Material advancement has been crucial in shaping the development of mankind. Progress in Material Science and Material Processing has always had a significant impact on society. From the Stone Age to the Composites and Alloy age, we as humans have always strived towards improving in the field of material science and its applications.
Currently, our society is advancing towards a new category of material, which will most likely be used in all kinds of industrial as well as homemade applications in the near future. This new category of smart materials is – Self-Healing Composites and Materials (SHCs).
What Are Self Healing Composites?
These composites are a huge breakthrough in material science and hold a lot of potential. Research and Development of materials that could mimic biological systems by healing themselves gradually have grabbed the interests of scientists and companies of various industries since the early 2000s. Such materials could be a major contributor to the aerospace industry as well.
The basic and most complete definition of a self-healing material is that it is an artificially or synthetically created substance that has the ability to repair itself from damage or can dynamically adapt to its surrounding environment for structural and mechanical integrity.
How Can SHCs be Beneficial to Us?
Depending on the constituent materials, they might or might not need an external stimulus for activation of the healing process. Such materials would be able to tackle problems such as wear and tear, aging effects, and defects like micro-cracks which pose a significant challenge in the field of aerospace and have not been overcome by composites and alloys currently used.
Self-healing materials can drastically increase life expectancy, durability, structural integrity and can simplify maintenance as remote maintenance without the use of manpower and human intervention is possible.
The concept of ‘self-healing’ is an inspiration from biological systems. The innovation comes under the category of biomimicry. The novel approach of such smart materials is based on how living organisms are able to self-heal from damage on their upper epidermal layer using blood cells like platelets which form clots and thus are able to seal any open wounds.
Classification of Self Healing Composites
SHCs by themselves form a vast category of composites but under a broader spectrum, they can be categorized into two distinct categories based on their method of implementation in products and approach for the healing process.
1. Extrinsic Approach
2. Intrinsic Approach
1. Extrinsic Approach of Self Healing:-
In the extrinsic approach of self-healing materials, the healing part of the composite could be in a different form and is held separately within the composite material. Extrinsic healing materials contain healing agents like epoxy in the form of microcapsules or artificial vascular tubes embedded in the matrix.
The basic functioning of the extrinsic approach is such that in the case of any damage imposed on the structure would lead to the release of the healing agent along with the catalyst (if any) into the developed cracks. The release of the healing agent triggers the healing reaction to start and heal the composite.
The main approaches to extrinsic healing are:-
- Encapsulation of healing agents and dispersion of the catalyst within a material matrix.
- The healing agent could be in a tubular form, while essentially functioning similarly to microcapsules, covering a larger surface area at the cost of structural rigidity.
- Using biomimicry as a tool and observing the cardiovascular network, a mesoporous network could be developed in order to supply the healing agent to the designated region of action.
2. Intrinsic Approach of Self Healing:-
This approach is reliant on the chemical and intermolecular properties of the constituent materials of the matrix itself. It is based on the chemical bonding and molecular structure of the material. Intrinsic materials usually require an external stimulus to initiate the healing process, but this is not necessary for all cases and types. Intrinsic healing is usually achieved from reversible covalent bonds, thermoreversible reactions, and supramolecular chemistry.
There are 3 modes of carrying out intrinsic healing:-
- Reversible covalent chemistry which implies that covalent bonds can dissociate and reassociate under deformation. Such a reaction mostly includes a ring-chain equilibrium.
- Thermoreversible physical interactions with a focus mainly on isomers.
- Supramolecular chemistry. Reversible supramolecular interactions are low-energy interactions that have an inﬂuence on the overall properties of a material, if well designed. Possible avenues for obtaining these interactions are based on hydrogen bonding or metal coordination. This has proven to be one of the more promising modes.
In a nutshell, when damage occurs, a crack is formed. Intrinsic self-healing is then achieved by the recovery of the former interactions prior to the deformation, with or without an external stimulus.
Applications in the Aerospace industry
The mechanical, physical, and chemical properties of self-healing materials are a perfect means of countering problems faced in the aerospace industry. We will be discussing some of its applications and uses related to aerospace in this section.
