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Citation: Hassan, A.M.; Elsayed, H.;
Awaad, M.; Saleh, A.M.; Naga, S.M.
Microstructure, Mechanical and
Thermal Properties of ZTA/Al2TiO5
Ceramic Composites. Ceramics 2023,
6, 1977–1990. https://doi.org/
10.3390/ceramics6040121
Academic Editor: Sergey Mjakin
Received: 8 August 2023
Revised: 25 September 2023
Accepted: 29 September 2023
Published: 4 October 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
ceramics
Article
Microstructure, Mechanical and Thermal Properties of
ZTA/Al2TiO5Ceramic Composites
A. M. Hassan 1, Hamada Elsayed 2, 3, * , M. Awaad 3, A. M. Saleh 1and S. M. Naga 3
1Materials Engineering Department, Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt;
ahmedsaleh.1993@yahoo.com (A.M.S.)
2Department of Industrial Engineering, UniversitàDegli Studi di Padova, 35131 Padova, Italy
3Refractories Ceramics and Building Materials Department, National Research Centre, El-Bohous Str.,
Cairo 12622, Egypt
*Correspondence: hamada.elsayed@unipd.it
Abstract:
Zirconia-toughened alumina (ZTA)/Al
2
TiO
5
composites were prepared via a sol–gel route.
The prepared samples were uniaxially pressed and pressurelessly sintered at 1650–1700
◦
C for 1 h. The
microstructure, densification, and X-ray diffraction patterns of the sintered ZTA/Al
2
TiO
5
composites
were investigated, and their mechanical properties, thermal coefficient, and shock resistance were
characterized. The addition of Al
2
TiO
5
hindered the grain growth of the alumina particles and
enhanced the relative density, Vickers hardness, and bending strength of the composites compared
with pure ZTA samples. The fracture toughness was improved by 19% upon the addition of 40 wt%
Al
2
TiO
5
. Moreover, increasing the Al
2
TiO
5
content resulted in an improvement in the thermal
shock resistance.
Keywords: ZTA; aluminum titanite; fracture toughness; thermal shock resistance
1. Introduction
Zirconia (ZrO
2
)-toughened alumina (ZTA) ceramics are being intensively investigated
due to their excellent mechanical and thermal properties, such as high strength, dimensional
stability, high-temperature strength, hardness, wear resistance, and thermal resistivity [
1
–
3
],
which render them suitable for various applications ranging from structural and mechanical
to biomedical applications. Many research studies have been devoted to enhancing the
properties of ZTA composites. Thus, approaches such as the modulation of sintering,
oxide addition, and the development of alternative synthesis routes have been explored
to improve the fracture toughness of ZTA composites [
4
]. In particular, the introduction
of second additives has been demonstrated to reduce the sintering temperature, tailor the
microstructure, and enhance the final mechanical and thermal properties of ZTA ceramics.
Many authors have addressed the effect of ceramic additives on the properties of ZTA
composites [
5
–
8
]. It was suggested that the addition of optimum amounts of additives in
addition to using suitable techniques in sintering ZTA ceramics, such as temperature and
soaking time, have an essential influence on improving both the mechanical and thermal
properties of ZTA composites by the enhancement of their microstructure and the formation
of secondary phases. For instance, the addition of titanium oxide (TiO
2
) promotes the
sintering and grain growth of alumina (Al
2
O
3
) [
9
] due to the improvement in diffusivity
that occurs as a result of the increasing concentration of Al
3+
vacancies generated by the
substitution of Al3+ with Ti4+ [10,11].
Aluminum titanate (Al
2
TiO
5
) is a well-known refractory material that offers high ther-
mal properties such as low heat conductivity, high thermal shock resistance, low thermal
expansion, and appropriate refractoriness [
12
,
13
]. Therefore, it is a good candidate as the
second phase in Al
2
O
3
ceramics to enhance their properties for tribological purposes [
14
,
15
].
Ceramics 2023,6, 1977–1990. https://doi.org/10.3390/ceramics6040121 https://www.mdpi.com/journal/ceramics
Ceramics 2023,61978
Aluminum titanate (Al
2
TiO
5
) possesses a high melting point (1860
◦
C), low thermal
conductivity (0.9–1.5 Wm
−1
K
−1
), low thermal expansion coefficient (8.6
×
10
−6
K
−1
), and
high thermal shock resistance, which is conducive to improving the alumina composites’
mechanical and thermal properties [
16
–
19
]. Moreover, the limited ability to form a TiO
2
solution enhances the densification of Al
2
O
3
[
20
]. Borrell et al. [
21
] studied the addition of
40 vol.% aluminum titanate to alumina to prepare Al
2
TiO
5
/Al
2
O
3
composites via an in-situ
sintering reaction. They concluded that there is an excellent enhancement in the mechanical
properties of the final composite (approximately 24 GPa, 424 MPa, and 5.4 MPa m1/2 for
Vickers’s hardness, bending strength, and fracture toughness, respectively) due to the ho-
mogenous and finer microstructure obtained. Meybodi et al. [
22
] prepared Al
2
O
3
/Al
2
TiO
5
composites by reaction sintering of Al
2
O
3
and TiO
2
nano powders. They investigated that
high sintering temperatures improved the densification and hardness of the composite. On
the other hand, Moritz and Aneziris [
23
] stated that the presence of Al
2
TiO
5
in alumina-rich
magnesium aluminate improves the thermal shock resistance of the formed composite as a
result of the micro-cracked structure of Al2TiO5ceramics obtained.
This study describes the investigation of the effect of the addition of Aluminum titanate
(Al
2
TiO
5
) on the microstructure, mechanical, and thermal properties of ZTA/Al
2
TiO
5
composites. The composite structure was investigated via X-ray diffraction (XRD) and
scanning electron microscopy (SEM) analyses. Moreover, composite’ properties such as
densification parameters, the thermal expansion coefficient, shock resistance, fracture
toughness, and bending strength were evaluated and interrelated.
