Microwave Sintering of Silicon Nitride with Zirconia as a Secondary Additive
Date
2007-04
Authors
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Publisher
New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Abstract
The effect of 3 mol % Y2O3 stabilized ZrO2 in microwave sintered Si3N4 was
investigated in the present study. A Finite Difference Time Domain (FDTD) numerical
technique was used to estimate the microwave power absorption and temperature
distribution within Si3N4 samples with MgO and Y2O3 sintering aids and different levels
of ZrO2 during microwave sintering. Higher temperatures were developed in the sample
containing the highest dielectric loss additive (4 wt% MgO, 6 wt% Y2O3 and 2.5 wt%
ZrO2) when compared to the lower loss additive (4 wt% MgO and 6 wt% Y2O3) or pure
Si3N4. The numerically calculated temperatures within the samples were used to correct
the values experimentally measured by an optical pyrometer, which were 75-125 ºC
lower.
Isothermal densification and α → β phase transformation rates of Si3N4 containing
4 wt% MgO, 6 wt% Y2O3 and between 0 and 5 wt% ZrO2 were examined. An addition of
2.5 wt% ZrO2 enhanced the densification rate during microwave sintering. Complete
α→β phase transformation occurred at a lower temperature during microwave sintering
than conventional sintering. Grain boundary phases of microwave sintered samples were
characterized by transmission electron microscopy (TEM). Small ZrO2 grains (d<200
nm) were remained dispersed in the matrix. Both tetragonal and cubic Zr-rich phases
were identified by selected-area electron diffraction. Additions of ZrO2 improved the
hardness of Si3N4. However, no variation in fracture toughness was observed despite
differences in microstructures. The strong bonding between the grains and grain
boundary phase resulted in predominantly transgranular crack propagation paths.
Description
Advisory committee members: Vasantha Amarakoon, James Varner, William Carty. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University
Type
Thesis