Enhancing biocompatibility, mechanical properties, and corrosion resistance of laser cladding β-TiNb coatings

Ti6Al4V (TC4) is diffusely applied in orthopedic biomaterials for as much as its lower density, high tensile, easy formation of a dense TiO₂ passivation film on the surface, and good biocompatibility.¹ However, V⁺ (Ref. 2) and Al³⁺ (Ref. 3) contained in the TC4 alloy may cause malignant tissue reaction in vivo and eventually lead to the surgical failure of long-term implantation.⁴ In consequence, some elements without obvious adverse reactions to the human tissues can be applied to Ti alloy materials to improve their biocompatibility and mechanical properties.

At present, extensive amounts of research studies have been carried out on materials with good biocompatibility, corrosion resistance, and wear resistance.^5,6 At the same time, TiNb-based alloys have attracted widespread attention^3,7 and become the first choice for medical implant metal materials.^8,9 Previous studies pointed out that Ti and Nb elements are biologically inert elements and have highly nonreactive and biocompatible properties when used as biomedical implants in vivo.^10,11 The Nb element contributes to the formation of osseous tissue in animal intramuscular implantation experiments⁹ and is conducive to cell adhesion, proliferation, and differentiation in cell experiments.¹² However, the theoretical calculation results showed that Nb is a stable element of the β titanium alloy,¹³ which can improve the mechanical strength and reduce the elastic modulus.^14,15 The mechanical strength, hardness, hydrophilicity, and corrosion resistance of the titanium alloy with Nb element were significantly improved.^16–18 

It is pointed out in the relevant literature that Al, O, N, C, and other elements in the titanium alloy promote the α phase transformation of the titanium alloy, while Mo, V, Nb, Ta, and other elements promote the β phase transformation of the titanium alloy.^13,19–21 Studies have pointed out that the addition of Nb will transform the microstructure and properties of the TiNb alloy. When the Nb content increases to 20 wt. %, there are three different phases in the alloy (α, β, and ω). Simultaneously, the β-TiNb alloy has a suitable elastic modulus, as well as equivalent mechanical and biological properties to human bone tissue.²² At the same time, related research on Ti45Nb has also been proposed.²¹ The β-type Ti45Nb alloy not only has analogous elastic modulus and higher strength to osseous tissue, but also exhibits excellent corrosion resistance in various corrosive solutions and has bone tissue compatibility comparable to the CP-Ti alloy in in vivo implantation experiments.¹⁹ The inoculated L-929 and MG-63 cells had good adhesion on the surface of the Ti45Nb alloy.^10,19 TixNb (x = 50, 80, and 90 wt. %) binary alloys with the cubic crystal phase had lower corrosion velocity and better cell viability than those of CP-Ti.²³ Tames et al. analyzed the interaction between different phase transformations and elastic modulus of the TiNb alloy with a large Nb content. The results indicate that the phase composition of the TixNb alloy affects the mechanical properties to a large extent.²⁴ However, other α + β low Young’s modulus TiNb alloys, such as Ti6Al7Nb (E = 105 GPa),²⁵ Ti5Al2.5Fe (E = 110 GPa),²⁶ Ti13Nb13Zr (E = 80 GPa),²⁷ and Ti35Nb5Ta7Zr (E = 55 GPa),²⁸ have excellent properties.

TiNb series alloy prepared by common melting, casting, rolling, and forging methods has excellent comprehensive properties. However, the performance of the TiNb alloy prepared by the above methods needs to be further improved, and the above-mentioned steps of the smelting alloy process are complex and costly. In contrast, the laser cladding process is convenient to process. In the process of laser cladding preparation, the rapid heating and cooling process can inhibit the growth of grains and achieve the effect of fine-grain strengthening.²⁹ At the same time, the related research of laser cladding TiNb alloy coating is rarely reported. At present, the surface modification methods of the titanium alloy mainly include magnetron sputtering, acid etching, sandblasting, anodic oxidation, and microarc oxidation. However, the coatings prepared by this method have low wear resistance and poor stability, and are prone to wear failure during implantation.^30–33 Therefore, we chose laser cladding technology to prepare the TiNb alloy coating with good metallurgical bonding.³⁴ 

Accordingly, the laser cladding coating of TixNb (x = 12.5, 17.5, 22.5, and 27.5 wt. %) alloy coatings was deposited on the TC4 surface by laser cladding technology in this project and focuses on the influence of different material ratios on the mechanical performance, corrosion resistance, and biocompatibility of the TixNb alloy coatings. The relationship between the composition and material microstructure properties of TixNb alloy coatings was revealed. TixNb laser cladding coating with excellent mechanical properties and biocompatibility is expected to be obtained, which provides a research basis for the application of orthopedic biomaterials.

