Metal-Matrix
Composites
By Simon Chambers and Martin Todman

 Why MMC's?   <Top><Next>

The idea behind Metal Matrix Composites (MMC's) is to adjust the properties of common metals by selectively adding reinforcing agents (such as ceramic particles or fibres) to produce a composite material whose density is close to that of the metal.  This can be cheaper and/or less hazardous than resorting to exotic metals (like beryllium).  The properties that can be adjusted include strength, stiffness, thermal expansion, wear resistance and electrical conductivity.  The goal is to produce a very light, strong and cheap material.

There are two categories of reinforcement materials:

• Discontinuous Materials - particles and short fibres.
• Continuous Filament - long reinforcing fibres.

In recent years, fibre-reinforced composites have been produced by the directional solidification of eutectics, near eutectics and monotectiecs.  However, the compositions, shapes, sizes and relative amounts of the two phases are restricted by the limits shown in equilibrium or phase diagrams.

In the production of MMC's, the particles or fibres are mechanically mixed into a molten alloy before it solidifies.  Therefore, none of the above restrictions apply.

An important consideration when immersing particles into a melt is the energy required to push the particles into the melt and have them stay there.  i.e. buoyancy effects, energy to create new surfaces, etc.  The immersion process needs to be energetically favourable.

Solid particles and short fibres can be introduced into molten alloys by:

• injecting powders entrained in an inert gas - particles are transferred into the melt as bubbles rise,
• adding the particles to the molten stream which is filling the mould,
• stirring the molten alloy with an impeller to create a vortex in which the solid dispersoids are added,
• forming pellets or small briquettes of particle powder, plunging them into the melt and stirring,
• using centrifugal acceleration to disperse particles in the melt,
• pushing the particles into the melt by reciprocating rods and
• using high-intensity ultrasound with or without mechanical force.

The easier it is for a particle to be immersed in a molten alloy, the greater is its "wettability".

Wettability can be improved by:

• using metal coatings such as Ni and Cu on refractory particles,
• adding reactive elements such as Mg, Ca, Ti, Zr and P to the melt,
• heat treating the particles and
• irradiating ultrasonic waves in the melt.

It is desirable to obtain a fine and homogenous distribution of particles in the melt for the best strength and machining properties.  This can be acheived by:

• using external forces to localise segregation or
• creating colloidal suspensions by using surface-treated or -coated mutually repelling particles.

• Continuous filament MMC's exhibit their optimum tensile properties when stressed parallel to the longitudinal direction of the fibres.
• Strength in the transverse direction is comparatively poor, but can be improved (at the expense of some of the longitudinal strength) by interweaving the sets of fibres at right angles to each other.
• To maximise the use of the higher strength of the fibres, it is desirable to have greater ductility in the metal matrix than in the fibres.  This ensures that the composite will fail at a higher stress.
• These MMC's can only be effectively processed into sheets; like a sandwich: with metal, a bunch of fibres, then metal again.
• The drilling of holes through these sheets can have adverse effects on the strength of the composite.

Failure of a metal matrix composite by debonding of the fibres from the matrix can be catastrophic.
Bonding between matrix and fibre/particle can be improved by:

• The same methods as improving wettability described above, and
• Introducing into the melt a substance that will bond well with both the particle/fibre and the matrix.

Production of discontinuous material MMC's is accomplished by:

• Sand Casting,
• Permanent Mould Casting,
• Compocasting,
• Pressure Die-Casting, and
• Centrifugal Casting.

• Although excellent properties can be achieved, the main problem with certain MMC's is that they are very energy- and time-consuming to produce.  There are also limitations on the size of the components made by the abovementioned process.
• Another major problem with continuous filament MMC's is that they have very poor resistance to fatigue.
• Machinability is a problem as machining the continuous fibres can destroy their load-bearing properties.

Current successful or experimental applications of Metal Matrix Composites Include:

• Anti-friction in high abrasion, high temperature applications such as Piston Rings, Bearings, Electric Current Collectors, Cylinder Liners;
• Military applications: some turbine blades, light-weight frames and portable supports; and
• Space shuttle / satellite parts - the low to zero thermal expansion of MMC's ensures that there is no distortion in the precision parts due to the changing exposure of the material to sun radiation and shadow of space.

• Civil aircraft parts - if more confidence is put into these materials as a result of further development.
• Self-lubricating dry bearings - dispersed graphite particles can rub off the bearings and provide lubrication without grease or oil.
• Static motor vehicle parts - parts where size is important but strength isn't as important - save weight; e.g. carburettors.
• High-performance internal combustion engine parts.

Although some problems have been encountered with MMC's, the future looks promising.  The development of cheaper and more energy-efficient production methods continues.  It is likely that soon, at least cast metal matrix composites will be a common entity in the engineering world.  Continuous filament MMC's still need a lot of work.

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