Characterization of Additively Manufactured Titanium-Based Alloy with a Micro-Arc Oxidation Coating and Overlying Polyurethane Layer

Abstract

Titanium alloys are widely used in the aerospace, automotive, chemical, and biomedical industries due to their excellent corrosion resistance, mechanical properties, and biocompatibility. However, the surface properties of titanium alloys are often insufficient to meet the increasingly complex requirements of certain applications. Therefore, enhancing the surface performance of titanium alloys in physiological environments has become a key focus of research. In this study, a porous oxide layer was generated on the surface of a titanium substrate through micro-arc oxidation (MAO). This layer served as an intermediate layer for a subsequently deposited polyurethane (PU) coating, providing a strong foundation for adhesion. The high porosity of the MAO layer not only facilitated the adhesion of the PU coating but also protected the titanium alloy, further enhancing its corrosion resistance. The surface microstructure after MAO treatment and the morphological changes after application of the PU coating were characterized using scanning electron microscopy. The PU layer uniformly covered the surface of the MAO layer, significantly improving the smoothness and uniformity of the surface. The increase in surface smoothness due to the PU coating on top of the MAO layer was verified through white light interferometry. Additionally, surface hydrophobicity was assessed through water contact angle measurements. The PU layer over the MAO coating significantly enhanced the hydrophobicity of the titanium alloy’s surface, which is crucial for reducing biofouling and improving the effectiveness of biomedical implants. Finally, electrochemical analysis was conducted to study the corrosion resistance of the titanium alloy after MAO and PU treatment. The titanium alloy with an MAO–PU composite coating exhibited the highest corrosion resistance. The findings revealed that the combination of the MAO layer and PU coating provides an excellent multifunctional protective layer for titanium alloys, not only enhancing their durability but also their ability to adapt to physiological and harsh environments.

1. Introduction

Titanium (Ti) alloys have become key materials in various high-performance applications because of their excellent biocompatibility, low density, high specific strength, and favorable corrosion resistance. Moreover, their exceptional strength-to-weight ratio renders them an ideal choice for use in industries requiring light yet strong materials, such as the aerospace and automotive industries, in which reducing weight can notably improve fuel efficiency and performance [1,2,3,4,5,6].

Polyurethane (PU) is an ideal material for protective coatings, especially in environments that require durability and reliability, because of its high wear resistance, energy absorption, flexibility, and chemical resistance [7,8,9]. PU-coated metals, particularly titanium-based materials, are widely used in the biomedical, aerospace, and industrial sectors because of their favorable mechanical properties and biocompatibility [10].

Micro-arc oxidation (MAO) is an electrochemical surface modification technique [11,12,13] used to deposit hard and wear-resistant ceramic coatings on metals [14,15]. Using MAO, a porous oxide layer can be generated on the surface of titanium alloys; this layer enhances the hardness, corrosion resistance, and bioactivity of the alloy’s surface [16,17,18]. However, the porous nature of the MAO coating can lead to increased surface roughness, promoting the initiation of corrosion. Consequently, micropores and cracks on MAO-coated surfaces must be repaired [19,20].

The MAO layer provides a rough and porous surface, which facilitates the adhesion of polymer materials while offering valuable properties, including hardness, corrosion resistance, and hydrophobicity [21,22,23]. Therefore, the MAO layer can enhance the durability of an entire composite structure, and its excellent electrical insulation properties can protect the substrate from electrochemical corrosion. The elasticity and flexibility of polymer materials can compensate for the brittleness of the MAO layer and produce a durable and crack-resistant coating. The strength of interfacial bonding between an MAO layer and a polymer material crucially influences the formation of a composite coating. Selection of a suitable polymer material and surface treatment method is critical for enhancing the strength of this interfacial bonding. Precise control over the electrical parameters of MAO can result in the formation of an ideal intermediate layer and ensure the adhesion of polymer materials [24,25,26,27,28,29].

This study explored the adhesion, mechanical properties, and corrosion resistance of a PU/MAO composite coating on additively manufactured titanium-based materials. Coating MAO-treated surfaces with PU was found to improve the surface roughness and to repair micropores and cracks [30]. The results of this study provide valuable insights into the potential applications and advantages of the composite coating method in various industries.