TC4 (dimensions of 50 × 30 × 8 mm³) served as substrates (Dongguan Shengshi Metal Materials Co., Ltd., China). The chemical composition of TC4 is shown in Table I. The dirt and grease were removed by sandpaper polishing and then cleaned with alcohol. The characteristics of the powder are shown in Table II. Laser cladding powder was composed of Ti and Nb (purity > 99%, powder size 50–100 μm) (Beijing Ryubon New Material Technology Co., LED, China). Nb content was x = 12.5, 17.5, 22.5, and 27.5 wt. % in laser cladding TixNb alloy coating, Ti content as a balance. The powder was weighed with an electronic balance with an accuracy of 0.001 mg (Mettle Toledo, China), and the electronic balance was calibrated before weighing, then mixed by a QM-3SP2 ball grinder at room temperature for 1 h, and then dried at 120 °C for 2 h before vacuum packaging. We use the stainless steel grinding ball and dry ball milling method.

Laser cladding coatings were fabricated by the fiber laser system (Wuhan Ruike Laser Co., Ltd., RC-LMS-6000-R, China). To prevent oxidation, the laser cladding process is carried out in a high-purity Ar atmosphere, and the air in the sealing bag was extruded out as high-density argon flowed at the bottom with a flow rate of about 5 l/min. The optimum process parameters are as follows: laser power P = 3000 W, scanning speed V = 3 mm/s, preset powder thickness d = 1.2 mm, overlap rate λ = 40%, spot diameter 2.5 mm, and defocusing amount 300 mm. The schematic diagrams of the fiber laser system and the powder sample preparation are shown in Fig. 1.

The macroscopic morphology of the surface of the laser cladding coatings is shown in Fig. 2. The middle part of the laser cladding sample was used to observe and test the microstructure and mechanical properties of the coatings. In order to study the microstructure of the coating using a small area sample, the sample containing the coating was cut into a size of 10 × 10 × 5 mm³, then mechanically polished with 320—2000 particle size SiC sandpaper, mirror-polished with 0.5–1.5 μm diamond polishing pastes, and finally chemically etched with a Keller reagent (H₂O:65%HNO₃:32%HCl:40%HF = 95:2.5:1.5:1) at room temperature for approximately 10 s. The microstructure and microelement distribution of the laser cladding coatings has been investigated with an SEM (TESCAN MIRA LMS, Czech Republic) equipped with an energy dispersive spectroscope (EDS, Oxford energy spectrum, Britain). The phase identification of laser cladding coatings was carried out by x-ray diffraction (XRD, DX-2700 B, Dandong Haoyuan Instrument Co., Ltd., China) using Cu Kα radiation. The diffraction angle (2θ) varies from 30° to 90°, and the scanning rate is 0.03°/s.

The microhardness of the cross section of the coatings was measured by an HV-50 (Shanghai Shangcai Testing Machine Co., Ltd., China) Vickers microhardness tester with a load of 1.0 N and residence time of 10 s. In each sample, every 100 μm interval continuously selected ten different points to measure the microhardness value, and each sample measures three sets of data on average. The interval is adjusted according to the instrument scale, and the first point is measured from 100 μm from the top of the coating. The indenter is a diamond square cone with a top angle of 136°.

The dry sliding friction and wear test at room temperature was carried out on an HSR-2M (Lanzhou Zhongke Kaihua Technology Development Co., Ltd., China) high-speed reciprocating friction and wear tester. The friction ball is Si3N4, φ is 5 mm, the wear load is 15 N, the friction distance is 3 mm, and the duration is 15 min. The morphology and wear trajectory profile of the cladding layer were observed by a laser scanning microscope (Keyence, VK-X1000, Japan). The 3D optical profiler (ContourGT-X3, Bruker, Germany) was used to analyze the worn surface morphology. The mass change of the sample before and after wear was measured and recorded by a high-precision electronic balance (weighing accuracy: 0.0001 mg).

