Fibres of high thermal stability

Fibres of high thermal stability J. A V E S T O N *

A number of alternatives to glass fibre have been developed over the last decade with the object of improving the stiffness-to-weight ratio of resinand aluminium-based composites. They are all at least one hundred times more expensive than glass, so their use is currently confined to areas where cost is of secondary importance such as the boron-epoxy wings for the F 111 aeroplane and top quality sports equipment, or where the cost can be recovered in operating economies, as in the RB211 engine.

Their room temperature strengths are generally inferior to glass fibre and so they offer no advantage for improving the strength of resin or ductile-metal matrices. On the other hand their stiffness is some six times higher than glass, ie greater than most bulk ceramics and virtually all metals, so that increases in both the strength and stiffness of these materials become feasible. However all the fabrication processes for ceramics (and most of those for metals) require high temperatures, so the fibre must be thermally stable even when the materials are intended for roomtemperature application. If the material is required for high temperature use, the problems are more acute: the fibre must have satisfactory long-term mechanical properties at the operating temperature, be wetted by the matrix and yet be compatible with it over long periods, and unless it can be completely encapsulated it must also be resistant to oxidation. There are no fibres that fulfil these conditions above 1 000°C, but if such a fibre can be developed it will have far-reaching effects, particularly in the aerospace industry. For example an increase in the operating temperature of a gas-turbine blade from the present limit (around 950 °) to 1 200 ° would enable the specific thrust to be increased by 10% for only a small increase in fuel consumption and could result in a single manufacturer dominating the market. Moreover if the fibre could be produced at a reasonable cost our whole concept of ceramics and their associated brittleness might eventually be changed. The microstructural properties required for a strong solid have been considered in detail by Kelly 1 . In this article we shall briefly consider those factors that affect high-temperature strength of fibres and then describe those fibres that are commercially available and have at least some thermal stability. We shall not be concerned with various low-strength fibres that have been developed for insulation and ablation applications. * National Physical Laboratory, Teddington, Middlesex, England 290

COMPOSITESSeptember 1970

STItENG TH A rough estimate of the theoretical cleavage strength of a perfect solid, o,,, is given by m

10 where E is Young's modulus, ~ the surface energy and a is the distance between cleavage planes. If the material contains surface cracks or internal flaws the theoretical strength is reduced to the Griffith value, which is roughly (depending on crack geometry) that obtained by substituting the crack length for the atomic spacing (a) in the above equation. When the crack is sharp, as is normally the case with glass and ceramics, the material will fail at the Griffith stress, so that for example a crack a mere four atomic spacings deep can halve the strength of a brittle material. The common metals are less sensitive to cracks because they contain mobile dislocations that permit a blunting of the crack tip, but only at the expense of an approximately ten-fold reduction in strength from the theoretical and of introducing other problems such as fatigue. We can limit the crack dimensions and control the weakening mechanism of dislocations by preparing the material in the form of a fine fibre. Metals that are substantially free from dislocations can be grown in whisker form and metal wires can be strong when the grains are elongated and the dislocation motion is limited by the high degree of work hardening. However if we are to control the flaw size by making a fine fibre we may as well use a ceramic, which at any given temperature has fewer dislocations than a metal. Of course at temperatures greater than about one half the absolute melting point single crystals of ceramics do start to undergo plastic flow to a limited extent. This plastic flow allows the stresses at the crack tips to be reduced as with metals and a useful gain in short-term strength can be retained. The strengths of some polycrystalline materials, notably aluminia, also pass through a

maximum as the temperature is raised but eventually fail in a brittle manner owing to crack nucleation at grain boundaries where slip is blocked. However, for engineering applications at high temperature, the stress rupture life and creep behaviour are more important than the short-term strength obtained from a conventional tensile test. The designer wants to know how long a material will withstand a specific stress at a given temperature and the corresponding dimensional changes after this time. For long-term strength at high temperatures we require a material with a high melting point, a large E, a small a and containing a high density of strongly directional bonding. Fortunately these requirements are all combined in some of the lightest and most abundant elements and their compounds, ie those of boron, carbon, nitrogen, oxygen, aluminium and silicon, and the problem thus reduces to producing these in fibrous form. Attempts have been made to produce fibres from virtually all the binary combinations of these elements that yield a chemically stable solid, as well as from the solid elements themselves, and the fibres that are commercially available are all derived from these materials. The choice of grain size is less straightforward: the sources of internal stress concentration should decrease with increasing grain size and in practice the strength of ceramics generally increases with decreasing grain size. On the other hand the Notarro-Herring creep of a polycrystalline ceramic is inversely proportional to the square of the grain size. Table 1

