Creep of ceramics

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. 

Models for the Creep of Ceramic Matrix Composite Materials... 

As previously noted, this chapter has been concerned mainly with those models for the creep of ceramic matrix composite materials which feature some novelty that cannot be represented simply by taking models for the linear elastic properties of a composite and, through transformation, turning the model into a linear viscoelastic one. If this were done, the coverage of models would be much more comprehensive since elastic models for composites abound. Instead, it was decided to concentrate mainly on phenomena which cannot be treated in this manner. However, it was necessary to introduce a few models for materials with linear matrices which could have been developed by the transformation route. Otherwise, the discussion of some novel aspects such as fiber brittle failure or the comparison of non-linear materials with linear ones would have been incomprehensible. To summarize those models which could have been introduced by the transformation route, it can be stated that the inverse of the composite linear elastic modulus can be used to represent a linear steady-state creep coefficient when the kinematics are switched from strain to strain rate in the relevant model. 

In general, the steady-state creep of ceramics may be expressed in the form " ... 

S. M. Wiederhorn, Creep of ceramics, pp. 123-36 in Introduction to Mechanical Behaviour of Ceramics, edited by G. de Portu, CNR/IRTEC, Faenza, Italy, 1992. 

The deformation and damage mechanisms in creep of ceramics and hard materials are similar to those in metals [150,151]. Under normal loading conditions (in the absence of severe elastic constraint) ceramics fracture at room temperature before any significant plastic flow. Dislocation glide in ionically bonded ceramics is complicated by the presence of both anions and cations, which create electrostatic (Coulombic) barriers to shear. As in metals, three creep regimes have been identified. The initial high strain-rate, observed on applying the load, decreases rapidly... 

J.A. DiCarlo and H-M. Yun, Creep of Ceramic Fibers Mechanisms, Models, and Composite Implications, Creep Deformation Fundamentals and Applications, eds. R.S. Mishra, J.C. Earthman, and S.V. Raj, The Minerals, Metals, and Materials Society, Warrendale, PA, 2002, pp. 195-208. 

The reader is referred to [ 160,161 ] for extensive reviews of creep of monolithic ceramics and ceramic composites. Models for creep of ceramic matrix composites are given in [161-163]. 

The Interaction between cyclic fatigue and creep of ceramic materials Is an area virtually unexplored. Although It has been demonstrated that the high-cycle fatigue life of silicon nitride can be enhanced by coaxing at high temperatures, little Is known about the influence of precoaxlng to the subsequent creep behavior. 

G. Fantozzi, J. Chevalier, C. Olagnon and J. L. Chermant, Creep of Ceramic Matrix Composites, in Comprehensive Composite Materials, edited by A. Kelly and C. Zureben, (Elsevier, London 2000)... 

D. J. Clough, Irradiation induced creep of ceramic fuels . Ibid. 

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