Study on the Influence and Mechanism of Different Micro-Texture Parameters on the Tribological Properties of Brass Under Multi-Working Conditions

Highlights

•

UV laser micro-texturing is employed to modify the H62 brass surface, which effectively improves its tribological properties under dry friction, wet friction, and oil-lubricated friction conditions.

•

The hexagonal pit micro-texture with a size of 200 μm and an area density of 10% achieves the optimal friction reduction and anti-wear performance in all working conditions.

•

The tribological performance of different micro-textures ranks in the order: hexagonal > circular > rectangular, which is determined by stress distribution, debris capture, and lubricant regulation.

•

The wear mechanisms of H62 brass under multi-working conditions are revealed, including adhesive and abrasive wear (dry), oxidative wear (wet), and slight abrasive wear (oil lubrication).

•

This work provides an experimental basis and technical support for the surface optimization design of brass components used in seals, valves, and cooling systems.

Abstract

Aiming at the problems of high friction coefficient, severe wear, and unsatisfactory service life and operational reliability of brass under complex working conditions such as dry friction, wet friction, and oil-lubricated friction, H62 brass was taken as the research object to improve its friction and wear properties via surface micro-texture technology. This study systematically compares the tribological performance of three typical geometric micro-textures under three coupled working conditions for the first time. Circular, rectangular, and hexagonal micro-dimple textures were fabricated on the brass surface using ultraviolet laser micromachining. The control variable method was adopted to systematically investigate the effects of micro-texture parameters including shape, size, and area density on the friction and wear properties of brass under the three typical working conditions, combined with reciprocating friction and wear tests and ultra-depth-of-field microscope characterization. The results show that the hexagonal micro-dimple texture (200 μm in size, 10% in area density) exhibits the optimal friction-reducing and anti-wear performance. Compared with the smooth surface, the friction coefficient decreases from 0.51 to 0.43, and the wear rate of the GCr15 steel ball is reduced by 2.8% under dry friction; the friction coefficient decreases from 0.43 to 0.12 with an 11.8% reduction in wear rate under wet friction; and the friction coefficient decreases from 0.29 to 0.24 with an 8.3% reduction in wear rate under oil lubrication. Relative to dry friction, the wear rates are further reduced by 16.7% and 8.3% under wet friction and oil lubrication, respectively. Different from most existing studies that only focus on a single texture type or a single friction condition, this paper systematically reveals the coupling regulation mechanism between texture parameters and working conditions, clarifies the optimal micro-texture design strategy for multi-working conditions, verifies that hexagonal micro-textures can significantly improve the wear resistance of brass, and provides technical support for the surface optimization design of brass workpieces under complex working conditions.

1. Introduction

Brass is widely used in the manufacturing of sealing rings, valves, and cooling system components due to its excellent mechanical properties, corrosion resistance, and good machinability. However, in practical service conditions, brass components are often subjected to complex operating environments, including dry friction, wet friction, and oil lubrication. These conditions frequently lead to high friction coefficients and severe wear, resulting in reduced service life and compromised operational reliability. Therefore, improving the tribological performance of brass under multi-operating conditions has become a critical issue that urgently needs to be addressed.

In the field of surface modification for friction and wear reduction, surface micro-texturing technology has been extensively applied to enhance tribological performance. Cai Y [1] systematically reviewed the applications of laser surface texturing in tribology and pointed out that micro-scale textures can effectively reduce the real contact area, trap wear debris, and regulate lubricant distribution, thereby significantly decreasing the friction coefficient and wear rate. However, their work mainly focused on macroscopic performance summaries and lacked quantitative analysis for specific material systems. Mohit V [2] compared various surface texturing fabrication methods and found that different texture types exhibit significant differences in tribological performance, but did not further investigate their adaptability under multi-condition coupling environments. Mao B [3] summarized the application of laser surface texturing in engineering materials and highlighted the critical role of texture parameters in determining friction behavior; however, their study mainly focused on steels and ceramics, with limited attention to copper alloys. In addition, Wang Zexiao [4] investigated the coupling effects of texture geometry and operating conditions, revealing that texture shape and load significantly influence tribological performance, although the study was limited to specific conditions without systematic multi-environment analysis.

Regarding fabrication methods, laser processing has been widely recognized as a core technique for micro/nano-scale surface structuring. Wang J [5] elaborated on the mechanisms of laser processing from micro- to atomic scales, demonstrating its capability for high-precision structural control, yet without linking it to specific tribological applications. Fomicheva I [6] investigated the formation mechanism of laser-induced periodic surface structures (LIPSS) on titanium alloys using picosecond lasers, revealing the influence of laser energy density and pulse number on structure size and morphology; however, its applicability to copper-based materials remains unclear. Hongfei S [7] systematically analyzed the effects of femtosecond laser parameters on metallic surface properties, indicating that pulse mode and energy input are key factors governing texture precision and functional performance, but did not address tribological evolution mechanisms. Guillemot F [8] conducted early studies on ultraviolet laser surface treatment of titanium alloys, confirming its capability for precise structural fabrication, although focusing mainly on biocompatibility rather than engineering tribology. Meanwhile, Balage P [9] demonstrated high-aspect-ratio micro-hole fabrication in glass using femtosecond lasers, highlighting the advantages of laser processing in complex structure fabrication, but with limited relevance to metallic tribological behavior.

In terms of bio-inspired micro-textures and functional surfaces, Wu S [10] constructed bio-inspired microstructures on Ti6Al4V surfaces using femtosecond lasers and found significant improvements in friction reduction and wear resistance; however, the study was limited to single materials and specific conditions. Chen Z [11] reviewed laser-fabricated special wettability surfaces, suggesting that bio-inspired structures can indirectly improve tribological performance by regulating interfacial wettability, although quantitative relationships remain unclear. Yong J [12] developed super-lubricated surfaces via femtosecond laser processing, enabling controllable droplet manipulation, which demonstrates the potential of micro-textures in interfacial control but focuses mainly on fluid behavior rather than solid friction and wear. Li Z [13] investigated the corrosion resistance of bio-inspired wettability surfaces, confirming the potential of laser micro-texturing in functional surface engineering, but without in-depth analysis of wear mechanisms.

Additionally, studies on other material systems provide valuable insights into this work. Fernandez G J [14] reported that micro-textures on tool surfaces can significantly improve machining performance, though such studies mainly focus on cutting processes rather than sliding friction. Peixuan L [15] summarized the effects of laser texturing on wettability and tribological behavior, emphasizing the importance of texture geometry but lacking systematic comparisons under different operating conditions. Bonse J [16] demonstrated the effectiveness of femtosecond laser texturing in reducing friction, yet did not establish a unified relationship between texture parameters and tribological performance. Demófilo M [17] experimentally compared different texture geometries and confirmed their significant influence on friction behavior, although limited to single-condition testing. Hu Y [18] achieved nanoparticle embedding in flexible substrates using femtosecond lasers, enhancing surface functionality and indicating the broad applicability of laser micro/nano-structuring, but without addressing metallic tribology.

