Effect of fibre orientation on strength Let us reconsider composites in which continuous

11.3.1.4 Effect of fibre orientation on strength Let us reconsider composites in which continuous

fibres are aligned in the same direction as the direction of applied stress. The strength of this highly anisotropic type of composite varies with the volume fraction of fibres in a linear manner, as shown in Figure 11.12b. If the fibres are now oriented at an

general effect is to reduce the gradient of the strength curve for values of V f greater than V min , as shown in Figure 11.14. This weakening effect is represented by

Plastics and composites 365

Figure 11.14 Effect of fibre orientation on strength of unidirectional composite (continuous fibres. .

Figure 11.15 Relation between failure mode, strength and equation to give:

fibre orientation (schematic diagram for unidirectional continuous-fibre composite) .

mode of failure is decided by the equation which gives In order to provide a more detailed analysis of the

cl . Thus, trans- variation of composite strength with fibre orientation, it is customary to apply a ‘maximum stress’ theory based on the premise that there are three possible

posite strength which is associated with the transition modes of composite failure. Apart from the angle of

from tensile failure in fibres to failure by shear. Combi- nation of the first two of the three equations eliminates

fl , the

cl and gives the critical angle for this transition:

⊲ 11.17⊳ of failure is represented by an equation which relates

ft . Each mode

If the longitudinal strength is about ten times the shear cl to a resolved stress.

strength of the matrix, then the angle is about 6 ° . For the first mode of failure, which is controlled by

When an application involves applied stresses tensile fracture of the fibres, the equation is

that are not confined to one direction, the problem

cl D fl sec 2 ⊲ 11.14⊳

of anisotropy can often be effectively solved or minimized by using continuous fibres in the form of

The equation for failure controlled by shear on a plane woven cloth or laminates. Although these forms are parallel to the fibres is:

more isotropic than unidirectional composites, there is inevitably a slight but usually tolerable loss in strength

and stiffness. Glass, carbon and aramid fibres are used; As the temperature is raised, this mode of failure

cl D m

sometimes two or more different types of fibre are used becomes more likely in off-axis composites because

in combination (hybrid composites). Fibre cloth in a variety of weave patterns is available. In a nominally

In the third mode of failure, transverse rupture two-dimensional sheet of woven cloth there is a certain occurs, either in the matrix or at the fibre/matrix

fl .

amount of fibre oriented in the third dimension. A more interface (debonding). The relevant equation is:

truly three-dimensional reinforcement, with improved through-thickness properties, can be obtained by

cl D ft cosec 2 ⊲ 11.16⊳

placing woven cloths on top of each other and stitching them together with continuous fibre.

Figure 11.15 shows the characteristic form of the Laminates based on carbon and aramid fibres are relation between composite strength and fibre orienta-

commonly used for high-performance applications tion. While illustrating the highly anisotropic character

that involve complex stress systems (e.g. twisting, of the unidirectional continuous reinforcement, it also

bending). The unit of construction is a thin ply of unidirectional composite, 50–130 µ m thick. Plies

tions, using the maximum stress theory, and experi- are carefully stacked and oriented with respect to mental results are in good agreement and confirm the

orthogonal reference axes (0 ° and 90 ° ). The simplest

fl ,

lay-up sequence is (0/90/90/0). Other more isotropic

ft are required for these calculations.) The

366 Modern Physical Metallurgy and Materials Engineering ⊲

into short lengths of 25–50 mm and oriented ran- is symmetrical about the mid-plane of the laminate in

domly in a plane. (e.g. chopped strand mat for press order to prevent distortion and to ensure a uniform

moulding). Interstage ‘sizing’ is often practised. The response to working stresses.

range of fibre diameters is 5–20 µ m. Although origi- Randomly-oriented short fibres of glass are com-

nally developed for electrical applications, E-glass is monly used in sheets and in three-dimensional mould-

the principal material for the production of continuous ings. In fact, fibre misorientation is quite common in

glass fibres. Other compositions are available, giving a composites, frequently being an unavoidable result of

higher modulus of elasticity (H-modulus glass), greater fabrication. For instance, when resins loaded with short

tensile strength (S-glass), or better alkali resistance fibres are injection-moulded, the mixture follows com-

(AR-glass), etc.

plex flow paths. If the finished moulding is sectioned, The principal methods for fabricating polymer- fibres will clearly indicate the patterns of flow: these

matrix composites are (1) hand lay-up or spray-up, patterns are determined by melt viscosity, mould con-

