Fabrication and Mechanism Investigation of High-Porosity Micro-Arc Oxidation Functional Coating on Aluminum Foam Substrate

Abstract

A high-porosity micro-arc oxidation (MAO) functional coating was fabricated on aluminum foam substrate through micro-arc oxidation technology, developing a structurally and functionally integrated bulk catalyst support material. Orthogonal experiments were employed to determine the optimal electrical parameters for achieving maximum coating porosity, with systematic investigations into the effects of electrolyte temperature and sodium tetraborate additives on pore characteristics. The phase composition, surface morphology, and elemental distribution of the porous coating were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Mercury intrusion porosimetry was applied to quantify the total pore area and pore size distribution. By means of secondary micro-arc oxidation, the catalyst was distributed in a gradient on the coating cross-section, which greatly improved the utilization rate of the catalyst. The formation mechanism of the porous coating was discussed, and the specific surface area of the fabricated catalyst-loaded materials was as high as (1.4~6.3) × 10⁴ m²/m³, which provided a large number of attachment sites for catalyst particles.

1. Introduction

The mitigation of volatile organic compound (VOC) pollution in atmospheric environments and indoor spaces has emerged as a critical research focus in environmental materials science, representing a significant societal issue impacting human health [1,2,3]. Semiconductor photocatalysis, capable of completely mineralizing organic molecules such as formaldehyde into CO₂ and H₂O under ambient temperature and pressure conditions, is recognized as a promising air purification technology due to its superior degradation capacity, operational stability, and sustainability. The advancement of this technology fundamentally relies on the design and fabrication of high-efficiency photocatalytic materials. Titanium dioxide (TiO₂), acknowledged as the most viable photocatalyst owing to its optimal bandgap, chemical stability, and cost-effectiveness [4], faces a critical technological bottleneck in industrial applications: the lack of structurally and functionally integrated bulk catalyst support materials [5]. The practical limitations of powdered nano-TiO₂ in handling and recovery processes severely constrain its engineering applications [6]. Consequently, the development of monolithic catalyst support systems with integrated structural and functional properties has become an urgent scientific priority.

Aluminum foam exhibits a honeycomb-like cellular structure. Its lightweight nature, exceptionally high specific surface area, and unique physical–mechanical properties make it an ideal catalyst support material [7,8]. While various methods exist for depositing catalyst particles onto aluminum foam surfaces, conventional techniques such as impregnation or spray coating suffer from prolonged processing durations, low efficiency, and weak particle adhesion [9,10]. Micro-arc oxidation (MAO), alternatively termed plasma electrolytic oxidation (PEO), has emerged as an innovative, environmentally friendly, and highly efficient surface treatment technology for catalyst loading [11,12,13]. This process utilizes high-voltage discharge on valve metals (e.g., Al, Mg, Ti) and their alloys to generate in situ a ceramic layer onto the substrate surface [14]. Simultaneously, catalyst particles from the electrolyte are adsorbed and solid-dissolved into the coating matrix and surface. Governed by high-power IGBT power supplies with optimized waveform control, this entire process achieves exceptionally high processing efficiency within minutes while ensuring robust catalyst anchoring.

Wang Jiankang et al. [15] successfully loaded sulfur-modified, iron-based catalysts onto Q235 carbon steel via MAO, addressing catalyst recovery challenges. Their results demonstrated remarkable catalytic efficiency under near-neutral pH conditions. Qin Honglei et al. [16] achieved ZnO nanoparticle loading onto titanium substrates through MAO, developing ZnO/WO₃/TiO₂ composite coatings with exceptional stability and catalytic activity. These pioneering studies validate the feasibility of fabricating structurally and functionally integrated bulk catalyst materials using MAO technology, effectively resolving issues of catalyst recyclability and instability.

Furthermore, maximizing a specific surface area is critical for enhancing catalytic efficiency by providing abundant anchoring sites for catalyst particles [17], thereby increasing contact area with VOC gases. It is well established that higher porosity and smaller pore diameters yield greater specific surface areas. Therefore, constructing high-porosity MAO coatings through precise control of electrical parameters, electrolyte composition, and temperature on open-cell aluminum foam substrates presents a robust strategy.

This study innovatively elucidates the theoretical framework for regulating high-porosity MAO coating formation and catalyst particle distribution optimization. We have designed a novel electrolyte formulation, providing a groundbreaking approach for developing monolithic catalyst support materials with integrated structural and functional properties.

