Synthesis and Characterization of Silica-Titanium Oxide Nano-Coating on NiTi Alloy

Abstract

To functionalize the surface of the NiTi alloy, hybrid layers comprising nanometric silica and titanium oxides were synthesized. The TiO₂–SiO₂ nanosystem was chemically prepared and utilized for electrophoretic deposition (EPD) to create multifunctional layers on the alloy surface. The impact of pH on Zeta potential and ceramic particle size was explored to ensure a stable colloidal suspension for EPD, with optimal parameters established at a pH of approximately 6. A uniform layer was formed by applying a voltage of 40 V for 3 min, appearing as a thin film interspersed with regularly spaced larger agglomerates. The thin film primarily consisted of a minor fraction of defective rutile nanoparticles, accompanied by silica and carbon agglomerates from the nanosystem synthesis process. Heat treatment at 800 °C for 2 h induced significant structural changes, developing a novel-generation material with a different structure. An interlayer with strong Si–O–Ti connections was formed. Moreover, the mechanism of layer formation was extensively discussed.

1. Introduction

Titanium oxides find broad applications in the field of regenerative medicine. These compounds demonstrate the ability to enhance tissue integration and promote cell adhesion, both crucial for successful implantation procedures. By accelerating bone healing and encouraging the growth of new bone tissue, titanium oxides significantly contribute to the stability of implants within the patient’s body [1,2]. Additionally, titanium oxides display indirect antibacterial effects. While they are not inherently toxic to bacteria and do not directly destroy microorganisms, they alter the surface properties of medical implants, leading to decreased bacterial adhesion and diminished biofilm formation [3,4]. This effect is instrumental in minimizing the risk of infection and other complications commonly associated with implants. Moreover, titanium oxides have applicability in drug delivery systems, allowing for the precise release of active substances like antibacterial agents, anti-inflammatory drugs, or compounds that promote tissue healing. It is significant in localized therapy, where drugs are released in specific areas, such as around the implant [5].

Silica, commonly called silicon dioxide (SiO₂), is another material that has been applied in regenerative medicine. Silica-based systems exhibit potential utility in fostering bone structure formation, facilitating calcification processes, and supporting regeneration after fractures [6]. Silica nanoparticles, distinguished by their high surface area, ease of functionalization, and biocompatibility, are widely employed in drug delivery applications due to their numerous advantages. Moreover, some papers reported that SiO₂ can produce anti-biofilm coatings on implants [7,8,9]. Despite the numerous benefits of silica, its application as a coating material might face limitations in surface engineering methods that necessitate elevated temperatures. It is well known that silica materials undergo various polymorphic transformations due to temperature, leading to a substantial change in material volume [10]. This phenomenon can result in cracks and delamination in the context of layers, rendering them unsuitable for medical applications. A similar effect was observed in hybrid layers incorporating nanometric silica and other materials, like hydroxyapatite [11]. Nonetheless, incorporating nano-silica as an additive to ceramic layers may positively impact their adhesion to the metallic substrate. It is attributed to the forming entirely new phases with a glass-like structure during deposition [12].

The synthesis of composite layers with distinct properties from the component materials holds significant potential in the surface functionalization of metal alloys employed in regenerative medicine. TiO₂ is predominantly employed in the surface modification of titanium alloys. One of the titanium alloys widely used in short-term implantology is the NiTi shape memory alloy (SMA). These materials, characterized by a chemical composition close to equilibrium, exhibit exceptional properties, including one-way and two-way shape memory effects (SME) and superelasticity. Owing to these features, they are the most widely used SMAs in a broad spectrum of biomedical applications, particularly orthopedic implants [13].

Nevertheless, the extended presence of NiTi implants in the body poses a significant constraint. It is associated with the high nickel content in the alloy, posing a risk of releasing ions into the organism due to the aggressive nature of body fluids [14]. Addressing this issue involves modifying the alloy surface and creating protective layers that act as a mechanical barrier for the released nickel ions, extending the implant’s viability within the patient’s body. Concerning SMA, the ceramic layers applied to their surface must be relatively thin. Thick and rigid layers may be susceptible to damage during the shape changes induced by the shape memory effect.

