Improving the Bio-Tribological Properties of Ti6Al4V Alloy via Combined Treatment of Femtosecond Laser Nitriding and Texturing

Abstract

This paper presents a compound laser surface modification strategy to enhance the tribological performance of biomedical titanium alloys involving femtosecond laser nitriding and femtosecond laser texturing. First, high-repetition-rate femtosecond pulses (MHz) were used to melt the surface under a nitrogen atmosphere, forming a wear-resistant TiN coating. Subsequently, the TiN layer was ablated in air with low-repetition-rate femtosecond pulses (kHz) to create squared textures. The effects of the combined nitriding and texturing treatment on bio-tribological performance was investigated. Results show that compared with the untreated samples, the single femtosecond laser nitriding process increased the surface hardness from 336 HV to 1455 HV and significantly enhanced the wear resistance of titanium, with the wear loss decreasing from 9.07 mg to 3.41 mg. However, the friction coefficient increased from 0.388 to 0.655, which was attributed to the increased hardness, roughness within the wear scars, and the formation of hard debris. After combined treatment, the friction coefficient decreased to 0.408 under the optimal texture density of 65%. The mechanisms for the improvement in friction behavior are the reduction in contact area and the trapping of hard debris.

1. Introduction

Titanium alloys are widely utilized in biomedical applications, particularly in orthopedic implants such as scaffolds, joint replacements, and dental prosthetics due to their excellent mechanical properties and biocompatibility, closely mimicking natural bone [1]. However, their relatively low surface hardness and poor wear resistance limit their broader use in such applications [2]. To improve the wear resistance of titanium alloys, various surface modification techniques based on lasers have been developed including laser nitriding [3], laser alloying [4], laser shock processing [5,6], and laser texturing [7]. Among these, laser nitriding and texturing have been widely studied in recent years.

Laser nitriding technologies have demonstrated significant efficacy in enhancing wear resistance through the formation of hard ceramic coatings on titanium surfaces. Compared to other metals, such as magnesium alloys and steel, titanium alloys exhibit relatively high chemical reactivity. The nitriding reaction can be easily conducted by exposing titanium alloys to nitrogen-rich gases with temperatures slightly below or above the melting point. Zong et al. [8] employed a fiber laser to nitride a TC11 titanium alloy, achieving a dense and uniform hard nitride layer on the surface. Following laser treatment, the hardness of the TC11 alloy increased from 372 HV to 824 HV, and the wear resistance was enhanced by a factor of 13. The surface of the TiZrAlV alloy was also strengthened through laser nitriding by Feng et al. [9], where the improvement in wear resistance was mainly attributed to the formation of hard nitrides composed of Ti(Zr)N and Ti(Zr)N_0.3. In biomedical applications, laser nitriding not only improves the wear resistance, but also confers antibacterial properties to the surface [10]. Various types of lasers have been employed in the nitriding process, with high-power continuous lasers being the most frequently reported. The thickness of the nitriding layer is closely correlated with the laser parameters and types utilized. For continuous wave (CW) lasers operating at a high-power of 3000 W, the nitriding layer thickness may exceed 100 μm; however, this can result in substantial thermal defects such as the formation of cracks and heat affected zones [3,11]. In contrast, low-power nanosecond lasers, which offer improved thermal control, typically yield a nitriding layer thickness of less than 10 μm due to simultaneous ablation during the nitriding process [12]. Femtosecond laser nitriding provides advantages including low thermal input and suitable nitriding layer thickness, effectively enhancing the hardness of titanium surfaces [13]. Therefore, compared to continuous and nanosecond lasers, femtosecond lasers are particularly well-suited for biological applications. Nonetheless, it is essential to recognize that laser nitriding may also increase the friction coefficient, posing challenges for its practical application [14,15].