For copper-based materials, previous studies have shown that micro-texturing can effectively improve tribological performance. Li P [19] fabricated micro-textures on copper alloy surfaces and filled them with solid lubricants, demonstrating the formation of a stable lubricating film during friction and a significant reduction in friction coefficient and wear rate; however, the study focused on specific conditions without systematic validation under varying environments. Gao Q [20] developed a micro-pore texture-based lubrication storage and release system, suggesting that micro-textures can synergistically interact with lubricants to continuously improve tribological performance, though lacking in-depth mechanistic analysis and systematic experimental validation. Zhu L [21] analyzed wear behavior and friction film evolution of copper-based materials from a microscopic perspective, revealing the involvement of multiple complex wear mechanisms, but most existing studies still focus on macroscopic evaluation, with insufficient correlation to micro-scale mechanisms.

Despite extensive research, several limitations remain. First, most studies focus on single texture geometries or single friction conditions, lacking systematic comparisons under multi-operating conditions. Second, research predominantly targets steels, ceramics, and titanium alloys, with relatively limited studies on brass and other copper alloys under complex environments. Third, although the influence of laser processing parameters on micro-textures has been widely recognized, their coupling with real service conditions is not yet well understood. Furthermore, environmental factors such as humidity and lubrication have been partially considered, but a comprehensive analytical framework is still lacking [22,23].

Based on the above research status, this paper takes H62 brass as the research object. Three typical micro-dimple textures—circular, rectangular, and hexagonal—are fabricated on its surface using an ultraviolet laser, and systematic comparative studies are conducted under three typical working conditions: dry friction, wet friction, and oil lubrication. Different from existing studies that mainly focus on a single working condition or a single texture type, this paper adopts a multi-dimensional coupling perspective of “texture shape–parameter–working condition” to systematically reveal the adaptation laws and synergistic mechanisms of different micro-textures under multi-working conditions. Through reciprocating friction and wear tests, combined with characterization of the worn surfaces using a digital microscope, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), a unified mechanism framework for friction reduction and wear resistance of micro-textures under multi-working conditions is established, and the optimal combination of micro-texture parameters suitable for complex working conditions is identified. This study provides new experimental evidence and mechanistic support for the surface structure design of key brass components under complex service environments.

To further improve the structural clarity and goal orientation of this research, the following research objectives are explicitly proposed:

Objective 1: To clarify the influence of laws of different micro-texture shapes (circular, rectangular, and hexagonal) on the tribological properties of brass under multi-working conditions including dry friction, wet friction, and oil lubrication;

Objective 2: To reveal the regulation mechanisms of micro-texture parameters such as size and area density on the friction coefficient and wear behavior, and to determine the optimal parameter combination;

Objective 3: To analyze the wear mechanisms under different working conditions and the friction-reducing and anti-wear mechanisms of micro-textures based on SEM and EDS characterization.

2. Materials and Methods

2.1. Specimen Preparation and Micro-Texture Design

H62 brass specimens with dimensions of 30 mm × 8 mm were used in this study. Prior to laser processing, the specimen surfaces were polished using a polishing machine (LAP-2000S, Naibo Precision Machinery Co., Ltd., Dongguan, China) with W10 diamond suspension to achieve a surface roughness of approximately 0.3 μm. The specimens were then ultrasonically cleaned using an ultrasonic cleaner (YA008G, Yuny Ultrasonic Equipment Co., Ltd., Shenzhen, China) in 99% anhydrous ethanol for 10 min, dried with compressed air, and sealed for storage to prevent surface contamination. Surface pretreatment plays a critical role in the fabrication of microstructures on copper-based materials. Previous studies on hierarchical microstructures of pure copper have shown that a low surface roughness (≤0.3 μm) after polishing can effectively reduce laser energy dissipation during processing and improve the dimensional accuracy and morphological consistency of micro-textures. Therefore, the pretreatment procedure adopted in this study (polishing to 0.3 μm followed by ultrasonic cleaning) was based on established processing strategies for copper-based materials, providing a reliable foundation for subsequent micro-texture fabrication.

Subsequently, a Cypress-355-15 ultraviolet nanosecond laser system (Huaray Precision Laser Co., Ltd., Wuhan, China) was employed to fabricate micro-textures on the brass surface. Laser processing parameters exert a significant influence on the formation of micro-textures. In this study, fixed laser parameters were adopted to ensure the comparability among different textures: laser power P = 15 W, repetition frequency f = 50 kHz, scanning speed v = 10 mm·s⁻¹, and spot diameter d ≈ 0.03 mm. By controlling the scanning spacing and scanning path, the consistency and repeatability of texture morphology were guaranteed.

The laser energy density can be expressed as:

$$E={\displaystyle \frac{P}{v·d}}$$

(1)

where P is the laser power, v is the scanning speed, and d is the spot diameter. In the preliminary experiment, the effects of different energy densities on the edge integrity of micro-textures were compared, and the current parameters were finally selected. All experimental parameters were controlled within the stable processing range to ensure the consistency of texture morphology, so that the research could focus on the influence of geometric structure on tribological properties. The laser processing setup is illustrated in Figure 1a, and the micro-textures were distributed within a 20 mm × 20 mm square region, as shown in Figure 1b. After laser processing, the specimen surfaces were re-polished to remove the recast layer generated during laser ablation [24]. The samples were then ultrasonically cleaned again to eliminate residual debris and molten particles, ensuring the integrity of the micro-texture morphology.

The accuracy of laser surface texturing is strongly dependent on the laser intensity distribution. Studies on femtosecond laser processing have demonstrated that a uniform intensity profile can reduce edge defects and minimize dimensional deviations. This principle is also applicable to ultraviolet nanosecond laser processing. By optimizing the laser intensity parameters, the continuity of micro-texture edges and dimensional consistency can be effectively ensured, thereby providing reliable samples for subsequent tribological testing [25]. During nanosecond laser processing of metallic materials, high-energy pulses tend to induce the formation of recast layers and residual slag on the surface. If not properly removed, these defects can adversely affect both the geometric accuracy of micro-textures and their tribological performance. Previous studies on nanosecond laser processing of polycrystalline diamond have shown that combining laser processing with subsequent grinding or polishing can effectively eliminate the influence of recast layers on surface quality. This combined processing strategy is also applicable to brass micro-texture fabrication and provides a valuable reference for the post-treatment procedures adopted in this study.

To comprehensively investigate the effects of micro-texture parameters on the tribological performance of brass, nine groups of micro-texture schemes with varying parameters were designed. In addition, a non-textured specimen was set as a control group (denoted as T10). The specific parameters are listed in

Table 1. Design of micro-texture parameters.

Group	Dimple Shape	Area Density/(%)	Size/(μm)
T1	Circular	5	100
T2	Circular	7.5	200
T3	Circular	10	150
T4	Rectangular	5	200
T5	Rectangular	7.5	150
T6	Rectangular	10	100
T7	Hexagonal	5	150
T8	Hexagonal	7.5	100
T9	Hexagonal	10	200

. The designed schemes include three texture geometries: circular dimples, rectangular dimples, and hexagonal dimples. For each geometry, different diameters (or side lengths) and area densities were applied to enable a controlled variable study.

The design of laser surface texturing schemes should be based on the synergistic effects of multiple parameters. Previous review studies have indicated that the combination of texture shape, size, and area density must be properly coordinated to maximize wear resistance, and optimization of a single parameter alone is insufficient to fully exploit the benefits of surface texturing. Therefore, the multi-geometry and multi-parameter design adopted in this study follows this principle, ensuring the identification of optimal texture parameters suitable for different operating conditions.

