Tantalum/Tantalum Oxide Coatings for Cardiovascular Stents: Enhancing Mechanical Performance, Corrosion Resistance, and Hemocompatibility

Highlights

What are the main findings?

• Tantalum-based coatings evaluated as protective surface layers for cardiovascular stents.
• Reactive magnetron sputtering used to deposit Ta, Ta₂O₅ and Ta/Ta₂O₅ coatings on 316L steel.
• Amorphous Ta₂O₅ coatings contrast with highly crystalline β-phase tantalum layers.
• Superior performance achieved with Ta/Ta₂O₅ bilayer due to combined mechanical and chemical effects.
• Ta-based coatings show hydrophilic character and surface energy supporting hemocompatibility.

What are the implications of the main findings?

• Synergistic effects in Ta/Ta₂O₅ bilayer maximize coating performance.
• Bilayer Ta/Ta₂O₅ coatings provide fracture resistance, corrosion protection and hemocompatibility.
• Integrated mechanical and chemical studies support next-generation stent coating design.

Abstract

This study delivers a comprehensive evaluation of tantalum-based coatings designed as protective surface layers for cardiovascular stents, focusing on their mechanical durability, corrosion resistance, and surface properties relevant to hemocompatibility. Coatings consisting of tantalum (Ta), tantalum oxide (Ta₂O₅), and a bilayer Ta/Ta₂O₅ system were deposited onto 316L stainless steel using plasma-assisted reactive magnetron sputtering. Structural characterization confirmed a nanocrystalline β-phase for Ta, while Ta₂O₅ exhibited an amorphous, dense, grain-boundary-free morphology that provided superior crack resistance together with enhanced corrosion protection. The bilayer configuration demonstrated the highest overall performance by combining the hardness and mechanical support of Ta with the chemical inertness and stability of Ta₂O₅. This architecture achieved the greatest hardness (861.5 HV), improved toughness proxies expressed as H/E = 0.08 and H³/E² = 0.06 GPa, and a favorable modulus gradient that effectively reduced interfacial stress and increased adhesion. Electrochemical testing in Hanks’ Body Fluid showed a dramatic 1000-fold reduction in corrosion current when compared with uncoated stainless steel, surpassing the performance of both individual monolayers. Assessments of surface properties further demonstrated that hydrophilic, oxide-rich surfaces limited protein adsorption and platelet activation, with Ta₂O₅ and Ta/Ta₂O₅ coatings performing strongly. Overall, these findings indicate that Ta/Ta₂O₅ bilayers provide a multifunctional surface solution for next-generation stents.

1. Introduction

Each year, an increasing number of medical grafts are implanted into the human body. To ensure their long-term functionality, these implants must not only endure the chemically and biologically aggressive environment of the human organism but also exert minimal adverse effects on surrounding tissues. A key strategy for meeting these dual demands is the application of protective, bio-tolerant surface coatings. Among the wide variety of implants, stents represent a distinct category that imposes a unique set of stringent requirements on coating materials and properties [1,2]. Currently, 316L stainless steel (316L, SS), nitinol (Ni-Ti alloy) and cobalt–chromium alloy (Co-Cr) are used as standard stent materials. They exhibit outstanding mechanical properties and improved corrosion resistance [3]. Although metal stents have demonstrated favorable outcomes in promoting vascular healing, restenosis has been reported in 20%–30% of patients within 6 to 12 months following implantation [4].

Experimental and clinical studies indicate that inflammation plays a critical role in restenosis after stent implantation. Contemporary stents are composed of biomaterials, which contain substantial quantities of chromium, cobalt, nickel, and in some cases, molybdenum. These elements have been associated with sensitivity issues and carcinogenic properties, leading to significant adverse reactions, including immunoreactions and inflammation [5]. Up to 15% of the population has an allergy to Ni, Cr or Mo, triggered by the reaction of immune system cells to ions of these metals. Such allergies are therefore likely to contribute to restenosis [6,7]. Moreover, clinically used metallic stents have limitations because of their mechanical properties that may complicate the insertion and positioning of stents at the site of obstruction.

Certain unfavorable features of stents can be eliminated or controlled by appropriate surface treatment. Novel types of oxide and oxynitride-coated stents have demonstrated low rates of platelet aggregation and fibrin deposition and no adverse effects on the endothelization process. These factors contribute directly to lower rates of myocardial infarction and stent thrombosis [8,9,10,11]. For instance, titanium oxynitride coating of stents has been proven to inhibit neointima hyperplasia in a porcine restenosis model and to reduce the incidence of adverse cardiac events compared to stainless steel stents of equivalent construction [11].

Corrosion and corrosion-wear are the major processes that cause problems during long-term implantation of prostheses and stents [12]. Corrosion in the aqueous medium of body fluids takes place via electrochemical reactions [13]. The physiological fluid environment is known to reduce the fatigue strength of metallic implants. The corrosion resistance of protective coatings plays a critical role in ensuring the long-term performance, biocompatibility, and structural stability of metallic stents. The stents are exposed to highly corrosive environment (blood, ions and proteins). Corrosion can lead to ion release (Ni, Cr, Fe), causing inflammation, restenosis, cytotoxicity. The structural integrity degrades with time, resulting in stent fracture, fatigue, or migration. The subsequent damage to metal products due to stress corrosion, cracking, and poor wear resistance has also been reported [14,15]. Protective coatings play a key role in surface passivation and enhancing corrosion resistance, thereby reducing the risk of pitting and galvanic corrosion. Coatings form a barrier to ion diffusion and electron transfer. The metal ion release minimization improves hemocompatibility. Current research is focused on enhancing biocompatibility, reducing thrombogenicity, and improving the overall safety and effectiveness of stent-based cardiovascular treatments [1].

An important criterion of stents long-term performance is general biocompatibility: the coating must not provoke immune responses or exhibit cytotoxic effects. For vascular stents in particular, hemocompatibility is a critical parameter, the surface must effectively minimize platelet adhesion and thrombus formation. Additionally, the coating should exhibit anti-inflammatory properties, avoiding the induction of chronic inflammation and minimizing the likelihood of restenosis.

Equally important are the mechanical and structural properties of the coating. High adhesion strength and long-term durability are essential to prevent delamination under physiological loading conditions. The coating must also demonstrate sufficient elasticity and flexibility to accommodate the significant deformations involved in stent deployment and vessel expansion. Furthermore, surface smoothness and low friction are desirable characteristics that facilitate safe navigation, delivery, and accurate positioning of the stent, while minimizing trauma to the vessel walls.

Given these multifaceted requirements, the selection of coating materials for stents is a critical design consideration. While various polymers, metals, and ceramic-like compounds have been explored, many present trade-offs between biocompatibility, mechanical performance, and long-term stability. In this context, tantalum (Ta) [16,17,18,19] and tantalum pentoxide (Ta₂O₅) [20,21,22,23,24,25] have emerged as promising materials for coating stents due to their exceptional combination of chemical stability, biocompatibility, and corrosion resistance. Unlike polymer-based or drug-eluting coatings, which may degrade or provoke inflammatory responses over time, tantalum oxide offers a stable and inert surface that minimizes adverse tissue reactions. Its high density and radiopacity also provide advantages in clinical imaging, facilitating accurate stent placement and follow-up monitoring. Moreover, recent studies suggest that tantalum-based surfaces can support endothelization while reducing thrombogenicity, making them particularly suitable for long-term vascular applications. As such, tantalum oxide coatings represent a compelling alternative or complement to conventional coating strategies, especially in demanding cardiovascular environments.