2. Experimental

2.1. Materials and Preparation of Specimens

The additive manufactured Ti-based materials, Ti-6Al-4V, with a composition (all in wt.%) of 5.5%–6.5% Al, 3.5%–4.5% V, 0.25% Fe, 0.13% O, 0.08% C, 0.05% N, and 0.012% H; the remainder was Ti as specified in ASTM F136. Pieces of additive manufactured Ti-based materials (50 mm × 50 mm × 3 mm) were used as substrates for MAO. Prior to MAO being performed, the substrates were polished using sandpaper of grit sizes ranging from 400 to 1200, after which they were rinsed with distilled water. The MAO equipment comprised a bipolar pulse power supply, a 5 L stainless steel vessel serving as the electrolytic cell, a stirring system, and a cooling system. A schematic of the MAO setup is shown in Figure 1. The cooling system maintained the temperature of the electrolyte below 15 °C during the process to ensure that a dense MAO coating formed on the substrate surface. The stainless steel vessel acted as the cathode, allowing the samples to be irregularly shaped, whereas the pure titanium alloy served as the anode. The input voltage was set at 430 V with a frequency of 50 Hz, and the process was conducted in the bipolar mode. The composition of the electrolyte is detailed in

Table 1. Composition and concentrations of the electrolytes used in MAO.

Electrolyte Formula	Na2SiO3	NaH2PO2	NaOH	NaF
Concentration	0.049 M	0.035 M	0.035 M	0.035 M

. The parameters of the MAO process are detailed in

Table 2. Parameters of MAO process.

Operating Parameters	Voltage (V)	Frequency (Hz)	Temperature (°C)	Time (s)
Value	430	50	below 15	600

. After completion of the MAO surface treatment, the substrates were rinsed with distilled water and dried using a pneumatic air gun.

2.2. Coating Characterization

The surface morphology and cross-section microstructure of the various coatings were described by using a scanning electron microscope (SEM; HITCHI S-3400N, Tokyo, Japan) and a focused ion beam chamber (Hitachi NX2000, Tokyo, Japan). Using a 3D surface measurement instrument (Chroma, 3D Optical Profiler Model 7503, Glasgow, UK) to measure the surface roughness of various coatings with a scanning area of 200 μm × 200 μm. And, mountains Map Imaging Topography (Chroma, Glasgow, UK, version 8.2) was used to analyze the surface roughness of the specimens. The apparent contact angle of the deionized water droplets on each sample was measured at room temperature using a contact angle instrument (Attension, Theta Lite, Espoo, Finland). Both the standard deviations and the mean values for the contact angles were determined by using more than five measurements obtained at different positions on each specimen. The abrasion resistance of the MAO coatings was tested using a scratch tester (Anton Paar RST3) with the following operating parameters: a normal load of 0.5–30 N, a length of 5 mm, and a vibration frequency of 50.0 Hz (speed of 10 N/s). Fourier transform infrared (FTIR, PerkinElmer Spectrum one) spectra of the synthesized CS/WPU composites were obtained at a resolution of 4 cm⁻¹ with 32 scans over the spectral range from 350 to 4000 cm⁻¹. The potentiodynamic polarization curve test was analyzed using an electrochemical workstation VersaSTAT 4 (AMETEK Scientific Instruments, Berwyn, PA, USA). A saturated calomel electrode served as the reference electrode, a platinum plate served as the counter electrode, and the test sample served as the working electrode. The corrosive medium was a 3.5 wt.% NaCl solution.

3. Results

3.1. Morphology Analysis

Figure 2 displays SEM micrographs of the bare substrate and the substrates with an MAO coating, a PU coating, or a MAO coating with a PU layer. The bare substrate had a surface covered with spherical particles; these particles imparted a relatively rough and uneven texture to the base material. The surface of the MAO coating was distinctly marked by highly dense microcracks and micropores, which reflected the inherent porosity and structural imperfections introduced during MAO. These features were indicative of the coating’s rough surface topology, which could affect its performance and durability. The SEM images of the substrate with a PU coating revealed that the PU layer only partially covered the substrate’s surface, with noticeable gaps and uneven areas. This incomplete coverage compromised the protection and sealing of the underlying surface. Conversely, the SEM images of the MAO coating with a PU layer indicated that PU had effectively filled in the microcracks and micropores in the MAO coating and thus led to more uniform and comprehensive coverage. This resulted in a more seamless and continuous surface, which would enhance the overall integrity and protective capabilities of the coating. The uniform distribution of the PU layer ensured better sealing and protection, notably enhancing the resistance of the surface to environmental factors and potential degradation.

