I. Research Background: The Urgent Need for Surface Modification of Biomedical Metals

In the field of medical implants, metallic materials are widely used; however, the complex physiological environment of the human body readily induces material corrosion, which not only degrades mechanical properties but also poses health risks due to the release of metal ions. Extensive research has demonstrated a strong correlation between corrosion resistance and biocompatibility. Consequently, constructing oxide coatings to modify the surface characteristics of metals has become the prevailing approach for enhancing the overall performance of biomedical materials.

Niobium pentoxide (\(\ce{Nb2O5}\)) coatings exhibit excellent corrosion resistance, thermal stability, and biocompatibility, effectively enhancing the performance of substrate materials. Pure niobium itself demonstrates superior biocompatibility, with cellular compatibility comparable to medical-grade titanium and even better than stainless steel; however, it suffers from relatively low surface hardness. By leveraging… Atmospheric thermal oxidation The technology forms an oxide layer on the niobium surface, which both enhances its mechanical properties and further improves its corrosion resistance. Currently, the industry often combines this approach with ultrasonic powder‑making techniques to produce ultrafine niobium powders, suitable for a variety of applications such as coatings and composite biomedical materials. However, the thermal stability of the niobium matrix is limited, and the oxidation temperature emerges as a critical factor determining the quality of the coating and the material’s practical value—this constitutes the central premise of the present study.

II. Experimental Protocol: Thermal Oxidation of Niobium at Gradient Temperatures

In this experiment, a pure niobium sample with a purity of 99.9% was used, and all samples underwent standardized pre‑treatment steps, including grinding, polishing, and ultrasonic degreasing. Under ambient atmospheric conditions, four temperature levels—400°C, 425°C, 450°C, and 500°C—were selected; the samples were held at each temperature for 4 hours to allow oxidation to proceed, followed by furnace cooling.

Researchers employed a suite of advanced characterization techniques, including SEM, EDS, X-ray diffraction, optical profilometry, Vickers microhardness testing, contact angle measurement, and an electrochemical workstation, to comprehensively analyze the samples. These methods were used to examine the cross-sectional morphology of the oxide layer, phase composition, elemental distribution, surface roughness, microhardness, and surface wettability. Additionally, corrosion tests were conducted in simulated body fluids (Ringer’s solution) to thoroughly evaluate the material’s overall performance.

III. Core Research Findings: Temperature Gradients Drive Divergent Performance Evolution

(1) Phases and Microstructure: Temperature Determines the Composition and Integrity of the Oxide Layer

The oxidation temperature directly alters the phase composition of the oxide layer, exhibiting distinct stage‑dependent characteristics:

1. 400℃ : Only formed Niobium–oxygen interstitial solid solution Nb(O) , no independent niobium pentoxide phase is present; no distinct layering is observed in the cross-section, and the lattice exhibits a 1.8% distortion.
2. 425℃ : The inner layer consists of an Nb(O) solid solution, while an extremely thin \(\ce{Nb2O5}\) layer forms at the surface, with lattice distortion increasing to 2%; the cross-section still exhibits no distinct oxide layer.
3. 450℃ The oxide layer is predominantly orthorhombic \(\ce{Nb2O5}\), exhibiting uniformity and compactness with an average thickness of 22.56 μm; however, microcracks first appear within the layer.
4. 500℃ : The main phase remains \(\ce{Nb2O5}\), but due to niobium’s poor thermal stability, the oxide layer suffers severe degradation, exhibiting numerous cracks, porosity, and spalling. The oxide layer thickness is drastically reduced to 9.95 μm, with an uneven microstructure and markedly increased brittleness.

Overall, once the temperature exceeds 450°C, the number of defects in the oxide layer increases sharply, and the thermal stresses induced at high temperatures are the primary cause of coating cracking and spalling.

(2) Surface Roughness and Hardness: As temperature increases, both values rise in tandem.

1. Surface roughness : The Ra value of the pristine niobium surface is only 0.141 μm. After oxidation at 400°C and 425°C, the roughness increases slightly; at 450°C, the roughness increases markedly due to the formation of a fine-grained oxide structure. The roughness reaches a peak at 500°C. (Ra = 2.444 μm), significantly higher than in the low-temperature group, with severe surface roughness primarily caused by coating degradation and delamination.
2. Microhardness All oxidation treatments enhance the surface hardness of niobium, with hardness continuously increasing as the oxidation temperature rises. The initial hardness of pure niobium is approximately 1.01–1.1 GPa; at 400°C, the solid-solution layer exhibits the smallest increase in hardness, whereas at 500°C, the \(\ce{Nb2O5}\) ceramic layer attains the highest hardness. The observed hardening results from oxygen solubility, lattice distortion, and the formation of high-hardness oxide ceramic phases.

(3) Surface Wettability: A Sharp Transition from Hydrophilic to Hydrophobic Properties

The surface contact angle is directly related to the material’s bioadhesive properties, and the experimental results reveal a clear dichotomy:

1. Raw niobium : Contact angle 56.3°, exhibiting moderate hydrophilicity.
2. 400℃, 425℃, 450℃ : The contact angle gradually decreases, and hydrophilicity continuously increases. Among them, the sample sintered at 450°C exhibits the best hydrophilicity, with a fine-grained \(\ce{Nb2O5}\) structure being the primary contributing factor. This favorable hydrophilicity promotes cell adhesion, making it well-suited for medical implant applications.
3. 500℃ : The contact angle abruptly increased to 109.6°, rendering the material hydrophobic. This change was jointly caused by excessive surface roughness and coating‑induced cracks, making the material no longer suitable for direct contact with human tissues.

