217
27
Nanomechanical Behavior of ZTA
Sadanand Sarapure, Arnab Sinha, Arjun Dey,
and Anoop Kumar Mukhopadhyay
27.1 Introduction
In the present chapter, we will be describing the nanoindentation behavior
of a tough ceramic, i.e., zirconia-toughened alumina (ZTA). Generally, zirco-
nia is known to be a strong ceramic. It has failure strength (650 MPa) nearly
1.5times that of alumina. Similarly, it has a critical strain energy release rate
(≈153 Jm
−2
) about 7.5 times as high as that of alumina (20 Jm
−2
). To obtain
high toughness for structural application purposes, tetragonal zirconia is
incorporated in an alumina matrix that gets toughened [13]. When the alu-
mina matrix contains the tetragonal phase of appropriate amount, what we
get are the ZTA ceramics. These ZTA ceramics often show an R-curve (crack
resistance curve) behavior, which means that the material has an intrinsic
capacity to exhibit an increase in fracture toughness with indentation crack
length [4]. Transformation toughening occurs when the retained metastable
tetragonal-ZrO
2
(t-ZrO
2
) transforms to the stable monoclinic-ZrO
2
(m-ZrO
2
)
phase in the tensile stress eld around a propagating crack. The volume
expansion (4%5%) characteristic of the t m transformation introduces
a net compressive stress in the process zone around the crack tip. This
reduces the local crack-tip stress intensity and hence the driving force for
crack propagation, thereby increasing the effective toughness of the mate-
rial. The related issues were discussed in detail in an excellent review by
Basu [5].
In the present case, 10, 20, and 40 vol% zirconia have been incorporated
in an alumina matrix and are named 10ZTA, 20ZTA, and 40ZTA, respec-
tively. The microstructures are shown in Figures27.1a–c, especially in back-
scattered mode in SEM, to identify the alumina and zirconia phases. Here,
the brighter phase corresponds to the zirconia phase, and the gray phase
indicates the alumina phase.
218 Nanoindentation of Brittle Solids
27.2 Nanomechanical Behavior
The nanoindentation experiments have been conducted at 1000 mN in the
Fischerscope H100XY
p
nanoindentation machine equipped with a Vickerstip.
The details of this machine have already been described in Chapter 9.
Figure27.2 shows a typical nanoindentation array in 10ZTA. No signature of
cracking has been identied. Further, the typical Ph plots of different ZTA
composites are shown in Figure27.3.
The data on nanohardness and Young’s modulus are plotted in Figure27.4
as a function of the vol% of zirconia in the ZTA batch compositions. The
nanohardness of the alumina matrix was ≈14 GPa. However, incorporation of
10vol% zirconia degraded it to ≈11 GPa. In 20ZTA, a subsequent decrement was
observed as ≈10 GPa. Further, incorporation of zirconia (40 vol%) in the 40ZTA
sample showed almost 40% further reduction in nanohardness value to ≈6 GPa.
In the case of Young’s modulus (E), parent alumina showed a value of
≈403GPa, which indicates a dense structure. However, with incorporation of
10 vol% zirconia, the modulus value was drastically decreased to ≈325GPa.
In20ZTA, further decrement was seen, as E was ≈310 GPa. However, with
incorporation of 40 vol% of zirconia, almost three times reduction was
observed, as E was now only about 109 GPa.
(a) (b)
(c)
FIGURE 27.1
Microstructures of (a) 10ZTA, (b) 20ZTA, and (c) 40ZTA.
219Nanomechanical Behavior of ZTA
Our work would be incomplete if we didn’t measure fracture toughness
of such ZTA composites. The main objective is to develop ZTA for the pur-
pose of obtaining damage-tolerant characteristics. However, in the aforesaid
nanoindentation studies, our load is not high enough to be able to initiate the
cracks so that it would be possible to measure the indentation fracture tough-
ness (IFT). Therefore, we deliberately employed Vickers macroindentation at
a relatively higher load of ≈147 N. The indentation fracture toughness data as
a function of the volume percent of ZrO
2
in the ZTA batch compositions are
shown in Figure27.5. The matrix alumina showed a very low value of IFT
of ≈2.2 MPam
0.5
. But even with the small incorporation of 10 vol% ZrO
2
in
the ZTA batch composition, IFT increased dramatically by nearly 2.5times,
FIGURE 27.2
Nanoindentation array on 10ZTA at 1000 mN.
0 500 1000 1500 2000
2500
0
200
400
600
800
1000
Load (mN)
Depth (nm)
10 ZTA
20 ZTA
40 ZTA
FIGURE 27.3 (See color insert.)
Ph plots on different ZTA composites.

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