2. Materials and Methods
2.1. Materials and Synthesis Methods
Samples composed of Zirconia (ZrO
2
)-toughened alumina composition (ZTA), where
Zirconia was stabilized with 5 mol% yttrium oxide (Y
2
O
3
), were prepared using the follow-
ing starting materials: Al
2
O
3
(99.98% purity, provided by Almatis Gmbh, Ludwigshafen,
Germany), zirconium (IV) n-butoxide, and yttrium nitrate (made of yttria (Y
2
O
3
) provided
by Strem Chemicals, Newburyport, MA, USA). The synthesis of 5 mol% yttria (Y
2
O
3
)-
partially stabilized ZrO
2
(PSZ) was conducted according to reported procedures [
24
].
Briefly, 5 mol% Yttria-partially stabilized zirconia (Y-PSZ) was developed through hydrol-
ysis of zirconium (IV)-n-butoxide (ZR, Stream Chemicals USA) and with the addition of
yttrium nitrate (after the dissolution of yttrium oxide (Y
2
O
3
) in nitric acid HNO
3
). The
formed gel was dried at 120
◦
C overnight, and then was calcined at 900
◦
C for 2h, using a
heating rate of 5
◦
C, to remove all organic and nitrate materials. After calcination, the syn-
thesized powders were grounded, at 300 rpm for 2 h, in a zirconia jar using a planetary ball
mill, to obtain fine powder free of agglomerates. The zirconia (ZrO
2
)-toughened alumina
composition (ZTA) was made by mixing powders of 90 wt% Al2O3and 10 wt% Y-PSZ.
Aluminum titanate (Al
2
TiO
5
, AT) was prepared from aluminum isopropoxide (C
9
H
21
O
3
Al)
and titanium (IV) n-butoxide (C
16
H
36
O
4
Ti) following the procedure described by Naga
et al. [
25
]. Aluminum tri-isopropoxide ((C
3
H
7
O)
3
-Al, purity > 99%, Sigma-Aldrich,
Darmstadt
,
Germany) was hydrolyzed in distilled water at a ratio of 1:10 (Al:H
2
O), under vigorous
stirring at 80
◦
C for 3 h. After that, a stable and homogeneous sol was attained upon the
addition of nitric acid (2 mL) to produce (sol 1). In addition, the stoichiometric amount
of titanium tetrabutoxide (C
16
H
36
O
4
Ti, Strem Chemicals, Newburyport, MA, USA) was
dissolved in absolute ethanol. Distilled water was then added dropwise to the mixture with
a ratio of 1:20 (Ti:H
2
O). The mixture was kept under vigorous stirring at room temperature.
Also, Nitric acid was added to ensure the complete hydrolysis of the mixture and the
formation of stable and transparent sol (sol 2). At the end, sol 2 was added dropwise to
sol 1 under stirring until gelation occurred at 80
◦
C. The obtained gel was dried at 120
◦
C
overnight and then was calcined in an electric oven for 2 h at 900
◦
C, with a heating and
cooling rate of 5
◦
C/min. After that, the calcined Al
2
TiO
5
powders were grounded using a
planetary ball mill. The XRD analysis was performed to determine the phase assemblage
of the prepared ceramics.
Ceramics 2023,61979
The targeted composites of Aluminum titanate and Zirconia (ZrO
2
)-toughened alu-
mina were also achieved by a mechanical mixing route. The batch compositions are given
in Table 1. Cylindrical green ballets of ZTA/Al
2
TiO
5
, with dimensions of 13 mm in di-
ameter and 4 mm in height, were obtained via uniaxial pressing at 240 MPa followed by
pressureless sintering at different temperatures of 1650
◦
C, 1675
◦
C, and 1700
◦
C for a
holding time of 1 h, with heating and cooling rates of 5 ◦C/min.
Table 1. Composition of AT/ZTA batches, wt%.
Batch Symbol Composition (wt%)
ZTA Al2TiO5
Z0 100 0
Z1 90 10
Z2 80 20
Z3 70 30
Z4 60 40
2.2. Characterization
The densification behavior of the obtained composite samples was investigated ac-
cording to the liquid displacement method (ASTM C-20). The different crystal phases of the
powdered samples developed after sintering were characterized by means of X-ray diffrac-
tion analysis (XRD) with a Philips X-ray diffractometer, model PW1730, with a Cu target
and Ni filter. The XRD patterns were obtained at room temperature with a goniometric
range of 10–70◦2θ.
The microstructure of the polished surfaces of the sintered specimens was examined
with a scanning electron microscope (SEM-Jeol JSM-T20). The samples were thermally
etched at 50
◦
C lower than the sintering temperature for 20 min in air and coated with gold
(15 nm thickness) by means of electro-deposition in order to impart electric conduction.
2.3. Mechanical Testing
Vickers hardness measurements were conducted using a hardness tester (Omnimet
automatic MHK system Model Micro Met 5114, Buehler, Lake Bluff, IL, USA) according to
the method described by Anstis et al. [
26
]. The bending strength was determined using a
universal testing machine (Model LLOYD LRX5K of capacity 5 KN). Tested samples were
polished to a mirror surface. The measurements were conducted under a crosshead speed
of 0.5 mm/min and 25 mm support distance. Ten specimens were measured for each data
point. The following equation is used to evaluate the bending strength from the fracture
load obtained in the three-point bending test:
σ= 3PfL/2wt2
where
σ
is the bending strength, P
f
is the fracture load, Lis the length of the specimen, wis
the specimen width, and tis the thickness of the specimen t. The fracture toughness was
measured using the single-edge v-notched beam (SEVNB) technique [
27
]. For the SEVNB
method, ground and polished rectangular specimens (3
×
4
×
45 mm
3
) were notched
on the surface (3
×
45 mm
2
) using a diamond-charged cutting wheel, perpendicular to
the length of the rectangular bars. The depth of the notches was approximately 0.7 mm,
i.e.,
≤
20% of the height of the specimen in accordance with DIN 51109 [
28
]. The fracture
toughness was determined by applying the following equation [29]:
K1c= [Lmax/t(h1/2 )] ×[L0−Li/h]×[3RM(d/h)1/2/2(1 −d/h)3/2 ]
Ceramics 2023,61980
where L
max
is the maximum load, L
0
and L
i
are the outer and inner roller spans, respec-
tively, tand hare the thickness and height of the specimen, and dis the depth of the
sharpened notch.
