What are Composites? A combination of two or more materials (reinforcement, resin, filler, etc.), differing in form or composition on a macroscale. The constituents retain their identities, i.e., they do not dissolve or merge into each other, although they act in concert. Normally, the components can be physically identified and exhibit an interface between each other.

Introduction

Metal matrix composites (MMCs) have high specific strength and specific modulus and have the potential for high fracture toughness and environmental resistance. They therefore have the potential to replace metal alloys in many applications.

Matrix Reinforcement Interface in MMC

Many properties of metal matrix composites are strongly influenced by the nature of the

matrix reinforcement interface. This in turn depends upon factors such as pretreatment of

the fibres and reinforcement fabrication techniques. The nature of the matrix

reinforcement interface is most commonly studied by electron spectroscopic techniques,

which can give information about the degree of bonding between the matrix and

reinforcement, and the formation of additional phases in the matrix material or at the

interface.

It refers to a bounding surface between reinforcement and matrix across which there occurs a discontinuity in a parameter such as chemical composition,elastic modulus,coefficient of thermal expansion and thermodynamic properties such as chemical potential.Interface is very important in all composites. In metal matrix composite,the reinforcement and the matrix will not be in thermodynamic equilibrium,i.e., there will be a thermodynamic driving force for an interfacial reaction that will reduce the energy of the system

The load acting on the matrix has to be transferred to the reinforcement via. Interface

The reinforcement must be strongly bonded to the matrix if high stiffness and strength are desired in the composite materials

A weak interface results in low stiffness and strength but high resistance to fracture

A strong interface produces high stiffness and strength but often low resistance to fracture, i.e. brittle behavior

2 types of failure at interface

Difficult to measure the strength of interface, this is because sometimes failure occur at interface, and sometimes not.

2 types of failure at interface 1) Adhesive failure - failure occur at interface 2) Cohesive failure – failure occur close to the interface (either at the fiber or matrix)

For example we have a interfacial zone with a finite thickness and possibly consisting of multiple layers. The multiple layer boundary zone will be in equilibrium at the high temperature at which the components are originally brought together. At any other themperature, there exists a complex stress field in the boundary zone because of the mismatch in the coefficient of thermal expansion of the various layers. These stress will be proportional to the difference in the elastic moduli, difference in the coefficient of thermal expansion and of course the amplitude of temperature difference between the equilibrium temperature and final temperature. Thermodynamically ,the phase in the boundary zone will tend to change such that the free energy of the system in minimized.

Defects:Dislocation

Grain Boundary Migration

Crack nucleation

Formation of Crack

An ideal MMCs should promote wetting and bond the reinforcement and the matrix to a desirable degree.The interface should protect the reinforcement and allow load transfer from the soft metallic matrix to the strong reinforcement.

Other parameter that characterize the interface:

1. Geometry and Dimensions2. Microstructure3. Morphology4. Mechanical properties5. Physical,chemical and thermal characteristics

Reinforcement Fabrication Techniques

• These techniques are also important factors in determining the nature of the reinforcement-matrix interface.

• MMCs are generally fabricated using 3 techniques1. Solid State2. Liquid State3. Vapour state

Solid state fabrication

•Powdered metal and discontinuous reinforcement are mixed and then bonded through a process of compaction and thermo-mechanical treatment.

•Layer of metal foil are sandwiched with long fibres,and then pressed through to form a matrix.

Liquid state

•A solution containing metal ions loaded with reinforcing particles is co-doposited forming a composite metal.

•Discontinuous reinforcement is stirred into molten metal, which is allowed to solidify.

Vapour State

•Vapour Deposition: The fiber is passed through a thick cloud of vaporized metal,coating it.

Interfacial bonding

Once the matrix has wet the reinforcement, bonding will occur.

For a given system, more than one bonding mechanism may exist at the same time.

The bondings may change during various production stages or during services.

Types of interfacial bonding at interface

1) Mechanical bonding2) Electrostatic bonding3) Chemical bonding4) Reaction or interdiffusion bonding

Mechanical bonding1. Mechanical interlocking or keying of two interfaces can leads to reasonable bond.2.The rougher the interface, the interlocking is Greater, hence the mechanical bonding is effective.

Mechanical bonding is effective when the force is applied parallel to the interface.

If the interface is being pulled apart by tensile forces, the strength is likely to be low unless there is a high density of features (designated A).

Electrostatic Bonding

-Occur when one surface is positively charged and the other is negatively charge.

-Interactions are short range and only effective over small distances of the order of atomic dimensions.-Surface contamination and entrapped gases will decrease the effectiveness of this bonding.

Chemical bonding

The bond formed between chemical groups on the reinforcement surfaces (marked X) and compatible groups (marked R) in the matrix.

Strength of chemical bonding depends on the number of bonds per unit area and the type of bond.

Chemical bonding normally exist due to the application of coupling agents.

Reaction or interdiffusion bonding

The atoms or molecules of the two components may interdiffuse at the interface.

For system involving metals & ceramics, the interdiffusion of species from the two components can produce an interfacial layer of different composition and structure from either of the component.

The interfacial layers also will have different mechanical properties from either matrix or reinforcement.

In MMC, the interfacial layer is often a brittle intermetallic compound.

One of the main reason why interfacial layers are formed is in ceramic and metal matrices is due to the processing at high temperature- diffusion is rapid at high temp; according to the Arrhenius equation).

Methods for measuring bond strength

Single fiber test Fiber pull-out test Bulk specimen tests Micro-indentation test

Single Fibre test

Fiber pull-out test

Fiber pull-out test Involves pulling a partially embedded single fiber out

of a block of matrix material Difficult to be carried out especially for thin brittle fiber From the resulting tensile stress vs. strain plot, the shear

strength of the interface and the energy of debonding and pull-out may be obtained

Bulk specimen tests

The simplest method and most widely employed

The tensile strength and shear strength obtained from the 3-point bending test are found to depend on the volume of fibers- not a true values for the bond strength

Micro-indentation test

Employs a standard micro-indentation hardness tester

The indentor is loaded with a force, on to a center of a fiber, whose axis is normal to the surface, and caused the fiber to slide along the matrix-fiber interface

Strength

Difficult to predict the strength this is due to the sensitivity of strength towards the matrix and fiber structure

For example, matrix and fiber structurewill be changed during the fabricationprocess.

Toughness

Depends on few factors:

1) Composition and microstructure of the matrix2) Type, size and orientaion of fiber3) Processing of composite, effect the

microstructure, i.e. voids, distribution of fiber, etc.

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