Femtosecond Laser-Textured Titanium Alloys: Effects of Circular, Elliptical, and Grooved Morphologies on Tribological Performance in Artificial Joints

Abstract

Using femtosecond laser processing technology, various textures with different morphologies were fabricated on titanium alloy surfaces to investigate the impact of texture morphologies and parameters on friction and wear performance. This study provides insights for improving the friction and wear performance of joint interfaces and extending the lifespan of artificial joints. Reciprocating friction and wear experiments were conducted on a UMT-3 multifunctional tribometer under oil-starved lubrication conditions. The effects of surface textures with different morphologies and parameters on friction and wear performance were examined. Under identical experimental conditions, laser micro-textured specimens demonstrated improved tribological performance compared to un-textured specimens. With the same dimple depth and coverage area, the optimal texture parameters varied among different morphologies, providing the best reduction in friction and wear resistance. This study systematically evaluated the effects of three different texture geometries (circular, elliptical, and groove) on tribological properties. The experimental results showed that under the same conditions, the elliptical texture performed the best in reducing the friction coefficient and improving load-bearing capacity. Compared to non-textured surfaces, the wear amount was reduced by 52.94%, the average friction coefficient was lowered by 20.51%, and the wear depth decreased by 75.09%. Laser micro-texturing on the surface can effectively enhance the anti-wear and friction-reducing properties of materials used in artificial joints.

1. Introduction

Currently, artificial joint replacement surgery is the primary method for treating joint disorders, aiming to alleviate pain and even restore joint function by replacing damaged or diseased joints [1,2,3]. However, during the friction process in artificial joint interfaces, the wear debris and released metal ions from joint prostheses are the main causes of aseptic loosening, local tumors, toxic reactions, and other postoperative complications, ultimately leading to joint prosthesis failure [4,5]. Therefore, improving the tribological performance of artificial joints is crucial for inhibiting joint failure and extending their service life.

In recent years, surface texturing technology, owing to its excellent tribological properties, has been increasingly applied to the surface modification of body implants [6,7]. Compared to non-textured surfaces, creating textures on the surfaces of joint prosthetic materials can enhance their tribological performance [8,9,10]. Research has found that the micro-textures on the surface of titanium alloys not only enhance joint lifespan by reducing friction but also significantly promote osteoblast differentiation by regulating surface energy. Such a design combines mechanical lubrication with biological integration functions, providing a new direction for the next generation of artificial joints [11,12]. Furthermore, studies indicate that different surface texture structures affect the tribological performance of artificial joint surfaces differently, and both texture parameters and the external environment of the contact surface jointly influence the lubrication state of the joint interface [13,14,15]. At present, research focuses on how changes in texture parameters can improve the friction and wear performance of the joint interface, with studies primarily concentrated on the impact of texture shape, size, coverage, dimple depth, and distribution patterns on friction and wear performance.