In addition, tantalum coatings possess several other advantageous properties, including the ability to self-heal damage in both metallic tantalum and its oxide films [26]. There is a general belief that the biological properties of Ta are caused by its surface being coated with tantalum oxide [27]. The use of tantalum oxide, which has excellent corrosion resistance and biocompatibility, is shown to promote the growth, spreading, and adhesion of cells [28,29,30]. Ta coating deposition is one of the most useful techniques for improving the long-term stability of Co-Cr alloys by enhancing their in vitro biocompatibility in body fluid environments [27]. There has been reported [28] that the surface of NiTi alloy after implantation with Ta ions exhibits improved biocompatibility and corrosion resistance in 0.9 wt. % sodium chloride (NaCl) solution. The reduced corrosion rate and ions released from the surface indicate a high degree of biocompatibility of Ta coated NiTi stents [31].

This study investigates the potential application of tantalum-based coatings as protective surface layers for stents. Specifically, we present a comparative analysis of coatings composed of tantalum, tantalum oxide, and their bilayer combination, with an emphasis on key performance criteria including mechanical durability, surface properties relevant to hemocompatibility, and corrosion resistance. Such a comprehensive framework represents a novel contribution in the field of tantalum-based coatings and provides a basis for the rational design of protective coatings for cardiovascular stents.

The coatings were deposited onto stainless steel substrates using reactive magnetron sputtering enhanced by plasma activation of the reactive gas, a technique selected for its ability to produce uniform, adherent, and functionally graded thin films suitable for biomedical applications. 316L stainless steel was selected as the substrate material for the test coatings due to its widespread use in the manufacture of cardiovascular stents and heart valve components.

2. Materials and Methods

2.1. Deposition Technique

Ta, Ta₂O₅ monolayers, and bilayer Ta/Ta₂O₅ coatings were deposited onto stainless steel (AISI 316L) substrates using a multifunctional Cluster Ion-Plasma System (CIPS). This advanced system integrates compatible sources for the delivery of metal atoms, ions, and chemically active particles, enabling a complex and synergistic effect on the growing film.

A detailed description of the system configuration and deposition process is provided in [32], while the rationale for selecting this approach is discussed in Section 4.

A direct current (DC) magnetron equipped with a 170 mm diameter tantalum target (99.9% purity) was employed for film deposition. In this study, the magnetron discharge power ranged from approximately 3.0 kW to 3.6 kW, depending on the specific coating type, while the maximum available power of the supply unit was 6 kW. The radio-frequency (RF) power applied to the inductively coupled plasma (ICP) source was maintained at 500 W. The distance between the target and the substrate was set at 300 mm.

Argon and oxygen gases were introduced into the vacuum chamber and the plasma source, respectively, through mass flow controllers to ensure precise regulation of gas flow rates. The entire deposition process was continuously monitored and controlled using a computerized data acquisition system. Further details on the magnetron sputtering setup within the deposition chamber have been previously described in [33,34], and the specific process parameters used in this work are summarized in

Table 1. Parameters of Ta, Ta₂O₅ monolayers and bilayer Ta/Ta₂O₅ coating deposition.

Parameter/Coating	Ta	Ta2O5	Ta/Ta2O5
Argon pressure (Pa)	0.12	0.12	0.12
Magnetron voltage (V)	465	508	465/505
Magnetron current (A)	7.7	5.9	7.7/6.1
Oxygen gas flow (sccm)	0	30	0/30
Total pressure (Pa)	0.12	0.16	0.12/0.16
Layer thickness (μm)	0.72	0.98	0.42/0.58

.

The influence of technological parameters on target poisoning and discharge behavior has been previously investigated [33,35], particularly at high reactive gas flow rates required for the formation of stoichiometric crystalline Ta₂O₅ coatings. A key challenge lies in managing the sequence of magnetron discharge initiation and oxygen introduction, as improper sequencing can lead to unstable deposition conditions and substoichiometric films.

Figure 1 illustrates the evolution of the current–voltage characteristics of the magnetron discharge as a function of oxygen flow rate. A pronounced hysteresis is observed for all non-zero oxygen flow rates, indicating a strong dependence of the discharge mode on the gas feed sequence. If oxygen is introduced prior to the initiation of the magnetron discharge, the system enters the target poisoning regime, characterized by a significant reduction in the deposition rate and a tendency to form non-stoichiometric coatings.

To avoid these issues, the correct operational sequence begins with the ignition of the magnetron discharge at its nominal working current while the oxygen flow is still closed. Once the discharge is stable, the oxygen flow is increased to the desired operational level (30 sccm in this study), while keeping the magnetron shutter closed. Only after the discharge has stabilized in the reactive mode is the shutter opened to initiate film deposition. This approach ensures that sputtering proceeds under conditions that avoid target poisoning (as detailed in Table 1) while maintaining the stoichiometric composition of the deposited films. Moreover, it helps to suppress undesirable effects such as micro-arcing and the formation of micro-droplets, contributing to improved coating uniformity and surface quality.

Polished stainless-steel disks with a diameter of 32 mm and a thickness of 3 mm were used as substrates. Prior to deposition, the substrates were sequentially cleaned in an ultrasonic bath using acetone, ethanol, and deionized water, followed by drying in a laboratory oven. The cleaned substrates were then loaded into the deposition vacuum chamber, which was evacuated to a base pressure of approximately 1 × 10⁻⁴ Pa. Before coating deposition, the substrates underwent ion cleaning using a Hall-type ion source in an argon atmosphere. The ion cleaning was conducted at a working pressure of 7 × 10⁻² Pa, with the ion source operating at an anode voltage of 2 kV and a discharge current of 40 mA, for a duration of 5 min.

2.2. Mechanical, Wettability and Corrosion Measurements

The thickness of the as-deposited coatings was measured using the Calotest setup, The Łukasiewicz Research Network—Warsaw Institute of Technology, Poland, based on microscopic measurements on ball sections made using a 30 mm diameter steel ball and 1 µm diamond paste, in accordance with EN ISO 26423:2016 Standard [36]. Three cross-sections were made for each coating and the thicknesses given are the average value of three measurements. The coatings thickness was: 0.72 μm for Ta, 0.98 μm for Ta₂O₅, and 1.0 μm for Ta/Ta₂O₅ bilayer system. The surface roughness parameters were measured by a Jenoptik Hommel-Etamic T8000 profilometer, software TURBO WAVE V7.42, 2009. The roughness values Ra of the Ta, Ta₂O₅, and Ta/Ta₂O₅coatings were 0.018 μm, 0.011 μm, and 0.008 μm, respectively.

