Tuning the mechanical integrity and corrosion performance of open-cell copper foams: identification of an optimization threshold

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Download PDF Abstract In this work, Porous copper foam samples were fabricated by a space-holder technique with water-atomized copper powder and sodium chloride (NaCl) particles in weight fractions ranging from 10 to 35 wt%. The powders were mechanically mixed at a 10:1 ball-to-powder ratio and 150 rpm for 2 h. The effects of space-holder content on the physical, mechanical, tribological, and electrochemical performance were systematically investigated.

Increasing the NaCl fraction precisely controlled the total porosity, from 39.15% to 57.76%, and reduced the bulk density from 5.452 to 3.785 g/cm³. XRD and SEM analyses confirmed a single-phase FCC copper matrix and a structural transition from isolated pores to a highly interconnected open-cell network.

Consequently, Vickers hardness, compressive yield strength, energy absorption capacity decreased from 52.12 to 29.23 HV, 18.25 to 1.65 MPa, and 35.73 to 10.46 MJ/m³, respectively, following a modified Gibson–Ashby model (R 2 = 0.95). Concurrently, the coefficient of friction and specific wear rate increased (reaching a maximum of 0.73 mm³/N·m under 15 N for 35 wt% NaCl) due to strut thinning and localized delamination.

Electrochemical evaluation in 3.5 wt% NaCl solution revealed an optimization threshold for sample fabricated with 25 wt% NaCl, which exhibited the lowest corrosion rate of 0.918 mm/y, and the highest polarization resistance (Rp) of 267.28 Ω, due to a stable Cu 2 O passive film. Exceeding this threshold at 30 and 35 wt% NaCl led to structural instability and stagnant electrolyte zones, accelerating corrosion kinetics.

Overall, lower-porosity porous copper foam architectures offer superior structural energy absorption, whereas the 25 wt% NaCl configuration achieves an optimal balance between mechanical integrity and corrosion resistance.

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Energy science and technology Engineering Materials science Mechanical Behavior and Applications of Metallic Foams Introduction Historically, porosity was perceived as a structural deficiency in engineering materials. However, with precise architectural control over pore morphology, it has evolved into a strategic design tool for fabricating ultra-lightweight materials possessing unique functional properties that fully dense solids cannot replicate 1 .

Driven by the rapid advancement of industrial technologies, the demand for lightweight, multifunctional materials has surged across diverse sectors. In this context, metal foams have ascended to prominence as a critical research hotspot 2 . These cellular structures consist of a solid metallic matrix encompassing a high volume fraction of gas-filled voids, deriving distinctive physical and mechanical properties directly from their cellular topology 3 .

The engineering utility of metal foams is primarily governed by their cellular interconnectivity, which classifies them into closed-cell, partially open, and open-cell configurations. In closed-cell foams, the voids remain isolated within the metallic matrix, whereas open-cell architectures feature a continuous, interconnected network that permits fluid transport.

Consequently, closed and partially open foams are indispensable for structural roles, such as kinetic energy dissipation and acoustic damping in the aerospace and defense sectors. Conversely, open-cell foams are preferred for functional applications, including filtration, catalytic substrates, and thermal management systems 4 .

Several manufacturing methodologies are used to synthesize metal foams, depending on the precursor phase (solid, liquid, powder, or vapor) and the targeted structural architecture as summarized in Table 1 . Prevailing industrial routes encompass cellulose-based replication, gas entrapment, direct gas injection 5 , melt foaming via blowing agents, and versatile space-holder techniques integrated with powder metallurgy (PM) 6 , 7 .

While liquid metallurgy routes, such as melt gas injection, are widely used, they frequently encounter persistent challenges, including foam instability, structural defects, and heterogeneous pore distribution. Furthermore, achieving precise regulation over the final foam quality is often hindered by the complex nature of parameter optimization 8 , 9 , 10 .

In contrast, the PM route offers distinct advantages, notably high cell consistency, the capability to manufacture near-net-shaped components with complex geometries 11 , tailored pore morphology, superior compressive strength, cost-effectiveness, and easy processing 12 , 13 . Among various metallic matrices, copper foam is widely utilized owing to its high thermal and electrical conductivities and high melting point.

These properties have made porous Cu foam highly desirable for advanced applications in thermal management, environmental remediation, and energy conversion systems 14 , 15 . To achieve the required porous architectures, various space holders have been used, as compiled in Table 1 . In this context, the integration of recycled copper powder produced via water atomization offers a compelling approach to combine environmental sustainability with advanced structural engineering.