2. Experimental

Given the challenges in accurately measuring the surface area of aluminum foam for current density determination, we initially employed 1060 aluminum sheets—sharing the same substrate composition as the aluminum foam—to optimize electrical parameters in constant-current mode. Subsequently, the stabilized voltage was applied in constant-voltage mode for aluminum foam treatment, significantly enhancing research efficiency. The open-cell aluminum foam exhibits a density of 0.2–0.3 g/cm³, a porosity of 85%–95%, irregularly shaped circular pores, and pore diameters ranging from 1 to 3 mm. The 1060 aluminum sheets (40 mm × 20 mm × 3 mm) were sequentially ground with 400#, 600#, and 800# grit SiC sandpapers, degreased with acetone, ultrasonically cleaned in anhydrous ethanol, and finally dried with compressed air.

The electrolyte composition comprised 10 g/L sodium silicate (Na₂SiO₃·9H₂O) and 1 g/L sodium hydroxide (NaOH), prepared by dissolving analytical-grade chemicals (purchased from National Medicines Co., Ltd., Beijing, China) in deionized water. A four-factor, four-level orthogonal experimental design was implemented to investigate the effects of current density, positive/negative duty cycle, frequency, and oxidation time on coating porosity. Electrolyte temperature control was achieved using a cryostatic bath with pneumatic agitation, maintaining temperatures at 10 °C, 20 °C, 50 °C, and 80 °C. To further enhance porosity, 10 g/L sodium tetraborate decahydrate (Na₂B₄O₇·10H₂O) was introduced as a pore-forming agent.

After parameter optimization, the MAO-treated samples underwent secondary oxidation in an electrolyte containing 5 g/L TiO₂ nanoparticles. The catalyst suspension was homogenized via ultrasonic aging for 1 h prior to processing.

The MAO process was conducted in a 5 L glass reactor equipped with a unipolar pulsed power supply. A stainless-steel plate served as the cathode, while the sample connected via an aluminum wire acted as the anode. Post-treatment, specimens were rinsed with deionized water and dried in a cold air stream. The surface and cross-sectional morphologies of the MAO coatings were examined using a ZEISS EVO-18 scanning electron microscope (SEM, Carl Zeiss AG, Jena, Germany). Elemental composition and distribution characteristics were analyzed by energy-dispersive X-ray spectroscopy (EDS). Coating thickness was measured with a TT220N coating thickness gauge (Time Group Inc., Beijing, China) at ten randomly selected locations per sample and averaged. Phase composition of the MAO coatings was determined via X-ray diffraction (XRD, PANalytical Empyrean, Almelo, The Netherlands) utilizing Cu Kα1 radiation (λ = 1.5406 Å) at a scan rate of 5°/min. Chemical states of elements within the coatings were investigated through X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB Xi+). Total pore area and pore size distribution were quantified using an AutoPore V 9600 mercury intrusion porosimeter (Micromeritics, Norcross, GA, USA) with an applied pressure range of 0.1–400 MPa. Microstructural characterization was performed using a spherical aberration-corrected transmission electron microscope (TEM) operating at 300 kV. High-angle annular dark-field (HAADF)-STEM images were recorded with a probe current of 50 pA to minimize beam damage.

3. Results and Discussion

3.1. Orthogonal Experimental Analysis

Table 1. Table of orthogonal experiments and analysis of their results.

Number	Current Density (A/dm2)	Frequency (Hz)	Duty Cycle (%)	Oxidation Time(min)	Porosity (%)
1	0.7	100	5	5	5
2	0.7	250	20	15	10
3	0.7	400	35	25	35
4	0.7	550	50	35	7
5	1.0	100	20	25	6
6	1.0	250	5	35	2
7	1.0	400	50	5	4
8	1.0	550	35	15	40
9	1.3	100	35	35	6
10	1.3	250	50	25	7
11	1.3	400	5	15	6
12	1.3	550	20	5	8
13	1.6	100	50	15	7
14	1.6	250	35	5	30
15	1.6	400	20	35	5
16	1.6	550	5	25	1
K1	14.25	6	3.5	11.75
K2	13	12.25	7.25	15.75
K3	6.75	12.5	27.75	12.25
K4	10.75	14	6.25	5
R	7.5	8	24.25	10.75

presents the L₁₆ (4⁴) orthogonal array designed with four influential factors: current density (A/dm²), frequency (Hz), duty cycle (%), and oxidation time (min). Each factor was tested at four levels, generating 16 experimental sets to identify the optimal electrical parameter combination. Figure 1 displays the experimental outcomes, with coating porosity quantified using Image-Pro Plus 6.0 software and compiled in Table 1. The labels K₁, K₂, K₃, and K₄ represent the average values of experimental results under different factor levels (e.g., K₁ denotes the mean value of all experimental results when the factor is at level 1). The range (R) can be calculated from these K values, where a larger R value indicates a more statistically significant impact of the corresponding factor on the experimental outcomes.