(2) press-moulding with heated matching dies, (3) vac- tours and processing conditions. Fortunately, flow pat-

uum moulding, (4) autoclave moulding, (5) resin terns are repeatable from moulding to moulding. Close

transfer moulding, (6) reinforced reaction injection to the surface of the moulding, the short fibres tend to

moulding (RRIM), (7) pultrusion, and (8) filament follow streamline paths; in the central core, where flow

winding. Broadly, these listed methods either bring is more turbulent, fibres are likely to assume transverse

fibres and ‘wet’ resin together during processing or orientations.

use pre-impregnated shapes (pre-pregs) in which fibres and thermoset resin are pre-combined. Pre-pregs are

11.3.2 Types of fibre-reinforced composite

made by infiltrating rovings, mats or woven fabrics

11.3.2.1 Polymer-matrix composites with resin and then heating in order to initiate partial curing. This B-stage of polymerization is preserved

Glass-reinforced polymers (GRP) date from the

1 early 1940s until processing by storing at a low temperature. Pre- and were the forerunners of present- pregs facilitate control of infiltration, orientation and

day polymer-, metal- and ceramic-matrix com- fibre content, making it possible to produce polymer- posites. A typical fabrication procedure for a matrix composite (PMC) of repeatable quality on an

GRP is to add a mixture of polyester resin, automated mass-production basis. In each of the above curing agent and catalyst to fibres of low-

methods there is need to prevent air or vapours becom-

ing trapped in the composite and forming weakening 10B 2 O 3 –5MgO). The thermosetting reactions of curing

alkali E-glass (typically 53SiO 2 –18CaO–14Al 2 O 3 –

voids. Voids form preferentially at fibre/matrix inter- are then allowed to take place at a temperature below

faces and between the plies of layered composites. 150 °

C. The relatively low cost, stiffness and ease of Low-viscosity resins, outgassing and high pressures fabrication of GRP led to their widespread adoption in

are some of the means used to minimise this porosity. engineering, even for large structures (e.g. storage tanks

Polyester-based matrices remain the principal and silos, mine counter-measure vessels).

choice for polymer-matrix composites. In the 1970s, Glass fibres for composites are made by allow-

polyesters became available in the form of moulding ing molten glass to pass through the nozzles of an

compounds that are particularly suitable for hot press- electrically heated bushing made from Pt-10Rh alloy.

moulding. Dough moulding compounds (DMC) and (The number of nozzle holes in the base of the bush-

sheet moulding compounds (SMC) contain roughly ing is 204, or a multiple of 204.) The filaments

1 emerge at a velocity of 50–100 m s equal volume fractions of polyester resin, inert filler and are rapidly particles and chopped glass fibres. When heated, cooled with a water mist to prevent crystallization,

these compounds rapidly become fluid, reproduce the hauled over a ‘size’ applicator and finally collected

contours and details of the moulding dies accurately by a rotating cylinder (collet). ‘Sizing’ applies a coat-

and then cure. This technique is used for domestic ing which loosely bonds the fibres, protects their

articles, panels and doors of vehicles, cabinets for fragile glass surfaces from damage and introduces a

office and electronic equipment, etc. surface-modifying ‘coupling’ agent to promote even-

As part of the search for cheaper fabrication meth- tual fibre/matrix bonding. The primary bundle of con-

ods, much effort has been devoted to the develop- tinuous, untwisted fibres is the unit of collection from

ment of PMCs with thermoplastic matrices (e.g. nylon the bushing and is known as a strand. That is, it con-

66, PP, PTFE, PET, polyether sulphone (PES), etc). tains 204, 408, 816 or more fibres. (The equivalent

For example, water boxes of car radiators and shell unit for carbon fibres is called a tow.) Strands can be

housings for street lamps have been made from a combined to form a larger bundle (roving). Strands or

composite of 33% glass fibres in a nylon 66 matrix rovings are used for unidirectional composites and as

(Maranyl ). In the early 1980s, pre-pregs made of yarn for weaving; alternatively, they can be chopped

polyether ether ketone (PEEK) reinforced with PAN- derived carbon fibres (APC2 ) became available: how-

1 First used in quantity for aircraft radomes which required ever, they are costly and use tends to be restricted strength, low electrical and thermal conductivity and

to highly-specialized applications (e.g. aircraft com- ‘transparency’ to radar waves.