Cross-sectional SEM images of the MAO coatings covered and not covered with PU indicated that the MAO layer was 11 μm thick in both these samples (Figure 3). The morphology of the MAO coating was characterized by an array of micropores and visible cracks. These imperfections indicated that although the MAO surface treatment created a robust and adherent oxide layer, it also introduced a certain degree of porosity and structural weakness. However, the cross-sectional SEM image of the MAO coating with a PU layer indicated that the PU created remarkable uniformity and smoothness. PU effectively infiltrated and filled the micropores and surface cracks in the MAO layer, thereby enhancing the overall continuity and integrity of the surface. The PU layer not only provided a protective barrier but also contributed to the uniformity of the surface, improving its resistance to environmental factors and potentially extending the lifespan of the underlying MAO coating. After performing EDS analysis (

Table 3. EDS analysis results for the three locations in the SEM image in Figure 2.

Point	Element–Atomic%
	O	Ti	Si	F	P	C	N	Total
a	58.1 ± 5.2	28.3 ± 3.7	5.5 ± 0.9	5.2 ± 0.3	2.9 ± 0.1	–	–	100
b	57.6 ± 4.4	28.5 ± 3.1	5.9 ± 0.8	4.6 ± 0.2	3.4 ± 0.3	–	–	100
c	19.1 ± 3.6	–	–	–	–	69.6 ± 6.8	11.3 ± 3.2	100

), elements such as titanium (Ti), oxygen (O), silicon (Si), fluorine (F), phosphorus (P), carbon (C), and nitrogen (N) are shown in different elemental distribution maps. From the data, it can be observed that the MAO layer contains higher concentrations of titanium (Ti), oxygen (O), silicon (Si), fluorine (F), phosphorus (P), and carbon (C), which indicate the structure and composition of the oxide layer. In contrast, the PU layer shows higher concentrations of carbon (C) and nitrogen (N), suggesting that the primary components of the PU layer are related to its chemical structure. The differences in elemental distribution clearly highlight the structural characteristics of the two layers and help to understand the interface features and overall coverage effect between the MAO and PU layers.

3.2. Properties of MAO Coating with a PU Layer

The surface roughness of the samples, which is a critical factor influencing a surface’s hydrophobic stability, was precisely measured using a three-dimensional white light interferometer. The measurements revealed that the R_a value of the composite MAO–PU coating was much lower than that of the MAO coating. This difference indicates that the PU covering improved the surface smoothness (Figure 4). However, the PU covering was noted to have only partially covered the surface; certain MAO areas were exposed. To address this issue and achieve optimal performance, the MAO coating was used as an intermediary layer. The MAO coating effectively acted as a foundational layer, enabling the PU layer to provide more uniform and complete coverage. This composite coating maximized the overall smoothness and hydrophobic stability of the surface while offering enhanced protection and functionality.

The water contact angle—the angle formed between a water droplet and the surface—is a key metric for evaluating the hydrophobicity of a coating [31]. This measurement provides insight into the extent to which a coating repels water. This measurement provides insight into the wettability of the coating, which is determined by both the interaction of the coating with an aqueous phase and the surface roughness. A mild increase in a contact angle occurred for the MAO coatings (Figure 5a,b) and can be explained by a decrease in the surface roughness (compare Figure 4a,b). In the course of the coating process, the PU particles enter into the micropores of the MAO and seal them. This leads to both an increase in the contact angle related to a decrease in the surface roughness and to the inhibition of the corrosive liquid penetration into the pores due to pore sealing. The latter results in better resistance of MAO coating with a PU layer to the corrosion and wear.

A cross-hatch adhesion test was conducted in accordance with the ASTM D3359 standard. The adhesion of the MAO coating was rated as 5B, that of the immersion PU coating was rated as 4B, and that of the intermediary MAO coating with the PU layer on top was rated as 5B (Figure 6). A rating of 5B indicates exceptional adhesion performance. These results thus demonstrated that the MAO layer significantly improved the adhesion of the PU coating, providing excellent stability and durability on the substrate surface.

