Fracture silicon nitride

When you pour boiling water into a cold bottle and discover that the bottom drops out with a smart pop, you have re-invented the standard test for thermal shock resistance. Fracture caused by sudden changes in temperature is a problem with ceramics. But while some (like ordinary glass) will only take a temperature "shock" of 80°C before they break, others (like silicon nitride) will stand a sudden change of 500°C, and this is enough to fit them for use in environments as violent as an internal combustion engine. 

The reinforcing fibers are usually CVD SiC or modified aluminum oxide. A common matrix material is SiC deposited by chemical-vapor infiltration (CVI) (see Ch. 5). The CVD reaction is based on the decomposition of methyl-trichlorosilane at 1200°C. Densities approaching 90% are reported.b l Another common matrix material is Si3N4 which is deposited by isothermal CVI using the reaction of ammonia and silicon tetrachloride in hydrogen at 1100-1300°C and a total pressure of 5 torr.l" " ] The energy of fracture of such a composite is considerably higher than that of unreinforced hot-pressed silicon nitride. 

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,... 

Becher PF, Hwang SL, Lin HT, Ticgs TN (1994) Microstructural contributions to the fracture resistance of silicon nitride ceramics. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics, NATO ASI Ser E Vol. 276, Kluwer Academic Publishers, Dordrecht, p 87... 

Klemm H, Herrmann M, Schubert C (1996) The Influence of the Grain Boundary Phase Composition on the High Temperature Properties of Silicon Nitride Materials. In Parilak L, Danninger H, Dusza J, Weiss B (eds) Proc Int Conf Deformation and Fracture in Structural PM Materials, IMR SAS Kosice 2 75... 

Trice, R.W., and Halloran, J.W. (1999), Influence of micro structure and temperature on the fracture energy of silicon nitride/boron nitride fibrous monolithic ceramics , J. Am. Ceram. Soc., 82(9) 2502-2508. 

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. 

Pezzotti, G., Tanaka, I. and Nishida, T., (1993), Intrinsic fracture energy of polycrystalline silicon nitride , Phil. Mag. Lett., 67 (2), 95-100. 

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... 

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