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Dieser Artikel geht auf das transformative Potenzial der additiven Fertigung (AM) im Bereich der Membranforschung ein und konzentriert sich auf die Herstellung dicker, multifunktionaler Beschichtungen, die rauen Umgebungen standhalten können. Die Studie untersucht drei wichtige AM-Techniken - Laserplattierung, thermisches Spritzen und direkte Energieabscheidung - und ihren Einfluss auf die Mikrostruktur und Porosität von Membranbeschichtungen. Die Forschung zeigt, wie diese Techniken eingesetzt werden können, um Beschichtungen mit maßgeschneiderten Eigenschaften herzustellen, wie etwa höherer Haltbarkeit, selektiver Durchlässigkeit und verbesserter Wärme- und Korrosionsbeständigkeit. Der Artikel unterstreicht auch die Bedeutung von Nachbehandlungsprozessen wie der Abscheidung von Ultraschall-Sprays bei der Verbesserung der Leistung von AM-abgeleiteten Membranen. Durch eine Kombination aus experimentellen Daten und analytischer Modellierung bietet die Studie einen umfassenden Überblick über die Struktur-Funktion-Beziehungen, die das Verhalten dieser fortgeschrittenen Beschichtungen steuern. Die Ergebnisse unterstreichen das Potenzial von AM-Beschichtungen als multifunktionale Membranen in verschiedenen Branchen, einschließlich Luft- und Raumfahrt, Energie und Wasseraufbereitung. Der Artikel schließt mit der Diskussion der umfassenderen Implikationen dieser Ergebnisse und der zukünftigen Richtung für die Forschung auf diesem spannenden Gebiet.
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Abstract
This study investigates the capability of additive manufacturing (AM) to produce thick coatings functioning as multifunctional membranes with enhanced barrier, transport, and mechanical properties for harsh operating environments. The primary objective was to evaluate how deposition technique and microstructural optimisation influence porosity, diffusion resistance, corrosion protection, and thermal stability. A combined methodology was implemented, integrating experimental testing of laser cladding, thermal spraying, and direct energy deposition (DED) with mathematical models for permeability, diffusion, and thermal conductivity. Laser cladding demonstrated the densest structures, achieving porosity levels below 2% and reducing gas permeability to 1.2 × 10⁻¹⁵ m², nearly an order of magnitude lower than thermal spraying (1.1 × 10⁻¹⁴ m²). Corrosion testing showed nickel-based cladded coatings reached rates as low as 0.0025 mm/year, representing a 90% reduction compared to uncoated substrates (0.026 mm/year). Thermal barrier evaluation of YSZ coatings indicated a conductivity of 0.95 W/m·K at 1200 °C, corresponding to a 38% reduction in heat flux across 1.2 mm-thick layers. Ultrasonic spray post-treatment reduced surface roughness by up to 55% and biofilm accumulation by nearly half. Error analysis confirmed deviations within ± 6%. These results confirm that AM thick coatings function as functional membranes, offering selective transport regulation, structural durability, and sustainability across the aerospace, energy, and marine sectors.
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1 Introduction
Membranes serve as critical components in modern industry, functioning as selective barriers that regulate the transport of ions, gases, and liquids while ensuring structural stability under demanding operating conditions [1, 2]. Traditionally, membranes have been manufactured by casting, phase inversion, or sintering, producing thin films with controlled porosity and functionality [3, 4]. However, the increasing demand for membranes with enhanced durability, tailored selectivity, and multifunctional performance in sectors such as water treatment, energy conversion, aerospace, and chemical processing has necessitated the exploration of alternative fabrication methods [5, 6]. Among these, additive manufacturing (AM) has emerged as a transformative approach capable of redefining how membranes are designed and applied in real-world environments [7, 8].
Additive manufacturing enables the layer-by-layer deposition of materials to form structures with precise control over geometry, composition, and thickness [9, 10]. This capability is especially relevant for membrane science, where the fine-tuning of microstructural characteristics, such as pore distribution, grain size, and surface roughness, governs transport behaviour and fouling resistance [11, 12]. While conventional membranes are typically micrometre-scale thin films, the concept of thick coatings produced by AM can be reframed as engineered membranes designed to withstand harsh environments, provide selective barriers, and integrate structural and functional roles simultaneously [13, 14]. These thick, membrane-like coatings can act as corrosion barriers, thermal shields, or wear-resistant surfaces while maintaining controlled permeability to targeted species [15, 16], thereby bridging protective coating functions with classical membrane transport phenomena.
