Oxide creep rate

Poly(ethyl methacrylate) (PEMA) yields truly compatible blends with poly(vinyl acetate) up to 20% PEMA concentration (133). Synergistic improvement in material properties was observed. Poly(ethylene oxide) forms compatible homogeneous blends with poly(vinyl acetate) (134). The T of the blends and the crystaUizabiUty of the PEO depend on the composition. The miscibility window of poly(vinyl acetate) and its copolymers with alkyl acrylates can be broadened through the incorporation of acryUc acid as a third component (135). A description of compatible and incompatible blends of poly(vinyl acetate) and other copolymers has been compiled (136). Blends of poly(vinyl acetate) copolymers with urethanes can provide improved heat resistance to the product providing reduced creep rates in adhesives used for vinyl laminating (137). 

Figure 5.50 Low-stress creep rates of several polycrystalline oxides. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

We have pointed out before that during creep, demixing of solid solutions is to be expected. Creep in compounds, however, occurs in such a way that the rate is determined by the slowest constituent since complete lattice molecules have to be displaced and the various constituent fluxes are therefore coupled. If extra fast diffusion paths operate for one (or several) of the components in the compound crystal, the coupling is cancelled. Therefore, if creep takes place in an oxide semiconductor surrounded by oxygen gas, it is not necessarily the slow oxygen diffusion that determines the creep rate. Rather, the much faster cations may determine it if oxygen can be supplied to or taken away from the external surfaces via dislocation pipes. 

Results on other composite materials are similar to those obtained by Morrell and Ashbee.56 Creep asymmetry has been demonstrated for two grades of siliconized silicon carbide,35,60,61 SiC whisker-reinforced silicon nitride,53 HIPed silicon nitride,29 and vitreous-bonded aluminum oxide.29 Again, stresses required to achieve the same creep rate were at least a factor of two greater in compression than in tension. In two grades of siliconized silicon carbide,35,58-61 the stress exponent changed from 4 at creep rates below... 

Use of ceramic materials in high temperature structural applications is often limited by creep resistance. However, several recent studies have shown that composite reinforcement can drastically reduce the creep rates compared to the unreinforced ceramic matrix.12,20,27-31 Most of these studies have been conducted in air, which is a strong oxidizing environment. In a few cases, investigators have attempted to isolate the effects of oxidation from creep by conducting parallel experiments in both air and inert atmospheres.27,28 The following is a description of the salient points made in these investigations. 

Table 11 Torsional creep rates of some polycrystalline oxides at 1,300°C and 124MPa applied stress (from [23], p. 755)...

FIGURE 3-20 Creep rate of Nextel 720 compared to other commercial oxide fibers. Source Wilson, 1997. 

Fundamental questions about factors that control the creep rates of ceramic materials have not been answered. The effects of carbon and solid solution dopants on the creep rate of SiC materials need to be better understood. The role of intragranular stacking faults on P-SiC creep rates should also be determined. Furthermore, a determination must be made as to whether the microstructure of a-SiC is intrinsically more creep resistant than the microstrueture of p-SiC. For oxide ceramics, the role of microstructure in controlling creep rate and creep rupture strength must he determined, partieularly for multiphase microstructures. 

Table 4. High-temperature properties of various Si3N4 materials (ai4oo c bending strength at 1400°C E - creep rate at 1400°C, 200 MPa - weighted gain during oxidation at 1500°C CTox - residual strength after oxidation).

The combination of silica with alumina can retain transitional forms of alumina, as in the Altex fiber but the combination in the Nextel 480 fiber gives a mullite structure whereas the combination in the Nextel 720 fiber gives a mullite structure in which a-alumina grains are embedded. All three fibers, however lose strength above 1100°C, as shown in Figure 9. The fibers show very different creep behavior, as can be seen from Figure 10, with the Nextel 720 fiber showing the lowest creep rate of all oxide fibers. 

All the creep tests were performed under constant load to determine the creep rates, maximum strains and times to failure. Tests were performed at 566°C (1050"F) and at stresses of 55, 69, 83, and 96 MPa (8, 10, 12, and 14 ksi). Four types of specimens were tested. Composites with five and eight infiltrations were examined. Sample specimens infiltrated 5 times were given specimen designations beginning with A while those infiltrated eight times were designated with B . Specimens were tested after a 600°C oxidation exposure in flowing air for 100 hours. The results of this work are summarized in Table 6. 

Table9.4 Values ofm [the p02 exponent in the creep rate of Eq. (7)] for various oxygen defects in the transition metal oxides with the rock-salt structure [44].

Primary (transient) creep can be considered as a consolidation process during which the structure of the material adjusts itself to the following steady-state creep stage. In some instances, like in cross-Unked elastomers at low stresses, the steady state is absent, with the creep rate decreasing to zero, and the total creep strain remaining constant. In this case, primary creep is a delayed response of the material to the applied stress. At higher stress levels, chain scission, oxidation effects etc. may influence this simple behavior. 

Polycrystalline-alumina-based fibres can at present not compete with silicon-carbide-ba.sed fibres when low creep rates are required. Fibres with higher resistance to creep by dislocation motion could be provided by oxides with high melting point and complex crystal structure, a tendency to order over long distances and the maintenance of this order to high fractions of the melting temperatures (Kelly, 1996). Experimental development of monocrystalline fibres by Czochralski-derived techniques from chrysoberyl... 

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