Harnessing Fly Ash as Particle Reinforcement in Nature-Inspired Multilayer Composites

Numerous biological organisms with ubiquitous soft-and-hard hybrid structures exhibit various length scales and adapted unique spatial arrangements responsible for diverse functionality and mechanical properties [1,2]. A nacre and foliated (FL) hierarchies from mollusk shells are well-known bio-inspiration models demonstrated to dissipate abundant energy by tablet sliding and delamination. Investigation insight that nacre structures exhibit 3000 times higher toughness than their miniature constitutes due to the three-dimensional (3D) arrangement of the wall (alternatively stacked soft and hard) combining inorganic–organic interaction [3–6]. This anomalous phenomenon notably relies on various geometry dimensions (i.e., diameter to thickness ratio, layer height, number of layers, and sample thickness) and borderline circumstances (i.e., bending length and contact radius) [7,8], for instance, enhancing the tablet diameter to thickness ratio from 25 to 3500 triggered a notable modification in the modulus of clay-based films from 15 to 40 GPa [9]. Numerous devices have been fabricated and easily implemented with combined soft phase sliding, polymeric chain orientation manipulation, chain entanglement, and nanoparticle interaction in proper architectures at the macroscale. However, at the microscale mimicking similar functions and structure is albeit challenging as at the miniature length scale (micrometer and below), material arrangement and interaction are less practical [10,11]. Therefore, Lyu et al. developed a chain of a soft and hard alternative segment with a liquid droplet (as soft) and metal-organic framework (as hard) via valence-limited assembly which able to shift between curved and straight conditions through thermoresponsive deswelling/swelling mechanism [12]. Bu and team strategically design self-assembly of nacre-like structure in polymeric composite based on PET and metallic glass compound. Nacre-like design improved mechanical properties of composites up to 506.9 MJ/m³ toughness, 55.4 MPa tensile strength, and 1180% elongation due to strain induced crystallization and more chain orientation in arrangement [13]. Moreover, to strengthen the mechanical characteristics of carbon fiber rainforced polymers, Xiao et al. introduce brick-and-mortar arrangement of mollusk shell. They have employed hand layup, prepreg, and hot press approach to fabricate laminate of nacre to evolute absorption of impact energy and impact response. The research outcome reveals that a “bricks and mortar” arrangement in three-dimensional enriches the strength retention rate of laminates and impact resistance [14]. Figure 1(a) explain miniature arrangement and components of numerous mollusk shell structures including aragonite and calcite pattern at macrolevel, foliated, nacre sheet (NS), nacre columnar (NC) arrangement, and fibril array at microlevel and tablet structure, dimension, and material at nanolevel.

