Mayank Kumar
Yadav
a,
Riddhi Hirenkumar
Shukla
a,
K.
Praveenkumar
b,
Sagar
Nilawar
c,
Chandra Sekhar
Perugu
d,
Prabhukumar
Sellamuthu
e,
Kaushik
Chatterjee
c,
Satyam
Suwas
c,
J.
Jayaraj
fg and
K. G.
Prashanth
*ah
aDepartment of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia. E-mail: kgprashanth@gmail.com; prashanth.konda@taltech.ee
bFaculty of Materials Science and Technology, VSB—Technical University of Ostrava, 17. listopadu 2172/15, 70 800 Ostrava, Czech Republic
cDepartment of Materials Engineering, Indian Institute of Science (IISc), Bangalore 560012, India
dEmerging Nanoscience Research Institute (EnRI), Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
eDepartment of Mechanical Engineering, Presidency University, Bangalore, India
fMaterials Technology, Dalarna University, SE-79188, Falun, Sweden
gDepartment of Mechanical and Materials Engineering, Karlstad University, SE-65188, Karlstad, Sweden
hCentre for Biomaterials, Cellular and Molecular Theranostics (CBCMT), Vellore Institute of Technology, School of Mechanical Engineering, Tamil Nadu 632014, India
First published on 14th April 2025
This study investigates the microstructural, mechanical, corrosion, and biological behaviors of spark plasma sintered (SPS) zinc (Zn) samples for biomedical applications. The findings reveal that SPS significantly refines the grain structure of pure Zn compared to the conventional casting method. The SPS process, conducted at a lower sintering temperature of 300 °C and a high uniaxial pressure of 50 MPa, produces fine and uniform equiaxed grains with an average size of 19 μm. The resulting Zn samples exhibit a calculated density of 7.1 g cc−1 due to complete densification. The sintering process disrupts the initial texture strength, and the uniform grain orientation achieved during SPS contributes to an isotropic microstructure, enhancing the mechanical properties. The compressive yield strength and ultimate strength of the SPS samples are 115 ± 4 MPa and 191 ± 6 MPa, respectively. The long-term biodegradation behavior of SPS Zn in simulated body fluid indicates controlled and gradual corrosion, supporting its potential for biodegradable implant applications, while potentiodynamic polarization analysis further confirms similar corrosion rates compared to cast Zn due to the formation of a stable corrosion product film. In vitro studies with MC3T3-E1 preosteoblast cells show healthy proliferation in culture media containing the degradation products of SPS Zn. Due to its unique microstructural, mechanical, and corrosion properties, along with its biocompatibility, SPS-processed Zn is a promising candidate for tissue engineering applications.
As an essential metallic element, Zn plays a crucial role in bone metabolism, to stimulate the formation of osteoblasts and inhibit osteoclast differentiation, thus enhancing bone strength.28,29 Consequently, biodegradable Zn alloys offer significant advantages over biodegradable materials (polymers, Mg, and Fe-based alloys) in orthopedic applications.30,31 For instance, Bowen et al.32,33 investigated the in vivo biodegradation behavior by inserting pure Zn wire into the arteries of rats, which degraded at an optimal rate (20 μm per year) for biodegradable stents during the initial three months. Despite Zn's numerous suitable properties for biomedical applications, its use is limited due to the coarse microstructure of as-cast pure Zn offering poor mechanical strength and strong anisotropy.34,35 Researchers have attempted to enhance Zn's properties through mechanical alloying or other similar processing techniques to reduce grain size and improve strength and ductility. Forming techniques such as rolling, forging, equal channel angular pressing (ECAP), high-pressure torsion (HPT), and additive manufacturing (AM) have been employed to refine Zn's microstructure.36–39 For example, Wen et al.18 optimized the selective laser melting (SLM) processing parameters to produce high-density pure Zn with enhanced mechanical properties (hardness of 46 ± 2 HV, Young's modulus of 20 ± 6 GPa, yield strength of 122 ± 3 MPa, ultimate strength of 138 ± 3 MPa, and elongation of 8 ± 1%). Similarly, Salehi et al.40 used a two-step 3D printing technique to create Mg–5.9Zn–0.13Zr components, achieving functional parts with increased relative density (69 ± 0.5%) and compressive yield strength (31 ± 3 MPa) through liquid phase sintering. Lu and Li et al.41,42 employed a high-pressure phase transition method to enhance the mechanical and corrosion behavior of Zn–1.5Mn and Zn–Li alloys through solid solution strengthening. Lin et al.43 fabricated MgxZny/Zn composites via accumulative roll bonding (ARB), achieving enhanced mechanical properties, along with an enhanced elongation of 12%, and improved corrosion resistance after 15 cycles. Demirtas et al.44 demonstrated that multiple equal channel extrusions reduced the grain size (2.0 mm) of Zn-0.3 wt% Al and the presence of Al-enriched precipitates, ranging from 50 to 200 nm, significantly enhancing the alloy's superplasticity at room temperature. Yang et al.31 developed binary zinc-based materials with elements like Ca, Mg, Li, Sr, Fe, Mn, Ag, and Fe improving cytocompatibility, osteogenesis, and osseointegration. However, the selection and proportion of alloying elements must be carefully considered to achieve good biocompatibility.