Various techniques are recognized for enhancing the surface properties of alloys intended for medical applications, creating bioactive and corrosion-resistant layers. Among the commonly employed methods for NiTi alloys are the sol-gel method, thermal or plasma spraying, electrophoretic deposition (EPD), electrochemical deposition, plasma sputtering, chemical vapor deposition (CVD), physical vapor deposition (PVD), ion or magnetron sputtering, oxidation, glow deposition, laser treatments, hydrothermal methods, biomimetic deposition, and ion implantation [15]. Surface modification methods should exhibit simplicity, repeatability, and a swift process. Considering economic factors, including the acquisition and usage costs of potentially expensive equipment, is equally crucial.

One technique that enables the production of thin ceramic layers is the electrophoretic deposition method. This method offers several advantages, including cost-effectiveness and achieving multifarious morphology and thickness coatings by adjusting deposition parameters [16]. Furthermore, the electrophoresis process can potentially develop novel materials with active surfaces that can profoundly influence bioactivity and biocompatibility [12,17].

This work applied electrophoresis to functionalize a NiTi alloy’s surface by developing innovative, multifunctional hybrid coatings. These coatings were deposited using a stable colloidal suspension of a chemically synthesized nanometric molecular system, which included a rutile-silica (TiO₂–SiO₂) nano-composite with a small amount of silica (approximately 1.3 wt.%). The optimal parameters for the stable colloidal suspension were selected by measuring the Zeta potential and ceramic particle size at different pH levels. The heat-treatment conditions for the deposited layers were chosen based on the results obtained from dilatometry, high-temperature microscopy, and thermogravimetric analyses. The modified alloy surface was characterized by morphology, topography, and structure following deposition and heat treatment. Detailed structural studies using Raman spectroscopy allowed the determination of the mechanism of the formation of the novel coating, which was thoroughly discussed.

2. Materials and Methods

2.1. NiTi Substrate

The NiTi shape memory alloy in the β-phase (B2), featuring characteristic temperatures of martensitic transformations below room temperature, served as the substrate for the coating deposition. The samples underwent polishing with SiC papers up to 2000-grit. Before deposition, the sample surfaces were thoroughly cleaned in acetone using an ultrasonic bath to eliminate trace amounts of impurities. Subsequently, the samples were passivated in an autoclave at 134 °C for 30 min for enhanced corrosion resistance, forming a thin, corrosion-resistant amorphous titanium oxide layer on their surfaces [18].

2.2. Synthesis of TiO₂–SiO₂ Nano-Composite

The synthesis of the SiO₂–TiO₂ nanosystem with a small amount of SiO₂ occurred in two stages. Initially, 3 g of titanium dioxide (TiO₂) (US Research Nanomaterials Inc., Huston, TX, USA) with a rutile structure (particle size approximately 30 nm) was dissolved in 100 mL of mixture of distilled water and isopropyl alcohol (Biomus sp. z.o.o., Lublin, Poland) in a ratio of 2:5. This solution was then subjected to stirring in a magnetic stirrer at a temperature of 200 °C for 1 h. Subsequently, a 10% aqueous NaOH solution was added dropwise, and the mixture was stirred for an additional 2 h at 100 °C. In a separate vessel, 3 g of silicon dioxide (SiO₂, s-type) was combined with 100 mL of distilled water. These components were mixed and exposed to a 250 W microwave field for 1.5 min. The resulting colloid was then dropped into the titanium dioxide and NaOH mixture. The entire composition was stirred at 100 °C on a magnetic stirrer for 1 h. The resultant nano-composite was subsequently filtered through a paper filter and dried. As a result of the synthesis, the TiO₂–SiO₂ nano-composite, with 4.0 ± 0.2 wt.% of silica, was produced.

2.3. Preparation of Suspension, Electrophoretic Deposition, and Heat Treatment

The deposition of coatings employed the electrophoresis technique, utilizing a colloidal suspension with a concentration of 0.1 wt.% of nano-composite SiO₂–TiO₂ powder in 75% ethanol (Avantor Performance Materials, Gliwice, Poland). Before deposition, the suspensions were stirred into a magnetic stirrer for 1 h and treated in an ultrasonic bath for 1 h.

The optimal parameters for the stable colloidal suspension were chosen based on measuring the Zeta potential and ceramic particle size at different pH levels. The suspension’s pH was adjusted by adding NaOH and HCl (Avantor Performance Materials, Gliwice, Poland). The electrophoretic deposition from the colloidal suspension with pH ca. 6 was conducted under a 40 V range of 1–10 min. A schematic outlining the procedure for preparing the colloidal suspension and the subsequent electrophoretic deposition process is presented in Figure 1. Next, the coatings underwent a 24-h room temperature drying process, followed by a 2-h annealing at 800 °C in a low vacuum.