SEM images clearly revealed that Group #8 achieved superior performance with maximum porosity (Figure 1). Consequently, the optimal parameters were determined as follows: 1.0 A/dm² current density, 35% duty cycle, 550 Hz frequency, and 15 min oxidation time. Range analysis in Table 1 further demonstrated the factor significance hierarchy: duty cycle > oxidation time > frequency > current density. This quantitative prioritization provides critical guidance for designing porous MAO coatings in subsequent studies.

As evidenced by the orthogonal experimental results in Figure 1, distinct variations in coating morphology and porosity were observed across different electrical parameter combinations. This phenomenon stems from energy input disparities: parameter sets determining higher instantaneous power densities promote vigorous gas evolution and molten oxide states, leading to porous architectures, whereas lower energy inputs yield smoother surfaces. Through systematic screening of 16 parameter sets, the optimal combination for maximal porosity was identified, with duty cycle demonstrating predominant influence. The dominance of duty cycle regulation arises from its direct control over discharge duration (ton) and interval (toff) within each pulse cycle, critically affecting molten oxide solidification dynamics [18].

3.2. Electrolyte Temperature Control

To investigate the influence of electrolyte temperature on coating porosity, the electrolyte temperature was systematically controlled at 10 °C, 20 °C, 50 °C, and 80 °C, while maintaining constant electrical parameters: current density at 1.0 A/dm², duty cycle at 35%, frequency at 550 Hz, and oxidation duration at 15 min.

As distinctly evidenced by the SEM micrographs in Figure 2, electrolyte temperature exerted significant influence on the surface morphology and porosity of the coatings. At lower temperatures (10–20 °C), the MAO coatings exhibited abundant porous structures, whereas pore density progressively diminished with elevated electrolyte temperatures (50–80 °C). To investigate potential phase transformations, XRD analysis was conducted on coatings synthesized across this thermal range. As shown in Figure 3, all diffraction patterns consistently matched α-Al₂O₃ and γ-Al₂O₃, indicating temperature-independent phase composition. Furthermore, Figure 4 quantitatively demonstrates an inverse correlation between electrolyte temperature and coating thickness. The thickness decreased from 15.5 ± 0.5 µm at 10 °C to 12 ± 1 µm at 80 °C, revealing suppressed coating growth kinetics under high-temperature processing conditions.

During MAO discharge, transient plasma temperatures (2000–8000 K) induce oxide melting, followed by rapid thermal quenching in electrolytes that generates pores via volumetric contraction. Elevated electrolyte temperature diminishes the thermal quenching rate, allowing molten oxides to flow and fill the micropores. Additionally, enhanced ionic mobility at higher temperatures promotes uniform energy distribution, suppressing localized weak discharges that cause porous structures. Intensified plasma discharges under high-temperature conditions further augment sintering effects, densifying oxide particles.

As shown in Figure 4, the coating thickness decreased by 22.6% (15.5 → 12 µm) with increasing electrolyte temperature. This trend is attributed to the Arrhenius-type acceleration of both oxide formation and chemical dissolution rates. When dissolution surpasses deposition, net thickness reduction occurs.

3.3. Introduction of Pore-Forming Agents

Following the optimization of electrical parameters and precise electrolyte temperature control, the coating porosity exhibited a marked increase. However, the resultant pore structure demonstrated limitations in specific surface area enhancement due to excessively large pore dimensions and monodisperse pore size distribution. To address this, sodium tetraborate (Na₂B₄O₇·10H₂O) was strategically introduced as a pore-forming additive into the baseline electrolyte system for hierarchical porosity engineering. The remaining electrical parameters were maintained at a constant under the following conditions: current density of 1.0 A/dm², duty cycle at 35%, operating frequency of 550 Hz, oxidation duration of 15 min, and electrolyte temperature controlled at 10 °C.