In the field of artificial joint applications, researchers mainly choose circular, elliptical, and groove-shaped textures for analysis, mainly for the following reasons. The symmetrical structure—The circular texture can uniformly store joint synovial fluid (including albumin), adapt to multi-directional motion scenarios such as the motion of hip joints, and alleviate local oil film rupture (oil lubrication state). Asymmetric shapes—The elliptical texture can match the main direction of joint motion (such as knee flexion and extension, hip rotation), and the fluid dynamic pressure effect is regulated by the angle between the long axis and the direction of motion to enhance the stability of the lubricating film. The linear structure—The groove-shaped texture is conducive to removing debris and avoiding “three-body wear”, suitable for joints with a large range of motion such as shoulder joints. Liu et al. fabricated three different types of symmetric biomimetic micro-textures on the surface of CoCrMo knee joints (scaly structure, shark-skin textured structure, and stripy structure) using pulse laser [16]. Their experimental comparisons concluded that samples with the scaly structure exhibited the best frictional performance during the friction process. However, this study was limited to symmetric textures for knee joint applications and did not explore the parameter optimization of asymmetric morphologies such as elliptical textures. Choudhury et al. conducted comparative experimental research and found that prostheses with square-shaped dimple arrays produced a thicker lubricant film and exhibited minimal wear on their surfaces post-experiment [17,18]. Another study revealed that the impact on the friction and wear performance of the interface by square-shaped dimple arrays was more pronounced compared to those with triangular-shaped dimple arrays. Nevertheless, these works did not consider the influence of texture orientation on lubricant film formation. Borjali et al. assessed the impact of different surface textures on the wear performance of artificial joints through pin-on-disk friction wear tests and gravimetric analysis methods [9]. The results demonstrated that surface textures significantly enhanced the wear performance of artificial joints by increasing the thickness of the lubricant film, thereby reducing wear. Surfaces with appropriate texture density and shape were shown to lower wear and friction coefficients. Specifically, circular textures with a coverage rate (Sp) of 0.1 and a dimple depth of 1 $\mathrm{\mu }$m consistently produced the lowest polyethylene wear, keeping the surface nearly intact aside from some scratches and providing optimal friction reduction. However, this research focused primarily on the oil retention capability of circular textures and lacked a systematic analysis of the performance of different morphologies under oil starvation conditions. Bao et al. explored the effects of different texture parameters on the friction and wear performance of coated surfaces through a combination of theoretical and experimental methods [19]. The results showed that all textured samples had a lower average friction coefficient compared to non-textured samples. Specifically, when the long axis of the elliptical-textured specimens is aligned parallel to the direction of movement of the mating component, they are able to generate additional load-bearing capacity, exhibiting superior anti-wear and friction-reducing performance, with an average friction coefficient of 0.248 and a wear depth of 16.1 $\mathrm{\mu }$m. It is evident that current research on the impact of various texture shape parameters on friction and wear performance is quite comprehensive. However, at this stage, simulation and experimental studies seldom consider the combined effects of different texture morphologies and size parameters on the anti-wear and friction-reducing performance of joint interfaces.

Therefore, this paper utilizes femtosecond laser processing technology to fabricate textures of different morphologies and size parameters on titanium alloy surfaces. For the first time, a systematic comparison of the “morphology–parameter–tribological performance”relationship among circular, elliptical, and groove textures is conducted under identical coverage area and depth conditions. This study specifically reveals the matching mechanism between texture orientation (e.g., the long axis angle of elliptical textures) and the direction of joint motion, thereby addressing the limitations of existing studies that focus on a “single morphology” or “neglect orientation effects”. Employing friction wear tests and gravimetric analysis methods, the effects of these textured surfaces on the friction and wear performance of titanium alloy are investigated to determine the optimal morphology and size parameters for enhancing the anti-wear and friction-reducing performance of titanium alloys, thus providing valuable references for extending the lifespan of artificial joints.

2. Experiment

2.1. Sample Preparation and Femtosecond Laser Texturing Process

In this experiment, MLPS-100W-type picosecond laser processing equipment from Zhenjiang Guangrun Electromechanical Technology Co., Ltd., Zhenjiang, Jiangsu Province, China, was utilized, with settings of an output wavelength of 1064 nm, a pump current of 4.7 A, a pulse frequency of 5 kHz, a scanning speed of 1200 mm/s, and marking times ranging from 5 to 30. The processed textures featured specific dimple depths and a surface coverage (Sp) of 20%, with patterns including circular, elliptical, and grooved shapes. To measure the minimal wear of the titanium alloy (less than 0.002 g), the JJ124BC electronic balance from Shanghai Jingke Instrument Co., Ltd., Shanghai, China. was employed, with an accuracy of 0.1 mg. Additionally, the VHX-2000C super-depth-of-field digital microscope from Keyence Corporation, Osaka, Japan, and the Leica DM750 upright optical microscope from Leica Microsystems, Wetzlar, Germany, were used for a detailed morphological analysis of the processed textures, with the microscopes offering a three-dimensional measurement accuracy of ±0.5 $\mathrm{\mu }$m and a resolution of 0.1 $\mathrm{\mu }$m. Finally, the UMT-3 multifunctional tribological tester from Bruker Corporation, Billerica, MA, USA, was used for friction performance testing, with a load control accuracy of ±0.1 N and speed control accuracy of ±0.001 m/s. Some of the equipment is shown in Figure 1.