2.2. Friction and Wear Test Conditions and Characterization Methods

Reciprocating ball-on-disk tribological tests were carried out using a reciprocating friction and wear testing machine (MFT-5000, Rtec Instruments Inc., San Jose, CA, USA). The friction pair consisted of an H62 brass specimen and a GCr15 steel ball with a diameter of 6 mm, forming a typical hard-soft contact configuration, as shown in Figure 1b. The test parameters were set as follows: a normal load of 5 N, a frequency of 2 Hz, a stroke length of 10 mm, and a test duration of 10 min for each run, ensuring consistent experimental conditions. Three typical operating conditions were investigated: dry friction, wet friction, and oil-lubricated friction. For the dry condition, tests were conducted at room temperature without any lubricant or abrasive particles. For the wet condition, deionized water was used as the medium and uniformly introduced into the contact interface to ensure a fully wetted state, simulating humid working environments. For the oil-lubricated condition, PAO4 polyalphaolefin lubricant (Mobil SHC Series, ExxonMobil Corporation, Irving, TX, USA) was applied to the contact surfaces and filled into the contact region to ensure the formation of a stable lubricating film during sliding, thereby simulating practical oil-lubricated conditions and reducing direct asperity contact. Prior to the formal tests, the reciprocating friction and wear tester was rigorously calibrated to ensure measurement accuracy. The resolution of the friction coefficient was 0.001, and the repeatability error was controlled within ±3%, which enabled reliable identification of the small differences in friction coefficients among different groups.

After the tests, the experimental data were sorted and analyzed. Firstly, the mass change of the GCr15 steel ball before and after the test was measured by the weighing method to calculate its wear loss and wear rate. Meanwhile, a VHX-1000 ultra-depth-of-field optical microscope (Keyence Corporation, Osaka, Japan) was used to observe the worn morphology of micro-textures on the surface of brass specimens. Profile scanning was performed on the wear scar regions perpendicular to the friction direction to obtain the cross-sectional profile curves of the wear tracks, and the wear scar width W, maximum depth h max, and cross-sectional area A were measured. Finally, scanning electron microscopy (SEM) was adopted to observe the micro-morphology of wear scars, and energy-dispersive spectroscopy (EDS) was used to analyze the elemental distribution on the worn surfaces, so as to reveal the wear mechanism.

The volume wear loss of the brass specimen V_brass is calculated by the following formula:

V_brass = A × l

(2)

where l is the wear scar length (l = S = 10 mm). On this basis, the volume wear rate W_R-brass of the brass specimen is further calculated.

To quantitatively evaluate the wear performance of the friction pair, the volume wear rate (W_R) is adopted as the evaluation index, and its calculation formula is as follows:

$$W_R = {\displaystyle \frac{V}{F \times L}}$$

(3)

where:

W_R—volume wear rate, unit: mm³·N⁻¹·m⁻¹;

V—volume wear loss of the specimen. The volume wear loss of the GCr15 steel ball is calculated from the mass loss Δm and material density ρ, namely $V_{\mathit{GCr}15} = {\displaystyle \frac{∆m}{\rho }}$ In this work, the density of GCr15 steel ball is ρ = 7810 kg·m⁻³;

F—applied test load, set as 5 N in this experiment;

L—total friction sliding distance, which can be calculated according to friction frequency and test time: L = 2 × S × f × t = 24 m.

This parameter is used for the comparative analysis of wear performance under different working conditions and micro-texture parameters in the subsequent study. All experimental data are uniformly calculated and processed based on the above methods to ensure the comparability of results under various test conditions. To guarantee the reliability of experimental data, each group of tests is repeated three times, and the average value is taken as the final result. The experimental error is controlled within ±3%, indicating that the test system possesses favorable stability and repeatability.

The experimental scheme in this paper corresponds one-to-one with the research objectives:

Objective 1: Investigate the effect of texture shape through friction tests under three conditions: dry friction, wet friction, and oil lubrication.

Objective 2: Analyze the influence of texture parameters and wear performance using nine sets of textures with different sizes and areal densities.

Objective 3: Complete the analysis of wear mechanisms and anti-friction/wear resistance mechanisms through SEM morphology and EDS energy spectrum.

2.3. Texture Morphology Characterization and Dimensional Measurement

A VHX-1000 digital microscope (Keyence Corporation, Osaka, Japan) was used to quantitatively characterize the surface micro-textures after laser processing. Geometric parameters such as diameter, depth, and surface roughness (Sa) of different textures were measured and analyzed (Figure 2). For each specimen, measurements were taken at three different positions, and the average values were taken as the final results. The results show that due to the melting and re-solidification of the material during the laser processing, certain deviations exist between the actual texture dimensions and the design values. The deviation between actual and design dimensions is less than 3%, which does not affect the observed trends. The measured geometric parameters of various typical textures are presented in

Table 2. Geometric parameters and surface roughness test results of typical micro-textured specimens.

Group	Texture Shape	Design Size/(μm)	Measured Size/(μm)	Measured Depth/(μm)	Area Density/(%)	Surface Roughness/Sa
T3	Circular	150	150.7 ± 2.0	17.1 ± 1.3	10	0.19 ± 0.04
T5	Rectangular	200	204.8 ± 1.2	16.2 ± 1.5	7.5	0.28 ± 0.02
T9	Hexagonal	200	201.2 ± 1.5	18.3 ± 1.1	10	0.22 ± 0.03
T10	Smooth surface	—	—	—	—	0.34 ± 0.02

.

2.4. Theoretical Calculation and Analysis Method

To quantitatively characterize the regulation mechanism of micro-textures on contact stress and lubrication state, relevant calculations are performed based on the Hertz contact theory and elastohydrodynamic lubrication (EHL) theory. The corresponding formulas are given below.

2.4.1. Contact Stress Calculation

The contact behavior of the ball-on-disk friction pair follows the Hertz contact theory. The contact radius a is calculated as:

$$a =\sqrt[3]{{\displaystyle \frac{3\mathit{FR}}{4E^{*}}}}$$

(4)

where:

F—normal load;

R—radius of the steel ball (R = 3 mm in this work);

E^∗—equivalent elastic modulus, determined by the elastic modulus and Poisson’s ratio of the two materials:

$${\displaystyle \frac{1}{E^{*}}}={\displaystyle \frac{1-v_1^2}{E_1}}+{\displaystyle \frac{1-v_2^2}{E_2}}$$

(5)

where E₁, v₁ are the elastic modulus and Poisson’s ratio of brass, respectively (E₁ = 100 GPa, ν₁ = 0.34); E₂, v₂ are the elastic modulus and Poisson’s ratio of GCr15 steel, respectively (E₂ = 210 GPa, v₂ = 0.3).

The formula for the maximum contact stress P_max is expressed as:

$$P_{\mathit{max}} ={\displaystyle \frac{3F}{2\pi a^2}}$$

(6)

2.4.2. Lubrication State Judgment

The lubrication state of the friction pair is evaluated by the film thickness ratio λ:

$$\lambda ={\displaystyle \frac{h_{\mathit{min}}}{\sqrt{R_{q1}^2+R_{q2}^2}}}$$

(7)

where h_min—minimum lubrication film thickness; R_q1, R_q2—root-mean-square surface roughness of the brass specimen and steel ball, respectively (approximately 0.3 μm for polished brass and 0.05 μm for the GCr15 steel ball).

