1 Introduction

In recent years, the development of lightweight and high-performance materials for impact mitigation has garnered significant attention across various industries, ranging from aerospace to sports equipment [1,2,3]. Protection of both structures and individuals from sudden impacts is crucial. Research has focused on developing innovative composite materials that offer lightweight and flexible features while enhancing impact resistance. One promising exploration avenue combines cork and shear thickening fluids (STFs) to create advanced impact-resistant structures [4,5,6,7,8,9,10].

STFs, colloidal suspensions that exhibit a remarkable increase in viscosity under shear stress, have garnered significant interest due to their ability to transform from a fluid to a solid-like state upon impact. This unique rheological behaviour offers potential applications in impact mitigation, as STFs can effectively dissipate and distribute energy during sudden loading events. Previous studies have demonstrated the effectiveness of STFs in enhancing the impact, puncture, ballistic, and stab resistance of materials such as fabrics and foams [11,12,13,14,15]. Recently, new concepts of multi-functional STFs have been introduced. Sheiki et al. [16] designed sandwich structures enhanced with a multi-functional STF to improve vibration damping and electrical conductivity.

Cork, a natural cellular material, presents innate properties and characteristics, which include low density, compressibility, and capability of absorbing impact energy [17], making it an excellent choice for integration with STFs in composite structures. Laminated cork composites interfacially enhanced with STF have been previously developed, presenting marginal gains in impact force mitigation [4, 7, 8]. These are based on laminated agglomerated cork sheets, interfacially reinforced with STF [4, 7].

Gürgen et al. [4] investigated the enhancement of impact resistance in cork composites employing a shear thickening fluid as an interfacial layer in 20-mm-thick samples, going up to ten cork layers. The ten-layer structure achieved 36% impact force reduction for 5 J impact. For 10 J impact, mitigation of the same design achieved less than 15% force reduction. With a four-layer design, this value was even lower than 10%. More recently, Oliveira et al. [7] investigated different STF formulations based on polyethylene glycol (PEG) and fumed silica nanoparticles and then subjected the samples to 20 J impact tests with a flat impactor. Although the reductions achieved were marginal, the STF based on PEG 200 and 30 wt% silica content revealed promising results.

Galindo-Rosales et al. [8] proposed a concept based on laminated cork sheets, which were laser engraved with a network of microchannels and filled with STF. The proposed solutions achieved marginal impact force reductions in very low-energy impacts (less than 5 J). The authors explored engraving depths from 0.1 to 1 mm and focused mainly on the flow behaviour of STF and the network of microchannels. Nevertheless, the authors did not explore the potential effect of laser texturing on the impact behaviour and interfacial enhancing mechanisms.

Laser texturing has demonstrated promising results in enhancing materials’ interfacial adhesion and mechanical properties [18, 19]. Laser surface texturing has also been used as a sustainable tool to modify the wettability of materials [20]. Laser texturing is a promising technique since it may provide an alternative to chemical ones, which can be toxic. Additionally, it can be employed to various types of materials. However, applying laser texturing to natural materials, e.g. wood, is scarcely addressed in the literature and is non-existent in the case of cork. In addition, the relevance of the interface in the STF performance in multilayered cork composites has been recently highlighted [21], settling the motivation for this research.

Therefore, this study aims to develop multilayered cork-STF composite structures for impact mitigation and investigate the effect of laser texturing on cork surfaces. Therefore, these structures will be subjected to low-energy impact tests, focusing on determining the resulting impact force. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) are also employed to investigate the interactions with STF and modifications caused by the interaction with the laser beam, correlating relevant observations with the impact performance.

2 Materials and methods

In this study, various multilayered agglomerated cork-STF composite structures were explored. In addition, the potential of laser texturing was also explored to optimise STF interaction with the cork surface.

2.1 Materials and specimen preparation

Figure 1 illustrates all the configurations along with their materials. The multilayered structures consist of two 10-mm-thick agglomerated cork sheets. This compound has a density of 170 kg/m3 and was supplied by Unicor. The supplied boards were cut into 50 mm × 50 mm square specimens. For a detailed breakdown of their properties, please refer to Table 1.

