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Surface modifications of biometallic commercially pure Ti and Ti–13Nb–13Zr alloy by picosecond Nd:YAG laser

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  • Corresponding author:

    Ivana Cvijović-Alagić    E-mail: ivanac@vinca.rs

  • Received: 21 February 2020Revised: 7 April 2020Accepted: 9 April 2020Available online: 12 April 2020
  • The effects of picosecond Nd:YAG laser irradiation on chemical and morphological surface characteristics of the commercially pure titanium and Ti–13Nb–13Zr alloy in air and argon atmospheres were studied under different laser output energy values. During the interaction of laser irradiation with the investigated materials, a part of the energy was absorbed on the target surface, influencing surface modifications. Laser beam interaction with the target surface resulted in various morphological alterations, resulting in crater formation and the presence of microcracks and hydrodynamic structures. Moreover, different chemical changes were induced on the target materials’ surfaces, resulting in the titanium oxide formation in the irradiation-affected area and consequently increasing the irradiation energy absorption. Given the high energy absorption at the site of interaction, the dimensions of the surface damaged area increased. Consequently, surface roughness increased. The appearance of surface oxides also led to the increased material hardness in the surface-modified area. Observed chemical and morphological changes were pronounced after laser irradiation of the Ti–13Nb–13Zr alloy surface.
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Surface modifications of biometallic commercially pure Ti and Ti–13Nb–13Zr alloy by picosecond Nd:YAG laser

  • Corresponding author:

    Ivana Cvijović-Alagić    E-mail: ivanac@vinca.rs

  • 1. Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia
  • 2. Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia

Abstract: The effects of picosecond Nd:YAG laser irradiation on chemical and morphological surface characteristics of the commercially pure titanium and Ti–13Nb–13Zr alloy in air and argon atmospheres were studied under different laser output energy values. During the interaction of laser irradiation with the investigated materials, a part of the energy was absorbed on the target surface, influencing surface modifications. Laser beam interaction with the target surface resulted in various morphological alterations, resulting in crater formation and the presence of microcracks and hydrodynamic structures. Moreover, different chemical changes were induced on the target materials’ surfaces, resulting in the titanium oxide formation in the irradiation-affected area and consequently increasing the irradiation energy absorption. Given the high energy absorption at the site of interaction, the dimensions of the surface damaged area increased. Consequently, surface roughness increased. The appearance of surface oxides also led to the increased material hardness in the surface-modified area. Observed chemical and morphological changes were pronounced after laser irradiation of the Ti–13Nb–13Zr alloy surface.

1.   Introduction
  • Titanium-based materials are widely used as hard-tissue replacements due to their excellent mechanical and tribo-corrosive properties combined with titanium’s low-ion release tendency [113]. When exposed to air, titanium can passivate and form a stable corrosion-protective surface oxide film favorable for hard-tissue implant osseointegration [56,1314].

    Prior to application, the surface of titanium-based materials as implants must be additionally processed, and implant biocompatibility and osseointegration must be improved by optimizing the surface chemistry, surface roughness, and surface topography of implants [1522]. Several surface treatments are used for such purpose; however, laser irradiation is considered as the most promising method [1522].

    Different research groups studied the morphological changes that occur during the laser irradiation interaction with the metallic surface and their effect on hard-tissue implant properties [2245]. Gnilitskyi et al. [34] and Götz et al. [35] observed that laser treatment is a highly desirable technique because it improves bone cell adhesion and proliferation, as shown by the appearance of highly oxidized surface films and open pores, which allow bone ingrowth during the early osseointegration stages. Moreover, Hao et al. [38] showed that laser surface treatment not only improves surface roughness but also increases the surface oxygen content and surface energy generation, leading to a high work adhesion with biological fluids. Chan et al. [44] showed that the laser surface modification of β-type Ti–Nb–Zr–Ta alloys resulted in the appearance of a hard wear-resistant surface layer, which improved the material corrosive behavior. The corrosion resistance of the Ti–6Al–4V (wt%) implant alloy in simulated physiological conditions is increased by excimer laser surface treatment in argon atmosphere as a result of elemental microstructural partitioning [42].