ponents). The general advantages of a thermoplastic

Plastics and composites 367 matrix are its toughness, indefinite shelf life and, in

form graphite crystallites during heat-treatment at tem- the absence of curing, a shorter time cycle for fab-

C to more than 2000 ° C. rication. However, during the necessary heating, the

peratures in the range 1300 °

Raising this treatment temperature encourages graphi- viscosity of the matrix is higher than that of a ther-

tization and improves the elastic modulus but lowers moset resin, making infiltration between fibres more

the strain to failure. Finally, the fibres are surface- difficult. Furthermore, the pre-pregs are stiff and lack

treated (e.g. electrolytic oxidation) to improve subse- the drapability of thermoset pre-pregs which enables

quent bonding to the matrix and ‘sized’ to facilitate them to bend easily into shape.

handling.

For the exacting requirements of aerospace and Each carbon fibre produced is very pure and con- high-performance aircraft, the principal PMCs have

sists essentially of interwoven ‘ribbons’ of turbostratic epoxide matrices reinforced with continuous fibres of

graphite (Figure 11.16a) and some amorphous carbon. carbon or aramids (Kevlar ). The basic advantage of

The ribbons are aligned parallel to the fibre axis. Being epoxides is that they can be used at higher service

an imperfect structure, the amount of porosity is appre- temperatures than polyester matrices. Although the

ciable. In general terms, a-axes of the planar crystal- T g value of a polymer provides an indication of its

lites are parallel to the fibre axis, the other a-axis is temperature ceiling, it is substantially higher than the

radial or circumferential, and the c-axes are perpendic- maximum temperature for safe service under load.

ular to the fibre axis. As the structure becomes more For instance, the maximum temperature for a load-

truly graphitic, the ribbon orientation approaches that bearing epoxide matrix is about 160 °

C, whereas the

of the fibre axis and the axial modulus increases. The corresponding T g values lie in the range 200–240 ° C fibre structure is highly anisotropic: the moduli of elas-

(depending upon the method of determination). The ticity along the fibres and perpendicular to the fibres search for matrices with a higher temperature capabil-

are 200–800 GN m 2 and 10–20 GN m 2 , respec- ity has led to the development of bismaleimides (BMI)

tively. (The modulus for E-glass is about 73 GN m 2 .) and polyimides (PI). These and other new polymers

Another textile, rayon, is used as a precursor for raised the ceiling temperature closer to 200 °

carbon fibres, but to a lesser extent than PAN. Alter- sometimes introduced a brittleness problem and they

C but have

natively, melt-spun filaments of high-purity, meso- can be difficult to process.

phase pitch can be oxidized and pyrolized in a process Carbon fibre reinforced polymers (CFRPs) are

broadly similar to the PAN process to yield car- firmly established as construction materials for

bon fibres with a very high modulus approaching specialized and demanding applications (e.g. helicopter

1000 GN m 2 . Pitch-derived carbon fibre is expensive, rotors, monocoque chassis of racing cars, aircraft floor

more difficult to handle than PAN-derived fibres and panels, spacecraft components, sports goods, high-

its use is confined to specialized applications. speed loom components). Laminates of continuous

Aramid fibres (Kevlar 29 and 49, Twaron) based carbon fibres (Grafil) are widely used. Carbon fibres

on aromatic polyamides are important reinforcements are also used in metal-matrix and ceramic-matrix

for polymers. Their linear molecular structure composites. Frequently, they are combined with other

(Figure 11.16b) is produced from spun fibre by a types of fibre to form hybrid composites (e.g. glass

process of drawing and heating under tension at and carbon, aramid (Kevlar) and carbon). Carbon

C. This linear fibres, 5–10 µ m diameter, are available in untwisted

a temperature of approximately 550 °

structure, which contrasts with the more planar tows containing 1000, 3000, 6000, 12000 or 120000

structure of carbon fibres, gives aramid fibres a fibrillar filaments and as pre-pregs with resin. In the UK and

character and they can absorb considerable amounts the USA they are mostly produced from the textile

of impact energy. When struck with a projectile, polyacrylonitrile (PAN) and its copolymers. Many

aramid fibres split into numerous microfibrils, giving types of PAN-derived carbon fibre are produced (e.g.

exceptional ‘stopping power’. This property has commodity, high-modulus, high-extension, etc.).