Adhesion properties were evaluated with a Revetest^® scratch tester (CSM Instruments, Peseux, Switzerland), software version 4.52, 2017, equipped with a Rockwell type C diamond indenter (spherical tip radius: 200 μm). Coating adhesion was characterized based on microscopic observation of scratches and the determination of critical normal load values. L_C1 is the load at which cohesive coating damage typically occurs, L_C2 is the load that causes regular adhesive coating damages, and L_C3 is the load value at which complete coating removal/delamination occurs [37]. Three scratch tests were performed for each coating. Based on microscopic observations of the resulting coating damage, the average critical load (L_C3) values were determined. The Rockwell test, which involves the pressing of a diamond indenter into the surface of the material under investigation using a standard hardness tester, was also employed in order to assess the adhesion of the coating. Three tests were performed on each sample. A comparison was made between the damage at the point of coating penetration and the reference samples, with the type and extent of damage being classified on a scale of HF1–HF6, according to the VDI 3198 [38] classification [39].

Hardness and elastic modulus were determined using a Fischerscope^® HM2000 XYp nanoindenter, Helmut Fischer GmbH Institut für Elektronik und Messtechnik, Sindelfingen, Germany, with WIN-HCU^® ver. 4.3.0.0, 2009, measurement software based on the ISO 15577-1:2015 Standard [40]. A Berkovitch diamond indenter was used. At least 10 measurements were taken for each sample. Based on these measurements, average values of hardness and modulus of elasticity were determined. To minimize substrate influence on the results, the maximum indentation depth was limited to less than 10% of the average coating thickness. Hardness and modulus values were calculated from the resulting load–displacement curves using the Oliver and Pharr method [41].

The phase composition was analyzed by X-ray diffraction (XRD) using an Empyrean PANalytical diffractometer (Malvern, UK) with Cu Kα radiation.

Surface characteristics, including wettability and surface free energy (SFE), were investigated using the tensiometric method with a Krüss K12 instrument (Krüss, Hamburg, Germany) at 20 °C. Before measurement, the samples were ultrasonically cleaned in acetone and deionized water, then dried. Contact angles were measured using standard test liquids with known surface tension and polar/dispersive components: water, formamide, diiodomethane, ethylene glycol, α-bromo-naphthalene, and glycerol. The total surface free energy and its polar and dispersive components were calculated using the Owens–Wendt–Rabel–Kaelble (OWRK) method [42].

Potentiodynamic polarization tests [43] and registration of open circuit potential (OCP) changes in time were performed by equipment: ATLAS 0531 Electrochemical Unit (Atlas-Sollich, Rębiechowo, Poland), conventional electrochemical three-electrode cell. All samples were washed in ethanol, in the ultrasonic bath before the measurements. Saturated calomel electrode (SCE, Hg/Hg₂Cl₂/KCl) was applied as reference electrode, and platinum sheet as counter electrode. The active area of the sample (working electrode) was 0.292 cm². Prior to polarization, the samples had remained submerged in electrolyte for 1 h (in case of linear potentiodynamic polarization tests) or 12 h (in case of cyclic potentiodynamic polarization tests) in order to stabilize their surfaces under open circuit conditions. The Hanks’ Body Fluid was used as corrosive medium (electrolyte) of a chemical composition given in mmol/dm³: Na⁺—142.00; K⁺—5.81; Mg²⁺—0.81; Ca²⁺—1.25; HPO₄²⁻—0.78; Cl⁻—145.00; SO₄²⁻—0.81; CO₃²⁻—4.27 and pH = 7.22. During the linear potentiodynamic polarization tests, the potential was changed in the range from −0.7 to 0.8 V for the reference sample (316L stainless steel) and from −0.5 to 1.0 V for all samples with deposited coatings. Sweep rate and reading density were 1 mV/s and 3 mV, respectively. Temperature during the measurements was nearby 25 °C. Cyclic potentiodynamic polarization measurements were conducted by scanning in the direction of higher potentials until the value of 0.6 V or until the anodic current density value reached 5 × 10⁻³ A/cm². Then, the scanning direction was reversed toward lower potentials. Linear polarization tests were carried out three times to confirm the repeatability of the results obtained. Values of corrosion potential (E_corr), corrosion current density (i_corr), and subsequently polarization resistance (R_pol) were estimated by extrapolation of tangents to the cathodic and anodic branches in the Tafel region of polarization curves. In cases where the anodic curves were dominated by passivation reactions, only the cathodic curves were used to determine i_corr, according to [44]. For each sample, measurements were repeated until three similar results were obtained. The corrosion parameter values are thus the average of three measurements. Assessment of surface morphology of the samples investigated, within the corroded areas and those not treated with the corrosive medium, was performed by means of scanning electron microscope (SEM, JSM-5500LV, JEOL, Tokyo, Japan).

3. Results

3.1. Structural Properties of the Ta and Ta₂O₅ Coatings

The microstructure and elemental composition of the samples were examined in this research; however, the results are consistent with those reported in our previous studies. Repeated measurements confirmed the stability and reproducibility of the findings. Therefore, only a brief summary is provided here, while detailed descriptions can be found in the following references.

Energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of the main constituent elements in the films, specifically tantalum and oxygen [45]. To further analyze the surface chemical composition of the Ta₂O₅ and Ta/Ta₂O₅ coatings, X-ray photoelectron spectroscopy (XPS) was employed in a related study [33]. The photoelectron spectra corresponding to Ta 4f, O 1s, and C 1s core levels were recorded. For the monolayer Ta₂O₅ coating, the Ta⁵⁺ oxidation state, characteristic of stoichiometric tantalum pentoxide, was identified by the Ta 4f₇/₂ and Ta 4f₅/₂ peaks located at binding energies of 26.8 eV and 28.7 eV, respectively. In contrast, the XPS profile of the bilayer Ta/Ta₂O₅ coating revealed the presence of multiple tantalum oxidation states. In addition to the Ta⁵⁺ peaks, a Ta⁰ doublet was observed at 21.8 eV (Ta 4f₇/₂) and 23.7 eV (Ta 4f₅/₂), which is indicative of metallic tantalum [46]. Notably, the Ta⁵⁺ peaks in the bilayer coating exhibited a slight shift toward lower binding energies, with the Ta 4f₇/₂ peak shifting by approximately 0.5 eV, likely due to changes in the local chemical environment at the oxide–metal interface. The O 1s peak observed at approximately 530.9 eV further supports the formation of tantalum oxide [44].

The morphology of the coatings was also previously evaluated by the atomic force microscopy (AFM, Bruker, Santa Barbara, CA, USA). According to AFM-images, the polished surface of the stainless steel has a microstructure with irregularities and protruding particles of alloying phases of 20–200 nm in diameter. Surface uniformity was increased after tantalum deposition on the stainless steel surface. The growth mechanism of tantalum oxide is different compared to the one of tantalum films. Ta films demonstrated crystalline growth, while tantalum oxide coatings are characterized by amorphous structure.