2.4. Method of Testing

The particle size and Zeta potential values were measured using the Malvern Zetasizer Nano ZS particle size analyzer with a 633 nm He-Ne laser (Malvern, UK). All measurements were performed at room temperature. Particle size was determined using a polystyrene cuvette. Five independent measurements were carried out at each pH. A U-shaped cuvette (DTS1070 from Malvern) was used for the Zeta potential values. Each measurement was performed on three samples.

The scanning electron microscopy (SEM) analysis was conducted using the TESCAN Mira 3 LMU instrument (TESCAN, Brno, Czech Republic) complemented by an Energy Dispersive Spectrometer (EDS) provided by Oxford Instruments-Aztek (Abingdon, UK). Imaging was achieved by collecting secondary electrons (SE). A 5 nm Cr layer was sputtered onto the samples using the Quorum Q150T ES equipment (Quorum Technologies, East Sussex, UK). Surface chemical composition analysis was conducted in five regions of the sample, followed by computation of the average.

Thermal analysis methods: differential thermogravimetry (DTG), differential thermal analysis (DTA), thermogravimetry (TG), and measurement of gases evolved from the TiO₂–SiO₂ nano-composite (EGA) were conducted. The samples were investigated with a simultaneous STA 409 PC thermal analyzer from NETZSCH and a QMS 403 C Aëolos quadrupole mass spectrometer (Selb, Germany). A total of 28.4 mg of the sample was put inside the Al₂O₃ crucible and subjected to heating from 40 °C to 1100 °C at a rate of 20 °C/min in an airflow of 30 mL/min.

The thermal expansion and shrinkage of the TiO₂–SiO₂ nanopowder were evaluated using Thermo-Mechanical Analysis (TMA 92 Setaram, Caluire, France). TMA tests were performed at temperatures between 20 and 1200 °C, the heating rate was 7 °C/min, and the air atmosphere was at 1.5 bar. The powder sample was put in a small container. The sample was loaded with a high-purity Al₂O₃ probe with a load of 5 g to maintain surface uniformity. The TMA curves for the samples were corrected for the result of the corundum expansion curve.

Leitz’s high-temperature microscope performed the analysis of linear changes during the heating process of TiO₂–SiO₂ nanopowder. A sample in the form of a cube, with a side of 3 mm, was prepared. The samples were put on the corundum support and heated up to 1200 °C at a rate of 7 °C/min.

The investigation into the linear changes and simulation of phenomena occurring during the heating process of TiO₂–SiO₂ nanopowder was conducted using a Leitz high-temperature microscope. A manually pressed sample, forming a cube with an approximate side length of 3 mm, was the focal point of the analysis. The samples placed on a corundum pad were heated up to 1200 °C at a rate of 7 °C/min.

Through the microscope’s eyepiece, images of the sample were captured at temperature intervals corresponding to changes in its shape. Continuous monitoring and photographic documentation of the sample’s morphological changes and dimensional variations in response to temperature fluctuations facilitated the acquisition of multiple images depicting the material’s behavior during heating. Analysis of these images enabled the determination of the relative change in the cross-section area δ(T) concerning temperature, allowing for identifying the starting temperature of sintering.

Fourier-transform infrared (FTIR) and Raman scattering (RS) techniques were applied to examine the structure and structural changes within the SiO₂–TiO₂ electrophoretically deposited coatings and subsequent heat treatment.

A Thermo-Scientific IS50 spectrometer equipped with a standard source, DTGS Pelti-er-cooled detector, and KBr beamsplitter (Thermo Fisher Scientific, Waltham, MA, USA) was employed for FTIR analysis. Spectra of NiTi substrates functionalized with SiO₂–TiO₂ coatings were acquired using Grazing Incident Attenuated Total Reflection (GIATR) and a diamond accessory. Measurements were conducted in the 400–3600 cm⁻¹ range, averaging 64 scans with a spectral resolution of 4 cm⁻¹. Subsequent post-processing included baseline correction and adjustments for water and carbon dioxide interference, with additional corrections applied for attenuated total reflection (ATR) using the refractive index of titanium oxide (n~2.49). Band fitting analysis was also performed using the GRAMS software package (version 9.2, Thermo Fisher Scientific, Waltham, MA, USA).

