Gecko-inspired nanocomposite coating based on setae-spatulae structure and dynamic adsorption-desorption mechanism

2.1. Contact theory analysis

The design of gecko-inspired nanocomposite coatings not only need to have a certain understanding of biological principles, but also need to study the mechanics of micro scale adhesive contact theory. There are two celebrated models that dominate the field of contact mechanics, the Johnson-Kendall-Roberts (JKR) model for elastic solids and the Derjaguin-Muller-Toporov (DMT) model for hard solids, respectively. In general, the JKR model is for biological systems [7]. It is assumed that the spatula of gecko have semi-spherical extremities, and its radius is set to $R$.

Combined Hertzian elasticity theory with Linear elastic fracture mechanics, regardless of the rate effect, the attractive force $P$ between the two bodies of radius $R$ for producing a contact area with radius $a$ can be calculated as follow [8]:

where $E^{*}$ is the Young’s modulus of the contact pair, which can be obtained by:

where $E_1$ is elastic modulus of contact surface 1, $E_2$ is elastic modulus of contact surface 2, ${\upsilon }_1$ is Poisson’s ratio of contact surface 1, ${\upsilon }_2$ is Poisson’s ratio of contact surface 2.

The pull-off force $P_c$ is obtained by taking $\frac{\partial P}{\partial a}=0$ [9]:

where ${\gamma }^{*}$ Adhesion work.

The minus sign in Eq. (3) is used to denote tensile forces. When the contact area reaches to $R$, the adhesion stress can also be calculated by the Eq. (3).

When apparent contact area is fixed, the adhesion stress will increase by $n$^1/2 times if it is subdivided into $n$ smaller contacts [5]. It explains the reason why the hierarchical structure of gecko foot produces such a strong adhesion force.

2.2. Research contents

The contact theory analysis of gecko illustrated that the pull-out force/adhesion force at the contact point is independent of Young’s modulus of semi-spherical extremities structure, the hierarchical structure is more critical. This is because the surface of the contact pair is uneven in microscopic conditions. The contact theory analysis provides inspiration for the selection and preparation of anti-corrosion coating materials.

Multicomponent biomimetic nanocomposite coating based on setae-spatulae structure of gecko was innovatively proposed. The realization of the dynamic adsorption desorption function of gecko-inspired nanocomposite coating depends on the size and type of the selected nanoparticles. In order to imitate “setae” and “spatulae” of the gecko toe pad, magnetic chiral nanoparticles were introduced, doped with other multi-component, multi-dimensional and multi-scale nanoparticles. Utilizing the unique two functions of chiral molecules (e.g., reversible double bonds, multiple grippers) and the synergy of multi-component nanoparticles, the design of setae-spatulae structure can be completed to achieve the ideal effect.

2.3. Construction of gecko like foot structure model

It is proposed to introduce multi-component nano additives of Al₂O₃, ZnO and Fe₂O₃, follow the principle of the lowest energy spontaneously, and coordinate uniformly according to the priority of bond energy under the joint action of van der Waals force and surface energy. “Interstitial” and “intercalation” self-assembly and “homogeneous dispersion” can be realized. With the help of multi-scale and multi-dimensional nano particles, the “gecko like keyhole structure” was imitated, The multi-scale TiO₂ modification was used as the connector between large particles and chiral molecules, the chain structure on the keyhole structure was imitated by the spiral rod like ZnOnano particles, and the “sucker” small hand on the keyhole structure was imitated by the multi-dimensional high wear-resistant Al₂O₃. Finally, the design of dynamic adsorption-desorption effect of gecko foot was finished.

Fig. 1Design of multi-component nano particle Compatibility Model of gecko-inspired nanocomposite coating

2.4. Experimental details

Firstly, blending method was used to prepare gecko-inspired nanocomposite coatings with one component, two-component and multi-component, respectively. Using blank epoxy resin (EP) coating and other multi-component coatings as a control group, the corrosion behavior of nanoparticles (e.g. Fe₂O₃, Fe₃O₄, Al₂O₃, TiO₂) with different scales and components in EP coating was investigated. Secondly, the potential polarization curve of multi-component nanocomposite coating immersed in 5 wt % NaCl environment, e.g. EP/Fe₂O₃($\gamma$-type), EP/Fe₃O₄, EP/Al₂O₃, EP/TiO₂, EP/ZnO/Fe₂O₃($\gamma$-type), was tested by electrochemical workstation respectively. Blocking and protective properties of all nanocomposite coatings were analyzed using electrochemical impedance spectroscopy (EIS). Finally, the accelerated corrosion effect of salt spray test was investigated.