In this study, particular attention is given to the differences in crystallinity between the tantalum and Ta₂O₅ coatings. Although two different compounds were deposited under identical conditions onto an unheated substrate, the resulting coating structures differed markedly. To investigate these differences, X-ray diffraction (XRD) analysis was conducted. Figure 2 presents the XRD patterns of the Ta and Ta₂O₅ coatings.

The tantalum oxide coating exhibits an amorphous structure, as indicated by the absence of distinct diffraction peaks attributable to the coating itself. The three observed peaks at 43.67°, 50.85°, and 74.76° correspond to diffraction signals from the stainless-steel substrate, specifically from iron-containing phases. Their presence is attributed to the limited thickness of the coating, which allows X-rays to penetrate through to the underlying substrate. This interpretation is supported by the fact that the same peaks are observed in the XRD pattern of the uncoated substrate. In contrast, the tantalum coating shows a relatively high degree of crystallinity. The observed diffraction peaks correspond to the β-phase of tantalum, with characteristic reflections at 33.7° (002) and 70.9° (004). However, the broadness of these peaks suggests a nanocrystalline structure. Using the Scherrer equation, the average crystallite size was estimated to be approximately 6.2 nm.

3.2. Mechanical Properties of the Ta, Ta₂O₅, and Ta/Ta₂O₅ Coatings

Table 2. Mechanical characteristics of Ta, Ta₂O₅ and Ta/Ta₂O₅ coatings deposited on the stainless steel (316L) substrates.

Coating	Mechanical Parameters (Average Results of 10 Tests)
	Hardness, H (GPa)	Young Modulus, E (GPa)	H/E	H3/E2 (GPa)	Adhesion LC3 (N)
316L	4.26 ± 0.29	169.6 ± 15	0.025	0.003	-
Ta	6.81 ± 0.13	127.3 ± 4	0.054	0.019	7.4 ± 0.4
Ta2O5	9.15 ± 0.13	108.1 ± 11	0.078	0.052	24.1 ± 0.7
Ta/Ta2O5	9.31 ± 0.19	109.0 ± 15	0.079	0.054	15.9 ± 0.5

summarizes the average value of mechanical properties of the coatings deposited on 316L stainless steel substrates, including hardness (H), Young’s modulus (E), derived ratios (H/E and H³/E²), and adhesion strength (L_C3). The Ta-based coatings exhibit relatively high hardness and low elastic modulus, indicative of substantial elastic recovery and enhanced resistance to cracking. Among the studied systems, the bilayer Ta/Ta₂O₅ coating demonstrated the highest hardness, whereas the monolayer Ta₂O₅ showed superior adhesion strength.

The experimental data reveal distinct mechanical advantages among the evaluated coatings:

• The Ta/Ta₂O₅ bilayer coating exhibits the highest hardness (9.31 GPa) and superior derived mechanical ratios (H/E = 0.079; H³/E² = 0.054 GPa), indicating enhanced resistance to both elastic and plastic deformation. These values suggest improved mechanical resilience, essential for maintaining coating integrity during stent expansion and cyclic loading. Despite its moderately high adhesion strength (L_C3 = 15.9 N), the bilayer’s performance reflects an effective balance of stiffness, toughness, and interfacial stability.
• The Ta₂O₅ monolayer coating shows slightly lower hardness (9.15 GPa) but the strongest adhesion to the stainless-steel substrate (L_C3 = 24.8 N), which is crucial in preventing delamination under physiological stress. Its relatively high H/E and H³/E² values (0.078 and 0.054 GPa, respectively) affirm its capacity to withstand cracking while maintaining surface functionality. These metrics support its suitability in applications prioritizing mechanical compliance and strong substrate bonding.
• The pure Ta coating, although biocompatible and moderately hard (6.81 GPa), demonstrates lower toughness indicators (H/E = 0.054; H³/E² = 0.019 GPa) and reduced adhesion strength (L_C3 = 14.6 N). These factors suggest limited energy dissipation ability and reduced tolerance to mechanical strain, which may compromise its long-term protective role in dynamic vascular environments.

Overall, both the bilayer Ta/Ta₂O₅ and monolayer Ta₂O₅ coatings demonstrate mechanical characteristics that are favorable for cardiovascular stents, particularly in terms of elasticity, crack resistance, and adhesion. Their performance metrics notably exceed those typical of conventional implant materials, reinforcing their potential as robust protective coatings for stent technology.

Table 3. Example damage images of Ta, Ta₂O₅ and Ta/Ta₂O₅ coatings recorded at critical loads.

Coating	Critical Load (N)
	LC1	LC2	LC3
Ta	0.9	4.3	7.4
Ta2O5	0.9	10.8	24.9
Ta/Ta2O5	0.9	8.1	14.2

shows examples of coating damage images after the scratch test, recorded at critical load values L_C1, L_C2 and L_C3. The observed lower adhesion strength of Ta and Ta/Ta₂O₅ coatings (Table 2 and Table 3), relative to the monolayer tantalum oxide coating, may be attributed to the incorporation of oxygen and potential contamination within the transitional interfacial layers during deposition. As noted in prior studies [47], a very thin Ta₂O₅ layer is known to form spontaneously on the surface of metallic Ta due to its high oxygen affinity. Ta, being a transition metal with strong reactivity toward oxygen, readily forms oxides when residual oxygen is present in the vacuum chamber during deposition. This uncontrolled oxide overlayer may introduce compositional heterogeneity and interfacial discontinuity, compromising adhesion performance. Additionally, contamination or intermixing in the transition regions between coating and substrate can increase mechanical brittleness and reduce the cohesive integrity of the coating system [16,47]. These factors collectively may explain the diminished adhesive strength observed in the Ta-bottom coatings when compared to the more chemically stable and structurally uniform Ta₂O₅ layer.

Example images of Rockwell test indentations and adhesion assessments of Ta, Ta₂O₅, and Ta/Ta₂O₅ coatings to 316L steel substrates are provided in

Table 4. Evaluation of Ta, Ta₂O₅ and Ta/Ta₂O₅ coatings adhesion by Rockwell test.

Coating	Rockwell Test (HF1 ÷ HF6 Scale)
Ta
	HF2–HF3
Ta2O5
	HF2
Ta/Ta2O5
	HF1–HF2

. Minor damage and chipping (HF2–HF3) were observed for the Ta and Ta₂O₅ coatings, but this level of damage is still acceptable. Despite significant conformal deformation of the coating and substrate, no coating damage was visible around the indentations made on samples with the Ta/Ta₂O₅ coating, indicating good adhesion, rated at HF1-HF2 on the Rockwell scale. Significant deformation of the substrates with coatings during the Rockwell test may simulate the behavior of coatings deposited on stents, which are subjected to significant deformation during their application.

3.3. Wettability of Ta, Ta₂O₅, and Ta/Ta₂O₅ Coatings

Surface wettability plays a pivotal role in the hemocompatibility of cardiovascular stents, directly influencing protein adsorption, cell–material interactions, and thrombogenic responses. Upon implantation, plasma proteins such as fibrinogen and albumin rapidly adsorb to the stent surface, forming a conditioning layer to which platelets adhere via integrin-mediated mechanisms. This interaction initiates platelet activation, the release of pro-thrombotic factors, and potentially thrombus formation. In general, the adhesion morphology and degree of platelets spreading are important indicators of their activation status [48].