RM= [1.9887 −1.326 (d/h)−[3.49 −0.68 (d/h) + 1.35 (d/h)2] (d/h) (1 −(d/h)]/(1 + (d/h))2.
2.4. Thermal Measurements
Thermal expansion experiments were performed on triangle bars with dimensions
of 25 mm
×
5 mm
×
5 mm, which were heated from ambient temperature up to 1100
◦
C
at a rate of 5
◦
C/min for both heating and cooling processes employing a NETZSCH DIL
402 PC dilatometer. The thermal shock resistance was measured using specimen bars with
dimensions of 60 mm
×
6 mm
×
6 mm. The bars were heated from room temperature
to 1000
◦
C at a rate of 5
◦
C/min, followed by holding the top temperature for 30 min.
The hot specimens were then immersed for 10 min in water at room temperature. The
abovementioned cycle was repeated up to 5, 10, 15, and 20 times for five specimens to
measure the retained bending strength, and the mean values were calculated.
3. Results
3.1. Phase Assemblage of the PSZ and Al2TiO5Ceramics
The XRD patterns of the obtained 5 mol% yttria (Y
2
O
3
)-partially stabilized ZrO
2
(PSZ)
and Aluminum titanate (Al
2
TiO
5
) powders are shown in Figures 1a and 1b, respectively.
Figure 1a confirms the synthesis of PSZ with pure Zirconia (ZrO
2
) as the main crystalline
phase’ where t-ZrO
2
represents the principal obtained phase with 89% of the overall ZrO
2
phase. Figure 1b shows The X-ray diffraction (XRD) patterns of the Al
2
TiO
5
ceramics
synthesized via the sol–gel technique after calcination at 900
◦
C. The interpretation of
XRD demonstrated the presence of Al
2
TiO
5
as the main crystalline phase in addition to
trace residues of alumina (Al
2
O
3
) and titania (TiO
2
), as previously reported in [
11
]. The
relatively lower calcination temperature was favored due to its role in the assemblage of
the nearly pure phase. In addition, the lower calcination temperature enhanced the thermal
stability by avoiding the decomposition of Al
2
TiO
5
into Al
2
O
3
and TiO
2
that occurs at high
calcination temperatures [30,31].
Ceramics 2023, 6, FOR PEER REVIEW 4
K
1c
= [L
max
/t (h
1/2
)] × [L
0
− L
i
/h] × [3R
M
(d/h)
1/2
/2(1 − d/h)
3/2
]
where L
max
is the maximum load, L
0
and L
i
are the outer and inner roller spans, respec-
tively, t and h are the thickness and height of the specimen, and d is the depth of the
sharpened notch.
R
M
= [1.9887 − 1.326 (d/h) − [3.49 − 0.68 (d/h) + 1.35 (d/h)
2
] (d/h) (1 − (d/h)]/ (1 + (d/h))
2
.
2.4. Thermal Measurements
Thermal expansion experiments were performed on triangle bars with dimensions of
25 mm × 5 mm × 5 mm, which were heated from ambient temperature up to 1100 °C at a
rate of 5 °C/min for both heating and cooling processes employing a NETZSCH DIL 402
PC dilatometer. The thermal shock resistance was measured using specimen bars with
dimensions of 60 mm × 6 mm × 6 mm. The bars were heated from room temperature to
1000 °C at a rate of 5 °C/min, followed by holding the top temperature for 30 min. The hot
specimens were then immersed for 10 min in water at room temperature. The abovemen-
tioned cycle was repeated up to 5, 10, 15, and 20 times for five specimens to measure the
retained bending strength, and the mean values were calculated.
3. Results
3.1. Phase Assemblage of the PSZ and Al
2
TiO
5
Ceramics
The XRD paerns of the obtained 5 mol% yria (Y
2
O
3
)-partially stabilized ZrO
2
(PSZ)
and Aluminum titanate (Al
2
TiO
5
) powders are shown in Figure 1a and 1b, respectively.
Figure 1a confirms the synthesis of PSZ with pure Zirconia (ZrO
2
) as the main crystalline
phase’ where t-ZrO
2
represents the principal obtained phase with 89% of the overall ZrO
2
phase. Figure 1b shows The X-ray diffraction (XRD) paerns of the Al
2
TiO
5
ceramics syn-
thesized via the sol–gel technique after calcination at 900 °C. The interpretation of XRD
demonstrated the presence of Al
2
TiO
5
as the main crystalline phase in addition to trace
residues of alumina (Al
2
O
3
) and titania (TiO
2
), as previously reported in [11]. The rela-
tively lower calcination temperature was favored due to its role in the assemblage of the
nearly pure phase. In addition, the lower calcination temperature enhanced the thermal
stability by avoiding the decomposition of Al
2
TiO
5
into Al
2
O
3
and TiO
2
that occurs at high
calcination temperatures [30,31].
Figure 1. XRD paerns for (a) PSZ and (b) Al
2
TiO
5
calcined at 900 °C.
3.2. Densification of the ZTA/Al
2
TiO
5
Ceramic Composites
Figure 1. XRD patterns for (a) PSZ and (b) Al2TiO5calcined at 900 ◦C.
Ceramics 2023,61981
3.2. Densification of the ZTA/Al2TiO5Ceramic Composites
Figure 2a,b shows the physical properties of the ZTA/Al
2
TiO
5
composites as a function
of the sintering temperature. Increasing the Al
2
TiO
5
content from 10 to 40 wt% increased
the bulk density of the studied samples when sintering at 1650
◦
C. Increasing the sintering
temperature to 1700
◦
C and 1750
◦
C positively affected the bulk density of all samples. The
apparent porosity showed the opposite trend, decreasing with both the Al
2
TiO
5
content and
the sintering temperature. This suggests that an alternative solid solution was created due
to the presence of small Ti
4+
ions (0.064 nm) compared with the larger Zr
4+
ions (0.079 nm),
which disrupted the crystalline structure, forming defects and enhancing the densification
process [32].