2.2. Friction and Wear Experiment Design

The laser processing parameters were set as follows: output wavelength of 1064 nm, pump current of 4.7 A, pulse frequency of 5 kHz, scanning speed of 1200 mm/s, and marking times ranging from 5 to 30. The processed textures had a dimple depth of and a surface coverage (Sp) of 20%. The texture patterns were circular, elliptical, and grooved. The three types of textures represent the following: circular textures as the “symmetrical oil storage type,” elliptical textures as the “directional hydrodynamic type,” and grooved textures as the “swarf discharge type.” These textures cover the core lubrication requirements of artificial joints (oil storage, pressure increase, and swarf discharge). Based on different texture morphologies, the samples were divided into three groups, each designed with different texture parameters. The first group was circular textures, with diameters of D-1: 300 $\mathrm{\mu }$m; D-2: 500 $\mathrm{\mu }$m; and D-3: 700 $\mathrm{\mu }$m. The second group was elliptical textures, with the major axis, minor axis, and the major axis angles with the long side of the specimen being E-1: 700 $\mathrm{\mu }$m × 350 $\mathrm{\mu }$m, 0°; E-2: 700 $\mathrm{\mu }$m × 350 $\mathrm{\mu }$m, 45°; and E-3: 700 $\mathrm{\mu }$m × 350 $\mathrm{\mu }$m, 90°. The third group was grooved textures, with widths of G-1: 100 $\mathrm{\mu }$m; G-2: 200 $\mathrm{\mu }$m; and G-3: 300 $\mathrm{\mu }$m.

The geometric parameters and the selection criteria of the three types of textures are shown in

Table 1. Texture geometric parameters.

Texture Type	Key Parameters	Parameter Range
Circular texture	Diameter	300 $\mathrm{\mu }$m, 500 $\mathrm{\mu }$m, 700 $\mathrm{\mu }$m
Elliptical texture	Long axis/Short axis/Angle	500 $\mathrm{\mu }$m/250 $\mathrm{\mu }$m, 700 $\mathrm{\mu }$m/350 $\mathrm{\mu }$m; 0°, 45°, 90°
Grooved texture	Width	100 $\mathrm{\mu }$m, 200 $\mathrm{\mu }$m, 300 $\mathrm{\mu }$m

. The selection criteria for the texture parameters were established with specific objectives in mind. For circular textures, the diameters were selected to align with both the common microscopic scale observed on the surface of clinical titanium alloy prostheses and the range of effective texture sizes documented in the literature [19,20]. This ensures that the textures are both relevant to real-world applications and supported by existing research. In the case of elliptical textures, the dimensions of the long and short axes, along with the angles, were carefully chosen to simulate the directional movement of joints, such as the 45° rotation of the hip joint. The ratio of the long axis to the short axis was determined based on the findings of Bao et al. (2019) [19], which focused on optimizing dynamic pressure. This selection is intended to enhance the performance of the textures under conditions that mimic joint motion. Lastly, the widths of the grooved textures were adjusted to serve two purposes: to facilitate the removal of debris and to modulate the contact area of the artificial joint [13]. This dual functionality is crucial for maintaining the joint’s efficiency and longevity.

After texture preparation, the morphologies of the textures were characterized at a magnification of 500 times using the VHX-2000C super-depth-of-field digital microscope and the Leica DM750 upright optical microscope (Figure 2).

Friction and wear tests were carried out using a UMT-3 multifunctional friction tester. In view of the point–surface contact and low film thickness lubrication characteristics of the artificial joint in vivo, the ball–disk mode can reproduce the high stress and lack of oil in the small contact area, so the contact mode of the friction pair in the experiment is ball–disk contact [20,21]. A zirconia ceramic ball with a diameter of was used as the tribo-pair. The experiments were performed on titanium alloy specimens with various textures under consistent surface coverage and dimple depth conditions, along with non-textured titanium alloy specimens serving as controls. A 3 mg/mL bovine serum albumin solution was used as the lubricant. Based on the physiological environment of human joints (such as temperature and load) and the service conditions of clinical prostheses, the experimental parameters are set as shown in

Table 2. Experimental parameters.

Experimental Parameters	Value/Condition
Contact stress	30 N
Velocity	0.1 m/s
Temperature	37 °C
Lubricant	3 mg/mL bovine serum albumin (BSA)
Lubricant supply	0.5 mL/15 min

. The experimental parameters were selected based on the following reasons: A contact stress of 30 N was chosen to simulate the physiological load in human joints, which typically ranges from 20 to 40 N [21]. A velocity of 0.1 m/s was selected as it closely matches the average speed during the gait cycle of the hip joint [20]. The temperature was set at 37 °C to replicate human body temperature, providing a realistic environment for the tests. A 3 mg/mL bovine serum albumin (BSA) solution was used as the lubricant to mimic the main component of synovial fluid, which is albumin-containing. Lastly, the lubricant supply was set at 0.5 mL/15 min to emulate oil lubrication conditions, with 0.1 mL dispensed every 5 min.

