Post 180 ⇒   by Gautam Shah 


Matrix-Filler Interface in Composites

A composite material is a complex entity. Understanding its constituents and their role helps in understanding of its strength and weaknesses. Here in very simple terminology this has been explained.



Plasan Sand Cat light (5t) military vehicle featuring integrated composite armoured body


In a composite material, the Filler in the form of particles, fibres and sheets, is expected to take up the stresses in unison with the Matrix, due to the strong interface provided by the later.


Composite materials with weak interfaces have low strength and stiffness, but high resistance to fracture, On the other hand materials with strong interfaces have high strength and stiffness but are brittle. The bonding between the matrix and the filler is dependent on the atomic arrangement and chemical properties of filler and on the molecular conformation and chemical constitution of the matrix.


A crack that starts in a monolithic material generally continues to propagate until that material fails, whereas the filler-matrix combination reduces the potential for a complete fracture.


Bonding at the interface: In a simple system the bonding is due to adhesion between filler and the matrix.


Adhesion can be attributed to following FIVE main mechanisms:


1 Adsorption and wetting: When two electrically neutral surfaces are brought sufficiently close, there is a physical attraction. Most solids, have surfaces that are rarely perfectly in level, blemish, and without any contamination. So a wetting agent that substantially covers all hills and valleys, displaces all air, and overcome the effects of contamination, is required.


2 Interdiffusion: It is possible to form a bond between two polymer surfaces by the diffusion of the polymer molecules on one surface into a molecular network of the other surface. The bond strength will depend on the amount of molecular entanglement and the number of molecules involved. Interdiffusion may be promoted by the presence of solvents and Plasticizing agents.


3 Electrostatic attraction: When one surface carries a net positive charge and the other surface, a net negative charge, electrostatic attraction occurs (as in acid+base reaction). Electrostatic attraction has no major role in contribution of bond strength, but has importance on how things initially begin to get mixed.


4 Chemical bonding: It is formed between a chemical group of filler and a chemical group of a matrix. The bond formation or breakage usually involves thermal activity.


5 Mechanical adhesion: Some bonding occurs by the mechanical interlocking of two surfaces (e.g. fibre shape-section).


Shocks, impact, loadings or repeated cyclic stresses can cause the Individual fibres to separate from the matrix, e.g. a fibre pull-out. In case of laminated or layered construction there could be a separation at the interface between two layers, a condition known as de-lamination.


Composite material: Radar absorbent material for stealth air craft



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