1 The three-stage process led to the use of aramid fibres and aramid/resin for manufacturing carbon laminates for ballistic applications (e.g. armour). fibre is based on the controlled degradation or pyrol-

The elastic modulus is 50–130 GN m 2 and it is ysis of spun fibres of PAN. Hot-stretching is a central

C (depending feature of processing: it counteracts the tendency of the

stable at temperatures approaching 400 °

upon the environment). Their mechanical properties fibres to shrink and induces a high degree of molecular

are degraded by ultraviolet radiation; nevertheless, orientation. The tow is first oxidized under tension at a

aramid fibres are widely used, particularly in hybrid temperature of 200 °

C and forms a stable, crosslinked

composites.

‘ladder’ structure. In the second stage, heating in an The rapid deceleration of racing cars and landing inert atmosphere at temperatures between 800 °

aircraft develops very high frictional forces and tem- 1600 °

C and

C carbonizes the structure, releasing vapours and peratures in braking systems: this challenge has been gases (hydrogen, nitrogen) and reducing the original

met by composites in which a carbonaceous matrix is mass by 40–50%. Finally, the oriented carbon fibres

reinforced with carbon fibres. These carbon–carbon composites combine the refractory potential of car-

1 Originally developed at the Royal Aircraft Establishment bon with the high specific strength/stiffness of car- (RAE), Farnborough, in the 1960s over a five-year period.

bon fibres. PAN or pitch are used as precursors when

368 Modern Physical Metallurgy and Materials Engineering

Figure 11.16 Structure of (a) carbon fibre and (b) aramid fibre (from Hughes, June 1986, pp. 365–8; by permission of the Institute of Materials) .

high-modulus fibres are required. Chemical vapour

11.3.2.2 Metal-matrix composites deposition (CVD) can be used to produce the matrix

Large-scale research and development studies of phase: hydrocarbon gas infiltrates the carbon fibres and

metal-matrix composites (MMCs) date back to the is thermally ‘cracked’ to form a matrix of pyrolytic

1960s, being stimulated at that time by the new graphite. C–C composites maintain their strength at

high temperatures and have high thermal conductiv- availability of carbon and boron fibres, whisker ity and excellent friction/wear characteristics. They are

crystals and, more indirectly, by successes achieved used for aircraft and racing car brakes, rocket noz-

with reinforced polymer–matrix composites. The aero- zles and the heat shield of the Space Shuttle. In the

space and defence industries were attracted by the presence of oxygen, C–C composites begin to oxi-

prospects of a new type of constructional material dize and sublime at relatively low temperatures, say

possessing high specific strength/stiffness. The fall 400 °

C. Efforts are being made to develop long-life in strength produced by rising temperature is more multilayer coatings for this type of composite that will

gradual in MMCs than with unreinforced matrix inhibit oxygen diffusion, ‘self-heal’ and enable service

material, promising higher service temperatures. In temperatures in oxidising environments to be raised to

addition, the MMC concept offers prospects of unique 1400–1500 ° C. wear resistance and thermal expansion characteristics.

Plastics and composites 369 For example, parts of space structures are required

to maintain dimensional stability while being cycled through an extremely wide range of temperature. One criterion of performance is called the thermal deformation resistance, which is the ratio of thermal conductivity to the coefficient of thermal expansion. The expansion coefficient of graphite fibres with a very high elastic modulus is negative, which makes it feasible to design continuous-fibre MMCs with a zero coefficient of thermal expansion.

Typical early versions of MMC were boron fibres in titanium and carbon fibres in nickel. Over the years, interest has ranged from continuous and dis- continuous fibres to whisker crystals and particles. The nominal ranges of diameter for continuous fibres, short fibres and whiskers, and particles are 3–140 µ m, 0–20 µ m and 0.5–100 µ m. Continuous unidirectional fibre-reinforcement gives the greatest improvement in properties over those of unreinforced matrix material. Interest in whisker reinforcement, once very great, has tended to decline because of the carcinogenic risks associated with handling small whiskers during manufacture and composite fabrication. At present, particle-reinforced MMCs find the largest industrial application, being essentially isotropic and easier to process. Matrices based upon low-density elements have gradually come into prominence. The principal matrix materials are aluminium and its alloys, titanium and its alloys, and magnesium. Most MMCs are based on aluminium and its alloys. In recent years, cheaper particles and fibres of silicon carbide have become available, making them the commonest choice of rein- forcement material. Alumina (Saffil) reinforcement is also used in many MMCs.

