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Article

Optimization of the Electrophoretic Deposition Parameters and Mechanism of Formation of Ag-TiO2 Nanocoatings on a NiTi Shape Memory Alloy: Part I

by
Karolina Dudek
1,*,
Mateusz Dulski
2,*,
Jacek Podwórny
1,
Magdalena Kujawa
1 and
Patrycja Rawicka
3
1
Łukasiewicz Research Network–Institute of Ceramics and Building Materials, Cementowa 8, 31-983 Kraków, Poland
2
Institute of Materials Engineering, University of Silesia, 75 Pułku Piechoty 1A, 41-500 Chorzów, Poland
3
Institute of Physics, Faculty of Science and Technology, University of Silesia in Katowice, 75 Pułku Piechoty 1a, 41-500 Chorzów, Poland
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(1), 44; https://doi.org/10.3390/coatings14010044
Submission received: 22 October 2023 / Revised: 16 November 2023 / Accepted: 22 December 2023 / Published: 28 December 2023
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Abstract

:
This paper reports research findings on the functionalization of NiTi shape memory alloy through the electrophoretic deposition of innovative complex layers comprising a silver-rutile (Ag-TiO2) nanocomposite. A colloidal suspension of a chemically synthesized Ag-TiO2 nanosystem prepared with a 59.4 ± 0.9 mV Zeta potential for anaphoretic deposition. Employing a design of experiment method (DoE), the optimal parameters for uniform coating depositions were identified as 40 V/3 min. Dilatometric tests and high-temperature microscopy determined that the deposited layers’ heat treatment temperature should not exceed 800 °C. Raman spectrometry and scanning electron microscopy (SEM) provided comprehensive structural and morphological insights into the resulting continuous and crack-free layer. The article extensively explores the impact of annealing on structural changes within the layer, proposing mechanisms for its formation. The findings affirm the feasibility of creating a highly reactive layer on the NiTi alloy, holding potential significance in implantation medicine.

Graphical Abstract

1. Introduction

Titanium oxides, particularly in nanometric dimensions, find extensive applications in the surface modification of alloys employed in implantation medicine. Their adoption is attributed to the exceptional mechanical strength and biocompatibility they exhibit. Moreover, these materials possess the unique capability to promote tissue integration and cell adhesion, critical factors for the success of implantation procedures. Facilitating the bone healing process and fostering the growth of new bone tissue, these materials contribute significantly to ensuring the stability of implants within the patient’s body [1,2].
Titanium oxides exhibit indirect antibacterial effects, as they are not inherently toxic to bacteria or capable of microorganism destruction. Rutile coatings, in particular, influence the surface properties of medical implants, resulting in decreased bacterial adhesion and biofilm formation [3,4,5]. This effect can contribute to lowering the risk of infection and mitigating other complications associated with implants. Titanium oxide layers, especially titanium oxide nanotubes, can be engineered to release controlled amounts of therapeutic, anti-inflammatory, or antibacterial substances [6]. This feature enables localized pharmacological therapy on the implant surface.
Additionally, the surface of titanium oxide can be modified with other substances, such as copper or silver, renowned for their antibacterial effects. Notably, nano silver and antibiotics share a similar impact on microorganisms, but microorganisms do not develop resistance to silver. Silver is among the most effective elements against a broad spectrum of bacteria and fungi. Its antibacterial action disrupts the cellular structure of bacteria, influences the functions of bacterial enzymes, and damages the cell membrane, ultimately resulting in bacterial destruction and growth inhibition [7,8].
Titanium oxides as layers are primarily used in the surface modification of titanium alloys. NiTi alloys are one of the titanium alloys widely used in short-term implantology shape memory alloys [9,10]. NiTi alloys with a composition close to that of the equiatomic ones reveal a unique property: the shape memory phenomenon. However, long-term implantation of NiTi alloys entails a severe limitation. It is related to the alloy’s high nickel content and the risk of releasing its toxic ions into the organism due to the aggressive environment of body fluids [11]. The solution to the problem is to modify the alloy surface by creating surface layers to provide a mechanical barrier for the released nickel ions, enabling the implant to stay in the patient’s body for longer.
The contemporary challenge in scientific research is to engineer multifunctional layers for implants, enhancing the biocompatibility of metallic alloys and imparting antibacterial properties. These layers must remain thin and flexible for shape memory alloys to avoid limiting or obstructing the shape memory effect. Various surface engineering methods facilitate the creation of thin ceramic layers using nanometric or submicrometric particles for deposition [12]. Among these methods, electrophoretic deposition (EPD) stands out, allowing for the generation of layers with diverse thicknesses and morphologies by precisely controlling deposition parameters, such as voltage/current or deposition time [13,14,15]. Electrophoresis can yield new materials with intriguing properties, such as a very active surface, which translates into wettability, bioactivity, and biocompatibility [16].
This study employed the electrophoretic deposition method to fabricate innovative multifunctional coatings on a shape memory alloy, utilizing a chemically synthesized nanometric molecular systems rutile-silver nanocomposite (Ag-TiO2) as the initial material. To optimize deposition parameters efficiently, a well-established experimental design method, the factorial method 22 of planning experiments, was employed [17], significantly streamlining this experimental stage. Subsequently, sintering conditions for the deposited layers were carefully selected. The resulting novel coating was thoroughly characterized in its structure and morphology, and the layer formation mechanism was extensively discussed.