From an eco-economic perspective, upcycling recycled copper reduces raw material costs, energy consumption, and carbon footprint, aligning with circular economy strategies. From a material processing standpoint, water atomization imparts a characteristically irregular and rough particle morphology to the recycled Cu powder, unlike the smooth spherical shapes produced by gas atomization.

This irregular geometry provides enhanced mechanical interlocking during cold compaction, resulting in superior green strength. Furthermore, the higher specific surface area of water-atomized particles promotes accelerated solid-state diffusion and robust neck formation during vacuum sintering, enabling high structural integrity even in highly porous frameworks.

Table 1 Comparative analysis of recent studies about the processing techniques, microstructural properties, mechanical properties, and industrial uses of Cu-based foams. Full size table Despite the extensive research on porous copper foams summarized in Table 1 , a critical challenge remains in achieving a precise balance between tailored porosity and mechanical integrity.

Most conventional studies target ultra-high porosity levels > 60%, which inherently leads to a severe compromise in load-bearing capacity and structural connectivity.

Furthermore, while various space-holder materials have been explored, the utilization of sustainable, low-cost recycled water-atomized copper powder to establish a controlled transition from isolated pores to an interconnected open-cell network and its direct impact on mechanical robustness, wear mechanisms, and electrochemical corrosion resistance remains underreported.

Therefore, the present study aims to address this scientific gap by fabricating high-performance porous copper foams via a reliable powder metallurgy route utilizing commercial water-atomized copper powder and NaCl as a space holder. The NaCl weight fractions were varied systematically from 10 to 35 wt% to precisely regulate the porosity and density.

In this study, the recycled water-atomized copper powder was used for the manufacturing of a porous Cu foam material possessing an optimal porosity ratio and acceptable high strength using 35 wt% NaCl space holder material. This work comprehensively investigates the synergistic effects of microstructural evolution and pore interconnectivity on the resulting mechanical properties (Vickers hardness and compressive yield strength) and tribological behavior.

Crucially, a novel focus is directed toward determining the optimization threshold of porosity that balances structural stability with enhanced electrochemical corrosion resistance in a 3.5 wt% NaCl environment, providing a solid framework for the utilization of these foams in functional engineering applications. Experimental procedure In this work, porous copper (Cu) foams were fabricated by the PM route using the space holder sintering technique.

Six distinct batches were prepared by varying the NaCl ratios from 10 to 35 wt% with a 5% step increase, utilizing copper powder synthesized from recycled scrap via atomization. Materials Pure Cu powder was experimentally synthesized from high-purity electrical copper scrap using the water atomization process, in which a nozzle producing a conical water jet with an apex angle of 20–25° was employed to achieve efficient atomization.

The melting process was carried out at 1100 °C to ensure complete melting of the copper scrap. Furthermore, shaping the molten copper stream into a flat, rectangular cross-section can produce finer copper particles. Water-atomized copper powder inherently exhibits an irregular morphology and an elevated surface oxygen content as copper oxides.

Consequently, thermal reduction in a hydrogen atmosphere was performed at 450 °C for 30 min to eliminate residual oxides and yield the final high-purity metallic copper powder. The multi-stage synthesis process, including atomization and subsequent hydrogen reduction, is schematically illustrated in Fig. 1 . Commercial NaCl was selected as the sacrificial space-holder material and sieved to a controlled particle size range of 40–120 μm to dictate the final pore architecture.

Fig. 1 Full size image Synthesis stages of high-purity Cu powder through water atomization and hydrogen reduction. The SEM micrographs, particle size distributions, and EDX purity analysis of the starting materials are presented in Fig. 2 . The NaCl space holder particles shown in Fig. 2 a and b display a coarse, angular morphology with a significantly larger size distribution, as presented in Fig. 2 c, centered between 40 and 120 μm.

In contrast, the copper powder produced via water atomization exhibits a predominantly irregular, non-spherical morphology, as shown in Fig. 2 d and e, which is characteristic of rapid solidification and the high impact energy associated with water jets. These irregular shapes are beneficial for increasing the green strength of the compacts due to better mechanical interlocking. The particle size distribution analysis in Fig.

2 f confirms that the copper powder has a fine average size ranging from 6 to 14 μm. This substantial size difference between the fine Cu matrix and the coarse NaCl space holders is vital for achieving a stable porous structure during the sintering process. Furthermore, high-magnification SEM Fig. 2 g and EDX quantitative elemental analysis Fig.