The superior energy dissipation ability of the nacre-inspired composite component renders the soft phase material, which can utilize rich micro and nanoparticles/materials such as graphene, MXene, clay, aluminum oxide, fly ash (FA), and its derivatives [16–19]. For instance, Guan et al. designed brick-and-mortar arrangement in a transparent composite film of bacterial cellulose and nanoclay platelets with excellent toughness (17.71 MJ m⁻³) and strength (482 MPa) [20]. Gao and team constructed hard brushite platelets and soft component sodium alginate that achieved a flexural strength of 267 MPa [8]. Ma and group synthesized nacre-like interlayer binders with graphene-based fibers, which involved both hydrogen bonds and strong π−π interactions and achieved a high toughness of 9.44 MJ/m³ and strength of 724 MPa to surpass natural nacre strength (80–135 MPa) [21]. Nevertheless, these nanoparticles are of metallic compound, ceramic, or two-dimensional materials comprising higher density and are relatively expensive, and need to be replaced with cost-effective and recyclable particles to manufacture strong composites [22]. The fly ash incorporation into the polymer for mortar phase will be feasible to produce significant mollusk shell structure, which can reduce immense FA waste (annual fly ash generation will be 437 MT in India by 2030), flies from flue gas emission during coal burning process in thermal power plants, heating for warmth, metal smelting, and so forth [23–25]. FA structure is decorated with various active components such as SiO₂, Al₂O₃, and Fe₂O₃ with approximately 20-μm particle size, and the glass-like smooth surface has been deeply implemented in cement production, building materials, composites, chemical industry, and agriculture [26,27]. The major components of fly ash are mullite (Al₆Si₂O₁₃) and quartz (SiO₂) phases, which comprise around 25 wt. % and 55 wt. %, respectively. Xue et al. designed a composite of polylactic acid (PLA) matrix reinforced with fly ash to reduce environmental waste and facile use of FA in high-strength composites. The introduction of FA has minimum effect on the crystallinity of PLA as according to crystal phase planes (200)/(100) and (203), the diffraction peaks were observed at 2θ  =  16.5 deg and 18.9 deg. Moreover, in differential scanning calorimetry measurements, curves' double melting behavior was noticed, which could render to melt-recrystallization model and endothermic peaks shifted vertically toward higher temperature due to which crystallinity decreased drastically from 44.6% and 26.8% owing to the introduction of FA particles diminishing the mobility of PLA chains and followed by retard the crystallization of PLA. The thermogravimetric analysis graph describes that the addition of FA's thermal stability of the composite improves with the addition of FA content due to the FA filler's barrier effect against the PLA chain mobility [28]. Apart from that, Batistella and group analyzed thermal stability and flame-retardant properties of PLA/FA composite and observed uniform dispersion of FA particles in the matrix without dispersion. The thermogravimetric analysis analysis reveals that thermal degradation temperature increases with increasing FA content up to 18.75%, and further deterioration was studied with FA enhancement [29]. The metallic catalytic constituents present in FA introduce a hindrance that results in a slight enhancement in the thermal stability of the composite material. It was noted that the FA particles exhibit effective dispersion and interconnection within the PLA matrix through Si–O or Al–O bonding, thereby serving a dual function of coupling the FA with the PLA matrix and imparting strong mechanical attributes. Nevertheless, there exists a need to comprehensively explore the mechanical performance of the FA composite within PLA featuring bio-inspired architectures. The primary objective of this research is to acquire a deeper understanding of how fractures manifest, the mechanisms that lead to failures, and the degree of improvements in material properties. Furthermore, the literature describes that FA can be a prominent additive in thermoplastic and thermoset polymers for enhancing mechanical properties [30,31]. Figure 1(b) portrays the molecular arrangement of brick (PLA) and mortar (FA) phases and their possible interaction between the C–H or C=O group of PLA and the metallic compound of FA.

This work introduces a novel strategy for creating a multilayered composite architecture comprising both rigid and flexible segments. The multilayered composites are fabricated using two distinct additive manufacturing approaches, namely, filament-based fused deposition modeling (FDM) and direct ink writing (DIW) printing. Fabricated composites comprise microscale arrangements of mollusk seashell structures such as nacre sheet, nacre columnar, and foliated architecture. Furthermore, composite incorporates microscale particles of environmental pollutants or industrial waste FA to enable the convenient utilization of fly ash while simultaneously mitigating environmental pollution. In this work, PLA-based stacked pallets or laminates infilled with the hybridized solution of organic and inorganic component (PLA + FA) mort structures have been fabricated using FDM and Polyjet 3D printing methods, the potential for material utilization remains constrained. As a result, achieving a varied amalgamation of mechanical constituents, particularly involving flexible polymers and inflexible filler materials, presents challenges. The recent research literature demonstrates that the dispersion of FA within PLA exhibits a minor aggregation, facilitating robust interfacial adhesion. Moreover, the existence of FA particles contributes rise to obstructions in the molecular mobility of PLA chains. A bionic composite material cantered around FA demonstrates impressive performance in scenarios involving impact, tension, and wear. This renders it highly suitable for a wide array of applications, including but not limited to, the production of bulletproof vests, components within the aerospace and automobile sectors, as well as various sporting equipment. The study introduces an innovative approach aimed at creating structures reminiscent of nacre and foliated patterns, which possess a harmonious blend of flexibility and rigidity. Remarkably, this is achieved by incorporating industrial pollutant fly ash as a filler material. The goal here is to optimize both the modulus of strength and resistance against impacts through this hybrid arrangement.