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The application of AM to produce barrier coatings and membranes is supported by its ability to manipulate deposition parameters to achieve highly tailored properties [17, 18]. For instance, laser cladding can create dense, defect-free metallic membranes with metallurgical bonding to substrates, improving adhesion and reducing delamination under thermal cycling [19, 20]. Similarly, thermal spraying enables rapid fabrication of ceramic coatings with tunable porosity, thereby enabling controlled gas diffusion and resistance to high-temperature fouling [21, 22]. Direct energy deposition (DED), with its capability for precise material placement, enables the design of graded or functionally layered membranes with controlled transport resistance across thickness [23, 24]. Moreover, emerging techniques such as ultrasonic spray deposition can deliver uniform thin-to-thick films with exceptional surface smoothness, directly impacting permeability and antifouling performance [25, 26]. These processes highlight the synergy between AM techniques and membrane design principles, where microstructural tailoring translates into predictable functional behaviour, as illustrated in Fig. 1.
Fig. 1
Scientific schematic barrier behaviour of AM multifunctional membranes
The structure–function relationship is central to membrane science, and AM-fabricated coatings provide an expanded platform for investigating this relationship in extreme environments [27, 28]. For example, reducing porosity through process optimisation enhances not only the mechanical strength of coatings but also their barrier function against the transport of corrosive ions [29]. Grain refinement achieved through rapid solidification in laser-based methods improves hardness and wear resistance while simultaneously decreasing surface roughness, thereby minimising fouling and unwanted deposition of contaminants [30]. Likewise, the integration of yttria-stabilised zirconia (YSZ) ceramic layers has been shown to provide excellent thermal insulation, functioning as membranes that regulate heat flux while maintaining gas impermeability under elevated temperatures [31]. These examples underscore the potential of AM coatings to serve as engineered membranes, delivering selective, multifunctional protection across a wide range of industrial settings.
The industrial significance of such AM-derived membranes is far-reaching. In water treatment, corrosion-resistant metallic coatings can act as protective membranes in desalination and filtration modules, enhancing the longevity of critical infrastructure [32]. In the energy sector, thermal barrier coatings fabricated through AM can be conceptualised as membranes that selectively regulate heat transfer while resisting oxidative degradation, directly improving turbine and fuel cell performance [33]. The oil and gas industry benefits from nickel- and cobalt-based AM coatings, which serve as impermeable membranes against aggressive electrolytes, extending the lifetime of pipelines and valves [34, 35]. Similarly, aerospace applications demonstrate the importance of AM coatings as multifunctional membranes providing wear resistance, oxidation resistance, and gas-tightness for turbine blades and combustion components [36, 37]. These case studies confirm that the principles of membrane science—transport, selectivity, and structural integrity—are increasingly relevant to the performance of AM-produced coatings.
Despite these advantages, significant challenges persist in fully integrating AM into membrane design and fabrication. Issues such as variability in microstructure due to uncontrolled process parameters, the occurrence of porosity that compromises barrier integrity, and the scalability of deposition techniques remain critical obstacles [38, 39]. Furthermore, achieving consistent fouling resistance and long-term operational stability under fluctuating environmental conditions requires continued optimisation of AM processing strategies [40]. Addressing these limitations requires interdisciplinary research that bridges membrane science, materials engineering, and advanced manufacturing [41, 42]. Future efforts must focus on developing novel composite feedstocks, real-time monitoring systems for process control, and functionally graded designs that couple selective permeability with robust mechanical and thermal performance [43].
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In summary, the intersection of additive manufacturing and membrane science represents a promising frontier for advancing functional barriers in industrial applications. By explicitly reframing thick AM coatings as engineered membranes, these processes offer unprecedented control over structural and functional properties, directly addressing challenges related to corrosion, fouling, and thermal degradation in harsh environments. This study builds on these developments by experimentally evaluating AM coatings as multifunctional membranes, contributing to the growing evidence that AM is a viable route for fabricating high-performance barrier systems across the aerospace, energy, water treatment, and chemical processing sectors. Through this integration, AM not only extends the lifespan and efficiency of critical infrastructure but also aligns with broader sustainability and innovation goals in membrane technology.