The three-electrode system of the CHI660E electrochemical workstation (CHI660D; CH Instruments. Company, Ltd., China) was used to evaluate the electrochemical properties, the test sample was the working electrode, the platinum sheet was the counter electrode, the saturated calomel electrode was the reference electrode, and the voltage range was −1 to 1 V. The electrochemical performance of the sample was evaluated by measuring the polarization curve of the sample in simulated body fluid (SBF) at room temperature. The specific surface area of the sample exposed to the electrolyte is 100 mm², and the newly replaced electrolyte was used before each measurement. The configuration sequence, quantity, purity, and the formula of SBF are shown in Table III.³⁵ In the experiment, each group has three parallel samples.

The laser cladding samples were cut into a block of 8 × 8 × 5 mm³. After mechanical polishing with 320–4000 SiC abrasive paper, the surface oil and impurities were removed by ultrasonically cleaning with three solutions (acetone, ethanol, and de-ionized water) for 10 min and drying with a strong blower. The optical contact angle system (JC2000D1, Shanghai Zhongchen Digital Technology Equipment Co., Ltd. China) was used to evaluate the contact angle of SBF on the coating surface at room temperature. The test method was as follows: The needle tube containing SBF solution was placed on the contact angle test system, and 10 μl SBF solution was slowly screwed down on the surface of the sample. The contact angle of the droplets on the surface was measured by elliptic curve fitting after the water droplets were allowed to stand for 10 s.

The biocompatibility of the TixNb alloy coating was evaluated by human osteosarcoma cells (MG-63, Hunan Fenghui Biological Technology Co., Ltd., China). First, the prepared samples were preplaced in a 24-well cell culture plate (Corning, USA) after sterilization and drying in a 120 °C autoclave. Then, the 24-well plate containing the sample was placed on the sterile operating table and further sterilized under ultraviolet light for 24 h. Second, MG-63 cells with good growth were inoculated on the surface of the material in the cell culture plate at the concentration of 5 × 10³ cells/cm³. The negative control group was the group containing only cells and medium, and the positive control group was the specific blank hole that only added medium and did not inoculate cells. Then, the cell culture plate was placed in a 37 °C, 95% humidity CO₂ incubator and replaced with a new medium every 2 days. Modified essential medium (containing nonessential amino acides, Procell Life Science & Technology Co., Ltd., China) supplemented with 10% fetal bovine serum [FBS, Sigma-Aldrich (Shanghai) Trading Co., Ltd., China] and 1% penicillin-streptomycin (Gibco, USA) was used as the medium.

On the first, third, and seventh days of culture, the cells were stained by Calcein fluorescein diacetate (FDA) (Sigma, Germany) staining and incubated for 5–10 min. Then, 20× live cells were photographed under a fluorescence microscope (Olympus IX-73, DP-80 digital camera, Japan, OBJ). The staining formula was 5 mg/μl of FDA mother liquor. Calcein AM was used to label esterase-active substances in live cells and emit green fluorescence. In each hole of the sample, five locations were randomly selected for digital microscopic photography and the images were analyzed using ImageJ software.

The cells were taken out from the incubator and washed with PBS (Sigma, USA) buffer three times after 7 days of cell culture. The cells were fixed overnight with 2.5% glutaraldehyde (Chengdu Jinshan Chemical Reagent Co., Ltd., China). Then, the gradient concentration of ethanol solution was prepared with anhydrous ethanol (ethanol concentration 100%) and distilled water (ethanol concentrations were 30%, 50%, 70%, 90%, 95%, and 100%), and the cells were soaked for 15 min at room temperature for dehydration. After dehydration, the samples were dried in a vacuum drying oven, the surface was coated with gold by a sputtering coating machine, and the cell adhesion morphology was further observed and photographed by scanning electron microscopy.