Natural nacre structure imitates as microplatelets of PLA with a diameter of 2 μm (accessible on a large scale) with equal sizes manifest as organic building blocks, and microparticle disappeared PLA + FA solution utilized in interface region (Fig. 2). While layered PLA structure (FDM printed), stacking with FA mixture of organic and inorganic particles (DIW printed) with various FA content such as 5 wt. %, 10 wt. %, and 15 wt. % adhering in vertical sequence utilized for mimicked laminated or foliated architecture [33]. Biocompatible PLA was utilized as the prime biopolymer matrix owing to its polymeric chain flexibility and mechanical strength, along with the ample hydroxyl and carboxyl groups at the chain's molecular level, which facilitate the interlaminar interface amid fly ash compound of Al and Si [34,35].

To investigate the ability of FA filler and mimicked geometry of the function under impact loading, Izod impact tests were performed on the monolithic structures, nacre-like assemblies, and foliated constructions under diverse impression velocities [41]. The specimens, according to ASTM D256 (63.5 × 12.7 × 12.7 mm³) with the pristine, foliated, and nacre-like architecture printing, were made of five PLA engraved 2 μm platelets diameter and 2 mm thick plates with 1 mm mortar space for polymeric mixture and four 200-μm-thick fly ash incorporated PLA solution in a 3D staggered “mortar-to-brick” configuration akin to natural shell (Fig. 4).

A total of three efficacious composite dog-bone shaped as per ASTM D638 type IV foliated and nacre specimens were created with four layers (two 3D-printed and two polymeric solution layers) (Fig. 5). The tensile characteristics of multilayered mollusk seashell structures are shown in Fig. 5(b) where the initial stress–strain curve indicates an almost equivalent nature to yielding or low-strain applications. Additionally, promoting further load on structures, NS manifests a noteworthy enhancement in force dissipation up to 4 MPa of ultimate stress, although NC ends at 3.2 MPa and the clean sample reaches 1.8 MPa. In contrast, Young's modulus (E) of the NS (274 MPa) describes a superior value to the NC (132 MPa) sequentially compared to the pristine structure (171). As per the representative volume element model, a similar behavior shows that under the action of uniaxial force, cracks propagate in the direction that grips low energy; hence, an NC skeleton made of uniform stacking yields a crack deflection path, while at the same point, breaks encounters with hard bricks in an asymmetrical network, which urges for surpassing energy in NS [49,50]. The stress–strain curve shows that higher strength is observed for higher filler additions when penetrated for the foliated and monolithic architecture; however, owing to low deformations, both fail in a brittle fashion (Figs. 5(c)–5(e)). With the increasing FA content, out-of-plane delamination of foliated structure is decreased, while minimal failure displacement is retained by monolithic structures [51]. By contrast, with high strength and large deformation, more ductile behavior is exhibited in nacre-like columnar and sheet structures. In bulk specimens loaded with 15 wt. % industrial waste FA, an elastic modulus of 418 MPa has been reported for nacre sheet, 1.5 fold higher than that single pristine, 169 MPa, and it reached the 217 MPa and 343 MPa Young's modulus for FDM printed foliated and columnar nacre frameworks. A study found that the modulus of elasticity of nacre sheet, columnar, and laminated composite strengthened by 12%, 17%, and 14% simultaneously when FA content changed from 5 wt. % to 15 wt. %. As the stress–strain curve indicates, multiple crack formation and delamination due to less holding of polymeric-micro-FA interaction are the prime reasons for lower strength in a laminated structure.

Meanwhile, in nacre sheet and columnar structures, the polymeric interlayers, microcracks, and large-scale tablet sliding hold the fragments and maintain the integrity for higher-containing FA filler composites [8,52]. At the same time, Xue et al. achieved maximum impact strength of 49.5 MPa with 20 wt. % FA microparticles incorporation in PLA without bionic structures [28]. Apart from that, the highest impact strength and tensile strength of 35 kJ/m³ and 200 MPa, respectively, for nacre-inspired PEEK composite in a study by Zhu and teams indicate that industrial waste FA amalgamation could drastically enhance the fracture behavior of composites [44]. It was observed that initially, a stress–strain curve for all the fly ash containing NC, NS, and FL followed similar trajectories to the elastic region, whereas in the plastic region, drastic change in fracture pattern and strength was indicated. In the elastic region, the deformation was flexible, and no crack formation was reflected where modulus enhanced due to tablet sliding, stacking, and interlocking over the mortar phase and fracture phenomena starting from plastic deformation. Figure 5 indicates that fly ash contains improves the modulus; however, the difference is not significant with 5 wt. %, 10 wt. %, and 15 wt. % due to sliding and tablet reorientation. Peng et al. experimented with thermoplastic polyurethane and carbon fiber-based 3D-printed nacre composite where they examined similar behavior in the elastic region and fracture strengthening was prominent in the plastic region [37,53,54].