Fig. 1
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Three types of sample configurations—from left to right: C-10 (neat cork); CSTF-10 (cork with STF between layers); CLESTF-10 to CLESTF-40 (cork with STF between laser textured layers)

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Table 1 Properties of the agglomerated cork (ISO 7322)
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This study increments from knowledge acquired in previous studies, mainly the sample design and test conditions [4] and the production and optimization of the STF formulations [7]. The better suspension achieved in Oliveira et al. [7] is here replicated, employing an STF that consists of polyethylene glycol (PEG) with a molecular weight of 400 g/mol from Thermo Scientific Acros (Table 2) and fumed silica (SiO2) nanoparticles with a specific surface area of 200 ± 25 m2/g from Merck (Table 3).

Table 2 Properties of polyethylene glycol [7]
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Table 3 Properties of fumed silica nanoparticles (SiO2) [7]
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As illustrated in Fig. 1, three types of samples were manufactured: (i) C-10, two 10-mm cork layers without STF on the interface; (ii) CSTF-10, two 10-mm cork layers with STF on the interface; (iii) CLESTF, two 10-mm cork layers, which were laser texturized, and STF was applied on the interface. In the latter configuration, the textured area is also a variable, exploring 10 mm, 20 mm, 30 mm, and 40 mm squares textured by a laser beam.

2.1.1 Shear thickening fluid

The STF used in the experiment is a suspension based on poly(ethylene glycol) (PEG) and fumed silica, previously characterised [7]. Specifically, PEG with a molecular weight of 400 g/mol and a density of 1.1275 g/mL (Thermo Scientific Chemicals), and fumed silica nanoparticles (SiO2) at a concentration of 30 wt% (Merck). The fumed silica nanoparticles have a specific surface area of 200 ± 25 m2/g. Stirring and homogenisation of silica nanoparticles in PEG were performed using an IKA T 18 brushless digital Ultra-Turrax. The dispersing stage was carried out at 15,000 rpm for 2 h.

Figure 2 depicts the typical rheological behaviour of STF, which was obtained using the following cone-plate setup carried out in the Kinexus rehometer at room temperature (Table 4), as described in Oliveira et al. [7].

Fig. 2
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Viscosity-shear rate of the STF

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Table 4 Cone-plate setup
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2.1.2 Laser texturing

The CLESTF group was designed to determine the effect of textured cork surfaces on STF interaction. Four types of samples were developed to investigate the textured area’s effect. This marked square is concentric to the sample. Figure 3 illustrates the dimensions of each layer, including textured area, and their values are presented in Table 5.

Fig. 3
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The dimensions of each layer and textured area of CLESTF samples: L1, sample side length; L2, textured surface side length; t, layer thickness. The top and bottom figures represent the front and top views, respectively

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Table 5 Dimensions of the laser textured areas
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Laser texturing was carried out using a pulsed 200-W Ytterbium-doped fibre laser (from IPGphotonics coupled to a RayLase galvanometric head) with a wavelength of 1070 nm. Table 6 presents the processing parameters for each sample type, which were experimentally optimised to achieve the best outcome. Overall, the scanning speed was adjusted to address the different textured areas between samples CLESTF-10 to CLESTF-40.

Table 6 Laser processing parameters for each CLESTF sample
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2.1.3 Sample preparation

The assembly process was carried out meticulously, with prior measuring of the mass of each component or material. The aim of the strategy behind the STF deposition was to deposit the maximum possible amount and then, by compressing both cork sheets together, remove the excess fluid that was draining out. The average STF mass achieved with this deposition method and the average surface density are presented in Table 7.

Table 7 STF mass measurements
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Although slight differences exist, a clear ascendant trend exists in the STF quantity with the textured area. It is plausible that the laser texturing of the cork surface created some superficial degradation. Through visual inspection, no engraving was observed, and only surface marking was achieved, which was the original aim. Nevertheless, this is probably the reason why samples with a larger textured area (CLESTF-40) retain an average of 29.6% more STF than the non-textured ones, which reflects on a surface density of 1.156 mg/mm2 against 0.892 mg/mm2.

2.2 Impact tests

The impact tests were conducted using an Instron 9440 drop tower, employing a hemispherical impactor. Table 8 presents an overview of the impact test conditions defined for 10 J impacts. Five samples of each type were tested to guarantee repeatability and statistically significant results.

