Fracture toughness enhancement of zirconia toughened alumina (ZTA) through additions of combination of CaCO3 and CaO
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Date
2016-09-01
Authors
Zhwan Dilshad Ibrahm Sktani
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Abstract
Zirconia Toughened Alumina (ZTA) is a successful ceramic compound in new
technological industry. However, its low fracture toughness limited the usage of ZTA
in many engineering applications. Therefore, the enhancement of fracture toughness is
necessary. Nonetheless, it is crucial to maintain high hardness and enhance the fracture
toughness to make ZTA more reliable for technological applications. In-situ formation
of elongated CaAl12O19 grains inside ZTA ceramics during sintering process is
preferred. This is due to its easy to be sintered, safety from health hazards and to avoid
more complicated and less economical methods of sintering. CaO and CaCO3 powders
were added into ZTA, wet-mixed, uniaxially pressed and ZTA samples formed by
pressureless solid state sintering. The current study is divided to three parts. The first
part is the addition of CaCO3 alone into ZTA. The fracture toughness was improved
from 5.95 MPa.m1/2 for pure ZTA to 6.3 MPa.m1/2 for ZTA added with 0.5 wt. % of
CaCO3 due to crack deflection mechanism along elongated CaAl12O19 grains. However,
the hardness decreased due to emission of CO2 which creates porous microstructure.
Hence, CaO and CaCO3 added together into ZTA to reduce the porosity and
simultaneously, obtain elongated CaAl12O19 grains which enhances the fracture
toughness of ZTA through crack deflection and crack bridging mechanisms. In the
second part, combination addition between CaO with CaCO3 was fixed at 0.5 wt. %.
The hardness was improved and better fracture toughness was obtained due to less
porous microstructure of ZTA and control of the size and shape of CaAl12O19 grains.
The optimum ZTA composition was added with 0.4 wt. % CaO combined with 0.1 wt.
% CaCO3. This composition has the maximum fracture toughness (6.51 MPa.m1/2) and
reasonable hardness (1592 HV). Therefore, the CaO/CaCO3 ratio of 4:1 was selected as
the base for the third part. The third part is to study the interaction effects of three
parameters: sintering temperature, soaking time and combination addition between CaO
and CaCO3 on the two responses: fracture toughness and Vickers hardness of ZTA. The
Design of Experiments (DOE) was implemented to reduce the number of tests and
meanwhile the optimisation process was employed to optimise the effective range of
responses. After applying Response Surface Methodology (RSM), it was proved that
sintering temperature is the most influence parameter on the fracture toughness and
hardness of ZTA ceramics. The optimum results (fracture toughness of 6.84 MPa.m1/2
and hardness 1615 HV) obtained from sintering temperature at 1600 °C, combination
addition between CaO and CaCO3 of 0.95 wt.%, and soaking time for 2.14 hr. The other
designs such as Analysis of Variance (ANOVA), Lack of Fit (LOF), 3D mapping and
Residual Analysis have also confirmed the adequacy of the result. However,
confirmation results obtained from experiments found that the optimum results (fracture
toughness 7.1 MPa.m1/2 and Vickers hardness was 1584 HV) were obtained from 1.05
wt.% of combination addition between CaO and CaCO3 into ZTA samples, sintering
temperature of 1600 °C and soaking time of 2.9 hr. These values are comparable with
empirical formulas from RSM.