Given the challenges of developing defect-free, fine micro-structured pure Zn for tissue engineering applications, this research employs the spark plasma sintering (SPS) technique to produce dense and fine micro-structured Zn without alloying or mechanical processing. SPS is an advanced sintering methodology that applies uniaxial pressure and pulsed current to heat the powder, facilitating rapid densification at lower sintering temperatures.45–49 The relatively fast cooling rate of SPS (compared to conventional cast) results in a refined, non-textured microstructure.50 Capek et al.51 developed porous Zn for implantation into trabecular bone using SPS, achieving a compressive yield strength (∼31 MPa) like that of trabecular bone (1–12 MPa). Based on earlier investigations, various studies have improved the mechanical and biological properties of Zn and its alloys using SPS.52,53 However, a comprehensive study that examines the microstructural, textural, mechanical, corrosion, and biological behavior of SPS-processed Zn has yet to be reported. To address this research gap, the present investigation aims to thoroughly understand these properties in SPS-processed pure Zn and compare them systematically with cast Zn samples. This comparison will help assess the effectiveness of the SPS process for the fabrication of Zn for biomedical applications.
The microstructural characterization of the samples including optical microscopy (OM) and scanning electron microscopy (SEM) fitted with electron backscattered diffraction (EBSD) was carried out after the samples were subjected to standard metallographic procedures. After mounting the sample, they are subjected to grinding (sequentially varied up to 4000 μm grit paper) followed by electropolishing and etching. The electropolishing of the samples was performed to obtain a mirror-like surface finish operating under 20 V for 35 s in an electrolyte consisting of orthophosphoric acid and ethanol (3:
5 ratio). The polished surface was etched for 10 s in 10% nital to characterize the grain morphology using an OM (Leica Microsystem) and SEM (Zeiss Gemini SEM 450) equipped with an EDAX EBSD detector. The room temperature compressive strength of the samples was tested by using an Instron 5567 screw-driven universal testing machine at a strain rate of 10−3 s−1. The in vitro biodegradation behavior of the samples was assessed by immersing them in simulated body fluid (SBF) for 7, 14, and 21 days, respectively. The SBF solution was prepared according to the method outlined elsewhere but in.54 Before immersion, all samples were polished using the previously described procedure and subsequently ultrasonicated for 30 min to eliminate any surface contaminants. The samples were then incubated at 37 °C with a 5% CO2 supply for the specified durations. The mass change was recorded to determine the degradation rate. The electrochemical corrosion behavior of the as-cast and SPS Zn samples was studied using a standard 3-electrode cell setup using a potentiostat (C.H. Instruments, CHI604E, Texas, USA) with standard calomel and platinum (Pt) as the reference and counter electrodes, respectively. Zn samples with an exposure area of 1 cm2 were used as the working electrode, and simulated body fluid (SBF) was employed as the electrolyte.55,56 All the samples were mechanically ground up to #4000 grit followed by cloth polishing to obtain scratch-free surfaces of an average roughness of 0.08 μm. The open circuit potential (OCP) for the Zn samples was continuously monitored for 1 h. To understand the corrosion behavior, the samples were covered by the SBF solution for 1 h and 24 h, respectively. After 1 h and 24 h, the electrochemical impedance spectroscopy (EIS) information was recorded. The EIS measurement was performed in the frequency range of 10 kHz to 0.01 Hz by applying 10 mV perturbation to OCP values. Similarly, the potentiodynamic polarization (PDP) tests were carried out for 1 h under the potential range of −0.3 V vs. SCE to 0.3 V vs. SCE offset to the rest potential with a scan rate of 1 mV s−1. All these studies were repeated three times to ensure repeatability. After PDP and EIS, all the samples were cleaned according to ISO 8407:2009,33 and the surface morphology of the corroded surface was observed via SEM (Model – Zeiss Gemini SEM 450). The corrosion rate (CR) was calculated according to eqn (1).