Under oil lubrication conditions, the minimum film thickness h_min is estimated by the Hamrock–Dowson formula:

$$h_{\mathit{min}}\approx 3.63\cdot {\left(\eta U\right)}^{0.68}\cdot {\left(\alpha \right)}^{0.49}\cdot {\left(E^{*}\right)}^{-0.073}\cdot {\left(R\right)}^{0.466}\cdot F^{-0.073}$$

(8)

The dynamic viscosity η ≈ 0.04 Pa·s, the pressure-viscosity coefficient α ≈ 1.5 × 10⁻⁸ m²·N⁻¹, and the entrainment velocity U ≈ 0.06 m/s.

3. Results and Discussion

3.1. Effect of Texture Geometry

Due to space limitations, only representative micro-texture morphologies on the brass specimens are presented in this paper, as shown in Figure 3, including circular, rectangular, and hexagonal dimples, as well as the smooth surface, both before and after the friction tests. The dimensions of the initial micro-textures were measured, and the results indicate that the deviation between the actual and designed sizes is within 3%. In addition, the micro-texture edges are continuous and free from obvious defects, demonstrating that the precision of ultraviolet nanosecond laser processing is sufficient to meet the experimental requirements and ensure the reliability of subsequent test results. Furthermore, by comparing the morphologies before and after testing, the wear degree of the micro-textures, as well as the adhesion and accumulation of wear debris under different operating conditions, can be directly observed, providing important morphological evidence for the subsequent analysis of tribological performance.

3.1.1. Dry Friction Condition

First, the friction coefficient curve of the untextured surface under dry friction conditions (Figure 4, T10) was analyzed. The friction coefficient gradually increased to 0.88 and then stabilized, with an average value of 0.51. In dry friction, the brass surface easily forms adhesive junctions with the GCr15 steel ball. These junctions continuously rupture and reform during sliding, resulting in unstable friction forces [26]. Additionally, the heat generated during friction raises the temperature in the contact area, reducing the hardness of the brass surface and intensifying plastic deformation, ultimately forming pits of varying depths. Compared with the micro-textured samples, all micro-textures reduced the amplitude of friction coefficient fluctuations and lowered the overall friction coefficient. Among the three types of micro-textures, the minimum friction coefficients are ranked from highest to lowest as follows: rectangular (0.45), circular (0.44), and hexagonal (0.43).

The rectangular micro-textures exhibited the least effective friction reduction. Their right-angled structures and fixed-direction grooves tend to become the main sources of friction resistance under dry conditions. The sharp corners generate significant stress concentration, leading to uneven contact pressure distribution and localized adhesive wear. Meanwhile, the relatively enclosed rectangular grooves allow wear debris generated during friction to accumulate without timely removal. This trapped debris acts as a “third body,” exacerbating abrasive wear and further increasing interfacial sliding resistance, resulting in the highest friction coefficient. Circular micro-textures performed better. Their curved surfaces have no pronounced corner stress concentration, effectively distributing contact pressure and reducing adhesive wear. The circular pits can accommodate some wear debris, preventing additional resistance from debris accumulation. Their smooth surfaces also reduce interfacial sliding resistance. However, the spherical pits have limited debris drainage capacity, so some debris may still remain, causing the friction coefficient to be slightly higher than that of hexagonal textures—lower than rectangular but higher than hexagonal. Hexagonal micro-textures exhibited the best performance. Their regular polygonal distribution and rounded transitions maximize optimization of the contact state. The rounded corners effectively prevent stress concentration, evenly distribute contact pressure, and reduce adhesive wear. Additionally, the hexagonal pits combine storage and drainage functions, both accommodating wear debris and guiding it smoothly out along the polygonal gaps, avoiding exacerbation of abrasive wear. The uniform polygonal distribution also ensures a more even contact area, minimizing interfacial sliding resistance. As a result, hexagonal textures have the lowest friction coefficient, the greatest reduction in friction, and the best dry-friction performance.

3.1.2. Wet Friction Condition

In practical water-lubricated environments without abrasive particles, the friction behavior of brass workpieces is influenced by the wetting effect of pure water, which differs significantly from ideal dry friction or abrasive-laden wet friction conditions. Wet friction tests without abrasives use pure water as the sole medium, simulating actual scenarios of particle-free lubrication, thereby providing a more accurate assessment of the anti-friction performance of different micro-textured brass surfaces. The specific experimental data are shown in Figure 5. The blank control group (T10) exhibited an average friction coefficient of 0.43, which is higher than that of all micro-textured samples under particle-free wet friction. The main reason is that, although the pure water medium provides cooling and mild lubrication to reduce direct adhesion between friction pairs, the blank sample’s surface lacks micro-textures to guide fluid flow. Consequently, the water film formed in the contact gap is stable, and small amounts of wear debris generated during friction are not effectively removed, accumulating at the contact interface and increasing frictional resistance. Meanwhile, the untextured surface has a larger contact area with uneven pressure distribution, which can cause local stress concentrations, maintaining a relatively high friction coefficient and leading to uniformly distributed minor surface scratches.

Comparing circular, rectangular, and hexagonal micro-textured samples, all three effectively reduced the friction coefficient under wet conditions. The optimal friction coefficients ranked from highest to lowest are: rectangular (0.16), circular (0.13), and hexagonal (0.12). Among them, hexagonal micro-textures achieved the greatest reduction, followed by circular, while rectangular textures were the least effective. The rectangular micro-textures performed worst because their regular right-angle structures and linear distribution, while able to store some water, have narrow, fixed-direction channels that create high flow resistance, making it difficult to form a stable hydrodynamic film. Moreover, the edges and corners of the rectangular textures are prone to stress concentration, which can cause micro-deformation during friction, increasing interfacial resistance. Small amounts of residual wear debris may also remain on the surface, further raising the friction coefficient. Circular micro-textures perform better than rectangular ones due to their smooth curved surfaces, which reduce stress concentration and sliding resistance. The spherical concavities can store water, generating some hydrodynamic effect that guides the fluid along the friction direction, preventing fluid stagnation and local pressure peaks, while accommodating small amounts of wear debris to reduce friction [27]. However, their anti-friction performance is still slightly inferior to that of hexagonal textures.

Hexagonal micro-textures exhibit the best performance. Their regular polygonal structures and uniform distribution provide superior fluid storage and drainage capabilities, guiding water to flow evenly in the micro-gaps and forming a thin but stable water film, greatly reducing direct contact between friction pairs. Rounded transitions avoid stress concentration, minimizing additional friction loss, while efficiently removing surface wear debris. As a result, the friction coefficient decreases the most, achieving the optimal anti-friction performance, which is fully consistent with the ranking of the experimental data discussed above.