The XRD patterns of the four groups of TixNb alloy coatings and the TC4 substrate are shown in Fig. 3. Due to the rapid heating and cooling process in the laser cladding process, the coatings were composed of α′, α″, and β phases with the increase in Nb contents under nonequilibrium conditions.^29,36 Nb is a promoting element of the β phase in the titanium alloy,³⁷ which can reduce the β phase transformation temperature and induce the transformation of the microstructure of the coatings from the hexagonal close packed α phase to the α′ and α″ martensite phases (the transition hexagonal structure with high a twist³⁸) and then to the BCC β phase. When Nb is added as a β stabilizer, the β phase remains metastable at room temperature.³⁹ Furthermore, it can also be demonstrated from the XRD analysis that when the element of Nb is added from 12.5 to 27.5 wt. %, the β diffraction peak at 38.5° diffraction peak is significantly enhanced, while the α′ diffraction peak weakened and transformed into α″ phase with the increase in the Nb content. It is mainly composed of the β phase and a small amount of the α″ phase in the TixNb coatings when the Nb content reaches 27.5 wt. %. The dilution effect of the molten pool during the laser cladding process causes the precipitation of Al³⁺ in the TC4 substrate, which promotes the transition of the α phase to the α″ phase in the coating and remains at room temperature.^19,20 In the process of laser cladding, the β phase transformation promoting element Nb and the α phase transformation element Al exist at the same time. As the cladding temperature increases, the alloy undergoes an incomplete phase transformation of α → α′ → α″ → β, forming a β and α dual-phase titanium alloy. At the same time, the cooling rate of the laser cladding process is fast, and the Al element promotes the β → α″ → α′ → α reverse incomplete phase transformation of the titanium alloy during the cooling process. Therefore, the TiNb coating obtained by cladding on the TC4 surface is mainly composed of β, α′, and α″ phases.

The XRD phase shows that Ti22.5Nb coating has more α″ and β phases, so we choose Ti22.5Nb coating as the research object to analyze its microstructure. SEM images of the microstructure of the coatings are shown in Fig. 4. It exhibits irregular α′′ and elongated β columnar crystals in the Ti22.5Nb composition. Inhomogeneous cellular martensite grows around β columnar grains with a width of about 0.1–0.3 μm and a length of about 1–3 μm [Fig. 4(d)].⁴⁰  Figures 4(e)–4(h) show the atomic structure transformation inside the coatings during laser cladding and cooling. Steady-state β phases appear in the TixNb system, which depended on the formation conditions and Nb content.

The EDS spectrum of the Ti22.5Nb coating microstructure is shown in Fig. 5. The results prove that the TixNb alloy coating is relatively uniform without microsegregation. The EDS results of the four groups of coating surfaces are shown in Table IV. It can be seen that with the increase in the Nb content, Al and V on the coating surface decrease. It can be seen that during the laser cladding process, the actual Nb content is lower than the theoretical Nb content due to the oxidation and the precipitation of the substrate composition, which inhibits the β phase transformation to a certain extent. Usually, the mechanical properties and biocompatibility of the β phase are better than that of the α phase and its metastable phase, which can effectively improve the overall performance of the implant. At the same time, the stability of the β phase is determined by the composition of alloy elements.⁴¹ 

The microhardness of the TixNb alloy coatings and the TC4 alloy is given in Fig. 6. It can be analyzed from the figure that TixNb alloy coatings have excellent microhardness compared with the substrate, while the average microhardness increases from 340 to 460–480 HV. When the Nb content is less than 22.5 wt. %, the microhardness increases with the increase in the Nb element content. However, the microhardness decreases when the Nb element content continues to increase to 27.5 wt. %. The results show that Ti22.5Nb has the highest microhardness.

Figure 7 shows the friction and wear coefficient, the three-dimensional morphology of the wear scar, and the wear quality of the coatings and TC4. It can be analyzed from Fig. 7(f) that the average friction coefficient (k) of the coatings is slightly lower than that of TC4. The friction coefficient k can be used to describe the coefficient of the wear resistance of the coating. The smaller the friction coefficient is, the more wear-resistant the coating. Both the coatings and the substrates showed abrasive wear. The wear resistance of the coating is improved due to the emergence of a highly strong uniform β phase in the coatings. The wear quality of coatings with different Nb contents was lower than that of TC4. The friction coefficient k and wear scar width of the coatings and the substrate are shown in Table V.

