Objective To improve the wear resistance of titanium alloy surface. Methods TiZrHfCrMoW coating was prepared on TC4 substrate by laser cladding technology. The phase composition and microstructure of the coating were analyzed by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The friction and wear behavior of TiZrHfCrMoW high entropy alloy (HEA) coating and TC4 alloy were studied by electrochemical workstation and friction and wear tester in air and 0.9%NaCl simulated body fluid environment at 37 ℃. Results The laser cladding HEA coating was uniform and dense without obvious defects. It was mainly composed of two BCC phases and one unknown phase. The average hardness of the coating was 584.6HV0.2, which was about 1.6 times the hardness of TC4 substrate. When sliding in air, the wear rate of HEA coating at 0.3, 0.5, and 1 N is lower than that of TC4 substrate, and the wear rate of coating increases with the increase of load, while the wear rate of TC4 is the opposite. In 0.9% NaCl solution at (37±0.5) ℃, the wear rate of TC4 under 0.5 N load is 6 times that of HEA coating. Compared with TC4 titanium alloy substrate, HEA coating has higher self-corrosion potential and lower corrosion current density. The main wear mechanism of HEA coating in simulated body fluid environment is layer-by-layer peeling and corrosive wear. Conclusion Laser cladding HEA coating can effectively improve the wear and corrosion resistance of TC4 alloy.

Titanium alloy has the advantages of low Young’s modulus, high specific strength, good corrosion resistance and good biocompatibility, and is an ideal biomedical implant material [1]. However, titanium alloy has low hardness and poor wear resistance. TC4 is one of the commonly used medical titanium alloys, which contains 6%Al and 4%V. Aluminum and vanadium improve the oxidation resistance and strength of TC4, but also cause some adverse biological tissue reactions. The low wear resistance of TC4 leads to the release of aluminum and vanadium ions, which may cause cancer and other neurological diseases [2-3]. These shortcomings limit the application of TC4 in long-term implants and medical devices in the human body. Surface modification technology is considered to be an effective way to overcome and improve the surface properties of titanium alloys [4-6]. Currently, the widely used surface modification technologies include physical vapor deposition (PVD), plasma spray, sol-gel and electrochemical deposition [7], but these technologies have some limitations, such as the coating is too thin, easy to fall off, and poor density [8-10]. Laser cladding is a surface modification technology that uses laser radiation to deposit a coating on the substrate surface. The coating material and the substrate surface are melted simultaneously during cladding and then rapidly solidified to form a coating, so that the coating and the substrate are metallurgically bonded at a very low dilution, thereby improving the surface properties of the substrate [11-14].

Although biometallic materials have been continuously developed and applied, there is an urgent need for a new generation of biometallic materials with excellent biocompatibility and mechanical properties. In recent years, due to the unique mechanical properties brought by the complex atomic arrangement and high mixing enthalpy of high entropy alloys (HEAs) [12-18], HEA coatings have also attracted widespread attention in the field of biometallic materials. However, the available elements of human implant materials are limited by biocompatibility factors. Moreover, while meeting corrosion resistance and wear resistance, the material also needs high yield strength and ductility as well as low Young’s modulus to avoid stress shielding. Using laser cladding technology, Li Qingyu et al. [12] prepared NbMoTaTi coating on pure Mo substrate and obtained an average microhardness of 397.6HV. The coating had fine grains and uniform structure. Zhang et al. [19] prepared refractory HEA coating on the surface of 45# steel substrate, with a microhardness of up to 700HV0.5. Due to the solid solution strengthening of the body-centered cubic matrix and the increase in the amount of β-TixW1‒x precipitation, a maximum hardness of 1300HV0.5 can be obtained after heat treatment at 800℃. Tüten et al. [20] prepared TiTaHfNbZr high entropy alloy coating with equal material ratio on Ti6Al4V substrate by radio frequency magnetron sputtering. The coating has high hardness〔(12.51±0.34) GPa〕, high elastic modulus〔(181.3±2.4) GPa〕 and high wear resistance. It can be seen that HEA coating has outstanding application prospects in the field of biometallic materials. In addition, Nagase et al. [21] prepared TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo high entropy alloys by arc melting. Both alloys showed good biocompatibility and obtained a high hardness of about 531HV, with potential high wear resistance. On this basis, in order to further improve the hardness and wear resistance, this paper added W element [22] and used laser cladding technology to prepare TiZrHfCrMoW high entropy alloy coating on the surface of TC4 alloy, and then analyzed the microstructure of the coating, and studied the friction and wear behavior of the HEA coating and TC4 alloy substrate in the atmospheric environment and 0.9%NaCl simulated body fluid environment at 37 ℃, providing theoretical support for the surface strengthening coating of metal implants. 1 Experiment