3.1. Electrochemical performance analysis

Fig. 2 depicts the corrosion rate of steel plate and EP composite coating samples at different times. As the immersion time elapsed, the corrosion growth rate of EP/Fe₃O₄ gecko-inspired nanocomposite coating was fastest, the highest $C_{rr}$ value is up to 0.00464 mm/a after168 hours immersion. This is due to Cl^- reacts with pigments and plasticizers in the coating, the micropore of the coating is enlarged and the penetration speed of Cl^- is accelerated. The corrosion growth trend of EP/Al₂O₃, EP/TiO₂, EP/ZnO/Fe₂O₃ ($\gamma$ type) coatings is relatively stable, among which the average corrosion rate of EP/ZnO/Fe₂O₃ ($\gamma$ type) coating is the lowest, and the lowest $C_{rr}$ value is 0.0005 mm/A. Compared to EP/Fe₃O₄ coating, its corrosion rate is reduced by an order of magnitude. The reason lies in that the coating with multi-component and multi-scale nano particles follows the principle of minimum free energy, which spontaneously realizes “interstitial” and “intercalation” self-assembly and compacts the structure. The corrosion rate of the coating decreases with the doping of multi-component and multi-scale nano additives, which proves that the addition of nano particles increases the contact area with metal substrate, and has the best anti-corrosion protection effect on steel members.

Fig. 2Corrosion rate of steel plate and EP composite coating samples at different times

3.2. Results of EIS

The IMF in Bode diagram is often used to evaluate the anti-corrosion performance of coatings. The impedance modulus at 0.01 Hz |Z|_0.01Hz can directly reflect the corrosion protection properties of the coating on the internal migration of electrolyte [10]. The |Z|_0.01Hz values of EP, EP/Fe₂O₃$\text{(}\gamma$-type), EP/Fe₃O₄, EP/TiO₂, EP/Al₂O₃, EP/ZnO/Fe₂O₃$\text{(}\gamma$-type) in the impedance modulus-frequency Bode diagram (Fig. 3(b)) are 1.13×10⁵ Ωcm², 1.17×10⁵ Ωcm², 2.09×10⁵ Ωcm², 3.02×10⁵ Ωcm², 4.24×10⁵ Ωcm² and 5.84×10⁵ Ωcm² respectively. The IMF of each coating is above 1×10⁵Ωcm². The IMF of EP is the lowest, the IMF of EP/ZnO/Fe₂O₃ ($\gamma$-type) nanocomposite coating is the highest, which is 5.17 times of that of EP, showing better electrolyte barrier effect. This is attributed to the incorporation of multi-scale and multi-component nanoparticles resulting in the increase of contact area with steel plate, and the electrolyte diffusion length also increase.

The Bode phase angle-frequency curve in Fig. 3(c) also shows that the phase angle growth rate of EP/ZnO/Fe₂O₃ ($\gamma$-type) composite coating increases faster than other coatings. It indicates that the time taken for EP/ZnO/Fe₂O₃ ($\gamma$-type) composite coating to form a stable and dense passivation coating is shorter. Besides, it has a higher degree of adhesion to the steel plate surface. In conclusion, all of these results indicate higher degree of protection of EP/ZnO/Fe₂O₃($\gamma$-type) than other composite coatings.

Fig. 3EIS diagram of gecko nanocomposite coating immersed in 5 wt% NaCl solution after 168h immersion

a) Model diagram

b) Impedance modulus diagram

c) Phase angel Nyquist e-frequency curve of Bode diagram -frequency curve of Bode

3.3. Salt spray test

The accelerated corrosion effect of salt spray test for about 150 days is as follows.

Fig. 4Pictures of salt spray test

a) Blank EP

b) EP/Fe₂O₃ ($\gamma$-type)

c) EP/ZnO/Fe₂O₃

It can be seen from Fig. 4 that after a long time of salt spray test, the surface of samples change obviously, accompanied by wrinkles, cracks, pitting and pits. With the increase of corrosion time, the number and depth of corrosion pits increase. The results show that blank EP is most severely corroded by salt spray, with obvious rust and peeling. However, the multi-component nanocomposite coating still maintains good gloss and uniformity, and only a few rust spots appeared, showing good corrosion resistance.