Highly hydrophobic surfaces tend to promote denaturation of plasma proteins. Notably, fibrinogen, a key plasma protein involved in mediating platelet adhesion to artificial materials, binds more strongly to hydrophobic surfaces. Upon adsorption, fibrinogen undergoes conformational changes that enhance platelet adhesion, spreading, and activation [2].

In contrast, hydrophilic surfaces attract water and polar molecules, thereby reducing nonspecific protein adsorption. This leads to decreased platelet adhesion and activation, effectively lowering the risk of thrombus formation. Experimental data from both in vitro and in vivo studies consistently show that hydrophilic surfaces exhibit significantly reduced platelet adhesion compared to hydrophobic ones. However, one drawback of hydrophilic surfaces is their potential to inhibit endothelial cell adhesion, which is essential for re-endothelization, healing, and long-term stent performance.

Thus, the surface properties of stent coatings should maintain a balance between minimizing thrombogenicity and promoting endothelial cell adhesion. Achieving this dual function (reducing platelet activation while enhancing endothelization) is key to preventing both thrombosis and restenosis, thereby supporting vascular healing.

As previously reported [49], increasing the reactive gas flow during deposition results in less metallic, more oxygen-rich films with lower contact angles, indicating higher surface hydrophilicity. A highly hydrophilic crystalline Ta₂O₅ coating was achieved via rapid thermal annealing at 700 °C. This surface showed a surface free energy (SFE) of approximately 30 mN/m, which correlates with minimal bacterial adhesion. Notably, the lowest Escherichia coli adhesion was observed for SFE values in the range of 21–29 mN/m [50].

In the present study, all Ta-based coatings exhibit SFE values in the range of 31.12–34.23 mN/m. As shown in

Table 5. Surface free energy and its polar and dispersion parts values of Ta-based coatings deposited on 316L substrate.

Coating	SFE and Its Polar and Dispersion Parts by Owens–Wendt–Rabel–Kaeble Method
	γ (mN/m)	γd (mN/m)	γp (mN/m)
Ta	31.12	20.05	11.07
Ta2O5	33.37	21.08	12.29
Ta/Ta2O5	34.23	21.95	12.38

, the oxide-coated substrates exhibit a shift toward increased hydrophilicity, with elevated SFE values. These findings are consistent with previously reported data [49,51,52] and further support the enhanced biocompatibility and antibacterial potential of Ta-based oxide coatings.

3.4. Anti-Corrosive Properties of the Ta, Ta₂O₅, and Ta/Ta₂O₅ Coatings

In order to assess the anti-corrosive properties of the Ta-based coatings, polarization tests (linear and cyclic) and the observation of OCP over time were carried out. Subsequently, changes in surface morphology due to electrochemical corrosion in the Hanks’ Body Fluid environment were examined by SEM. Results for the substrate/coatings systems were compared to those obtained for the uncoated stainless steel.

Figure 3 presents representative linear polarization curves for 316L stainless steel and substrates coated with Ta, Ta₂O₅ (as-deposited), and Ta/Ta₂O₅, while the corresponding corrosion parameters are summarized in

Table 6. Electrochemical parameters (mean values together with standard deviations) characterizing corrosion processes occurring in Hanks’ Body Fluid on 316L and the steel substrates with Ta, Ta₂O_5, and Ta/Ta₂O₅ coatings.

Substrate/Coating	Ecorr (V)	Eb (V)	icorr (A/cm2)	Rpol (Ωcm2)
316L	−0.292 ± 0.014	0.364 ± 0.016	(3.0 ± 0.9) × 10−7	(3.4 ± 1.1) × 105
316L/Ta	−0.198 ± 0.026	0.481 ± 0.079	(1.0 ± 0.3) × 10−8	(2.1 ± 0.2) × 106
316L/Ta2O5	−0.181 ± 0.011	0.285 ± 0.007	(1.2 ± 0.2) × 10−9	(3.9 ± 1.0) × 107
316L/Ta/Ta2O5	−0.320 ± 0.028	0.450 ± 0.069	(3.3 ± 0.1) × 10−10	-

. Analyzing the polarization curve obtained for the bare substrate, one can distinguish three specific anodic areas observed also by other authors during electrochemical studies carried out for 316L stainless steels in various media [53,54,55]. They correspond to: (I) dissolution/oxidation of the alloy components, which extends from the corrosion potential E_corr = −0.292 V up to about E = −0.240 V, (II) active-passive transition and (III) unstable passive region observed until the breakdown potential (E_b) value of 0.364 V is reached. The absence of a clear current plateau within the third range may indicate an active dissolution of the steel occurring through the oxide/hydroxide layer (based on Cr₂O₃/Cr(OH)₃) that ultimately leads to a break of passivity and instant rise in the anodic current density. However, a transformation of p-type Cr₂O₃ to n-type CrO₂ occurring within the solid state cannot be excluded [56].

Deposition of the Ta coating onto 316L steel substrate resulted in a shift in E_corr toward more noble potentials, up to −0.198 V, which proves higher corrosion resistance of this system in Hanks’ Body Fluid. Simultaneously, the corrosion current density (i_corr) decreased significantly, and the value of the polarization resistance (R_p) increased, as evidenced by the data presented in Table 6. According to Faraday’s law, i_corr reflects the rate of reactions responsible for electrochemical corrosion. R_p as a value inversely proportional to i_corr has a similar meaning. Therefore, both quantities indicate a slower oxidation kinetics of the Ta coating as compared to the stainless-steel substrate. Further, the shape of the polarization curve recorded for the 316/Ta system significantly differs, particularly within the anodic range, from that characteristic of 316L steel. Due to the strong affinity of Ta for oxygen, the coating surface undergoes systematic, slow oxidation, which results in a wide active range extending from E_corr to about 0.160 V. The generated passive/oxide layer is stable until the value of the breakdown potential, i.e., 0.481 V, is reached. The higher E_b in relation to 316L steel and the absence of the characteristic inflection originated from the chromium compounds (between 0.100 and 0.200 V in the passive range) indicate that the corrosive processes occur mainly within the coating, without a significant contribution of the substrate. This observation is confirmed by the SEM images shown in Figure 4b and Figure 5a. Local corrosion centers most likely develop in the vicinity of coating defects, as shown in Figure 4a. However, the image of an interior of the produced pitting (Figure 5a) indicates a uniform dissolution of the Ta coating rather than interference with the substrate structure.