2. Materials and Methods

2.1. Substrate

The commercially available NiTi alloy in the β-phase (B2) with characteristic temperatures of martensitic transformations below an ambient temperature was used as a substrate for coating deposition. SiC papers polished the samples up to 2000 grit. Before deposition, the surface of the samples was washed in acetone in an ultrasonic bath to remove trace amounts of impurities. Then, to improve the corrosion resistance, they were passivated in an autoclave at 134 °C for 30 min to form a protective corrosion-resistive thin amorphous TiO2 layer on their surface [18,19].

2.2. Synthesis of Ag-TiO2 Nanocomposite

The sample was synthesized by a chemical reduction method in an air atmosphere. All reagents used to prepare the Ag-TiO2 nanocomposite were commercially available and analytically graded. The silver nitrate (AgNO3), sodium hydroxide (NaOH), and isopropyl alcohol were purchased from Biomus sp. z.o.o. (Lublin, Poland). Titanium dioxide (TiO2) was produced by US Research Nanomaterials Inc. (Huston, TX, USA). The first stage of the synthesis was the dissolution of a 10 g titanium dioxide carrier with a rutile structure with a particle size of approximately 30 nm in 150 mL of a mixture of distilled water and isopropyl alcohol in a ratio of 2:5. Then, the mixture was stirred on a magnetic stirrer for 30 min at 200 °C. In a separate vessel, 15 mL of a 4% aqueous solution of NaOH was prepared. After 30 min, an aqueous NaOH solution was added dropwise to the support and stirred at 100 °C for 2 h. For depositing Ag nanoparticles onto the TiO2 matrix, 20 mL of a 4% aqueous solution of AgNO3 was added dropwise to create a nanocomposite. The solution was stirred at 100 °C for 1 h. The last synthesis stage was the filtration of the nanosystem through a paper filter and drying it. As a result of the synthesis, the Ag-TiO2 nanosystem, with 4.7 ± 0.6 wt.% of silver, was obtained.

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

The coatings were deposited using the electrophoresis (EPD) technique from a colloidal suspension having a concentration of 0.1 wt.% of the nanocomposite Ag-TiO2 powder in 75% ethanol (Avantor). Before deposition, the suspensions were placed into a magnetic stirrer for 1 h and then in an ultrasonic bath for 2 h. The Zeta potential of the suspension was −59.4 ± 0.9 mV, which allowed for anaphoretic deposition to be performed. The electrophoretic deposition was carried out under a 30–50 V range and 1–5 min. Next, the coatings were dried at room temperature for 24 h and then subjected to a heat treatment at 800 °C in a low vacuum for 2 h.