The average friction coefficient and wear scar width of coatings are lower than those of TC4. The 3D profile shows that the surface wear trace of TixNb alloy coatings is elliptical and the wear trace of the width of the TC4 surface is larger than that of TixNb alloy coatings. It can be shown in the figure that the friction coefficient curves of Ti12.5Nb, Ti17.5Nb, and Ti27.5Nb are stable. The results indicated that the average friction coefficient, average wear loss, and wear scar width of Ti12.5Nb and Ti27.5Nb are the lowest. Compared with the substrate, the friction coefficient decreased by 0.137 and 0.149, and the wear scar width decreased by 0.078 and 0.089. Therefore, the wear rate of the Ti12.5Nb and Ti27.5Nb coatings is lower, which has excellent wear resistance. At the same time, Ti22.5Nb has the highest microhardness, but the friction and wear coefficients are larger. It may be due to the adhesive wear on the surface of the grinding ball caused by the residual wear debris with strong hardness during the wear process, resulting in the maximum wear quality. However, its wear rate is still lower than that of TC4, so the wear resistance of Ti22.5Nb coating is still higher than that of TC4. The results show that with the increase in the Nb element, the wear resistance first increases and then decreases. Ti12.5Nb has excellent wear resistance. With the increase in the Nb content, the wear resistance decreased of the coating. The wear resistance is enhanced when the Nb content increases to 27.5 wt. %.

The corrosion characteristic of the coatings and TC4 in SBF was evaluated by the dynamic potential polarization method. Figure 8 shows the electrochemical polarization curves and impedance spectra of the coatings and the TC4 in the potential range of −1 to 1 V in SBF. All polarization curves show similar general characteristics, indicating that the corrosion process of all alloys is analogous during the increase in potential. Table VI lists the self-corrosion potential (E_corr) and self-corrosion current density (I_corr) of the coatings and substrates. Compared with TC4, the coatings have higher E_corr and lower I_corr in the SBF solution. I_corr decreases with the increase in the Nb content. Combined with the Tafel curve in Fig. 8(a) and Table IV, it can be seen that Ti27.5Nb has the smallest I_corr [(1.68 ± 0.03) × 10⁻⁷ μA/cm³]. The E_corr of the coating is about −0.5 V, which is much lower than that of TC4 (−1.23 V). At the same time, with the increase in the Nb content, the passivation of the four groups of TixNb coatings showed a widening trend, which proved that the addition of the Nb content effectively enhanced the corrosion resistance of the coatings.

To further analyze the anticorrosion mechanism and anticorrosion ability, electrochemical impedance spectroscopy (EIS) was performed for the TC4 substrate, TixNb coatings using the analog circuit shown in Fig. 8(b).⁴²  Figure 8(b) shows that the radius of the TiNb coating is much larger than that of the composite coating, indicating that the TiNb coating is much larger than the composite coating. As shown in Fig. 8(c), the Bode plots of the TiNb alloy coatings with different Nb contents have only one time constant, indicating that a similar chemical reaction occurred on the surface of the coating. In addition, in the high frequency region, the phase angle of the TiNb coating is higher than that of TC4, which also shows that the TiNb coating has good corrosion resistance. The |Z|_{0.01 Hz} impedance modulus corresponding to the 0.01 Hz value is used to evaluate the barrier characteristics of the passivation film. Obviously, the |Z|_0.01 Hz of the TiNb coating is higher than that of TC4, indicating that the TiNb coating has better shielding performance, as shown in Fig. 8(d). In summary, the TiNb coating can greatly improve the corrosion resistance of the TC4 surface.

It can be seen from Fig. 8(b) that the impedance radius of all coatings is larger than that of the TC4 substrate. With the increase in the Nb content, the impedance radius shows an upward trend. In addition, from Table VII, it can be seen that with the increase in Nb contents, R_f increases from 14.3 to 27.83 Ω cm² and is much larger than 13.6 Ω cm² about TC4, and C_f (F cm⁻²) also shows a decreasing trend. In general, the coatings have better corrosion resistance than that of the TC4 substrate.

Figure 9 shows the typical SEM morphology and EDS spectrum of the coating surface after corrosion at low magnification. Some pore regions were observed on the coating’s surface. Those pores are irregular in shape and vary in size between 1 and 5 μm. Figure 10 shows the EDS diagram of the corroded surface. It is exhibited that the surface O element content increased significantly. Concurrently, the oxygen content increased to more than 50 wt. % in the corrosion pit.