Before and after heat treatment, Raman measurements on modified NiTi alloy substrates were conducted using a WITec confocal alpha 300R Raman microscope (CRM) equipped with a solid-state laser (λ = 532 nm) and CCD detector. Laser radiation was delivered through a polarization-maintaining single-mode optical fiber (50 μm diameter) and directed into the microscope. Scattered radiation was focused on a mul-ti-mode fiber (50 μm diameter) and a 600 line/mm grating monochromator. The microscope utilized an Olympus MPLAN 100×/0.9 NA objective to optimize lateral (LR) and depth (DR) resolution [19]. Lateral resolution was estimated according to the Rayleigh criterion LR = 0.61λ/NA, while depth resolution was DR = λ/(NA)2. LR stands for the minimum distance between resolvable points (in X- and Y-directions), DR is the minimum distance between resolvable points (in the Z-direction), NA refers to the numerical aperture, and λ is the wavelength of laser excitation. As a result, lateral resolution is estimated at LR = 0.36 μm and depth resolution at DR = 0.65 μm.

Surface Raman imaging maps in the X- and Y-directions were collected in a 50 μm × 50 μm area using 150 × 150 pixels (=22,500 spectra) with an integration time of 500 ms per spectrum, and the precision of moving the sample during the measurements was ±0.5 μm. The depth scan Raman imaging map was gathered at +5.0 up to −5.0 μm in the Z-direction in a 50 μm × 10 μm area using 150 × 40 pixels (=6000 spectra) with an integration time of 500 ms per spectrum, and the precision of moving the sample during the measurements was ±0.5 μm. Spectra were collected in the 75–4000 cm⁻¹ range with ten mW laser power and 3 cm⁻¹ spectral resolution. The spectrometer monochromator was calibrated using a Ne lamp’s emission lines, while the silicon plate’s signal (520.7 cm⁻¹) was provided for checking the beam alignment. Post-acquisition, Raman data underwent baseline correction and automatic cosmic ray removal. A basis analysis in the WITec Pro-jectFive Plus Software (v5.1.1) was utilized for chemical and structural differentiation of the coat-forming material and the band fitting analysis on individually selected samples after the basis analysis; Raman data were analyzed using the GRAMS software package (version 9.2, Thermo Fisher Scientific, Waltham, MA, USA).

3. Results and Discussion

3.1. Determination of Optimal Parameters and Characterization of TiO₂–SiO₂ Colloidal Suspension

The Zeta potential and particle size were determined as a function of pH for the colloidal suspension, and the results are illustrated in Figure 2. Zeta potential is a crucial indicator of suspension stability and particle mobility and gives information about the degree of particle dispersion within the suspension. Typically, the highest positive values (or the lowest negative values, contingent on cataphoresis or anaphoresis) signify improved dispersion [20]. The colloidal suspension of the TiO₂–SiO₂ nano-composite revealed a positive Zeta potential value in the pH range from 3 to 7, transitioning to a negative value for higher pH. A notable decrease in value occurred between pH 7 and 8, indicating the likely location of the isoelectric point. Furthermore, Zeta potential values increased for pH 11 and 12, correlating with the observed enlargement of particle diameter or their aggregates in these environments.

In the pH range from 3 to 10, the TiO₂–SiO₂ nano-composite powder diameter values, considering standard deviation, ranged from 362.6 nm (pH 9) to 413.7 nm (pH 4). Given that the initial titanium oxide ceramic particles were approximately 30 nm in size, it can be inferred that agglomeration occurred. However, at pH levels 11 and 12, there is a higher likelihood of sample aggregation, leading to an increase in diameter values and a higher standard deviation. This results in a noticeable differentiation in the sample, particularly regarding the size of particles suspended in the suspension. The observed pH-dependent behavior significantly impacts the stability and aggregation tendencies of the TiO₂–SiO₂ nano-composite.

Considering the obtained results, the optimal pH range for the electrophoretic deposition of TiO₂–SiO₂ nanopowder in colloidal suspension appears to be approximately 5–6 or 8–10. Notably, the tests revealed that the initial pH of the suspension formed post-solution preparation stood at 5.7, falling comfortably within the specified range. Consequently, there was no imperative need to adjust its pH. Deliberately elevating the pH to higher values (within the 8–10 range) would necessitate introducing minute quantities of extraneous ions from the pH-adjusting substance (NaOH) into the solution. Hence, the initial colloidal suspension was used for deposition.