1.1 Coating preparation

Ti, Zr, Hf, Cr, Mo, W pure metal powders with a particle size of 10~150 μm and a purity higher than 99% were used, prepared according to the same material ratio, and the powders were mixed evenly using a ball mill. The substrate was a TC4 alloy plate with a size of 200 mm×100 mm×3 mm. The mixed metal powder was coated on the substrate using the pre-set powder method. The powder was dried before the experiment to remove the moisture. Then, under the protection of high-purity argon, a high-power ytterbium-doped Nd-YAG (YSL-3000IPG) continuous wave laser was used to clad the surface of the TC4 alloy. Optimized
Laser cladding parameters: laser power 900 W, scanning rate 100 mm/s, laser flux 19.10 J/mm2, laser beam spot diameter 6.0 mm, track overlap 0.2 mm. The prepared samples were cut into 15 mm × 15 mm blocks, and then ground and polished for use. The PW3710 X-ray diffractometer was used to analyze the physical phase of the cladding coating, with a scanning range of 20°~100°. The JSM-6360 scanning electron microscope was used to observe the microstructure and wear morphology of the coating. The hardness test was carried out using an HX-1 000TM/LCD automatic turret microhardness tester with a loading load of 200 g and a holding time of 20 s. 1.2 Performance test and friction test The dry friction test used Si3N4 with a diameter of 6 mm as the grinding ball, and the loads were 0.3, 0.5, and 1 N respectively. According to the Hertz contact stress formula, the maximum contact stress under these three loads was 231, 274, and 346 MPa, respectively, which is much higher than the maximum contact stress of human joint implants. Other test parameters: sliding speed 20 mm/s, reciprocating frequency 2 Hz, reciprocating stroke 5 mm, friction time 100 min. The potential and polarization curves of the coating and substrate in 0.9% NaCl solution were tested using a Gamry 1010E electrochemical workstation. During the electrochemical test, the working electrode was the sample, the reference electrode was a saturated calomel electrode, and the auxiliary electrode was a platinum electrode. The exposed area of ​​the sample in the corrosion solution was 10 mm× 10 mm, the scanning speed was 1 mV/s, and the scanning range was ‒1.7~1.5 V. The electrochemical corrosion wear was tested using a multifunctional friction tester and a Gamry 1010E electrochemical workstation, using a 0.9% NaCl solution at (37±0.5) ℃. After the sample was welded with wires, it was encapsulated with epoxy resin, with an exposed area of ​​1 cm’2, and other areas were covered with insulating glue and epoxy resin. The corrosion test process under static load and friction conditions is: first stand still for 25 minutes, then load and slide for 100 minutes, and finally stand still for 20 minutes. The total test time is 145 minutes. The wear scar volume is obtained by scanning the wear scar with a profilometer.
The Hertz contact stress formula [23] is: See formula (1) in the figure
Where: F is the applied normal load; E1 and E2 are the elastic moduli of the two contact body materials; μ1 and μ2 are the Poisson’s ratios of the two contact materials. The wear rate calculation expression is: See formula (2) in the figure
Where: V represents the wear volume; L represents the total friction sliding length; F represents the normal load.