At the macroscopic level, the ultimate flexural strength declined when the laminated structure was glued with polymeric PLA + FA solution in additive manufacturing, which enhanced for sheet and columnar nacre where microparticle bonds form microbridges and interfacial interaction between Al–O–Al and Si–O–Si molecules [55–57]. In addition, bulk material's mechanical properties are also influenced by secondary building film thickness, FA restrains as mortar, and the tablet stacking direction of the print. To evaluate the resistance to crack initiation, the flexural fracture was measured to be lower for all the artificial hierarchy, which turns into a brittle fracture [58,59] Additionally, Fig. 6(b) also provides the stress–strain curve of dynamic mechanical flexural bending flexural results of FL, NC, and NS architecture. As per Fig. 6(b), from the initial loading until the fracture, the NC structure deforming slightly subordinately corresponds to a nonuniformed nacre array, whereas the pristine geometry trails an identical path. For columnar stacking, the crack propagates at the interface of brick, which can be separated quickly, leading to crazing in the zone, while the sheet also has an equivalent tendency to a generation of crack, yet the propagation hinders subsequent bricks in layered position [60,61] The bending strengths of 3D-printed immaculate geometry and gastropod architecture achieved the maximum levels of 32.05 and 33.79 MPa, respectively, whereas bivalve shell geometry reached the ultimate power of 43.75 MPa. However, it has been noticed that total energy absorbance capacity was superior for the NC, attributed to minor interlayer deviation and interface tablet cohesion against bending load conditions [62]. The bulk nacre mimic's ultimate flexural strength can be up to ∼882 MPa after 15 wt. % FA, while with FL and NC architecture, ∼441 MPa and ∼615 MPa were the maximum value for the same, further surpassing that of pristine printed PLA (∼261 MPa) (Fig. 6(a)). A rise of mineral content within the 10 wt. % and 15 wt. % range enhances the fracture strength of laminated hierarchy by 33% and 40%, respectively, than that 5 wt. % FA (Fig. 6(b)). In addition to this strengthening effect, the sheet nacre composites with 10 wt. % and 15 wt. % of mineral contents reach flexural moduli, respectively, 849 MPa and 882 MPa which are higher relative to 5 wt. % (780 MPa) addition and similar behavior attributed to nacre columnar structures (Figs. 6(c) and 6(d)). This damage phenomenon can be attributed to interlayer delamination and intralayer string breaking in laminated structures, whereas nacre sheet and columnar undergo polymeric interlayer plastic deformation in heterogeneous brick mortar structures [63–65]. Liu et al. performed finite elemental studies to understand flexural loading behavior and vertical strain pattern for nacre-like brick and mud structures, interlocking structures, and foliated structures. When structures are subjected to bending deformation and vertical impact, the neutral layer exhibits the highest horizontal normal stress $σxx$ and horizontal normal strain $εxx$ in nacre sheet and columnar hierarchy, while vertical normal stress $σzz$ and vertical normal strain $εzz$ were extreme for the layer which directly contact the bending roller means strain energy was higher for outer layers and reduce as reaches to central layer. When foliated structures create bending deformation, in the lengthwise direction interlaminar sliding of soft materials occurs due to higher shear stress $σxz$ and shear strain $εxz$⁠. Hence, limiting effect of intralaminar soft material of nacre-like architecture to relative sliding of foliated structure's interlaminar is strengthen, which leads to lower shear strain region and larger vertical normal stress–strain of samples. The strain energy stored by the nacreous structures is slightly higher than those of foliated structures as portrayed in Ref. [66].

