Whisker composites

FlQure 15 Fractwine microstmcture of hot pressed AloCK SiC whisker composite tested at room ton wature. Note clear Imprlntr dt whiskers on the surface showing crack deflection, whisker pull-out, or both occurring along the crack surfaces. 

Limited work on HIPIng whisker composites has begun. Thus, Becher and Tleges HIPed (at 1600 C) some of the AI2O3-SIC whisker composites they sintered, but such efforts are limited by the limited levels of whiskers In composites that can be sintered to closed porosity for HIPIng. However, as discussed later, these restrictions do not necessarily apply to reaction processed matrices. 

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For the remainder of this book, fiber-reinforced composite laminates will be emphasized. The fibers are long and continuous as opposed to whiskers. The concepts developed herein are applicable mainly to fiber-reinforced composite laminates, but are also valid for other laminates and whisker composites with some fairly obvious modifications. That is, fiber-reinforced composite laminates are used as a uniform example throughout this book, but concepts used to analyze their behavior are often applicable to other forms of composite materials. In many Instances, the applicability will be made clear as an example complementary to the principal example of fiber-reinforced composite laminates. 

Cook, J. (1970). Mechanical testing of whiskers. Composites, March, 176—180. 

HIPing is considerably more complex and expensive than hot-pressing but for monolithic silicon nitride it normally produces a superior product with near-isotropic properties at low additive contents. However, with silicon nitride whisker composites, whisker bridging and the formation of whisker nests can be more of a problem.5... 

Rossignol, F., Goursat, P. and Besson, J.L. Microstructure and mechanical behaviour of self-reinforced Si3N4 and Si3N4-SiC whisker composites , J. Eur. Ceram. Soc., 13 (1994) 299-312. 

Pezzotti, G., Tanaka, I. and Okamoto, T. Si3N4/SiC-whisker composites without sintering aids III, High temperature behaviour , J. Am. Ceram. Soc., 74[2] (1991) 326-332. 

The first composite system is a barium osumilite composition reinforced with 30 wt% (25 vol%) No. 1 whiskers. The second composite system evaluated is a barium-stuffed cordierite matrix 30 wt% No. 1 whisker composite. The room-temperature properties of these two composites are given in Table 3.16. 

Collin, K.M., Rowcliffe, D.J. (2001), Influence of thermal conductivity and fracture toughness on the thermal shork resistance of alumina-silicon-carbide-whisker compositer Journal of the American Ceramic Society, 84(6), 1334—1340. 

Fig. 2.1 Fracture surface of an alumina/20 vol.% SiC whisker composite showing microscopically rough surface. Presence of whiskers readily evident due to debonding along the matrix-whisker interface.

Fig. 2.2 Flexural strength of alumina/SiC whisker composite at elevated temperatures and different whisker volume contents.

Fig. 2.3 Elevated fracture toughness of alumina/20 vol.% SiC whisker composite showing a linear response up to temperatures of 1000°C. At temperatures 1100°C, creep damage results in apparent increases in toughness.

Thermal shock testing of an alumina/20 vol.% SiC whisker composite showed no decrease in flexural strength with temperature transients up to 900°C.33 Monolithic alumina, on the other hand, shows significant decreases in flexural strength with temperature changes of >400°C. The improvement is a result of interaction between the SiC whiskers and thermal-shock induced cracks in the matrix, which prevents coalescence of the cracks into critical flaws. 

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... 

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. 

Table 2.3 Summary of short-term fracture toughness of silicon nitride/SiC whisker composites at elevated temperatures... 

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. Hayami, K. Ueno, I. Kondou, N. Tamari, and Y. Toibana, Si3N4-SiC Whisker Composite Material, in Tailoring Multiphase and Composite Ceramics, eds. R. E. Tressler, G. L. Messing, C. G. Pontano, and R. E. Newnham, Materials Science Research Series, Plenum Press, New York, NY, 1986, pp. 663-674. 

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). 

Whisker Composites High Temperature Properties, in Proc. 24th Auto. Tech. Dev. Contractors Coord. Meeting, Vol. P-197, Society of Automotive Engineers, Warrendale, PA, 1987, pp. 279-283. 

G. Pezzotti, et al., Processing and Mechanical Properties of Dense Si3N4-SiC-Whisker Composites without Sintering Aids, J. Am. Ceram. Soc., 72[8], 1461-1464 (1989). 

Fig. 7.3 Examples of crack profiles in AI2O3/33 vol.% SiC whisker composite subjected to static and cyclic tensile loads (in a four-point bend configuration) in 1400°C air. From Ref. 22. Crack growth direction is from right to left, (a) Static crack growth at K 4-5 MPaVnT (b) Cyclic crack growth at R = 0.15 and vc = 0.1 Hz in the AK range 3.5-5 MPaVfiT...

Fig. 7.4 Fatigue crack growth rate, da/dN, plotted as a function of stress intensity factor range, AK in AI2O3/33 vol.% SiC whisker composite in 1400°C air at cyclic frequencies of 0.1 Hz and 2 Hz at R = 0.15, 0.40 and 0.75. After Ref. 22.

Fig. 7.5 Data for R = 0.15 from Fig. 7.4 replotted in terms of da/dt versus Kmax and compared with experimentally measured static crack velocity data for AI2O3/33 vol.% SiC whisker composite in 1400°C air. Also indicated are cyclic crack velocities computed from static fracture data using Eqn. (14).

Fig. 7.7 (a) Transmission electron micrograph of the as-received, untested microstructure of AI2O3/33 vol.% SiC whisker composite, (b-d) Transmission electron micrographs of the fatigue crack tip region in the composite in 1400°C air (R = 0.1S and vc = 0.1 Hz) showing the nudeation of interfacial cavities. Part (e) shows the development of a diffuse cavitation zone ahead of the fatigue crack tip. From Ref. 25. 

Fig. 7.8 Profiles of cracks in AI2O3/33 vol.% SiC whisker composite, (a) An example of the formation of a diffuse microcrack zone ahead of a main crack in the air environment at 1500°C. (b) An example of microcracking damage at 1400°C. From Refs. 22 and 25.

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