Ceramics 2023, 6, FOR PEER REVIEW 5
Figure 2a,b shows the physical properties of the ZTA/Al
2
TiO
5
composites as a func-
tion of the sintering temperature. Increasing the Al
2
TiO
5
content from 10 to 40 wt% in-
creased the bulk density of the studied samples when sintering at 1650 °C. Increasing the
sintering temperature to 1700 °C and 1750 °C positively affected the bulk density of all
samples. The apparent porosity showed the opposite trend, decreasing with both the
Al
2
TiO
5
content and the sintering temperature. This suggests that an alternative solid so-
lution was created due to the presence of small Ti
4+
ions (0.064 nm) compared with the
larger Zr
4+
ions (0.079 nm), which disrupted the crystalline structure, forming defects and
enhancing the densification process [32].
Figure 2. Densification parameters for AT/ZTA composites sintered at different sintering tempera-
tures in terms of (a) bulk density g/cm
3
and (b) apparent porosity %.
Pure ZTA samples showed a low relative density of 93% when sintered at 1700 °C for
1 h (Figure 3), likely because of their poor sinterability stemming from the deficiency of
defects in the ZrO
2
laice [33]. In contrast, the addition of 40 wt% Al
2
TiO
5
increased the
relative density of the composites at the same sintering temperature. In general,
ZTA/Al
2
TiO
5
samples displayed higher relative density than pure ZTA samples.
Figure 2.
Densification parameters for AT/ZTA composites sintered at different sintering tempera-
tures in terms of (a) bulk density g/cm3and (b) apparent porosity %.
Pure ZTA samples showed a low relative density of 93% when sintered at 1700
◦
C
for 1 h (Figure 3), likely because of their poor sinterability stemming from the deficiency
of defects in the ZrO
2
lattice [
33
]. In contrast, the addition of 40 wt% Al
2
TiO
5
increased
the relative density of the composites at the same sintering temperature. In general,
ZTA/Al2TiO5samples displayed higher relative density than pure ZTA samples.
Ceramics 2023,61982
Ceramics 2023, 6, FOR PEER REVIEW 6
Figure 3. Relative density of AT/ZTA composites sintered at 1700 °C.
3.3. Phase Composition of the ZTA/Al
2
TiO
5
Ceramic Composites
The XRD diffraction paerns of the samples sintered at 1700 °C are shown in Figure
4, which revealed the presence of α-Al
2
O
3
, t-ZrO
2
, and m-ZrO
2
as the main phase constit-
uents and Al
2
TiO
5
as a minor phase. The absence of any crystalline phases other than the
starting phases proves that the sintered samples did not undergo dissociation even at the
high sintering temperature of 1700 °C.
It was noticed that although the ZrO
2
content decreased gradually with the increase
in the Al
2
TiO
5
content, the t-ZrO
2
content was almost constant. It may be due to the Ti
4+
enhancement of t-ZrO
2
formation. It is well known that Ti
4+
ions are smaller than Zr
4+
ions.
During the sintering process, some Ti
4+
is partially substituted in the Zr
4+
laice, leading
to the laice distorting and enhancing t-ZrO
2
retention [34,35]. Additionally, the presence
of Al
2
TiO
5
reduced the particle size of ZrO
2
to be smaller than the critical size needed for
the transformation from the t-ZrO
2
to m-ZrO
2
[36,37].
Figure 3. Relative density of AT/ZTA composites sintered at 1700 ◦C.
3.3. Phase Composition of the ZTA/Al2TiO5Ceramic Composites
The XRD diffraction patterns of the samples sintered at 1700
◦
C are shown in Figure 4,
which revealed the presence of
α
-Al
2
O
3
, t-ZrO
2
, and m-ZrO
2
as the main phase constituents
and Al
2
TiO
5
as a minor phase. The absence of any crystalline phases other than the starting
phases proves that the sintered samples did not undergo dissociation even at the high
sintering temperature of 1700 ◦C.
Ceramics 2023, 6, FOR PEER REVIEW 7
Figure 4. XRD diffraction paern for the samples sintered at 1700 °C.
3.4. Microstructure of ZTA/Al
2
TiO
5
Ceramic Composites
Representative SEM micrographs of the ZTA/Al
2
TiO
5
composites sintered for 1 h at
various sintering temperatures are shown in Figure 5a–d. Large equiaxed Al
2
O
3
grains
were observed for the samples with the lowest Al
2
TiO
5
content (Z1 having 10% Al
2
TiO
5
).
Meanwhile, increasing the Al
2
TiO
5
content resulted in smaller and more homogeneously
distributed Al
2
O
3
grains (Figure 5d). Observation of the sample microstructure revealed
that the grain size increased with the Al
2
TiO
5
content, leading to the anisotropy of the
matrix structure [38]. Microcracks were observed on all samples, which may be due to the
anisotropy of Al
2
TiO
5
. Microcrack formation and Al
2
TiO
5
grain growth have been previ-
ously described as very prevalent phenomena in Al
2
TiO
5
ceramic bodies and are respon-
sible for the matrix structure anisotropy leading to the lowering of the thermal expansion
of the resultant composites [38–41]. The difference in the thermal expansion coefficient of
the components of the sample matrix, i.e., Al
2
TiO
5
, ZrO
2
, and Al
2
O
3
, and the consequent
stress could be a second factor promoting the formation of microcracks.
Figure 4. XRD diffraction pattern for the samples sintered at 1700 ◦C.
Ceramics 2023,61983
It was noticed that although the ZrO
2
content decreased gradually with the increase
in the Al
2
TiO
5
content, the t-ZrO
2
content was almost constant. It may be due to the Ti
4+
enhancement of t-ZrO
2
formation. It is well known that Ti
4+
ions are smaller than Zr
4+
ions.
