Self-Healing Epoxy Composites Get a Titanium Dioxide Boost Against Wear and Corrosion

Composite materials have transformed modern engineering, from aircraft fuselages to wind turbine blades, but they carry a persistent weakness: once internal cracks begin to form, they tend to grow silently until catastrophic failure occurs. Unlike metals, which can be inspected for fatigue cracks and repaired by welding, polymer composites often hide their damage deep within the laminate, where no visual inspection can reach. A research team from institutions across Tamil Nadu and Andhra Pradesh in India has now reported a strategy to tackle this problem at its source, embedding tiny self-healing capsules into an epoxy matrix and reinforcing both the capsules and the surrounding material with carbon-based nanomaterials. Their study, published in the Journal of Materials Science, describes how a titanium dioxide sol–gel coating further elevates the mechanical, tribological and corrosion performance of these hybrid composites, pointing toward structural materials that can actively fight back against the damage they accumulate in service.

The core of the innovation lies in the microcapsules themselves. The researchers fabricated capsules using waterborne polyurethane as the shell material, with the polyurethane content varied between 5 and 15 percent. Waterborne polyurethane is an attractive choice for encapsulation because it forms stable films, adheres well to epoxy matrices and avoids the volatile organic solvents associated with conventional polyurethane chemistry. Inside each capsule sits a healing agent, held in reserve until the moment a crack arrives. The team produced these capsules using a two-step encapsulation technique, a widely used approach in self-healing materials research that allows independent control over the core and shell formation. When a crack propagates through the composite, it ruptures the capsules in its path, releasing the healing agent into the crack plane where it can polymerize and bond the fractured faces back together, effectively filling the damage before it can grow further.

What distinguishes this work from earlier microcapsule studies is the deliberate reinforcement of the capsule system with two carbon nanomaterials: multi-walled carbon nanotubes, incorporated at levels between 0.5 and 1.5 percent, and graphene oxide, incorporated at levels between 2 and 6 percent. Multi-walled carbon nanotubes are celebrated for their extraordinary axial stiffness and strength, and when dispersed in a polymer they act as molecular-scale rebar, bridging microvoids and transferring load across weak interfaces. Graphene oxide brings a complementary set of properties: its two-dimensional sheets provide barrier characteristics that slow the penetration of corrosive species, while its oxygen-containing functional groups improve interfacial bonding with the epoxy matrix. The combination of the two creates a synergistic hybrid network, with the tubular nanotubes and planar graphene sheets interlocking to form a more continuous reinforcement architecture than either nanofiller could achieve alone.

The researchers systematically varied the composition of their epoxy hybrid composites and evaluated the internal and external morphology of the resulting microcapsules to confirm successful encapsulation. Among the formulations tested, one stood out: the epoxy hybrid composite designated EHFC2, containing 10 percent of the MWCNT–graphene oxide hybrid system combined with a 5 percent titanium dioxide sol–gel coating. This composition exhibited superior crack-filling characteristics, meaning that when fractures were introduced, the released healing agent and the surrounding reinforced matrix worked together to close the damage more completely than in the other formulations. Crack-filling ability is a critical metric for self-healing composites, because an unfilled crack continues to act as a stress concentrator and a pathway for moisture and aggressive ions, undermining both mechanical integrity and corrosion resistance.

The mechanical results reported for the coated hybrid composite are striking. After application of the 5 percent titanium dioxide sol–gel coating, the tensile strength of the material reached 97.2 megapascals, while the hardness rose to 54 on the Brinell hardness number scale. These figures represent meaningful improvements over the uncoated counterparts and demonstrate that the sol–gel layer is not merely a passive film but an active contributor to load-bearing capacity. Sol–gel coatings are produced through a wet-chemical route in which titanium alkoxide precursors hydrolyze and condense into a continuous ceramic-like network at relatively low temperatures. This process yields a dense, well-adhered titanium dioxide layer that can be applied to complex geometries without the high thermal budget of conventional ceramic processing, making it compatible with temperature-sensitive polymer substrates such as epoxy.