2 Results and analysis

2.1 Microstructure of HEA coating

The X-ray diffraction pattern (XRD) of TiZrHfCrMoW (HEA) coating is shown in Figure 1. The cladding layer is mainly composed of two BCC phases and one unknown phase. Figure 2 is a cross-sectional morphology of the HEA coating. The HEA coating is mainly composed of white fishbone-like structure and light gray blocky structure embedded in the whole gray structure. From the diffraction peak area corresponding to each diffraction peak in the XRD spectrum, it can be seen that the content of BCC1 phase is the largest, followed by BCC2 phase, and the unknown phase is the least. From the cross-section of the HEA coating close to the upper surface (Figure 2a), it can be seen that the gray matrix phase content is relatively the largest, followed by the light gray blocky precipitate phase, and the white fishbone precipitate phase is the least. Through the corresponding relationship between the content of each phase in the coating, it can be inferred that the gray matrix phase, light gray blocky precipitation phase and white fishbone-shaped precipitation phase correspond to the BCC1 phase, BCC2 phase and unknown phase respectively. As can be seen from Figure 2c, there is no clear interface between the coating and the substrate, indicating that the coating and the substrate are in a typical metallurgical bond, and there are no obvious cracks and pores. The phase composition at the bottom of the coating is mainly BCC1 and BCC2 phases, with a small amount of unknown phase. It can be inferred that the BCC1 phase is the matrix phase of the HEA coating. However, at the top of the coating, the BCC2 phase and the unknown phase increase significantly. This change may be caused by the segregation of the alloy components (especially the W element) and the different heat dissipation rates of each part [24]. Due to the high mixing enthalpy and atomic radius differences of the multi-principal element, there is a serious lattice distortion in the BCC solid solution, which improves the solid solution strengthening effect. The average surface hardness of the coating reached 584.6HV0.2, which is significantly higher than the average surface hardness of the TC4 alloy substrate (about 366.6HV0.2).

2.2 Friction and wear behavior of HEA coating in atmospheric environment

The friction factor-time curves of HEA coating and TC4 substrate in atmospheric environment under different loads are shown in Figure 3. Compared with TC4, the friction factor of HEA coating is larger under all loads. The friction factor of TC4 is relatively stable, while the friction factor of HEA coating fluctuates relatively more. Under loads of 0.5 N and 1 N, the initial friction factor of HEA coating is low, maintaining at around 0.1 for the first 600 s, then rising rapidly, and finally fluctuating around 0.7. This may be because during the initial friction sliding process, a moderately high load can form a smooth and dense oxide layer on the friction surface (the friction heat formed under a load of 0.3 N is not enough to meet the needs of the oxidation reaction), thereby showing a low friction factor. When sliding continues for a period of time, as the amount of wear increases, more abrasive particles are generated at the sliding contact interface, causing three-body wear between the friction pairs, which sharply deteriorates the flatness of the contact interface, and thus causes the friction coefficient to increase rapidly. The friction coefficient is affected by the lubrication effect of the contact interface, but a low friction coefficient does not necessarily mean a low wear rate [25]. As shown in Figure 4, the friction coefficient of the HEA coating is higher than that of the substrate, but the wear rate under the same load is lower than that of the substrate, and as the load increases, the layer-by-layer peeling phenomenon gradually increases, and the locally peeled layers are crushed and oxidized to form more wear debris. These wear debris play a role in reducing friction, resulting in a decrease in the friction coefficient. From the average friction coefficient and wear rate of the HEA coating and TC4, it can be seen that the wear rate of TC4 decreases with the increase of load, while the coating is the opposite. This may be because the increase in load causes more heat to be generated during the friction process, and the oxidized friction layer containing titanium dioxide has a lubricating effect. At the same time, Mao et al. [26] found that TC4 has different types of friction layers at different temperatures, and the type of friction layer affects the wear rate. If the peeling rate of the friction layer generated within a certain temperature range is less than its generation rate, it will show a protective effect on the contact surface. In the atmospheric environment, as the load increases, the friction heat effect increases, the surface temperature of the titanium alloy rises to form a friction oxide layer, and at the same time, adhesion and abrasive wear occur on the friction surface in the part not covered by the friction oxide layer. Due to the friction reduction effect of the friction oxide layer, the wear rate of TC4 decreases with the increase of load, which is a typical characteristic of mild oxidation wear [27].