During the sintering process, some Ti
4+
is partially substituted in the Zr
4+
lattice, leading
to the lattice distorting and enhancing t-ZrO
2
retention [
34
,
35
]. Additionally, the presence
of Al
2
TiO
5
reduced the particle size of ZrO
2
to be smaller than the critical size needed for
the transformation from the t-ZrO2to m-ZrO2[36,37].
3.4. Microstructure of ZTA/Al2TiO5Ceramic Composites
Representative SEM micrographs of the ZTA/Al
2
TiO
5
composites sintered for 1 h at
various sintering temperatures are shown in Figure 5a–d. Large equiaxed Al
2
O
3
grains
were observed for the samples with the lowest Al
2
TiO
5
content (Z1 having 10% Al
2
TiO
5
).
Meanwhile, increasing the Al
2
TiO
5
content resulted in smaller and more homogeneously
distributed Al
2
O
3
grains (Figure 5d). Observation of the sample microstructure revealed
that the grain size increased with the Al
2
TiO
5
content, leading to the anisotropy of the
matrix structure [
38
]. Microcracks were observed on all samples, which may be due to
the anisotropy of Al
2
TiO
5
. Microcrack formation and Al
2
TiO
5
grain growth have been
previously described as very prevalent phenomena in Al
2
TiO
5
ceramic bodies and are
responsible for the matrix structure anisotropy leading to the lowering of the thermal
expansion of the resultant composites [
38
–
41
]. The difference in the thermal expansion
coefficient of the components of the sample matrix, i.e., Al
2
TiO
5
, ZrO
2
, and Al
2
O
3
, and the
consequent stress could be a second factor promoting the formation of microcracks.
Ceramics 2023, 6, FOR PEER REVIEW 8
Figure 5. SEM micrographs of the polished, thermally etched sintered samples, (a) 10% AT, (b) 20%
AT, (c) 30% AT, and (d) 40% AT.
When increasing the Al
2
TiO
5
content, the Al
2
TiO
5
crystals adopted an elongated mor-
phology (rod-like shape). Figure 6a shows the microstructure of the Z4 sample (containing
40 wt% Al
2
TiO
5
), in which three types of Al
2
TiO
5
crystals could be identified, namely, very
fine crystals spaered all over the matrix, short rod-like crystals, and nonuniform crystals
with no specific shape. An energy-dispersive X-ray spectroscopy analysis (Figure 6b) con-
firmed the formation of the three crystalline shapes of the Al
2
TiO
5
phase.
Figure 5.
SEM micrographs of the polished, thermally etched sintered samples, (
a
) 10% AT, (
b
) 20%
AT, (c) 30% AT, and (d) 40% AT.
Ceramics 2023,61984
When increasing the Al
2
TiO
5
content, the Al
2
TiO
5
crystals adopted an elongated mor-
phology (rod-like shape). Figure 6a shows the microstructure of the Z4 sample (containing
40 wt% Al
2
TiO
5
), in which three types of Al
2
TiO
5
crystals could be identified, namely, very
fine crystals spattered all over the matrix, short rod-like crystals, and nonuniform crys-
tals with no specific shape. An energy-dispersive X-ray spectroscopy analysis (Figure 6b)
confirmed the formation of the three crystalline shapes of the Al2TiO5phase.
Ceramics 2023, 6, FOR PEER REVIEW 9
Figure 6. SEM micrograph of Z4 (40 mass % AT) sintered samples (a) and the EDS spectra (b–e) of
the different AT shapes marked on subfigure (a).
3.5. Mechanical and Thermal Properties of the ZTA/Al
2
TiO
5
Ceramic Composites
The mechanical properties of the sintered samples, including bending strength, Vick-
ers hardness, and fracture toughness, were determined as a function of the Al
2
TiO
5
con-
tent.
3.5.1. Three-Point Bending Strength Test Results
The addition of up to 20 wt% Al
2
TiO
5
resulted in a steady increase in the bending
resistance of the sintered bodies, as shown in Figure 7. While increasing the Al
2
TiO
5
con-
tent to 30 wt% significantly increased the bending strength. The microstructure of bodies
containing 30 wt% Al
2
TiO
5
and the pore’s shape and number reduction considerably en-
hanced the bending strength by lessening the loading area and thus enhanced the stress
concentrations [42]. It is evident from Figure 2 that increasing the Al
2
TiO
5
wt% reduced
the porosity % from 6.3% to 4%. Less pronounced increases were observed upon further
increasing the Al
2
TiO
5
content to 40 wt%. This lower increase rate in the mechanical
strength at Al
2
TiO
5
content over 40 wt% can be aributed to the fact that Al
2
TiO
5
became
a major phase, and Al
2
O
3
is known to be denser and have higher mechanical strength than
Al
2
TiO
5
. In addition, according to Ewais et al. [43], the difference in the thermal expansion
coefficient between Al
2
O
3
and Al
2
TiO
5
is the main factor limiting the bending strength of
the samples with higher Al
2
TiO
5
content.
Figure 6.
SEM micrograph of Z4 (40 mass % AT) sintered samples (
a
) and the EDS spectra (
b
–
e
) of
the different AT shapes marked on subfigure (a).
3.5. Mechanical and Thermal Properties of the ZTA/Al2TiO5Ceramic Composites
The mechanical properties of the sintered samples, including bending strength, Vickers
hardness, and fracture toughness, were determined as a function of the Al2TiO5content.