Beyond static mechanical properties, the study examined how the composites behave under sliding contact, a crucial consideration for components such as bearings, gears, seals and pump parts that experience repeated surface loading. The researchers measured the specific wear rate, a normalized quantity that expresses material loss per unit of applied load and sliding distance, allowing fair comparison across test conditions. In polymer composites, wear resistance typically improves when stiff nanofillers carry part of the contact load, reduce the real area of contact and promote the formation of a stable tribofilm on the sliding surface. The hybrid network of carbon nanotubes and graphene oxide, further protected by the hard titanium dioxide coating, provides exactly this kind of load-sharing and surface protection, and the reported wear data confirm that the coated hybrid formulation resists material removal more effectively than the baseline epoxy.

Corrosion performance received equally careful attention. The team conducted pH analysis to assess the chemical environment and the material’s response to it, complementing the corrosion-resistance enhancement provided by the titanium dioxide layer. Corrosion of polymer-coated metal structures usually begins when water, oxygen and chloride ions diffuse through microscopic defects in the coating and attack the underlying substrate. A sol–gel titanium dioxide layer acts as a tortuous barrier that lengthens the diffusion path for these aggressive species, while the graphene oxide within the composite adds a second line of defense through its own platelet-based barrier effect. The self-healing microcapsules add a third mechanism: if the coating is scratched or cracked, ruptured capsules release healing agent that seals the breach before corrosion can initiate. This triple-layered defense strategy, combining barrier coating, nanocomposite reinforcement and autonomous repair, reflects a broader trend in protective materials research toward multifunctional systems that address several degradation modes simultaneously.

The implications for engineering practice are considerable. Epoxy composites are ubiquitous in structural applications where maintenance access is difficult or expensive, including offshore platforms, pipelines, marine vessels, aerospace components and civil infrastructure. A material that can heal its own microcracks while resisting wear and corrosion could extend service intervals, reduce lifecycle costs and improve safety margins in all of these settings. The authors of the study, based at Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology in Avadi, Rajalakshmi Institute of Technology in Chennai, Aditya University in Surampalem and Sri Venkateswara College of Engineering and Technology in Chittoor, acknowledge the support of IIT Tirupati, which provided laboratory facilities for the work. Their findings add to a rapidly growing literature on microcapsule-based self-healing systems, which in recent years has expanded from simple single-shell capsules toward sophisticated multi-layered and nanofiller-reinforced designs.

Challenges remain before such materials reach widespread deployment. The long-term stability of microcapsules during processing and service, the consistency of capsule rupture under realistic loading conditions, and the scalability of sol–gel coating deposition are all active areas of investigation. The concentration of nanofillers must also be optimized carefully, because excessive loading can increase viscosity during manufacturing, promote agglomeration and even degrade toughness. The composition window explored in this study, spanning 5 to 15 percent waterborne polyurethane, 0.5 to 1.5 percent multi-walled carbon nanotubes, 2 to 6 percent graphene oxide and a fixed 5 percent titanium dioxide sol–gel coating, provides a useful map of how these variables interact and where the optimum lies.

Nevertheless, the reported combination of a 97.2 megapascal tensile strength, a Brinell hardness of 54, improved crack-filling behavior and enhanced wear and corrosion resistance in a single material system represents a notable step forward. It illustrates how rational design at multiple length scales, from molecular sol–gel chemistry to nanoscale carbon reinforcement to microscale capsule architecture, can be integrated into one coherent material strategy. As self-healing composites continue to mature, the vision of structures that monitor, resist and repair their own damage moves steadily closer to everyday engineering reality, promising a future in which the materials around us quietly maintain themselves rather than waiting for human intervention.

Subject of Research: Self-healing epoxy composites reinforced with GO/MWCNT microcapsules and TiO2 sol–gel coatings for tribological and corrosion performance

Article Title: Self-healing epoxy composites with GO/MWCNT-reinforced microcapsules: influence of TiO2 sol–gel coating on tribological and corrosion performance

Article References: Self-healing epoxy composites with GO/MWCNT-reinforced microcapsules: influence of TiO2 sol–gel coating on tribological and corrosion performance. (n.d.). https://doi.org/10.1007/s10853-026-13795-5

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13795-5

Keywords: self-healing composites, epoxy, microcapsules, graphene oxide, multi-walled carbon nanotubes, titanium dioxide, sol-gel coating, tribology, corrosion resistance, waterborne polyurethane, nanocomposites, mechanical properties