Si3N4/SiCN nanocomposites: Tensile ductility and rupture behaviour

Tensile ductility and rupture behaviour of Si3N4/SiC composites were studie d in the 1595 and 1640 degrees C temperature range under strain rates of 5x 10(-5) s(-1) and 10(-5) s(-1). The behaviour was compared to that of Si3N4 monoliths containing the same initial alp Si3N4 ratios with reference to an alpha-phase rich superplastic silicon nitride. The ductility of the compos ites was much smaller than that of the corresponding monoliths. It decrease d when the SiC content increased and increased when the temperature was rai sed. Under a strain rate of 5x10(-5) s(-1), ductility remained limited (eps ilon<25%) whatever the composition or the temperature, and failure occurred by slow crack growth. Contrary to the case of the alpha-phase rich superpl astic silicon nitride for which the hardening was strain-enhanced and essen tially time-independent, a dramatic change in hardening appeared and the du ctility was markedly enhanced when the strain rate was lowered to 10(-5) s( -1). It was concluded that, under the higher strain rate, deformation was c ontrolled by grain boundary sliding of the micro-sized Si3N4 grains. In thi s case, the SiC nanoparticles located along the grain boundaries hampered s liding of the Si3N4 grains, leading to a poor ductility. On the contrary, u nder the lower strain rate, the SiC nanoprecipitates contributed to the def ormation and the ductility is improved. (C) 1998 Elsevier Science Limited. All rights reserved.