2.4. Method of Testing

Zeta potentials were measured on a Malvern Zetasizer Nano ZS particle size analyzer with a 633 nm He-Ne laser. The analyzer measures electrophoretic mobility using the M3-PALS (Mixed Mode Measurement–PhaseAnalysis Light Scattering from Malvern PANalytical, Malvern, UK) technique to measure electrophoretic mobility. Then, it uses Henry’s equation to determine the value of the Zeta potential. Measurements were performed at room temperature in a U-shaped cuvette (DTS1070 from Malvern PANalytical, Malvern, UK).
Scanning electron microscopy (SEM) data were obtained by a TESCAN Mira 3 LMU with an Energy Dispersive Spectrometer (EDS) from Oxford Instruments. Aztek was used to determine the microstructure and perform the chemical analysis. Images were collected by secondary electrons (SEs) and backscattered electrons (BSEs). The measurements were carried out on the samples covered by a carbon layer using Quorum Q150T ES equipment.
Thermo-mechanical analysis (Setaram TMA 92, Caluire, France) was used to perform measurments of the powder thermal expansion and shrinkage. TMA was performed up to 1200 °C at a heating rate of 7 °C/min in an air atmosphere at 1.5 bar. The powder sample was placed in a tiny crucible and loaded using a flat-ended alumina probe with an applied load of 5 g (to maintain the sample’s integrity before starting the experiment). The TMA curve was corrected for the blank measurement, i.e., the expansion of the corundum elements.
Linear changes during heating of the Ag-TiO2 nanocomposite were examined in a Leitz high-temperature microscope as an approximate simulation of the phenomena occurring during heat treatment. A sample was formed in a hand press and a cube with a side of approx. 3 mm was obtained. The sample was placed on a corundum pad and then heated to 1200 °C at a 7 °C/min rate. An image of the sample was recorded in the microscope eyepiece at temperatures corresponding to changes in shape. Thanks to the continuous observation of the sample and the photographic recording of changes in its shape and dimensions as a function of temperature, several photos were obtained showing the material’s behavior during heating. Based on the sample photos, the relative change in the sample cross-sectional area δ(T) as a function of temperature and the sintering start temperature was determined.
The Raman measurements on NiTi alloy substrates with functionalized surfaces in the presence of Ag-TiO2 before and after the heat treatment were performed using a WITec confocal alpha 300R (WITec Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) Raman microscope (CRM) equipped with an air-cooled solid-state laser (λ = 532 nm) and CCD detector. Laser radiation in such solution was directed into the microscope through a polarization-maintaining single-mode optical fiber, boasting a 50 μm diameter. The scattered radiation was focused on a multi-mode fiber (50 μm diameter) and a 600 line/mm grating monochromator. An Olympus MPLAN 100×/0.9NA objective was used to preserve the optimal parameters between lateral and depth resolution [20]. In this context, lateral resolution (LR) was estimated according to the Rayleigh criterion LR = 0.61λ/NA, while depth resolution (DR) 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, LR = 0.36 μm, while DR = 0.65 μm. 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. 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 +7.0 up to −7.0 μm in the Z-direction in a 50 μm × 15 μm area using 150 × 45 pixels (=6750 spectra) with an integration time of 500 ms per spectrum, and the precision of moving the sample during the measurements was ±0.5 μm. All spectra were collected in the 75–4000 cm−1 range at 10 mW on the sample and 3 cm−1 spectral resolution. The output data were manipulated by performing a baseline correction using the auto-polynomial function of degree 3 and were submitted to an automatic cosmic ray removal procedure. A basis analysis implemented in the WITec ProjectFive Plus Software (version 5.1.1, WITec Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany) was applied to differentiate the coat-forming material chemically and structurally. Finally, a band fitting analysis using a Lorentz–Gauss function with the minimum number of components was performed using the GRAMS (version 9.2, Thermo Fisher Scientific, Waltham, MA, USA) software package on the averaged spectrum originating from the individual sample. A similar procedure was performed five times to analyze the whole NiTi substrate coating statistically.

3. Results and Discussion

3.1. Optimization of Coating Deposition Parameters, Characterization of the Deposited Coatings, and Mechanism of Formation