    The laser irradiation method is a contactless technique; therefore, direct surface contamination during the material surface modification cannot occur [1718]. Moreover, laser beam interaction with the metallic surface is often accompanied by plasma formation, leading to additional surface sterilization which is beneficial to the applicability of a material for medical purposes [40]. The most commonly used material processing lasers are CO2 laser, Nd:YAG laser, diode, and other lasers operating in the impulse mode, although the typical pulse duration may vary [1921]. Nd:YAG laser is a type of a solid-state laser that uses a coherent photon beam as the source of irradiation, and it can emit radiation from the infrared region through visible to ultraviolet regions and therefore can be successfully used for the modifications of biometallic material surfaces [24].

    The interaction of laser irradiation with metallic targets involves a series of complex processes that depend on the characteristics of laser irradiation, the physical and chemical properties of the target, and surrounding gas composition and pressure [2533]. Recent studies in this field were directed to the usage of diverse laser pulse regimes (nano-, pico-, and femtosecond) to achieve the optimal surface morphologies of Ti-based targets; the obtained results indicated that the usage of picosecond laser radiation shows potential regarding the formation of different surface hydrodynamic features favorable to biomedical applications of such materials [19,46].

    Commercially pure titanium (CP-Ti) is widely used in orthopedic and dental implantology, and improvement of its characteristics is considered to be a cost-effective means to achieve durable hard-tissue replacements [14,8]. On the other hand, the Ti–13Nb–13Zr (wt%) alloy is a promising metallic material for high-quality implants with enhanced properties because this β-type alloy shows significantly excellent mechanical biocompatibility combined with the simultaneous elimination of possible cytotoxic reactions [35,1213,4750]. Given such properties, the present research aimed to investigate an easy-to-apply method for the enhancement of CP-Ti and Ti–13Nb–13Zr alloy with tribo-corrosive, biocompatible, and osseointegration properties utilizing the Nd:YAG laser irradiation.

2.   Experimental
  • Laser irradiation was performed on CP-Ti and Ti–13Nb–13Zr (wt%) alloy samples. Grade 2 CP-Ti was supplied by Goodfellow, Germany, whereas Ti–13Nb–13Zr alloy was laboratory-produced at the Vinča Institute of Nuclear Sciences from 99.9% pure titanium, niobium, and zirconium (A.D. Mackay Inc., Denver, USA) by arc melting in a protective atmosphere and subsequent thermomechanical treatment. For the present investigation, Ti–13Nb–13Zr alloy was in hot-rolled condition, and its production route was explained in detail elsewhere [5,12]. Previous investigations revealed that the CP-Ti used in this study is a single α-phase material with equiaxed α grains present in the microstructure [8], whereas the hot-rolled Ti–13Nb–13Zr alloy is characterized with an acicular “basket weave” microstructure with martensitic α' plates present within the large prior β grains [1213].

    Before irradiation, the CP-Ti and Ti–13Nb–13Zr alloy samples were metallographically prepared, i.e., they were ground with SiC paper, polished with diamond suspension, cleaned in an ultrasonic bath with ethanol, and dried in air. Laser surface modification was performed using the Nd:YAG system EKSPLA SL 212/SH/FH operating at 1064 nm wavelength in the fundamental transverse mode (TEM00 mode) under different gas atmospheres, i.e., air and a flow atmosphere enriched with argon. The shot-to-shot energy variation was ≤5%. The target surface was irradiated by focusing the laser beam with a quartz lens of 20 cm focal length on the sample surface area with a diameter of 0.03 cm. The angle of incidence of the laser beam with respect to the sample surface was 90°. Table 1 presents the laser output parameters used in this study.

