Nextel 610 alumina fibers

Lanthanum phosphate coatings were deposited onto NEXTEL 610 alumina fibers of up to 3 feet in length. The coatings are dense and... 

Fig. 4 Scanning electron micrograph of 300 - 400 nm thick Nextel alumina fibers coated with lanthanum phosphate. The lower figure is a 5x magnification of the inset in the upper figure.

An important polycrystalline a-alumina fiber has been developed by 3M Co. This a-alumina fiber, trade name Nextel 610, is made via a sol-gel route. The sol-gel process of making fibers involves the following steps common to all sol-gel processing ... 

The Nextel series of fibers produced by the 3M Company consists of a variety of aluminosilicate fibers. These contain mainly AljOj+SiOj and some B2O3. The compositions and properties of Nextel 312, Nextel 440, Nextel 550, Nextel 610, and Nextel 720 fibers are given in Table 6.3. Nextel 610 is, of course, the a-alumina fiber described above. The sol-gel manufacturing process used by 3M Co. has metal alkoxides as the starting materials. Metal alkoxides are M(OR)j -type compounds where M is the metal and n is the metal valence, and R is an organic compound. Selection of an appropriate organic group is very important. It should provide sufficient stability and volatility to the alkoxide so M-OR... 

Figure 6.14 A comparison of various polycrystalline alumina and aluminosilicate fibers in the Nextel series. Note the superior creep resistance of the Nextel 720 fiber. 3M data.

A prerequisite for an effective fiber coating is chemical stability with the fiber and matrix phases of a composite. If reactions occur, the fiber-coating-matrix interface would become strongly bonded, and the composite would exhibit brittle fracture behavior. Based on currently available phase diagrams, oxide compounds have been identified that do not show compound formation with commercially available oxide fibers. Particular attention has been paid to fibers that show the highest temperature capability (alumina fibers, such as Fiber FP, PRD-166, and Nextel 610). Oxide compounds identified as stable with alumina included tin oxide, zirconia, titania (at temperatures below 1,150°C [2,102°F]), zirconia titanate (ZrXi04, also at temperatures below 1,150°C [2,102°F]), and possibly zirconia tin titanate (Zr(Sn,Ti)04). 

Many layered oxide coatings show promise as crack deflecting interfaces for oxide-oxide composites. Although the jl-aluminas have been shown to be unstable with Nextel 720 fibers, they are still candidate fiber coatings (especially the less volatile RbAlnOiy) for ceramic composites if other creep-resistant oxide fibers become available. Research into an improved creep-resistant Nextel 610 fiber that incorporates additions of rare-earth oxides or garnet-based fibers, may make the (5-... 

Other manufacturers have modified the production technique to reduce the diameter of the alpha-alumina fibers that they have produced. This reduction of diameter has an immediate advantage of increasing the flexibility and hence the weaveability of the fibers. Mitsui Mining and 3M Corporation have introduced polycrystalline fibers, the Almax and the Nextel 610 fibers with diameters of 10 p.m, that is half the diameter of Fiber FP. 

The Nextel 650 fiber is continuous and circular in cross-section with a diameter of 11.2 p,m and is composed of a-alumina and 10% wt of cubic zirconia stabilized by 1 %wt of... 

Figure 7 compares the creep behavior of the three nearly pure a-alumina fibers. It can be seen that Fiber FP crept at the lowest rate at each temperature. The smaller grains in the Nextel 610 fibers allowed them to creep faster whilst the porosity and residual stresses in the Almax fibers meant that they crept the fastest (23). 

F igure 8 shows the creep behavior from 1100 to 1300°C of several fibers FP and Nextel 610 represent pure alumina fibers, PRD-166 and Nextel 650 zirconia-alumina fibers and... 

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. 

FIGURE 8. A comparison of the creep rate of the Nextel 650 and PRD-166 fibers, both of which are composed of a-alumina and zirconia, the Nextel 720 which is composed of a-alumina and mullite and the pure a-alumina Fiber FP. (Reprinted, from reference 12, "Mechanical and Microstructural Characterization of Nextel 650 Alumina-Zirconia Fibres , with kind permission of Elsevier). 

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