The surface roughness of printed structure relies on two aspects, first the upper geometry of the printed part such as hexagonal with fly ash loaded interface in nacre case and flat liquid polymer printing of PLA and fly ash. Second, the printing parameter and resolution play a key role in the surface roughness of fabricated components. The biomimetic printed structures can potentially be employed in mechanical components like bearings, gear, and shafts, and in aerospace structures such as wings, flips, tail, and small unmanned aerial vehicle bodies where the printed structure needs to undergo various coating and paints to improve thermal resistance, EMI shielding and esthetic perspective which urge for optimal surface roughness. In support of that, Ulrich et al. examined the increment in surface roughness by adjusting the tablet size in wood-based nacre structures [67]. Moreover, Tian and team observed that artificial nacre-inspired brick-and-mortar meshes, namely, graphene oxide, polyethylenimine, SiO₂, and halloysite nanotubes structures show higher surface roughness due to the geometric arrangement of nacre resulting in hydrophobic structures, whereas all four are hydrophilic individually [68]. Additionally, Moradi et al. experimented with the influence of the layer width and infill percentage of 3D-printed PLA-based fiber-reinforced composites on surface texture behaviors. They concluded that higher layer high and lower infill percentages render in enhancement of surface profile where 0.6 mm layer width and 40% infill emerge as optimal criteria based on their optimization [69]. Portoacă et al. performed surface profile analysis on 3D-printed PLA and acrylonitrile butadiene styrene composites in various printing parameters conditions. They have utilized the design of experiment method to optimize parameters which analysis reveals that for PLA material, an infill percentage of 50%, layer thickness of 0.15 mm, and for acrylonitrile butadiene styrene material an infill percentage of 50% and layer thickness of 0.1 mm achieve minimum surface roughness [70]. It was concluded that surface roughness is a prime parameter to provide adhesion of secondary material such as paint and coating which relies on surface architecture and printing parameters [71]. The layered structure is deemed an ideal applicant according to the brick wall structures surface in nacre and laminated, which can render better the mechanical properties when the stronger surface interaction is obtained. During the sliding, the brick structure's surface roughness will act as interlocking mechanism sites which yield higher mechanical performance, and by tailoring the surface morphology (sheet, columnar, and laminated), various roughness can be achieved. On the other hand, improving the surface roughness of the 3D-printed brick structures was adept at creating additional anchor points, which were widely considered to strengthen the interfacial interaction with the polymer binder [72,73]. The surface unevenness value of the top layer of additively produced virgin PLA, laminated PLA, nacre sheet, and nacre columnar substances are calculated, and the mean maximum profile height (R_z) and the mean average of surface roughness (R_a) for each bio-inspired skeleton is described in Fig. 7(a). As designated in Fig. 7(b) for the two-dimensional roughness profiles, it is perceived that the roughness profile is more uneven when a difference in the height of peak-to-valley is a rise which is perfectly suitable with the NS; nevertheless, it gradually deteriorations for NC, FL, and neat sample. The naturally inspired component comprises a superior Ra value (arithmetic average of four different positions of two samples) of 11.1 μm for NS architecture with 25 wt. % FA progressively decreases to 5.525 μm, to 3.607 μm, to 2.448 μm for NC, FL, and pristine hierarchy, respectively. In the case of filler addition, 15 wt. % was the highest value in all the structures due to more particle distribution which was progressively higher than that of 10 wt. % and 5 wt. % comprise composites. Miscellaneous environments such as water, temperature, adhesives, and coating play a crucial role in surface texture, a surface roughness component defining the composite interaction. Roughness is an excellent gauge of the prospective functioning of a mechanical component owing to irregularities on the plane that could form a nucleation site for corrosion or cracks [74,75].