Contact angle measurement is a convenient method for evaluating the hydrophobicity of materials. The contact angle of the sample surface was measured by SBF. Figure 11 shows the comparison of the SBF contact angle between coatings and the TC4 surface. As shown, the SBF contact angles of TixNb alloy coatings are about one-third lower than those of TC4. The decrease in the surface contact angle and the increase in the hydrophilicity are beneficial to cell adhesion, proliferation, and differentiation.

Figure 12 shows the proliferation and number of adherent cells under a fluorescence microscope on days 1, 3, and 7. Figure 13 shows the cell proliferation on days 1, 3, 5, and 7. The fluorescence image was combined with cell proliferation to analyze the experimental results. As shown in the figure, the cells on the coating’s surface are uniformly distributed in spindle shape. On the first day, the cells were evenly distributed on the surface and spread spindle shape on the surface, which proved that the cells effectively adhered and grew after inoculation. The cell density on the coating’s surface is slightly higher than that on the substrate’s surface. Cell proliferation data also showed that the cell proliferation rates of the four groups of the coating’s surface were higher than those of the substrate and negative control groups. On the third day, four groups of TixNb alloy coating surface cells proliferated and covered entirely the surface. There is still a gap between the cells on the TC4 surface compared with the observed coatings. On day 7, the coatings and substrate surfaces were completely covered by cells. However, the OD value analysis showed that the cell proliferation rate of the coating’s surface was still higher than that of the substrates. Cell proliferation showed higher fecundity on TixNb coatings, and Ti22.5Nb and Ti27.5Nb showed good cell compatibility.

Within 7 days of culture, the cell survival rate and proliferation rate of MG-63 cells on the surfaces of TixNb alloy coatings were excellent. The cell’s coverage area increased significantly with the extension of time, representing that the coatings can promote cell proliferation and good biocompatibility. The cells on the coating’s surface were uniformly distributed and grew well, showing an obvious osteoblastlike morphology and spindle-shaped growth along the grinding direction.

To analyze the cell compatibility of the samples, the cell adhesion morphology was studied. Figure 14 shows the typical morphology SEM image of MG-63 cells cultured on TixNb alloy coatings and the TC4 surface for 7 days. Four groups of TixNb alloy coatings’ surface cell proliferation, adhesion, and expansion effect are more excellent compared with TC4 groups. The results of the direct observation of the cell adhesion morphology were consistent with those of the indirect measurement of cell viability. Ti and Nb alloys are considered to be biocompatible heavy metals, which have been used in orthopedic and medical devices. Ti and Ni form a stabilization oxide film on the coating’s surface, effectively inhibiting the release of metal ions. Consequently, TixNb alloy coatings were nontoxic and have good biocompatibility.

When T_C> T₀, β → α′ transition occurs. The higher the instantaneous temperature is, the greater the driving force. The instantaneous temperature in the molten pool upgrade with the increase in the energy density, increasing the driving force of the martensite transformation; that is, the content of the α′ phase is increased with the enhancement of martensite transformation. For all that, the increased heat is sufficient to melt more Nb particles with the continuous increase in the energy density, thus promoting the formation of more β (Ti, Nb) solid solubility and inhibiting the transformation of β → α′′ → α′ during cooling. The interaction between the two promotes the formation of the β phase. In summary, with the addition of the Nb content, β phase transformation was promoted and α phase transformation was inhibited.

