Whisker reinforced composites alumina

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... 

The SiC whisker-reinforced alumina composite, a model for engineered materials, has opened new vistas for tool material development. Whereas SiC whisker-reinforced alumina is used extensively for the machining of nickel-base superaHoys, SiC whiskers react chemically with steel, causing rapid wear on the rake face. Attempts are underway to replace SiC whiskers with less reactive whiskers such as TiC or TiN. 

SiC whisker-reinforced alumina is a major advance in tool material development, as it provides a means to increase the fracture toughness of the material via the composite material approach. It is entirely possible that in the next century many new whiskers of refractory, hard materials will be made... 

Small single crystals, such as those of potassium titanatc, are being used at an annual rate of over 10,000 tons for the reinforcement of nylon and other thermoplastics. These composites are replacing die-cast metals in many applications. Another microfiber, sodium hydroxycarbonate (Dawsonite), also improves the physical properties and flame resistance of many polymers. Many other single crystals, called whiskers, such as alumina, chromia, and boron carbide, have been used for making high-performance composites. 

Dogan, C.P. and Hawk, J.A. Influence of whisker reinforcement on the abrasive wear behavior of silicon nitride- and alumina-based composites , Wear, 203-204 (1997) 267-277. 

Lee, W.J., Case, E.D. (1989), Cyclic thermal shock in SiC-whisker-reinforced alumina composite , Mater. Sci. Eng., A119, 113-126. 

C. A. Anderson, P. Barron-Antolin, A. S. Fareed, and G. H. Schiroky, Properties of Fiber-Reinforced Lanxide Alumina Matrix Composites, in Whisker- and Fiber-Toughened Ceramics, eds. R. A. Bradley, D. E. Clark, D. C. Larsen, and J. O. Stiegler, ASM, Materials Park, PA, 1988, p. 209. 

K. Xia and T. G. Langdon, The Mechanical Properties at High Temperatures of SiC Whisker-Reinforced Alumina, in High Temperature/High Performance Composites, Materials Research Society Symposium Proceedings, Vol. 120, eds. F. D. Lemkey, S. G. Fishman, A. G. Evans, and J. R. Strife, Materials Research Society, Pittsburgh, PA, 1988, pp. 265-271. 

It is well established that improvements in the room-temperature fracture toughness of ceramics can be achieved by whisker reinforcement, and various mechanisms have been proposed to explain the phenomenon. Recently, two studies have examined the fracture behavior of SiC whisker-reinforced alumina composites at high temperatures to determine the dependence of fracture toughness on temperature, the effect of test atmosphere on fracture toughness, and mechanisms of fracture.33,34 In both studies, single-edged-notched bars... 

R. Chaim, L. Baum, and D. G. Brandon, Mechanical Properties and Microstructure of Whisker-Reinforced Alumina-30 vol% Glass Matrix Composite, J. Am. Ceram. Soc., 72[9], 1636-1642 (1989). 

High stiffness ceramic fibers such as alumina, alumina-silica, silicon carbide, boron, etc. are used as reinforcement fibers for polymeric, metallic, and ceramic matrix composites (Chawla, 1987). Silicon carbide whisker reinforced alumina composites are used as high speed cutting tools (Chawla, 1993). 

Among the oxide composites, SiC whisker-reinforced alumina deserves some discussion. It was first introduced in the 1980s, and there was high level of enthusiasm for whisker composites. In addition to SiC whiskers, work had been done with whiskers of TiC, TiCN, and TiB2 showing improved performance in laboratory (Pettersson and Johnsson 2003) (Fig. la), but these did not get much commercial success due to lack of significant differentiation in performance and lack of whisker availability. Commercially available composites have used SiC as whisker for the last two decades. Improvements have been made in xmderstanding the whisker-matrix relationship. Whisker is the cost driver and is a health hazard. 

C.A. Andersson, P. Barron-Antolin, G.H. Schiroky, and A.S. Fareed, Properties of Fiber-reinforced Lanxide Alumina Matrix Composites, pp. 209 15 in ASM Proc. Int. Conf. Whisker- and Fiber-toughened Ceram., 1988. 

As a class of ceramic materials, SiC whisker-reinforced ceramic composites were developed for potential structural applications because of the significant improvements in the mechanical properties these materials offered as compared to the monolithic materials. The incorporation of SiC whiskers into alumina ceramics resulted in increases in strength, fracture toughness, thermal conductivity, thermal shock resistance and high temperature creep resistance. These discoveries initiated several years of intense study into this class of composites. 

SiC whiskers were initially developed in the early 1960s. However, their use was originally to be applied to the reinforcement of metal matrices, such as aluminum. These metal matrix composites were only a small commercial success, mainly because of the high cost of the whiskers. The first application of whisker reinforcement to ceramics, and in particular alumina, did not occur until the 1980 s. For the purposes of this article, SiC whiskers will be defined to be acicular or needle-like shaped, discontinuous, nearly single crystals. 

Because SiC and aliunina have significantly different properties, the composites have physical properties, which are a combination of both [19]. The addition of SiC whiskers to an alumina matrix increases the hardness and thermal conductivity as shown in Figs. 10 and 11, respectively. Conversely, the electrical resistivity and thermal expansion are observed to decrease with SiC whisker additions as shown in Figs. 12 and 13, respectively. Other detailed property summaries on SiC whisker reinforced alumina can be found in Reference 20. 

SiC whisker reinforced alumina has significantly improved mechanical properties compared to monolithic alumina ceramics. The composites can be fabricated by several methods using typical conventional powder processing techniques. Cutting tools are the main application for the composites at the present time. 

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