    Irradiation atmosphereAir; argon
    Laser output energy / mJ5; 30
    Laser pulse duration / ps150
    Number of accumulated laser pulses50
    Irradiation time / s5
    Repetition rate / Hz10

    Table 1.  Parameters of laser irradiation of CP-Ti and Ti–13Nb–13Zr alloy samples using the Nd:YAG system

    Surface morphology was analyzed by field-emission scanning electron microscopy (FE-SEM; TESCAN Mira3 XMU operated at 20 kV). Variation in the surface composition with laser irradiation parameters was observed using energy dispersive spectrometry (EDS; Oxford Inca 3.2 coupled with the SEM JEOL JSM 5800 operated at 20 kV). Estimation of the surface damage degree and surface topographic modifications obtained during irradiation was performed by noncontact optical profilometer ZYGO NewView 7100.

    The possible effect of laser irradiation on the appearance of a hard wear- and corrosion-resistant surface layer was estimated by conducting surface microhardness measurements. The microhardness was determined before and after laser surface modifications using a Vickers indentation hardness tester (Buehler Micromet 5101) by applying a load of 1961 mN (200 gf) for 10 s. For the laser-irradiated samples, microhardness indentations were made in the central zone of the laser-modified area. All Vickers microhardness measurements were conducted in triplicate with excellent reproducibility.

3.   Results and discussion
  • During the interaction of the laser beam with the surface of a solid body, part of its energy is reflected, and another part is absorbed. Absorbed laser irradiation energy causes chemical and morphological changes in the surface layer, influencing the implant biointegration properties.

  • In the present study, at a specific laser wavelength, the pulse duration and the number of applied pulses were maintained at fixed values; changes in the target surface morphology depend on the presence of the reactive gas atmosphere and laser fluence at energies above the damage threshold [31]. Figs. 18 present the morphological modifications that emerged on the surface of investigated titanium-based implant materials after laser irradiation under different irradiation conditions.

    Figure 3.  (a, c) 3D and (b, d) linear profilometric analyses of the CP-Ti surfaces after irradiation in air under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Figure 4.  (a, c) 3D and (b, d) linear profilometric analyses of the CP-Ti surfaces after irradiation in argon under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Figure 6.  FE-SEM micrographs showing the surface damage of the Ti–13Nb–13Zr target during the irradiation in argon under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Figure 7.  (a, c) 3D and (b, d) linear profilometric analyses of the Ti–13Nb–13Zr alloy surfaces after irradiation in air under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Figure 8.  (a, c) 3D and (b, d) linear profilometric analyses of the Ti–13Nb–13Zr alloy surfaces after irradiation in argon under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Fig. 1 shows the morphology of the CP-Ti target surface after irradiation in air under different laser pulse energies.

    Figure 1.  FE-SEM micrographs showing the surface damage of the CP-Ti target during irradiation in air under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Under low-light conditions, the alterations caused by laser interaction with the target surface were superficial (Figs. 1(a) and 1(b)). The low laser pulse energy and short-term irradiation were insufficient to induce the formation of a distinct crater on the CP-Ti surface. However, these irradiation parameters were enough to cause the formation of low-level hydrodynamic effects and visible microcracks on the outskirts of the damaged area (Fig. 1(b)). The initially small damage, which was caused by low-energy laser irradiation, can assume the shape of the crater by increasing the irradiation time and/or laser pulse energy and the number of accumulated pulses [28]. In the case of the CP-Ti surface, the increased laser pulse energy in the air atmosphere caused rapid melting of the target material and formation of a crater with melted and solidified surrounding area (Figs. 1(c) and 1(d)). Increased laser pulse energy also resulted in hydrodynamic effects in the form of periodic wave-like structures and pronounced microcracks in the damaged area (Fig. 1(d)). Microcracks with approximately 0.2–0.3 µm width probably emerged on the laser-treated surface due to the rapid cooling of the melted area after irradiation [31].

    Fig. 2 presents the FE-SEM micrographs showing the morphology of the CP-Ti target surface after irradiation in argon atmosphere.

