Self-healing materials Flashcards
How can self-healing be defined?
It is the ability of a material to heal (recover/repair) damages.
How can self-healing materials be classified?
1) Autonomic and Non-Autonomic
i) Autonomic
Self-healing materials do not require any additional external trigger, the damage itself is the stimulis for the healing. The detection of the damage (by a sensor), as well as the repair (by an actuator), proceeds autonomously within the material structure. There is a stored energy that guide the healing process without the request of energy from the environment.
ii) Non-Autonomic
Self-healing materials that require a modest external trigger, like a change in temperature, in pressure, in environmental conditions (ph), or exposure to light. When there is damage in the system , it is not able to start the healing process alone but it needs an external input of energy.
2) Extrinsic or Intrinsic
i) Extrinsic
Healing process based on external healing components (intentionally embedded into the matrix materials). The material has a special feature which only functionality is the healing of the material. The most used approach are healing systems that have inside a capsule or vascular network that provide healing. Their only function is healing. A downside of inserting this type of structure is the decrease of the mechanical properties (they can be seen as defects).
ii) Intrinsic
No separate healing agents, formation of covalent or non-covalent chemical bonds as well as physical interactions. In this case, we have specific chemical bonds or specific physical bonds inside the material that give it the capability of repairing. The bonds are reversible so they can be open and closed to seal the damage.
Do all materials have self healing?
Yes, all materials have the ability of self-healing, such as polymers, composites, ceramics, concrete and metal. SH is hard on metals though.
What is self-healing based on?
Self-healing is based on the same common general principle:
The generation of a mobile phase, which can close the crack.
What damages can be healed?
1) Delamination
2) Scratches
3) Debris impacts
How can you achieve optimal healing?
Different damages may require different healing strategies (or even a combination of them) to achieve optimal healing.
The healing efficiency is measures as: Healed material properties/Original Material Properties * 100. For example, if we consider a delamination in a composite, the property we can use for the healing efficiency is the fracture toughness. To measure the effiency with a crack, we can mesaure the scratch depth/volume before and after the damage.
How do hollow vessels or fibers containing reactive healing work? (extrinsic self healing)
1) Damage of the material, crack propagates
2) Rupture of vessels of capillaries
3) Release of the reactive healing agent into the crack. The reactive agent is in a liquid state and its a non reacted epoxy. The healing agent reacts with a catalyst embedded in the matrix
4) Repair of the sample
Multicapsule systems with separate capsule types for both catalyst and healing agents exist.
What are the disadvantages of Microcapsules?
1) Small amount of healing agent
2) One healing event
3) Brittle phase in the composite (reduction of mechanical properties)
4) The creation of the tiny capsule.
What are the Vascular systems? (extrinsic self healing)
There are three possible approaches for vasular systems:
1) Hollow fibers filled with one part resin
2) Hollow fibers filled with resin and hardener
3) Hollow fibers filled with resin and harderner dispersed in the matrix
The difficulties of this approach is the creation of the chanelling network inside the material and filling them with the agent and the catalyst. 1D, 2D and 3D networks exist. The 1D is the 1D but it is also the system the decreases less the mechanical properties.
Creating a pressurized vascular system with capillary tube of small produce creates an increment of healing efficiency, because we can push the healing agent inside the crack. The introduction of a pumping system also increases the healing efficiency.
How can we protect the self-healing coating from corrosion?
We can have a coating with capsule on top (self-healing coating). Creating a scratch on the coating leads to the breaking of the capsule with the healing agent which can react with the environment and seal the scratch avoiding corossion of the layer below. The problem of this approach is the life of the system, so how much the capsule can survive inside the system without having polymerization inside them. The healing agent should have the same efficiency in reaction for all the life of the coating.