3.5.1. Three-Point Bending Strength Test Results
The addition of up to 20 wt% Al
2
TiO
5
resulted in a steady increase in the bending
resistance of the sintered bodies, as shown in Figure 7. While increasing the Al
2
TiO
5
content to 30 wt% significantly increased the bending strength. The microstructure of
bodies containing 30 wt% Al
2
TiO
5
and the pore’s shape and number reduction considerably
enhanced the bending strength by lessening the loading area and thus enhanced the stress
concentrations [
42
]. It is evident from Figure 2that increasing the Al
2
TiO
5
wt% reduced
the porosity % from 6.3% to 4%. Less pronounced increases were observed upon further
increasing the Al
2
TiO
5
content to 40 wt%. This lower increase rate in the mechanical
strength at Al
2
TiO
5
content over 40 wt% can be attributed to the fact that Al
2
TiO
5
became a
major phase, and Al
2
O
3
is known to be denser and have higher mechanical strength than
Ceramics 2023,61985
Al
2
TiO
5
. In addition, according to Ewais et al. [
43
], the difference in the thermal expansion
coefficient between Al2O3and Al2TiO5is the main factor limiting the bending strength of
the samples with higher Al2TiO5content.
Ceramics 2023, 6, FOR PEER REVIEW 10
Figure 7. Effect of Al
2
TiO
5
wt% content on the bending strength of AT/ZTA composite.
3.5.2. Vickers Hardness Measurements
The results of the Vickers hardness measurements for the sintered composite samples
are illustrated in Figure 8, which shows that the increase in the Al
2
TiO
5
content enhanced
the Vickers hardness. The sintered samples containing 40 wt% Al
2
TiO
5
displayed the high-
est hardness value (3652). Specifically, adding 40 wt% Al
2
TiO
5
increased the hardness of
the ZTA samples by 39%. The effect of the sintering behavior on the hardness must be
taken into consideration. The improvement in the hardness with the increase in the
Al
2
TiO
5
content is due to the enhancement in the physical properties. Figure 5d shows the
decrease in the porosity and the crack content in Z4 samples containing 40 wt% Al
2
TiO
5
,
which also displayed the best hardness value, compared with other samples. Accordingly,
it can be concluded that the hardness improved when improving the densification param-
eters of the samples, i.e., increasing the bulk density and decreasing the content of cracks
and pores [9].
Figure 8. Effect of Al
2
TiO
5
wt% content on the Vicker’s hardness of AT/ZTA composite.
Figure 7. Effect of Al2TiO5wt% content on the bending strength of AT/ZTA composite.
3.5.2. Vickers Hardness Measurements
The results of the Vickers hardness measurements for the sintered composite sam-
ples are illustrated in Figure 8, which shows that the increase in the Al
2
TiO
5
content
enhanced the Vickers hardness. The sintered samples containing 40 wt% Al
2
TiO
5
dis-
played the highest hardness value (3652). Specifically, adding 40 wt% Al
2
TiO
5
increased
the hardness of the ZTA samples by 39%. The effect of the sintering behavior on the
hardness must be taken into consideration. The improvement in the hardness with
the increase in the Al
2
TiO
5
content is due to the enhancement in the physical proper-
ties. Figure 5d shows the decrease in the porosity and the crack content in Z4 samples
containing 40 wt% Al
2
TiO
5
, which also displayed the best hardness value, compared
with other samples. Accordingly, it can be concluded that the hardness improved when
improving the densification parameters of the samples, i.e., increasing the bulk density
and decreasing the content of cracks and pores [9].
Ceramics 2023, 6, FOR PEER REVIEW 10
Figure 7. Effect of Al
2
TiO
5
wt% content on the bending strength of AT/ZTA composite.
3.5.2. Vickers Hardness Measurements
The results of the Vickers hardness measurements for the sintered composite samples
are illustrated in Figure 8, which shows that the increase in the Al
2
TiO
5
content enhanced
the Vickers hardness. The sintered samples containing 40 wt% Al
2
TiO
5
displayed the high-
est hardness value (3652). Specifically, adding 40 wt% Al
2
TiO
5
increased the hardness of
the ZTA samples by 39%. The effect of the sintering behavior on the hardness must be
taken into consideration. The improvement in the hardness with the increase in the
Al
2
TiO
5
content is due to the enhancement in the physical properties. Figure 5d shows the
decrease in the porosity and the crack content in Z4 samples containing 40 wt% Al
2
TiO
5
,
which also displayed the best hardness value, compared with other samples. Accordingly,
it can be concluded that the hardness improved when improving the densification param-
eters of the samples, i.e., increasing the bulk density and decreasing the content of cracks
and pores [9].
Figure 8. Effect of Al
2
TiO
5
wt% content on the Vicker’s hardness of AT/ZTA composite.
Figure 8. Effect of Al2TiO5wt% content on the Vicker’s hardness of AT/ZTA composite.
Ceramics 2023,61986
3.5.3. Fracture Toughness
The fracture toughness results shown in Figure 9indicate that the addition of Al
2
TiO
5
up to 20 wt% enhanced the fracture toughness. Specifically, the fracture toughness increased
from 5.48 to 5.98 MPa m
1/2
when increasing the Al
2
TiO
5
content from 0 and 20 wt%. Further
addition of Al
2
TiO
5
up to 40 wt% increased the fracture toughness by 19% (6.51 MPa m
1/2
).
The following factors can be invoked to explain the increase in the fracture toughness:
(1) An increase in the content of Al
2
TiO
5
rod-like grains, as shown in Figure 6, results
in the movement of the cracks in many planes and around the grains composing the
microstructure, crack bridging, which increases the energy required for fracture and boosts
the fracture toughness [
44
,
45
]. At the same time, a transgranular fracture would need
more energy in the presence of rod-like grains than with flat and plane grains. (2) Another
factor is the presence of a secondary phase, Al
2
TiO
5
, which helps in increasing the fracture
toughness through internal thermal effects [
46
–
49
]. With the increase in the Al
2
TiO
5
content
to 30 and 40 wt%, ZrO
2
could be partly dissolved in the Al
2
TiO
5
phase, forming a thin grain
boundary of an Al
2
O
3
–TiO
2
–ZrO
2
amorphous solid solution. Such a grain boundary solid
solution would promote grain boundary diffusion [
50
]. (3) Another factor is the decrease in
the porosity content of the sintered bodies and the reduction in the alumina grain size with
the increase in the Al2TiO5content (Figures 2and 5) also increases the fracture toughness.
Porosity causes localized stresses at the grain boundaries, which weaken the intracrystalline
binding and give rise to boundary looseness, enhancing the stress concentricity and the
tendency to fracture [51].
