6 - The interfacial region

Summary

In composites the role of the interface is crucial. Stiffening and strengthening rely on load transfer across the interface, toughness is influenced by crack deflection/fibre pull-out, and ductility is affected by relaxation of peak stresses near the interface. Unfortunately, however, a great deal of confusion surrounds the question of how best to characterise, and then optimise, the mechanical response of the interface to stresses arising from an applied load. In this chapter, a brief outline is given of the meaning and significance of interfacial bond strength, followed by a summary of the methods used to measure interfacial mechanical properties, with particular reference to fibrous MMCs. Some attention is then devoted to interfacial chemical reactions. Finally, the production and characteristics of fibre coatings are briefly examined.

The significance of interfacial bond strength

In the previous two chapters it has become clear that many important phenomena can take place at the matrix/reinforcement interface. For polymer-based composites, although the chemistry involved may be complex, the objectives in terms of interfacial properties are often the rather straightforward ones of a high bond strength (to transfer load efficiently to the fibres) and a good resistance to environmental attack. In designing ceramic composites, on the other hand, the aim is usually to make the interface very weak, as the prime concern is in promoting energy dissipation at the interface so as to raise the toughness. For MMCs, a strong bond is usually desirable, but there may be instances where inelastic processes at the interface can be beneficial.