Table 8 Impact test parameters and conditions
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2.3 Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS)

In order to search for phenomena that might result from the interaction between the different materials and laser radiation, SEM and EDS analyses were carried out. The TM4000 PLUS from Hitachi was used to carry out these analyses. These enable surface analysis, yielding high-resolution images and revealing the chemical composition. The following specimens were analysed using SEM and EDS: (i) neat cork layer without laser texturing (C-10), (ii) laser-textured cork layer without STF, (iii) non-textured cork layer with STF (CSTF-10), (iv) and laser-textured cork with STF (CLESTF).

3 Results and discussion

3.1 SEM and EDS analysis

Figures 4, 5, 6, and 7 present the SEM micrographs of four distinct specimens: (i) neat cork layer without laser texturing and STF (Fig. 4), (ii) laser-textured cork layer without STF (Fig. 5), (iii) non-textured cork layer with STF (Fig. 6), (iv) and laser-textured cork with STF (Fig. 7). Figure 4 depicts a standard cork microstructure, revealing hexagonal prismatic closed cells. On the other hand, Fig. 5 presents a microstructure where the honeycomb pattern is still visible, but clearly, there are alterations to the surface. The higher magnification reveals some sort of fibres deposited on the surface, which might indicate degradation of the outer cell walls. After suberin, lignin, cellulose, and hemicelluloses are among the highest constituents of cork [22]. Due to the degradation or even decomposition of the cell walls, one can hypothesise about their presence on the cork’s textured surface. Additionally, due to the high temperatures achieved on the surfaces irradiated with the laser beam, there will be a place for volatilisation and carbonised material. In fact, recent studies have been investigating the development of lignin carbon fibres after high-temperature pyrolysis [23,24,25].

Fig. 4
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Neat agglomerated cork layer: a 100× magnification; b 500× magnification

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Fig. 5
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Laser textured agglomerated cork layer: a 100× magnification; b 500× magnification

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Fig. 6
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Agglomerated cork layer with STF interface: a 200× magnification; b 500× magnification

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Fig. 7
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Laser-textured agglomerated cork layer with STF interface: a 200× magnification; b 500× magnification

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Comparing the micrographs from STF-cork samples with and without laser textured surfaces, Figs. 6 and 7, respectively, STF is more uniformly spread across the open cells of the textured layer. Additionally, there is more retention of STF at the interface. This correlates well with mass measurements presented in Table 7 since laser engraving was avoided, aiming to mark the sample, i.e. just texturization. In fact, the sample with a smaller textured area presented an average STF surface density 13.9% higher than the non-textured one, which presents a potential key-enhancer mechanism to retain STF on cork surfaces.

In Fig. 8, it is possible to see the region of interest (ROI) where EDS scans were made. These correspond to neat and textured cork samples, both without STF. As expected, the main chemical elements present in the samples are carbon and oxygen (Table 9). Cork is a closed-cell material, and its cells typically entrap air during bark growth. Table 9 indicates 29.81% oxygen for the neat cork and 16.64% for the textured one. This 44% reduction is probably caused by the collapse and potential degradation of the cells’ walls on the sample surface. Additionally, the increase in carbon amount caused by laser irradiation supports the cell degradation hypothesis, reinforcing the possible occurrence of mechanisms behind lignin carbon fibres obtained by high-temperature pyrolysis, as reported in the literature [23,24,25].

Fig. 8
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EDS scans ROI with 500x magnification: a neat agglomerated cork layer; b laser-textured agglomerated cork layer

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Table 9 Elemental composition (mass percentage) [%]
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3.2 Impact tests

The results showcase the average values obtained for five samples tested of each configuration. Regarding the maximum force experienced (as shown in Fig. 9), it is evident that cork-neat samples achieved higher peak forces. Therefore, the presence of at least 2.23 g of STF on the 50 mm × 50 mm cork interface (average STF surface density of 0.893 mg/mm2) makes it possible to achieve impact force reductions higher than 15%.

Fig. 9
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Peak impact force [N]

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On the other hand, laser-textured samples CLESTF-10 and CLESTF-20 did not show significant improvement compared to CSTF-10. Comparing the average peak force, CLESTF-10 and CLESTF-20 presented slightly higher forces than CSTF-10. Additionally, although there was a slight improvement in impact force reduction throughout the progression of the textured area in CLESTF samples, this evolution is non-linear.