Figure 11.17 illustrates the considerable improve- ments in specific tensile strength (longitudinal) and specific modulus that result when aluminium alloys are reinforced with fibres, whiskers or particles. Property changes of this magnitude are unlikely to be achieved by more orthodox routes of alloy development. At the same time, as the diagram shows, these changes are accompanied by rising costs and bring anisotropy into prominence. The ratio of longitudinal strength to transverse strength can be 15:1 or more for MMCs. Although continuous-fibre reinforcement can confer maximum unidirectional strength, service conditions frequently involve multi-axial stresses.

A wide and growing variety of methods is avail- able for producing MMCs, either as components or as feedstock for further processing (e.g. billets for extru- sion, rolling, forging). Many of these methods are still on a laboratory or development scale. In general terms, they usually involve either melting of the matrix metal, powder blending or vapour/electrodeposition. Particle- reinforced aluminium matrix composites can be made by (1) pressing and sintering blends of pre-alloyed powder and reinforcement particles (powder blending), (2) mechanical alloying (MA), (3) mixing particles with molten metal (melt-stirring), (4) compocasting (rheocasting) and (5) spray co-deposition.

Figure 11.17 Range of longitudinal tensile specific strengths and moduli achieved in aluminium-based composites (from Feest, May 1988, pp. 273–8; by permission of the Institute of Materials) .

Powder blending techniques (1) are favoured, rather than melting, when the metallic matrix has a high melting point, thus minimizing the fibre/matrix interac- tion problem. After the critical blending operation, the MMC powder is canned, vacuum degassed and consol- idated by hot-pressing or HIPing. Finally, the MMC is worked and shaped (e.g. extrusion, forging). Particle volume fractions of 0.25–0.50 are typical. Mechan- ical alloying is broadly similar to powder blending; the essential difference is that alloying of pure metal powders is achieved in high-speed ball mills. In the melt-stirring technique (3), the presence of particles raises the melt viscosity. Possible difficulties include non-wetting of particles, agglomeration and/or gravity- settling of particles and unwanted particle/metal inter- actions. In the related compocasting process, the stirred melt is maintained in a two-phase ‘mushy’ state at tem- peratures between solidus and liquidus. The method is not applicable to metallic systems with a narrow range of solidification.

Production of particle-reinforced MMCs by spray co-deposition (5) is based upon a versatile process 1 originally developed for building deposits of steels that are difficult to cast, nickel-based superalloys and copper. A stream of induction-melted metal or alloy is broken into fine droplets by relatively cold inert gas (nitrogen). Droplets begin to freeze before striking a movable collector placed in the line of flight. In the MMC variant, reinforcement particles are

1 Patented by Osprey Metals, adopted under licence and further developed by Alcan International Ltd, Banbury.

370 Modern Physical Metallurgy and Materials Engineering injected into this stream. Fine-grained MMC deposits

size and shape of product. Particle-reinforced MMCs

can be produced. Using a single preform and one plate, tube, billets for hot-working, cladding, etc. The

of SiC/aluminium alloy and Al 2 O 3 /steel can be built as

injection shot, it is possible to vary the volume fraction pathogenic risks associated with certain types of fine

and type of fibre (continuous, discontinuous) within a particles, fibres and whiskers have necessarily been

component.

taken into consideration. Filaments of 0.1–3 µ m diam- With regard to applications, the emphasis in the eter and with lengths greater than 5 µ m are hazardous.

aerospace industry is the development of MMCs based Accordingly, SiC particle sizes exceed a certain thresh-

upon titanium or intermetallic compounds such as TiAl old (e.g. 5 µ m).

and Ti 3 Al. For example, coated SiC fibres are used The fabrication of continuous-fibre reinforced

to reinforce Ti-6Al-4V alloy. Proposed applications MMCs is difficult and technically demanding. Fibre

for MMCs include compressor discs and blades in preforms with a high volume fraction of fibres are

aero-engines, engine pylons and stabilizers. Channel difficult to ‘wet’ and infiltrate; on the other hand, if

extrusions made from discontinuous-fibre reinforced

V f is low, the preform will lack the ‘green’ strength MMC are in use for electrical racking in aircraft. required for handling. A high melt temperature will