The well-known factorial design of experiment type 22 was used to select the most favorable parameters for applying the Ag-TiO2 layers on the NiTi alloy [17]. The first series of planned experiments was based on the authors’ experience selecting the parameters for layer production. The most optimal parameters for layer preparation were sought in the voltage range of 40 V plus and minus 10 V and the layer deposition time of 3 min plus and minus 2 min. The experimental design is presented in Table 1. The plus sign indicates the assumed value of the parameter at the higher level, and the minus sign indicates the value at the lower level. The response function was based on a subjective evaluation by an SEM observation of the quality of deposited layers on three test pieces from each deposited sample. Among those test pieces, a few areas were observed. The response function was constructed with a subjective evaluation of the coating quality based on microscopic examination and the team’s experience. The numerical parameters related to the size of the agglomerates and the distances between them were not determined during this evaluation. Obtaining a uniformly applied layer without cracks and a uniform distribution of small agglomerates was essential. Figure 1 shows example SEM images for layers deposited under the conditions in Table 1. Table 1 shows the plan of experiments with the given range of variability of the optimized parameters X1 (deposition time), X2 (deposition current-voltage), and X12 and the interaction of both these parameters. The plus and minus signs indicate the higher and lower values of the X1 and X2 parameters, respectively. The correlation coefficients b1, b2, and b12 determine the impact of parameter variability within the adopted limits on the course of the layer deposition process. The evaluation criterion was the formation of a thin film (made of the finest fraction of a colloidal suspension; it is deposited first, and then larger particles forming agglomerates are deposited), nanocomposite agglomerates, uniform distribution of small and big agglomerates, and density of agglomerates. The scale of points used to assess the quality of the obtained layer was from 0 to 5 points: 0 means no film was formed, and 5 means uniformly distributed small agglomerates.
Microscopic observations revealed the influence of applied voltage and deposition time on the morphology and quality of the produced coatings. Coatings deposited at low voltage and a short deposition time (30 V/1 min) were characterized by the uneven distribution of small agglomerates of Ag-TiO2 particles (Figure 1a–c). However, it was found that a thin ceramic film was formed between the agglomerates on the surface of the NiTi alloy during deposition. Visible darker places in SEM images (Figure 1b,c) originate from places where larger agglomerates were detached, confirming the presence of the film. Increasing the deposition time to 5 min increased the number of deposited agglomerates that formed clusters or islands (Figure 1d).
Moreover, a higher voltage of 50 V caused an increase in the size of the islands and the deposition of more large agglomerates, visible in Figure 1e. Microscopic examinations also showed that the result of extending the time while applying the same voltage is the deposition of large agglomerates in the coating (Figure 1d,f). This phenomenon is unfavorable in the case of coatings on shape memory alloys because, during deformation, more significant stresses and cracks appear at the boundaries of large agglomerates, which may result in the delamination and chipping of such parts of the layer.
The results in Table 1 indicate that the most favorable parameters are similar to those from experiment number 1 (time 1 min and voltage 30 V) but not entirely satisfactory. From the determined correlation coefficients b1 and b2, the deposition time and voltage strongly influence the quality of the deposited coating. However, the correlation coefficient b12 obtained for the interaction of both parameters has a lower value than the coefficients determined for each parameter separately. It indicates that optimizing both parameters should allow finding the most favorable deposition conditions. Based on this conclusion, the initial values for optimizing the deposition parameters were those from experiment 1, shortening or extending the deposition time by 30 s and the current voltage by 5 V in each subsequent experiment. The optimization experiment plan is presented in Table 2.
The optimal parameters for the deposition of Ag-TiO2 coatings on the NiTi alloy using electrophoretic deposition were found in experiment 10, which gave a satisfactory result. The layer production parameters were then set at 40 V and 3 min. Microscopic observations of this coating unveiled that such production conditions result in a regular arrangement of small agglomerates with a thin film between them (Figure 2). Furthermore, it was noticed that the agglomerates were composed of titanium oxide nanoparticles, which deposited larger agglomerates of silver nanoparticles (bright spots in Figure 2c). In turn, the element distribution map (Figure 2d) and SEM-BSE observations (Figure 2c) revealed that fine Ag particles were evenly dispersed over the entire sample surface, confirming the presence of a thin film. Ni originating from the NiTi substrate in the analysis suggests that the formed layer is relatively thin.
Raman spectroscopy played a pivotal role in validating the formation of coatings resulting from the electrophoretic modification of the NiTi surface. In this context, the NiTi surface, featuring an as-prepared Ag-TiO2 coating, was subjected to 2D Raman imaging in the X- and Y-directions and X- and Z-directions. This approach comprehensively illustrated the chemical and structural phase diversity across the coating. The following test results are consistent with the microscopic observation, chemical composition analysis, and element distribution maps recorded using the SEM-EDS method (Figure 2).
An in-depth analysis of the X-Y Raman maps revealed a complex four-phase system with varying material packing and distribution degrees. Statistically, the dominant component constituting the coating was determined to be rutile, evident from the distinctive band arrangement with maxima at approximately 265 cm−1 (indicative of multiple-phonon scattering processes), 429 cm−1 (Eg symmetry), and 614 cm−1 (A1g symmetry) [21,22,23,24]. Rutile exhibited a uniform coverage of the NiTi surface, with variable material aggregation densities, as indicated by variations in color intensity across the coating (observable as brighter and darker orange areas in Figure 3a). Notably, the positions of the prominent rutile bands due to the specific synthesis procedure of the coat-forming nanocomposite were slightly upshifted compared to typical literature-derived band positions [21,22]. In this process, the surface of the titanium oxide underwent modification through the introduction of silver ions, resulting in structural distortion of the octahedra and the formation of Ag–O–Ti interconnections or Ag–O–Ag complexes chemically sorbed onto the distorted titanium dioxide surface [25]. The Raman analysis further unveiled small clusters of another titanium dioxide polymorph (evident as green spots in Figure 3a,b), which exhibited a characteristic anatase band arrangement featuring maxima at 156 cm−1 (Eg symmetry), 391 cm−1 (B1g symmetry), 511 cm−1 (A1g symmetry), and 636 cm−1 (Eg symmetry) [26,27,28]. Research shows that it occurred in trace amounts.
However, the most intriguing aspect of the chemical analysis of the Ag-TiO2 coating pertained to the presence of micrometer-sized silver carbonate-related islands (appearing in red/pink in Figure 3a,b). These islands were identified through bands centered around 90, 130, 150, and 200–300 cm−1 (indicative of silver lattice vibrational modes) and 1560 cm−1 (corresponding to CO32−) [29]. The origin of these silver carbonates can be explained in two ways: one hypothesis posits that a highly reactive chemical environment, brought about by the surface modification of the TiO2 nanocomposite in the presence of silver, led to the incorporation of carbon into the distorted titanium oxide octahedra, resulting in the formation of Ti-O-C interconnections. The other hypothesis relates to carbon dioxide adsorption and its reaction with silver ions, culminating in the crystallization of silver carbonate with an aragonite structure, potentially hydrogen-bonded within (Ag-…-OHAg2CO3) complexes [30]. Remarkably, the absence of some silver carbonate bands could only be explained by assuming a simultaneous combination of the previously mentioned mechanisms. It leads to the atypical spatial coordination of carbonate units chemically bonded to the distorted titanium dioxide with tailored silver ions or particles. Finally, the coexistence of silver carbonates and metallic silver nanoparticles was notably observed due to the presence of prominent bands at 670 and 740 cm−1, which are indicative of molecularly chemisorbed oxygen species [31,32,33,34], contribute to the surface’s structural complexity, and enhance its reactivity (see remark O*/Ag in Figure 3a,b).
Valuable insights were also derived from the fundamental analysis of the Raman cross-sections conducted in the X-Z-directions. This examination unveiled that rutile and silver carbonate are the predominant materials forming the coating. Rutile particles tended to aggregate into irregular clusters of varying sizes, with isolated silver carbonate particle aggregates dispersed throughout the coating (Figure 3c). Additionally, cross-sections obtained from different locations demonstrated the presence of signals from the coat-forming components along the entire profile, extending to the interface with the NiTi substrate. Notably, the coating’s thickness exhibited significant variations, with the thickness estimation conducted by calculating the full width at half maximum of the Raman signal (Figure 3d). This analysis pointed to a two-stage process in forming the composite coating. In the initial stage, a thinner layer composed of Ag2CO3 and Ag-TiO2 nanocomposites, with an approximate thickness of ca. 1 µm, was formed close to the NiTi surface during the electrophoretic deposition process. Subsequently, as the deposition process continued, the particles exhibited a propensity to aggregate, resulting in a rougher coating with a thickness ranging from approximately 2.2 µm to 3.2 µm (Figure 3d).