COMMERCIAL FIBRES Having considered the conditions for thermal stability we may now ask what is actually available at present? The answer, it must be admitted, at least for very high temperatures, is very little. The mechanical properties of the majority of strong non-metaUic fibres that have been produced in significant quantities are listed in Table 1 and the limiting temperature for long-term use under oxidizing conditions for those fibres that are commercially available is roughly related to the cost in Fig 1. As the fibres were developed essentially to provide increased strength and stiffness in composites for use at ambient temperatures, it is hardly surprising that market forces should dictate that these properties should be closely related to price, but it is a rather unfortunate coincidence that the thermal stability should be likewise related. The cost of the raw materials is, at the most, a few shillings per kilogram and so the great expense of the majority of the fibres clearly stems from slow production rates coupled with high unit costs of the plant. Glass can be cheaply converted to fibres because its viscosity decreases continuously as the temperature is increased through the melting point to give a viscous liquid from which a 200 filament tow can be drawn at several thousand metres a minute. Silica can similarly be drawn into extremely strong fibres but the price is an order of magnitude greater as there is no material that will contain silica in air at its melting

Properties of thermally stable fibrous reinforcements

Fibre

Meltin_g° point [ C]

Continuous and semi continuous E glass Silica Alumina, single-crystal Carbon type I Carbon type II Boron nitride Boron tungsten Silicon-carbide/boron /tungsten Silicon-carbide/ tungsten Boron-carbide/ tungsten Titanium diboride/ tungsten Tungsten wire Beryllium wire Discontinuous and whiskers Chrysotile asbestos

loses H20

Density

Approximate

Strength [GN/m 2] *

Modulus [GN/m 2 ]

Specific strength [GN/m 2 ]

Specific modulus [GN/m 2 ]

Producer

700 1 660 2 072 3 650 3 650 2 980 2 300

2.55 2.19 3-96 1-90 1.90 1-90 2-63

3.5 6.0 2-0 2.0 2.6 1-4 2-8

72 72 470 390 240 90 380

1.4 2.7 0.5 1.1 1.4 0.7 1.1

28 33 118 264 126 47 145

Generally available General Electric Tyco Cou rtau Ids Cou rtau Ids Carborundum United Aircraft

2 300

2.70

2.8

380

1-0

140

United Aircraft

2 200 (sub)

3-35

2.3

470

0.7

140

General Technologies

2 450

2.36

2-3

470

1.0

200

General Technologies

NA

2 980 3 400 1 280

4.48 19-4 1-83

1-0 4.0 1.3

510 410 240

0.2 0-2 0.7

114 210 131

General Technologies

NA

General Electric Beryllium Corporation

2.55

4-5

164

64

Turner Bros Asbestos

0.1

Cape Asbestos

0" 1

--~500 loses H20 --~ 300 2 072

3"37

2.8

180

53

3-96

2--20

470

118

Thermokinetic Fibres

whiskers

2 200 (sub)

3-17

2--20

470

150

Carborundum

Silicon-nitride whiskers

1 900 (sub)

3-18

1--10

380

120

ERDE

Crocidolite asbestos Alumina whiskers Silicon-carbide

cost [£/kg]

0.5 28 72 000 60 50 280 270 400 2 700

90 9 000

7 000 100

Key * 1GN/m 2 = 145 000 Ibf/in 2

COMPOSITES September 1970

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FIG 1 Cost and limiting operating temperature in air for fibres that are commercially available