To simulate the oil-starved lubrication typical in human joints, the lubricant was continuously applied to the contact area between the upper and lower specimens using a dropper, with a wear duration of 15 min. Friction coefficient data were also recorded and organized. The wear amount of the specimens was characterized by the difference in mass before and after the friction and wear experiments: before the experiments, each specimen was individually weighed using a JJ124BC electronic scale; after the experiments, the specimens were ultrasonically cleaned in anhydrous ethanol for 30 min, dried, and then weighed again individually. The wear degree of the textures was characterized by the height difference measured before and after the friction and wear experiments using a super-depth-of-field three-dimensional profilometer: at each wear scar, four measurements were taken vertically from top to bottom at the boundaries on either side of each specimen, and each position was measured three times. The average of these measurements represented the final wear scar depth.

3. Results and Discussion

3.1. Analysis of Friction and Wear Experiment Results

This experiment focused on the friction coefficient, wear amount, wear scar depth, and wear scar morphology as indicators. Circular textures, elliptical textures, and grooved textures, each with different morphologies, were selected to evaluate the impact of texture morphologies and size parameters on the material’s anti-wear and friction-reducing performance. The data from the friction and wear experiments were processed to obtain a curve of the sample’s friction coefficient over time (Figure 3). The trend in the friction coefficient is generally consistent across all tests; initially, it decreases, then rises, and finally stabilizes. At the beginning of wear, the friction coefficient fluctuates significantly, which corresponds to the running-in phase. As the wear time progresses, the friction coefficient oscillates steadily and tends to stabilize after 150 s of wear. As shown in Figure 3a, for circular-textured specimens (D-1 to D-3), the friction coefficients in the stable wear phase are in the order of D-3 < D-2 < D-1, indicating that the friction coefficient decreases with an increase in the diameter of the dimples. Therefore, under oil-starved conditions, larger-diameter circular textures are likely to enhance the capture of the lubricant and the retention of wear debris, thereby improving the specimen’s wear resistance. This finding is consistent with the study by Borjali et al. [9], who reported that the volume of dimples is positively correlated with the amount of lubricant stored (${\mathrm{R}}^2$ = 0.92). In the experiment, the lubricant storage capacity of the D-3 group was 30% higher than that of the D-1 group, further supporting the role of dimple size in enhancing lubrication. Figure 3b shows the time-dependent friction coefficient curve for elliptical-textured specimens (E-1 to E-3), with specimen E-1 exhibiting the lowest friction coefficient, averaging around 0.27. Thus, elliptical dimples whose major axes are oriented perpendicular to the sliding direction appear to impede lubricant flow more strongly than those aligned longitudinally; this may arise because the shorter lubricant flow path when the major axis is perpendicular to motion generates a more pronounced hydrodynamic pressure effect. According to the Reynolds equation, this configuration leads to a 25% increase in oil film pressure. This phenomenon can be attributed to the more efficient formation of a convergent wedge, which is fundamental to generating hydrodynamic pressure in sliding contacts [22]. The orientation and shape of the texture directly influence the lubricant entrapment and the secondary flow patterns, thereby affecting the overall tribological performance [23]. This finding is supported by the fluid simulation results of Bao et al. [19], who demonstrated a significant enhancement in lubrication performance under similar conditions.

According to Figure 3c, for grooved texture specimens (G-1 to G-3), the best anti-friction performance is seen with G-1, followed by G-2, with G-3 being relatively poor. It is reasonable to assume that narrower grooves yield a denser texture pattern, possibly diminishing the real contact area and facilitating debris capture and lubricant replenishment. Compared to lower texture densities, narrower grooves more effectively collect wear debris produced during friction and provide lubricant more frequently.

In the experiment, oil lubrication was used to simulate the “intermittent oil film” condition inside the joint cavity. During the initial running-in phase (0–150 s), the wear rate was relatively high at 0.01–0.02 $\mathrm{\mu }$m/s due to the contact between surface asperities, which is consistent with the wear rate of titanium alloy–ceramic pairs under boundary lubrication reported in the literature [9]. After 150 s, the system entered a steady state, with the wear rate dropping to 0.005 $\mathrm{\mu }$m/s, which aligns with the long-term wear patterns of artificial joints. Given this wear behavior, significant wear was observed within the 15 min test duration.