Laser texturing has also been proven to improve the friction performance of titanium alloys in both dry and lubricative conditions [16,17]. In particular, femtosecond pulses have gained attention due to the advantages of the nonthermal effect and high precision. However, the femtosecond laser does not change the surface composition or mechanical properties of the material [7]. Textures created by femtosecond lasers are prone to degradation due to the pore load-bearing capacity, which limits the long-term effectiveness of textures. Therefore, enhancing the hardness and mechanical properties of textures is crucial for femtosecond laser texturing. Combining surface hardening and texturing is a trend for enhancing the tribological properties of titanium alloys, but it typically requires two different types of processing equipment [18,19,20,21].

In this study, laser nitriding and laser texturing were integrated into one femtosecond processing system. Femtosecond laser nitriding (FLN) is recognized as a thermal processing method; under the effect of the heat accumulation of high repetition rates, a hard TiN coating can be formed by melting the surface in nitrogen. Subsequently, the TiN layer was ablated by femtosecond laser texturing (FLT), which can be recognized as cold processing under a repetition rate of 200 kHz. Hence, the untreated, single nitrided, and combined nitrided/textured samples were compared and characterized in detail, and the effect of texture density on bio-tribological performance was systematically studied.

2. Materials and Methods

2.1. Materials

A TC4 (Ti6Al4V) alloy was utilized in this study, and its chemical composition is shown in

Table 1. Chemical composition of the TC4 alloy (wt%).

Fe	C	N	H	O	Al	V	Ti
≤0.30	≤0.10	≤0.05	≤0.015	≤0.20	5.5–6.8	3.5–4.5	Bal.

. The TC4 rods were cut into 20 mm × 20 mm × 5 mm blocks and pre-treated by grinding with 400-grit sandpaper to remove oxidation from wire cutting, followed by 15 min of ultrasonic cleaning in anhydrous ethanol (99.7%). The surface roughness of the pre-treated samples was 0.605 μm.

2.2. Femtosecond Laser Nitriding-Texturing Combined Treatment

The TC4 alloy was modified using a combined nitriding and texturing approach with a femtosecond laser system. This system is characterized by an adjustable repetition rate ranging from 50 kHz to 19 MHz and a pulse duration of 270 fs. Detailed specifications of the laser system can be referenced in our previous research [13,22]. The schematic of the laser modification process is illustrated in Figure 1. In the initial step, femtosecond laser nitriding (FLN) was carried out at a repetition rate of 19 MHz. This process involved melting the surface of the titanium alloy in a nitrogen atmosphere, leading to the formation of a hard nitrided layer through the interaction between molten titanium and nitrogen. The laser scanning paths were parallel. To mitigate crack formation and ensure a uniform nitrided layer, the FLN parameters were optimized as follows: single pulse energy of 1.74 μJ (well below the ablation threshold of titanium), scanning speed of 40 mm/s, scan spacing of 20 μm, and nitrogen flow rate of 20 L/min. In a subsequent step, femtosecond laser texturing (FLT) was performed at a repetition rate of 200 kHz to ablate the nitrided layer, thereby creating a micron-scale square array structure. This FLT process was conducted in air, eliminating the need for a protective chamber. The scanning path for FLT is shown in Figure 1b. The texturing parameters were a single pulse energy of 20 μJ and scanning speed of 200 mm/s. A summary of the laser and scanning parameters for each step is provided in

Table 2. Processing parameters of FLN and FLT.

Processing Parameters	FLN	FLT
Pulse width (τp)	270 fs	270 fs
Repetition rate (frep)	19 MHz	200 kHz
Single pulse energy (Ep)	1.74 μJ	20.00 μJ
Laser power (P)	33 W	4 W
Scanning speed (ν)	40 mm/s	200 mm/s
Scanning distance (d1)	20 μm
Scanning distance (d2)		10 μm
Scanning distance (d3)		200 μm/250 μm/300 μm

.

The geometrical characteristics of the FLN-FLT modified coating is illustrated in Figure 1c, where the height of the squared textures was fixed at 35 μm, and the spacing between adjacent texture was 70 μm. The density of the textures was adjusted by varying the width of the squares W. The texture density D can be calculated by D = (n × s/A) × 100%, where n is the total number of squared textures on the surface, s is the area of a single squared texture, and A is the total surface area of the sample [23].

