Lecture 1 - MMC's

Question 1

What is an example of a composite cutting tool?

Answer 1

Cemented carbide, which is a hard metal.

Question 2

Give 4 applications and 4 composite materials used in industry.

Answer 2

Borsic aluminium: used in fan blades of aircraft engines

Kevlar-epoxy: used in space shuttle

Graphite-polymer: sporting goods

Glass-polymer: lightweight automotive applications.

Question 3

Describe the term monolithic

Answer 3

A monolithic material has a uniform and continuous microstructure, formed from a single material and more than one micro constituent may be present.

Whereas the microstructure of a composite is non-uniform, discontinuous and multiphase.

Question 4

What advantages do advanced composites provide over conventional monolithic materials?

Answer 4

Monolithic materials present limitations in achieving a good combination of strength, stiffness, toughness and weight (density).

Composites provide significantly improved properties, including high specific strength, specific modulus, damping capacity and wear resistance comped to unreinforced alloys (such as monolithic materials).

Question 5

Why are composites preferred over metals? (6 reasons)

Answer 5

Specific strength and stiffness

Tailorable design

Fatigue life

Dimensional stability

Corrosion resistance (more for CMC and PMC)

Cost-effective fabrication

Question 6

How can composites be tailored for a design? (4 possible choices)

Answer 6

Type of reinforcement

Fibre/particulate volume fraction

Reinforcement orientation

Layer stacking sequence

Question 7

How do you provide dimensional stability in composites?

Answer 7

Temperature changes can result in overheating of components, thermal fatigue due to cyclic temperature changes and rendering structures structures inoperable.

Low coefficient of thermal expansion can maintain dimensional stability of composite structures in a thermal environment.

Question 8

What are the main challenges associated with composites? (6 main challenges)

Answer 8

Lack of automation techniques

High cost of fabrication (MMC’s more expensive than PMC’s)

Complex mechanical characterisation

Complex repair

May not satisfy all required properties

Limited standardisation

Question 9

What are the 5 geometrical and spatial characteristics of a fibre phase that may influence a composites properties?

Answer 9

Concentration of the fibre

Size of the fibre

Shape of the fibre

Distribution of the fibre

Orientation of the fibre

Question 10

What are the 3 main groups of composite types and give a explanation of how they can be classified?

Answer 10

Particle Reinforced: The particle dimensions are approximately the same in all directions.

Fibre Reinforced: Large length to diameter ratio. Can be continuos (long aligned) or discontinuos (short aligned or randomly orientated)

Structural: Combinations of composites and homogeneous materials (laminates or sandwich panels).

Question 11

Explain the difference between PMC, CMC and MMC composites.

Answer 11

They are classified by their primary phase (matrix).

PMCs: thermosetting resin / epoxy with fibre reinforcement and are widely used. Relatively soft and flexible matrix, reinforcement provides strength and stiffness. Bond must be strong to transfer load from matrix to reinforcement.

CMCs: Embedded with fibres to improve properties, especially in high temperature applications. Relatively hard and brittle, but excellent high temperature properties. Reinforcement enhances fracture toughness and strength. Poor bonding rather than good is required.

MMCs: Mixture of metals and ceramics, such as cemented carbides (cutting tool) and other cermets.

Question 12

What is the aerospace industry dictated by?

Answer 12

By high performance NOT COST!

Question 13

Give the 8 requirements for MMC’s

Answer 13

Low density

High compressive & tensile strength

Mechanical compatibility

Chemical inertness

Thermal stability

High young’s modulus

Good processability

Economic efficiency

Question 14

What are the 3 most important MMC systems used?

Answer 14

Aluminium Matrix: Continuos/discontinuos, whiskers and particulates

Magnesium Matrix: Continuos, whiskers and particulates

Copper Matrix: Diverse reinforcements

Question 15

How do you produce an MMC? (give typical volume fraction and a crucial point about properties).

Answer 15

Disperse a reinforcing material into a metal matrix. Volume fraction is typically 10-70%. The interface plays a crucial part in determining properties.

Question 16

What advantages are gained by reinforcing a typical ductile metal matrix? (7 to get)

Answer 16

Higher specific strength and specific stiffness

Abrasion resistance

Creep resistance

When carbon/graphite fibres are reinforced it substantially reduced CTE.

Enhanced performance at higher temperatures

Better irradiation performance

Commonly lower density

Question 17

What advantages are gained by MMC’s over PMC’s? (4 to get)

Answer 17

Higher operating temperatures

Non-flammable

More resistant to degradation by organic fluids

No moisture absorption

Question 18

What is a major drawback to MMC’s compared to PMC’s?

