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Original Article
Development and characterization of alumina-
toughened zirconia (ATZ) ceramic composites
doped with a beneficiated rare-earth oxide
extracted from natural ore
Eduardo de Sousa Lima a,*, Camila Catalano Gall a,
Manuel Fellipe R.P. Alves b, Jos�e Brant de Campos c,
Tiago Moreira Bastos Campos d, Claudinei dos Santos a,e
a Materials Science Department, Military Institute of Engineering, Praça Gen. Tibúrcio, 80, Praia Vermelha, 22290-
270, Rio de Janeiro, RJ, Brazil
b Mechanical and Energy Department, Rio de Janeiro State University, UERJ-FAT, Rodovia Presidente Dutra, Km 298,
27537-000, Resende, RJ, Brazil
c Mechanical Engineering Department, Rio de Janeiro State University, Rua Fonseca Teles, 121, S~ao Cristov~ao, 20940-
200, Rio de Janeiro, RJ, Brazil
d Physics Department, Aeronautics Institute of Technology, Praça Marechal Eduardo Gomes, 50 Vila Das Ac�acias,
12228-900, S. J. Campos, SP, Brazil
e Federal Fluminense University - UFF/EEIMVR, Av. Dos Trabalhadores,420, V. S.Cecı́lia, 27255-125, Volta Redonda,
RJ, Brazil
a r t i c l e i n f o
Article history:
Received 27 October 2021
Accepted 25 November 2021
Available online 4 December 2021
Keywords:
Ce-TZP/Al2O3 composites
Rare-earth oxides
YAG - Y3Al5O12
Characterization
Mechanical properties
Brazilian xenotima
* Corresponding author.
E-mail address: sousalima@ime.eb.br (E.S
https://doi.org/10.1016/j.jmrt.2021.11.141
2238-7854/© 2021 Published by Elsevier B.V. T
licenses/by-nc-nd/4.0/).
a b s t r a c t
This work analysed the effect of the addition of rare-earth mixed oxide (RE2O3), a solid
solution of Y2O3 and other rare earths benefited from a Brazilian natural ore called Xen-
otime, on the sintering and properties of a commercial nanopowder composed of Ce-
tetragonal polycrystal zirconia (TZP) þ 15 wt.% Al2O3. Powder mixtures were prepared,
adding 10 wt.% of Y2O3 or 10 wt.% RE2O3 in Ce-TZP/Al2O3 powder, which were compacted
and sintered at 1500 �C for 2 h. Sintered samples were characterized by X-ray diffraction,
scanning electron microscopy, and relative density. Structural analyses and phase quan-
tification were performed using the Rietveld refinement method. Mechanical character-
ization - Vickers hardness and fracture toughness - of the samples was carried out by
Vickers indentation. The results indicated that the RE2O3 is composed of a solid Y2O3 so-
lution with lattice parameters slightly lower than those of pure Y2O3 due to the presence of
other oxides such as Yb2O3 (19.7%), Er2O3 (13.9%), or Dy2O3 (10.2%). During sintering, the
oxides added to the composite were completely consumed by the Ce-TZP/Al2O3 matrix to
form two different crystalline phases: ZrO2-Cubic and Y3Al5O12. As a consequence, multi-
phase composites with relative density of 93.8 ± 1.2 (Y2O3 reinforced) and 94.5 ± 1.7
(RE2O3 reinforced), average hardness in the order of 10.5 GPa, and fracture toughness of 7.1
e8.5 MPa m1/2 were obtained. The high fracture toughness observed was a consequence of
. Lima).
his is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
mailto:sousalima@ime.eb.br
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https://doi.org/10.1016/j.jmrt.2021.11.141
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https://doi.org/10.1016/j.jmrt.2021.11.141
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j o u r n a l o f ma t e r i a l s r e s e a r c h a nd t e c h no l o g y 2 0 2 2 ; 1 6 : 4 5 1e4 6 0452
Table 1 e Specification of the materials
Material
Manufacturer (commercial specification)
ZrO2
CeO2
Al2O3
SiO2 þ Na2O
TiO2
Y2O3
Other rare-earth elements
Density (g/cm3)
Average particle size (mm)
Specific surface area (m2/g)
a Not determined.
the presence of different coupled toughening mechanisms resulting from the multiphase
characteristic of these composites.