2 h confirm the high chemical purity of the reduced copper powder, exhibiting exclusively pure copper peaks 100 wt% Cu with zero detectable impurities. Fig. 2 Full size image SEM micrographs, particle size distributions, and EDX analysis: ( a ) NaCl at 200 μm, ( b ) NaCl at 50 μm, ( c ) NaCl size distribution, ( d ) Cu at 100 μm, ( e ) Cu at 20 μm, ( f ) Cu size distribution, ( g ) single Cu particle, and ( h ) EDX elemental spectrum.

Foam synthesis To fabricate the porous copper foams, the as-received water-atomized Cu powder was homogeneously blended with varying weight fractions of NaCl as space-holder particles by 10, 15, 20, 25, 30, and 35 wt% using a planetary ball milling machine model (PQ-N2, China).

The blending process was conducted at a moderate rotating speed of 150 rpm for 2 h under a protective argon gas atmosphere to achieve a uniform distribution without inducing severe plastic deformation or altering the initial morphology of the constituent powders, also to protect the Cu powder from any oxidation. A ball to powder weight ratio of 10:1 was maintained.

To ensure uniform blending and effectively prevent cold welding or any agglomerations of the ductile copper particles, 2 wt% of n-hexane was incorporated as a process control agent (PCA). The milling/blending operation was performed in intermittent 15 min cycles, followed by cooling intervals to preclude thermal accumulation inside the vials.

After the mechanical milling step, the volatile PCA was allowed to evaporate, and the mixed Cu-NaCl powders were uniaxially cold-compacted in a hardened steel die at a compaction pressure of 250 MPa utilizing a universal hydraulic press to produce green compacts. These green samples were subsequently transferred to a vacuum furnace for isothermal solid-state sintering according to the thermal profile illustrated in Fig. 3 .

The compacts were heated with a controlled rate of 3 °C/min up to 250 °C and held at this temperature to guarantee the complete removal and degassing of residual volatiles and organic PCA constraints.

After that, the temperature was further elevated at a heating rate of 5 °C/min to the final solid-phase sintering temperature of 900 °C, where the samples were isothermally held for 60 min to facilitate adequate metallurgical neck growth and consolidation of the copper matrix around the space-holder templates. The cycle concluded with controlled furnace cooling at a constant rate of 10 °C/min until the sample reached room temperature.

To generate the porous architecture, the sintered Cu-NaCl composites were subjected to a water-leaching process. The samples were immersed in a continuously stirred distilled water bath maintained at 60 °C for several cycles to dissolve and extract the sacrificial NaCl templates completely. The leaching water was refreshed periodically until no silver chloride (AgCl) precipitate was detected upon testing with a silver nitrate (AgNO 3 ) solution.

This verified the macroscopic dissolution and thorough extraction of the space-holder templates from the interconnected open-pore network. However, minor micro-traces of NaCl may remain encapsulated within isolated micropores or sintering neck junctions where fluid circulation is geometrically constrained. Finally, the resulting interconnected open-cell porous Cu foams were dried in an oven at 80 °C for 4 h to eliminate any entrapped moisture.

The fabrication stages of the porous Cu foams are schematically illustrated in Fig. 4 . Fig. 3 Full size image Heating cycle for the vacuum sintering process of green samples. Fig. 4 Full size image Schematic flow diagram of the Cu foam fabrication process using the space holder technique.

Characterization and performance analysis The actual bulk density of the sintered Cu foams (ρ f ) was determined based on Archimedes’ principle 28 , 34 , where the average value of three samples per batch was recorded to ensure statistical reliability.

To calculate the total porosity, the measured foam density was compared to the theoretical density of fully dense solid copper (ρ Cu ), taken as 8.96 g/cm³, according to the following relationship 35

(1) For microstructural observation, the sintered samples were systematically ground using a series of silicon carbide (SiC) papers grades of 600, 800, 1000, 2000, and 3000, respectively.

Following the grinding stages, a final polishing step was performed using 3 μm alumina paste to achieve a mirror-like surface finish, ensuring the samples were suitable for high-resolution imaging. Morphological homogeneity and elemental distribution of the consolidated samples were investigated using a Field Emission Scanning Electron Microscope (FE-SEM, ZEISS-Sigma 500, Sohag, Egypt).

This was coupled with Energy-Dispersive X-ray Spectroscopy (EDX) and elemental mapping to verify the chemical composition, the absence of any undesirable phases, and to ensure the homogeneity of the Cu matrix. Phase purity and crystalline structure were analyzed by X-ray diffraction (XRD, X’Pert PRO PANalytical) utilizing Cu kα radiation (λ = 0.15406 nm). The diffraction patterns were recorded over a 2θ range from 10° to 100°, employing a step size of 0.02°.

