Characterization of zirconia-alumina composites sintered using 2.45 GHz radiation

The composite material 80 wt% ZrO₂ (+3 mol% Y2O3) 20 wt% AI₂O₃ was sintered in a Cober S6F microwave oven. Power was supplied to the multi-mode resonant cavity through a WR-284 waveguide by a 6 KW variable power microwave generator operating at 2.45 GHz microwave radiation.

The microwave heating process has shown potential for uniform material heating on a macroscopic scale, and internal and volumetric heating, and selective heating on the both a macroscopic and microscopic scale with rapid heating rates possible. This work showed how a zirconia-alumina composite interacts with microwave radiation and how the loss characteristics of this material vary as a function of temperature.

The microwave heating of the zirconia-alumina composite samples was done in two groups; one was heated with no susceptor and the other was heated with a susceptor to allow enhanced coupling. The relative occurrence in time of various power levels and the sample surface temperature were used to control the sample sintering. For all work the system was operated in the manual mode. The microwave heating was described as a three-step process, namely 1) preheating, 2) transition, and 3) stable sintering. Experimental results on optimum heating conditions, temperature profile, and transient heating characteristics of materials were presented and explained.

Microwave processed materials were densified much more rapidly and at much shorter time and at higher heating rates than the conventionally processed materials. A comparison between microwave processed and conventionally processed materials revealed no difference in measured thermal-shock resistance. This was probably due to how the samples were prepared and further work will need to be done in this area before a final opinion can be given. However, a comparison in phase analysis of zirconia and microstructure revealed a significant difference.

Unusual microstructure of microwave processed materials was explained by microwave heating theory in an earlier paper by Meek. Also, smaller grain size of the microwave processed materials was explained by Brook's theory.