![]() | (1) |
The in vitro cytocompatibility of the SPS Zn samples was studied by indirect method by using MC3T3-E1 cells (the MC3T3-E1 preosteoblast cells were obtained from an established commercial supplier, meeting all necessary regulatory and quality standards), and the results were compared with the as-cast Zn samples. Initially disc shaped Zn samples (20 mm diameter and 2 mm thickness) were polished uniformly followed by sterilizing in ethanol and exposed to UV for 1 h. To remove residual ethanol, the samples were further washed thrice using a mixture of phosphate buffer saline solution and 1% antibiotic. Since Zn degrades in the solution media, the effect of leaching out from the material on the cellular response was observed by using conditioned media. Conditioned media was prepared by incubating the sterilized samples in a complete culture medium, i.e., the mixture of α-MEM (minimum essential medium), 10% fetal bovine serum (FBS, Gibco, Life technologies) and 1% antibiotic (Sigma Aldrich) for 24 h and 72 h at 37 °C in the presence of 5% CO2. A constant amount of 15 μL mm−2 of complete media was used to prepare the conditioned media. After the mentioned incubation period, the samples were removed. The conditioned medium was centrifuged (5000 rpm for 20 min) to remove the presence of any debris.
MC3T3-E1 cells were cultured in complete media using a 48-well plate with 3 × 103 cells per well and allowed to attach for 24 h at 37 °C with 5% CO2. After 24 h of incubation, the media was replaced with a conditioned medium and further incubated for 24 h and 72 h separately. The condition media of the as-cast and SPS Zn were used at two different dilutions: 1× (i.e., 100% conditioned medium) and 8× (i.e., 12.5% concentration of the condition medium and the remaining complete medium). For the positive control, cells were cultured in fresh media and incubated for the same period as that for the conditioned media sample. After the incubation period (1 day and 3 days), the conditioned medium was aspirated, and the cells were rinsed with PBS. Subsequently they were incubated in complete media containing WST-1 (Invitrogen) solution with 1:10 dilution for 3 h to measure the cell viability. The optical density of the resultant medium was analyzed using a plate reader (Biotek Gen 5, Santa Clara, CA, USA) at 440 nm. The cell viability was reported in the form of relative growth rate (RGR), which can be calculated using eqn (2)
![]() | (2) |
The viability of the cells was studied with live/dead assay by staining with Calcein AM (Thermo Fischer Scientific, India) and Ethidium Homodimer dye (Thermo Fischer Scientific, India). This is followed by imaging of the samples using an inverted epi-fluorescence microscope (Olympus IX-53, Tokyo, Japan). The cell morphology was visualized by fixing the cells with 3.7% formaldehyde in PBS solution at room temperature for 30 min, followed by PBS washing. The cell membrane was permineralized by incubating in a 0.2% Triton X-100 solution (Sigma, Germany) for 8 min at room temperature, followed by PBS washing. The washed cells were incubated with 25 μg mL−1 Alexa Fluor 488 (Invitrogen) for 30 min at room temperature for actin staining. The cell nuclei were stained by incubating in 0.2 μg mL−1 DAPI (Invitrogen) for 3 min at room temperature. The stained cells were examined using an inverted epi-fluorescence microscope.
