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January 28th, 2010

Sintering Behavior and Microstructure Development of Porous Silicon Nitride Ceramics Prepared in an Air Atmosphere Furnace

Published on January 28th, 2010 | By: pwray@ceramics.org

Volume 6 Issue 6, Pages 702 – 716

 

Kevin Plucknett
Published Online: Nov 5 2008 3:08PM

DOI: 10.1111/j.1744-7402.2008.02309.x

 

ABSTRACT
Porous silicon nitride (Si3N4)
ceramics have been sintered in a conventional air atmosphere furnace at
temperatures between 1500°C and 1700°C, with sintering additions of
either 5 or 10 wt% yttria (Y2O3).
The use of a protective α-Si3N4
powder bed helps to prevent significant oxidation of the samples during
sintering in air, keeping oxidation weight gains below 4 wt% (typically
<2.5 wt%). This compares with more typical weight
losses
when sintering in nitrogen. Samples prepared with a
protective α-Si3N4
powder bed exhibited sintered densities up to ∼89% of the theoretical,
which was notably higher than for similar compositions prepared in a
controlled nitrogen atmosphere (i.e., ∼70% of theoretical). Increased
densification is proposed to arise from the formation of a thin SiO2
surface layer on individual α-Si3N4
particles, through passive oxidation, which is expected to promote
viscous-flow densification at lower temperatures. The excess of SiO2,
relative to the starting composition, is also reflected in the final
phase assembly, with Si2N2O
formed in addition to the transformation of α- to β-Si3N4,
when sintering above ∼1550°C. A Si2N2O
gradient occurs, with the surface being approximately 80 vol% Si2N2O
and the bulk predominantly Si3N4,
for samples sintered at 1600°C and 1700°C. Conversely, Si2N2O
is not observed in any measurable volume for similar samples sintered
in nitrogen.

 

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