Figure 5 shows the wear surface morphology of the HEA coating and TC4 substrate under atmospheric conditions. The wear surface of TC4 under different loads has similar plastic deformation characteristics. Due to the low hardness, plastic deformation is induced by extrusion during reciprocating sliding, resulting in grooves parallel to the sliding direction and adhesion marks. The large number of small white particles near the grooves indicate that during the wear process, the grinding balls squeeze and cut the titanium alloy surface to produce abrasive chips, which are further crushed under the action of cyclic stress and embedded in the friction surface to produce furrows. The oxide film formed by tribochemical reaction inhibits wear, but adhesive wear tends to increase with increasing load. The friction layer formed during the wear process causes the wear rate to continue to decrease. The mechanical properties of HEA coatings are affected by their structure. Due to the effects of high entropy, lattice distortion and hysteresis diffusion caused by multi-principal element characteristics, HEA with BCC structure usually has a higher hardness, which effectively improves the wear resistance of the cladding layer. Compared with TC4, the wear surface of HEA coating is smoother and flatter, without deeper furrows. Under loads of 0.3 N and 0.5 N, there are black areas on the wear surface of HEA coating, and there are obvious cracks on the surface of the black area in Figure 5d, which may be because the HEA coating undergoes oxidation reaction during dry friction and wear, and forms a composite oxide film on the surface. From the wear rate results, the high hardness of the oxide protects the wear surface and reduces the wear rate, but due to its low fracture toughness, the oxide film is more and more likely to break with the increase of the applied load (Figure 5d). The wear debris generated by the rupture of the oxide layer will increase the surface roughness, resulting in fluctuations in the friction factor. The hard oxide wear debris produces a similar effect to abrasive particles at the friction interface, and the wear mechanism mainly changes from layer-by-layer peeling to abrasive wear, as shown in Figure 5f, which leads to an increase in wear rate with increasing load.

2.3 Corrosion and wear behavior of HEA coating in simulated body fluid environment

The evolution of OCP potential and friction factor of TC4 and HEA coatings over time is shown in Figure 6. It can be observed that at the beginning of corrosion wear, after the passive product film is removed, the active material is exposed to the corrosive environment, and the potential drops sharply to a more negative value, and then fluctuates up and down within a certain range. These fluctuations represent the periodic removal of the passive product film by mechanical action (depassivation) and the growth of the passive product film in the friction area (repassivation). The remaining relatively flat OCP curves also undergo a cycle of depassivation and repassivation, which may be due to the changes in the thickness, morphological characteristics, roughness, phase composition and hardness of the passivation product film, which establishes a balance between the mechanical depassivation and electrochemical repassivation rates, so that a relatively stable state is achieved in terms of potential.

The OCP curve of TC4 in Figure 6a shows periodic fluctuations. Under a load of 0.3 N, as the potential increases, the friction factor of TC4 decreases instead. It is speculated that the formation of the corrosion product film plays a certain lubricating role. Moreover, the fluctuation of the friction factor is relatively violent, which means that the surface contact state is strongly affected by both wear and corrosion, and the formation and destruction of the corrosion product film have not yet reached a balance. When the passivation product film is generated, the direct contact between the friction pair surfaces can be reduced, thereby reducing the friction factor; when the passivation product film is broken, the peeled passivation product film is crushed into wear debris, and the flatness of the contact surface deteriorates sharply, resulting in an increase in the friction factor. Under low loads, the passivation product film plays a protective role on the friction surface. As the load increases to 0.5 N, the surface structure of the passivation product film is rapidly destroyed during the friction process, generating a large amount of wear debris, increasing the contact area with the friction pair, intensifying abrasive wear, and increasing the friction factor. Under 1 N load, the relationship between potential and friction factor changes, and the friction factor increases with the increase of potential, which may be due to the change of the structure and composition of the passivation product film under the dual effects of friction heat and corrosion. Compared with TC4 alloy, HEA coating exhibits high friction factor and low wear rate in both atmospheric environment and NaCl solution. This is because HEA coating has higher hardness, increases the ability of the surface to resist plastic deformation, and improves the wear resistance of the cladding layer, but under the test load, the friction factor of the coating is slightly higher than that of the TC4 substrate, indicating that the surface lubrication state of the material is different. From the wear morphology of HEA coating and TC4, it can be seen that a large amount of wear debris is accumulated on the surface of TC4, which plays the role of solid lubricant and reduces the surface friction factor. However, there are only a small amount of wear debris on the surface of HEA coating and no deep grooves, which cannot collect and store wear debris well, and there is no effective friction reduction form, forming a lubrication state different from TC4.

