Flexural SiAlONs

Fig. 2.5 High temperature flexural strength of sialon (Si4 5Al15015N65-2%3Y203-5AI203) and silicon nitride (Si3N4-6% Y203-2% A1203) materials with and without SiC whiskers. Whiskers are from American Matrix.

A number of w-crack configurations have been experimentally observed at triple points. Figure 6.79 shows crack nucleation at grain boundaries, formed when shear stress acts along the boundaries. Here, a wedge-type cavity was formed under creep in a SiAlON-YAG ceramic in a flexural bar studied at 1170 °C. Figure 6.80 shows the cavitation strain and the total strain as functions of the normalized beam height for the applied moments of M — 0.165 N.m and M = 0.247 N.m (i.e., at the initial, maximum applied stresses of 80 and 120 MPa). Both tests were carried out at 1170 C and interrupted before failure. For M = 0.165 N.m (a = 80 MPa), the maximum cavitation strain is 20 % of the total strain, whereas for M = 0.247 N.m (b = 120 MPa), the cavitation strain... 

Sialons, as ceramic alloys of silicon nitride and aluminum oxide, were developed as an economically and fimctionally superior alternative to H PSN (see above). This alloying imposes increased high-temperature mechanical (flexural and tensile strengths, fracture toughness, hardness, wear resistance), thermal (thermal shock resistance) and chemical properties (corrosion and oxidation resistance) compared to unalloyed silicon nitride (see Table 11.13). 

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