Ceramics 2023, 6, FOR PEER REVIEW 11
3.5.3. Fracture Toughness
The fracture toughness results shown in Figure 9 indicate that the addition of Al
2
TiO
5
up to 20 wt% enhanced the fracture toughness. Specifically, the fracture toughness in-
creased from 5.48 to 5.98 MPa m
1/2
when increasing the Al
2
TiO
5
content from 0 and 20 wt%.
Further addition of Al
2
TiO
5
up to 40 wt% increased the fracture toughness by 19% (6.51
MPa m
1/2
). The following factors can be invoked to explain the increase in the fracture
toughness: (1) An increase in the content of Al
2
TiO
5
rod-like grains, as shown in Figure 6,
results in the movement of the cracks in many planes and around the grains composing
the microstructure, crack bridging, which increases the energy required for fracture and
boosts the fracture toughness [44,45]. At the same time, a transgranular fracture would
need more energy in the presence of rod-like grains than with flat and plane grains. (2)
Another factor is the presence of a secondary phase, Al
2
TiO
5
, which helps in increasing
the fracture toughness through internal thermal effects [46–49]. With the increase in the
Al
2
TiO
5
content to 30 and 40 wt%, ZrO
2
could be partly dissolved in the Al
2
TiO
5
phase,
forming a thin grain boundary of an Al
2
O
3
–TiO
2
–ZrO
2
amorphous solid solution. Such a
grain boundary solid solution would promote grain boundary diffusion [50]. (3) Another
factor is the decrease in the porosity content of the sintered bodies and the reduction in
the alumina grain size with the increase in the Al
2
TiO
5
content (Figures 2 and 5) also in-
creases the fracture toughness. Porosity causes localized stresses at the grain boundaries,
which weaken the intracrystalline binding and give rise to boundary looseness, enhancing
the stress concentricity and the tendency to fracture [51].
Figure 9. Effect of Al
2
TiO
5
wt% content on the fracture toughness of AT/ZTA composite.
3.5.4. Reversible Thermal Expansion
Al
2
TiO
5
exhibits a very low thermal expansion due to its large thermal expansion
anisotropy [52]. In contrast, PSZ has a high thermal expansion because of the increase in
its laice constant with the addition of Y
2
O
3
, which decreases the laice binding energy.
Under normal conditions, the thermal expansion coefficient increases as the laice con-
stant increases [53] because the binding energy between cation and anion decreases with
increasing the ionic distance. Furthermore, more oxygen vacancies in PSZ are formed by
increasing the Y
2
O
3
content, which decreases the binding energy of the crystal. Therefore,
the thermal expansion coefficient of ZrO
2
becomes larger with the Y
2
O
3
addition [54].
As expected, the thermal expansion coefficient decreases with the increase in the
amount of aluminum titanate phase, which has a lower thermal expansion compared to
both alumina and zirconia, according to the simple rule-of-mixtures for a two-phase com-
posite, as shown in Figure 10. The previous results agree with the report by Zhu et al. [55],
Figure 9. Effect of Al2TiO5wt% content on the fracture toughness of AT/ZTA composite.
3.5.4. Reversible Thermal Expansion
Al
2
TiO
5
exhibits a very low thermal expansion due to its large thermal expansion
anisotropy [
52
]. In contrast, PSZ has a high thermal expansion because of the increase in
its lattice constant with the addition of Y
2
O
3
, which decreases the lattice binding energy.
Under normal conditions, the thermal expansion coefficient increases as the lattice con-
stant increases [
53
] because the binding energy between cation and anion decreases with
increasing the ionic distance. Furthermore, more oxygen vacancies in PSZ are formed by
increasing the Y
2
O
3
content, which decreases the binding energy of the crystal. Therefore,
the thermal expansion coefficient of ZrO2becomes larger with the Y2O3addition [54].
As expected, the thermal expansion coefficient decreases with the increase in the
amount of aluminum titanate phase, which has a lower thermal expansion compared
to both alumina and zirconia, according to the simple rule-of-mixtures for a two-phase
composite, as shown in Figure 10. The previous results agree with the report by Zhu
et al. [
55
], which deals with the addition of TiO
2
to Nd
2
O
3
–Al
2
O
3
–SiO
2
glass ceramics. In
Ceramics 2023,61987
addition, the increase in the crystal structure anisotropy of the samples containing higher
Al2TiO5content contributes to lowering the thermal expansion (Figure 5).
Ceramics 2023, 6, FOR PEER REVIEW 12
which deals with the addition of TiO
2
to Nd
2
O
3
–Al
2
O
3
–SiO
2
glass ceramics. In addition,
the increase in the crystal structure anisotropy of the samples containing higher Al
2
TiO
5
content contributes to lowering the thermal expansion (Figure 5).
Figure 10. Thermal expansion of the AT/ZTA composites as a function of Al
2
TiO
5
content.
3.5.5. Thermal Shock Resistance (TSR)
The TSR can be evaluated by measuring the bending strength after performing
quenching experiments in water [56]. A three-point bending test was conducted after 5,
10, 15, and 20 cycles of TSR, and the results are shown in Figure 11.
Figure 11. Reduction % in bending strength of AT/ZTA samples after 5, 10, 15, and 20 cycles of ther-
mal shock.
After 20 thermal shock cycles, there was no sign of cracking or breakdown in any of
the tested samples. The thermal shock of ceramic materials is affected by diverse factors,
such as heat transmission, the sample form and size, and the mechanical and thermal
properties and porosity % [57–59]. In general, ceramics are sentient to thermal shock be-
cause of their poor thermal conductivity properties. Consequently, sudden heat variations
cause temporal thermal fatigue in ceramics, which results in either microcracking or even
total failure [56].