The polarization curve recorded for the 316L/Ta₂O₅ system revealed a different nature of the Ta₂O₅ coating compared to the metallic Ta deposit. As Ta₂O₅ itself is a chemically inert material [57], the observed corrosion processes can only develop within the coating defects, visible in Figure 4c. Restricted access of electrolyte to the steel surface resulted in the increased value of corrosion potential, low corrosion current density and the high value of polarization resistance (Table 6). The corrosion centers reached relatively small sizes (Figure 4d), which may be contributed by the good adhesion of the monolayer Ta₂O₅ coating to the substrate. However, in the case of open porosity, Hanks’ Body Fluid remains simultaneously in contact with the coating material and steel, initiating breakdown processes at the bottom of the pores and leading to damage to the substrate material. Figure 5b showing an interior of the exemplary corrosion center revealed characteristic changes in the surface structure of 316L steel resulting from localized corrosion—intercrystalline and pitting (micropits).

The use of the Ta/Ta₂O₅ bilayer system effectively eliminates this disadvantage. The coating surface is smooth and nearly defect-free, as evidenced by both the roughness parameters presented in Table 1 and Figure 4e. The SEM images obtained for the corroded areas of the sample (Figure 4f and Figure 5c) disclosed changes that expanded within Ta/Ta₂O₅ coating and did not affect the steel substrate. The polarization curve registered for the 316L/Ta/Ta₂O₅ system exhibits strong current density fluctuations both in the cathodic and anodic regions. This feature does not arise from the material properties but rather from the very low current density values, which approach the detection limit of the measurement device (ATLAS 0531 Electrochemical Unit). For this reason, the corrosion parameters (E_corr, i_corr) could be only roughly estimated for this specimen. Nevertheless, both the SEM images and the lowest i_corr value (among the systems tested) prove that Ta/Ta₂O₅ constitutes a barrier coating, effectively protecting 316L stainless steel against the corrosive medium.

The findings derived from the linear polarization tests are consistent with the trends in OCP changes recorded during 12 h immersion in Hanks’ Body Fluid (Figure 6,

Table 7. OCPs values recorded for AISI 316 and the steel substrates with Ta, Ta₂O_5, and Ta/Ta₂O₅ coatings after 12 h immersion in Hanks’ Body Fluid.

Substrate/Coating	OCP After 12 h (V)
316	−0.194
316L/Ta	−0.084
316L/Ta2O5	0.049
316L/Ta/Ta2O5	0.093

). In the case of the 316L/Ta system, the open circuit potential was increasing linearly, confirming the strong tendency of the Ta metallic coating to passivate in an oxidizing corrosive environment. Ta₂O₅ and Ta/Ta₂O₅ coatings showed similar characteristics, aiming, after the initial stage of immersion, to achieve a steady-state potential value. The curve obtained for the 316L/Ta₂O₅ system exhibited numerous drops followed by the potential increases, which correspond to the transpassivation and passivation processes occurring within the Ta₂O₅ coating defects. All substrates covered with Ta-based coatings achieved higher OCP values in the final stage of measurement compared to the bare stainless steel (Table 7). This demonstrates their greater chemical stability in Hanks’ Body Fluid, and particularly the systems with the inert oxide coatings.

After 12 h immersion/stabilization in the electrolyte, all substrate/coating systems were subjected to cyclic polarization tests to verify their potential pitting repassivation capacity. Figure 7a shows the cyclic polarization curve obtained for 316L/Ta. One can observe that the forward scan is similar in its course to the anodic curve recorded during the linear measurement, and the corrosion potentials assume similar values, regardless of the immersion time. The reversed scan revealed a distinct hysteresis loop, which indicates pits’ formation and their further growth. These processes are stopped when the potential becomes lower than the protection potential (E_prot) [58]. E_prot, defined as the intersection of the forward and reverse scans, takes the value of −0.177 V in this case. For the 316L/Ta sample, an additional value of the potential can be distinguished, appearing as the inflection of the reversed scan. This is the so-called pit transition potential E_ptp (−0.058 V) at which the small pits are repassivated. In contrast to the Ta coated steel substrate, the 316L/Ta₂O₅ system (Figure 7b) showed a shift in the corrosion potential towards higher values compared to E_corr registered after 1 h immersion in Hanks’ Body Fluid (Figure 3, Table 6). This phenomenon is likely linked to the passive layer (Cr₂O₃/Cr(OH)₃) development within the coating defects that has also been observed during the 12 h OCP recording experiment (Figure 6). To confirm the E_corr ennoblement effect, cyclic polarization tests were performed twice and showed high compliance. Both cyclic polarization curves revealed the presence, in the forward scan, of an inflection assigned to the transformation of p-type Cr₂O₃ to n-type CrO₂. This confirms the participation of the steel substrate in the observed corrosion processes. Moreover, E_prot takes values lower than E_corr, indicating formation of stable pits and inability of the system to their repassivation (similarly to 316L). The cyclic polarization curve obtained for 316L with the Ta/Ta₂O₅ bilayer coating (although affected by strong current fluctuations) displayed E_prot > E_corr and very low current density due to the presence of a sealed Ta₂O₅ surface layer.

4. Discussion

4.1. Deposition of Oxide Coatings

A significant challenge faced by the reactive magnetron sputtering method relates to the so-called “target poisoning” effect, which occurs when the reactive gas (typically oxygen or nitrogen) forms a compound layer on the target surface. This reduces the sputtering rate and can lead to abrupt transitions between metallic and compound deposition modes, often making the process unstable and sensitive to minor fluctuations in gas flow or power input. Achieving precise stoichiometry and reproducible film properties requires fine control over the reactive gas partial pressure, which is further complicated by hysteresis effects in the deposition system. These issues are particularly pronounced when operating in the transition region between metallic and compound sputtering, where even small variations can significantly affect the microstructure and functional performance of the resulting coatings.

Another key limitation of reactive magnetron sputtering is the low sticking coefficient of molecular oxygen on the surface of the growing film. This issue arises from the strong double bond in the O₂ molecule, which has a dissociation energy of approximately 5 eV. Consequently, molecular oxygen must first be dissociated before participating in chemical reactions with the target or film surface, and the energy barrier cannot be overcome by thermal dissociation under typical deposition conditions. For instance, in [59], the sticking coefficient of molecular oxygen on an atomically clean aluminum surface under ultrahigh vacuum was measured experimentally. The results showed that the probability of a ground-state O₂ molecule adhering to the aluminum surface at room temperature was a mere 2%. Converging conclusions were reported in [60], wherein molecular dynamics simulations substantiated the minimal sticking probability under analogous conditions. It has been demonstrated in further studies [61,62] that even under conditions more representative of industrial environments, the sticking coefficient of molecular oxygen during reactive magnetron sputtering remains limited to just a few percent. Consequently, the fabrication of stoichiometric oxide films frequently necessitates the utilization of a substantial excess of reactive gas.

In order to address the issue of the low sticking coefficient of molecular oxygen, a new approach involving the directed delivery of plasma-activated reactive gas to the surface of the growing film was developed [32]. The physical principles and technical details of this method are described in [63]. In this technique, the reactive gas is introduced through a dedicated plasma source containing a dense inductively coupled plasma, while argon is supplied separately near the magnetron target. The concentration of high RF power within a confined volume has been demonstrated to enable the generation of a high-density plasma, resulting in a significantly elevated degree of gas dissociation and activation. This setup produces a directed flow of activated reactive species that can be delivered precisely to the substrate surface. The localized delivery enhances the reactivity at the film surface while minimizing the oxidation of the magnetron target. The approach effectively decouples the plasma chemistry from the sputtering source, thereby enabling independent control of metal and reactive gas interactions. The experimental results demonstrate a substantial increase in the oxygen sticking coefficient, from values below 0.1 without activation to approximately 0.9 when plasma activation is applied. This advancement enables more efficient and controlled film growth, facilitating the synthesis of stoichiometric oxide coatings under otherwise challenging conditions.