Table 3. Sample number and corresponding texture density.

Sample	Type of Laser Modified	Texture Width (W, μm)	Texture Density (D, %)
A0	–	–	–
N0	FLN	0	0
T1	FLN-FLT	130	55%
T2	FLN-FLT	180	65%
T3	FLN-FLT	230	75%

summarizes the square width W and the corresponding texture densities. For convenience, samples with texture densities of 55%, 65%, and 75% are referred as T1, T2, and T3, respectively. The untreated and single FLN-treated sample are referred to as A0 and N0, respectively.

2.3. Surface Topography and Microstructure Characterization

The surface morphology was observed by a confocal laser scanning microscope (CLSM, VK-X250K, KEYENCE, Osaka, Japan) and a scanning electron microscope (SEM, S-7800, Hitachi, Tokyo, Japan). The chemical composition was determined through energy dispersive spectrometry (EDS). Surface roughness measurements were performed with the CLSM at a 20× magnification over an area of 200 μm × 200 μm. Before cross-sectional microstructure characterization, samples were polished with #2000 sandpaper and then etched with Kroll’s solution for 20 s. Phase composition was analyzed using X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Saarbrucken, Germany). The microhardness of both the surface and cross-sections was measured by a micro Vickers hardness tester (FM ARS900, Xinci, Zhenjiang, China) under a load of 500 g (4.9 N) for 15 s. Each measurement was performed five times, and the average value was recorded.

2.4. Bio-Tribological Tests

The tribological performance of the femtosecond laser treated samples was evaluated using a ball-on-disc tribometer (MFT-5000, Rtec, Silicon Valley, CA, USA), as illustrated in Figure 2. All tests were conducted at room temperature with the samples fully immersed in simulated body fluid (SBF) [24]. A Si₃N₄ ball, with diameter of 5 mm and hardness of 1500 HV, was used as the counterbody. The tests were conducted for 30 min with a motion length of 5 mm and a frequency of 10 Hz, corresponding to a sliding speed of 100 mm/s and a total sliding distance of 180 m. A normal load of 5 N was applied, resulting in a Hertzian contact pressure of approximately 1.7 GPa. Mass loss before and after the friction tests was measured using a digital balance with 0.01 mg precision, and each measurement was repeated three times for accuracy. Wear volume loss (V) was calculated from the wear scars using the formula: V = L × S, where L is the motion length (5 mm) and S is the cross-sectional area of the wear tracks [23]. The specific wear rate K was then determined using K = V/(F × D), where F represents the load (5 N) and D is the total sliding distance (180 m). The drafting of this manuscript was supported by GPT-4, a language model from OpenAI, used solely for language refinement. This tool contributed to the enhancement of textual expression and organization without affecting the analysis or interpretation of the research data.

3. Results and Discussion

3.1. Surface Morphologies

The surface morphology of the untreated and FLN-treated samples is shown in Figure 3. The surface roughness Sa of the untreated sample was 0.605 μm, which increased to 0.964 μm after femtosecond laser nitriding. Periodic “ridge-like” microstructures formed due to the rapid solidification of femtosecond laser melting, as shown in Figure 3b, with an amplitude of approximately 7 μm. The surface roughness nitrided by the femtosecond laser was significantly lower than that of conventional CW lasers [4] (with an Sa of approximately 10 μm). Figure 4 shows the morphology of the FLN-FLT-treated samples at varying texture densities and depicts the advantages of femtosecond lasers over nanosecond lasers for surface texturing, where defects (such as bulges) were found around the textures [25,26].

The XRD diffraction patterns are presented in Figure 5. The XRD results showed that the composition of the FLN-treated and FLN-FLT-treated surfaces were TiN and α’-Ti. The TiN phase displayed four main diffraction peaks at 2θ angles of 36.861°, 42.823°, 62.165°, and 78.441° (PDF card No. 38-1420), while the α’-Ti phase exhibited peaks at 35.170°, 38.455°, 40.751°, 52.092°, 63.106°, and 70.767° (PDF card No. 44-1294). As shown in Figure 5b, compared to sample A0, the diffraction peaks of Ti in sample N0 and T1 shifted slightly to the higher 2θ value. This was due to the effect of the nitrogen solid solution and residual tensile stress in rapid cooling during femtosecond laser nitriding. Furthermore, the TiN intensity in T1 was lower than in the N0 sample, which was caused by the material removal of the nitrided layer after femtosecond laser ablation.