3.1.3. Oil-Lubricated Friction Condition

In practical oil-lubricated environments, the use of PAO4 poly-α-olefin lubricant significantly alters the surface friction behavior of brass workpieces, differing fundamentally from dry or particle-free wet friction conditions. Oil-lubricated friction tests use PAO4 poly-α-olefin as the lubricating medium to simulate real-world oil-lubricated scenarios, allowing a more comprehensive evaluation of the anti-friction and anti-wear performance of micro-textured brass surfaces. The specific experimental data are shown in Figure 6. The blank control group (T10) exhibited an average friction coefficient of 0.29, higher than all micro-textured samples under oil lubrication. This is mainly because, although PAO4 poly-α-olefin forms a basic lubricating film to reduce direct contact between friction pairs, the untextured surface lacks micro-textures to assist in storing and guiding the lubricant. Consequently, the lubricating film is prone to rupture or loss during friction, failing to maintain a stable lubrication state. Additionally, the uneven contact pressure on the untextured surface can cause local film failure, leading to increased friction. The surface shows only very slight wear marks; although the wear is minor, it is still higher than that of micro-textured samples.

Comparing circular, rectangular, and hexagonal micro-textures, all three further reduced the friction coefficient under oil-lubricated conditions. The optimal friction coefficients are ranked from highest to lowest as follows: rectangular (0.27), circular (0.26), and hexagonal (0.24). Hexagonal micro-textures achieved the greatest reduction, followed by circular, while rectangular textures were the least effective, consistent with the trend observed under wet friction. Rectangular micro-textures performed the worst. Their right-angled structures and fixed-direction grooves can store some PAO4 poly-α-olefin lubricant, but the oil has poor mobility within the grooves, making it difficult to quickly replenish the lubricating film lost from the contact gap. The sharp corners also create stress concentrations, leading to uneven local film thickness and film rupture. Furthermore, small amounts of wear debris can remain in the rectangular grooves and are not efficiently removed by the lubricant, forming localized micro-wear points and further increasing friction resistance, resulting in the highest friction coefficient.

Circular micro-textures performed better. Their curved surfaces reduce stress concentration, and the smooth surfaces lower both lubricant flow resistance and interfacial sliding resistance. The circular pits can store PAO4 poly-α-olefin lubricant, forming a relatively thick hydrodynamic film that guides the lubricant along the sliding direction, replenishing the contact gap and simultaneously accommodating and flushing small amounts of wear debris, thereby lowering friction. As a result, the friction coefficient is slightly lower than that of rectangular textures, with better anti-friction performance but still inferior to hexagonal textures.

Hexagonal micro-textures exhibited the best performance. Their regular polygonal distribution and rounded transitions combine excellent lubricant storage with efficient drainage. They can stably store lubricant while guiding its uniform flow across the micro-textured gaps, rapidly replenishing the lubricating film lost during friction and forming a stable, uniform lubrication system. The rounded corners alleviate stress concentrations, completely preventing local film failure. Additionally, the flow of lubricant efficiently removes surface wear debris, minimizing additional frictional losses. Consequently, hexagonal textures have the lowest friction coefficient, the greatest reduction in friction, and the best anti-friction and anti-wear performance.

3.2. Effect of Texture Parameters

Using the friction coefficient as the core evaluation index, the regulatory effects of size and areal density on the tribological performance of three shaped micro-textures were systematically analyzed, and the optimal parameter combinations under multiple operating conditions were screened.

3.2.1. Effect of Micro-Texture Size on Tribological Performance

Circular micro-texture: At a size of 200 μm, the friction coefficient under all three operating conditions was lower than that of the 100 μm and 150 μm groups. The larger size provides greater wear debris storage space and lubricant storage capacity, resulting in better friction reduction.

Rectangular micro-texture: At a size of 200 μm, the friction coefficient was slightly lower than that of the 150 μm and 100 μm groups, but due to the limitation of the right-angle structure, the size optimization effect was weaker than that of the circular and hexagonal textures.

Hexagonal micro-texture: At a size of 200 μm, the friction coefficient reached its lowest value under dry friction, wet friction, and oil lubrication conditions. The 200 μm size balances the contact area, wear debris storage, and lubricant capability, offering the best adaptability.

3.2.2. Effect of Areal Density on Tribological Performance

At an areal density of 5%, the micro-textures are sparsely distributed, resulting in limited friction reduction and wear resistance.

At an areal density of 7.5%, the performance improved but did not reach the optimum.

At an areal density of 10%, the friction reduction effect of all three micro-texture shapes reached its optimum, among which the hexagonal micro-texture showed the greatest decrease in friction coefficient under the 10% areal density. A 10% areal density reduces the actual contact area while avoiding structural overlap and stress concentration caused by excessive texture density, thus achieving optimal parameter matching.

Comprehensive analysis of size and areal density: The hexagonal dimple with a size of 200 μm and an areal density of 10% (T9) is the optimal micro-texture parameter combination under multiple operating conditions.

3.3. Analysis of Wear Volume and Wear Rate

This study adopts the volumetric wear rate (W_R) to evaluate the wear performance of the tribo-pair, as calculated in Section 2.2. Based on this index, the wear performance under various operating conditions and micro-texture configurations is comparatively analyzed.

3.3.1. Wear Performance Analysis of GCr15 Steel Ball

The optimal micro-texture T9 (hexagonal, area density 10%, size 200 μm) was selected for comparison with the blank control group T10 (see

Table 3. Comparison of wear mass and volumetric wear rate of GCr15 steel balls under different working conditions. Note: ↓ indicates the reduction in wear rate compared with the smooth surface (T10).

Operating Condition	Group	Mass Before Wear/(g)	Mass After Wear/(g)	Mass Loss Δm/(g)	$\mathbf{Wear} \mathbf{Rate} {\mathit{W}}_{\mathit{R}}$/(mm3·N−1·m−1)	Change Relative to T10
Dry friction	T10	0.8874	0.8837	0.0037	3.95 × 10−3	—
	T9	0.8878	0.8842	0.0036	3.84 × 10−3	↓ 2.8%
Wet friction	T10	0.8877	0.8843	0.0034	3.63 × 10−3	—
	T9	0.8880	0.8850	0.0030	3.20 × 10−3	↓ 11.8%
Oil-lubricated friction	T10	0.8866	0.883	0.0036	3.84 × 10−3	—
	T9	0.8876	0.8843	0.0033	3.52 × 10−3	↓ 8.3%

). The results show that the T9 group effectively reduced the wear rate of the GCr15 steel ball under all three friction conditions: dry friction, wet friction, and oil-lubricated friction. Under dry friction, the wear rate of T9 was 3.84 × 10⁻³ mm³·N⁻¹·m⁻¹, a 2.8% reduction compared with T10. Under wet friction, the wear rate decreased to 3.20 × 10⁻³ mm³·N⁻¹·m⁻¹, with a reduction of 11.8%, representing the best anti-wear effect among the three conditions. Under oil-lubricated friction, the wear rate was 3.52 × 10⁻³ mm³·N⁻¹·m⁻¹, 8.3% lower than T10; since the lubricant itself provides good lubrication, the enhancement effect of the micro-texture is moderately diminished.

A lateral comparison of the T9 group under different friction conditions was conducted (see

Table 4. Comparison of the wear rate of steel balls corresponding to the optimal micro-texture T9 under different working conditions. Note: ↓ indicates the reduction in wear rate compared with the dry friction condition.

