Ceramic-matrix composites Reinforcement of cements and concretes with short fil-

11.3.2.3 Ceramic-matrix composites Reinforcement of cements and concretes with short fil-

aments of glass, steel or carbon is applied in the build- ing and construction industries. Costs are very closely scrutinized in these industries and the increase in cost associated with fibre reinforcement is often regarded as unacceptable unless there is promise of exceptional properties. In one example, a 2% v/v dispersion of steel wires (low-carbon or austenitic stainless) in a

Figure 11.19 Stages of failure in CMC reinforced with

continuous fibres (from King, Dec 1989, pp. 720-6; by resistance to crack propagation and greater flexural

concrete matrix (Wirand Concrete 2 ) confers improved

permission of the Institute of Materials) . and compressive strength. These wires are typically 0.25–0.5 µ m diameter, with an aspect ratio of 100:1. The resistance of this composite to salt water and to

in a gradual (‘graceful’) manner, rather than suddenly pebble abrasion have led to its use for 40 t twin-fluked

and catastrophically. This overload capability is illus- shapes (dolosse) for shore defences against heavy seas.

trated schematically in Figure 11.19, which gives the Since the 1980s there has been a sustained inter-

tensile stress versus strain curve for a unidirectional est in the development of ceramic-matrix compos-

composite. It will be noted that the fracture strain ites (CMCs) for high-temperature applications that

of the unreinforced matrix is considerably less than demand exceptional strength and chemical stability

that of the composite. Superimposed on the curve (e.g. advanced engine designs, cutting tools). Ceramics

for the composite are the principal micromechanisms in general are inherently notch-sensitive and brittle: the

of failure, namely (1) microcracking of the matrix, introduction of fibres is primarily intended to improve

(2) interfacial failure as fibre pull-out takes place and the toughness of the matrix and reliability in service.

(3) fibre fracture.

(In polymer-matrix and metal-matrix composites, rein- In general terms, events taking place in the wake of forcement is used to improve strength, stiffness and,

each crack tip become more significant than flaws in if possible, toughness.) Currently, the principal matri-

the matrix. Thus, unbroken fibres bridge the cracks in ces are inorganic glasses and glass-ceramics but it

the matrix, oppose opening separation of these cracks is also possible to use engineering ceramics, such as

and reduce the effective stress intensity factor at the tip alumina and silicon nitride. Reinforcement materials,

of the crack. As the matrix cracks, load is increasingly which include silicon carbide (Nicalon, Tyranno), alu-

transferred to the fibres. A high E f /E m ratio and a high mina and carbon, have taken the form of continuous

content of fibres favour this transfer. Volume fractions fibres, short fibres, whisker crystals, woven fibre cloth

up to 0.5–0.6 are quoted in reports. and laminates.

Debonding at the fibre/matrix interfaces is clearly Failure in CMC structures is complex in character

an important aspect of the overall fracture process. and a variety of mechanisms has been identified. It is

Strong bonding assists the transfer of load from matrix not possible to apply directly the theory of fracture

to fibres and increases the amount of energy absorbed mechanics that was developed for monolithic ceramic

during fibre pull-out. However, if this bond strength bodies. Researches on CMC systems using continuous

is exceptionally strong, cracks may travel in an unhin- fibres primarily aim to achieve substantial improve-

dered, brittle manner through matrix and fibres alike, ments in toughness and reliability. To this end, there

with the CMC behaving like a monolithic ceramic. On is particular interest in structures that have a capac-

the other hand, if bonding is relatively weak, the course ity for accumulating damage and that eventually fail

of an incident crack may be deflected into the interfa- cial zone, with useful absorption of energy. For these

1 In the early 1980s, MMCs were classed as strategic reasons, special attention is given to ways of control- materials by the US government: samples and detailed

ling and/or modifying the surface character of fibres. information are consequently unavailable.

The degree to which the linear coefficients of ther- 2 Patent based on researches of Professor James P. Romualdi

mal expansion ⊲˛⊳ of fibre and matrix materials match of the Carnegie-Mellon University and the Battelle

is an important aspect of CMC design. For instance, Development Corporation, USA, in the 1960s).

a mismatch between ˛-values can favour the type of

Plastics and composites 373 crack-deflection process just described. Again, if short

fibres are used and there is a substantial mismatch in ˛ -values, cracks will tend to be initiated at the ends of the fibres. Thermal cycling will aggravate this type of damage. A mismatch in the expansion characterisics of well-bonded fibres and matrix can also introduce a ‘pre-stressing’ effect that inhibits crack growth. Thus,

if ˛ f >˛ m , then cooling from a high temperature will induce residual tensile stress in the fibres and residual compressive stress in the matrix. This imbalance has

a toughening effect because microcracks advancing in the matrix have to overcome compressive stress. Scanning electron microscopy provides valuable information on deformation and failure mechanisms in CMC structures. A sample of composite can be imaged in situ with high resolution while being sub- jected to tensile, bending, compressive or cyclic stress. Figure 11.20a shows a SiC fibre in a matrix of cal- cium aluminosilicate during an early stage of fatigue failure, with the fibre bridging the microcrack in the matrix. Eventually, as Figure 11.20b shows, the fibre

(a)

itself fractures after a certain amount of pull-out from the matrix.

Fabrication of CMCs is expensive, demanding very close control. During fabrication of a continuous fibre CMC, it is necessary to infiltrate the fibres, which may

be in the form of woven cloth, with a fluid form of the matrix-building material. The most common method uses a slurry of ceramic particles, a carrier liquid (water or alcohol), an organic binder and, sometimes,

a surface-wetting agent. After drying, the shape is hot- pressed. A range of alternative production routes is now available (e.g. sol –gel reaction, melt infiltration, chemical vapour deposition (CVD), reaction-sintering, transfer-moulding). Unique fabrication problems can arise. For instance, during high-temperature firing, chemical reaction at the fibre/matrix interfaces may cause weakening intermediate phases to form. With regard to industrial utilization, there is still a widely- accepted need to improve toughness and reliability further. Although some CMCs can be used in air at temperatures approaching 1000 °

C, oxidation is a

potential problem when carbon and SiC fibres are

(b)

used; fibre oxidation may occur at temperatures as low as 350–400 °

C. Coatings have proved effective in Figure 11.20 Field-emission scanning electron micrographs of fatigue cracking in SiC fibre/Ca aluminosilicate composite preventing this wasting attack and in raising the ceiling

(fibre diameter 15 µ m) (courtesy of M. D. Halliday, temperature for the composite. Alternatively, an inert

Interdisciplinary Research Centre, University of gaseous environment may be used but is obviously not

Birmingham, UK) .

always feasible. Glassy matrices are favoured because processing is generally simpler; unfortunately, it is possible for certain glasses to begin to soften in the

glass (substrate of laser mirrors), and graphite/glass- temperature range 500–800 ° C. ceramic (bearings, seals and brakes). The SiC

As the foregoing remarks indicate, a number of fibre/lithium aluminosilicate (LAS) glass composite is problems are associated with the production and uti-

a candidate material for new types of heat-engine. lization of ceramic-matrix composites. Commercial

exploitation of CMCs to date has consequently been rather restricted. Combinations that have found appli-