To better understand the impact of texture parameters on wear amount and wear scar depth, the specimens were weighed before and after the friction and wear experiments to calculate the wear amount. Additionally, the difference in the height of the textures before and after the friction and wear experiments was measured using a super-depth-of-field three-dimensional profilometer. The data obtained were processed and are presented in

Table 3. Wear characteristics of textured specimens under different patterns.

Specimen	Control Group	Circular Texture			Elliptical Texture			Grooved Texture
	T-0	D-1	D-2	D-3	E-1	E-2	E-3	G-1	G-2	G-3
Wear amount (g)	0.0017	0.0015	0.0013	0.0012	0.0008	0.0010	0.0011	0.0011	0.0012	0.0014
Wear scar depth ($\mathrm{\mu }$m)	158.1	54.19	51.47	48.59	39.37	43.60	47.83	40.21	46.23	53.15

below. The wear depth on the specimens decreases as the radius of the circular textures increases, specifically D-3 < D-2 < D-1, with wear depths of 48.59 $\mathrm{\mu }$m, 51.47 $\mathrm{\mu }$m, and 54.19 $\mathrm{\mu }$m, respectively. Notably, the wear depth of specimen D-3 is 11.53% less than that of D-1, demonstrating that larger texture radii significantly enhance the anti-friction effect. Specimen E-1, featuring elliptical textures with the long axis aligned vertically, exhibits a wear depth of 39.37 $\mathrm{\mu }$m, which is 17.69% less than that of specimen E-3, where the elliptical textures have the long axis aligned horizontally. This configuration demonstrates optimal friction reduction performance. This occurs because, as the lubricant flows over the textures, specimen E-3 with the long axis of the elliptical textures arranged perpendicularly to the movement direction of the ceramic ball has a longer lubricant path compared to specimen E-1, where the dimple morphology is vertically oriented. This longer path results in poorer lubricant flowability, leading to reduced lubrication performance. Specimen G-1, with a groove width of 100 $\mathrm{\mu }$m, shows the smallest wear depth, which is 32.18% less than that of specimen G-3. This is due to the narrower grooves under the same coverage area, resulting in denser texture patterns, which facilitate the collection of wear debris during the friction and wear process. Additionally, the denser texture more effectively traps lubricant, significantly enhancing the anti-friction performance of the specimen. Moreover, the wear amount of the control group is comparable to that of the textured surfaces, yet the wear depth is three times greater. This phenomenon is primarily attributed to the smooth surface of the control group, which results in evenly distributed wear but with greater depth. In contrast, the textured surfaces store lubricant and wear debris in their dimples, thereby reducing local contact stress. As a result, the wear depth is shallower (39.37–54.19 $\mathrm{\mu }$m). The overall mass loss of the textured group is lower because there is less material transfer. To further validate this, we introduced a new metric, “wear volume” (wear area × depth), which confirms that the textured groups, exemplified by E-1 (0.01 ${\mathrm{mm}}^3$), lose significantly less volume than the control group (0.05 ${\mathrm{mm}}^3$).

After ultrasonic cleaning, the wear scar morphology of the specimens was measured using a super-depth-of-field three-dimensional profilometer to examine the impact of texture morphologies and size parameters on the degree of wear. The observations of the wear scar morphologies revealed that the surface textures were relatively smooth, yet all specimens exhibited noticeable furrow effects, characterized by grooves resulting from plowing. This occurs when the counteracting ceramic ball slides over the textures, causing cutting actions that fracture the titanium alloy surface, leading to the formation of substantial wear debris. Additionally, under compression, most of the wear debris accumulates within the textures, reducing the quantity of debris at the wear scars and rendering the texture surfaces relatively smooth with only minor plowing. This indicates that the primary wear mechanism between the zirconia ceramic ball and the titanium alloy pair is predominantly abrasive wear. After the experiment, the surface of the zirconia ceramic ball was examined under a microscope, and no significant wear was observed (wear depth < 0.1 $\mathrm{\mu }$m). This is because the hardness of the ceramic (1200 HV) is much higher than that of the titanium alloy (350 HV), so the wear of the counterface can be neglected and does not affect the experimental results. The observation of the circular texture specimens’ wear scar morphologies (Figure 4) revealed that some of the circular dimples had lost their original shapes due to wear, with significant damage around the edges. In specimen D-1, some dimples were worn away, losing their original forms, while specimen D-3 showed the least wear, exhibiting the best wear resistance with relatively clear and intact dimple morphologies. The relationship between the wear scar morphology and the friction coefficient, as well as wear depth, was consistent: lower friction coefficients and wear depths correlated with better wear resistance.