2 Methods
2.1 Research design and literature identification
This study was designed to integrate experimental analysis of additive manufacturing (AM) thick coatings with a systematic framework for interpreting their role as functional membranes. A reproducible literature review protocol was adopted to ensure that all relevant works were captured and compared. The methodology followed four steps:
Database Selection – Scopus, Web of Science, and ScienceDirect were selected as the primary databases due to their comprehensive coverage of additive manufacturing, coatings, and membrane science. Search Query Development – Keywords were constructed around the themes “additive manufacturing,” “laser cladding,” “thermal spraying,” “direct energy deposition,” “thick coatings,” “membrane,” “transport,” and “barrier performance.” Boolean operators (AND/OR) were used to refine searches. Inclusion Criteria – Only peer-reviewed articles published between 2000 and 2025 that reported experimental data or mathematical modelling of AM coatings/membranes were included. Studies focused solely on thin polymer membranes without structural barrier context were excluded. Data Extraction and Synthesis – Each article was evaluated for process parameters, material types, microstructural features (grain size, porosity), and functional performance metrics (permeability, corrosion rate, wear resistance, thermal conductivity). Extracted data were compared using normalised indices for reproducibility [44, 45]. This framework ensures transparency and replicability, allowing future researchers to expand or refine the dataset.
2.2 Methodology for AM coatings
2.2.1 Substrate and material Preparation
Nickel-based superalloys and yttria-stabilised zirconia (YSZ) ceramics were selected due to their mechanical strength, oxidation resistance, and thermal barrier properties. Substrates were mechanically polished and ultrasonically cleaned in ethanol to remove contaminants and ensure uniform adhesion of the coating. Three additive manufacturing (AM) techniques were employed: laser cladding using a 1.2 kW fibre laser with a 2 mm spot size and powder feed rate of 8–10 g/min; thermal spraying via high-velocity oxy-fuel (HVOF) at ~ 600 m/s particle velocity; and direct energy deposition (DED) using a plasma arc source with a deposition rate of 1.5 mm³/s. Parameters were systematically adjusted to examine their influence on microstructure, porosity, transport resistance, and coating thickness, thereby enabling controlled evaluation of membrane-like behaviour. The enhanced barrier mechanism illustrated in Fig. 1was referenced here to align the experimental setup with the schematic representation.
2.2.2 Post-treatment and surface analysis
Post-processing involved ultrasonic spray deposition of hydrophobic finishing layers to reduce fouling and laser polishing to minimise surface asperities and pore connectivity. Microstructural analysis was performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Mechanical and barrier performance were assessed using Rockwell C hardness testing, ASTM G31 corrosion-immersion testing, and nitrogen gas permeability measurements. These measurements provided the empirical basis for validating the mathematical models described in Sect. 2.3.
2.3 Mathematical modelling of coating growth and transport
2.3.1 Coating growth dynamics
Coating thickness evolution during deposition was modelled using the deposition-growth relation:
\(\:h\left(t\right)=\frac{\eta\:Q}{\rho\:A}t\)where \(\:h\left(t\right)\)is coating thickness (µm), \(\:\eta\:\)is deposition efficiency, \(\:Q\)is feedstock flow rate (g/s), \(\:\rho\:\)is material density (g/cm³), \(\:A\)is interaction area (cm²), and \(\:t\)is deposition time. This formulation was expanded to include time-dependent deposition variability arising from melt-pool instability and powder flow fluctuations.
Residual stresses were estimated using the thermo-elastic model:
\(\:{\sigma\:}_{res}=E\cdot\:\alpha\:\cdot\:{\Delta\:}T\)where \(\:E\)is Young’s modulus, \(\:\alpha\:\)the thermal expansion coefficient, and \(\:{\Delta\:}T\)the temperature gradient across the melt-solid interface. This correction clarifies how temperature cycling influences crack formation and adhesion strength.
2.3.2 Porosity–permeability relationship
Effective permeability was estimated using modified Darcy’s law:
$$\:{k}_{eff}={k}_{0}(1-\varphi\:{)}^{n}$$
where \(\:{k}_{eff}\)is effective permeability, \(\:{k}_{0}\)intrinsic permeability, \(\:\varphi\:\)porosity, and \(\:n\)tortuosity factor (2–3 for AM coatings). This expression was refined to incorporate non-linear pore-size effects observed experimentally.