In view of the aforementioned arguments, the present study employed reactive magnetron sputtering, enhanced by plasma activation of the reactive gas, for the deposition of tantalum oxide coatings. The spatial arrangement of the magnetron, the RF plasma source for generating activated reactive gas species, and the ion source was carefully optimized to allow the simultaneous delivery of metal atoms, activated reactive gas, and inert or reactive ions to the substrate surface. The RF plasma source was positioned inside the vacuum chamber, providing flexibility in adjusting the relative distances between the sample, magnetron, and plasma source. This configuration enabled precise control over the composition and energy of the species reaching the substrate. Consequently, the CIPS setup allowed for the independent regulation of metal flux, ion bombardment, and reactive gas activation, offering a high degree of control over film growth mechanisms and resulting coating properties.

4.2. Functional Properties of Ta, Ta₂O₅, and Ta/Ta₂O₅ Coatings

Surface engineering approaches enable the development of multifunctional materials through the integration of tailored physicochemical properties of blood-contacting materials with biological functionalities [64,65]. As highlighted in the Introduction, stent coatings are required to satisfy a complex set of performance criteria; in this context, corrosion resistance and hemocompatibility emerge as critical factors that directly influence metal ion release, inflammatory responses, and long-term implant degradation. Equally critical are the mechanical properties, as mechanical failure—most notably cracking—can directly compromise the protective function of coatings. Therefore, fracture toughness is recognized as a key parameter in assessing coating reliability. A number of studies [66,67,68,69] have investigated the potential correlation between fracture toughness and mechanical properties, such as hardness (H) and elastic modulus (E), in thin-film systems. However, a universal relationship has not yet been established, since fracture toughness is also affected by factors such as microstructure, residual stress, coating thickness and interfacial adhesion.

Despite this complexity, several empirical models and heuristic trends have emerged to relate fracture toughness to mechanical parameters and derived ratios, notably H/E and H³/E², especially within hard coatings. These metrics serve as indirect indicators of the coating’s mechanical resilience and offer a practical framework for the screening of coating materials in stent applications, where both mechanical adaptability and long-term durability are critical:

• H/E ratio—reflects elastic strain to failure. A higher H/E ratio suggests superior elastic recoverability, allowing the coating to accommodate substrate deformation without cracking. Thus, increased H/E is generally associated with enhanced resistance to brittle fracture.
• H³/E² ratio—quantifies the resistance to plastic deformation and is often interpreted as a toughness surrogate. It encapsulates the balance between material hardness and compliance, indicating the coating’s ability to absorb and dissipate mechanical energy. Higher values imply improved toughness and diminished risk of delamination or structural failure under load.

The results obtained for the coatings under investigation and their implications for the rational design of efficient protective coatings for stents are discussed below. The measured H/E and H³/E² ratios for the coatings fall within the ranges of 0.054–0.079 and 0.019–0.054 GPa, respectively, with peak values observed for the bilayer Ta/Ta₂O₅ configuration. It is noteworthy that these values exceed those typical of bulk metals (∼0.01) and other contemporary implant materials [68], thereby underscoring the coatings’ favorable balance of stiffness, elasticity, and toughness, which are crucial for stent performance under physiological mechanical stress.

Another important observation is that, despite its relatively high hardness, the elastic modulus of the amorphous Ta₂O₅ coating (108.1 GPa) is substantially lower than that of the AISI 316L stainless steel substrate (158–188 GPa). This mismatch in stiffness can potentially result in interfacial stress concentrations under mechanical loading. The metallic Ta interlayer, with an intermediate Young’s modulus of 127.3 GPa, serves to mitigate this modulus disparity by introducing a gradual transition between the substrate and the top oxide layer in the bilayer Ta/Ta₂O₅ system. It should be noted here that the crystalline structure is a key factor influencing the properties of Ta coatings. Tantalum has been found to exist in two distinct phases: the stable α-phase and the metastable β-phase. The α-tantalum phase is characterized by a body-centered cubic (bcc) structure and is commonly observed in bulk materials. It is distinguished by its high ductility and low hardness, characteristics attributable to its well-ordered lattice, which enables dislocation motion. In contrast, the β-tantalum phase has a complex tetragonal structure and is metastable. It is commonly observed in thin films obtained by deposition techniques such as magnetron sputtering. This distorted structure hinders dislocation glide, resulting in significantly increased hardness and reduced ductility. X-ray diffraction analysis (see Section 3.1) revealed that only the β-phase of tantalum was synthesized under our deposition conditions. This is consistent with tantalum’s known tendency to adopt the β-phase during low-temperature physical vapor deposition processes. Gradient architectures (316L/Ta/Ta₂O₅) therefore promote mechanical compatibility between the substrate and the coating and reduce the likelihood of delamination or crack propagation, as demonstrated in Rockwell indentation tests (see Section 3.2). As Musil and Jirout [70] have demonstrated, this approach facilitates the exploitation of the “combined action of the thin film and the substrate,” thereby enhancing the overall fracture resistance of the coating. Moreover, the mechanical superiority of multilayer systems over monolayer Ta-based coatings for long-life implantable components has been corroborated in previous studies [71], thereby supporting the rationale for bilayer architecture in cardiovascular stent applications.