The engine is constantly subjected to high temperatures and a corrosive environment. Therefore corrosion and temperature resistance play a major part in the selection of materials for making engines. Current materials used in engines include ceramic composites which are resistant to high temperatures. However, they are brittle and extremely susceptible to impact damage.
Further, nickel alloys that are used for the manufacture of compressor and turbine blades have a low melting point and therefore, are not viable in high-temperature working conditions. This results in lower efficiency of the engine and the manufacturer have to give temperature constraints.
Research and the prospects of using self-healing composites which involves using multi-layered matrix containing boron are being looked into. The healing process includes the formation of particular healing phases that form when boron compounds are oxidized. These phases then flow into the cracks and seal them. Such boron-bearing composites have experimentally been proved to have high corrosion and thermal resistance under large mechanical and impact loads. Thus, they are a suitable option to replace nickel superalloys in the development of turbine blades.
Modern aircraft employ extensive use of fiber-reinforced polymers for making fuselage. The FRPs currently in use are susceptible to impact damage. Furthermore, they require frequent maintenance as structural damage and integrity is difficult to detect. Self-composites provide a promising alternative for the manufacture of lightweight, low maintenance fuselage.
The application potential of the use of self-healing hollow glass fiber-epoxy composites for making fuselage is huge. Recent studies and research has shown that composites made by molding of such materials have a strong recovery up to 47% after healing from damage by three-point bend impact stress. Also, compressive strength recovery was found out to be 92%. The results validate that self-healing FRPs can be efficiently used for the future development of aircraft fuselages.
Research has also been done on the preparation of an ionomeric polymer(an EMAA copolymer) which showed excellent self-healing abilities and resistance to high-velocity impacts. Their properties make them a much better, efficient, and viable option to replace aluminum alloy, which is a common material in the space industry.
Coatings have a vital role in the aerospace industry. They are responsible for the protection of aerostructures, fuselage, wing, engine cascade, etc. from external environmental conditions which can lead to corrosion. Corrosion damage can further lead to mechanical and structural failure due to the weakening of the outer layer of the structure.
Therefore, it is crucial to come up with solutions to counter corrosion. Self -healing coatings appear to be a promising alternative to coatings and paints currently in use due to their ability to self-heal automatically without the need for human intervention. Use of such coatings would drastically reduce the use of manpower and frequency of maintenance cycles. Automatic damage recovery would also reduce maintenance costs.
Examples of such coatings include an epoxy resin composite, which when applied on steel samples, provide the sample with excellent corrosion and damage protection. Furthermore, self-healing vanadia composites have also been synthesized and tested on aerospace-grade aluminum and magnesium for protection from corrosion. The results were satisfactory and it was concluded that it would be a good alternative to chromate coating which is currently in use.
Recent developments in such composite sciences have propelled self-healing materials into the limelight. These developments could pave the way to several applications, speciﬁcally in the ﬁeld of aerospace. Self-healing composites are likely to be an important component in aerospace applications, particularly in addressing fatigue and impact resistance problems.
Corrosion and barrier properties can also be efﬁciently recovered after healing. Applications in the aerospace sector include fuselage and aerostructures, engine blades, combustion chambers, anti-corrosion coatings, smart paints, and impact-resistant space structures. The current manufacturing infrastructure to supply such materials must also develop simultaneously to support future demand, otherwise, this promising sector could collapse. Military applications will prove to be a major driving force as it has proven for many other products and propel us into a new age of smart self-healing materials.
However, even though there is a lot of development and potential sector, it will take some time for such materials to become economical and effective in the aerospace sector. Currently, composite material development is a highly desirable sector in the aviation sector with military development companies investing the most. Research and Development can cost up to billions in dollars as it accounts for aspects like the properties expected from the material, the availability and feasibility to attain the materials to produce it and keep up with increasing demand. New production techniques might need to be implemented and companies might have to invest more into training their employees on handling and using the material. There is a considerable risk factor that comes with R & D in this field. With current technology, the desired composite material might not be able to be developed, or even if it is designed, it may have a single application that may not make it profitable. At the end of the day, profit and financial security is a significant aspect in the development of a composite material.
Written by Aryaman Singh Samyal and Nathan Nash Barretto for AeroMIT
Edited By Rahul Alvares
• https://www.ijiert.org/admin/papers/1483537113_Volume 4, Issue 1.pdf