Friction and wear functioning are crucial and are of foremost interest for the new structures or material's relevance, especially when they are subject to contact stress conditions with mutual motion. The relevant damage mechanism under friction and wear of bionic architecture like nacre columnar, sheet, and foliated composite remains to explore more despite the recent attempts [76,77]. In particular for structural applications like gears, crankshafts, bearings, roller, car bonnets, etc., of our developed different nature-inspired architecture and various FA content composites, it is necessary to examine friction and wear performance against base material, PLA in this case. The wear attributes evolution sample was developed with 10 × 10 × 10 mm³ dimensions and examined by pin-on-disk friction (Magnum Engineers, Bangalore, India). The applied load of 10 N, constant sliding speed of 0.5 m/s with a sliding distance of 20 m, and at 200 rpm with track radius of 10 mm for about 10 min has been employed. Figure 8 represents the variation in coefficient of friction with time for all the fabricated architecture composites wearing against base materials PLA. The friction nature revealed large variations depending on their specific hierarchy types and the addition of industrial waste fly ash. For the 15 wt. % FA objects, the composites display an evident running-in period where the friction coefficient incessantly rises compared to 5 wt. % and 10 wt. % FA filler with time before reaching a steady plateau stage (Fig. 8). The nacre sheet structure exhibits the highest plateau steady behavior for 15 wt. % FA around 0.45 compared to those nacre columnar, laminated, and monolithic, which limits at 0.32, 0.18, and 0.11 until around 300 s. For the various FA incorporation, the coefficient of frictions rapidly reach a steady plateau stage after a sharp peak at an initial stage. Figures 8(b)–8(d) show a direct comparison of 5–15 wt. % by considering the values corresponding to 150 s to exclude the initial increment. The bio-inspired composites with FA demonstrated higher coefficient of frictions, such as 0.13–0.22 in FL, 0.13–0.19 in NC, and 0.14–0.26 in NS, as the FA percentage increased in the mortar phase. Despite the difference in the hierarchy, FA can induce higher mechanical strength in bionic structures, which also introduces higher wear tendencies and surface roughness.

The fracture behavior of fabricated composites was studied in more detail by performing field emission scanning electron microscopy characterization using ∼10 kV high electron beam and secondary electron scattering at different widths and magnifications. For field emission scanning electron microscope imaging, fractured objects were gold sputter-coated to produce the thin film without polishing the surfaces, followed by being examined using a Zeiss, Germany, Scanning Electron Microscope. After examination, sheet specimens demonstrate a classical 45-deg helical fracture perpendicular to the principal tensile stress, while columnar geometry exhibits sharp fracture throughout the interfaces between aragonite tablets. Moreover, the foliated structure behaves as breaking of layered printed and polymeric interfacial bonding when delamination prompts at a very early stage; cracks kink from one interface to another, eventually separating the specimen, and neatly printed architecture observed catastrophic fracture surface. Detailed scanning electron microscope image (Fig. 9) describes that tablets interlaced with broken edges and huge regions of tablets are exposed that transit spirally from one layer to the next.

Additionally, the broken edges integrate the brutal breakage of the transtablet and the delamination of the smooth intertablet. Both laminated and staggered patterns initiate pseudo-ductility above a threshold mortar phase volume and stacking, with the foliated design consequential in higher pseudo-ductility in impact, whereas the staggered design was subsequent in larger tensile strength. This distinction is because of the design pattern affecting the amount of stable delamination crack growth and shear lag strengthening between the tablets. The percentage of pseudo-ductility in foliated architecture improves with the mode II interlaminar toughness. However, platelet delamination under bending and tensile force occurs due to excessive interlaminar toughness suppressing delamination cracking, and this demotes the pseudo-ductility amount in laminate components [78].

The application of natural skeletons, nacreous, and foliated hierarchy offers an unusual combination of modulus and toughness simultaneously, rendering solutions for futuristic composite structures. The embodiment of industrial waste fly ash can help further strengthen mechanical behavior and provide sustainable waste management solutions. This research showcases the capacity of synergistically employing two distinct additive manufacturing methodologies (FDM and DIW) at the micron scale for the fabrication of nacre-inspired sheet, columnar, and laminated structures. These techniques offer valuable insights into the manipulation of both rigid and pliant material phases within fly ash waste, thereby influencing consequential changes in mechanical properties. The fly ash addition reveals the dominance of texture characteristics, mineral bridges, and micro-asperities on the strength and toughness of composites. As more fly ash contains supplement, for instance, 5–15 wt. %, a tensile modulus and flexural modulus improvises with 1.17 and 1.4 times for FL, 2.3 and 1.2 times for NC, and 1.2 and 1.4 times for NS geometries, respectively. Moreover, during the impact loading, energy dissipation in the multiple cracks and delamination of foliated structures are more prominent than the fully saturated and activated tablet sliding of a nacre-like structure over the immense region. Under the numerous stacking arrangements, materials composition, and boundary conditions, finding anomalous applicability of biomimetic skeletons as load-carrying materials remains vital. However, in the future, it will be intriguing to explore the role of nanodimension fabrication via additive manufacturing and surface treatment of fly ash particles for better adhesion and dispersion to address the crack formation, propagation, and delamination challenges related to composite fabrication and fractures. Additionally, to high requirement of waste utilization, accessibility, high obligation, and green manufacturing render the path for vital applications in aerospace, defense, automobile, and structural components. The mild surface roughness, wear, friction performance against base polymer, excellent fracture toughness, and machinability make FA-based bionic composite particularly appealing for gear, shaft, bearing, and dental applications.