Answer 18

Much more expensive to fabricate (therefore MMC’s are much less widespread).

Question 19

What is the main benefit of MMC’s?

Answer 19

They have a very good specific modulus vs specific strength ratio.

Question 20

Give the 6 main challenges of MMC’s?

Answer 20

New technology not fully understood

High cost of some reinforcement fibres

Complex and extensive fabrication methods

Machining processing challenges

Reinforcing material may reduce ductility and fracture toughness

Fibre-matrix interactions at high temperature degrade fibres

Question 21

What are the 2 main concerns arising during manufacture of MMC’s?

Answer 21

Concern (1): Some matrix and reinforcement combinations are highly reactive at elevated temperatures

• Composite degradation may be caused by high temperature processing or by subjecting the MMC to elevated temperatures during service.
• Resolved by applying a protective surface coating to the reinforcement or by modifying the matrix alloy composition.

Concern (2): possible galvanic corrosion. Example: carbon (graphite) fibres in aluminium or magnesium matrix

• Materials are at opposite ends of the galvanic (electrochemical) series.
• If exposed to a corrosive environment, an electric potential may develop between the two materials.
• The reinforcement surface must be suitably treated or coated, hence separated (insulated) from the matrix.

Question 22

Give a definition of the matrix material and list the 5 principal matrix systems.

Answer 22

The matrix is the monolithic (bulk) material into which the reinforcement is embedded, completely continuous. Usually a light metal (weight considerations) supporting the reinforcements. The matrix shares & transfers an applied load to the secondary
phase.

Principal alloy matrix systems:

Aluminium

Magnesium

Titanium

Copper

Superalloys (Fe-Ni, Ni or Co based alloys)

Question 23

State the 5 key functions of the matrix.

Answer 23

Provide a level of ductility

Binding the fibre together.

Transmitting and distributing an externally applied stress to the fibres.

Protecting individual fibres from surface damage. (abrasion or chemical)

Separating the fibres and preventing the propagation of brittle cracks from fibre to fibre.

Question 24

What the 4 key types of MMC’s characterised by the reinforcement?

Answer 24

Continuous fibre

Discontinuously reinforced (short fibres, whiskers or particulates)

Whiskers

Particulates

Question 25

What is the need for the reinforcement within a matrix? Give 7 objectives that are achieved.

Answer 25

Increase specific yield and tensile strength while maintaining acceptable ductility and/or toughness. Enhance creep resistance (at high temp). Higher fatigue strength (especially at high temp). Improves thermal shock resistance. Strengthens the corrosion resistance. Increases young's modulus. Reduces the thermal elongation (low CTE)

Question 26

What are the two ways to classify the reinforcing phase?

Answer 26

1. Large particles: particle/matrix interactions not on the atomic or molecular scale. * Particulate phase is harder and stiffer than the matrix. * Reinforcing particles restrain movement of the matrix phase in the vicinity of each particle. * Matrix transfers part of the applied stress to the particles, which bear a fraction of the load. Hence, degree of reinforcement (property improvement) depends on strong bonding at the matrix/particle interface. 2. Dispersion strengthened: particles normally much smaller, 0.01–0.1 μm diameter. * Particle/matrix interactions, i.e. strengthening, on an atomic or molecular level. * Strengthening mechanism similar to precipitation hardening. * Matrix bears the major portion of an applied load, the fine dispersed particles hinder or impede dislocation motion. Thus, plastic deformation restricted → yield/tensile strength & hardness improve.

Question 27

What are the typical reinforcement requirements?

Answer 27

Low density / high Young’s modulus, compressive and tensile strength Mechanical compatibility (low CTE but the reinforcement should also adapt to the matrix). Chemical compatibility (i.e. leading to optimal matrix/reinforcement adhesion but not corrosion issues) Thermal stability Good processability Economic efficiency

Question 28

What group of reinforcements give a balanced combination of properties and cost?

Answer 28

Discontinuous reinforced MMC's seem to offer a balance. Cost is lower, has good properties (young's modulus, CTE and wear). Long fibres and other types can be more costly for a slightly better performance but it all depends on the specific application.

Question 29

Give some typical MMC configuration examples?

Answer 29

Fibre (Graphite) - Matrix (Aluminium, Magnesium, Lead, Copper). Fibre (Boron) - Matrix (Aluminium, Magnesium, Titanium). Fibre (Alumina) - Matrix (Aluminium, Lead, Magnesium) Fibre (Silicon Carbide) - Matrix (Aluminium, Titanium, Co based Superalloy)