3.2. Microstructure and Structural Characterization of the Deposited TiO₂–SiO₂ Coatings

Examinations using electron microscopy (SEM) unveiled the impact of the applied deposition parameters on the morphology of the deposited coating. Scanning electron microscopy (SE-SEM) observations and element distribution maps (Figure 3) showed that the coatings manifested as a thin film comprising the smallest nanoparticles deposited initially. Within this film, agglomerates comprising larger nanoparticles were observed (Figure 3b). The even distribution of these agglomerates was obtained by employing a voltage of 40 V and a deposition time of 3 min. The chemical composition analysis further revealed a homogenous distribution of silicon derived from nano-silica, not confined solely to the agglomerates but also within the thin film. The silicon content was measured at 0.8 wt.%, which, upon conversion to silica content, equates to 1.3%. A chemical analysis unveiled the existence of nickel (Ni) derived from the NiTi substrate, implying that the deposited layer is comparatively thin.

Raman spectroscopy played a crucial role in confirming the formation of the SiO₂–TiO₂ coatings during the electrophoretic deposition. The modified NiTi alloy surface underwent 2D Raman imaging in the X- and Y-directions and X- and Z-directions. This approach provided comprehensive insights into the coating’s chemical and structural phase diversity, aligning with microscopic observations and chemical composition analyses (Figure 3).

A thorough examination of the X-Y Raman maps revealed a complex four-phase system with varying material packing and distribution degrees. The dominant component comprising the coating was rutile statistically, co-existing with two variations: a more defective and a more ordered phase. Raman spectrum of one rutile feature maxima at approximately 262 cm⁻¹ (indicative of multiple-phonon scattering processes), 414 cm⁻¹ (E_g symmetry), 539 cm⁻¹ (A_1g + B_1g symmetry), and 607 cm⁻¹ (A_1g symmetry) and more ordered with bands centered around 245, 443, 535 and 617 cm⁻¹ [21,22,23,24,25]. The Eg-rutile band position, indicative of the in-plane mode of the O–O bond, strongly correlated with the O/Ti ratio, suggesting an oxygen-deficient TiO₂ structure [26,27,28]. An observable band shift toward lower frequencies around 39 cm⁻¹ with a rise a full width at half maximum of an E_g mode around 12 cm⁻¹ strongly indicated the presence of oxygen vacancies and structurally distorted octahedra. Notably, two structurally different rutile phases co-existed and seemed to form a more or less uniform coverage, as evidenced by variations in color intensity across the coating (observable as brighter and darker blue regions in Figure 4a). Additionally, Raman analysis revealed tiny clusters related to anatase, another titanium dioxide polymorph (evident as yellow spots in Figure 4a,b) presented a band arrangement with maxima at 147 cm⁻¹ (E_g symmetry), 390 cm⁻¹ (B_1g symmetry), 521 cm⁻¹ (A_1g symmetry), and 632 cm⁻¹ (E_g symmetry) [29,30,31] as well as carbon-coated silica particles due to bands around 259, 439 cm⁻¹ (bending motions of Si–O–Si bonds in large n-membered silicate rings (n ≥ 5)) as well as 1366 and 1598 cm⁻¹ (D and G carbon-related bands) [32].

Valuable insights were also obtained from the fundamental analysis of the Raman cross-sections conducted in the X-Z-directions. This examination revealed that rutile and defective rutile tended to aggregate into irregular clusters of varying sizes, with carbon-silica particles spread more locally in smaller aggregates (Figure 4c). Coat-forming rutile forms a continuous coating with variable thickness but lower than 1 µm, as illustrated in cross-sections from different locations. This layer formed close to the NiTi surface (Figure 4d). The aggregated rutile with carbon-silica particles increased coating roughness, resulting in localized thickening in areas ranging from 1.3 µm to 1.7 µm (Figure 4d).

FTIR spectroscopy was employed to verify the presence of silica within the coat-forming materials. However, interpreting the low-lying bands (below 900 cm⁻¹) posed a slight challenge due to the overlapping nature of silica and titanium oxide modes. Despite this, the FTIR spectrum of the SiO₂–TiO₂ coating spanning the 400–1200 cm⁻¹ range, prominently featured bands at approximately 642, 675, 788, 1104, and 1184 cm⁻¹, originating from symmetric Si–O–Si and asymmetric vibrations of siloxanes [33,34,35] as well as bands centered at 517, 588, 714, 755, and 788 cm⁻¹, attributed to Ti–O and/or Ti–O–Ti bonds within multiphase-TiO₂ nanoparticles [36,37]. Comparing the gathered spectrum with reference spectra of titanium dioxides (anatase, rutile) revealed noticeable changes in band positions, full width at half maximum, and intensity, indicating a distortion within the octahedral TiO₆ and the emergence of oxygen vacancies that enhance surface reactivity [32,38].