From the standpoint of corrosion resistance, the performance of stent coatings is primarily determined by their chemical composition and microstructural characteristics. In this regard, the amorphous structure of Ta₂O₅ coatings is particularly advantageous. Amorphous materials are characterized by the absence of long-range atomic order and grain boundaries, which serves to minimize pathways for crack initiation and propagation. Consequently, Ta₂O₅ coatings demonstrate enhanced fracture toughness, attributable to their homogeneous isotropic structure, which is capable of more effective dissipation of mechanical stress without the formation of intergranular cracks. Such cracks, if formed, could otherwise expose the underlying substrate to corrosive attack. Grain boundaries are also known to act as sites where electrochemical reactions occur preferentially, due to their higher energy state. They simultaneously can serve as fast diffusion channels for corrosive species, such as oxygen and chloride ions, which are commonly present in physiological fluids. Therefore, the amorphous Ta₂O₅ coatings, with their dense and uniform atomic arrangement, provide a more effective barrier against ion penetration, electrolyte ingress, and chemically driven degradation processes [72,73]. In contrast, the nanocrystalline Ta coating, despite its higher crystallinity and potential for improved hardness, suffers from reduced corrosion resistance due to enhanced grain boundary diffusion. The small grain size (estimated at ~6.2 nm from the Scherrer analysis) implies a large volume fraction of grain boundaries, which can compromise the integrity of the coating under corrosive environments. However, although Ta₂O₅ is chemically inert [57], corrosion can occur at coating microdefects. This has been demonstrated by SEM observations of areas treated with Hanks’ Body Fluid (see Section 3.4). In the presence of open porosity, penetration of the electrolyte through coating defects enables direct contact with both the coating and the underlying steel, promoting localized electrochemical reactions at pore bases. Such conditions favor the formation of micro-galvanic cells and accelerate anodic dissolution of the substrate, ultimately compromising corrosion protection. The Ta/Ta₂O₅ bilayer effectively suppresses these processes by combining the high electrochemical stability of metallic tantalum with the strong barrier properties of the Ta₂O₅ oxide layer. The oxide layer limits ionic transport and oxygen diffusion, while the tantalum interlayer enhances adhesion and reduces the likelihood of galvanic coupling at defect sites. As previously discussed, the corrosion of samples coated with tantalum oxide is primarily localized at microdefects in the coating, where the stainless-steel substrate becomes directly exposed to the electrolyte. However, in the presence of the Ta sublayer, the material exposed at such defects is tantalum rather than the substrate. Tantalum has been shown to form a thin, dense, and stable native oxide film (Ta₂O₅), which provides efficient passivation and effectively suppresses further corrosion propagation. This synergistic electrochemical effect is clearly reflected in corrosion current measurements: tantalum alone reduces the corrosion current by approximately 30-fold, whereas Ta₂O₅ achieves a 250-fold reduction. In contrast, the Ta/Ta₂O₅ bilayer produces a remarkable 1000-fold decrease, demonstrating its superior ability to inhibit charge transfer reactions and provide long-term corrosion resistance.

The surface properties of coatings are equally as important as corrosion resistance. Balancing surface free energy (SFE) to regulate platelet adhesion and endothelialization remains a key challenge in blood-contacting biomaterials. The optimal range depends on both the total SFE and its polar and dispersive components. Intermediate SFE values (25–40 mN/m) are associated with reduced platelet adhesion, while balanced polarity promotes specific protein adsorption and endothelialization. Optimal ranges of 20–30 mN/m for the dispersive component and 10–15 mN/m for the polar component provide a favorable balance between these processes in vitro [74].

The hydrophilic properties of the surface enhance the desired properties. Section 3.3 demonstrates that Ta₂O₅- and Ta/Ta₂O₅-coated substrates exhibit a slightly increased tendency towards hydrophilicity. These results are consistent with those obtained in previous studies and with literature data, including for other types of oxide.

In vitro measurements of platelet adhesion and fibrinogen adsorption on titanium oxide (TiO₂) and zirconium oxide (ZrO₂) coatings have revealed significantly lower values compared to diamond-like carbon (DLC) coatings. Furthermore, in vivo studies comparing oxide-coated, DLC-coated, and uncoated stainless steel stents demonstrate that TiO₂- and ZrO₂-coated stents evoke a reduced inflammatory response and more complete endothelization. Titanium oxide and oxynitride coatings also showed improved outcomes in terms of mass loss, restenosis, and target lesion vascularization, while reducing platelet adhesion and fibrinogen binding. These improvements are attributed to the enhanced hemocompatibility and cytocompatibility of the oxide coatings [75].

In our previous investigation [45], a comparative analysis was conducted of platelet adhesion on Ta-based ceramic layers. The study focused on correlating platelet behavior with the chemical composition and surface characteristics of Ta and Ta₂O₅ films. Surfaces coated with elemental tantalum showed uniform platelet aggregation, indicating stronger platelet adhesion. In contrast, Ta₂O₅ coatings exhibited minimal platelet aggregation, with individual platelets displaying weak activation, spherical morphology, and only minimal pseudopodia formation, which are suggestive of a reduced pro-thrombotic response. Interestingly, some platelets appeared damaged or disrupted, which may indicate localized surface-mediated effects on cell integrity.

5. Conclusions

This study evaluates tantalum-based coatings as protective layers for cardiovascular stents, with emphasis on mechanical performance, corrosion resistance, and surface properties relevant to hemocompatibility. Monolayer Ta and Ta₂O₅ coatings, as well as a bilayer Ta/Ta₂O₅ system, were deposited on 316L stainless steel using plasma-assisted reactive magnetron sputtering, enabling the formation of uniform, adherent, and functionally graded films. The work establishes a design strategy linking mechanical parameters (hardness, Young’s modulus, H/E, H³/E², adhesion) with surface chemistry and wettability to reduce cracking, delamination, thrombosis, and restenosis.

Particular attention is given to the differences in crystallinity between the tantalum and Ta₂O₅ coatings. Ta coatings exhibit a nanocrystalline β-phase structure, while Ta₂O₅ is amorphous. The amorphous Ta₂O₅ coatings demonstrate superior corrosion resistance and fracture toughness due to the absence of grain boundaries, which limits crack propagation and diffusion of corrosive species. In contrast, nanocrystalline Ta coatings, despite higher hardness, are more susceptible to ion diffusion and stress concentration.

The bilayer Ta/Ta₂O₅ coating combines the advantages of both materials, showing the highest hardness (9.31 GPa) and improved toughness (H/E = 0.079; H³/E² = 0.054 GPa). The presence of a Ta interlayer provides a gradual transition in elastic modulus, reducing interfacial stresses, improving adhesion, and enhancing mechanical durability.

The superior corrosion resistance of the bilayer Ta/Ta₂O₅ coating confirms its suitability for stent applications. Electrochemical testing in Hanks’ Body Fluid and post-exposure SEM analysis showed that, although defects in Ta₂O₅ may induce localized corrosion, the bilayer structure significantly limits this effect, reducing current density by three orders of magnitude compared to uncoated steel and ensuring effective substrate protection.

Surface wettability measurements showed that all coatings exhibited surface free energy in the range of 31–34 mN/m, with higher polar components for oxide-rich surfaces. Increased surface polarity enhances hydrophilicity, which, in turn, correlates with reduced non-specific protein adsorption and platelet adhesion.

In summary, the bilayer Ta/Ta₂O₅ coating offers a synergistic combination of high hardness, enhanced toughness, superior corrosion resistance and surface properties related to hemocompatibility. These attributes make it a promising candidate for advanced cardiovascular stent applications, where resistance to mechanical fatigue, corrosion, thrombosis, and restenosis is essential.

Based on the considerations outlined above, the 316L/Ta/Ta₂O₅ system has been selected for further investigation. Future work will focus on advanced electrochemical characterization, particularly electrochemical impedance spectroscopy (EIS), to provide deeper insight into the long-term stability and barrier performance of these coatings under physiologically relevant conditions. In addition, the mechanical performance of Ta/Ta₂O₅ coatings will be evaluated on fully fabricated stents, including assessments of flexibility, fatigue resistance, and coating integrity during expansion and cyclic loading. Preclinical studies will also be considered, with emphasis on both in vitro and in vivo (animal model) evaluations of hemo/biocompatibility, such as metal ion release and inflammatory response as well as restenosis rates, in order to validate the clinical potential of the Ta/Ta₂O₅ system and support its translation into biomedical applications.