The main impact force reduction between samples with STF occurs between CLESTF-20 and CLESTF-30. In addition, the difference in STF mass is the lowest between CLESTF-20 and CLESTF-30, which makes it possible to conclude that this higher impact force reduction is not mainly attributed to a higher STF surface density but probably to the textured area. Therefore, since these samples have almost the same mass of STF, a hypothetical reason behind this decrease could be related to the textured area. The hemispherical impactor has a diameter of 20 mm, which means that the CLESTF-30 sample is the first to ensure that the hemispherical impactor radius of action is within the textured area. On the other hand, although CLESTF-40 ranked second in impact performance, the force values reached were slightly higher than CLESTF-40. The only explanation for this phenomenon is a potential threshold that was crossed, countering the positive evolution achieved with CLESTF-30, potentially caused by damage to the structure since the irradiated area by the laser beam is larger.

Figure 10 depicts the displacement reached at maximum force. In fact, the sample CLESTF-40 reached the highest displacement from all the samples with STF, which shows it is easier to penetrate by the hemispherical impactor. It is also worth highlighting that the displacement results indicate a marginal increase in penetration throughout the CLESTF samples from 10 to 40, successively. In addition, the average displacement reached between CLESTF-20 and CLESTF-20 is almost the same.

Fig. 10
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Displacement at the peak [mm]

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Overall, the results highlight the benefit of including STF on these composite structures by achieving impact force reductions of 16.1% for CSTF-10 and 19.7% for CLESTF-30. Therefore, although this is not linear, surface texturing triggers a 3.6% impact force reduction.

Figure 11 presents the absorbed energy, showing that except for the reference specimen (C-10), all samples registered less than 8 J. This aspect confirms that STF positively influences the impact force reduction, leading to a steadier mitigation as it is possible to confirm by the stress-strain curves depicted in Figs. 12, 13, 14, 15, 16, and 17.

Fig. 11
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Absorbed energy (J)

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Fig. 12
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Force-displacement curve: C-10

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Fig. 13
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Force-displacement curve: CSTF-10

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Fig. 14
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Force-displacement curve: CLESTF-10

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Fig. 15
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Force-displacement curve: CLESTF-20

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Fig. 16
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Force-displacement curve: CLESTF-30

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Fig. 17
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Force-displacement curve: CLESTF-40

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Figures 12, 13, 14, 15, 16, and 17 present all the samples’ force-displacement average curves and their respective standard deviation. A noticeable difference between samples C-10 and CSTF-10 is observed when the force approaches its maximum. The reference specimen’s curve presents a linear behaviour, whereas in the presence of STF, the force increase rate drops near its peak, almost remaining stagnant. This trend is observed in all the STF-based specimens (including CLESTF). This observation further confirms the STF’s positive impact on the structure’s impact performance.

4 Conclusions

This study investigated the multilayered cork-STF composite structures for impact mitigation. It also investigated the effect of laser texturing on cork surfaces that interact with STF. The results indicate a significant impact force mitigation caused by STF incorporation on the cork composite structures, reaching a 16.1% reduction with just STF (CSTF-10) and a maximum reduction of 19.7% with a textured sample (CLESTF-30).

Although the impact force difference between the best-performing sample (CLESTF-30) and the one with just STF (CSTF-10) is marginal, SEM micrographs of textured surfaces revealed a more homogenous distribution of the STF throughout the cork microstructure of textured interfaces. Therefore, further investigation is needed to determine the optimum thresholds while employing laser processing techniques on cork for this application.

In addition, the laser processing of cork revealed another potential use for this sustainable material since the laser interaction might produce exciting results for manufacturing lignin carbon fibres based on high-temperature pyrolysis. Thus, the phenomenon resulting from the laser processing of cork might open new avenues for cork waste utilisation for lignin carbon fibre production. Also, in a potential upscaling and industrial application, it would be necessary to optimise laser power and scanning speed for each scenario to achieve the desired textured quality while saving energy and time.

Nevertheless, the main key element in the multilayered composite was the STF interface, which made it possible to reduce impact force significantly. At this stage, and before future developments employing laser technology, STF demonstrates a high potential to be employed in cork composites, particularly in laminates, without interfering in the assembly or manufacturing processes, resulting in an excellent cost-benefit relationship. Based on the main findings, these cork-STF composite structures can be employed in protective equipment and structures, having prominent shock absorption and damping roles.