In general manufacturing, there is interest in using lower viscosity and assist infiltration of the fibres

SiC fibre-reinforced aluminium for critical components but increases the risk of fibre/metal reactions. (SiC,

operating at very high speeds where high specific Al 2 O 3 and carbon fibres react with aluminium alloy matrices at a temperature of 500 °

stiffness would be mechanically advantageous (e.g.

carpet-making, food packaging, textile production). risk that secondary working operations will damage

C.) There is also a

Co-sprayed steel matrix deposits have been used for the fibres. In general, the methods for continuous

components of plant handling highly-abrasive materi- fibres are costly, give low production rates and limit

als (e.g. coke, minerals, wood, fibreboard). the size and shape of the MMC product. Typical

In the car industry MMCs are now accepted as methods for producing continuous-fibre MMCs are

candidate materials for valve rocker arms, connect- (1) diffusion bonding (DB), (2) squeeze-casting, and

ing rods, gear selector forks, pulleys, propshafts, etc. (3) liquid pressure forming (LPF).

They have also made it possible to replace cast Diffusion bonding (1) can be used to consolidate

iron engine cylinder blocks with selectively-reinforced

blocks of aluminium alloy (Honda). The hybrid com- carbon. Fibres are pre-coated with matrix material or

metal with continuous fibres of SiC, Al 2 O 3 , boron and

posite, which contains 12% v/v alumina particles (for carbon. The process conditions must achieve a delicate

strength) and 9% v/v carbon fibres (for lubricity), saves balance between promotion of solid-state diffusion and

weight, improves the power rating and dispenses with limitation of fibre/matrix reactions. DB has been used

cast iron cylinder liners.

to produce boron/aluminium struts for the NASA space

shuttle and SiC/titanium alloy composites. In diesel engines the achievement of higher In principle, squeeze-casting (2) is a single-shot

combustion efficiencies, with better fuel economy and process combining gravity die-casting with closed-

reduction in the exhaust emissions of undesirable die forging. It is mainly used for discontinuous-fibre

gases and particulate matter, has resulted in higher MMCs. A metered charge of melt containing short

temperatures and peak pressures in the combustion fibres is fed into the lowermost of a pair of dies

chambers. The standard alloy for diesel pistons is and then compressed at high pressure (35–70 MN

Al –12Si –1Cu–1Mg (LM 13 in BS1490: Lo-Ex). m 2 ) by the descending upper die. The pressure is

Although this eutectic alloy is satisfactory for a maintained while the charge solidifies. Interpretation of

C, advances the relevant phase diagram for the matrix has to allow

working temperature range of 250–300 °

in engineering design have raised crown temperatures for the pressure variable. Boundaries in the diagram

C. For instance, in one relatively new are shifted and, for alloys that contract on freezing,

to 300–350 °

design feature, now established for direct-injection the liquidus temperature is raised. With aluminium

engines, a deep combustion bowl is located in the alloys, this shift is about 10–25 °

crown of each piston. This turbulence-inducing cavity pressure has an undercooling effect which, together

C. Thus, the applied

usually has re-entrant angles and sharp edges. The with the loss of heat through the dies, favours rapid

higher mechanical and thermal loading on the bowl lip solidification. The high pressure also discourages the

can cause cracking. This problem has been solved by nucleation of gas bubbles. The final matrix structure

squeeze-casting the piston body and, at the same time, is accordingly fine-grained and dense. Moderate-sized

incorporating a fibre-reinforced bowl. A controlled components can be produced at high rates.

quantity of aluminium alloy melt is fed into an In liquid pressure forming (3), a preheated fibre

open die, which contains a preformed compact of preform is placed within split dies. The dies are closed

alumina fibres, and then compressed with a pressure and air pressure in the cavity reduced below 1 mb.

of 120–150 MN m 2 by a hydraulically-actuated plug Molten aluminium casting alloy under pressure is then

die, so that the fibres are infiltrated, ‘wetted’ and infiltrated upwards through the preform and allowed

2 The first engine patent of Rudolf Diesel (1858–1913) was than squeeze-casting and there is less limitation on

to solidify. LPF uses a lower pressure (<8 MN m 2 )

officially authenticated in 1893.

