Silicon nitride fracture toughness

Siaions consist of three-dimensional arrays of (Si—Al) (O.N)4 tetrahe-dra. These oxynitrides are traditionally fabricated with silicon nitride An example is beta-sialon, where the O and Si are partially replaced by N and Al, respectively. Advanced sialons are now being researched to enhance fracture toughness and iinpioved cieep properties,... 

Blugan, G., Orlovskaya, N., Lewis, M., Kuebler, J., Multilayer silicon nitride laminates exhibiting high fracture toughness and crack deflection, Key Eng. Mater., 280-283, 1863-1868, 2004. 

Ellen, Y.S. etal. (1998), Microstructural design of silicon nitride with improved fracture toughness II, Effects of yttria and alumina additives , J. Am. Ceram. Soc., 81(11), 2831-40. 

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 addition of SiC whiskers to silicon nitride matrices resulted in only moderate increases in the fracture toughness a compared to monolithic materials. Fracture strengths of composite materials showed increases in some cases and slight decreases in others. Summaries of the variations in toughness and strength are given in Tables 2.3 and 2.4. The moderate increases in... 

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

A. S. Kobayashi, A. F. Emery, and B. M. Liaw Dynamic Fracture Toughness of Reaction Bonded Silicon Nitride, Journal of the American Ceramic Society, 66[2], 151-155 (1983). 

A new type of liquid-phase sintered SiC using yttria [1314-36-9J, Y20, as the oxide additive and submicrometer SiC powder for enhanced densification, produces a material which can be densified without the application of pressure (13). This material, sintered from cold isostatically pressed billets, appears to be comparable to silicon nitride in strength and fracture toughness. 

Silicon nitride ceramics should have high flexural strength and fracture toughness at room temperature and good creep resistance at elevated temperatures. Because these properties are determined by the microstructure of these ceramics, further improvement of these ceramics is possible through alloy design. The following sections will be devoted to micro-... 

Lai and Tien [26] (Fig. 15) have shown that the flexure strength and fracture toughness of silicon nitride ceramics depends on the aspect ratio... 

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