© 2021 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
With the advancement of synthesis techniques in nano-
crystalline ceramic powders, many crystallographic and
microstructural phenomena and mechanical, thermal and/or
chemical properties have been explored in recent years aim-
ing to increase the potential use of these materials within the
engineering supply chain. Among these materials, those
intended for use as structural ceramics [1e5] that present high
fracture toughness, hardness, thermal shock and corrosion/
degradation resistance are of particular importance in appli-
cations such as ballistic shielding or protective layers for ar-
tefacts in marine environments.
ZrO2eAl2O3 composites [6e9], also known as alumina-
toughened zirconia (ATZ) ceramics, have great potential for
structural applications. These ceramic materials are usually
composed of a tetragonal zirconia polycrystalline (TZP) matrix
stabilized with yttria (Y-TZP). They are aimed at generating
solid bodies mainly composed of tetragonal grains (t-ZrO2)
that present a peculiar toughening mechanism through
tetragonal-to-monoclinic (t/m) phase transformation
[10e13]. When combined with alumina (Al2O3), this ATZ
composite presents higher hardness and fracture toughness
than those of other conventional ceramics such as monolithic
alumina (Al2O3), silicon carbide (SiC), silicon nitride (Si3N4),
among others [14e16]. On the other hand, these tetragonal
zirconia grains present in the ATZ composite have low resis-
tance to hydrothermal degradation, and thus are not indicated
for use in aqueous media for long periods. Therefore, Ce-TZP/
Al2O3 composites, which present ceria oxide (CeO2)-stabilized
grains in the TZP matrix and superior fracture toughness and
used in this work (man
Ce-TZP/Al2O3
Saint-Gobain,
UprYZr-Shock (France)
Balance
6 ± 0.7 wt.%
15 ± 1 wt.%
<0.04 wt.%
<0.005 wt.%
e
e
5.65
0.2
8.5
hydrothermal degradation resistance, are a technological
advance in these ATZ ceramics [17e19].
The yttrium aluminium garnet (YAG - Y3Al5O12) compound
is formed by the combination of yttrium oxide (Y2O3) and
alumina (Al2O3). This formation of Y3Al5O12 occurs under
thermal treatments at >1300 �C, and the YAG widely used in
optical applications [1,20,21], as Al2O3 toughener [22], as well
as the component used in the formation of the liquid phase
required for sintering/densification of covalent ceramics
based on silicon carbide (SiC) and/or silicon nitride (Si3N4)
[23,24], promote significant improvement in the mechanical
properties of these materials.
Recently, a process was developed in Brazil to obtain an
yttrium and rare-earth mixed oxide (RE2O3) with structural
characteristics close to those of pure Y2O3, but at a much
lower cost. RE2O3 production is based on the use of a very
abundant mineral called xenotime [25e27]. Xenotime, which
is basically an yttrium and rare-earth phosphate (Y,RE)PO4, is
mixed with NaOH and heated up to 700 �C, allowing the
reaction 1 to occur:
(Y,RE)PO4 þ 6NaOH / Na3(Y,RE)O3 þ Na3PO4 þ 3H2O (1)
This material is milled, lixiviated, precipitated with oxalic
acid, and calcined again to obtain a solid solution of Y2O3,
Yb2O3, Er2O3, Dy2O3 and smaller amounts of other rare earths
[28,29].
This study aimed to obtain the in situ formation of the
Y3Al5O12 phase during sintering of the Ce-TZP/Al2O3 com-
posite using a comparative form: addition of Y2O3 or RE2O3 to
the reaction with Al2O3 present in the chemical composition
of the mixture. Microstructural and crystallographic aspects,
ufacturing data).
Y2O3 RE2O3
H.C.Starck (Germany) University of S~ao Paulo (USP)
(Brazil)
e e
e e
e e
e e
99.9 wt.% 44.6mol%
Yb2O3 (19.7%), Er2O3
(13.9%), Dy2O3 (10.2%),
Ho2O3 (3.2%), Tm2O3 (2.8),
Lu2O3 (2.6%); (Gd2O3 þ Tb2O3
þ Sm2O3 - balance) (mol %)
5.01 5.12
0.5 0.8
a a
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Fig. 1 e XRD patterns of the starting powders: a) (Ce,Y)-TZP/
Al2O3. b) Y2O3; c) RE2O3.
j o u r n a l o f m a t e r i a l s r e s e a r c h and t e c hno l o g y 2 0 2 2 ; 1 6 : 4 5 1e4 6 0 453
aswell as the presence of three simultaneous phases and their
effects on densification, microstructure, and mechanical
properties were discussed.