(4) Corrosion Resistance: Improved across the entire temperature range, with optimal performance in the low-temperature segment.

Electrochemical corrosion tests were conducted using 37°C Ringer’s solution to simulate human body fluids. All oxidized samples exhibit superior corrosion resistance compared to the pristine pure niobium. ：

1. 400℃, 425℃ : Exhibits the best overall corrosion resistance. The corrosion potential shifts positively, while the corrosion current density and the passive current density are reduced by two orders of magnitude compared to the base material; the polarization resistance increases by two orders of magnitude, and a stable protective oxide layer can form. At 425°C only, due to the extremely thin surface \(\ce{Nb2O5}\) layer and its slight structural discontinuities, the performance is slightly inferior to that at 400°C.
2. 450℃, 500℃ : Affected by defects such as oxide‑layer cracks, porosity, and spalling, the protective performance is significantly reduced, and the corrosion resistance is markedly lower than that of the low‑temperature group; however, it remains superior to that of unoxidized niobium.

IV. Research Conclusions: Optimal Oxidation Temperature Selection for Biomedical Niobium Materials

By integrating the four core properties—microstructure, mechanical performance, wettability, and corrosion resistance—the study arrives at a clear application‑oriented conclusion:

1. 400℃ and 425℃ are the preferred temperatures for biomedical applications. . At this temperature, the oxide layer exhibits a well‑defined structure with no obvious defects, and it also possesses Excellent corrosion resistance and good hydrophilicity , with outstanding cell adhesion and biocompatibility, it is an ideal process for fabricating low‑load medical implants.
2. There are application limitations at 450°C. Although it exhibits excellent hydrophilicity and a significant increase in hardness, microcracks form within the oxide layer, resulting in insufficient structural stability and precluding its direct clinical application.
3. 500°C is not suitable for biomedical applications. Although the hardness and surface roughness reach their maximum values, the oxide layer exhibits high brittleness and severe spalling, with a hydrophobic surface, leading to simultaneous degradation in both structural integrity and biological performance. This process could be considered for non‑medical applications such as corrosion protection and self‑cleaning.

Overall trend: As the thermal oxidation temperature increases, both the surface hardness and roughness of niobium increase concurrently; once the temperature exceeds 450°C, the material’s inherent thermal‑oxidative instability becomes pronounced, leading to rapid degradation of the oxide layer. If these materials are to be used in high‑temperature environments, it is recommended to apply an additional anti‑oxidation protective coating.

V. Industry Value

This study elucidates the relationship between thermal oxidation temperature and the biomedical properties of niobium, thereby defining an optimal process window for the low-cost, high-efficiency fabrication of niobium oxide coatings for biomedical applications. Leveraging the straightforward air‑thermal oxidation process, it is possible to precisely tailor the surface characteristics of niobium; when combined with advanced powder‑processing techniques such as ultrasonic milling, this approach can further diversify the product portfolio of niobium‑based biomaterials. These advances not only facilitate the practical deployment of niobium and niobium oxide in implantable devices for orthopedics, dentistry, and other fields but also provide valuable guidance for surface‑modification research on other medical metals.

Image and Text Guide

Figure 1. Relationship between polarization resistance and biocompatibility of metals and alloys

Table 1: Process Parameters for Niobium Oxidation

Figure 2. Scanning electron microscope images of the cross-section of niobium after oxidation at the following temperatures: a) 400°C; b) 425°C; c) 450°C; d) 500°C

Figure 3. a) Niobium and its applications in b) 400°C, c) 425°C, d) 450°C, e) Scanning electron microscope (SEM) images of the surface after oxidation at 500°C.

Table 2 Roughness of Niobium and Its Oxide Layers at 400°C, 425°C, 450°C, and 500°C

Figure 4. a) Niobium and in b) 400°C, c) 425°C, d) 450°C, e) X-ray diffraction patterns of the layers obtained by oxidation at 500°C

Figure 4. Continued Figure

Table 3 Chemical compositions [atomic percent] measured at the surface of the niobium substrate and in the oxide layers formed at 400°C, 425°C, 450°C, and 500°C.

Figure 5. In a) 400°C 、 b) 425°C, c) 450°C, d) Distribution of oxygen and niobium in the cross-sections of the layers oxidized at 500°C.

Figure 6. Microhardness of niobium and the oxidation‑treated layers at 400°C, 425°C, 450°C, and 500°C.

Figure 7. a) Niobium surface and b) 400°C, c) 425°C, d) 450°C, e) Image of water droplets on the oxide layer surface at 500°C.

Figure 8. Contact angles on the niobium surface and on its oxide layers at 400°C, 425°C, 450°C, and 500°C.

Figure 9. Open-circuit potentials of niobium and the oxide layers formed at 400°C, 425°C, 450°C, and 500°C.

Figure 10. Polarization curves of niobium and the oxide layers formed at 400°C, 425°C, 450°C, and 500°C.

Table 4 Electrochemical Parameters of Niobium and Its Layers After Oxidation at 400°C, 425°C, 450°C, and 500°C