The presence of a distorted titanium octahedron may contribute to the formation of Ti–O–C interconnections, as suggested by bands around 960 and 1053 cm⁻¹ [39]. Alternatively, some studies propose that the intense band at 960 cm⁻¹ corresponds to a tetrahedrally coordinated framework within the titanium dioxide structure [40]. Other hypotheses point to the presence of hydroxyls and Si–O groups at defect sites or tetrahedrally coordinated Si–O [41,42,43,44], perturbed by the presence of the Ti atom [39]. Additionally, some authors suggest that the appearance of such a band in the SiO₂–TiO₂ infrared spectrum is related to the antisymmetric stretching of the Si–O–Ti bridge. These hypotheses underscore the formation of a SiO₂–TiO₂ or similar system with carbon incorporated into such connections [35,45,46].

Notably, the lack or very low signal intensity of the silica bands, coupled with the relatively strong signal in the infrared spectrum, indicates the presence of very tiny silica particles well distributed within the coating.

3.3. Determination of the Heat-Treatment Temperature of the TiO₂–SiO₂ Coating

The heat treatment plays a pivotal role in forming a chemical bond between electrophoretically deposited ceramic layers and the metallic substrate [47]. This phase also induces the sintering of ceramic particles. However, temperature significantly influences ceramic layers’ thermal expansion, leading to either contraction or expansion and subsequently causing the buildup of internal stresses within the material. It can potentially result in the formation of cracks and the delamination of the deposited coatings [11,19]. Therefore, it is crucial to identify the characteristic temperatures and understand the behavior of ceramic particles designed for coating materials. The dilatometric (Figure 4a) and high-temperature microscope tests (Figure 4b) were performed on TiO₂–SiO₂ nano-composite powder.

Dilatometric analysis (Figure 5a) unveiled that the TiO₂–SiO₂ nano-composite powder exhibited minimal thermal expansion, measuring 0.28%, up to 856 °C, followed by a notable rapid shrinkage observed up to 1050 °C. The maximum recorded shrinkage, reaching 13.97%, occurred at 1200 °C, contributing to an overall total shrinkage of 14.25%. This shrinkage phenomenon is intricately tied to the sintering process undergone by the sample.

High-temperature microscopy studies provided insights into the size and rate of linear transformations exhibited by TiO₂–SiO₂ nanopowder as temperature increased. The graph depicting linear changes concerning temperature and accompanying images taken at selected temperatures during the tests are presented in Figure 5b and Figure 5c, respectively. The findings revealed that changes in the cross-sectional area of the nano-composite sample commenced beyond 800 °C, pinpointing the initiation of sintering at 920 °C. In the temperatures between 900 and 1100 °C, a consistent change in the cross-sectional area in a linear form was noticed, followed by a gradual decrease in shrinkage. The initial shrinkage at the onset of sintering was approximately 6%, progressively increasing to 53% at 1050 °C. The peak change in the cross-sectional area occurred at 1200 °C, reaching 55%. The solid cube-shaped sample obtained may imply the presence of a molten phase at the grain boundary. However, no distinctive temperatures were recorded, such as softening, melting, or flow points.

The results obtained from the tests suggest that the heat treatment temperature for TiO₂–SiO₂ layers should be maintained below ca. 920 °C. Exceeding this temperature threshold may induce shrinkage, causing damage to the ceramic coating. In contrast, subjecting NiTi SMA to high heat treatment temperatures can lead to the decomposition of the alloy into equilibrium phases, consequently reducing transformation enthalpy and adversely impacting the shape memory effect. Previous research and literature data showed an optimal high annealing temperature of 800 °C for ceramic coatings applied to NiTi alloys [48]. At this temperature, the TiO₂–SiO₂ nano-composite undergoes slight thermal expansion (Figure 5), and these changes should not break the continuity of the produced layer.