3.2. Determination of the Heat Treatment Temperature of Ag-TiO2 Coatings

The heat treatment of electrophoretically deposited ceramic layers is necessary to create a chemical bond between the coating and the metallic substrate [35]. The temperature also causes the sintering of ceramic particles. However, in the case of shape memory alloys, a high heat treatment temperature may lead to the decomposition of the NiTi alloy into equilibrium phases, lowering transformation enthalpy and consequently negatively affecting the shape memory effect. Therefore, heat treatments of layers on this type of alloy are carried out to a maximum temperature of 900 °C [36,37]. Moreover, the temperature affects the thermal expansion of ceramic layers and their shrinkage results in the induction of stresses in the material.
Consequently, this may lead to cracking and delamination of the coatings [16]. Therefore, it is crucial to determine the characteristic temperatures and know the ceramic’s behavior for use as a coating material. For this purpose, dilatometric and high-temperature microscope tests were performed.
The outcomes of the dilatometric analysis of the Ag-TiO2 nanocomposite are shown in Figure 4. It was found that the material reveals a small thermal expansion up to 560 °C of 0.46% and subsequent slow shrinkage up to 800 °C, related to the beginning of sintering. A maximum shrinkage of 12.12% was registered at 1200 °C.
Studies using a high-temperature microscope allowed for estimating the size and speed of linear changes accompanying structural changes in the nanocomposite. Photographs of the sample at selected temperatures, recorded during the examination, and a graph of linear changes as a function of temperature are shown in Figure 5. The change in the cross-sectional area of the sample begins at a temperature ca 800 °C, while the beginning of sintering was determined at 850 °C (Ts). In the 850–1050 °C range, a linear change in the sample cross-sectional area was observed, consistent with dilatometric studies (Figure 4). It was observed that above 1050 °C, the shrinkage slowed down. Shrinkage at the sintering temperature was approximately 7%. Then, it gradually increased to almost 45% at a temperature of 1050 °C (T1). The maximum change in the cross-sectional area of the sample was 55% at 1200 °C. A solid shape in the form of a cube was obtained, which may indicate the appearance of a liquid phase at the grain boundary. Still, no other characteristic temperatures were recorded, i.e., softening, melting, or flowing temperatures.
Considering the results of the above tests, the formation of cracks in the deposited Ag-TiO2 coating due to the heat treatment temperature should appear above 800 °C. Hence, 800 °C was applied for layers deposited at 40 V for 3 min, and then the influence of temperature on changes in microstructure and structure was analyzed.