2. Methods
2.1. Materials
Table 1 presents the specifications of the starting powders
used in this work.
2.2. Processing
Two compositions of the (Ce,Y)-TZP/Al2O3 composite were
studied in this work: one containing 10 wt.% Y2O3 and another
containing 10 wt.% RE2O3. Preparation of the ceramic bodies
started with mixing the powders with isopropyl alcohol for
2 h, aiming homogenization. After mixing, the powder
batches were dried in oven at 100 �C for 24 h. The powder
mixtureswere sieved through a 60-mesh screen. Themixtures
were compacted into a cylindrical matrix of 15 mm in diam-
eter by cold uniaxial pressing at 70 MPa for 30 s. Then the
green compacts were sintered in a MoSi2 furnace (Fortelab
F1600, Brazil). The heating rates varied as follows: 2 �C/min up
to 700 �C and 5 �C/min up to 1500 �C, with no sintering plateau.
The cooling rate was 10 �C/min. Comparatively, the as-
Table 2 e Crystallographic characteristics of the Y2O3, RE2O3 ox
Parameters Y2O3 RE2O3
Present phase Y2O3 Y2O3 (solid solution)
Percentage (%) 100 100
System Cubic Cubic
Space group Ia e3 (n� 206) Ia e3 (n� 206)
a (�A) 10.6079 (3) 10.588 (1)
Rp (%)/Rwp (%)/Rexp (%) 9.2/15.4/11.3 11.3/14.0/10.9
c2 1.88 1.66
received composite powder, was compacted, sintered under
similar processing conditions, and characterized.
2.3. Characterization
The apparent density and consequent relative density of the
composites were measured by the immersion in distilled
watermethod using the Archimedes principle. The theoretical
density of the samples was calculated according to the rule of
mixtures.
The crystalline phases were determined by X-ray diffrac-
tion (XRD) on a PANalytical EMPYREAN diffractometer
equippedwith a graphitemonochromator and Cu Ka radiation
(l ¼ 1.5406 �A), operating at a voltage of 40 kV and a current
emission of 40 mA, with a q-2q scanning system configured in
BraggeBrentano geometry. The data were obtained over a 2q
range of 10e90� at a scan rate of 0.2�/min and step size of 0.6 s/
step. Quantitative and present phase calculations and basic
parameters were obtained by applying the Rietveld refine-
ment method.
For microstructural analysis of the sintered samples, pol-
ished surfaces were thermally etched at 1400 �C for 15 min
using a furnace (FE1750-MAITEC, Brazil). The etched surfaces
were metalized using an Emitech K550X Sputter Coater
(Quorum Technologies - Kent, UK) with 30 mA for 2 min.
Microstructural analyses were performed in a scanning elec-
tron microscopy (JEOL® FEG JSM 7100FT) with an 80 mm2 EDS
detector (Oxford X-Max, UK). Microstructural analysis of the
sintered sampleswas performed using the ImageJ software on
a population of approximately 500 grains.
Hardness and fracture toughness (KIc) were determined by
the Vickers indentationmethod [30,31] using hardness testing
equipment (Emcotest DuraScan). In each group of samples, 20
indentations were measured under a load of 98 N for 30 s.
Fracture toughness was calculated according to Equation (2),
valid for Palmqvist-type cracks [32,33].
KIC ¼0:035
�
l
a
��1
2
�
Hv
EF
��2
5
 
Hva
1
2
F
!
(2)
where: KIC ¼ fracture toughness (MPa.m1/2); “l” ¼ length of the
crackmeasured from the tip of the indentation to the tip of the
crack (mm); “a” ¼ half length of the indentation diagonal (mm);
“HV” ¼ Vickers hardness (GPa); “E” ¼ Young's modulus (GPa);
"F" ¼ dimensionless constant developed by Niihara (F ¼ 2.7).
