Bioinspired self-healing nickel coating

Abstract

We present a study of self-healing mechanisms including their kinetics and thermodynamics in nickel coatings. The bioinspired self-healing coating is designed to enhance the durability of structural metal components exposed to harsh conditions. Microcapsules, reminiscent of natural healing reservoirs, were synthesized via in situ polymerization in an oil-in-water emulsion to encapsulate linseed oil, a healing agent, within poly(urea-formaldehyde) (PUF) shells. Nickel coatings incorporating PUF shell microcapsules were electrodeposited on mild steel substrates to assess their effectiveness in self-healing, mimicking nature's ability to provide on-demand healing. Comprehensive characterization of the microcapsules and coating was performed using techniques including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Thermogravimetric Analysis (TGA). The self-healing performance of the coating was evaluated using SEM and EDS after scratches simulating damage were made on the surfaces of the samples. Corrosion resistance and self-healing ability were evaluated through an immersion test, and additional corrosion resistance tests such as Open Circuit Potential (OCP) and Linear Polarization (LP) were conducted. The results indicate that the nickel coating containing PUF shell microcapsules confers corrosion resistance to the substrate and, upon damage to that coating, induces a self-healing response analogous to natural systems, highlighting the potential of bioinspired designs in advanced material solutions.

Fig. 1. Schematic mechanism for the co-deposition of metal-microcapsule coating using electrodeposition methods. The figure illustrates three main stages: (a) the initial condition of the electrodeposition medium before applying direct current, (b) the bonding of metal ions to the microcapsules under the influence of the electric field, and (c) the movement of the capsules and ions towards the cathode.

Fig. 2. Optical microscope image of (a) emulsion of linseed oil in water (size of the scale bar: 100 μm), (b) synthesized micro-capsules after two hours of reaction time of their constituents (size of the scale bar: 100 μm).

Fig. 3. SEM micrographs of microcapsules (a) single microcapsule (size of the scale bar: 10 μm), (b) aggregates of microcapsules (size of the scale bar: 20 μm).

Fig. 4. SEM micrographs of nickel-PUF shell microcapsules self-healing coatings with different microcapsule concentrations: (a) 0 g L⁻¹, (b) 10 g L⁻¹, (c) 20 g L⁻¹, and (d) 30 g L⁻¹ in the bath.

Fig. 5. Cross-section of (a) nickel coating (thickness ∼43.5 μm), (b) nickel coating with PUF shell microcapsules (thickness ∼36.7 μm).

Fig. 6. SEM micrographs of microcapsules embedded in the nickel coating with a line scale. The roughness of the shell is likely caused by tiny urea-formaldehyde particles sticking together and accumulating at the edge of the capsule.

Fig. 7. SEM micrographs of X-scratched samples after 24 hours of making the scratch (a) pristine nickel coating, (b) self-healing coating with 10 g L⁻¹ microcapsules, (c) self-healing coating with 20 g L⁻¹ microcapsules, (d) self-healing coating with 30 g L⁻¹ microcapsules in the bath.

Fig. 8. EDS analysis of the X-scratched samples after 24 hours of making the scratch (a) pristine nickel coating, (b) self-healing coating with 10 g L⁻¹ microcapsules, (c) self-healing coating with 20 g L⁻¹ microcapsules, (d) self-healing coating with 30 g L⁻¹ microcapsules in the bath. EDS analysis reveals that linseed oil has formed a self-healing protective barrier after scratching.

Fig. 9. Mass loss (percentage) vs. time (days) graph for self-healing nickel coatings.

Fig. 10. OCP diagram of (a) pristine nickel coating, (b) self-healing coating with 10 g L⁻¹ microcapsules, (c) self-healing coating with 20 g L⁻¹ microcapsules, (d) self-healing coating with 30 g L⁻¹ microcapsules, in the bath and (e) mild steel substrate without coating.

Fig. 11. LP diagram of (a) pristine nickel coating, (b) self-healing coating with 10 g L⁻¹ microcapsules, (c) self-healing coating with 20 g L⁻¹ microcapsules, (d) self-healing coating with 30 g L⁻¹ microcapsules, in the bath and (e) mild steel substrate without coating.

Fig. 12. OCP diagram of X-scratched samples (a) pristine nickel coating, (b) self-healing coating with 10 g L⁻¹ microcapsules, (c) self-healing coating with 20 g L⁻¹ microcapsules, (d) self-healing coating with 30 g L⁻¹ microcapsules, in the bath and (e) mild steel substrate without coating.

Fig. 13. LP diagram of X-scratched samples (a) pristine nickel coating, (b) self-healing coating with 10 g L⁻¹ microcapsules, (c) self-healing coating with 20 g L⁻¹ microcapsules, (d) self-healing coating with 30 g L⁻¹ microcapsules, in the bath and (e) mild steel substrate without coating.