The crystallite size (D) of the samples was estimated using the Scherrer equation 36

(2) Where K is the shape factor (0.9), λ is the X-ray wavelength, θ is the Bragg angle, and FWHM is the full width at half maximum of the diffraction peaks.

The lattice strain (ε) was evaluated using the following expression 37 , 38

(3) Additionally, the lattice constant (a) for the cubic structure of copper was determined according to the following interplanar spacing relationship (4) Where (hkl) are the Miller indices of the reflecting planes.

Surface microhardness of the consolidated Cu foams was measured using a Vickers microhardness tester model (HV-10MPTA, Laizhou Huayin, China) under an applied load of 500 g for 15 s, averaging five distinct indentations per sample. Dry sliding wear tests were conducted at room temperature using a pin-on-disk tribometer under the conditions summarized in Table 2 . Table 2 Wear test conditions.

Full size table To prepare samples for tribological testing, the contacting surfaces of the cylindrical Cu foam samples and the SiC counterface ring were sequentially ground with SiC abrasive papers up to 1200-grit to achieve a uniform baseline surface roughness.

Subsequently, the samples were ultrasonically cleaned in an acetone bath for 10 min to remove any loose wear particles or grease contaminants, followed by thorough drying with warm air prior to initial mass measurement and testing. Mass loss was tracked using a high-precision electronic balance with a resolution of 0.0001 g to compute the specific wear rate (W sp ) by the following empirical relationship 39 .

(5) Where Δm is the wear mass loss, ρ f is the actual bulk density of the foam, SD is the total sliding distance, and N is the normal load. Tests were performed in triplicate to calculate mean values, while the dynamic coefficient of friction (COF) was recorded in real time.

Uniaxial compression tests were performed at room temperature on an Instron 3369 universal testing machine under quasi-static loading conditions at a constant crosshead speed of 1 mm/min (corresponding to an initial strain rate of 1 × 10⁻³ s⁻¹), in accordance with the ISO 13,314 standard for porous and cellular metals.

The energy absorption capacity per unit volume (W absorbtion energy ) was determined by integrating the area under the compressive stress-strain curve up to a 50% strain limit (ε* =0.5) according to the following integral equation 40

(6) Where σ and ε represent the compressive stress and strain, respectively.

Electrochemical corrosion evaluation was performed using an Autolab PGSTAT 302 N potentiostat in a 3.5 wt% NaCl aqueous solution at room temperature under a relative humidity of 38% ± 5%. Before testing, all porous copper foam samples were cold-mounted in non-conductive epoxy resin, leaving a flat cross-sectional surface with a controlled apparent geometric exposure area (A geom ) of 0.445 cm² exposed to the electrolyte.

The exposed working surface was sequentially ground using SiC papers (grades 600, 800, 1000, 1200, and 2000), fine-polished with 3 μm alumina suspension to a mirror finish, degreased in an ultrasonic ethanol/acetone bath for 5 min, rinsed with distilled water, and dried with warm air immediately before electrochemical immersion.

Utilizing the nominal geometric surface area provides a standardized, reproducible baseline to evaluate and compare the electrochemical parameters across samples with varying porosity levels (39.15% to 57.76%). A standard three-electrode cell was utilized, consisting of the mounted Cu foam sample as the working electrode, a platinum wire auxiliary electrode, and a saturated Ag/AgCl reference electrode. The electrolyte was continuously agitated via a magnetic stirrer.

Before polarization, samples were immersed for 1 h to establish a stable open-circuit potential (OCP). Potentiodynamic polarization scans were performed from ± 250 mV relative to the OCP at a scan rate of 1 mV/s. Corrosion potential (E corr ) and corrosion current density (i corr ) were extracted by the Tafel extrapolation method.

Electrochemical Impedance Spectroscopy (EIS) and Bode measurements were conducted over a frequency range from 10 5 to 10 − 2 using a sinusoidal perturbation amplitude of 10 mV.

The annual corrosion rate (R corr , mm/y) was calculated based on Faraday’s law 36 , 41

(7) Where i corr is the corrosion current density (µA/cm 2 ), EW represents the equivalent weight of copper, and ρ f signifies the actual bulk density of the foam (g/cm 3 ).

Source

- Nature (2026-10-16) - Original article: Tuning the mechanical integrity and corrosion performance of open-cell copper foams: identification of an optimization threshold - https://www.nature.com/articles/s41598-026-72455-5