Noteworthily, the TSR was improved by increasing the Al
2
TiO
5
content. The Al
2
TiO
5
-
free ZTA samples showed the worst results, which are in agreement with some research-
ers [59,60]. The TSR enhancement could be due to several factors, one of them being the
pre-existing flaws in Al
2
TiO
5
that enabled the flaw-tolerant during thermal shock. Also, it
could be due to the mismatch in the thermal expansion coefficient of the constitutive
phases, i.e., 8.2 × 10-6 K
−1
for α-Al
2
O
3
, 6.5 × 10
−6
K
−1
for m-ZrO
2
, 10.5 × 10
−6
K
−1
for t-ZrO
2
,
Figure 10. Thermal expansion of the AT/ZTA composites as a function of Al2TiO5content.
3.5.5. Thermal Shock Resistance (TSR)
The TSR can be evaluated by measuring the bending strength after performing quench-
ing experiments in water [
56
]. A three-point bending test was conducted after 5, 10, 15, and
20 cycles of TSR, and the results are shown in Figure 11.
Ceramics 2023, 6, FOR PEER REVIEW 12
which deals with the addition of TiO
2
to Nd
2
O
3
–Al
2
O
3
–SiO
2
glass ceramics. In addition,
the increase in the crystal structure anisotropy of the samples containing higher Al
2
TiO
5
content contributes to lowering the thermal expansion (Figure 5).
Figure 10. Thermal expansion of the AT/ZTA composites as a function of Al
2
TiO
5
content.
3.5.5. Thermal Shock Resistance (TSR)
The TSR can be evaluated by measuring the bending strength after performing
quenching experiments in water [56]. A three-point bending test was conducted after 5,
10, 15, and 20 cycles of TSR, and the results are shown in Figure 11.
Figure 11. Reduction % in bending strength of AT/ZTA samples after 5, 10, 15, and 20 cycles of ther-
mal shock.
After 20 thermal shock cycles, there was no sign of cracking or breakdown in any of
the tested samples. The thermal shock of ceramic materials is affected by diverse factors,
such as heat transmission, the sample form and size, and the mechanical and thermal
properties and porosity % [57–59]. In general, ceramics are sentient to thermal shock be-
cause of their poor thermal conductivity properties. Consequently, sudden heat variations
cause temporal thermal fatigue in ceramics, which results in either microcracking or even
total failure [56].
Noteworthily, the TSR was improved by increasing the Al
2
TiO
5
content. The Al
2
TiO
5
-
free ZTA samples showed the worst results, which are in agreement with some research-
ers [59,60]. The TSR enhancement could be due to several factors, one of them being the
pre-existing flaws in Al
2
TiO
5
that enabled the flaw-tolerant during thermal shock. Also, it
could be due to the mismatch in the thermal expansion coefficient of the constitutive
phases, i.e., 8.2 × 10-6 K
−1
for α-Al
2
O
3
, 6.5 × 10
−6
K
−1
for m-ZrO
2
, 10.5 × 10
−6
K
−1
for t-ZrO
2
,
Figure 11.
Reduction % in bending strength of AT/ZTA samples after 5, 10, 15, and 20 cycles of
thermal shock.
After 20 thermal shock cycles, there was no sign of cracking or breakdown in any of the
tested samples. The thermal shock of ceramic materials is affected by diverse factors, such
as heat transmission, the sample form and size, and the mechanical and thermal properties
and porosity % [
57
–
59
]. In general, ceramics are sentient to thermal shock because of
their poor thermal conductivity properties. Consequently, sudden heat variations cause
temporal thermal fatigue in ceramics, which results in either microcracking or even total
failure [56].
Noteworthily, the TSR was improved by increasing the Al
2
TiO
5
content. The
Al
2
TiO
5
-free ZTA samples showed the worst results, which are in agreement with some
researchers
[59,60]
. The TSR enhancement could be due to several factors, one of them
being the pre-existing flaws in Al
2
TiO
5
that enabled the flaw-tolerant during thermal
shock. Also, it could be due to the mismatch in the thermal expansion coefficient of
the constitutive phases, i.e., 8.2
×
10
−6
K
−1
for
α
-Al
2
O
3
, 6.5
×
10
−6
K
−1
for m-ZrO
2
,
10.5
×
10
−6
K
−1
for t-ZrO
2
, and 1
×
10
−6
K
−1
for Al
2
TiO
5
. Such a mismatch can pro-
mote the formation of microcracks to some extent, which consumes stress energy and
Ceramics 2023,61988
prevents sample failure. In addition, these microcracks provide the transformation from
t-ZrO
2
to m-ZrO
2
with sufficient space to occur, avoiding crack formation and sample
failure [
61
,
62
]. Additionally, the microstructure toughening of the ZTA matrix containing
Al
2
TiO
5
particles as a second phase modulates the microstructure and enhances the TSR.
4. Conclusions
The following conclusions can be extracted from the investigation of the effect of
Al
2
TiO
5
addition on the microstructure, mechanical, and thermal properties of ZTA/Al
2
TiO
5
composites:
(1) Three types of Al
2
TiO
5
crystals were observed in the microstructure of the com-
posites, i.e., very fine crystals spattered all over the matrix, short rod-like crystals, and
nonuniform crystals with no specific shape, which affected the mechanical properties of
the composites.
(2) The fracture toughness of the composites increased upon increasing the Al
2
TiO
5
content up to 40% most likely due to an increase in the content of rod-like grains, which
caused the displacement of cracks.
(3) The mismatch in the thermal expansion coefficient between the constitutive phases
(
α
-Al
2
O
3
, ZrO
2
, and Al
2
TiO
5
) significantly enhanced the thermal shock resistance of the
ZTA/Al2TiO5composites compared with pure ZTA.
Author Contributions:
Conceptualization, S.M.N. and M.A.; formal analysis, A.M.H. and H.E.;
investigation, A.M.H., H.E., M.A., A.M.S. and S.M.N.; methodology, A.M.H. and A.M.S.; supervision,
A.M.H. and S.M.N.; validation, A.M.H., H.E. and S.M.N.; writing—original draft, A.M.H., H.E.
and A.M.S.; writing—review and editing, H.E. and S.M.N. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data presented in this study are available upon request from the
corresponding author.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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