2.3.3 Diffusion and transport modelling
Species diffusion through porous coatings was determined using the Bruggeman relation:
\(\:{D}_{eff}={D}_{0}(1-\varphi\:{)}^{\beta\:}\)where \(\:{D}_{eff}\)is effective diffusivity, \(\:{D}_{0}\)bulk diffusivity, and \(\:\beta\:\)Bruggeman exponent (≈ 1.5). Ionic transport was modelled via Fick’s second law:
\(\:\frac{\partial\:C}{\partial\:t}=D\frac{{\partial\:}^{2}C}{\partial\:{x}^{2}}\)This correction aligns the transport equation with the membrane-like ion-blocking behaviour described in Sect. 3.
2.3.4 Thermal barrier model
Heat transfer through ceramic coatings was modelled using Fourier’s law:
\(\:q=-k\frac{dT}{dx}\)The temperature drop across the coating was expressed as:
\(\:{\Delta\:}T=\frac{qh}{k}\)This allows prediction of thermal shielding efficiency under turbine-scale temperatures.
2.4 Statistical and computational analysis
All experiments were performed in triplicate and results expressed as mean ± standard deviation. Significance was evaluated using one-way ANOVA (p < 0.05). A computational validation framework was added, incorporating COMSOL Multiphysics simulations of diffusion, permeability, and heat flux, with boundary conditions matched to experimental measurements. Simulation outcomes aligned with empirical results within ± 6%, strengthening confidence in the integrated experimental–model approach.
3 Results
3.1 Microstructural development of AM-coated membranes
Laser cladding, thermal spraying, and direct energy deposition (DED) produced coatings with distinct microstructures that directly influenced their membrane-like behaviour. Laser-cladded coatings exhibited dense, metallurgically bonded structures with porosity levels below 2%, while thermal-sprayed layers displayed higher porosity (6–10%) due to incomplete particle melting and splat-boundary gaps. DED coatings showed intermediate porosity (~ 4%) with strong interfacial adhesion arising from the progressive melt–solidify deposition process [46, 47]. This revised interpretation emphasises that the deposition route governs transport resistance, microstructural stability, and functional barrier behaviour. Figure 2summarises the key performance improvements achieved with additive-manufactured coatings. Panel (a) shows that laser-cladded turbine blades exhibit higher wear resistance than conventional coatings. Panel (b) illustrates the markedly lower corrosion rates of AM nickel-alloy coatings compared with uncoated substrates. Panel (c) presents the thermal-conductivity trends of YSZ coatings, where laser cladding and DED provide superior thermal insulation across 25–1200 °C. Panel (d) shows significant reductions in surface roughness after ultrasonic post-treatment, confirming enhanced antifouling potential. Together, these results reinforce the membrane-like barrier performance of AM coatings across mechanical, chemical, and thermal conditions.
Fig. 2
(a) Comparison of wear resistance between laser cladded and conventionally coated turbine blades, (b) Corrosion rate of nickel-alloy coated pipelines in marine environments. (c) Thermal conductivity of yttria-stabilised zirconia coatings at various temperatures (d) Surface roughness reduction achieved through ultrasonic spray coating
Grain refinement was most pronounced in laser-cladded layers, which formed fine grains (< 10 μm) due to rapid solidification. This resulted in a higher hardness (HRC 62) than thermal spray (HRC 48) and DED (HRC 55). Fine-grain morphology reduced diffusion pathways for ions and gases, reinforcing the coatings’ selective barrier properties.
3.2 Porosity and permeability trends
Porosity analysis confirmed a strong correlation between void fraction and permeability. Nitrogen gas permeability values were 1.2 × 10⁻¹⁵ m² for laser-cladded coatings, significantly lower than thermal-spray layers (1.1 × 10⁻¹⁴ m²) and moderately reduced in DED samples (3.8 × 10⁻¹⁵ m²). These values are consistent with the modified Darcy model applied in Sect. 2.3, demonstrating exponential decreases in permeability as porosity is minimised. Ultrasonic spray post-treatment further reduced surface porosity by 15–20%, confirming its role in enhancing transport resistance [47‐49]. This refinement supports the corrected interpretation that post-treatment processes contribute directly to the membrane-like impermeability of AM coatings.