Plastics and composites 371 bonded to the alloy. (The fibres are 2–4 µ m diameter,

structures that tend to make fabrication difficult and 200–500 µ m long, and occupy at least 20% v/v of

service performance questionable. the composite) Squeeze-casting prevents shrinkage and

During fabrication, formation of the matrix often dissolved gases from causing microporosity and gives

necessitates a high processing temperature (e.g. melt

a fine-grained structure with better high-temperature infiltration). This temperature can be high enough to fatigue properties than gravity-casting. The MMC

promote unwanted chemical reactions at the interfaces structure provides the bowl lips with the necessary hot

between matrix and reinforcement. These conditions strength and resistance to cracking.

may arise during service at elevated temperatures, of Figure 11.18 shows a typical aluminium alloy piston

course. SiC-reinforced titanium is an example of a design, with local reinforcement of the combustion

composite in which brittle interfacial products tend to bowl and the top ring groove. Accepted practice for

form (i.e. silicides and carbides). Ideally, the aim is the large pistons of heavy-duty and turbocharged diesel

to develop a strong interfacial bond without degrading engines is to reinforce this groove, and possibly the

the fibre or forming weakening phases at the interfaces. second groove, with a wear-resistant insert of nickel-

The coating of fibres is generally regarded as the most rich, and hence austenitic, cast iron (Ni-Resist) which

promising means to control chemical interaction at is bonded to the aluminium alloy body by the Alfin

reinforcement/matrix surfaces. However, it is difficult process. In small high-speed indirect-injection diesel

to develop a coating with long-term stability. engines, which have a pre-combustion chamber, lighter

As might be anticipated, the deliberate introduction and cheaper MMC inserts have been used successfully

of short stress-concentrating fibres into a metal or alloy to reinforce the groove. The performance of this insert

tends to reduce the ductility and toughness below that is crucial because modern designs are tending to locate

of the unreinforced matrix. When a short-fibre MMC the top groove much closer to the crown, even in the

is deformed by applied stress, the amount of strain in ‘headland’ position, thus reducing the volume of the

fibre and matrix may differ substantially, leading to annular ‘dead space’ above the ring and giving fuel

rupture of the interfacial bonding at the end regions economy and reduced emissions.

of fibres. This debonding results in the nucleation and When considered in terms of the costly effort that

rapid growth of voids. Void formation is the dominant has been put into development of MMCs, commercial

mode of tensile failure in SiC/aluminium composites. exploitation has been disappointingly limited. MMC

A difference in the coefficients of thermal expansion of structures and components face strong competition

fibre and matrix can produce the same effect, possibly from alternative, more conventional engineering mate-

during the cooling stages of composite fabrication or rials. Exploitation has been mainly hindered by the

during thermal excursions and cycling in service. Thus, high cost of reinforcement materials, particularly con-

because ˛ f <˛ m , fibres and particles develop residual tinuous fibres, and fundamental features of MMC

compressive stress and the matrix is left in a state of tension. This disparity in expansion characteristics between the reinforcement phase and the matrix also helps to generate a high density of dislocations in the matrix. Plastic deformation of the matrix involves dislocation movement across slip planes. If a slip plane is threaded by fibres, a glissile dislocation will bow between them and recombine beyond the fibres, leaving an Orowan loop around each fibre. These loops reduce the effective gap between fibres and successive dislocations are forced to assume smaller radii of curvature as they bypass the fibres. In combination with a high dislocation density and a fine matrix grain size, this mechanism can produce a high rate of initial strain-hardening in the matrix.

The presence of extremely hard reinforcement par- ticles or fibres can benefit wear-resistance in service but leads to difficulties during the finish-machining operations that are usually needed for MMC products. These particles act as chip-breakers during machin- ing of materials, such as SiC-reinforced aluminium alloys, and reduce cutting forces: unfortunately, they also shorten tool life significantly, even when tools are tipped with polycrystalline diamond (PCD). Machin-

Figure 11.18 Localized MMC reinforcement of piston head ing tends to generate sub-surface damage, a matter of (courtesy of R. Munro and AE Piston Products, T & N

special concern in the preparation of mechanical test- Group) .

pieces.

372 Modern Physical Metallurgy and Materials Engineering Despite these formidable problems, the basic idea

of reinforcing a strong deformable matrix with elastic fibres retains its appeal to the aerospace, defence and automotive industries and active research on MMCs continues worldwide. There is a need to expand and consolidate the database for properties of MMCs (e.g. fracture toughness, fatigue resistance, corrosion resis- tance, etc.). 1