Second phase particles alumina

Another notable example of a reduction in creep rate through the addition of second-phase particles concerns nanocomposites . In alumina-SiC, systems, several investigations have reported significant reductions in creep rate compared with monolithic alumina [48, 49], Figure 4.9 shows the results of Ohji and co-workers [48], At 1200°C the creep rate of an AI203-17vol%SiC nanocomposite was less than that of alumina for a given stress by a factor of 250, and the time to rupture at 50 MPa was increased from 120 h to 1120 h. The SiC inhibits creep primarily because it is difficult to remove or deposit... 

Although the nucleation of cavities does not seem to be well understood at present it is clear that cavitation depends on microstructure. Pbrosity and second-phase particles, which are sources of stress concentration (see Chapter 18), can act as nucleation sites for cavitation and subsequent crack growth. Remember pores can be found in most ceramics even pore-free materials such as hot-pressed alumina may contain small pores. Cavitation also occurs in ceramics with IGFs. Nucleation of the cavities will usually occur at regions where the IGF is not homogeneous, e.g., nonwetted regions, gas bubbles, and impurity particles. 

Structure. This particle of P" -alumina contains a sheet of spinel the spinel is like a sheet of a second phase. As we have seen in Section 7.12, we can also think of the structure of the P" -Al203 as being blocks of spinel that twin on every six 111 oxygen planes. The attractive part of this description is that we can then understand the fault in the structure where the twin occurs after only four 111 oxygen planes. Remember that the twin planes are not actually spinel twin planes because the chemistry is different on the twin plane. 

The creep of alumina based fibers can be reduced by adding a second phase. For example, the steady state creep rate of a PRD-166 a-alumina/t-ZrOa (Y) fiber is about one order of magnitude lower than that of the single phase a-alumina fiber (Fiber FP) [78]. This effect is thought to be related, at least at low temperatures (T<1100°C), to the fact that the dispersed zirconia particles at grain boundaries limit the mobility of intergranular dislocations [67],... 

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