hypothesis is that the retardation in creep results from inhibition of the grain boundary diffusion kinetics by a simple “steric hindrance ” effect. There is also some evidence in the literature that the presence of dopant ions can significantly increase the proportion of special (i.e., coincident site lattice) grain boundaries in alumina (Lartigue-Korinek et al., 1994). Perhaps the diffusivities along these boundaries are diminished, resulting in grain boundary sliding rates that are also diminished. A comprehensive understanding of the mechanism by which oversized isovalent cations inhibit creep, however, remains elusive.

Although the findings described above regarding creep in fine-grained polycrystals have been limited to oxides, it is anticipated that analogous effects are possible for non-oxide ceramics. For example, certain additives and second-phase particles (e.g., Ti, C, and B ₄C) enhance the creep strength of polycrystalline SiC, while others (e.g., B and Al) degrade it (recall Chapter 3). The mechanisms, however, are not understood.

The development of an amorphous fiber in the Si-N-C-B system has revealed two unexpected possibilities. First, this fiber appears to be stable in the amorphous state up to ~1,600°C (2,912°F), even in air, and retains its tensile strength up to 1,600°C (2,912°F). Second, oxidation of the Si-N-C-B fiber forms silica at rates comparable to the rate that silica is formed upon oxidation of crystalline SiC, and also creates a thin (~50 nm [0.002 mils]) buried interphase of hexagonal BN. It has been hypothesized that this in-situ BN would act as a crack deflection layer in a CMC. If so, the regenerative nature of the deflection interphase could avert composite degradation by stress oxidation.

These findings imply that there is a realistic potential for discovering materials with superior creep strength, through a mechanism-based research approach. Complex oxides and carbides, as well as multiphase materials, could be tailored to create attractive combinations of properties.

Fine-grained (grain size < 1µm [0.04 mils]) oxide poly-crystals creep in accordance with diffusive deformation mechanisms, accompanied by sliding of the grain boundaries. (Note that the stress/temperature domain in which dislocation is the dominant creep mechanism has yet to be rigorously identified). Therefore, if all other parameters remained fixed, the creep rate would simply scale with the grain boundary diffusivity (D_b) and vary inversely with the grain size (d) to the first, second, or third power (n) depending on whether the process is controlled by interface control (or sliding), lattice diffusion, or grain boundary diffusion (see Equation 1).

One implication that has been adequately validated by experiment is that complex oxides with low D_b, particularly mullite, have creep strengths superior to single-phase oxides, such as alumina, yttria, and magnesia. However, the findings that solid solutions and nanoparticles profoundly affect creep rates in bulk polycrystalline oxides was not obvious from

Front Matter (R1-R16)

Executive Summary (1-5)

1 Introduction (6-11)

2 Current and Future Needs (12-19)

3 State of the Art in Ceramic Fiber Performance (20-36)

4 Ceramic Fiber Processing (37-48)

5 Materials and Microstructures (49-53)

6 Interfacial Coatings (54-74)

7 Cost Issues (75-82)

8 Recommendations and Future Directions (83-86)

References (87-92)

Biographical Sketches of Committee Members (93-95)