3.3. Characterization of the Coatings after Heat Treatment, Phase Transitions, and Mechanism of Formation

Microscopic observations provided information about the impact of the heat treatment on changes in the microstructure of the produced layers (Figure 6). By comparing the appearance of the spaces between the agglomerates after deposition, where a thin film was formed (Figure 2b), with the sample after annealing (Figure 6d), crystallization and grain growth were observed. The surface became rougher. Moreover, observations using the BSE detector revealed changes in the distribution of silver particles in the coating. As a result of the temperature, a higher number of Ag agglomerates visible in Figure 2c decomposed. However, the surface of the heat-treated layer was still covered by a thin layer consisting of silver (light areas in Figure 6c,e). Element distribution maps (Figure 6f) confirmed the uniform distribution of silver over the entire sample surface. The outcomes also showed no discontinuity in the layer, which confirms the well-selected heat treatment temperature.
Following a similar analytical approach as previously applied to the electrophoretically deposited Ag-TiO2 coating on the NiTi alloy, Raman spectroscopy was used to determine structural changes. Raman imaging maps were obtained from various areas across the entire coating. These maps were analyzed both in the X- and Y-directions to recognize the spatial phase diversity of the materials and in the X- and Z-directions to understand the mechanisms behind coating formation after heat treatment. The results indicated slight differences in chemical and structural composition compared to the as-prepared Ag-TiO2 coating, with three dominant coat-forming phases observed (Figure 3 and Figure 7). Like the as-prepared Ag-TiO2 coating, one characteristic anatase-related band around 154 cm−1 (Eg symmetry) appears after heat treatment, suggesting its distribution as tiny particles around the entire coating [26,27,28]. In turn, rutile as a dominant coat-forming phase was determined due to characteristic bands and maxima at approximately 270 cm−1 (indicative of multiple-phonon scattering processes), 422 cm−1 (Eg symmetry), and 609 cm−1 (A1g symmetry) [21,22,23]. The positions of the bands at 270 and 609 cm−1 closely matched those found in the literature for rutile (Figure 7a,b). However, the most prominent Eg mode exhibited a downshift toward lower frequencies. This behavior can be explained by examining the role of the Eg mode, attributed to in-plane vibrations, which are highly sensitive to the O/Ti ratio [38,39]. Previous studies have demonstrated that a decrease in the O/Ti ratio is correlated with a significant increase in oxygen vacancies in TiO2 and structural distortions that strongly modify the characteristics of rutile [39].
Consequently, the initially existing Ag-TiO2 composite appeared to be subjected to temperature-induced decomposition due to the breaking of Ag–O–Ti interconnections [25]. It leads to the release of non-stoichiometric and highly reactive silver oxide, silver ions, or reactive oxygen species. A similar thermal degradation effect was applied to the Ag–O–Ag complexes. Moreover, the primary silver carbonate within the rutile coating undergoes a phase transition and a gradual degradation due to high temperature. This transformation involved a minor thermal expansion in the b-axis and a positive expansion in the a- and c-axes [40]. In the initial stages of the annealing, up to approximately 180 °C, this process led to the growth of α, β-Ag2CO3 particle agglomerates. Subsequently, α, β-Ag2CO3 decomposed into silver oxide (ca. 230 °C) and then into metallic silver (ca. 380 °C), with the diffusional removal of CO2 through the surface product layer [41,42].
Furthermore, due to the highly reactive environment within the coating, carbon dioxide was entrapped within silver nanoparticle cores, forming a layered carbon system with unsaturated bonds. It was observed through bands between 200 and 400 cm−1 (C-Ag bending) and around 1340 and 1580 cm−1 (D- and G-planes) [43,44]. On the Raman chemical maps, the thermal decomposition products of carbonates appeared as isolated and randomly distributed islands. Moreover, the thermal expansion of the carbonates triggered the reorganization of titanium dioxide particles, which tended to order around the carbonate islands (Figure 7a,b).
The reorganization of rutile particles was associated with forming a phase featuring an unusual band arrangement around 280, 340, 1350, and 1590 cm−1 (Figure 7a,b). The exact explanation for this band arrangement is not evident. Still, it can be attributed to a combination of factors, including the NiTi surface’s passivation and coat-forming materials’ thermal decomposition products (especially Ag-O-Ti interconnections, Ag-O-Ag complexes, or Ag2CO3 nanoparticles). Consequently, the disruption of such structures generated highly reactive silver ions, which, at the applied annealing temperature (800 °C) and with the remnants of carbon dioxide, formed thermal hotspots, favoring the formation of non-stoichiometric TiyO1−x with unsaturated bonds. In turn, non-stoichiometric titanium dioxides allowed for the anchoring of silver and enforced the formation of an interlayer with core-shell carbon-covered silver nanoparticles [39,40]. An alternative hypothesis presumes to incorporate of silver ions into the outermost layer of the NiTi substrate and form intermetallic TiAg joints according to the thermodynamic conditions and Ti-Ag phase diagram [45]. These coatings will enhance bonding strength and flexibility or eliminate the formation of an undesirable intermetallic or brittle phase at the interface between the coating and substrate. Similar results, obtained at comparable sintering temperatures of composite coatings, have been reported for the silver interlayer placed between stainless steel and a titanium substrate, resulting in a significant increase in the bonding strength between the coating and the substrate [46,47].
In line with the deposited coating, post-annealing Raman cross-sections in the X-Z-direction provided clear evidence of the segregation of coat-forming phases with one layer uniformly coating the NiTi substrate. As determined by Raman analysis, the outermost layer primarily comprised rutile particles, while the interlayer featured compacted Ag, AgxO, and TiyO1-2 particles (Figure 7b,c). Additionally, particle agglomerates corresponded to core-shell structures of carbon-layered silver, unevenly distributed and displaying irregular shapes detected within both layers (Figure 7b,c).
Like the as-prepared coating, cross-sections from various locations exhibited consistent signals from the coat-forming materials throughout the profile, resulting in a relatively uniform thickness (Figure 7d). The thickness in this instance was estimated by calculating the full width at half maximum of the Raman signal. According to this analysis, the interlayer near the NiTi substrate surface reached an approximate thickness of around 2 µm. Notably, this layer was formed after the decomposition of the initially thick Ag-TiO2 and Ag2CO3 layers. In turn, the doubling of its thickness was attributed to the high mobility of titanium dioxide particles. These tiny anatase particles tended to migrate into available spaces and consolidate with the interlayer material.
Consequently, the thickness of the outer layer decreased, spanning a range from approximately 1.2 to 2.2 µm (Figure 7d). This consolidation of the interlayer significantly improved the surface roughness, which was visible during SEM observation (Figure 6d).

