Creep behavior ceramics

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. 

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

Coble creep -for ceramics [CERAMICS - MECHANICALPROPERTIES AND BEHAVIOR] (Vol 5)... 

Particulate ceramic matrix composites are two-phase ceramics, in which hard refractory particles or fibers are embedded in a ceramic matrix which is usually less creep resistant. Creep begins when the matrix begins to flow hence, the more refractory the matrix, the more creep resistant the composite. However, creep resistance of the composite is determined by both the creep behavior of the matrix and the concentration of particles in the matrix. At low particle concentrations, creep is determined primarily by the rheological... 

In this chapter, we discuss the creep behavior of ceramic matrix composites in terms of their microstructure. The role of interparticle interactions during creep is emphasized. The relationship between creep rate and lifetime is also discussed. 

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. 

Elevated Temperature Creep Behavior of Continuous Fiber-Reinforced Ceramics... 

To gain a better understanding of the creep behavior of fiber-reinforced ceramics, a simple 1-D analytical approach will be used to examine the effects of constituent behavior on composite creep deformation and changes in internal stress. Since the derivation of the model provides valuable insight into the parameters that influence composite creep behavior, the derivation of the 1-D concentric cylinder model will be outlined first. 

To illustrate a key point concerning the creep behavior of fiber-reinforced ceramics, the primary creep behavior of the constituents was purposely omitted in the above analysis. As shown in Fig. 5.2, even though it was assumed that the constituents undergo only steady-state creep, a protracted transient creep... 

Although analytical models are very effective tools for understanding how changes in the elastic and creep behavior of the constituents of a composite influence overall creep behavior, one must not blindly assume that accurate predictions of composite creep behavior can be obtained based upon creep experiments conducted on the individual constituents. For instance, even if a monolithic ceramic and a composite were processed under identical conditions, the fracture and creep behavior of the monolithic ceramic may be... 

---