    Figure 2.  FE-SEM micrographs showing the surface damage of CP-Ti target during irradiation in argon under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    Short-term irradiation of the CP-Ti target with low output energy resulted in the melting of the target material in the central zone of the irradiated area and shallow crater formation (Figs. 2(a) and 2(b)). The spherically shaped rise of the material, centrally located in the crater area, was visible and probably arisen from the pool of molten material that formed due to the high level of absorbed energy exceeding the melting latent heat at the damage spot [40]. Microcracks were also observed around the damaged area (Fig. 2(b)). An increase in the laser pulse energy led to the formation of a pronounced crater with microcracks and periodic wave-like structures in the form of ripples as a result of the melt material pool creation and spillage of the liquid metal toward the periphery (Figs. 2(c) and 2(d)) [30].

    Characterization of the laser irradiation-modified and damaged surfaces was additionally carried out using optical profilometry. Three-dimensional (3D) and linear profilometric analyses have been utilized to determine the effect of laser irradiation conditions on the resulting surface area modifications. The laser-induced morphological changes of CP-Ti in air atmosphere under low-laser-output energy conditions were reflected in the superficial surface modifications of the material and appearance of surface damage up to 1.5 µm below the material surface (Figs. 3(a) and 3(b)).

    The 3D and linear damage profiles obtained on the CP-Ti surface irradiation-affected area confirmed that no distinct crater was formed during the irradiation in air with lower laser output energy (Fig. 3(a)). However, the target material visibly accumulated in the affected area. The results of profilometric analysis indicate that the surface damage caused by short-term interaction of laser irradiation of the CP-Ti surface was almost superficial with the damaged area width of ~300 µm (Fig. 3(b)). However, an increase in the laser pulse energy led to the formation of distinct and deep surface damage in the shape of a crater with 7 µm depth, resulting from the high ablation rate (Figs. 3(c) and 3(d)). The energy increase also caused the pronounced ejection of the material, which accumulated at the periphery reaching 5 µm in height and led to an increase in the damaged area diameter to ~450 µm (Fig. 3(d)). These results are in accordance with the FE-SEM observations (Fig. 1).

    In consideration of the laser irradiation of the CP-Ti sample in argon atmosphere, profilometric analysis confirmed the appearance of deeper damage features under the same irradiation conditions than after the irradiation in air (Fig. 4).

    Craters (5 and 10 µm deep) with diameters of 300 and 400 µm formed during the laser irradiation under output energies of 5 and 30 mJ, respectively (Figs. 4(b) and 4(d)). However, the laser modification conducted in different surrounding atmospheres caused no major changes in the crater diameter (Figs. 3 and 4). Comparison of the surface alterations achieved during the irradiation in air and argon atmosphere shows that under the same irradiation conditions (laser pulse energy and irradiation duration), the damage induced by irradiation in argon was characterized by pronounced craters with higher depth. Thus, as a consequence of the irradiation in argon atmosphere, craters with different characteristics (dimensions and shapes) were formed, confirming the obtained FE-SEM findings (Fig. 2).

    Figs. 5 and 6 present the laser-modified surfaces of the Ti–13Nb–13Zr alloy after the irradiation in air and argon, respectively. The FE-SEM analysis of the Ti–13Nb–13Zr alloy surface showed that during alloy irradiation in air with a low pulse energy, the alloy surface morphology changed slightly, and damage in the central zone of the irradiated area appeared in the form of microcracks and minor hydrodynamic effects (Figs. 5(a) and 5(b)).

    Figure 5.  FE-SEM micrographs showing the surface damage of the Ti–13Nb–13Zr target during the irradiation in air under laser output energies of (a, b) 5 mJ and (c, d) 30 mJ.

    With the increase in irradiation energy, a more distinct surface crater with melted and solidified material formed in the peripheral zone (Figs. 5(c) and 5(d)). The increase in irradiation energy caused the appearance of an increased number of distinct surface damage features in the form of microcracks on the irradiated alloy surface (Fig. 5(d)).