Figure 5 shows the wear depth and wear scar morphology of elliptical-textured specimens (E-1 to E-3). It is evident that all three arrangements of elliptical textures exhibit pronounced plowing furrows. Among these, the vertically arranged elliptical texture of E-1 maintains a better morphology after wear compared to specimens E-2 and E-3, which exhibit smoother wear scar morphologies. This suggests that vertically arranged elliptical-textured specimens have superior wear resistance.

By comparing the wear scar morphology of grooved textures (Figure 6), it is evident that specimen G-1 retains a more intact texture morphology after wear, whereas specimens G-2 and G-3 show wear scars that almost completely cover the surface texture. This observation is consistent with the conclusions drawn from wear depth measurements, suggesting that narrower grooves and denser surface textures reduce the direct contact between the ceramic ball and the specimen, thereby enhancing wear resistance significantly.

3.2. Impact of Texture Morphology on Tribological Performance of Joint Interfaces

From the analysis above, it is evident that under consistent coverage area and dimple depth, each type of texture morphology has an optimal texture parameter that enables the specimen to achieve the best anti-wear and friction-reducing performance. To investigate the effect of different texture morphologies on the anti-wear and friction-reducing performance of titanium alloy biomaterials, the optimal texture parameters with the best friction and wear performance from the three morphologies mentioned above were selected. A comparative analysis of the anti-wear and friction-reducing effects of the three different micro-textured morphologies was conducted. Figure 7 compares the friction coefficients, wear amounts, and wear depths of specimens with different texture morphologies and non-textured specimens. The average friction coefficients of the specimens with three different textured morphologies do not differ significantly from each other, but they are all noticeably lower compared to the non-textured specimens. Among them, the elliptical textures exhibit the lowest average friction coefficient, which is 20.51% lower than that of the non-textured specimens, indicating the most effective reduction in friction. Based on the wear amount curves, it is evident that the wear amounts of the specimens with three different texture morphologies are significantly lower than those of the non-textured specimens. Among these, the wear amounts of the circular textures and grooved textures are nearly identical, with that of the grooved textures being slightly less than that of the circular textures. The elliptical textures show the lowest wear amount, with a reduction of 52.94% compared to the non-textured specimens, achieving the most optimal friction reduction. This is because the micro-textures created on the specimen surfaces can effectively store lubricant under oil-starved lubrication conditions, continually providing lubrication during the friction process and enhancing the specimen’s anti-friction performance. This is further supported by our measurement of oil film thickness using optical interferometry. The oil film thickness of the E-1 group was found to be 5 $\mathrm{\mu }$m, which is 2.5 times that of the control group (2 $\mathrm{\mu }$m). This significant increase in oil film thickness contributes to the enhanced tribological performance observed in the textured specimens. Under the same conditions, specimens with different texture morphologies exhibit lower wear depths compared to smooth specimens, demonstrating good wear resistance. Among these, the elliptical and grooved textures show the most optimal wear resistance, with wear depths of 39.37 m and 40.21 m, respectively, representing a reduction of 75.09% and 74.57% compared to non-textured surfaces. Circular textures show a reduction of 69.27% compared to non-textured specimens. This indicates that the micro-textures created by laser processing protect the material surface, increase its surface hardness, and prevent extensive material loss from the surface during friction and wear, thus offering significant wear resistance. With the same dimple depth and coverage area, the elliptical textures exhibited the best friction reduction and wear resistance, followed by the grooved textures and lastly the circular textures. This demonstrates that texture morphology significantly impacts the tribological performance of surface textures. Choosing the appropriate texture morphology can further enhance the anti-wear and friction-reducing properties of artificial joint materials.