Figure 7 shows the results of a scratch test, which is a method used to evaluate the surface hardness and adhesion of samples by applying a certain force and observing the consequent surface damage. At a force threshold of 15 N, surface damage was observed only in the samples with an MAO coating and samples with a PU coating. The PU-coated samples, in particular, exhibited damage at 1.56 N owing to the thinness and local coverage of the PU layer. The highest performance was exhibited by the samples with an intermediate MAO layer and PU layer on top, showing initial damage at 23.01 N, indicating that this composite coating provided the hardest layer with the best adhesion (peeling or cracking occurs at the latest).

The Fourier transform infrared spectra presented in Figure 8 indicate remarkable similarities between the spectral features of the PU layer alone and the PU layer applied over the MAO coating. Specifically, both spectra have absorption peaks corresponding to C–N (1048 cm⁻¹), N–H (1526 cm⁻¹), C=O (1717 cm⁻¹), and C–H (2919 cm⁻¹). Moreover, the presence of these characteristic peaks confirmed that the chemical composition and structural integrity of the PU coating were preserved.

3.3. Corrosion Resistance Test

The corrosion resistance of the titanium-based alloys was evaluated using potentiodynamic polarization tests. Figure 9 displays the polarization curves obtained for the bare substrate, substrate with an MAO coating, substrate with a PU coating, and substrate with an MAO–PU coating in 3.5 wt.% NaCl solution. The values of the current density i_corr and corrosion potential E_corr are listed in

Table 4. Corrosion potentials and current densities of the various samples.

	Substrate	MAO Coating	Immersion PU Coating	MAO Coating Immersion PU
Ecorr (VSCE)	−0.38 V	−0.16 V	−0.29 V	−1.56 V
icorr (µA/cm2)	1.06 × 10−5	1.11 × 10−6	1.31 × 10−6	1.91 × 10−8

. E_corr represents the corrosion potential, the point at which the current density markedly increases, reflecting the behavior of metal dissolution until a critical value is reached; i_corr represents the corrosion rate. Higher corrosion potential and lower corrosion current density indicate more favorable corrosion resistance. The following i_corr values were obtained for the different samples investigated: bare substrate, 1.06 × 10⁻⁵; substrate with an MAO coating, 1.11 × 10⁻⁶; substrate with a PU coating, 1.31 × 10⁻⁶; and substrate with an MAO coating and PU layer, 1.91 × 10⁻⁸. The PU–MAO composite coating exhibited the lowest current density, suggesting that this composite coating offered the highest corrosion resistance among the samples investigated.

The superior performance of the MAO + PU coating compared to single-layer coatings is due to the combined benefits of composite layers. The PU coating formed a strong mechanical and physical interlock with the MAO coating due to the porous structure. This results in a stronger bond between the layers. Additionally, the PU layer significantly improves the coating’s corrosion resistance by acting as a protective barrier, preventing corrosive agents from penetrating the MAO layer. It also enhances scratch resistance by absorbing impact and protecting the underlying MAO layer. Finally, these properties make the MAO + PU composite coating more durable and resistant to both mechanical wear and corrosion.

4. Conclusions

The surface of untreated titanium alloy was covered with spherical particles, and MAO treatment of this surface led to the creation of micropores and cracks. A PU coating alone failed to fully cover the bare alloy surface, but applying an MAO layer as an intermediate layer and then applying a PU coating resulted in the PU uniformly filling the defects in the MAO coating and enhanced adhesion to the surface. Cross-sectional analysis revealed that the composite coating had a total thickness of 11.5 µm, with the thickness of the MAO layer being 11 µm and that of the PU layer being 0.5 µm. White light interferometry and water contact angle measurements indicated that the PU coating increased the roughness and hydrophobicity of the surface. Scratch tests verified that the adhesion and hardness of the MAO–PU composite coating were superior to those of the individual single-layer coatings. Fourier transform infrared analysis further verified that the PU layer completely covered the MAO layer, and corrosion tests demonstrated that the MAO–PU composite coating was the surface with the highest corrosion resistance.