The microstructure analysis shows that the alloy will undergo martensite transformation under rapid cooling when the molybdenum equivalent is less than 10 wt. %. All β phases are retained in metastable when the molybdenum equivalent was greater than 10 wt. %. For TixNb alloy coatings, the β phase is more stable in the coatings with higher Nb content. There are two main factors affecting the growth of α particles. The first factor is the distribution of Nb solute atoms. The second factor is related to β-trans temperature. Nb is a β-stable element, which can reduce the β-trans temperature. Thus, when the Nb content is high, α has a lower transition temperature. With the increase in the Nb content, the β-trans temperature decreases. Simultaneously, the phase transformation diffusion process slowed down, inhibiting the growth of α precipitates, so the martensite structure was refined.⁴⁵ Higher energy density is needed to melt Nb powder in the process of laser cladding due to the Nb element has a higher melting point than the Ti element. Hence, large Nb particles are not completely melted and their diffusion is limited in the laser cladding process due to insufficient input energy or a fast cooling rate. Nb particles melted under the same energy input have higher dynamic viscosity, resulting in weak diffusion. Therefore, the enrichment of Nb in the molten pool is also one of the reasons for the precipitation of the α phase. In this study, due to the high energy density of the laser cladding process, enough Ti and Nb can be melted to make their coating surface uniformly distributed. Therefore, with the increase in the Nb content, the β-phase transformation promoting element Nb and the α-phase transformation element Al simultaneously affect, forming a TiNb alloy coating mainly composed of β, α′, and α″ phases.

On the one hand, the rapid heating and cooling in the laser cladding process produce the solid solution strengthening effect.²⁹ On the other hand, the β phase with better mechanical properties was formed in the laser cladding process. The addition of the Nb content reduced the β phase transformation temperature and increased the β phase content in the coatings. It is consistent with the previous research that the β phase strengthening increases the hardness of the coatings.³⁷ Furthermore, the increase in microhardness can be attributed to interstitial oxygen liquid strengthening.⁴⁵ The EDS results in Table IV show that the oxygen content of the coatings can attain 5–6 wt. %. Oxidation during the laser cladding process increases the interstitial oxygen content. Therefore, the TiNb coating composed of a large amount of the β phase and a small amount of α′ and α″ phases has stronger hardness. At the same time, as the content of the β phase increases with the content of Nb, the wear resistance of the coating is enhanced.

It is well known that the complex heating process in laser cladding coatings will cause stress concentration,⁴⁶ and the molten pool boundary and cladding coatings defects of laser cladding coatings are the weakest parts. Therefore, corrosion preferentially occurs at the molten pool boundary (stress concentration area) or the manufacturing defect (usually the hydrolysis reaction with high concentrations of H⁺ and Cl⁻).⁴⁷ These defects play a role in the formation of a microanode in the primary battery during the corrosion process. Accordingly, electrochemical corrosion preferentially occurs at the oxidation pores with the increase in the voltage, creating corrosion pits. The electrochemical polarization curve also showed obvious pitting corrosion. With the increase in the voltage, the passivation film is broken down from the defect.

In the redox reaction process, after Ti preferentially corroded, Nb and O react to form Nb₂O₅ with high corrosion resistance. The corrosion is inhibited; thus, the passivation area of the electrochemical polarization curve is obvious. During the corrosion process, the Nb-rich region in the coatings and the Ti substrate will form a substantial microprimary battery. It is reported that incomplete melting of Nb particles in the molten pool as a microcathode accelerates the corrosion of the microanode (Ti), resulting in preferential corrosion at Nb grain boundaries.⁴⁸ It can also be explained that by reducing the Ti content or increasing the Nb content, the Ti/Nb mass ratio is close to 1 to improve the pitting potential and achieve better corrosion resistance.⁵⁰ The corrosion principle is shown in Fig. 15. Figures 15(a)–15(d) show the corrosion steps 1, 2, 3, and 4, respectively. Step 1 indicates that the alloy coating has not reacted with the electrolyte. In step 2, due to the microcurrent effect, Ti preferentially reacts to form a defect oxide film. The oxide film is a point defect formed at the metal-film and film-solution interfaces (step 2). O²⁻ migration to the metal-film interface due to defect migration produces new oxides (step 3) and thickens the oxide film (step 4). The oxide film grew with the increase in electrochemical potential during the test. At this stage, Ti is hydrolyzed into TiO₂ in solution.⁵¹ Simultaneously, Nb is dissolved from the TixNb matrix to form Nb₂O₅.⁴⁹ 

The anodic branch of polarization curves exhibits typical passivation characteristics in all TixNb alloy coatings. Electrochemical impedance spectroscopy results showed that with the increase in the Nb content, the low-frequency impedance spectrum radius increased, indicating that the corrosion resistance is considerably enhanced. It is reported that when the metal is immersed in corrosive medium containing gas, the passive current density is less than 100 μA/cm² (Refs. 52 and 53) and the metal will be passivated spontaneously. In this study, the passive current density of the alloy was less than 100 μA/cm², so the coatings were passivated spontaneously during the corrosion process, and the corrosion resistance was significantly improved. All in all, these alloys have high corrosion resistance and are highly likely to be passivated in human environments. In summary, the Ti and Nb elements in the TiNb alloy coating can form a large number of TiO₂ and Nb₂O₅ passivation films during the corrosion process, which makes these alloys have high corrosion resistance. Alloys with a passivation film will enhance their corrosion resistance when implanted into the human environments.