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Fig. 2 Plot showing the spark plasma sintering cycles observed due to piston displacement and temperature as a function of time for the Zn samples. |
Considerable piston displacement with an average rate of 0.35 mm min−1 was observed. In the next stage II, the temperature increased quickly, resulting in the generation of plasma between the powder particles. The particle surface was cleaned, and thermally activated densification takes place.63 A piston displacement of 0.1 mm min−1 was observed during this stage. The temperature changes from 50 °C to 300 °C at the rate of 50 °C min−1 with a dwell time of 10 min, which leads to grain boundary diffusion, necking, and pore closure. In the last stage III, the cooling stage, the temperature decreases from 300 °C to 50 °C at a rate of 50 °C min−1. The piston displacement reduces to 0.01 mm min−1, and this piston motion is attributed to the thermal shrinkage of the sample and this shrinkage doesn’t play any role in densification.64
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Fig. 3 The X-ray diffraction patterns for the as-received zinc powder and the bulk Zn samples fabricated by both casting and SPS processes. |
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Fig. 4 Pole figures measured by X-ray texture goniometer for the as-cast and spark plasma sintered Zn samples along the following planes: planes (0002), (1010), and (1120). |
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Fig. 5 (a) and (c) Optical and (b) and (d) scanning electron microscopy images of the zinc samples fabricated by (a) and (b) casting and (c) and (d) spark plasma sintering processes. |
The detailed sintering mechanism and the effect of the process parameter on the microstructure can be explained as follows: the applied heating rate of 50 °C min−1 during the second stage of sintering (from 50 °C to 300 °C) induces a non-equilibrium condition, promoting the formation of fine recrystallized grains while minimizing the grain growth. Localized Joule heating at particle contact points enhances densification by increasing the driving force for diffusion, thereby restricting excessive grain coarsening.74–77 Moreover, the pulsed DC current in SPS induces localized heating at grain boundaries promoting sintering through grain boundary diffusion, while the short pulse duration effectively limits grain coarsening. This controlled diffusion mechanism leads to the formation of a finer and more uniform grain structure, further enhancing the material's mechanical properties.78 The application of 50 MPa uniaxial pressure throughout the sintering process facilitates plastic deformation, leading to a significant increase in dislocation density at grain boundaries. This, in turn, activates dynamic recrystallization (DRX), where newly nucleated grains replace deformed ones, resulting in a refined microstructure. Additionally, DRX disrupts the initial texture, contributing to a more isotropic grain orientation.79 These mechanisms collectively contribute to the superior microstructural characteristics of SPS-processed Zn, distinguishing it from conventionally cast counterparts.
For a better understanding of the crystal orientation and grain boundary characteristics, electron backscatter diffraction (EBSD) analysis was carried out on the Zn samples (Fig. 6). The orientation map developed on the sample surface represents the crystal direction normal to the surface and is referred to as the inverse pole figure (IPF). Most of the grains in the SPS samples are red-colored, suggesting that these grains have their c-axes positioned close to the specimen's normal direction, thus indicating that their basal planes {0001} are nearly parallel to the surface. Fig. 6(b) and (e) show the grain boundaries superimposed image quality (IQ) map for the bulk Zn samples. The blue lines in the IQ maps indicate high-angle grain boundaries (HAGBs) with angles exceeding 15°, while red and green lines denote low-angle grain boundaries (LAGBs) characterized by angles below 15°. The as-cast sample displays around 33% LAGBs, while the SPS sample shows ∼15% LAGBs. Notably, in the SPS sample, LAGBs primarily appear in sub-grains within coarse grains, while fine grains tend to be relatively free of LAGBs. The SPS samples have smaller grains (∼19 μm) compared to the as-cast sample (∼150 μm) with more HAGBs. Areas with fine-grain regions show more HAGBs, possibly due to the strained concentration. The transition from LAGBs to HAGBs is believed to occur mainly during dynamic recrystallization, which is triggered by the combination of applied pressure and temperature observed during the SPS process. Hence, a bimodal grain structure characterized by the coexistence of both coarse and fine grains is observed. On the other hand, the as-cast samples exhibit a predominantly coarser grain with pronounced twin formation. These differences in the microstructural characteristics between the as-cast and SPS samples highlight the distinct variations in the processing methods and thermal conditions/variations observed during the respective manufacturing processes. While the SPS process facilitates dynamic recrystallization and grain refinement, the as-cast samples retain a coarse grain structure with evident twin boundaries.
σyield = σ0 + kd−1/2 | (3) |
σyield ∝ d−1/2 | (4) |
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Fig. 7 (a) True compressive stress and strain and (b) strain hardening rate vs. strain plots for the bulk Zn samples fabricated by casting and spark plasma sintering. |
Under the SPS condition, the deformed sample showed uniform bulging, whereas in the as-cast condition, irregular deformation is observed (Fig. 7(a) inset). Bulging generally refers to increased absorption of energy due to uniform deformation of load with less twin activity. Fig. 7(b) shows the strain hardening rate (θ) vs. true strain behavior under compression. In the SPS sample, the strain hardening rate decreases rapidly with increasing strain, whereas in the as-cast condition, it reaches the plateau. The extended strain hardening behavior in the as-cast sample might be due to the presence of the coarse-grained microstructure and the formation of twins. Table 1 summarizes the mechanical properties of SPS Zn, along with a comparison to previously reported studies on pure Zn and its alloys fabricated using various manufacturing methods.