3.2. Microstructure and Microhardness

The microstructures after femtosecond laser nitriding are illustrated in Figure 6. The nitrided layer exhibited a distinct boundary from the substrate, with a uniform thickness of 73 μm and no obvious cracks. Higher magnification of the nitride microstructures is shown in Figure 7. Three distinct regions based on the TiN morphology are displayed: the outer region (corresponding to yellow rectangle in Figure 6b), characterized by a continuous thin TiN layer with a thickness of approximately hundreds of nanometers; the middle region (corresponding to red rectangle in Figure 6b), featuring TiN as dendritic structures; and the interphase region (corresponding to blue rectangle in Figure 6b), where TiN appears as fine particles. The lath-like α’-Ti can also be observed in the interdendritic region, as shown in Figure 7d. After femtosecond laser texturing, porous microstructures with diameters of approximately 10 μm could be found, as seen in Figure 6b,c. This porous morphology, also reported in previous studies, resulted from a combination of thermal accumulation effects, ion ejection, and shielding phenomena during femtosecond laser ablation with repetition rates ranging from several hundred kHz to a few MHz [27,28]. Notably, porous microstructures on the surface are beneficial for biomedical applications including implants, where enhanced cell adhesion and tissue integration are desired [29,30]. Additionally, the oxygen content outside and inside the textured area was tested using EDS. The results showed that the atomic percentage of oxygen on the surface of the textured area (Point A in Figure 6c) was approximately 3%. Inside the textured area (Point B in Figure 6c), this value decreased to less than 1%. This indicates that slight oxidation occurred on the surface of the textured area.

Figure 8 presents the hardness and nitrogen content distributions of different regions inside the nitrided layer. The nitrogen content at different positions along the depth was measured using EDS. The hardness of the near-surface region of the nitrided layer was 1455 HV, approximately four times that of the substrate (336 HV). The hardness of the nitrided layer decreased with increasing depth, and the values of the middle and interphase regions were 945 HV and 625 HV, respectively. The nitrogen content distribution was consistent with the hardness along the depth. In the dendrite arms (Point A), the nitrogen atomic content was close to 50%, indicating that nitrides were primarily in the TiN phase. In contrast, the nitrogen content of the nitrides in the middle and bottom regions (Point B and C) decreased to 36% and 24%, indicating that the nitrides were in the form of non-stoichiometric TiN_{x (x<1)}. Although the nitrides in both the outer (Point A) and middle (Point B) regions appeared as dendrites and shared the same crystal structure (FCC-TiN_{x (x<1)}), their hardness was closely linked to the nitrogen content. As the nitrogen content decreased, the hardness of the nitrides also decreased. Additionally, the nitrogen atomic content of α’-Ti (Point E) reached 15% due to the nitrogen diffusion into α’-Ti. Hence, the hardness of nitrogen-rich α’-Ti increased to 892 HV under the effect of solid solution hardening and phase transformation [31]. It is noteworthy that laser nitriding can improve both the surface hardness and wear resistance of biomedical titanium alloys. However, this will lead to a moderate increase in the elastic modulus of the alloy. The increase in modulus may contribute to the stress shielding effect, which may ultimately result in bone resorption and the loosening of implants.

