1.4 Concluding remarks We see that further progress in transportation systems and technique of energ
We see that further progress in transportation systems and technique of energy transformations,6 depends strongly on availability of structural materials with higher strength and fracture toughness values, higher rigidity, higher working temperatures, and lower density. If we look around, we shall see that technical potential of metal alloys that are the main class of the structural materials, are about exhausted at present. The technical history of the Iron Age which is full of brilliant innovations in metallurgy, paradoxically, has led to a search for non-metallic substances to be used in structural materials.
Despite ceramics having been known since the Stone Age, they are really a new class of the substances for structural materials. We reviewed briefly the ways of bringing ceramics closer to borders at which they will meet the requirements to structural materials. However, these ways do not yield universal solutions, improvements in fracture toughness are not satisfactory for an overwhelming majority of the applications.
Therefore, the advent of glass fibre reinforced polymers (GFRP) and then carbon fibre reinforced polymers (CFRP) is an enormous step in the right direction. Brittle substance, glass or carbon, has become a part of structural materials. High modulus and high strength graphite fibres being glued together with a weak polymer have spread over a large area of various application that demonstrate a possibility of overcoming the inherent brittleness of ceramic-like substances by combining their fibrous form and weak interfaces between the fibres!
However, a question arises when we look at a combination of an obviously high-temperature material like carbon and an obviously low-temperature organic. Is it a natural combination? The answer is not obvious. Look at the structure of natural materials: they are composed of ceramics and organics [110]. Perhaps, Nature has been optimizing the composition of biological structures limiting Itself with both available mineral components and a necessity to use organics. Certainly, the availability of both carbon fibres and a GFRP experience in 60s yields the idea of CFRP. This was a great idea. However, the time space in which Nature has acted, and for people to develop new structural materials, are drastically different. So the great idea will show and has already revealed drawbacks. Low modulus polymers provide weak interfibre bonds easily and this makes the crack to delaminate the interface and to change its configuration. This is good, but at the same time, it can yield uncontrolled delamination and change a configuration of the whole structural element. This is a failure. Hence, one needs to design a composite element very carefully, and this good for... mainly, for mechanical engineers involved in the process.
Thus, fibre reinforced polymers have shown both potential of fibrous structure and restrictions imposed by low modulus organics used as a binder. This has borne an idea to combine ceramic fibres with metal or ceramic matrix. With this, we drop the limitations of polymers but bring new problems. Not to say about technological difficulties as compared with polymer technology, an important problem is how to make fibre and matrix materials compatible, how to adjust the microstructure of a composite to a loading pattern, etc. However, it becomes clear, at least on a qualitative level, that combining ceramics and metals, ceramics and ceramics, we broaden enormously the usage of ceramics as structural materials provided we have succeeded in preventing brittle fracture. How to reach this goal, is a subject of Part III.
To conclude, we have to point out one more advantage of a wider usage of non-metallic substances. John Bernal was the first to emphasize that when we are using nitrides, carbides, etc. we are consuming chemical elements which are not so rare on Earth as metals used for structural alloys. Actually, changes in environmental problems which would arise as a result of a wide use of composites based on ceramics, have not been discussed yet. The time for such a discussion is still remaining.