Thixoforming : Semi-solid Metal Processing

esults. Chemical attack in melt corrosion tests was significantly higher than in
contact corrosion, exacerbating analysis of corrosion samples and identification of
reaction mechanisms. This is aggravated by the necessity to use casting powder to
prevent oxidation of the steel melt, in particular when applying cyclic immersion of
the samples. Chemical interaction of ceramic samples with the steel melt is overlaid
by reaction with the chemically aggressive, low-viscosity slag formed, acting as a
separating agent between the test sample and the liquid steel. Experimental results
hence allow for a rapid qualitative ranking of potential die materials, whereas phase
formation between Si3N4 and steel in the semi-solid state was examined by contact
corrosion tests. Significant differences in chemical interaction were observed depending
on the atmosphere present during testing. Under oxidizing conditions,
superficial oxidation of silicon nitride occurs, independently of the steel contact,
which was confirmed by EDS analysis of lateral parts of the Si3N4 corrosion samples.
Silicon/oxygen ratios of approximately 1:2 lead to the conclusion that Si3N4 decomposes
by releasing gaseous nitrogen and reacting with the surrounding oxygen to
form SiO2:
Si3N4 þ 3O2 ! 3SiO2 þ 2N2
ð8:2Þ
Nitrogen gas is either removed from the contact zone through cracks and channels in
the reaction layer or is entrapped in pores of different size within this layer
(Figure 8.43). Concurrently, the grain boundary phase of near-surface Si3N4, consisting
mainly of the sintering additives Y2O3 and Al2O3, is mobilized and segregates
in the contact zone in the form of a precipitation seam containing high amounts of
yttrium and aluminium. Both effects, release of nitrogen gas and mobilization of the
grain boundary phase, lead to depletion of the contact zone from intergranular phase
and the formation of considerable porosity.
Figure 8.43 (a) SEM image of the contact zone of Si3N4/
X210CrW12 contact corrosion at 1330 C/2 h in air. (b) Results of
EDS analysis.
8.6 Bulk Ceramic Forming Toolsj289