    The irradiation of the investigated Ti-based alloy surface in argon also resulted in the formation of microcracks, craters, and hydrodynamic effects in the form of periodic wave-like structures, which can be observed in the FE-SEM micrographs in Fig. 6. Comparison of the achieved surface modifications during the laser irradiation in air and argon atmosphere under the same irradiation conditions led to the conclusion that laser modification in argon resulted in the pronounced surface damage of the investigated alloy with pronounced craters, accompanied with an increased number of microcracks and highly visible hydrodynamic effects.

    As shown by Götz et al. [35], the increased surface roughness and texture of metallic biomaterials influence the improvement of cell adherence, bone–implant contact, and biomechanical interaction. Furthermore, the study conducted by Götz et al. [35] demonstrated that although laser-induced surface pores (craters) with at least 100 µm size can serve osteon formation in the rabbit transcortical model, the process of bone remodeling within the pores is delayed due to the decreased mechanical stability occurring in the small pores. Thus, laser irradiation parameters should be optimized in such a manner to ensure the obtainment of large surface pores (~300 µm), which will result in high bone–implant contact and mechanical stability within the first weeks after implantation.

    Given these goals, a profilometric analysis of the irradiated Ti–13Nb–13Zr alloy surface was conducted and showed that the irradiation of the alloy surface resulted in distinctive and versatile surface features (Figs. 7 and 8).

    Low-energy and short-term alloy irradiation in the air resulted in the formation of shallow craters (up to 2 µm in depth), with accumulated material in the outer sections of the modified area with a maximal height of 4 µm (Figs. 7(a) and 7(b)). Moreover, the presence of the spherically shaped rise of the material in the central damage spot area with a height of 1 µm and 0.2 mm in diameter was recorded. The increased laser pulse energy led to the formation of deep surface craters and enhancement of the damaged area dimensions (Figs. 7(c) and 7(d)). A 10 µm maximal crater depth was recorded by the optical profilometer after irradiation of the Ti–13Nb–13Zr alloy in air, whereas the maximal recorded crater diameter was approximately 400 µm. Fig. 8(b) shows that short-term alloy irradiation in argon with low laser pulse energy resulted in the appearance of a crater with a high depth that reached 5 µm. Crater width (~350 µm) was immutable after irradiation in both atmospheres (Figs. 7(a) and 8(a)). Thus, the irradiation atmosphere caused no effect on the crater width in this case. On the other hand, the laser pulse energy directly influenced the width of the formed crater, that is, the increase in output energy resulted in the increased crater diameter (Figs. 8(a) and 8(c)). A 13 µm maximal crater depth was observed after the laser modification of the Ti–13Nb–13Zr alloy in argon atmosphere, whereas the diameter was approximately 400 µm (Fig. 8(d)). The obtained results indicate that the high damage degree of the investigated alloy surface along the target depth occurred during irradiation in argon atmosphere (Figs. 8(b) and 8(d)).

    The recorded CP-Ti and Ti–13Nb–13Zr alloy surface modifications achieved under the same irradiation conditions indicated that the highest degree of damage along the target depth was achieved during the irradiation of the Ti–13Nb–13Zr alloy surface.

    Based on the obtained results, the hydrodynamic effects observed on the surface of the investigated Ti-based materials in the central and peripheral zones of the crater areas (Figs. 1, 2, 5, and 6) resulted from the high temperatures achieved by rapid heating of the small target surface [20]. In addition, the obtained results indicate that the laser pulse action on the implant material surfaces improved the surface roughness (Fig. 9). Thus, during the irradiation of CP-Ti and Ti–13Nb–13Zr alloy, the increase in laser pulse energy resulted in the increased surface roughness. The obtained results also showed that the increase in surface roughness was more pronounced after the laser irradiation in argon atmosphere irrespective of the target material composition. The increase in the implant material surface roughness is favorable to the enhancement of biointegration and can diminish the possibility of implant rejection after surgical implantation in the human body [28,33].

