Mullite whiskers

Mullite is a catenarian silicate mineral (Al Si2 O5 5 0 5 belonging to the rhombic system and is an excellent refractory material. The biggest advantage of mullite as a metal reinforced material is its chemical inertness. Mullite whiskers can be used as ultra high-temperature thermal insulation material and are used in industrial high-temperature furnaces and reinforcing material of fiber-reinforced metal. The industrial production of mullite whiskers is concentrated mainly in Japan 

Short whisker Imperial Chemical Industries (ICI) of British 95 5 —  

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Magnesium aluminosilicate glass composites with 3-D mullite whisker reinforcement Fracture strength 330 MPa. Fracture toughness 3.3 MPam Structural applications requiring isotropic mechanical properties [30]... 

Travitzky, N. A., Mechanical Properties and Microstmcture of Mullite Whisker-Reinforced Magnesiitm Aluminosilicate Glass with Cordierite Composition, 7. Mat. Sci. Lett. 17 (1998) 1609-1611. 

Liu Yonghe et al., Parameter Optimization for Aspect Ratio of Mullite Whiskers by Response Surface Method [J].( Journal of the Chinese Ceramic Society, 2011 39(3))403 408. 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

The flexural strength of a mullite/20 vol.% SiC whisker composite as a function of temperature is presented in Fig. 2.4. As shown, the significant increase in the strength of the composite is retained up to temperatures of at... 

Fig. 2.4 Fracture strength of mullite/20vol.% SiC whiskers at elevated temperature.

Shaw and Faber30 reported that the toughness of mullite/20 vol. % SiC whisker composites with either ARCO/ACMC or Tateho whiskers remained relatively constant up to temperatures of 1100°C. At temperatures >900°C they also observed considerable whisker pull-out lengths up to 100 /urn. However, the number of pull-outs was always <0.01% of the total whisker volume and only observed with whiskers that were perpendicular to the crack plane. 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

R. Ruh, K. S. Mazdiyasni, and M. G. Mendiratta, Mechanical and Microstructure Characterization of Mullite and Mullite-SiC-Whisker and Zr02-Toughened-Mullite-SiC-Whisker Composites, J. Am. Ceram. Soc., 71[6], 503-512 (1988). 

T. N. Tiegs, P. F. Becher, and P. Angelini, Microstructures and Properties of SiC Whisker-Reinforced Mullite Composites, in Mullite and Mullite Matrix Composites, Ceramic Transactions, Vol. 6, eds. S. Somiya, R. F. Davis, and J. A. Pask, American Ceramic Society, Westerville, OH, 1990, pp. 463-472. 

Fig. 8.4 Phase-contrast TEM image of SiC whisker in alumina matrix that has undergone oxidation. The silicate glass (G) has begun to react with alumina to form a mullite nucleus (M), and graphitic carbon (C) surrounds the crystallite.14...

The mechanisms of whisker growth vary depending on the substrate used for the reaction. For example, the Si3N4 whiskers grown on mullite show axial defects, characteristic of crystallization by an axial-screw-dislo-cation mechanism [29], In the presence of a catalyst, whisker growth occurs by means of a VLS mechanism, involving crystallization of the nitride from drops of binary or ternary alloys of silicon with iron and aluminum. The iron and aluminum act as solvents for silicon. 

R.D. Nixon, S. Chevacharoenkul, and R.F. Davis, Creep of hot-pressed SiC-Whisker-reinforced mullite, Ceram, Trans. 6, 579-603 (1990). 

I.E.Gonenli and GL.Messing, Texturing of Mullite by Templated Grain Growth with Aluminum Borate Whiskers, / Ear. Cer. Soc.. 21, 2495-2501 (2001). 

The slurry process can also be used to produce composites by tape casting. An example is the fabrication of laminated SiC whisker-reinforced mullite composites. 

Mullite is mixed with an organic binder in a ball mill for 24 h. SiC whiskers, between 10 and 50 vol%, are added and mixed for a further 24 h. 

Gonenli IE, Messing GL (2001) Texturing of mullite by templated grain growth with aluminum borate whiskers. J Eur Ceram Soc 21 2495-2501... 

With the development of some composite materials and the commercial success of some of these products, the demand for whiskers has increased. Numerous companies experimented with different whisker growth processes over the years and several patents have been issued in the area. At the present time, commercial sale of whiskers is limited to a few companies in the world with SiC whiskers the most predominant. In addition, whiskers ofTiC.TiN, Al203,mullite, Si3N4, and B4C have also been produced and studied. However, SiC remains the whisker of most interest and commercial success. 

Mullite-SiC Whisker and Mullite-Zr02-SiC Whisker Composites... 

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