4. Conclusions

Innovative complex multifunctional layers of silver-rutile (Ag-TiO2) nanocomposite was obtained using electrophoretic deposition (EPD) to functionalize the NiTi shape memory alloy surface. The chemically synthesized Ag-TiO2 nanosystem prepared a colloidal suspension for EPD. A design of the experiment and analysis of variance methods were applied to optimize the parameters of uniform coating deposition and reduce the time needed to conduct this research stage. The optimal parameters of production coatings were determined as 40 V/3 min.
The coating was formed as a thin film with a thickness of ca. 1 µm, with regularly spaced agglomerates and silver nanoparticles uniformly covering the entire layer surface. Some silver particles formed larger clusters between the rutile particles in the agglomerates. The thickness of the agglomerates ranged from ca. 2.2 µm to ca. 3.2 µm. The Raman investigation also revealed that the obtained coating is a complex system where rutile and silver carbonate are the predominant materials forming the coating. Moreover, during the EPD, the surface of the titanium oxide underwent modification by introducing silver ions.
Electrophoretically deposited coatings were subjected to a heat treatment at 800 °C in a low vacuum for 2 h to create a chemical bond with the metallic substrate and increase adhesion. As a result of the applied heat treatment, changes in the morphology and structure of the coatings were detected. A higher number of Ag and Ag2CO3 agglomerates decomposed. However, the fine silver particles still uniformly covered the entire coating surface. Raman analysis provided information about phase transformations and decomposition in the coat-forming material. The outermost layer of the coatings primarily comprised rutile particles, while the interlayer (the film) featured compacted Ag, AgxO, non-stoichiometric titanium oxide TiyO1-2 particles, and a TiAg-related interphase. Crystallization of the new phase and grain growth was observed, and the thin film between the agglomerates became rough and reached a thickness of ca. 2 µm. Particles with core-shell structures of carbon-layered silver were identified in both layers. As a result of transformations, the thickness of the outer layer decreased, spanning a range from approximately 1.2 to 2.2 µm. The produced layers were crack-free and showed no signs of delamination.
In summary, a very reactive layer was obtained on the NiTi alloy due to the applied processes, which could be very important in implantation medicine.

Author Contributions

Conceptualization, K.D.; methodology, K.D., M.D., J.P., M.K. and P.R.; validation, K.D., M.D. and J.P.; formal analysis, K.D.; investigation, K.D., M.D., J.P., M.K. and P.R.; resources, K.D., M.D., J.P., M.K. and P.R.; data curation, K.D.; writing—original draft preparation, K.D.; writing—review and editing K.D., J.P. and M.D.; visualization, K.D., M.D., J.P. and M.K.; supervision, K.D.; project administration, K.D.; funding acquisition, K.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Center in Poland (NCN), grant number 2020/39/D/ST5/01531.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

https://repod.icm.edu.pl/dataset.xhtml?persistentId=doi:10.18150/64KWUO (accessed on 20 October 2023).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the study’s design, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

