Smart composites: tougher and self-healing interfaces

Safety and durability in construction directly affect our daily lives. From civil buildings to major infrastructures, a material’s ability to maintain its performance over time is not only a technical issue but also a social one: protecting people and property means ensuring stability, reducing maintenance costs, and promoting sustainability.

In this context, fiber-reinforced composites (FRCs)—materials made of fibers (glass, carbon, basalt) embedded in a polymer matrix—play a key role. Their success lies in their excellent strength-to-weight ratio, making them ideal for structural applications. However, despite these advantages, FRCs have a well-known weak point: the fiber–matrix interface.

The interface: the Achilles’ heel of composites

The interface is the contact zone between fiber and matrix. Under load, interfacial debonding often originates here. Even if both fiber and matrix remain intact, poor adhesion between them can cause sudden and catastrophic failure. This is a major concern in structural applications where safety is critical.
Research has therefore focused on this weakness from two complementary perspectives:

1. Improving fiber–matrix adhesion to prevent premature failure.
   
2. Developing self-healing interfaces capable of restoring adhesion after damage.
   

Innovative techniques to enhance adhesion

Two original methods have been explored:

• Triboelectrification: Glass fibers rubbed against Teflon generated positive charges that attracted graphene oxide nanosheets, creating a stronger and more uniform bond with the matrix. The interfacial adhesion increased by about +45%.
  
• CO₂ laser treatment: Applied to basalt fibers, it increased surface roughness and promoted better mechanical interlocking with the matrix, improving adhesion by about +8%.
  

Both approaches proved effective, confirming that interface engineering is key to enhancing composite performance.

Toward self-healing composites

A second research line aimed to create self-healing composites, capable of restoring interfacial properties after damage. The idea was to coat the fibers with poly(ε-caprolactone) (PCL), a thermoplastic polymer that can reversibly melt at around 60 °C.

Two coating methods were used:

• Continuous PCL film, achieving over 90% self-healing efficiency.
  
• Nanostructured PCL coatings, reaching 100% recovery of interfacial properties.
  

This result is remarkable: a damage that would normally shorten the composite’s service life can be reversed with simple heating, reducing maintenance costs and increasing safety.

Toward real-world applications

The next step is testing these interfaces under realistic environmental conditions, since temperature, humidity, and thermal cycles can affect long-term performance. Understanding these effects is crucial to move from lab research to real applications in construction, infrastructure, and transportation.

The research, conducted by PhD candidate Laura Simonini, was presented at the AIMAT conference, where it won the Best PhD Thesis Award in the field of Materials Engineering.

IMAGES

Fig. 1: Laura Simonini visiting Tampere University in Finland.

Fig. 2: FEM modeling of interfacial debonding.