FIGURE 5-4 High resolution secondary ion mass spectroscopy compositional maps showing dopant segregation in yttrium and lanthanum doped alumina. Source: Chabala et al., 1997.

prior knowledge of the creep mechanism. The current theory is that (1) some solid solutions diminish the diffusivity, and (2) some intergranular nanoparticles inhibit the sliding and rotation of the grain boundaries. The specific mechanisms that dictate the diffusivity and sliding are not clear, but the findings are compelling and suggest trajectories for future research and development.

The best creep results on nanoparticle-reinforced alumina have been achieved with intergranular SiC (at about 5 volume percent). The effect of SiC nanoparticles was unexpected because simple theory indicates that diffusion would occur rapidly around SiC particles located along sliding grain boundaries because of their small size. However, there is no evidence that this occurs. The mechanism(s) responsible for this counterintuitive effect need to be understood so this behavior can be realized in other systems. Despite the promising results indicating excellent creep resistance in Al₂O₃ reinforced by SiC nanoparticles, SiC nanoparticle-reinforced Al₂O₃ does not appear to be appropriate for use as a fiber for reinforcement of CMCs subject to oxidizing conditions. That is, the oxidation characteristics of this material are problematic. Oxidation measurements indicate that a silica outer layer forms on Al₂O₃ reinforced with SiC nanoparticles at a rate comparable to the rate it would form on SiC. However, a much thicker SiO₂ layer forms below the surface of SiC nanoparticle-reinforced Al₂O₃, wherein the SiC particles have a modified chemistry and morphology. Such a thick SiO₂ layer would result in the degradation of a fiber made of this material. The benefit of creep strengthening with nanoparticles could be realized without oxidative degradation, if oxide ceramics were reinforced with oxide ceramic nanoparticles. The committee recommends that this possibility be investigated.

Certain solutes can also have a profound, beneficial effect on creep resistance. For example, it has recently been discovered that the addition of small quantities (less than 1,000 ppm) of rare earth dopants, such as yttrium, lanthanum, or neo-dymium, to fine grained Al₂O₃ (1µm to 2µm [0.04 to 0.08 mils] grain size) can reduce the creep rate by several orders of magnitude. The effect seems to be strongly correlated with the fact that these ions are highly oversized for the alumina lattice and thus segregate very strongly to the grain boundaries. Other isovalent dopant ions, such as chromium and iron, which are closer in size to aluminum and do not segregate strongly to the grain boundaries, do not affect creep strength.

The mechanism for creep reduction is not understood although the relationship between dopant concentration and creep resistance has been confirmed to be a true solid-solution effect. One school of thought is that creep is controlled by grain boundary diffusion and that large segregated ions simply hinder diffusion in the core sites at the boundary. This theory is supported by kinetic measurements of self-diffusion in undoped and yttria-doped alumina and by studies of the oxidation of aluminum alloys (Le Gall et al., 1995). There is