The crack lengths generated by the Vickers indentations
were observed by scanning electron microscopy (SEM) using a
TESCAN VEGA 3 XMU microscope in the secondary-electron
imaging mode.
ides and (Ce,Y)-TZP/Al2O3 composite from XRD analysis.
(Ce,Y)-TZP/Al2O3
ZrO2-m ZrO2-t CeO2 Al2O3
77.1 4.3 4.4 14.2
Monoclinic Tetragonal Cubic Hexagonal
Not evaluated
1.78
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Fig. 2 e a) XRD patterns of Ce-TZP/Al2O3 doped with 10% Y2O3 and 10% RE2O3 after sintering at 1500 �C for 2 h; b) expanded 2
q (38� ~ 42�); c) expanded 2 q (65� ~ 75�).
Fig. 3 e Quantification of the phases in the sintered
ceramic composites doped with 10% Y2O3 or 10% RE2O3.
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3. Results and discussion
3.1. Characterization of the starting powders
Figs. 1 (a-c) and Table 2 present the results of X-ray diffraction
and Rietveld refinement of the RE2O3 and Y2O3 oxides and the
Ce-TZP/Al2O3 powder.
The X-ray diffraction (XRD) patterns of the yttrium and
rare-earth mixed oxide powders show that all peaks are
related to the Y2O3-type structure. It can be noted that the
peaks are less intense and wider, Fig. 1 (a), than those of pure
Y2O3 (Fig. 1 (b)). The full width at halfmaximum (FWHM) of the
most intense peak (222) was determined to be 0.50�.
Comparing the diffraction patterns of RE2O3 (Fig. 1 (a)) and of
Y2O3 (Fig. 1 (b)), the intensity of the main peak (222) for the
mixed oxide is about 9 times smaller than that of pure yttria. It
can also be observed that the peaks of the mixed oxide were
well adjusted for a pseudo-Voigt profile function. The well-
defined peaks evidence the formation of a true solid solu-
tion, as noted in previous work [34].
The refined structural parameters for yttria and the mixed
oxide are shown in Table 2. It can be observed that the lattice
parameter of RE2O3 is slightly lower than that of pure Y2O3,
which can be explained by the smallermean lattice parameter
of the rare-earth oxides that constitute the studied mixture.
The (Ce,Y)-TZP/Al2O3 composite powder presents different
phases: m-ZrO2 (77.1%), t-ZrO2 (4.3%), CeO2 (4.4%), and Al2O3
(14.2%). These proportions are in accordance with data pro-
vided by the manufacturer.
3.2. Characterization of the sintered specimens
Figs. 2 (a-c) shows the XRD results of the sintered samples
containing 10 wt.% RE2O3 or 10 wt.% de Y2O3 as a dopant, and
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Table 3 e Rietveld refinement of sintered ceramic composites doped with 10% Y2O3 or 10% RE2O3.
Ce-TZP/Al2O3/Y2O3
Al2O3 (Corundum) Y3Al5O12 (YAG) t- ZrO2 c- ZrO2
Crystalline composition (%) 46.4 t-ZrO2
37.3 c-ZrO2
12.7% - Al2O3
3.6% - YAG
Crystalline structure Rhombohedral Cubic Tetragonal Cubic
Space group R-3cH Ia-3d P42/nmcZ e
Lattice parameters (Â) a ¼ 4.755
c ¼ 12.956
a ¼ 12.011 a ¼ 3.608
c ¼ 5.191
e
Phase density (g/cm3) 4.002 4.555 6.053 e
Crystallite size (nm) 326.641 93.834 72.226 e
Ce-TZP/Al2O3/RE2O3
Al2O3 (Corundum) Y3Al5O12 ZrO2 c- ZrO2
Crystalline composition 47.4% t - ZrO2
34% c - ZrO2
5.9% - Al2O3
12.7% - YAG
Crystalline structure Rhombohedral Cubic Tetragonal Cubic
Space group R-3cH Ia-3d P42/nmcZ e
Lattice parameters (Â) a ¼ 4.751
c ¼ 12.961
a ¼ 11.990 a ¼ 3.610
c ¼ 5.184
e
Phase density (g/cm3) 4.008 4.574 6.056 e
Crystallite size (nm) 355.640 138.767 54.836 e
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Fig. 3 presents the results of phase quantification in the sin-
tered samples. The Rietveld refinement results are presented
in Table 3.