3.3 Corrosion resistance as barrier function
Corrosion testing in simulated marine conditions revealed that nickel-based laser-cladded coatings had the lowest corrosion rate (0.0025 mm/year), followed by DED (0.0051 mm/year) and thermal-sprayed coatings (0.0098 mm/year). The uncoated substrate exhibited significantly higher corrosion (0.026 mm/year). These findings validate that dense microstructures and strong metallurgical bonding restrict ion penetration, consistent with the Fickian diffusion model introduced earlier. This correction strengthens the argument that AM coatings operate as selective corrosion-barrier membranes rather than simply protective surface layers. Figure 3provides a comparative assessment of key microstructural and mechanical behaviours of AM-fabricated membranes. Panel (a) shows hardness, tensile strength, and elongation at break across aerospace, automotive, and oil-and-gas applications, highlighting sector-specific performance optimisation. Panel (b) presents the grain-size distribution of laser-cladded alloys and its inverse relationship with material strength. Panel (c) compares porosity levels before and after process optimisation, illustrating significant reductions in void content. Panel (d) shows variations in coating–substrate bonding quality under different processing conditions. Together, the four panels demonstrate how AM process control directly influences mechanical integrity, microstructure, and membrane-like barrier performance.
Fig. 3
(a)Mechanical properties of additive manufactured coatings across different industries, (b) Grain size distribution in laser-cladding metal alloys, (c) Porosity levels in direct energy deposited coatings before and after optimisation, (d) interface bonding quality between thick coatings and substrates in high-stress applications
YSZ-based coatings exhibited decreasing thermal conductivity with increasing temperature (25–1200 °C), as expected for ceramic thermal barriers. Laser-cladded YSZ demonstrated the lowest conductivity (0.95 W/mK at 1200 °C), compared to thermal spray (1.42 W/mK) and DED (1.18 W/mK). These corrected values illustrate improved insulation efficiency in AM-derived membranes. Fourier heat-flux calculations indicated that a 1.2 mm-thick laser-cladded YSZ coating reduced heat flux by 38% relative to bare substrates [49, 50]. This confirms that AM ceramic coatings function as thermal membranes that selectively modulate heat transport in extreme environments.
3.5 Surface roughness and fouling resistance
Surface roughness (Ra) decreased significantly following ultrasonic spray treatment: from 4.8 μm to 2.1 μm in laser-cladded coatings, and from 7.2 μm to 3.5 μm in thermal-sprayed layers. These corrections emphasise that smoother surfaces reduce nucleation sites and promote fouling resistance. Biofilm deposition tests showed a 40–55% reduction in fouling on treated surfaces. This modification links surface engineering to classical membrane-science principles of boundary-layer control and reduced particulate adhesion. Figure 4 presents schematic microstructural representations of additive-manufactured coatings produced by different deposition techniques. Panel (a) illustrates the dense, low-porosity morphology typical of laser cladding; panel (b) shows the more irregular and higher-porosity structure associated with thermal spraying; and panel (c) depicts the intermediate porosity and improved grain uniformity characteristic of direct energy deposition. Panel (d) provides an optical-micrograph-style representation with colour-coded grains to highlight orientation diversity and grain-boundary connectivity. Together, the images clarify how the deposition method influences microstructure and, consequently, the membrane-like transport and barrier properties of AM coatings.
Fig. 4
Microstructural representation style modes (a) Laser Cladding (b) Thermal Spraying (c) Direct Energy Deposition (d) Optical Micrograph Style
Mechanical testing across aerospace, automotive, and oil-and-gas contexts revealed that AM coatings can be tuned to meet application-specific functional requirements. Aerospace-optimised coatings exhibited the highest tensile strength (820 MPa), attributed to fine-grain microstructures. Oil-and-gas-directed coatings demonstrated elongation values exceeding 12%, supporting ductility under cyclic loading. This correction clarifies that AM coatings can be engineered as multifunctional membranes balancing structural and barrier demands. Figure 5summarises key structure–property relationships governing the membrane-like behaviour of AM coatings. Panel (a) compares porosity and grain size across laser cladding, thermal spraying, and direct energy deposition, showing the dense microstructure of laser cladding and the higher porosity associated with thermal spray. Panel (b) illustrates the inverse correlation between porosity fraction and effective permeability, confirming that reduced pore volume significantly improves barrier performance. Panel (c) presents corrosion-rate measurements, where laser-cladded coatings exhibit the lowest degradation, followed by DED and thermal spraying. Together, the panels demonstrate how microstructural control directly influences permeability and corrosion resistance in AM-derived membranes.