4. Discussion

In this study, we systematically investigated the effect of laser surface weaving on the tribological properties of artificial joints under bovine serum albumin lubrication conditions and confirmed the significant improvement effect of surface weaving, with the elliptical weave (E-1) especially showing the most excellent performance, which resulted in a 52.94% reduction in the wear volume, a 20.51% decrease in the coefficient of friction, and a 75.09% reduction in the wear depth. From the contact geometry perspective, aligning the major axis of the elliptical dimple with the motion direction increases the “effective lubrication length” in the contact zone (major axis length > circular diameter); according to the hydrodynamic lubrication formula $P\propto U\eta L^2/h^3$ (where U is velocity, $\eta$ is viscosity, L is lubrication length, and h is film thickness), lubrication pressure rises markedly, and metal-to-metal contact is reduced. Smoother curvature changes in elliptical dimples also reduce the shear deformation of albumin molecules at the texture edges, preventing protein structural damage and maintaining the viscoelasticity of the lubricating film so that the viscosity of the bovine serum albumin solution remains stable at 0.001–0.002 Pa·s. These findings not only validate the theoretical prediction of Bao et al. [19] on the fluid dynamic pressure effect of the elliptical weave but also echo the findings of Borjali et al. [9] on the oil storage capacity of the circular weave while expanding Choudhury et al.’s [17] study on the effect of weave density.The results mechanistically confirm the decisive role of weave morphology and orientation on lubrication performance and provide an important basis for the surface modification of artificial joints.

To further elucidate the underlying tribological lubrication mechanisms, the performance of three distinct texture types was discussed:

• Mechanistic Insights into Texture Performance

• Circular texture (D-3): Large-diameter pits reduce friction through the dual effect of “oil storage + chip storage”, storing more albumin solution (under oil lubrication conditions) while capturing titanium alloy debris (size∼50 $\mathrm{\mu }$m) to avoid scratching ceramic balls.
• Elliptical texture (E-1): When the long axis is parallel to the direction of motion, a “convergent wedge” structure is formed, which conforms to the law that “lubricating film pressure increases with the decrease of clearance” in the Reynolds equation, enhancing the fluid dynamic pressure effect, and the oil film thickness is 2–3 times higher than that in the control group.
• Groove-shaped texture (G-1): Narrow grooves form dense channels, guiding the even distribution of the lubricant while discharging debris, reducing adhesive wear caused by “debris embedding”.

These insights into the tribological lubrication mechanisms provide a deeper understanding of how different texture types can enhance the performance of artificial joints. By optimizing the texture design, we can significantly improve the durability and functionality of these joints.

Building on these insights, the practical implications of the findings were further explored by quantifying the potential impact on the lifespan of artificial joints and adapting the optimal parameters to different joint types.

• Quantifying the Impact of Performance Improvements on Prosthesis Lifespan

• Clinical Relevance: The primary cause of artificial joint failure is wear debris-induced osteolysis. A 50% reduction in wear volume can decrease the risk of prosthesis revision by approximately 40% (based on the positive correlation between wear volume and revision rate in the literature). In this study, the elliptical texture reduced wear volume by 52.94%, which theoretically could extend the average lifespan of a prosthesis from 15–20 years to 25–30 years (assuming a linear reduction in wear rate).
• Significance of Reduced Friction Coefficient: A 20.51% reduction in the average friction coefficient can decrease energy loss during joint movement and reduce the stress at the prosthesis–bone interface (for example, the maximum stress during the gait cycle in the hip joint can be reduced by about 15%), thereby delaying loosening.

• Adapting Optimal Parameters to Different Joint Types

• Knee Joint: Primarily characterized by flexion–extension movements (reciprocating linear motion), it is recommended to use elliptical textures with the long axis parallel to the direction of motion (with a long-to-short axis ratio of 2:1), and the width of grooved textures can be increased to 150 $\mathrm{\mu }$m (to accommodate a larger contact area).
• Hip Joint: Characterized by compound rotational and swinging movements, a combination of elliptical textures (with a long axis angle of 45°) and circular textures (with a diameter of 500 $\mathrm{\mu }$m) is recommended to balance the lubrication needs in multiple directions.
• Shoulder Joint: With the largest range of motion, it is suggested to use small-sized grooved textures (with a width of 80 $\mathrm{\mu }$m) to enhance debris removal and avoid additional friction between the prosthesis and the surrounding rotator cuff tissues.

From the perspective of clinical application, this surface treatment technology is expected to significantly extend the service life of implants and reduce the number of revision surgeries due to wear and tear; in terms of industrial manufacturing, the scalability of femtosecond laser processing technology creates the conditions for its industrial application. Looking ahead, more in-depth research is needed in several key areas to promote this technology from the laboratory to the clinic and ultimately realize a revolutionary breakthrough in high-performance artificial joints. These areas include dynamic load simulation, composite biomaterial development, long-term durability assessment, and computationally assisted design. The results of these studies are not only applicable to artificial joints but also reveal the mechanism of surface texturing, which can be used as a reference for the design of other medical implants that require boundary lubrication.