Figure 2 (a) shows the presence of tetragonal (t-ZrO2) and
cubic (c-ZrO2) zirconia regardless of the use of Y2O3 or RE2O3.
In addition, difference in intensity betweenthe crystalline
peaks can be noticed: the ceramic composite doped with 10
wt.% Y2O3 presents more intense cubic phase (c-ZrO2) peaks,
whereas the ceramic composite doped with 10 wt.% RE2O3
shows more intense Y3Al5O12 peaks, as shown in magnifica-
tions of the diffractograms in Fig. 2 (b-c). An associated anal-
ysis of Fig. 2 and Fig. 3 shows that both materials have a
considerable amount of t-ZrO2 after sintering, in the order of
47%, from the reaction between monoclinic zirconia and
cerium oxide present in the starting powder, reaction 3.
Considerable differences are observed in the quantification of
the c-ZrO2 phase, where the composite doped with 10% Y2O3
(37.3%) presented larger amount of cubic phase than the
Fig. 4 e SEM micrographs of the sintered (Ce,Y)-TZP-Al2O3 comp
10,000£).
composite doped with 10% RE2O3 (34%). On the other hand,
there is a great difference between Al2O3 and Y3Al5O12, which
results from the greater propensity of RE2O3 to form REAG
(Y(ss)2Al5O12) compared with Y2O3, according to reaction 4.
ZrO2 (m) þ CeO2 / ZrO2 (t) (3)
3/2 Y(ss)2O3 þ 5/2 Al2O3 / Y(ss)3Al5O12 (4)
Assuming that, the use of Y2O3 as a dopant facilitates for-
mation of the c-ZrO2 phase (Eq. (5)); the original alumina
quantifications are maintained with a little YAG formation in
the structure, with excess Y2O3.
ZrO2 (m) þ Y2O3 / ZrO2 (c) (5)
Thus, when RE2O3 is used to dope the ceramic composite,
there isgreaterpresenceofYAGintheoverall compositionof the
osite highlighting hexaluminate platelets (magnification -
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sintered composite and, consequently, smaller fractions of
cubicphaseZrO2-(c)andAl2O3areobserved.Thisoccursbecause
RE2O3 is a mixture of oxides that presents greater atomic dis-
order, increasing the reactivity of the powder. In this case, the
associated energy is greater and, therefore, the reactions in the
solid state begin at temperatures lower than those of the high
purity oxides. In Table 1, the mean grain size of RE2O3 is on the
order of 0.8 mm, but the FWHMof the XRD peak in Fig. 1 is large,
although it is a true solid solution. As the RE2O3 particles are
formed fromfine crystallite aggregates, theywill have excellent
reactivity, which justifies the clear XRD peaks of Y3Al5O12
observed in the sample with the addition of RE2O3.
Figure 4 Present micrographs obtained by SEM of the
composite without additions of Y2O3 or RE2O3, indicating the
presence of submicrometric equiaxed grains of zirconia (light
phase) and alumina (dark phase). Some platelets larger than
2 mm are also seen. These platelets are formed during sinter-
ing, by the solid state reaction of alumina contained in the
initial powder mixture and cerium oxide, forming hex-
aluminates, and are indicated as reinforcements for these Ce-
TZP-Al2O3 composites [39]. Fig. 5 and Fig. 6 present the SEM
micrographs of composites containing 10 wt.% Y2O3 and
10 wt.% RE2O3 respectively. Furthermore, Fig. 7 (a-c) shows the
average grain size of the samples containing 10 wt.% Y2O3 and
10 wt.% RE2O3. It is possible to observe a zirconia matrix with
submicrometric characteristics, as well as equiaxial alumina
grains (Al2O3) distributed throughout the zirconia matrix,
which are preferentially found in triple junctions.
Fig. 5 e SEMmicrographs of the sintered (Ce,Y)-TZP-Al2O3 comp
indicate equiaxed grains of Al2O3 (dark phase, Fig. 5(b), and gro
The addition of Y2O3 (Fig. 5) or RE2O3 (Fig. 6) inhibits the
formation of hexaluminate platelets, therefore platelets are
not observed in these composites. Furthermore, the insertion
of Y2O3 as an additive promotes an increase in the number of
c-ZrO2 grains in a behaviour predicted by Li et al. [40] in
ZrO2eCeO2eY2O3 system, whose microstructure presents
grains remarkably larger than the tetragonal (t-ZrO2) grains.