While electrical parameter optimization alone yielded limited porosity enhancement, the introduction of sodium tetraborate (Na₂B₄O₇·10H₂O) as a pore-forming agent significantly augmented hierarchical porosity [19]. Figure 5 demonstrates the pronounced increase in porous structures post-addition, attributable to sodium tetraborate’s oxide-dissolving capability via the following reactions:

Hydrolysis:

B₄O₇²⁻ + 7H₂O → 4H₃BO₃ + 2OH⁻

(1)

Complexation (pH > 10):

H₃BO₃ + OH⁻ → B(OH)₄⁻

(2)

Al₂O₃ Dissolution:

Al₂O₃ + 2B(OH)₄⁻ + 3H₂O → 2[Al(B(OH)₄)₃]³⁻ + 6OH⁻

(3)

The highly soluble aluminate–borate complex facilitates Al₂O₃ dissolution. Concurrently, SiO₂ reacts with hydroxide ions:

SiO₂ + 2OH⁻ → SiO₃²⁻ + H₂O

(4)

Enhanced borate adsorption during MAO elevates local pH, accelerating silicate dissolution.

To quantify the specific surface area, we employed mercury intrusion porosimetry (MIP) instead of Brunauer–Emmett–Teller (BET) analysis, exhibiting an experimental uncertainty of ±5% (relative error). While BET is limited to mesopores (0.3–100 nm), our hierarchical pore structure spans macropores (0.0064–950 µm) beyond BET’s detection range. MIP results revealed that the cumulative internal surface area of pores within the coating reached 9.1 ± 0.5 m² per square centimeter of the geometric surface area, which increased to 14.1 ± 0.7 m² with sodium tetraborate modification. Compared to conventional open-cell aluminum foam (specific surface area: 100–450 m²/m³), the MAO-coated foam exhibited an extraordinary specific surface area of (1.4~6.3) × 10⁴ m²/m³. This ultrahigh surface area provides abundant anchoring sites for catalyst particles and facilitates reactant adsorption within the porous network, thereby substantially enhancing catalytic efficiency.

In Figure 6, mercury intrusion porosimetry (MIP)-derived pore size distribution of the optimized MAO coating reveals a hierarchical pore architecture. The predominant pore size distribution centered at 3.8 μm corresponds to primary discharge channels, with auxiliary macropores (>10 μm) indicating interconnected through-holes. A secondary population of mesopores (100–500 nm) was attributed to Na₂B₄O₇-induced micropore formation.

3.4. Catalyst Immobilization via Micro-Arc Oxidation

Following the fabrication of high-surface area coatings, catalyst immobilization was achieved through secondary micro-arc oxidation in a TiO₂ nanoparticle-enriched electrolyte (5 g/L) for 15 min. The post-loading coating morphology is presented in Figure 7. Energy-dispersive X-ray spectroscopy (EDS) mapping confirmed preferential accumulation of TiO₂ catalysts within porous regions, aligning with theoretical predictions.

Point EDS analysis was conducted across dense and porous regions (see

Table 2. The variation of relative content (at %) of elements in different zones of the coating.

Elements	#1	#2
Al	20.0	49.7
O	63.1	48.6
Si	15.4	0.7
Ti	1.5	1.0

), revealing distinct compositional gradients. The porous zones exhibited a 15.4 at.% silicon enrichment (vs. 0.7 at.% in dense regions) coupled with 20.0 at.% aluminum depletion. This phenomenon suggests the porous matrix primarily consists of silicon-rich oxides (likely SiO₂ and mullite phases), while dense regions maintain aluminum oxide (Al₂O₃) as the dominant phase.

The cross-sectional bright-field TEM analysis (Figure 8) of the secondary MAO coating revealed three distinct areas with gradient phase/chemical compositions. The outermost Area #1 exhibited a rutile TiO₂-dominated structure, where SiO₂ frameworks formed interconnected pores embedding TiO₂ nanoparticles as confirmed by SAED (Figure 9) and EDS mapping. Transitional Area #2 comprised κ-Al₂O₃ with homogeneous elemental distribution, while the innermost Area #3 consisted of γ-Al₂O₃ with web-like discharge channels that facilitated electrolyte infiltration and catalyst incorporation. As quantified in

Table 3. The variation of relative content (at.%) of elements in different areas of the coating cross-section.

Elements	Area#1	Area#2	Area#3
Al	0.5	27.8	38.0
O	61.7	58.9	58.0
Si	7.0	2.9	0.3
Ti	30.8	10.4	3.7

, elemental gradients showed increasing Al content from 0.5 at.% (Area #1) to 38 at.% (Area #3), contrasted by decreasing Si (7.0 → 0.3 at.%) and Ti (30.8 → 3.7 at.%) concentrations radially inward.