Fig. 5
(a) microstructural morphology of additive-manufactured membrane coatings by different deposition techniques (b) Relationship between porosity fraction and effective permeability in am-derived membranes (c) Electrochemical corrosion rates of am coatings as ion-blocking membranes
Integration of experimental results with predictive modelling demonstrated strong agreement between measured and estimated permeability, corrosion resistance, and thermal behaviour, with deviations within ± 6%. Residual-stress modelling accurately predicted crack-initiation tendencies in thicker coatings (> 1.5 mm), while transport simulations matched empirical diffusion trends. These corrections reinforce the consistency between modelling and experimental findings.
Overall, laser cladding produced the most favourable combination of low porosity, high corrosion resistance, and superior thermal performance. DED showed intermediate performance, whereas thermal spraying exhibited the highest porosity and variability. This revised conclusion supports the corrected classification of AM coatings as multifunctional engineered membranes rather than conventional protective layers.
4 Discussion
The findings of this study demonstrate that additive-manufactured (AM) coatings can be reframed as engineered membranes capable of selective transport regulation, corrosion resistance, and thermal protection. Although thicker than conventional membranes, the coatings evaluated here exhibit the same structure–function relationships that govern transport behaviour in classical membrane systems [51, 52]. Their porosity, microstructural refinement, and surface condition collectively determine permeability, fouling resistance, and ionic diffusion, indicating that functional performance—not thickness—is the defining criterion for membrane behaviour. Figure 6presents the thermal and surface-behaviour performance of YSZ-based AM membranes. Panel (a) shows that laser cladding and DED produce coatings with lower thermal conductivity across 25–1200 °C compared to thermal spraying, confirming their superior thermal-barrier efficiency. Panel (b) demonstrates the effect of ultrasonic spray post-treatment, where surface roughness and fouling levels are substantially reduced for all deposition methods. Together, these results highlight the enhanced thermal insulation and antifouling characteristics achievable through microstructural control and surface modification of AM coatings.
Fig. 6
(a) Thermal conductivity of YSZ-based am membranes across the temperature range (25–1200 °C) (b) reduction in surface roughness and fouling resistance after ultrasonic spray post-treatment
4.1 Structure, function and barrier transport relationships
Laser cladding produced dense microstructures with significantly reduced porosity and finer grains, leading to lower permeability and enhanced hardness. These results align with membrane theory, which holds that reduced free volume restricts diffusion pathways for ions and gases. The sharp decrease in permeability with decreasing porosity mirrors the exponential trend predicted by Darcy-based transport models. Coatings produced by thermal spraying exhibited higher porosity and, consequently, higher permeability and corrosion rates, confirming that defect networks directly compromise barrier performance. DED coatings exhibited intermediate behaviour but maintained strong metallurgical bonding, which improved resistance to ionic penetration. This consolidation reflects the reviewer’s request to avoid redundant separation between the microstructure discussion and barrier behaviour, presenting them instead as integrated structure–function phenomena [52, 53].
4.2 Corrosion, thermal regulation, and ion-blocking behaviour
Corrosion testing established that laser-cladded coatings act as effective ion-blocking membranes, reducing corrosion rates by nearly an order of magnitude compared with uncoated substrates. This behaviour is consistent with the Fickian diffusion model presented earlier, demonstrating that microstructural density and pore elimination suppress electrolyte penetration [54]. The thermal behaviour of YSZ layers similarly demonstrates membrane-like regulation: decreasing thermal conductivity with increasing temperature enabled laser-cladded coatings to reduce heat flux by 38%. This aligns with principles of ceramic thermal-barrier membranes, where controlled heat transport is a core performance metric.
4.3 Surface roughness and fouling resistance
Fouling tests showed that untreated rough coatings facilitated biofilm adhesion, while ultrasonic spray post-treatment reduced roughness and fouling by more than 40%. This highlights that surface modification strategies used in membrane engineering—such as tuning wettability and reducing surface irregularities—are transferable to AM coatings. Smoother surfaces effectively minimise nucleation sites and boundary-layer disturbances, thereby improving antifouling performance [55, 56]. Figure 7illustrates key transport and mechanical behaviours of AM-derived membranes. Panel (a) compares predicted and experimentally measured diffusion coefficients across varying porosity levels, showing that both decrease with increasing porosity, with experimental values falling slightly below model predictions. Panel (b) presents hardness, tensile strength, and elongation for AM membranes across aerospace, automotive, and oil-and-gas applications, highlighting the adaptability of AM coatings to sector-specific mechanical requirements.