The degradation mechanism of Zn in SBF can be explained as: during the initial stages of electrochemical corrosion, upon exposure to simulated body fluid (SBF), Zn begins to dissolve, initiating oxygen reduction at the cathode according to the following anodic and cathodic reactions (5) and (6):
Zn → Zn2+ + 2e− | (5) |
O2 + 2H2O + 4e− → 4OH− | (6) |
The by-product OH− of the cathodic reaction increases the pH value, according to the Pourbaix diagram;83 as the pH of the electrolyte increases, zinc ion (Zn2+) reacts with the hydroxide (OH−) ion by dehydration reaction to form Zn(OH)2, which further transforms into a thermodynamically stable zinc oxide (ZnO) layer according to reaction (7) and (8):
Zn2+ + 2OH− → Zn(OH)2 | (7) |
Zn(OH)2 → ZnO + H2O | (8) |
However, the zinc oxide layer tends to dissolve back into Zn2+ due to the leaching action of aggressive Cl− competing with the surface hydroxyl groups in Zn(OH)2 according to reaction (9), resulting in the formation of ZnCl2:
Zn(OH)2 + 2Cl− → Zn2+ + 2OH− + 2Cl− | (9) |
After 24 h of incubation, a thick corrosion product forms on the sample's surface. The change in surface morphology probably influenced both mass transport and the ionic diffusion. Thus, the corrosion mode varies, and the degradation of pure Zn accelerates progressively with the co-existence of calcium (Ca2+) and phosphate (PO43−) ions, leading to the formation of calcium phosphate precipitates according to reactions (10)–(12).84 A schematic representation of the corrosion mechanism of pure Zn immersed in a simulated body fluid solution is shown in Fig. 9.
5Zn(OH)2 + 2HCO3− + 2H+ → Zn5(CO3)2(OH)6 + 4H2O | (10) |
3Ca2+ + 2PO43− → Ca3(PO4)2 | (11) |
Ca2+ + HPO24− + 2H2O → CaHPO4·2H2O | (12) |
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Fig. 10 Open-circuit potential (OCP) behavior of Zn as-cast and SPS samples for an exposure period of 1 h in SBF. |
This, in turn, promotes a more stable electrochemical response compared to as-cast Zn. The formation and stability of surface corrosion products further influence the OCP behavior, as the SPS Zn surface tends to form a more homogenous and protective oxide layer at an early stage of immersion (Table 2).
Material designation | MR | GM | ρ/porosity | H | YS (MPa) | US (MPa) | E (GPa) | Ref. | |
---|---|---|---|---|---|---|---|---|---|
Size (μm) | Shape | ||||||||
WAAM – wire arc additive manufacturing; Hap – hydroxyapatite; SPS – spark plasma sintering; CP – coarse powder (600–850 μm); FP – fine powder (40–100 μm). | |||||||||
Pure Zn | WAAM | 14 ± 5 | Equiaxed | — | 35 ± 2 HV0.3 | — | — | — | 89 |
Wrought | 11 ± 4 | — | — | 41 ± 1 HV0.3 | — | — | — | ||
Ti–5Zn | Hot Pressing | — | — | — | 158 ± 18 HV | 651 ± 3 | — | 17 ± 2 | 90 |
Ti–10Zn | — | — | — | — | 934 ± 32 | — | 20 ± 2 | ||
Ti–20Zn | — | — | — | 390 ± 43 HV | 1136 ± 10 | — | 27 ± 2 | ||
Ti–30Zn | — | — | — | 270 ± 35 HV | — | — | 4 ± 1 | ||
Zn-16HAp (wt%) | SPS | — | — | 18% | 24 ± 5 HV5 | 46 ± 3 (Tensile) | 65 ± 4 (Tensile) | — | 53 |
CP Zn | — | — | 20 ± 2% | 29 HV0.3 | 43 ± 2 | — | — | 51 | |
FP Zn | — | — | 21 ± 2% | 17 HV0.3 | 31 ± 5 | — | — | 91 | |
Zn | — | — | — | 43 ± 2 | 53 ± 17 | 171 ± 13 | — | ||
Zn-5Ge | Cast | — | — | — | 38 ± 0.6 HV0.1 | 29 ± 3 (Tensile) | 34 ± 6 (Tensile) | — | 92 |
Hot rolled | — | — | — | 39 ± 1 HV0.1 | 84 ± 3 | 153 ± 3 | — | ||
Pure Zn | Cast | 150 | Coarse | 7 g cc−1 | — | 60 ± 16 | 274 ± 37 | — | Present work |
SPS | 19 | Fine/equiaxed | 7.1 g cc−1 | — | 115 ± 4 | 191 ± 6 | — | ||
Human cortical bone | — | — | — | 5–10% | — | 80–120 | — | 3–30 | 93 |