Comprehending the thermal stability of coat-forming materials is also crucial in establishing the suitable heat treatment temperature. Consequently, the thermo-gravimetric method was employed to assess the thermal stability of the TiO₂–SiO₂ nanopowder under varying temperature conditions (Figure 6). The results indicated that up to a temperature of 725 °C, the nano-composite sample underwent a 1.70% reduction in its initial mass in two distinct stages: 1.30% loss up to 340 °C and an additional 0.40% loss up to 725 °C. The initial 1.30% mass loss is attributed to the release of water (with the maximum release occurring at 94 °C) and carbon dioxide (with the peak release at 92 °C). In the second stage, from 300 °C to 700 °C, a continuous release of CO₂ was observed, with a maximum at 391 °C and 581 °C. The differential thermal analysis (DTA) curve exhibits an exothermic thermal effect, peaking at 457 °C, suggesting a potential combustion reaction. It should be considered that trace amounts of carbon in the TiO₂–SiO₂ nano-composite, originating from the synthesis process, could influence the progression of the reaction within the heat-treated coating and impact the types of formed new phases.

3.4. Microstructure and Structural Characterization of the Heat-Treated TiO₂–SiO₂ Coating

Microscopic observations yielded insights into the impact of annealing on the microstructure changes in the electrophoretically deposited TiO₂–SiO₂ nano-coatings (Figure 7a–c). A comparison of the inter-agglomerate spaces’ appearance after deposition, where a thin film had formed with the sample after heat treatment, revealed noticeable crystallization and grain growth (Figure 7b). Observations also detected changes in the microstructure of the agglomerate surface (Figure 7c). Consequently, the surface of the coatings showed increased roughness and surface development. Notably, the observations indicated that the heat treatment did not induce cracks or delamination in the layer, be it the thin film or the agglomerates. Element distribution maps (Figure 7d) further verified the uniform dispersion of silicon in the coating, while its content remained unchanged compared to the coating before heat treatment.

After sintering, Raman spectroscopy also examined structural changes within the SiO₂–TiO₂/NiTi. Raman imaging maps were analyzed in the X- and Y-directions to identify spatial phase diversity and in the X- and Z-directions to elucidate the mechanisms underlying coating formation.

The results indicated slight chemical and structural composition differences compared to the electrophoretically deposited coating (Figure 8a,b). Rutile remained the dominant coat-forming phase, evidenced by maxima at approximately 268 cm⁻¹ (indicative of multiple-phonon scattering processes), 437 cm⁻¹ (E_g symmetry), and 605 cm⁻¹ (A_1g symmetry) [22,23]. Similarly to the analysis conducted for the untreated coating, the E_g-related band is susceptible to the O/Ti ratio [25,26] being slightly downshifted, confirming a lower O/Ti ratio with some oxygen vacancies in the TiO₂ structure [26]. Additionally, owing to the highly reactive environment within the coating, carbon dioxide became trapped within the defective titanium dioxide structure, forming a layered carbon system with unsaturated bonds. It was visible through bands around 1345 and 1605 cm⁻¹ (D- and G-planes) [49,50]. This rutile phase co-existed with more locally dispersed carbon-modified titanium dioxide islands, exhibiting a much greater carbon concentration around the titanium core (Figure 8a,b).

The reorganization of silica and rutile particles was more significantly visible in the formation of a two-phase system constituting a matrix for more defected carbon titanium dioxide, featuring an unusual band arrangement around 257, 337, 1350, and 1590 cm⁻¹, co-existing with typical rutile bands around 449, 546, and 618 cm⁻¹ (Figure 8a,b). The exact explanation for the band arrangement, especially considering the two-phase system composed of SiO₂ and TiO₂ and a reduction atmosphere, is not evident and is difficult to explain. As a result, subjecting rutile to high temperatures (600–1000 °C) in a reducing environment induces various defects in the TiO₂ crystal structure, such as oxygen vacancies, which alter the Ti and O ratio and lead to the formation of different suboxides with the chemical structure Ti_yO_1−x [51,52].

Importantly, defects introduced in rutile lead to thermodynamic instability and atomic reorganization into more stable Magnéli phases under high-temperature processes. An alternative hypothesis presumes the incorporation of silica into the outermost layer of the NiTi substrate, forming in the presence of carbon, an intermetallic titanium–silicon–carbon system according to thermodynamic conditions and individual phase diagrams [53]. According to such a hypothesis, a low-lying band around 157, 257, 337 cm⁻¹ originated from symmetrical modes of C–Ti–Si bonds in Ti₃SiC₂ molecules or other similar [54,55]. The applied thermodynamic conditions may also stimulate the crystallization of fire opal or opal-like structures due to planar Si–O modes of three and four-member rings [56].

Unfortunately, an unambiguous interpretation of such Raman bands is complex and requires additional experimental techniques to confirm the formation of individual structures. Nonetheless, it appears that the co-crystallization of variable defected or structurally nonstoichiometric phases could enhance the adhesion between the coating and the NiTi substrate, potentially eliminating the formation of undesirable brittle phases.

