and 0.4 mm, the load-carrying capacity was at a maximum. The simulation results were the same as those for the changing trend in the lubrication experiment results. However, there were differences in the numerical values, which may be because the simulation test was in an ideal environment, and there are always many uncontrollable factors influencing the differences in data during an experiment.

Figure 8b shows the influence of different groove angles on the load-carrying capacity. The bearing capacity decreased with the increase in the groove angle, and the rotation velocity had no obvious effect. However, when the groove angle was 150°, the load-carrying capacity was lowest, as shown in the above lubrication experiment results. In addition, in the simulation experiment, the sample with a textured groove of 90° had the highest load-carrying capacity, followed by the sample with a textured groove of 120°. This was different from the lubrication experiment because, as shown in Figure 7, the peak pressure of the TiN-Ang-90° was the highest, and the cavitation area was large in the velocity contour.

3.3. Wear Experiments

Figure 9 shows the contour of the wear topography on the surface of the lower sample. Since the sample without coating and texture was copper and the sample with the pattern was stainless steel, the hardness of the upper sample was lower than that of the stainless steel, so the wear of the stainless steel was not obvious, and contour of the wear is not visible in the figure. Figure 9b shows the whole wear morphology on the surface of the coated and textured sample (first by the texturing process, then by the coating process); the ring wear morphology formed on the grinding surface during the entire wear experiment was not completely consistent in all locations. On the one hand, the reason for this phenomenon is that the two matching surfaces were not completely parallel at micro- and nano-scales, and the surface parameters such as the surface flatness and surface roughness caused micro-convex contact between the two surfaces. On the other hand, the wear debris generated in the process of wear caused the aggravation of local wear. Figure 9a shows the coated and textured sample’s local wear topography, where deep areas of wear marks appeared in the middle, because the texture stored wear debris during the grinding process. Figure 9c shows the local wear topography of the solely coated sample; there were deep wear marks on both sides, because the upper sample had no texture in which to store wear debris; therefore, the wear debris was extruded to both sides and caused the wear marks on both sides to deepen during the wear process. Because the hardness of the upper sample was lower than that of the lower sample, the wear depth was shallow, as shown in Figure 9d.

Figure 10 shows a schematic view of the wear during the wear experiment. Figure 10a shows the schematic diagram of the coated sample (solely coated) against grinding stainless steel. In the process of wear, a large amount of granular debris was generated to scratch the surface, because there was no texture in which to store the debris on the surface of the sample, and the grinding debris from the extrusion caused wear, which deepened the depth of the wear marks. This may be because there was more wear debris acting as a third-body abrasive [27]. During the wear process, granular debris accumulated on both sides with the extrusion, causing the wear marks to deepen. Figure 10b shows a schematic diagram of grinding stainless steel with the textured and coated samples (first by the texturing process and then by the coating process). Because the abrasive debris was washed into the grooves of the texture by the oil, the abrasive debris was accumulated in the grooves of the texture; therefore, the wear in the middle of the stainless-steel sample was aggravated. The reticular texture surface made it easier for wear debris to escape from the contact area into the texture grooves; therefore, the depth of the stainless steel worn by the textured and coated sample was less than that of the solely coated sample [8]. Figure 10c shows the grinding of the stainless steel by the textured samples (solely textured). It can be seen from the figure that because the hardness of the upper and lower samples was not consistent, and there was no protective effect of a coating, the wear debris was mostly in the upper sample, and the wear of the lower sample was reduced.

Figure 11 shows the wear volume variation in the sample with the reticular texture with different groove widths and different angles at a 15 N load and an 800 r/min rotation velocity. Figure 11a shows the change in the wear volume of the grinding stainless steel with different groove width textures, and Figure 11b shows the wear volume of the stainless steel with different groove angle textures. As shown in the figure, the sample that was solely coated had a much higher wear volume than the other samples. In addition, compared to the textured and coated sample, and compared with other grinding stainless steel, the wear volume of the solely textured sample to the grinding stainless steel was the lowest. The coated and textured samples with groove widths of 0.3 mm and 0.4 mm showed minimal wear volume, with groove angles of 120° and 90° having the smallest wear volume. On the one hand, according to the above lubrication experiments and simulation experiments, this may be due to the existence of the reticular texture, which reduced the contact area, resulting in hydrodynamic pressure and increased additional lift of the oil film. The texture effectively acted as a storage groove, storing wear debris and lubricant oil. On the other hand, due to the high hardness, low elastic modulus, and high abrasion resistance of the TiN coating, the wear volume of the coated and textured samples showed the lowest wear.

The lubricating properties of the reticular texture were verified by simulation and experiment in this study. Results showed that surface friction pair with reticular texture increased with the increase in velocity. Similar conclusions were found in [28,29], and dimensional parameters such as angle also had an effect on improving the load-bearing capacity and tribological properties. [3,30] showed that TiN coating had great advantages in improving wear performance, especially when the combination of texture and coating could better improve tribological properties. From reference [3] shows a combination of triangular surface texture and TiN coating, which had the smallest wear volume loss of 0.96 × 10⁻⁴ mm³. However, the combination of reticular surface texture and TiN coating in this study had the smallest wear volume loss of 1.148 × 10⁻⁶ mm³, demonstrating better wear properties.

In this study, other dimensional parameters of reticular texture were not studied. Additionally, there was no comparison with other shape textures; representing some limitations of this study. In the future, we will study the influence of different textures and different parameters on tribological properties. Beyond that, we will investigate another coating to observe the influence of wear performance.

4. Conclusions

In this paper, the effects of the reticular texture’s groove width, angle, and rotation velocity on the tribological properties of a TiN coating were studied using simulation and experimental methods. The conclusions were as follows:

(1)

The geometrical parameters of the reticular texture had significant effects on lubrication performance. In particular, the texture’s groove width and angle for the load-carrying capacity of each sample had a beneficial effect; with the increase in the groove width, the load-carrying capacity was larger, and with the increase in the angle (within a certain range), the capacity was reduced. The best lubrication performance was exhibited for the reticular texture’s groove width of 0.4 mm and a groove angle of 120°, and the higher speed was more conducive to producing a higher load-carrying capacity.

(2)

Through the establishment of the fluid model for the simulation experiment, the load-carrying capacities of different groove widths and angles were obtained using a fluid lubrication simulation, and a pressure cloud diagram and velocity contour were obtained. The simulation experiment not only verified the conclusion of the lubrication experiment, but also explained the mechanism of the bearing capacity of the reticular texture.

(3)

The coated and textured samples showed a smaller wear volume from the grinding stainless steel disc; in particular, the samples TiN-Wid-0.4, TiN-Ang-90°, and TiN-Ang-120° had the lowest wear volume, due to the high hardness and high abrasion resistance of the TiN coating. During the wear process, the abrasive wear caused abrasion and furrow marks on the surface of the stainless steel. The material loss was caused by the extrusion and movement of hard particles or hard convex bodies on the surface of the friction pair on the solid surface. These samples also showed the generation of hydrodynamic pressure and the synergy of the collection and storage of lubricating oil and wear debris due to the reticular texture.