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Fig. 11 Potentiodynamic polarization curves obtained after immersion in simulated body fluid for 1 h. |
Sample condition | E corr (V) vs. SCE | I corr (μA cm−2) | I p (μA cm−2) | E pp (V) vs. SCE | E pb (V) vs. SCE | CR (mm per year) × 10−4 |
---|---|---|---|---|---|---|
As-cast | −1.21 ± 0.02 | 8.61 ± 0.11 | 62.0 ± 3.1 | −1.16 ± 0.03 | −1.06 ± 0.17 | 0.60 ± 0.08 |
SPS | −1.20 ± 0.04 | 12.16 ± 0.90 | 52.9 ± 1.2 | −1.15 ± 0.03 | −1.03 ± 0.23 | 0.88 ± 0.05 |
The anodic branch of the Zn samples shows an active-to-passive transition in SBF. Initially, these samples undergo typical active anodic dissolution, followed by passivation at higher potentials. Due to the formation of a passive film, the current density remains constant during the passive region (as seen in Fig. 11). However, after breakdown potential (Epb), the current increases significantly, indicating transpassive corrosion. From the Tafel curve, the corresponding values of Icorr, Ecorr, Ip (passivation current density), Epp (passivation potential), Ebp (breakdown potential), and corrosion rate (CR) are calculated and are tabulated in Table 2. The Tafel extrapolation data show that both the as-cast and SPS zinc samples exhibit a similar trend in the cathodic region, with Ecorr observed to be −1.21 and −1.20 V and Icorr measured at 8.61 and 12.16 μA cm−2, respectively. Upon increasing the potential, a stable oxide layer forms, indicating the passive regime. The passivation potential (Eap) was recorded as −1.16 and −1.15 V for the as-cast and SPS samples, respectively. The passivation current density (Ip) was recorded as 62.0 and 52.9 μA cm−2 for as-cast and SPS samples, respectively. The stable passivation current in this regime suggests the formation of a passivation layer with a lower Ip indicating better corrosion resistance. Furthermore, the current starts to increase in both samples at the breakdown of the passive layer. The breakdown potentials (Epb) for the as-cast and SPS samples are observed to be −1.06 and −1.03 V, respectively.
Such differences in potential likely arise from the grain size differences (a smaller grain size of 19 μm is observed for the SPS Zn sample leading to a higher density of grain boundaries per unit volume, contrasting with the larger grain size of 150 μm observed in the as-cast condition). The grain boundaries, due to their elevated energy state and altered chemical composition, are inherently more susceptible to corrosion than the bulk material. Furthermore, the presence of fine grains facilitates higher hydrophilicity,97 increased diffusion, and deeper penetration into the material, potentially leading to localized pitting, as evident from SEM micrographs (Fig. 12). Additionally, it is worth noting that the texture of the samples also plays a significant role in influencing their corrosion behavior.88 As discussed above, Fig. 11 indicates the stable passivation layer between −1.15 V to −1.05 V in both the as-cast and SPS Zn samples; beyond that, both the samples indicate the dissolution of the passive layer and the inner material is directly exposed to the corrosive environment increasing the corrosion rate. As indicated above, with a decrease in grain size, an increase in corrosion rate was observed for the SPS sample. This disparity suggests a lower corrosion rate (CR) attributed to the formation of a stable oxide layer, as depicted in Table 2, and the corrosion results of the present work with previously reported work are compared in Table 3.