Operating Condition	$\mathbf{Wear} \mathbf{Rate} {\mathit{W}}_{\mathit{R}}$/(mm3·N−1·m−1)	Change Relative to Dry Friction
Dry friction	3.84 × 10−3	—
Wet friction	3.2 × 10−3	↓ 16.7%
Oil-lubricated friction	3.52 × 10−3	↓ 8.3%

), showing a clear dependence of wear rate on the operating condition. The lowest wear rate was observed under wet friction, decreasing by 16.7% compared with dry friction. This indicates a highly effective synergy between the lubricating and cooling effects of the pure water medium and the fluid drainage and debris-trapping functions of the hexagonal micro-texture, resulting in the most pronounced anti-wear performance. Under oil-lubricated conditions, the wear rate decreased by 8.3% relative to dry friction, with the overall wear level remaining lower than in dry friction. In dry friction, lacking external lubrication, the micro-texture relies solely on structural optimization to achieve anti-friction and anti-wear effects, yielding an intermediate wear rate. Overall, the volumetric wear rate of the GCr15 steel ball is closely related to the lubrication condition. The optimal hexagonal micro-texture exhibits the best compatibility with the pure water medium under wet friction, achieving the most significant anti-wear effect, followed by oil lubrication, while its structural anti-wear effect is relatively limited under dry friction.

3.3.2. Wear Performance Analysis of Brass Specimen

To further analyze the wear behavior of the brass specimen itself, the optimal texture T9 (hexagonal, areal density 10%, size 200 μm) and the blank control group T10 were selected. The wear scar width and depth of the brass specimen under three operating conditions were measured using a super-depth-of-field microscope (see Figure 7), from which the cross-sectional area and volume wear rate were obtained.

As shown in

Table 5. Comparison of volume wear rate of brass specimens under different operating conditions. Note: ↓ indicates the reduction in wear rate compared with the smooth surface (T10).

Operating Condition	Group	Wear Scar Width/(μm)	Wear Scar Depth/(μm)	Cross-Sectional Area A/(μm2)	Wear Rate WR-brass /(mm3·N−1·m−1)	Change Relative to T10
Dry friction	T10	741.5	15.3	11345	0.95 × 10−3	—
	T9	659.7	12.7	8378.2	0.70 × 10−3	↓ 26.3%
Wet friction	T10	311.3	6.6	2054.6	0.17 × 10−3	—
	T9	260.4	4.2	1093.7	0.09 × 10−3	↓ 47.1%
Oil-lubricated friction	T10	305.1	4.9	1495	0.13 × 10−3	—
	T9	253.3	3.8	926.5	0.08 × 10−3	↓ 38.5%

, under dry friction conditions, the volume wear rate of the non-textured brass specimen (T10) is 0.95 × 10⁻³ mm³·N⁻¹·m⁻¹, with a wear scar width of 741.5 μm and a maximum depth of 15.3 μm, indicating that severe adhesive wear and abrasive wear have occurred on the brass surface. After introducing the optimal hexagonal micro-texture (T9), the wear rate decreases to 0.70 × 10⁻³ mm³·N⁻¹·m⁻¹, a reduction of 26.3%, and the wear scar width and depth are reduced to 659.7 μm and 12.7 μm, respectively. This is because the micro-texture effectively reduces the actual contact area, while the dimples trap some of the wear debris, suppressing repeated cutting by abrasives.

Under wet friction conditions, the wear rate of the T10 group already drops to 0.17 × 10⁻³ mm³·N⁻¹·m⁻¹, and that of the T9 group further decreases to 0.09 × 10⁻³ mm³·N⁻¹·m⁻¹, a reduction of as much as 47.1%. The wear scar width is reduced from 311.3 μm to 260.4 μm, and the maximum depth decreases from 6.6 μm to 4.2 μm. This remarkable improvement is attributed to the excellent fluid storage and channeling capability (guiding/dispersion capability) of the hexagonal micro-texture: the pure water medium forms a stable liquid film in the dimple gaps, while wear debris is rapidly expelled, effectively suppressing oxidative wear and surface exfoliation.

Under oil lubrication conditions, the wear rate of the T9 group is 0.08 × 10⁻³ mm³·N⁻¹·m⁻¹, which is 38.5% lower than that of the T10 group (0.13 × 10⁻³ mm³·N⁻¹·m⁻¹). The wear scar width decreases from 305.1 μm to 253.3 μm, and the depth decreases from 4.9 μm to 3.8 μm. The micro-texture acts as “micro-oil reservoirs” that continuously release lubricating oil, maintaining a stable mixed lubrication state and reducing direct contact between the brass and the steel ball.

Comparing Table 3 (steel ball wear) and Table 5 (brass wear) reveals that the wear-reducing effect of the micro-texture on the soft brass substrate is much better than that on the hard steel ball. For example, under wet friction conditions, the reduction in wear rate of brass (47.1%) is more than four times that of the steel ball (11.8%). This further confirms that the micro-texture preferentially protects the softer material surface by reducing contact stress, storing lubricants, and guiding away wear debris.

Table 6. Comparison of wear rates of brass specimen with optimal micro-texture T9 under different operating conditions. Note: ↓ indicates the reduction in wear rate compared with the dry friction condition.

Operating Condition	$\mathbf{Wear} \mathbf{Rate} {\mathit{W}}_{\mathit{R}}$	Change Relative to Dry Friction
Dry friction	0.70 × 10−3	—
Wet friction	0.09 × 10−3	↓ 87.1%
Oil-lubricated friction	0.08 × 10−3	↓ 88.6%

shows a cross-comparison of the brass specimen in the T9 group under different operating conditions. Compared with dry friction, the wear rates under wet friction and oil lubrication are reduced by 87.1% and 88.6%, respectively, indicating that the synergistic effect of the lubricating medium and the micro-texture can greatly improve the wear resistance of brass. The oil lubrication effect is slightly better than that of wet friction, but both are significantly superior to dry friction.

3.4. Analysis of Friction-Reducing and Anti-Wear Mechanisms of Micro-Textures

3.4.1. Contact Mechanics Analysis

The contact behavior of the friction pair conforms to Hertz contact theory. The introduction of micro-textures significantly reduces the real contact area on the brass surface, changes the contact stress from a continuous distribution to a discrete distribution, greatly reduces the maximum contact stress, effectively lowers the interfacial shear strength, and suppresses the occurrence of adhesive wear. The hexagonal and circular micro-textures adopt rounded corner designs, which further uniformly disperse the contact stress and avoid stress concentration. In contrast, the right-angle structure of the rectangular micro-texture tends to cause obvious stress concentration, leading to local plastic deformation and intensified wear. This is the core mechanical reason why the tribological performance of the hexagonal micro-texture is better than that of the rectangular one.

Based on the contact mechanics calculation results, the maximum contact stress on a smooth surface under the test load is approximately 708 MPa. After introducing a micro-texture with an areal density of 10%, the actual load-bearing area is reduced, and the maximum contact stress can be reduced to about 637 MPa, a decrease of about 10%. This significant reduction in contact stress effectively weakens the interfacial shear strength and reduces adhesion. Due to structural differences, the stress concentration factor at the sharp corners of the rectangular texture can reach 2–3 times, and the local contact stress can exceed 1400 MPa, further aggravating wear. In contrast, the hexagonal and circular textures avoid sharp-corner stress concentration, resulting in a more uniform contact pressure distribution and better wear resistance.