    Figure 9.  Target surface roughness variation as a function of irradiation parameters.

  • Variations in the chemical composition of the crater area, which were induced on the investigated implant materials surfaces depending on the irradiation conditions, were analyzed by EDS and are summarized in Table 2. Results of the EDS analyses indicate that in addition to morphological alterations, the interaction of laser irradiation with the CP-Ti and Ti–13Nb–13Zr alloy surfaces was accompanied by various chemical reactions, changing the chemical compositions of the target surfaces. Oxide formation in the laser-modified area was analytically confirmed, whereas nitrogen was not detected at the surface of the investigated Ti-based materials after laser irradiation in air irrespective of the target material composition (Table 2). Moreover, the interaction of the laser beam with the CP-Ti and Ti–13Nb–13Zr alloy surfaces was accompanied by plasma formation, which is desirable when a contaminant-free surface should be ensured.

    Implant materialIrradiation atmosphereLaser output energy / mJChemical composition of the central crater area / wt%
    TiONbZr
    CP-TiAir586.7113.29
    3080.9319.07
    Argon588.4811.52
    3089.5310.47
    Ti–13Nb–13ZrAir562.6314.599.5313.25
    3067.1213.225.3714.29
    Argon571.396.608.6813.33
    3071.477.448.5012.59

    Table 2.  EDS elemental analyses of the central crater areas on the surfaces of CP-Ti and Ti–13Nb–13Zr alloy samples after irradiation treatment

    In general, the obtained EDS results show that irradiation in air resulted in passivated titanium oxide film formation on the surface due to the high titanium reactivity. The presence of oxides, especially titanium dioxide, on the implant material surface is favorable because it improves implant material osseointegration in the human body [13,17,19]. Thus, the induced surface oxygen content increase by laser treatment governs the increase in the biometallic wettability and improves osteoblast cell adhesion and proliferation [38]. On the other hand, the formation of considerable amount of titanium oxide film on the surface of the Ti-based target during the laser modification in argon was unexpected. However, the EDS analysis showed the significant presence of oxygen in the central zone of the irradiated area. The detection of oxygen on the target surface indicated the TiO2 surface layer formation in argon atmosphere and suggested that laser irradiation stimulated the material surface oxidation that is favorable for hard-tissue implant biointegration.

    Furthermore, the results of EDS analysis revealed that during the irradiation of CP-Ti in air, the oxygen content in the damaged surface area changed with the change in laser output energy. Thus, with the increase in laser pulse energy, an increased oxygen content was observed along with the increase in crater depth. In argon atmosphere, a high oxygen content was detected at low laser pulse energy. Thus, with the increased laser energy, the crater depth increased, whereas the oxygen content slightly decreased.

    Moreover, during the irradiation of Ti–13Nb–13Zr alloy in air, the oxygen content varied as the intensity of the laser pulse energy changed. EDS analyses showed that when the laser energy value increased, the oxygen content decreased. However, during the irradiation in argon, a completely opposite trend was observed, and the increased laser pulse energy resulted in the increased oxygen content. Furthermore, the formed surface oxide film was composed of titanium oxide and niobium oxide, and the increase in laser pulse energy induced the increase in niobium oxide content in the damage spot area. Consequently, after the laser irradiation in argon, the oxygen content in the analyzed crater area was lower than that observed after the irradiation in air.

  • The results of EDS analysis imply that the oxide layer, which was formed during laser irradiation, contributed to the establishment of a hard wear- and corrosion-resistant implant surface. Thus, attainable research results in this field indicate that the presence of the predominant titanium dioxide film at the implant material surface is beneficial not only for osteoblast cell proliferation [3435,38] but also for the formation of the hard surface zone favorable for the implant tribo-corrosive behavior [4445]. Given such condition, microhardness measurements were performed to investigate the possible hardening effect of the increased oxygen content on the implant surface area and to establish the irradiation parameter influence on the implant material surface hardness which consequently led to the low implant degradation induced by the harsh tribo-corrosive environment. From the results presented in Fig. 10, the Vickers microhardness of the irradiated implant surfaces was in accordance with the detected oxygen content and composition of the surface oxide film. The surface microhardness values showed that laser modification resulted in the significant hardening of the irradiation-affected zone.

