Whisker-reinforced silicon nitride ceramics

M D PUGH, Concordia University, Canada and M B R O C H U, McGill University, Canada 

The whiskers that are commonly added to silicon nitride to form composites are Si3N4 and SiC (common nomenclature is to add the suffix (w) to denote whiskers and this will be used where appropriate). Although other ceramic and metallic whiskers are available (e.g. BN, TaC, TiC, B4C and Fe), Si3N4 and SiC each have properties that make them prime candidates as reinforcements, not least of which is that they can sustain the high temperatures and reactivity of the sintering process without being degraded. 

Green fabrication and sintering of compacts containing second phases poses several problems and when the second phase is the form of whiskers these problems are magnified. These problems include inhomogeneous distribution 

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 

The only method currently available that avoids the whole problem of whisker impingement is reaction bonding of silicon powder to form the 

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S. R. Choi, et al., Dynamic FatigueProperty of Silicon Carbide Whisker-Reinforced Silicon Nitride, Ceram. Eng. Sci. Proc., 12[7-8], 1524-1536 (1991). 

The data base now contains 5497 test results on over 320 different batches of ceramic materials. Approximately 46% of these are on zirconia-based ceramics, 9% are on silicon carbides, 21% are on silicon nitrides, 6.7% are on whisker-reinforced silicon nitrides, 16.3% are on alumina-based ceramics (including whisker-reinforced aluminas and mullites), and 2% are on other ceramics. Table 1 gives a detailed breakdown, by material class, of the data stored in the system, A list of materials within a material class Is available on request. 

Pugh, M.D. and Gavoret, L. Nitridation of whisker reinforced reaction bonded silicon nitride ceramics , J. Mat. Sci., 35 (2000) 3257-3262. 

R. T. Bhatt, The Properties of Silicon Carbide Fiber-Reinforced Silicon Nitride 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. 199. 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

Ueno, K., and Toibana, Y. (1983). Mechanical properties of silicon nitride ceramic composite reinforced with silicon carbide whisker. Yogyo-Kyokai-Shi 91 491-497 (in Japanese). 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

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. 

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