3.4.2. Lubrication Zone Analysis

According to the Stribeck curve, the lubrication state of a friction pair can be divided into three regimes: boundary lubrication, mixed lubrication, and hydrodynamic lubrication. Under dry friction conditions, the friction pair operates in the boundary lubrication regime, where micro-textures improve tribological performance mainly by reducing the contact area and trapping wear debris. Under wet friction conditions, the micro-textures guide pure water to generate a hydrodynamic effect, shifting the interface toward the hydrodynamic lubrication regime and significantly reducing the friction coefficient. Under oil lubrication conditions, the micro-textures act as “micro-oil reservoirs” that store lubricating oil, maintaining a stable mixed lubrication state and preventing the lubricating film from rupturing and transitioning to boundary lubrication.

Quantitative determination of the lubrication state using the film thickness ratio (λ) shows that under oil lubrication, the smooth surface friction pair has a film thickness ratio of only 0.39, which corresponds to typical boundary lubrication. After introducing the hexagonal micro-texture, the equivalent film thickness increases to 0.4–0.6 μm, and the film thickness ratio rises to 1.3–2.0, shifting the lubrication state into the mixed or even hydrodynamic lubrication regime, thereby significantly reducing the friction coefficient. Under wet friction conditions, the smooth surface has a film thickness ratio of only 0.02, placing it in boundary lubrication; the micro-texture can increase the film thickness ratio to 0.5–1.0, entering the mixed lubrication regime, resulting in a prominent friction-reducing effect. Under dry friction conditions, the film thickness ratio is close to 0, indicating direct contact friction, where micro-textures function primarily by reducing the contact area and trapping wear debris.

3.4.3. Geometric Structure Mechanism

The significant performance difference between the hexagonal micro-texture and the rectangular micro-texture is primarily attributed to differences in connectivity, debris evacuation capability, and stress distribution arising from their geometric structures.

In terms of connectivity, hexagonal dimples are uniformly distributed in a honeycomb pattern, with multi-directional inter-texture gaps. The lubricating medium and wear debris can flow and be guided in any direction. In contrast, rectangular dimples are arranged linearly with single and closed channels, restricting the flow paths of fluid and debris.

Regarding debris evacuation capability, the rounded corners and multi-directional interconnecting gaps of the hexagonal structure enable rapid expulsion of wear debris from the friction interface, preventing debris accumulation and subsequent three-body wear. The rectangular grooves with right-angled corners tend to trap debris, which cannot be expelled promptly, thereby further aggravating interfacial wear.

With respect to stress distribution, the hexagonal texture has no sharp corners and thus exhibits uniform contact stress distribution. The rectangular texture, however, has pronounced stress concentration at its sharp corners, which readily induces local plastic deformation, microcracks, and material delamination.

To quantify the structural differences, the number of connecting channel directions (Nc) and the corner sharpness index (Sc) are introduced for comparison. Hexagonal dimples arranged in a honeycomb pattern provide more connecting directions, leading to superior debris evacuation capability. Rectangular dimples have few effective connecting directions, resulting in dead zones for flow. Moreover, after rounding the corners, the hexagonal texture exhibits a lower stress concentration factor, whereas the rectangular texture has a stress concentration factor of 2.5–3.0 at its sharp corners, making it more prone to deformation and cracking under load. Based on the geometric parameter analysis, the hexagonal micro-texture outperforms the rectangular one in debris evacuation efficiency, stress dispersion capacity, and fluid guidance capability, which is fully consistent with the experimentally observed ranking of friction coefficients (hexagonal < circular < rectangular).

3.4.4. SEM Morphology of Wear Tracks and EDS Analysis

To gain deeper insight into the effects of micro-textures on the friction and wear behavior of brass surfaces under different conditions, the wear track regions of representative samples were examined using scanning electron microscopy (SEM), coupled with energy-dispersive spectroscopy (EDS) to analyze elemental composition. The SEM and EDS results provide a microscopic basis for the previously described variations in friction coefficient and wear rate, facilitating an understanding of the anti-friction and anti-wear performance of different micro-textures under dry, wet, and oil-lubricated conditions.

Figure 8 shows the SEM morphology of the wear tracks under dry friction. On the plain sample (T10), significant plastic deformation is observed, with deep and wide wear tracks accompanied by material spalling and accumulation of blocky debris, indicating that both adhesive and abrasive wear mechanisms are at play. EDS analysis reveals that the wear tracks are primarily composed of Cu and Zn, with a small amount of O detected, suggesting slight oxidation under high contact stress and frictional heat.

For the micro-textured surface (T9 hexagonal), the wear tracks are significantly mitigated: the wear depth is reduced, the surface appears smoother, and adhesive and tearing phenomena are weakened. Debris accumulation is observed inside the pits, indicating that the micro-texture can effectively trap wear debris generated during friction, reducing its repeated cutting action at the contact interface. EDS analysis shows that the C and O content on the micro-textured surface is relatively high before friction, but decreases noticeably after friction, indicating that some surface-adsorbed carbonaceous contaminants and oxides are removed along with the debris during dry friction. While the micro-texture does not show significant storage of debris, its geometric structure effectively reduces local adhesive wear and the involvement of abrasive particles.

Figure 9 shows the wear characteristics under wet friction conditions. The untextured surface is generally smooth, but shallow grooves and localized micro-spalling regions are present, indicating that wear is dominated by mild abrasive wear with some adhesion. EDS results show a significant increase in O content, suggesting that the aqueous medium contributes to oxidative wear.

On the micro-textured surfaces, the wear tracks are further optimized, with significantly improved surface flatness and only minor shallow scratches, and no obvious material spalling is observed. Around the dimples, there is no noticeable accumulation of wear debris, indicating that debris can be effectively removed during friction. EDS analysis shows a uniform distribution of Cu and Zn, with moderate O content, indicating that the micro-texture helps store the aqueous medium and maintain a lubricating film, thereby reducing direct contact and suppressing oxidative wear.

Figure 10 shows the SEM morphology of wear tracks under oil-lubricated conditions. On the smooth surface, wear is mainly manifested as shallow scratches, with minor plowing in some localized areas, indicating that the lubricating oil provides partial protection to the contact interface during friction, but localized abrasive wear still occurs. In contrast, wear on the textured surface is much milder. The central regions of the dimples remain largely intact, and only a small amount of debris accumulates at the edges, indicating that the dimple structure can guide debris distribution, reduce local contact pressure, and maintain surface flatness.

EDS analysis shows that after wear, the C content in the dimple regions of the micro-textured surface is higher than that in the worn areas of the smooth surface, while the O content is lower. This indicates that the dimples can maintain effective local lubrication and slow down oxidation. Furthermore, the C and O contents in the dimple regions change little before and after wear, suggesting that the micro-texture stabilizes the distribution of the lubricating medium, preventing oil loss and reducing the risk of local high-temperature oxidation.

Based on the SEM morphology and EDS results, the anti-friction and anti-wear mechanisms of the hexagonal micro-texture can be summarized as follows: (1) Lubrication regulation: The dimple structures act as “micro oil reservoirs,” storing and releasing lubricating oil to form a stable lubricating film during friction, reducing direct metal contact. (2) Debris management: Edges of the dimples capture and guide debris distribution, weakening three-body wear and maintaining contact surface flatness. (3) Oxidation suppression: The maintenance of localized lubricating films and reduced contact stress effectively inhibits oxidative wear. (4) Stress distribution optimization: Hexagonal dimples with rounded corners evenly disperse contact pressure, preventing localized stress concentration and plastic deformation.

