A case study: thermal protection system in space shuttle orbiter

10.7 A case study: thermal protection system in space shuttle orbiter

Most ceramics are chemically very inert materials and their supreme integrity over metals and poly- mers at temperatures above 1000 ◦

C often makes them the only choice of candidate materials in applications at such high temperatures. By controlling the porosity during sintering, a wide range of ceramics can also be turned into very efficient heat conductors or insulators. Figure 10.25 shows the thermal conductivities of different ceramic materials. Crystalline and well-bonded ceramics such as graphite and bonded SiC are good conductors of heat because of effective phonon conduction in these materials. Porous, coarse ceramics such as firebrick or powdered ceramics are good insulators of heat because the heat transfer mechanism in these materials is mainly by radiation across the voids, which is an inefficient process.

The most well-known application of ceramic materials as high-temperature heat insulating materials is perhaps the thermal protection system in a space shuttle orbiter. A shuttle orbiter re-enters the earth’s atmosphere at a certain yaw angle, so that it travels as a bluff aerodynamic body with respect to the surrounding air in order to achieve the appropriate drag for speed control. The frictional effects are so intense that the air surrounding the orbiter is turned into a plasma state. During this time, the temperature on the orbiter surface is generally above 400 ◦

C at certain protruded parts, such as the nose and the wing and tail tips. To protect the Al- and Ti-based substructure from softening or even melting at such high temperatures, a thermal protection system (TPS), made principally of a range of ceramic materials, is constructed as an outermost skin of the orbiter. Localized damage of the TPS, however, can lead to the loss of the entire shuttle orbiter during re-entry, as witnessed by the Columbia tragedy on 1 February 2003.

C and can rise up to 1500 ◦

In the NASA Space Shuttle program, the TPS is designed to last for 100 missions without major refurbishment. The TPS is an excellent example of ceramic engineering in that it involves the use of

546 Physical Metallurgy and Advanced Materials

Platinum

Graphite Bonded 41.9 SiC

Pure, dense BeO

Pure, dense MgO 1 ⫺ ⬚C

cm)

Fireclay refractory

m 4.2

Clear, fused

Al 2 O 3 Dense, stabilized ZrO 2

Polyethylene

2800⬚F

mal conductivity (W

mal conductivity (cal s

0.04 Powdered MgO

Temperature, ⬚C (⬚F)

Figure 10.25 Thermal conductivities of different materials ( from Richerson, Modern Ceramic Engineering: Properties, Processing, and Use in Design, Marcel Dekker, 1992).

Coated tile Step

terminator Filler bar

Uncoated 352 kg m ⫺ 3 Tile – ⫺ (22 lb ft 3

) ( 9 lb ft ⫺ 3 ) Pure silica fiber – fired at

144 kg m ⫺ 3

~ 1370 C ( ~ 2500 F)

Coating – Borosilicate (Glass) for waterproof and thermal properties – fired at 1150⫺1205 C (2100⫺2200 F) SIP – Nomex felt Filler – Coated Nomex felt

Adhesive – RTV silicone

Figure 10.26 Silica tile system for thermal protection of a space shuttle orbiter ( from Korb, Morant, Calland and Thatcher, 1981, by permission of the American Ceramic Society).

different ceramic and glass materials, depending on the location to be protected. In areas exposed to the highest temperature during re-entry, i.e. the nose cap and the leading edges of the wings, reinforced carbon–carbon (RCC) is used, which can sustain temperatures up to 1650 ◦

C. RCC is a ceramic–ceramic composite consisting of a carbon matrix reinforced with graphite fibers, coated

Non-metallics I – Ceramics, glass, glass-ceramics 547 with silicon carbide and then impregnated with silica. RCC has sufficient strength and stiffness at

temperatures up to 1650 ◦

C to withstand the airloads and thermal stresses as a result of the large thermal gradients across the upper to lower vehicle surfaces during re-entry. In the major part of the orbiter surface where the temperature is between 400 and 1260 ◦

C, the TPS is in the form of silica (SiO 2 ) tiles bonded to the substructure by adhesive. Figure 10.26 shows the typical construction of the silica tile system. The tiles are made from high-purity amorphous silica fibers, which are felted from a slurry and sintered at 1370 ◦

C into blocks. The tile material is highly porous (93% void) and, because it is amorphous, the thermal conductivity is very low (0.017–0.052 W m −1 K −1 , cf. Figure 10.25). It also has very low thermal expansivity and so thermal shock resistance is high. The silica has to have high purity (>99.62%) to avoid devitrification, which will increase the thermal expansivity. The exposed surfaces of each tile are coated with borosilicate glass to achieve waterproof and the proper emittance properties (Figure 10.26).

Problems

10.1 Glass is extremely susceptible to small cracks. If the glass is stressed to 50 MPa, determine the maximum permissible surface flaw to avoid fracture (E = 70 GPa and the surface energy γ s = 0.3 J m −2 ).