    Figure 10.  Vickers microhardness of the target surface as a function of irradiation parameters: (a) CP-Ti; (b) Ti–13Nb–13Zr alloy. Error bars represent standard deviation.

    During the irradiation of CP-Ti in air, the increase in laser output energy was accompanied by the simultaneous increases in the surface oxygen content and surface hardness (Fig. 10(a)). In argon atmosphere, the slight decrease in the surface oxygen content along with the increase in irradiation energy was complemented with the discrete decrease of the irradiation-affected zone hardness. The obtained results confirm that the presence of the surface film with predominating TiO2 particles in the CP-Ti irradiated area was characterized by the increase in material hardness, and that the titanium oxide content in the laser-modified area was directly proportional to its hardness (Table 2 and Fig. 10(a)).

    The same dependence of the irradiated area hardness on oxygen content was observed in the case of the Ti–13Nb–13Zr alloy (Table 2 and Fig. 10(b)). In other words, the microhardness value of the irradiated area was directly proportional to the oxygen content in the laser-affected zone. However, a slight discrepancy was observed after the laser modification in argon atmosphere. Thus, although the overall oxygen content was low at the surface of the implant alloy after the laser modification in argon followed by modification in air (Table 2), the overall microhardness of the laser-modified surface area was significantly higher (Fig. 10(b)). This discrepancy can be explained by the increased niobium oxide content in the surface oxide film given that niobium oxide particles filled the pores in the passivated titanium oxide film, increasing the density and overall hardness of the film.

    However, changes in the irradiated surface microhardness can be influenced not only by the formation of surface oxide films but also by the appearance of microstructural alterations in the surface and subsurface region.

  • The laser irradiation parameters used in this study were not only sufficient to induce the morphological and chemical modifications of the investigated Ti-based materials surfaces but also sufficient to cause microstructural alterations of the target materials in the surface and subsurface region of the damaged area.

    Thus, based on laser pulse duration, estimation of the affected surface area diameter and laser output energy of the laser irradiation intensity can be conducted [20]. The calculated laser irradiation intensities were 0.5 × 1011 and 3 × 1011 W·cm−2 for the laser output energies of 5 and 30 mJ, respectively. Extremely high irradiation intensities resulted in plasma formation in front of the target surfaces and achievement of the extremely high temperatures at the laser beam and implant material interaction spot [20]. The extreme temperatures of the laser-induced plasma (approximately several thousand K [51]), rapid heating [33], and subsequent rapid cooling of the target material at the end of the interaction influenced the changes in the microstructural features in the surface and subsurface region of the laser-affected area.

    According to the literature [52] and our previous investigation [8], the microstructure of the single-phase CP-Ti will remain in the single-phase region, independent of the annealing parameters and the cooling rate. Thus, the CP-Ti processing above the β-transus temperature, Tβ (Tβ = 882°C), followed by rapid cooling from the β phase region cannot entirely suppress the β → α transformation. Hence, the cooling process will only affect the α grain size and shape in the final single α phase morphology. We assumed that rapid cooling of the material damage spot after the laser interaction was completed and could potentially lead to the formation of the unstable retained β structure; however, in this case, the formation of the α phase cannot be suppressed, and the α grains with serrated and irregular boundaries will appear in the microstructure [52]. On the other hand, the presence of oxygen in the damaged area may influence the microstructural morphology and mechanical properties of this single-phase material. The presence of oxygen, which can be interstitially dissolved in titanium, influenced the appearance of Widmanstätten morphology of the α-Ti formed from the β-phase region and at the same time led to the increased strength and hardness of the material, which was experimentally confirmed (Fig. 10(a)).