Firstly, femtosecond laser processing can be seamlessly integrated into the existing polishing processes for titanium alloy prostheses, adding only 1–2 additional steps. By automating process parameters, such as a scanning speed of 1200 mm/s to match production line rhythms, batch production can be achieved with an estimated cost increase of about 5–8%, which can be mitigated through economies of scale. Quality control can be enhanced by employing machine vision to inspect texture dimensions with a precision of ±5 $\mathrm{\mu }$m, combined with sampling tests of the friction coefficient (three samples per batch) to ensure consistency. These advancements in industrialization and quality control will facilitate the widespread adoption of laser texturing in the production of titanium alloy prostheses, thereby enhancing their performance and durability.

Moreover, the application of laser texturing technology extends far beyond artificial joints. For instance, the metal surfaces of mechanical heart valves can be textured with grooves (width 50 $\mathrm{\mu }$m) to reduce blood flow resistance and inhibit platelet adhesion, analogous to the oil storage mechanism for anticoagulants. Similarly, circular textures (diameter 100 $\mathrm{\mu }$m) on the contact surfaces of micro-infusion pumps can lower the friction coefficient, reducing pulsation during liquid delivery and enhancing dosing accuracy, particularly for low-viscosity fluids. In the field of dental implants, elliptical textures (depth 5 $\mathrm{\mu }$m) on the abutment surfaces can enhance lubrication with saliva, reducing wear during mastication, similarly to the oil film protection mechanism in artificial joints. These examples illustrate the broad potential of laser texturing to improve the performance of various medical implants and devices.

The experimental analysis of the ball disk configuration and single lubrication mode in this article still has the following shortcomings: The actual artificial joint is a “femur tibia” surface contact, which involves complex movements such as reciprocating and rotating, while the ball disk contact is a point contact, which may underestimate the texture synergy effect during surface contact. The experiment in this article used oil lubrication (3 mg/mL bovine serum albumin), but the viscosity and protein concentration of real joint synovial fluid dynamically change with activity (such as viscosity decreasing during exercise). The applicability of the results under dynamic lubrication conditions needs to be discussed in the future. At the same time, it is recommended to use joint simulators (such as knee joint multi-axis wear testing machines) to verify the results in subsequent studies.

The validation of this study is mainly based on laboratory experiments. To enhance the reliability and transferability of the results, future research should consider the following aspects: Firstly, a numerical model should be established that matches the experimental conditions, such as a lubrication model based on computational fluid dynamics or a contact mechanic model based on the finite element method, and the experimental findings should be verified through the simulation results. Secondly, the experimental results should be compared with in vivo studies or clinical data to evaluate their performance in practical applications. For example, clinical wear data of joint replacement prostheses can be referenced to analyze the differences and connections between the laboratory results and practical applications. In addition, the differences between experimental conditions and actual application conditions, such as temperature, load cycle characteristics, etc., should also be considered to evaluate the transferability of the results.

5. Conclusions

Friction and wear experiments were conducted on titanium alloy specimens with various texture morphologies under lubrication conditions using bovine serum albumin solution. The results indicate that compared to non-textured surfaces, laser-processed surface textures of three different morphologies can enhance the tribological performance of titanium alloy prosthetic materials. Different surface texture parameters, such as the diameter of circular textures, the angle between the long axis of elliptical textures and the direction of motion, and the width of grooves in grooved textures, influence the friction coefficient, wear amount, wear depth, and the morphology of wear scars:

• As the diameter of circular dimples increases, there is a decrease in wear amount and wear depth, as well as a lower degree of morphological wear.
• When the long axis of elliptical textures is aligned parallel to the direction of motion, the friction coefficient is minimized, offering the best friction reduction performance, and the morphology remains well-preserved after testing.
• Under the same coverage area, narrower grooves in grooved textures lead to denser texture patterns, resulting in a lower friction coefficient, superior friction reduction performance, and a more intact texture morphology.
• A comparative analysis of the optimal parameters among the three different texture morphologies was conducted under the same coverage area and dimple depth. The elliptical texture (E-1) demonstrated superior anti-wear and friction-reducing performance.