The adhesion mechanism of cells on the coating surface is shown in Fig. 16. The first step [Fig. 16(a)] indicates that the coating surface contains orientation scratches. The second step [Fig. 16(b)] represents the initial state of cell inoculation. The third step [Fig. 16(c)] indicates that the cell pseudopodia begin to adhere to the surface of the coatings. The fourth step [Fig. 16(d)] represents cell proliferation. The cells on the surface of TC4 and TixNb alloy coatings are stretched polygonal with many pseudopods and cellular plasticity. The surface of the coatings forms stable TiO₂ and Nb₂O₅ with Ti and Nb elements due to the oxygen contained in the laser cladding process, which has high biological activity and large liquid adhesion. Hence, the cells have better activity on the surface of the TixNb alloy coatings.

The attachment, proliferation, and differentiation of bone-derived cells on metal materials depend on the physical and chemical properties of the material surface. Cell adhesion is mediated by extracellular matrix (ECM) molecules, which are affected by surface properties such as surface energy, polarity, charge, wettability, morphology, and roughness. The specific properties of the material lead to protein adsorption at a specific position on the surface of the material, which is then recognized and combined by cell adhesion receptors.⁵⁴ Chemical composition on the surface of materials is another important factor affecting cell adhesion. The performance of the β-TixNb alloy coating is related to the existence of various oxygen-containing functional groups, which increases the surface energy, polarity, and wettability.⁵⁵ Therefore, the TixNb alloy with good wettability and low toxic ion content has high biocompatibility in theory. In summary, since the laser cladding coating reduces the content of the Al³⁺ and V⁺ as well as increases the hydrophilicity of the surface, the laser cladding coating exhibits more excellent biocompatibility.

In summary, TixNb alloy coatings were prepared using laser cladding technology on the surface of TC4 and applied to the field of bio-implant materials. It had excellent mechanical properties and corrosion resistance, as well as higher biocompatibility. First, the content of Al³⁺ and V⁺ in TixNb alloy coatings decreased obviously, and the surfaces were smooth without obvious defects and cracks. The microstructure mainly shows a small amount of cellular α″ martensite uniformly precipitated around the elongated β grains. Second, the rapid heating and cooling process of the laser cladding process promotes the formation of more β phases. The increase in the oxygen content in the cladding process forms more stable oxides with Ti and Nb elements, which effectively improves the microhardness and wear resistance of the TixNb alloy coating. In addition, during the SBF electrochemical corrosion process, the addition of the Nb element in the coatings induces the formation of a dense and stable passivation film on the surface, which effectively inhibits the corrosion of the coatings. Finally, in vitro cell experiments demonstrated that the cells cultured on the coatings had eximious cell viability and proliferation rate forasmuch as the good wettability of the surface and the formation of TixNb oxide bioactive sites on the surface, which was beneficial to the adhesion of cell protein mechanism on the surface. Therefore, the TixNb alloy coatings with excellent comprehensive performance were obtained by laser cladding technology, exhibiting more dramatic mechanical properties, corrosion resistance, and biocompatibility.

This study was funded by the Guizhou Provincial Science and Technology Foundation (QKHJC ZK [2021] general 241), the Natural Science Research Project of Guizhou Provincial Education Department (QJH KY Z [2021] 098), and the Fostering Projects of Guizhou University ([2020] 66).

The authors have no conflicts to disclose.

Yu Zheng: Formal analysis (equal); Methodology (equal); Writing – original draft (equal). Peng Xu: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal). Long Li: Supervision (equal); Writing – review & editing (equal). Qibin Liu: Supervision (equal). Shanzhu Guo: Methodology (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.