point and the fibres are usually drawn from a silica rod which is fed into an oxy-hydrogen flame. Patents exist for potentially cheaper processes, but silica, like glass, suffers from the disadvantage of a low Young's modulus so that metal-matrix reinforcement can only be obtained at the expense of high plastic strain of the matrix with its associated fatigue problems and no reinforcement of ceramic matrices is possible at all. At the top of the league of those materials that can be drawn from the melt comes alumina but here the conditions are very different. Alumina melts sharply at 2 072°C to give a mobile liquid. Container materials must be maintained in an inert atmosphere and as the growth takes place below the melt surface at the tip of the advancing filament the rate is limited to a few centimetres per minute. Boron and silicon-carbide fibres produced by vapour deposition are somewhat cheaper, but still expensive by ordinary standards and likely to remain so. The method (Fig 2) consists of decomposing a volatile compound such as boron trichloride, BC13 or trichloromethylsilane, SiCH3CI3 in hydrogen carrier gas onto an electrically heated tungsten wire. The expense stems from the laborintensive nature of the process and the high cost of the tungsten substrate. Efforts are being made to substitute cheaper silica or graphite substrates but there remains the difficulty of fibre breakage and laborious rethreading which limits the capacity of the apparatus to a single fibre. The limiting service temperature of boron under oxidizing conditions (around 500°C above which the protective boric oxide film breaks down) may be extended to 700°C by applying a thin coating of silicon carbide in a final plating stage, but for really high temperature applications the whole fibre must be produced in silicon carbide. By analogy with the mechanical and oxidation properties of bulk silicon carbide, this fibre should be useful in air to at least 1 400°C but in practice the tungsten core begins to react with the silicon carbide as low as 900°C and the strength of heat-aged fibres is reduced by stress concentration originating in this reaction zone (Fig 3). Silicon carbide whiskers are, at first sight, an attractive alternative, especially as prices as low as £2 per kilogram have been forecast2 but in common with all whiskers there are compensating disadvantages. With silicon carbide only a small weight fraction (the thin ones) are any stronger than the monofflament and the cost of sorting and alignment must be added to the cost of the crude product. A more serious problem, stemming from the low fibre pull-out

298

COMPOSITESSeptember 1970

length and correspondingly low work of fracture of whisker reinforced brittle matrices, suggests that whiskers may be more useful for the reinforcement of metals where the matrix can contribute to the toughness. Silicon carbide monofilament is more expensive than boron because the deposition rate is lower and it has been produced on a smaller scale but if the substrate problem can be overcome it should provide an attractive alternative to single crystal alumina for very high temperature applications. Boron on the other hand seems unlikely to survive in competition with high-modulus carbon fibre as a resin reinforcement, although it could have some future as a light alloy reinforcement in the medium temperature range. Carbon fibres would be expected to retain much of their strength and stiffness to around 2 000°C but they do not seem well suited for high-temperature composites. They react at high temperature with all technologically interesting metal matrices and their extremely anisotropic expansion coefficient precludes the application of a ceramic coating to provide a barrier layer and protect them from oxidation. They can with difficulty be incorporated into ceramics but the materials are only oxidation resistant if the fibres are completely enclosed. Thus the fibre content of the composite shown in Fig 4 could be estimated quite accurately from the weight loss after an hour at 700°C in air!

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FIG 2

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FIG 3 Fracture surface of heat-treated silicon-carbide fibre showing fracture origin at core

No article on thermally stable fibres would be complete without some mention of asbestos. From the Stone Age, when it was used to reinforce cooking utensils, to the introduction of glass fibre, it was the only heat-resistant fibre available, and even today the production far exceeds that of glass and all the remaining inorganic fibres put together. The mineral occurs as closely packed bundles of fibres with an ultimate size of 0.01 - 0.2#m (depending on the variety of asbestos) which have been formed by recrystaUisation of the host rock. The strength of even quite large bundles of fibres may be as high as 7 000MN/m 2 (106 lbf/in 2) because the adhesion between the fibres is low and a crack initiated at a surface flaw is repeatedly deflected along the fibre axis so that the fracture of the type shown on the left in Fig 6 results. Unfortunately when the fibres are heated, the bonding between them is strengthened to the point where a crack is no longer deflected, so that a loss of strength occurs at temperatures well below that at which any structural change in the fibre takes place.

FUTURE DE VEL OPMENTS We have shown that high specific strength at elevated temperature is most likely in the light elements and their compounds. Most of these have now been produced in fibre or whisker form, sometimes with strengths approaching the

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FIG 5 Relative cross sections o f the various fibrous reinforcements discussed

theoretical limit, and it seems probable that the future rests with one of these produced by a cheaper method rather than in any new material. Beryllium may be dismissed on account of its cost and toxicity and, with the exception of silicon, the carbides and nitrides are not resistant to high-temperature oxidation. For moderate temperatures or high temperatures under reducing conditions carbon (including some of the newer forms based on pitch) has a combination of potential cheapness and strength that will be hard to beat, but for high temperature oxidizing conditions we are left with silicon carbide, silicon nitride and alumina: a low cost manufacturing route to any of these in strong fibre form would herald the breakthrough into high temperature composites.

REFERENCES 1 Kelly, A. 'Strong solids', Clarendon Press, Oxford (1966) 2 Peters, D. The Engineer, Vo1230, p 43 (1970)

FIG 4 Fracture surface of carbon-fibre-reinforced Pyrex glass. The fibre content can be measured accurately.

FIG 6

Fracture surface of natural crocidolite fibre (left) and fibre which had been heated to 500 °C (right) COMPOSITESSeptember 1970 299