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. Th e toughening effects can be enhanced by increasing the diameter of the elongated grains. However, the composition of the additives and, henc e, the grain boundary amorphous phase can significantly alter this res ponse; the question remains as to whether this is due to interfacial s tructure and bonding or the state of residual stresses. Here studies o f debonding of the interfaces associated with beta-Si3N4 whiskers embe dded in oxynitride glasses are used to examine the role of oxynitride glass composition on the debonding behavior. Using indentation crackin g, measurements of interfacial debond lengths versus angle of incidenc e (i.e. between crack plane and interface plane) are used to determine the critical angle for debonding which can be related to debonding en ergy. In Si-Al-Y-O-N glasses, it is found that increases in the Y:Al a nd O:N ratios promote interfacial debonding. At the same time, the pro perties of the glasses are dependent upon composition. As a result, th e thermal expansion mismatch stresses imposed on the whisker by the gl ass also vary with composition. Thus, the influence of thermal expansi on mismatch stresses on the interface debonding was considered by anal yses of the radial and the axial stresses using the measured glass pro perties. These results show that thermal expansion mismatch stresses a re not a factor in interfacial debonding in the current experiments. R ather, interfacial compositional profile analyses and high resolution microstructural characterization of the interfaces reveal that the obs erved compositional effects on debonding behavior are related to forma tion of interfacial phases, in this case the growth of a beta'-SiAlON layer. Copyright (C) 1996 Acta Metallurgica Inc.