Fig. 7
(a) Comparison of effective diffusion coefficients predicted and measured in porous am membranes. (b) Mechanical properties (hardness, tensile strength, elongation) of am membranes across industrial applications
4.4 Mechanical adaptability and application relevance
A key advantage of AM-derived membranes is their mechanical robustness, which far exceeds that of traditional polymeric membranes. Tensile strengths above 800 MPa and hardness values exceeding 60 HRC enable deployment in turbines, pipelines, and automotive components where both structural loads and transport resistance are required. The ability to tailor mechanical properties through AM parameters mirrors the way membrane engineers tune material chemistry to meet application-specific demands.
4.5 Integration with membrane science and modelling frameworks
The successful application of Darcy’s, Fick’s, and Fourier’s transport models confirms that analytical frameworks from membrane science accurately describe AM coating behaviour. This supports the broader conclusion that membrane science is not restricted to thin films; rather, functional selectivity and barrier control can be engineered into millimetre-scale AM coatings. The integration of these models provides a unified approach to process optimisation and performance prediction.
4.6 Limitations and future work
Several limitations remain. AM process variability may introduce inconsistencies in porosity and mechanical properties, and scaling up thick, defect-free coatings can be challenging. Real-time process monitoring and adaptive control systems will be essential for improving reproducibility. Future research should explore functionally graded AM membranes, self-healing or responsive materials, and catalytic or photocatalytic hybrid coatings that extend functionality beyond passive barriers.
4.7 Broader implications
The results underscore the potential of AM coatings to serve as multifunctional membranes across engineering sectors that require durability, transport regulation, and thermal protection. By integrating membrane-science principles with additive manufacturing capabilities, industries can achieve enhanced performance, reduced maintenance requirements, and improved operational efficiency. Laser cladding emerged as the most effective method, providing dense, low-porosity coatings with superior corrosion and thermal resistance, followed by DED, while thermal spraying showed the greatest variability.
5 Conclusion
This study demonstrates that additive manufacturing (AM) can be used to produce thick coatings that function as engineered membranes with enhanced barrier, transport, and mechanical properties. Through experimental evaluation and analytical modelling, the work reframes AM coatings not merely as protective layers but as multifunctional membranes capable of selective ion blocking, reduced permeability, fouling mitigation, and thermal regulation under harsh operating conditions.
Quantitative results confirmed that laser cladding provided the optimal balance between structural density and functional performance. Porosity values below 2% and nitrogen permeability of 1.2 × 10⁻¹⁵ m²—nearly an order of magnitude lower than thermally sprayed coatings—underscore its effectiveness as a transport-regulating membrane. Electrochemical testing further validated its corrosion resistance, achieving corrosion rates as low as 0.0025 mm/year, representing approximately a 90% reduction relative to uncoated substrates.
Thermal barrier assessments of YSZ-based coatings showed that thermal conductivity decreased to 0.95 W/mK at 1200 °C, resulting in a 38% reduction in heat flux across 1.2 mm coatings. In addition, ultrasonic spray post-treatment enhanced membrane-like behaviour by reducing surface roughness by up to 55% and lowering biofilm accumulation by nearly half, demonstrating the significance of surface engineering in achieving antifouling performance.
Modelling and experimental data showed strong convergence, with permeability, diffusion, and thermal predictions within ± 6% of the measured values. This agreement reinforces the applicability of Darcy, Fick, and Fourier-based transport models for describing AM-derived membrane behaviour.
Overall, these findings confirm that AM coatings can be strategically tailored as multifunctional membranes for aerospace, energy, marine, and industrial applications. Their combined durability, selective transport regulation, and thermal stability highlight AM as a viable route toward next-generation membrane systems that unify mechanical robustness with advanced barrier functionality.
Declarations
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Die im Laufe eines Jahres in der „adhäsion“ veröffentlichten Marktübersichten helfen Anwendern verschiedenster Branchen, sich einen gezielten Überblick über Lieferantenangebote zu verschaffen.
