Biju Patnaik Central Library

Thesis (Ph.D)- National Institute of Technology, Rourkela

Al
2
O
3
-ZrO
2
composite precursor powder containing 5-15mol% ZrO
2
is prepared by wet
chemical route. The washed gel containi
ng pseudoboehmite and amorphous zirconia are
characterized with respect to DTA/TG, XRD and IR spectroscopy. In the calcined powder
phase evolution of Al
2
O
3
follows the sequence: pseudoboehmite
→
bayerite
→
boehmite
→
γ
-Al
2
O
3
→
θ
-Al
2
O
3
→
α
-Al
2
O
3
; while that of ZrO
2
follows amorphous ZrO
2
→
t-ZrO
2
→
(t +
m)-ZrO
2
. However, the crystallization behavior of aluminum-hydroxide is somewhat affected
with addition of zirconia. FTIR study reveal the number of M-OH and M-O bond increases on
increasing mol% of ZrO
2
due to a change in the cationic charge of the composite powder. The
formation of amorphous material before crystallization can also be predicted from TEM study,
where particle exhibits different morphology; and show smooth, distinct and faceted surfaces
at different temperature. A bi
modal particle size distribution is observed because of both
unagglomerated and partial agglomerated partic
les in the range from15 to 210nm for highest
zirconia content, however, the specific surface area of powder decreases with increasing
zirconia content. The addition of 5wt% uncal
cined precursor enhances the consolidation
process. The sintered zirconia toughened alum
ina nano-composite with 98% relative density
has been achieved at 1550
o
C for 4h. The average grain size ratio
d
Al2O3
/
d
ZrO2
varies in between
4.35 for Al
2
O
3
-5ZrO
2
and 2.29 for Al
2
O
3
-15ZrO
2
, whereas average grain size of alumina and
zirconia in Al
2
O
3
-10ZrO
2
composite are 360nm and 125nm respectively. EPMA confirms the
near uniform distribution of zirconia part
icles in the alumina matrix. For homogenous
dispersion of ZrO
2
in Al
2
O
3
, the optimum amount of ZrO
2
is found to be 10mol% and up to
the critical amount of ZrO
2
addition, flexural strength and fracture toughness increase and the
size of the intergranular zirconia strongly influences these properties. The room temperature
fracture strength of these composite varies within ~500 to 700MPa, but drastic decrease of
high temperature strength for lowest zirconia containing specimen is observed above 600
0
C,
where transformation toughening is no longer effective. The elastic modulus and hardness,
both the properties decrease with ZrO
2
content. Thermal shock study reveals the retained
flexural strength varies with zirconia content.
The hystersis effect in the dilatation curve due
to t
→
m transformation is also influenced by the grain size of ZrO
2
.