3.3. Tribological Properties in SBF

Figure 9 illustrates the 3D morphology and profiles of wear tracks after friction testing. The cross-sectional wear area (S) and wear depth (H_scar) were measured by CLSM software. For the untreated and FLN-treated samples with continuous wear tracks, S was measured directly. While, the wear track profiles of the FLN-FLT-treated samples were discrete, S was calculated by summing the wear areas at the textured sites, as shown in Figure 9c–d. The untreated sample had an S of 7784 μm² and an H_scar of 24.5 μm, while the FLN-treated sample showed an improved wear resistance with an S of 2375 μm² and an H_scar of 6.5 μm. Unlike the severe destruction reported in single femtosecond laser texturing [15], the textures of the FLN-FLT-modified samples only sustained partial damage, suggesting that nitriding significantly enhances the material’s mechanical properties. Therefore, the lifespan of textures is extended. Compared to the FLN-treated sample, the S and H_scar of the FLN-FLT-modified samples were further reduced, with optimal performance in the T2 (65% texture density), where S and H_scar decreased to 1893 μm² and 5.6 μm, respectively.

Figure 10a presents the friction coefficient (COF) curves before and after laser treatment, with Figure 10b summarizing the average COF during the steady state. The untreated titanium displayed a COF of 0.39 in simulated body fluid, which is consistent with the reported values of laser nitriding [29]. After single femtosecond laser nitriding (FLN), the average COF increased to 0.66. In contrast, the COF of the FLN-FLT-treated samples decreased to 0.45, 0.41, and 0.45 with texture densities of 55%, 65%, and 75%, respectively. This decrease highlights the effectiveness of texturing in improving the friction performance of laser nitriding surfaces. The mechanisms of the COF increasing (caused by nitriding) and reduction (caused by texturing) will be further discussed in the subsequent analysis.

The wear rate and mass loss after the friction test are shown in Figure 10c,d. The TC4 alloy recorded a wear rate of 0.43 × 10⁻³ mm³/Nm and a mass loss of 9.07 mg under SBF lubrication. After single FLN, these values improved to 0.15 × 10⁻³ mm³/Nm and 3.41 mg, indicating the effective enhancement of wear resistance. The FLN-FLT-treated samples achieved optimal performance at the texture density of 65%, with a wear rate of 0.11 × 10⁻³ mm³/Nm and a mass loss of 2.32 mg. These results suggest that single FLN significantly enhances the wear resistance of TC4 alloys, and the FLN-FLT combined treatment provides a notable advantage in improving both the wear and friction performance.

Figure 11 illustrates the worn surface morphology of the untreated and single FLN-treated samples after sliding tests. The FLN treatment resulted in a reduction in wear width from 0.693 mm to 0.551 mm, indicating a significant enhancement of FLN. As shown in Figure 11a–c, the untreated sample exhibited substantial adhesive wear debris, which is a typical characteristic of adhesive wear. The hardness difference between the untreated sample (336 HV) and the Si₃N₄ ball (1500 HV) leads to severe plastic deformation during sliding, resulting in pronounced ploughing and the generation of debris. The wear mechanisms for untreated titanium have been identified as severe adhesive and abrasive wear [14]. According to the EDS results in

Table 4. EDS results of the corresponding points in Figure 11.

Element (At%)	Point A	Point B	Point C	Point D
N	3.02	1.03	7.59	17.93
O	34.11	17.26	51.21	56.71
P	0.54	0.52	4.53	2.26
Ca	0.56	0.72	5.02	1.88
Si	2.08	1.27	10.25	3.13
Ti	59.13	78.48	20.24	17.01
Na	0.56	0.72	1.16	1.08

, the main elements of the worn surface of the untreated sample were Ti and O, with a little Si and N from the Si₃N₄ ball. The wear debris of the untreated sample was composed of soft material broken from the substrate and TiO_x particles.

The worn surface of the single FLN-treated sample displayed typical features of the brittle fracture of ceramic coatings (Figure 11d–f), which could be attributed to outstanding resistance to the plastic deformation of TiN [32]. A large number of microcracks and peelings were observed in the worn area. In the SEM images, the peeled and unpeeled areas are shown in different greyscales, with the peeled area in gray and the unpeeled area in black. The EDS results of the FLN-treated worn area revealed that the contents of O, Si, and N increased while the Ti content decreased compared to the untreated sample. The worn products of the FLN-treated samples were composed of hard ceramic debris from the TiN, oxide particles, and Si₃N₄. Hence, a hard protective layer formed on the worn area of the FLN sample, attributed to the aggregation of hard ceramic debris during sliding [12].