The crystallizing phase may have a complex crystal structure. Silicon (4+), due to its ionic radius, tends to occur in tetrahedral coordination in oxide systems, while titanium in both oxidation states (4+ and 2+) occurs in octahedral coordination. It should be expected that the possible incorporation of Ti into the spatial structure of SiO₄ tetrahedra will be interstitial and may cause defects in the spatial network and distortions of coordination figures. Comparable outcomes were observed with other composite coatings at similar heat treatment temperatures, demonstrating a significant enhancement in the bonding strength between the ceramic coatings and the metallic substrate [16,57,58,59].

Raman analysis after heat treatment conducted in the X-Z-direction revealed the reorganization of silica and titanium dioxide, forming a uniformly distributed coating around the NiTi substrate with locally segregated carbon-modified titanium dioxide structures (Figure 8c). Furthermore, cross-sections conducted at various locations consistently revealed signals from the phases across the profile, with some areas indicating slight inconsistency, suggesting the formation of a thin interlayered coating directly covering the NiTi substrate (Figure 8d). Consequently, the thickness of the thicker SiO₂–TiO₂ coating, estimated previously by calculating the full width at half maximum, reached approximately 1.4 µm. In comparison, the interlayer formed of defective or substoichiometric titanium oxide with the chemical formula Ti_nO_2n−1 after reduction reached a much lower thickness of less than 1.4 µm [51,60]. Notably, the variable thickness correlates well with the layer thickness before sintering, while the coating exhibits significantly higher continuity due to TiO₂–SiO₂ interconnections.

Upon sintering, the FTIR spectrum of the SiO₂–TiO₂ coating in the range of 400–1200 cm⁻¹ underwent slight modifications (Figure 8e). Bands related to titanium dioxide decreased in intensity, while those associated with silica intensified and became predominant in the spectrum. Specifically, bands linked to the symmetric stretching Si–O–Si bond and bending Si–O–Ti bonds shifted towards 799 cm⁻¹ and 968 cm⁻¹, respectively. The shift observed in the latter band is attributed to varying numbers of cross-linking Si–O–Ti bonds formed concurrently by the Ti atom [61], correlating with the dehydration and partial or complete decomposition of the Si–OH bonds around the Ti–O network. This effect significantly enhances the intensity of the Si–O–Ti bond and promotes intermolecular interaction between titanium dioxide and silica oxides. Some studies suggest that this observation leads to the slow growth of titania grain [62] as an amorphous phase composed of Si–O–Ti cross-linking bonds and -OH radicals emerge.

Moreover, the intensity of other bands, originating from the asymmetric stretching vibration of the Si–O–Si bond at 1071 and 1139 cm⁻¹, increases due to decreased defects in the SiO₄⁴⁻ tetrahedra. It is crucial to underscore that the more vigorous intensity of the silica bands correlates with a lower presence of titanium dioxide in the infrared spectrum, suggesting segregation of the coat-forming materials. Additionally, the initially observed very tiny silica particles begin to form a solid solution with titanium dioxide, indicating a smooth and continuous SiO₂–TiO₂ coating uniformly covering the NiTi substrate.

4. Conclusions

To functionalize the surface of the NiTi alloy, hybrid layers consisting of nanometric silica and titanium oxides were synthesized. The TiO₂–SiO₂ nanosystem was chemically prepared and utilized for electrophoretic deposition. Investigating the impact of pH on Zeta potential and ceramic particle size ensured a stable colloidal suspension for electrophoretic deposition, with optimal parameters identified at a pH of approximately 6. Employing a voltage of 40 V and a deposition time of 3 min resulted in the forming a consistent layer, appearing as a thin film interspersed with regularly spaced larger agglomerates. The research revealed that the thin film primarily comprised the most minor fraction of defected rutile nanoparticles. The agglomerates constituted silica and carbon (originating from the nanosystem synthesis process) with larger rutile nanoparticles, including defective ones. High-temperature tests demonstrated that to mitigate the risk of cracks in the coatings due to the shrinkage of the coat-forming material, the heat treatment of such layers should not exceed 920 °C. Because of the NiTi substrate, the process was conducted at 800 °C for 2 h. The annealing induced significant structural alterations in the layer, creating a novel-generation material characterized by a structure distinct from the initial materials. It was found that strong Si–O–Ti connections were formed, and an interlayer was created.