Creep in ceramics

Some other aspects of creep in ceramic matrix composites are also shown in Fig. 4.1. The failure strain in these materials is generally small, typically less than 1-2%. The strain at failure is also a function of the minimum strain rate the lower the minimum strain rate, the greater the strain to failure. This is easily seen in Fig. 4.1 where the failure strain of this Si/SiC is much greater for lower creep rates. This effect is illustrated more quantitatively in Fig. 4.3 for the same material.36 As can be seen, the failure strain varies from 0.5 to 1.5%, as the minimum strain rate varies from =10 7 s 1 to 10-8 s 1. This same type of behavior is obtained for other ceramic matrix composites. 

R. L. Coble, A Model for Boundary Diffusion Controlled Creep in Ceramic Materials, J. Appl. Phys., 34,1679-1682 (1963). 

As shown in Fig. 5.23c, creep in ceramics can also be caused by the diffusion along the grain boundaries, with the creep equation being given by [55, 56] ... 

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. 

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... 

Lofaj F, Wiederhorn SM, Long GG, Jemian PR (2001) Tensile Creep in the next Generation Silicon Nitride. In Singh M, Jessen T (eds) 25th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures A (Ceram Eng Sci Proc 22). Am Ceram Soc, Westerville, OH, p 167... 

Lofaj, F., Okada, A., and Kawamoto, H., Cavitation strain contribution to tensile creep in vitreous bonded ceramics , J. Am. Ceram. Soc, 1997, 80, 1619-23. 

Jin, Q., Wilkinson, D.S. and Weatherly, G.C., (1999), High-resolution electron microscopy investigation of viscous flow creep in a high-purity silicon nitride , J. Am. Ceram. Soc., 82 (6), 1492-1496. 

In this section we review experimental observations on the creep of ceramic matrix composites. Observations that apply to all ceramic matrix composites are discussed. Creep curves obtained on ceramic matrix composites are compared with curves obtained on metals and metallic alloys. The role of a second phase in increasing the creep resistance of composites is emphasized. Finally, a discussion of creep asymmetry is presented, wherein creep occurs more easily in tension than in compression. 

Comparing the type of information obtained on suspensions with that obtained on composites gives useful insight into the types of mechanisms that control creep of ceramic matrix composites. The very large increase in creep resistance of dense particulate composites, i.e., more than 65vol.% particles, suggests that the particle packing density is above the percolation threshold. Creep of particulate composites is, therefore, controlled by direct interparticle contract, as modified by the presence of relatively inviscid matrices. Mechanisms that control such super-threshold creep are discussed in Section 4.5. 

In this paper, the importance of particle and whisker reinforcement to creep and creep rupture behavior of ceramics is discussed. Particle and whisker additions generally increase both the fracture toughness and creep resistance of structural ceramics. These additions also act as nucleation sites for cavities. Cavities form preferentially in tensile specimens. This results in a creep asymmetry, in which composites creep faster in tension than in compression. As a consequence of cavitation, the stress exponent for creep in tension 6-10,... 

J. R. Porter, Observations of Non-Steady State Creep in SiC Whisker Reinforced Alumina, in Whisker- and Fiber-Toughened Ceramics, eds. R. A. Bradley, D. E. Clark, D. C. Larsen, and J. O. Stiegler, ASM International, Metals Park, OH, 1988, pp. 147-152. 

R. L. Tsai and R. Raj, Creep Fracture in Ceramics Containing Small Amounts of a Liquid Phase, Acta Metall., 30,1043-1058 (1982). 

S. Suresh and J. R. Breckenbrough, A Theory for Creep by Interfacial Flaw Growth in Ceramics and Ceramic Composites, Acta Metall. Mater., 38[1], 55-68 (1990). 

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