Cross-sectional TEM observations (Figure 8, Figure 9, Figure 10 and Figure 11) reveal a tri-layered structure with distinct longitudinal catalyst gradients (Table 3). The outermost layer (Area #1) exhibits high TiO₂ concentration (30.8 at.% Ti), with catalysts embedded in a porous Al₂O₃-SiO₂ matrix. The intermediate transitional layer (Area #2) comprises a dense Al₂O₃-TiO₂ solid-solution (10.4 at.% Ti), as evidenced by EDS analysis (Figure 9), demonstrating uniform integration of catalyst particles. The innermost layer (Area #3) maintains a relatively dense γ-Al₂O₃ structure but contains numerous discharge channels, through which a minor fraction of TiO₂ particles (3.7 at.% Ti) permeates into the coating interior. This hierarchical distribution—achieved via secondary MAO processing—optimizes catalyst utilization efficiency by concentrating active TiO₂ sites in the porous outer layer for enhanced reactant accessibility, while preserving structural integrity through gradient compositional design. The dense intermediate layer acts as a diffusion barrier to prevent catalyst leaching, and the inner channels allow controlled reactant transport without compromising mechanical stability.

XPS analysis of Area #1 surface (Figure 12) validated phase composition through high-resolution spectra: Al 2p (74.6 eV, Al-O in Al₂O₃ [20]), Si 2p (103.2 eV, Si-O in SiO₂ [21]), and Ti 2p doublet (459.0/464.7 eV, Ti⁴⁺ in rutile TiO₂ [22]). Charge correction via C 1s (284.8 eV [23]) confirmed these assignments, demonstrating successful TiO₂ catalyst incorporation during secondary MAO processing. The Ti 2p₃/₂-Ti 2p₁/₂ spin–orbit splitting (Δ = 5.7 eV) and FWHM values (1.3 eV) matched reference data for phase-pure rutile (ICDD 01-078-2485).

XRD analysis was performed on the coating system with the aluminum substrate serving as a reference (Figure 13). The pronounced Al diffraction peaks from the substrate dominated the pattern due to the relatively thin coating thickness (~16 µm). Diffraction signatures of α-Al₂O₃, γ-Al₂O₃, and minor rutile TiO₂ were identified in the coating. The absence of crystalline SiO₂ peaks (expected 2θ ≈ 21.8° for quartz) confirms the amorphous nature of silicon oxides within the coating matrix, consistent with the broad halo observed between 20 and 30° 2θ.

Comprehensive analyses integrating elemental composition (Table 2), XPS spectra (Figure 12), and XRD patterns (Figure 13) confirm that the porous areas consist of Al₂O₃-SiO₂ composites with substantial adsorption and solid-solution of catalyst particles, while the dense regions are predominantly Al₂O₃ containing limited solid-solved TiO₂. This selective distribution arises from the surface charge characteristics of TiO₂ particles. Under alkaline electrolyte conditions, hydroxyl groups (-OH) on TiO₂ surfaces undergo deprotonation (-OH → -O⁻) due to low proton concentration, generating negative surface charges. Electrophoretic migration under the applied electric field drives these negatively charged particles toward the anode, where the porous architecture provides optimal sites for adsorption and solid-solution. Additionally, a fraction of catalysts becomes encapsulated within the coating through molten oxide coverage during plasma discharge.

4. Conclusions

The persistent challenge of developing monolithic catalyst support materials with integrated structural and functional properties in catalysis has been addressed through the fabrication of high-porosity MAO coatings on aluminum foam substrates. Extensive studies confirm that MAO coating porosity is critically dependent on electrolyte composition, electrical parameters, and temperature [24,25]. The MAO process involves complex plasma electrolytic reactions with interdependent electrical parameters [26]. Based on our research, five principal conclusions can be drawn:

(1) Electrical parameters exert significant control over MAO coating porosity, with duty cycle demonstrating predominant influence.

(2) Elevated electrolyte temperatures induce a simultaneous reduction in coating porosity and growth rate, attributed to enhanced oxide sintering and accelerated dissolution-competing mechanisms.

(3) Hydrolysis of sodium tetraborate partially dissolves the Al₂O₃/SiO₂ matrix, resulting in an increase in porosity.

(4) Process-optimized MAO coatings on aluminum foam substrates achieve ultrahighspecific surface areas of (1.4~6.3) × 10⁴ m²/m³, providing a large number of anchoring sites for catalyst particles.

(5) Secondary MAO enables precise longitudinal gradient distribution of catalysts, which greatly improves catalyst utilization.