The construction of biomimetic structures via PLA (polylactic acid) owing to appropriate impact strength of 84 J/m, tensile strength 39.3 MPa, flexural strength 63.3 MPa, elongation 2.09%, low density 1.1 g/cm³, high flow, and glass temperature 65 °C which purchase from MakerBot Pvt. Ltd., New York, whereas fly ash microparticle was purchased from Vyankatesh Interlocking Fly Ash Bricks in Pune, Maharashtra, India. The FDM model Method X (MakerBot Pvt. Ltd.) was utilized to produce shell (brick pattern) architecture with 0.2 mm layer height, three shells, 100% infill density, and a linear pattern. Moreover, DIW technique was used to print the mortar phase in brick interphase with 0.2 mm luer tip syringe-based Hyrel System 30M DIW machine produced by AMS, India Pvt. Ltd., Bengaluru, India. Followed by that, PLA + Fly ash solution for printing mortar phase were prepared into DCM with 30 wt. % of matrix part (PLA) using stirrers which could provide suitable viscosity for the mortar phase, followed by fly ash dispersion ranging from 5 wt. % to 15 wt. % in an ultrasonicate machine as described in Fig. 2. Initially, the first layer of brick hexagonal was printed with PLA as per the discussed parameters, and then the solution was filled into a mortar lever using a direct ink writing printing technique. The second layer of the brick phase is printed on top of the dried first layer, using FDM and DIW, accompanied by all the layers printed up to the desired layers. Additionally, foliated structures were printed in stacking layers where the first layer was printed using PLA filament-based FDM and the second layer of PLA + Fly ash solution was manufactured using the DIW 3D printing technique (as shown in Fig. 2).

Characterizing intricate nacre geometry analyzed using Izod impact tester (Tinius Olsen Impact 503) as per ASTM D256, tensile testing, and flexural bending test (UTM. Instron) performed according to ASTM D638 and ASTM D790, respectively. The microstructure examination of fractured hierarchical and cryogenic fractures was analyzed using field emission scanning electron microscope (Zeiss, Germany). The surface roughness of FDM printed components was quantified using the RUGOSURF 90G surface roughness testers (TESA SWISS MAKE) for neat, foliated, and nacre parts to estimate the surface imperfection considering different designs. The mean surface roughness (R_a) and the mean maximum height of the profile (R_z) are quantified for the upper surface layer of the FDM printed neat PLA, foliated, columnar, and sheet samples. Three regions on a sample were assessed for surface roughness in each arrangement.

The authors acknowledge research support from DIAT (DU) and Deakin University, Australia, under the Deakin India Research Initiative (DIRI). The first author acknowledges NMRL and ARDE (DRDO) labs and all the lab members of the additive manufacturing lab, DIAT (DU), for their continuous and valuable technical support throughout the performing research and writing. The authors are thankful and acknowledge the efforts and consideration of Dr. Robin McIntyre, D.Phil., Director, Iconiq Innovation, Leicestershire, UK, in improving the technicality and refining the English language of the paper. The first author wants to acknowledge Dr. Amrita Nighojkar, Ms. J. P. Niranjana, Ms. Alsha Subash, Ms. Neelaambhigai Mayilswamy, and Ms. Shruti Gupta for their continuous motivation and support.

There is no conflict of interest.

No data, models, or code were generated or used for this paper.