These micro-mechanisms work synergistically, allowing the hexagonal micro-texture to significantly reduce friction coefficients and wear rates under oil-lubricated conditions, confirming its excellent anti-friction and anti-wear performance.

3.4.5. Comprehensive Analysis

Based on SEM and EDS observations, the wear mechanisms of brass differ significantly under various lubrication conditions. Under dry friction, wear is dominated by adhesive and abrasive mechanisms. For the T9 hexagonal micro-texture, the friction coefficient decreases from 0.51 to 0.43, and the wear rate of the GCr15 steel ball drops by 2.8% (Table 3); the wear rate of the brass specimen decreases by 26.3% (Table 5). The micro-texture effectively reduces local shear strength and the participation of abrasive particles by optimizing the contact state and capturing debris. Under wet friction, oxidative wear is enhanced, while the micro-texture forms a stable lubricating film through water storage and liquid film regulation, reducing the friction coefficient from 0.43 to 0.12 and the steel ball wear rate by 11.8%; the brass wear rate is reduced by 47.1%. The hydrodynamic effect of the liquid ensures uniform distribution of the friction medium at the interface, effectively alleviating local stress concentration and reducing oxidative wear. Under oil lubrication, the contact primarily experiences mild abrasive wear. The friction coefficient of T9 decreases from 0.29 to 0.24, and the steel ball wear rate drops by 8.3%; the brass wear rate decreases by 38.5%. The micro-texture acts as a “micro-oil reservoir”, storing lubricating oil to form a uniform film, while guiding small amounts of debris away, thereby reducing local stress and wear.

Based on friction coefficients, wear rates, and SEM/EDS analyses under different conditions, the anti-friction and anti-wear mechanisms of micro-textures can be systematically explained from three aspects: contact mechanics regulation, third-body behavior modulation, and lubrication control.

(1)

Contact State Regulation Mechanism

The introduction of micro-textures significantly alters the real contact state of the friction pair. On one hand, the surface dimple structures effectively reduce the actual contact area, changing the contact stress from a continuous to a discrete distribution, thereby lowering interfacial shear strength. On the other hand, hexagonal and circular textures with rounded transitions effectively avoid stress concentration, whereas rectangular textures with sharp corners tend to induce localized stress concentration, accelerating wear. Therefore, optimizing the contact state is a fundamental basis for micro-textures to reduce adhesive wear.

(2)

Third-Body Regulation Mechanism (Debris Storage Effect)

Under dry friction and certain wet friction conditions, wear debris acts as a “third body” and has a decisive impact on wear behavior. The dimples of micro-textures can effectively capture and store debris generated during friction, preventing its repeated participation in the cutting action at the interface, thereby reducing abrasive wear. Moreover, hexagonal textures, due to their multi-directional connected structure, are more favorable for guiding and removing debris, further weakening the three-body wear effect.

(3)

Lubrication Regulation Mechanism

Under wet friction conditions, micro-textures store water and generate local hydrodynamic effects during friction, ensuring uniform distribution of the liquid at the interface, forming a stable lubricating film, and reducing direct contact and oxidative wear. Under oil-lubricated conditions, micro-textures act as “micro oil reservoirs,” continuously storing and releasing lubricating oil. This induces hydrodynamic effects that promote the formation and regeneration of the lubricating film, enhancing its stability and reducing the transition from boundary lubrication to dry friction.

(4)

Oxidation and Interfacial Chemical Effects

EDS results indicate that O content significantly increases under wet friction, showing that oxidation participates in the wear process. Micro-textures suppress severe oxidative wear by reducing contact temperature and frictional intensity, while forming a relatively stable oxide layer that helps lower interfacial shear strength, further improving friction performance.

(5)

Synergistic Multi-Mechanism Effect

In summary, micro-textures operate through the synergistic action of multiple mechanisms under different conditions (

Table 7. Dominant anti-friction and anti-wear mechanisms under different operating conditions.

Operating Condition	Dominant Mechanism
Dry friction	Reduced contact + Debris storage
Wet friction	Stable liquid film + Debris storage + Mild oxidation
Oil-lubricated friction	Oil storage + Hydrodynamic lubrication

):

Among them, the hexagonal texture, owing to its excellent stress dispersion, debris storage, and lubrication regulation capabilities, exhibits the best anti-friction and anti-wear performance under all three operating conditions.

4. Conclusions

This paper takes H62 brass as the research object. To address the problems of high friction coefficient and severe wear under three typical working conditions—dry friction, wet friction, and oil lubrication—circular, rectangular, and hexagonal dimple micro-textures were fabricated on the brass surface using an ultraviolet nanosecond laser. The influence laws of micro-texture shape, size, and area density on the friction and wear properties of brass were systematically investigated through the control variable method. Furthermore, the wear characteristics under multiple working conditions and the friction-reducing and anti-wear mechanisms of the micro-textures were revealed using a digital microscope, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The main conclusions are as follows:

Corresponding to Objective 1 (Influence of texture shape): Surface micro-textures can significantly improve the multi-condition tribological performance of H62 brass, with the friction-reducing and anti-wear capacity following a clear hierarchy: hexagonal > circular > rectangular. Owing to their rounded transition structure and multi-directional interconnected geometric features, hexagonal micro-textures exhibit the best friction-reducing and anti-wear capability under all three working conditions—dry friction, wet friction, and oil lubrication.

Corresponding to Objective 2 (Influence of parameters): The geometric dimensions and area density of micro-textures have a significant coupling regulation effect on tribological performance. The hexagonal dimple texture (200 μm, 10% area density) is identified as the optimal parameter combination for multi-condition applications. Under this parameter configuration, the protective effect of the micro-texture on the soft brass substrate is far superior to that on the hard counterpart (GCr15 steel ball), confirming the core mechanism that micro-textures preferentially protect softer surfaces by reducing contact stress.

Corresponding to Objective 3 (Mechanism analysis): The friction-reducing and anti-wear effect of micro-textures results from multi-mechanism synergy: reducing contact stress and optimizing stress distribution based on Hertz contact theory; regulating the lubrication state from boundary lubrication to mixed/hydrodynamic lubrication according to the Stribeck curve; and achieving efficient debris evacuation and stress dispersion through geometric structure optimization. The dominant mechanisms vary under different working conditions: dry friction is dominated by “contact reduction + debris storage,” wet friction by “hydrodynamic lubrication + oxidation inhibition,” and oil lubrication by “hydrodynamic lubrication + oil storage and stabilization.”

This study confirms the optimization effect of UV laser-machined surface micro-textures on the multi-condition wear performance of H62 brass, systematically reveals the coupling mechanism between texture parameters and service environments, and provides a theoretical basis and data support for the anti-wear surface design of brass components under complex working conditions. Future work can further explore the design of groove-type and hybrid micro-textures, combined with finite element simulation and fluid dynamics numerical modeling, to deepen the understanding of laser processing parameter optimization and lubricant flow behavior, thereby promoting the refined design of micro-texture structures.