Debonding of interfaces between beta-silicon nitride whiskers and Si-Al-Y oxynitride glasses

Crack bridging mechanisms in self-reinforced silicon nitride ceramics rely on interfacial debonding and intergranular fracture processes. The toughening effects can be enhanced by increasing the diameter of the elongated grains. However, the composition of the additives and, hence, the grain boundary amorphous phase can significantly alter this response; the question remains as to whether this is due to interfacial structure and bonding or the state of residual stresses. Here studies of debonding of the interfaces associated with beta-Si3N4 whiskers embedded in oxynitride glasses are used to examine the role of oxynitride glass composition on the debonding behavior. Using indentation cracking, measurements of interfacial debond lengths versus angle of incidence (i.e. between crack plane and interface plane) are used to determine the critical angle for debonding which can be related to debonding energy. In Si-Al-Y-O-N glasses, it is found that increases in the Y:Al and O:N ratios promote interfacial debonding. At the same time, the properties of the glasses are dependent upon composition. As a result, the thermal expansion mismatch stresses imposed on the whisker by the glass also vary with composition. Thus, the influence of thermal expansion mismatch stresses on the interface debonding was considered by analyses of the radial and the axial stresses using the measured glass properties. These results show that thermal expansion mismatch stresses are not a factor in interfacial debonding in the current experiments. Rather, interfacial compositional profile analyses and high resolution microstructural characterization of the interfaces reveal that the observed compositional effects on debonding behavior are related to formation of interfacial phases, in this case the growth of a beta'-SiAlON layer. Copyright (C) 1996 Acta Metallurgica Inc.