Ionic interdiffusion as interaction mechanism between Al and Si 3 N 4
Al-Si 3 N 4 couples were heat-treated at 850-1150°C for 250 hours. The thickness of the interacted area was measured by scanning electron microscopy (SEM) and scanning/transmission electron microscopy (TEM/STEM). The interaction rate increases exponentially with inverse temperature, with an activation energy of 194.23 kJ/mol and diffusion pre-coefficient of 5 x 10 −9 m 2 /s, indicating that the interaction is diffusion-dependent. As the results showed, the interfacial area is comprised of Al alloy channels, Si precipitates, and AlN grains. Al-Si transfer through the solid solution (Si 3-x Al x N 4-y ) at the interface of Al alloy and β-Si 3 N 4 grains controls the kinetic of the interaction. When concentration of Al in solid solution exceeds a certain amount, it undergoes a topotactic phase transformation to form Al 1-x Si x N 1+y (viz., AlN). Next, the Al 1-x Si x N 1+y grains detach from the β-Si 3 N 4 grains and subsequently new Al-Si 3 N 4 interfaces are established. These interfaces repeat the interaction process, continuing until all the reactant is depleted. Thus, the interaction kinetics consist of a sequence of associated parabolic stages, precluding the observation of parabolic kinetics.