    However, when the surface of the two-phase Ti–13Nb–13Zr alloy interacted with the laser beam, the almost instantaneous increase of the surface temperature influenced the microstructural alterations in the surface and subsurface region. Rapid cooling of the material damage spot from the temperature, which is significantly higher than the Tβ of this alloy (Tβ = 735°C), at the end of the surface irradiation led to the formation of the as-cast alloy martensitic microstructure in the laser-irradiated area [12]. Thus, during irradiation, when the absorbed energy exceeded the melting latent heat, the alloy was melted at the interaction spot and subsequently solidified when the interaction was completed due to the rapid cooling of the damaged area [40]. As a consequence of the rapid β → α + β transformation, a diffusionless martensitic transformation occurred, and the martensitic microstructure appeared similar to the case during the alloy casting procedure [12]. In this acicular microstructure, the presence of martensitic α' needles with a close-packed hexagonal structure inside of the retained β grains with a body-centered cubic crystal structure was detected. In this case, the cooling rate was extremely high and cannot suppress the martensitic formation; however, on the other hand, the cooling rate influenced the shape of the martensitic needles appearing in the microstructure. Thus, the higher the cooling rate is, the finer the martensitic needles are [5,12]. Based on this assumption, the microstructure of the alloy in the damaged area after irradiation under the output laser energy of 30 mJ was characterized with finer α' needles present within the retained β grains than in the case of the utilization of the laser beam with the output laser energy of 5 mJ. The cooling rate also affected the elements partitioning between the phases. Given that the β phase of this alloy is enriched in with niobium and zirconium, and the solute redistribution is unexpected during the diffusionless martensitic transformation, the contents of these two elements in the α' phase will be low. Therefore, after the alloy irradiation under the high laser output energy, the formation of an acicular microstructure with fine martensitic structure and high hardness can be expected (Fig. 10(b)) [5,1213].

4.   Conclusions
  • Laser surface modification increases the possibility of the successful biomedical application of metallic materials. The surfaces of the CP-Ti and Ti–13Nb–13Zr alloys were subjected to Nd:YAG laser irradiation in this study, and the resulting alterations of their surface properties are summarized as follows:

    (1) Laser irradiation of both investigated materials in argon led to the formation of more pronounced craters with visible microcracks and hydrodynamic effects in the form of periodic wave-like structures than during the irradiation in air. Under the same irradiation conditions, the surface of the Ti–13Nb–13Zr alloy exhibited a higher degree of texturing than the CP-Ti surface.

    (2) Hydrodynamic effects in the form of periodic wave-like structures and ripples can be observed inside and at the outskirts of the formed crater area, whereas traces of melted and solidified material were noticed in the peripheral zones of the damaged areas when the irradiation energy was increased irrespective of the material chemical composition.

    (3) The appearance of favorable oxidized surface films, which were composed of the highly desirable titanium oxide or mixed particles of titanium oxide and niobium oxide, was noticed. High oxygen content and surface roughness obtained during the irradiation in argon were observed on the modified CP-Ti surface.

    (4) Laser surface modification of both investigated implant materials resulted in the increased material hardness, and the obtained surface microhardness values were directly proportional to the oxygen content in the irradiated area.

    (5) The highly desirable appearance of plasma in front of the samples was detected during the laser modification of CP-Ti and Ti–13Nb–13Zr surfaces.

    (6) The laser irradiation intensities were sufficient to induce microstructural alternations and microconstituent modification in the final microstructural morphology and consequently affected the damage area mechanical characteristics.

    In general, laser surface modification could lead to the significant improvement of the surface biointegration properties by reducing the risk of hard-tissue implant failure due to defective osseointegration. Thus, the results of the present study should be perceived as the starting point for the in vitro and in vivo investigations aiming to determine the influence of laser irradiation parameters on live cells and tissue response to laser-modified implant surfaces.

Acknowledgements
  • This work was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia through Project Nos. ON174004 and ON172019 and the PhD fellowship of Slađana Laketić.

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