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Fig. 12 Scanning electron microscopy images showing the microstructure of the zinc samples after potentiodynamic polarization immersion in simulated body fluid. |
Material | Manufacturing route | Electrolyte | Degradation parameter | I corr (μA cm−2) | E corr (V) vs. SCE | Ref. | |
---|---|---|---|---|---|---|---|
Corrosion rate (mm per year) | Immersion time (days) | ||||||
WAAM – wire arc additive manufacturing; SBF – simulated body fluid; Gln – glutamine. | |||||||
Pure Zn | Extrusion | SBF | 0.048 | 56 | 3.241 | −1.033 | 85 |
SBF + Gln | 0.023 | 4.135 | −1.055 | ||||
SBF + Glucose | 0.014 | 5.675 | −1.099 | ||||
SBF (Tris-HCl) | 0.033 | 4.119 | −1.024 | ||||
SBF (Tris-HCl) + Gln | 0.034 | 2.951 | −1.004 | ||||
SBF (Tris-HCl) + Glucose | 0.027 | 4.815 | −0.999 | ||||
Pure Zn | Wrought | SBF | 0.30 ± 0.10 | — | 6 ± 1 | −1.13 ± 0.16 | 98 |
Pure Zn | WAAM | 0.45 ± 0.20 | — | 9 ± 1 | −1.18 ± 0.03 | ||
Zn–1Mg | Cast | — | — | 1.2 | −0.98 | 99 | |
Zn–1.5Mg | — | — | 8.8 | −0.93 | |||
Zn–3Mg | — | — | 7.4 | −0.93 | |||
Mg–0.5Zn | Cast | SBF | — | — | 131 ± 6 | −1.87 ± 0.01 | 100 |
Mg–1Zn | — | — | 124 ± 10 | −1.83 ± 0.01 | |||
Mg–2Zn | — | — | 115 ± 10 | −1.81 ± 0.02 | |||
Mg–3Zn | — | — | 102 ± 8 | −1.71 ± 0.04 | |||
Ti–5Zn | Hot press sintering | SBF | — | — | 0.692 | −0.187 | 101 |
Ti–10Zn | — | — | 0.975 | −0.202 | |||
Ti–20Zn | — | — | 0.741 | −0.178 | |||
Ti–30Zn | — | — | 3.631 | −0.245 | |||
CP Zn | SPS | SBF | 0.61 ± 0.11 | 14 | — | — | 102 |
FP Zn | 0.75 ± 0.12 | — | — | ||||
Zn16HAp | SPS | SBF | 0.41 | 14 | — | — | 103 |
Zn-0HAp | SPS | Hank solution | 0.073 ± 0.042 | 1.5 h | 4.90 ± 2.810 | −0.942 ± 0.07 | 104 |
Zn-1HAp | 0.327 ± 0.050 | 21.07 ± 3.25 | −1.281 ± 0.03 | ||||
Zn-5HAp | 0.630 ± 0.011 | 39.12 ± 0.66 | −1.274 ± 0.01 | ||||
Zn-10HAp | 0.856 ± 0.031 | 51.04 ± 1.80 | −1.290 ± 0.01 | ||||
Pure Zn | As-cast | SBF | 0.0179 ± 0.002 | 1 h | 9.43 ± 0.11 | −1.21 | Present work |
0.0017 ± 0.002 | 24 h | 8.73 ± 0.42 | −1.09 | ||||
SPS | 0.0025 ± 0.001 | 1 h | 12.93 ± 0.9 | −1.2 | |||
0.0016 ± 0.002 | 24 h | 8.43 ± 0.27 | −1.07 |
Fig. 12 shows the SEM images and Fig. 13 shows the EDS images of the corroded Zn samples after 1 h and 24 h immersion time in SBF. The samples, after 1 h of polarization, show the localized form of corrosion with different pit sizes. The as-cast sample shows many pits of the uniform cross-section in a concentrated region/area; however, in the case of the SPS sample the corroded region spreads uniformly with the formation of deep grooves inside the pits (see the higher magnification image in Fig. 12) due to the larger density of grain boundaries. The SEM images reveal that the dissolution is more pronounced at grain boundaries, likely due to their higher activity at these boundaries. The regions adjacent to grain boundaries form micro-level galvanic couples, facilitating increased electron transfer and making them more susceptible to activation and localized corrosion.
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Fig. 13 Energy dispersive spectroscopy images showing the microstructure of the zinc samples after potentiodynamic polarization immersion in simulated body fluid. |
Both the as-cast and SPS samples display similar electrochemical corrosion behavior but differ in corrosion morphology, likely due to the differences in their microstructural feature (mainly grain morphology), where finer grains are observed in the SPS samples compared to their as-cast counterparts.
Q = Y0−1(jω)−n | (13) |
W = Y0−1(jω)−0.5 | (14) |
According to Huang et al.,105 the corrosion product covering the sample surface changes the surface condition of the electrode and acts as a time constant. Due to the formation of corrosion product (film) on the sample surface, resistance is induced, which effectively impedes the diffusion of ions during the electrochemical process, leading to the development of Warburg impedance.105 A similar circuit was also used by Huang et al.105 to study the early electrochemical characteristics of pure Zn in SBF. The fitted parameters obtained are shown in Table 4, where the solution resistance (Rs) value of the electrolyte is observed to be the same for all the samples. However, the Rf and Rct values are very high (1008 Ω cm2 and 790 Ω cm2, respectively, for the as-cast samples and 1180 Ω cm2 and 889 Ω cm2, respectively, for the SPS samples) during the first hour of the experiment, but these values decrease significantly after 24 h. Subsequently, the Rf and Rct values reach 600 Ω cm2 and 653 Ω cm2, respectively, for the as-cast and 658 Ω cm2 and 257 Ω cm2, respectively, for the SPS Zn samples. The sharp decrease in Rf and Rct values indicates a significant decline in the protective effectiveness of the corrosion product film and the bare metal surface, respectively.