As the load increases, the fluctuation amplitude of the OCP curve of the HEA coating gradually decreases, indicating that the influence of the amount of corrosion and wear product generation is gradually decreasing. As can be seen from Figure 7 and Table 1, the self-corrosion potential of the HEA coating under a load of 0.5 N shifts negatively as a whole compared with that of 0.3 N, and a small positive shift occurs under a load of 1 N, which is consistent with the potential change trend of the OCP curve under different loads. The average OCP potential of the HEA coating under loads of 0.5 N and 1 N is ‒0.69 V and ‒0.65 V, respectively. The corrosion potentials of the two loads are similar, indicating that with the increase of load, the degree of wear scar activation does not increase significantly. This may be due to the generation of a relatively stable passivation product film under a load of 1 N. The generated passivation product film is thicker, denser or more stable, covering the active surface, avoiding direct contact between the coating surface and the friction pair, and improving the corrosion resistance of HEA. However, as the load increases, the mechanical damage caused by friction to the contact surface is greater than the protective effect of the passivation product film, so the wear rate continues to rise.

Figure 7 shows the polarization curves of the HEA coating and TC4 substrate with and without sliding under different loads. It can be seen that friction affects the shape and position of the dynamic polarization curve compared with the pure corrosion curve. The polarization curves of the HEA coating and the substrate during the corrosion and wear process show obvious current oscillations. Table 1 gives the corrosion potential and corrosion current density. The corrosion current during the corrosion and wear process is higher than the corrosion current under pure corrosion conditions, which indicates that rapid dissolution occurs on the worn surface. Compared with TC4, the HEA coating has a lower corrosion current density, indicating that the effect of wear on TC4 corrosion is more significant than that on the HEA coating. The HEA coating shows a low wear accelerated corrosion effect, which may be the reason for its low OCP value [28]. In the dynamic polarization curve of the HEA coating in Figure 7b, passivation breakdown was observed at 1 V. This can be attributed to the microstructural characteristics and chemical composition of the HEA coating. Hf usually exhibits passivation breakdown in chloride-containing solutions [29].

Figure 8 shows the average wear rate and average friction coefficient of the HEA coating and TC4 under OCP conditions. Obviously, the average wear rate of the HEA coating is lower than that of the TC4 substrate. The TC4 wear surface mainly shows plowing grooves parallel to the sliding direction. This is because the hardness of TC4 is low, and the friction area produces extensive shear deformation and adhesive wear exists in the central area. Under a load of 0.3 N, it can be seen from Figures 9a and 9b that the surface of the TC4 substrate is severely scratched and has obvious wear marks, but the wear rates of the two are not much different. This may be because the test time is too short, resulting in insignificant differences in the results. Under a load of 0.5 N, the wear rate of TC4 in a corrosive environment is 6 times that of the HEA coating. The wear scar morphology of HEA coating under dry friction conditions and wet friction conditions has certain similarities. For example, there are obvious plastic deformation and shallow furrows on the wear surface. In the atmospheric environment, the main wear mechanism is layer-by-layer peeling caused by plastic deformation. With the increase of load, the abrasive wear formed by the crushing of the peeling layer has a tendency to gradually increase, as shown in Figure 5. In the simulated body fluid environment, the main wear mechanism is layer-by-layer peeling caused by plastic deformation, and due to the corrosive effect of the solution and the convenience of fluid to remove abrasive particles, the abrasive wear characteristics such as furrows have not been very significant. Therefore, there are many similarities between the main wear mechanisms in the two environments. The main difference is that the proportion of abrasive wear at a larger load (1 N) in the atmospheric environment increases significantly. In the simulated body fluid environment of open circuit potential, the main wear mechanism of HEA coating is layer-by-layer peeling and corrosive wear.

3 Conclusions

1) TiZrHfCrMoW coating was successfully prepared on TC4 surface by laser cladding technology. The coating is composed of BCC1, BCC2 and unknown phases, and is well bonded to the substrate. The surface hardness of the coating reached
584.6HV0.2, which is significantly improved compared with the hardness of TC4 substrate (366.6HV0.2).

2) Under loads of 0.3, 0.5, and 1 N, the wear rate of HEA coating is generally lower than that of TC4 substrate. In atmospheric environment, the wear rate of coating increases with the increase of load, while the wear rate of TC4 decreases with the increase of load. Under a load of 0.5 N, the wear rate of TC4 in a corrosive environment is 6 times that of HEA coating. The coating can improve the wear performance of the substrate.

3) In the atmospheric environment, the main wear mechanisms of the HEA coating are layer-by-layer peeling and abrasive wear, and the abrasive wear tends to increase with the increase of load; in the simulated body fluid environment, the main wear mechanisms of the HEA coating are layer-by-layer peeling and corrosive wear.