The enhancement of hardness in the titanium alloy resulting from femtosecond laser nitriding significantly improved its wear resistance. However, this increased hardness also necessitated greater traction forces in the contact area when sliding against the Si₃N₄ ball, leading to a notable rise in the coefficient of friction compared to the untreated samples. This effect was particularly pronounced during the running stage of the wear test, as illustrated in Figure 10a. Furthermore, the protective layer formed by the accumulation of hard wear debris separated the Si₃N₄ ball from direct contact with the nitrided layer. While this protective layer enhanced the wear resistance, it also resulted in an increase in the tangential force during the friction process, which contributed to a higher coefficient of friction in the stable stage compared to that observed in the running stage in Figure 10a. Additionally, microcracks propagated within the protective layer under fluid pressure and shear stress, leading to peeling of the layer, which increased the worn roughness and further contributed to a further rise in the COF during sliding.

Figure 12 presents the worn morphology of the FLN-FLT combined treatment at various texture densities. The wear widths of the FLN-FLT-treated samples were smaller than those of the untreated and single FLN samples, indicating a further enhancement of wear resistance caused by FLN-FLT. The microstructures in the worn area of the FLN-FLT-treated samples exhibiting brittle fracture features were characterized by microcracks and peelings. Compared to the single FLN, the extent of peeling was significantly restrained after FLN-FLT, suggesting that the textures effectively inhibited crack propagation and delayed the failure of the protective layer. Additionally, the textures can trap hard debris generated during friction, reducing the three-body abrasive wear of the nitrided surface [33]. During friction sliding, the friction coefficient is closely related to the contact area of the interface [34]. Compared to the single FLN samples, the textures diminished the actual contact area between the Si₃N₄ balls and the nitrided surface, which is beneficial for a decrease in the COF. Sample T2, with a texture density of 65%, showed the lowest COF of the FLN-FLT-treated samples, which was mainly be attributed to minimization of the actual contact area under the optimal texture density, as illustrated in Figure 9d.

Figure 13 illustrates the wear mechanisms of titanium following the single FLN and FLN-FLT composite treatments. For the untreated titanium, the titanium matrix experienced fatigue wear, generating soft debris and a small amount of oxide that adhered to the worn surface when sliding against hard Si₃N₄ balls. The wear mechanisms of the original sample were severe adhesive wear and abrasive wear. After single nitriding, the hard nitrided layer displayed pronounced brittle fracture characteristics, leading to the accumulation of spalled hard debris on the surface and the formation of a protective layer that mitigates adhesive wear. However, as this protective layer degrades, the roughness of the wear scar increases, resulting in elevated frictional forces and a corresponding increase in COF. The wear mechanisms of the singe FLN sample were mild adhesive wear and abrasive wear. Through the synergistic effects of debris trapping, the suppression of crack propagation, and reduction in the actual contact area, the FLN-FLT combined treatment not only enhanced the wear resistance of the material, but also lowered the friction coefficient.

4. Conclusions

This work employed a femtosecond laser nitriding-texturing combined treatment to enhance the bio-tribological properties of titanium alloys. The wear behavior of titanium alloys in simulated body fluid was systematically investigated before and after laser processing. The main conclusions are as follows:

(1)

After femtosecond nitriding, a nitrided layer composed of TiN_x(x≤1) and nitrogenrich α’-Ti formed on the surface with a thickness of 73 μm. The TiN_x(x≤1) phase appeared as fine dendrites and particles, while lath-like α’-Ti was distributed between the dendrites.

(2)

The surface hardness increased from 336 HV to 1455 HV with the wear resistance significantly improved after femtosecond laser nitriding. However, the COF of the nitrided surface rose from 0.388 to 0.655 due to the increased roughness and the generation of hard debris.

(3)

The COF of the nitride-textured surface decreased to 0.408 with the optimal texture density of 65% due to the synergistic effects of textures in debris trapping, the suppression of crack propagation, and reduction in contact area.