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Coatings 14 00044 g001
Figure 1. SEM-SE images of the electrophoretically deposited Ag-TiO2 coatings under different conditions (a,b,d–f) and SEM-BSE image (c) of coating deposited at 30 V/1 min.
Figure 1. SEM-SE images of the electrophoretically deposited Ag-TiO2 coatings under different conditions (a,b,d–f) and SEM-BSE image (c) of coating deposited at 30 V/1 min.
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Figure 2. SEM-SE images (a,b), SEM-BSE image (c), and element distribution map (d) of the electrophoretically deposited Ag-TiO2 coating at 40 V/3 min.
Figure 2. SEM-SE images (a,b), SEM-BSE image (c), and element distribution map (d) of the electrophoretically deposited Ag-TiO2 coating at 40 V/3 min.
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Figure 3. A chemical and structural differentiation image of the functionalized Ag-TiO2/NiTi surface after electrophoretic deposition visualized in the X- and Y-directions (a) and X- and Z-directions (c) with averaged Raman spectra corresponding to individual color-highlighted spots (yellow: rutile, violet: silver carbonate, red: silver carbonate with oxygen species, green: anatase) observed on the Raman maps (b) and depth scan profiles of the four exemplary places on the X- and Y-direction cross line (d). Dashed lines on the depth profiles determine the boundary of the Ag-TiO2 layer.
Figure 3. A chemical and structural differentiation image of the functionalized Ag-TiO2/NiTi surface after electrophoretic deposition visualized in the X- and Y-directions (a) and X- and Z-directions (c) with averaged Raman spectra corresponding to individual color-highlighted spots (yellow: rutile, violet: silver carbonate, red: silver carbonate with oxygen species, green: anatase) observed on the Raman maps (b) and depth scan profiles of the four exemplary places on the X- and Y-direction cross line (d). Dashed lines on the depth profiles determine the boundary of the Ag-TiO2 layer.
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Figure 4. Change in linear dimensions of the Ag-TiO2 nanocomposite with temperature.
Figure 4. Change in linear dimensions of the Ag-TiO2 nanocomposite with temperature.
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Figure 5. Dimensional changes in the Ag-TiO2 nanocomposite, where Ts is the sintering start temperature, T1 is the end of sintering temperature (a), and the pictures of the cube sample during the test in the high-temperature microscope (b).
Figure 5. Dimensional changes in the Ag-TiO2 nanocomposite, where Ts is the sintering start temperature, T1 is the end of sintering temperature (a), and the pictures of the cube sample during the test in the high-temperature microscope (b).
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Figure 6. SEM-SE images (a,b,d), SEM-BSE images (c,e), and element distribution map (f) of the Ag-TiO2 coating after heat treatment.
Figure 6. SEM-SE images (a,b,d), SEM-BSE images (c,e), and element distribution map (f) of the Ag-TiO2 coating after heat treatment.
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Figure 7. A chemical and structural differentiation image of the functionalized Ag-TiO2/NiTi surface after sintering visualized in the X- and Y-directions (a) and X- and Z-directions (c) with averaged Raman spectra corresponding to individual color-highlighted spots (yellow: rutile, black: Ag@C, and blue: Ag@C-TiyO1-x) observed on the Raman maps (b) and depth scan profiles of the four exemplary places on the X- and Y-direction cross line (d). Dashed lines on the depth profiles determine the boundary of the Ag-TiO2 layer.
Figure 7. A chemical and structural differentiation image of the functionalized Ag-TiO2/NiTi surface after sintering visualized in the X- and Y-directions (a) and X- and Z-directions (c) with averaged Raman spectra corresponding to individual color-highlighted spots (yellow: rutile, black: Ag@C, and blue: Ag@C-TiyO1-x) observed on the Raman maps (b) and depth scan profiles of the four exemplary places on the X- and Y-direction cross line (d). Dashed lines on the depth profiles determine the boundary of the Ag-TiO2 layer.
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Table 1. Design of experiments to determine the most favorable conditions for the deposition of Ag-TiO2 coatings on the NiTi alloy and the ranges of the tested parameters.
Table 1. Design of experiments to determine the most favorable conditions for the deposition of Ag-TiO2 coatings on the NiTi alloy and the ranges of the tested parameters.
Time (min.)Voltage (V)
Basic level 340
Change interval210
Higher level550
Lower level 130
SymbolX0X1X2X12Evaluation of the effectiveness of the selection of parameters
(response function) Y, an average value
Experiment 1+−−+4
Experiment 2++−−3
Experiment 3+-+−3
Experiment 4++++1
Regression coefficientb0b1b2b12
2.750−0.750−0.750−0.250
Table 2. Design of experiments in optimizing parameters for the deposition of Ag-TiO2 coatings on the NiTi alloy.
Table 2. Design of experiments in optimizing parameters for the deposition of Ag-TiO2 coatings on the NiTi alloy.
Symbol X1X2Evaluation of the Effectiveness of the Selection of Parameters (Response Function) Y, Average Value
Experiment 50.5251
Experiment 60.5302
Experiment 1 1.0304
Experiment 71.0352
Experiment 82.0353
Experiment 92.0404
Experiment 103.0405
Experiment 113.0454
Experiment 124.0451
Experiment 134.050-
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Dudek, K.; Dulski, M.; Podwórny, J.; Kujawa, M.; Rawicka, P. Optimization of the Electrophoretic Deposition Parameters and Mechanism of Formation of Ag-TiO2 Nanocoatings on a NiTi Shape Memory Alloy: Part I. Coatings 2024, 14, 44. https://doi.org/10.3390/coatings14010044

AMA Style

Dudek K, Dulski M, Podwórny J, Kujawa M, Rawicka P. Optimization of the Electrophoretic Deposition Parameters and Mechanism of Formation of Ag-TiO2 Nanocoatings on a NiTi Shape Memory Alloy: Part I. Coatings. 2024; 14(1):44. https://doi.org/10.3390/coatings14010044

Chicago/Turabian Style

Dudek, Karolina, Mateusz Dulski, Jacek Podwórny, Magdalena Kujawa, and Patrycja Rawicka. 2024. "Optimization of the Electrophoretic Deposition Parameters and Mechanism of Formation of Ag-TiO2 Nanocoatings on a NiTi Shape Memory Alloy: Part I" Coatings 14, no. 1: 44. https://doi.org/10.3390/coatings14010044

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