Sample condition | R s Ωcm2 | R f Ωcm2 | R ct Ωcm2 | CPE1 (Q1) μΩ−1 cm−2 Sn | CPE1 (n1) | CPE2 (Q2) μΩ−1 cm−2 Sn | CPE2 (n2) | W μΩ−1 cm−2 S0.5 | Error (%) |
---|---|---|---|---|---|---|---|---|---|
Zn (as-cast)-1 h | 40.39 | 1008 | 789.6 | 2.19 × 10−9 | 0.8 | 2.5 × 10−3 | 0.8 | 0.013 | 5.8 |
Zn (SPS)-1 h | 41.96 | 1180 | 889 | 7.62 × 10−6 | 0.76 | 6.6 × 10−4 | 0.8 | 0.005 | 7.9 |
Zn (as-cast)-24 h | 40.68 | 600 | 653 | 7.9 × 10−8 | 0.8 | 2.4 × 10−5 | 0.6 | 0.015 | 2.8 |
Zn (SPS)-24 h | 42.13 | 658 | 257 | 1.94 × 10−5 | 0.6 | 8.02 × 10−3 | 0.72 | 0.017 | 7.5 |
A live–dead assay was performed to further evaluate toxicity induced by the leachates from samples. Fig. 16 depicts the live–dead assay result, showing live cells in green color and dead cells in red color. 8-Fold dilution of both the samples showed a minimal number of dead cells for both day 1 and day 3 compared to live cells, confirming the cytocompatibility. It also depicts an increased number of cells on day 3 compared to day 1 in both samples confirming cell proliferation. In the case of 100% extract-conditioned media samples, most of the cells were detached after death, which agrees with the WST assay results. Cellular morphology was characterized by using fluorescence microscopy. Micrographs after day 1 and day 3 of incubation in conditioned media are shown in Fig. 17, where a well-developed cellular morphology with uniformly distributed active fibers is seen when exposed to 8-fold conditioned media. In both samples, the cellular morphology exhibits good spreading and appears like that of the control samples, which further corroborates the nontoxicity of the 8-fold dilution condition in both samples. In the case of the 100% conditioned samples (both cases), they showed hampered/stressed morphologies. Based on the above study it can be concluded that both the samples exhibited similar and favorable cytocompatibility. At 8-fold dilution, no toxicity was observed, with healthy cell proliferation and morphology comparable to the control. In contrast, 100% of extract-conditioned media showed reduced viability and stressed morphology. These findings confirm that both SPS and as-cast Zn samples exhibit favorable biocompatibility under 8× dilution, making them suitable for potential biomedical applications.
• SPS samples have much lower MRD values than their as-cast counterparts. The (0002) pole figure MRD drops from ∼17 in the as-cast sample to ∼3 for the SPS condition due to rapid heating, cooling, and pressure, causing recrystallization and grain growth.
• EBSD analysis reveals that the SPS samples have an average grain size of 19 μm, much smaller than that of the as-cast samples.
• The compressive yield strength of the SPS Zn samples notably increased to 115 MPa compared to the as-cast samples.
• Electrochemical analysis and long-term immersion studies collectively demonstrate that SPS Zn exhibits controlled and gradual degradation in simulated body fluid, similar to that of the cast-Zn. In addition, the formation of a stable and protective corrosion product layer is exhibited, supporting its suitability for biodegradable implant applications.
• The SPS samples exhibit cell viability and proliferation that are like their as-cast Zn counterparts. When exposed to leachates at a dilute concentration (8× dilution of conditioned media), the MC3T3 cells exhibited cell viability that was like that of fresh medium but lower viability for undiluted conditioned media.
• Cytocompatibility studies confirmed that both SPS and cast Zn support cell viability and proliferation, particularly at diluted concentrations.
• Given its refined microstructure, enhanced mechanical performance, and stable degradation behavior, SPS Zn emerges as a promising candidate for biomedical applications, particularly in tissue engineering and biodegradable implant development.
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