Creep ceramics

Like metals, ceramics creep when they are hot. The creep curve (Fig. 17.4) is just like that for a metal (see Book 1, Chapter 17). During primary creep, the strain-rate decreases with time, tending towards the steady state creep rate... 

Above 0.5 ceramics creep in exactly the same way that metals do. The strain-rate increases as a power of the stress. At steady state (see Chapter 17, eqn. 17.6) this rate is... 

Time-dependent hysteresis effects can also occur in crystalline materials and these lead to mechanical damping. Models, such as the SLS and the generalized Voigt model, have been used extensively to describe anelastic behavior of ceramics. It is, thus, useful to describe the sources of internal friction in these materials that lead to anelasticity. The models discussed in the last section are also capable of describing permanent deformation processes produced by creep or densification in crystalline materials. For polycrystalline ceramics, creep is usually considered from a different perspective and this will be discussed further in Chapter 7. 

Creep. The slow deformation that many materials undergo when continuously subjected to a sufficiently high stress. With most ceramics, creep becomes measurable only when the stress is applied at a relatively high temperature. A 20-50 h creep test for refractories is described in ASTM C832. BS 1902 Pt 4.10 describes a compressive creep test up to 1600 °C for refractories. See also... 

For ceramics, p is high and h is small. Ceramics are therefore not very deformable until a relatively high temperature is reached. It seems that in the domain where ceramics creep, the dislocation activity is very limited, so that the deformations are explained by diffusion type processes. The dislocation activity plays an important role only at very high temperatures, often unrealistic with respect to the temperature at which the material will be used. Among the covalent solids, it is the semi-conductors and in particular silicon and germanium which have been the most studied. 

SiHcon nitride (see Nitrides) is a key material for stmctural ceramic appHcations in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, exceUent abrasion and corrosion resistance, low density, and, consequently, a low moment of inertia. Additionally, siHcon nitride is made from abundant raw materials. 

A more extensive comparison of many potential turbine blade materials is available (67). The refractory metals and a ceramic, sHicon nitride, provide a much higher value of 100 h stress—mpture life, normalised by density, than any of the cobalt- or nickel-base aHoys. Several intermetaHics and intermetaUic matrix composites, eg, aHoyed Nb Al and MoSi —SiC composites, also show very high creep resistance at 1100°C (68). Nevertheless, the superaHoys are expected to continue to dominate high temperature aHoy technology for some time. 

Creep. The phenomenon of creep refers to time-dependent deformation. In practice, at least for most metals and ceramics, the creep behavior becomes important at high temperatures and thus sets a limit on the maximum appHcation temperature. In general, this limit increases with the melting point of a material. An approximate limit can be estimated to He at about half of the Kelvin melting temperature. The basic governing equation of steady-state creep can be written as foUows ... 

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

Poly(vinyl acetate) homopolymers adhere well to porous or ceUulosic surfaces, eg, wood, paper, cloth, leather (qv), and ceramics (qv). Homopolymer films tend to creep less than copolymer or terpolymer films. They are especially suitable in adhesives for high speed packaging operations. 

Fig. 5. Tensile elongation vs time demonstrating creep behavior of ceramics. Section I is primary creep II, secondary or steady-state creep III, tertiary...

In the steady-state creep regime of ceramics, almost aU creep mechanisms fit a strain rate dependence of the form (18) ... 

Figure 17.1 and Table 17.1 give melting points for metals and ceramics and softening temperatures for polymers. Most metals and ceramics have high melting points and, because of this, they start to creep only at temperatures well above room temperature... 

Creep tests require careful temperature control. Typically, a specimen is loaded in tension or compression, usually at constant load, inside a furnace which is maintained at a constant temperature, T. The extension is measured as a function of time. Figure 17.4 shows a typical set of results from such a test. Metals, polymers and ceramics all show creep curves of this general shape. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

Designing metals and ceramics to resist power-law creep... 

Ceramics, on the other hand, often deform predominantly by diffusional flow (because their grains are small, and the high lattice resistance already suppresses power-law creep). Special heat treatments to increase the grain size can make them more creep-resistant. 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

These requirements severely limit our choice of creep-resistant materials. For example, ceramics, with their high softening temperatures and low densities, are ruled out for aero-engines because they are far too brittle (they are under evaluation for use in land-based turbines, where the risks and consequences of sudden failure are less severe - see below). Cermets offer no great advantage because their metallic matrices soften at much too low a temperature. The materials which best fill present needs are the nickel-based super-alloys. 

A well-known example of this time-temperature equivalence is the steady-state creep of a crystalline metal or ceramic, where it follows immediately from the kinetics of thermal activation (Chapter 6). At a constant stress o the creep rate varies with temperature as... 

In compression, of course, the strength is greater. Most ceramics are about fifteen times stronger in compression than in tension, for the reasons given in Chapter 17. For ice the factor is smaller, typically six, probably because the coefficient of friction across the crack faces (which rub together when the ceramic is loaded in compression) is exceptionally low. At stresses below 6 MPa, ice loaded in compression deforms by creep at 6 MPa it crushes, and this is the maximum stress it can carry. 

Frost, H.J. and Ashby, M.F. (1982) Deformation-Mechanism Maps The Plasticity and Creep of Metals and Ceramics (Pergamon Press, Oxford). 

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