10.2 A relationship for the Young’s modulus E to the melting temperature T m is given by the approximation

kT m

E =A

where A is a constant ( ∼90). If diamond melts at 4200 K and has an atomic volume ( )

5.68 × 10 −30 m 3 , estimate the value for the modulus.

10.3 An expression relating porosity (p) to modulus is E =E 0 (1 − 1.9p + 0.91p 2 ). The modulus of elasticity for alumina (Al 2 O 3 ) with 25% porosity is 220 GPa. Estimate the modulus for a non-porous ceramic.

10.4 The activation energy for viscous flow of a glass is 250 kJ mol −1 . If the strain rate at 1000 ◦ C

(T 1 ) is 10 −2 s −1 , calculate the strain rate at a higher temperature of 1250 ◦

2 C (T ).

10.5 Rubber strained 100% requires a stress of 20 MPa at room temperature. To maintain the strain over a period requires a drop in stress, i.e. relaxation. What is the stress after 10 days if the stress drops to exp( −1) of the original value after 30 days?

3.1 and 1.1 eV respectively. How is this variation in energy gap reflected in the appearance of each material?

10.6 Diamond, silicon carbide and silicon have an energy gap of 5.6,

10.7 How is the polymorphic form of zirconia exploited in toughening?

10.8 The flexural strength of porous alumina is σ fs =σ o exp( −np), where σ o is the flexural strength for non-porous alumina, p is percentage porosity and n is a material constant. The room temperature flexural strength of alumina is 175 MPa for a specimen with 5% porosity and 75 MPa for one with 25% porosity. Determine the parameters σ o and n in the relationship for the influence of porosity on flexural strength.

10.9 From the temperature dependence of the viscosity curve for SiO 2 –Na 2 O–CaO glass (shown in Figure 10.13), calculate the activation energy for the process over the intermediate temperature range 600–900 ◦ C.

10.10 From Figure 10.13 determine the maximum temperature to which an SiO 2 –Na 2 O–CaO glass may be heated if the deformation of a cylindrical specimen 100 mm long subjected to a tensile stress 50 kPa is to be less than 1% over a period of a year.

548 Physical Metallurgy and Advanced Materials

10.11 The following table shows the tensile strengths measured from a series of 15 dense Al 2 O 3 (0–2% porosity) of identical sizes with a gauge section measuring 5 mm × 5 mm × 30 mm:

Rank, i

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Strength 192 203 212 224 230 235 243 250 253 260 276 282 286 295 301

(MPa)

(i) By plotting the data in an appropriate format, show that the data roughly obey the Weibull statistics. (ii) Calculate the Weibull modulus m and the stress normalization constant σ o for this batch of

Al 2 O 3 . (iii) If a survival probability of 95% is to be maintained, calculate the strength of a piece of Al 2 O 3 specimen made using a similar method with dimensions measuring

50 mm × 50 mm × 300 mm.

Further reading

Binner, J. (1992). Processing of advanced ceramic powders. Metals and Materials, October, pp. 534–537. Institute of Materials. Burkin, A. R. (ed.) (1987). Production of Aluminium and Alumina. Wiley, Chichester. Cahn, R. W. and Harris, B. (1969). Newer forms of carbon and their uses. Nature, 11 January, 221, 132–141. Creyke, W. E. C., Sainsbury, I. E. J. and Morrell, R. (1982). Design with Non-Ductile Materials.

Elsevier/Chapman & Hall, London. Davidge, R. W. (1986). Mechanical Behaviour of Ceramics. Cambridge University Press, Cambridge. Ichinose, N. (1987). Introduction to Fine Ceramics – Applications in Engineering. Wiley, New York. Inagaki, M. (2000). New Carbons – Control of Structure and Functions. Elsevier, New York. Jack, K. H. (1987). Silicon nitride, sialons and related ceramics. In Ceramics and Civilisation, Vol. 3.

American Ceramic Society, New York. Kingery, W. D., Bowen, H. K. and Uhlmann, D. R. (1976). Introduction to Ceramics, 2nd edn. Wiley- Interscience, New York. Laminated Glass Information Centre (1993). Laminated Glass. LGIC, London. McColm, I. J. (1983). Ceramic Science for Materials Technologists. Leonard Hill, Glasgow. Mantell, C. L. (1968). Carbon and Graphite Handbook. Wiley-Interscience, New York. Parke, S. (1989). Glass – a versatile liquid. Metals and Materials, January, pp. 26–32. Institute of Materials. Rawson, H. (1980). Properties and Applications of Glass. Elsevier Science, Oxford. Richerson, D. W. (1992). Modern Ceramic Engineering: Properties, Processing, and Use in Design. Marcel

Dekker, New York. Riley, F. L. (ed.) (1977). Nitrogen Ceramics. Noordhoff, Leiden. Saito, S. (ed.) (1985). Fine Ceramics. Elsevier, New York. Ubbelohde, A. R. J. P. and Lewis, F. A. (1960). Graphite and its Crystal Compounds. Clarendon Press,

Oxford. Wachtman, J. B. (ed.) (1989). Structural Ceramics, Vol. 29. Academic Press, New York.

Chapter 11