Furthermore, a microstructural heterogeneity due to prefer-
ential growth of cubic c-ZrO2 phase grains is observed. The use
of RE2O3 as a dopant favoured the formation of large YAG
(Y3Al5O12) grains, Fig. 6(c), characterized by the absence of
alumina grains in the vicinity due to having been consumed
according to the reaction (4), and in agreement with the re-
sults X-ray diffraction, Fig. 3, It is also noted that the residual
alumina present in this composite is composed of equiaxed
grains of submicrometric size, as indicated in Fig. 6(a,b).
Figure 7 (a) shows that the sintered composite doped with
Y2O3 has a large population of zirconia grains with average
size <1 mm. Moreover, a population of zirconia grains with
exaggerated growth is identified and, finally, the larger grains
(3.1e4.9 mm) are characteristic of Y3Al5O12. The use of RE2O3 as
a dopant also allows the grains of the zirconia matrix to
remain with submicrometric characteristics, with sizes be-
tween 0.1 and 0.7 mm, with a small population of larger cubic
grains in addition to YAG grains >2 mm (Fig. 7 (b)). The analysis
also showed that the Al2O3 grains, regardless of their average
size, do not present significant variations as a function of the
type of dopant used in the mixture (Fig. 7 (c)).
osite containing 10 wt.% Y2O3: a) 5000 x; b) 10,000 x. Arrows
wn grains of ZrO2-Cubic, Fig. 5(c).
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Fig. 6 e SEM micrographs of the sintered (Ce,Y)-TZP-Al2O3 composite containing 10 wt.% RE2O3: a) 10,000 x; b) 40,000 x; c)
10,000 x (highlighting gray YAG grains with no black Al2O3 grains on their boundaries).
j o u r n a l o f m a t e r i a l s r e s e a r c h and t e c hno l o g y 2 0 2 2 ; 1 6 : 4 5 1e4 6 0 457
3.3. Mechanical properties
The hardness and fracture toughness values of the samples
sintered at 1500 �C determined at room-temperature are
summarized in Table 4.
Results of the mechanical properties are probably corre-
lated with material densification. The composite doped with
Y2O3 had a relative density of 93.8 ± 1.2%, while the composite
doped with RE2O3 presented a slightly higher density value
(94.5 ± 1.7%) and were thus considered statistically similar.
The microstructural and crystallographic configuration pre-
sented by the materials, as well as the residual porosity in the
order of 5.5e5.2%, after sintering, led to mean Vickers hard-
ness values of 9.8 ± 1.2 GPa (Y2O3) and 10.1 ± 1.7 GPa (RE2O3), as
shown in Table 4.
The composites developed in this work showed that the
use of 10 wt.% Y2O3 induces the formation of the cubic phase
ZrO2-(c) as well as of the ZrO2-(t) phase stabilized with ceria
(Ce-TZP), an amount of Al2O3 close to the original proportion,
and a small fraction of transformed YAG. This complex
microstructural configuration, allowed high fracture tough-
ness values (8.5 ± 0.9 MPa m1/2). In contrast, the composite
doped with 10 wt.% RE2O3 induces formation of four distinct
phases: Ce-TZP (majority), cubic-ZrO2, Al2O3, and Y3Al5O12,
which associated with densification, enabled fracture tough-
ness values in the order of 7.1 ± 0.4 MPa m1/2.
As the densification and mechanical properties of the
original commercial composite without the addition of Y2O3
or RE2O3 oxides are slightly superior to doped composites,
even without the presence of hexaluminate platelets, new
sintering conditions involving higher final temperatures or
prolonged isothermal holding times may be suggested for
future investigations, aiming to improve densification and
mechanical properties, adjusting their properties to the
intended final application of this composite.
Figure 8 (a) shows the Vickers indentation and 8(b), the
radial cracks developed.
Results of the microstructural analysis suggest that the
use of rare-earth oxides enables greater refinement of the
Ce-TZP grain, responsible for the toughening of the material
due to toughening mechanisms by phase transformation.It
is known that tetragonal zirconia with average grain size
<0.8 mm has higher toughening capacity than zirconia with
larger grain size [35]. This toughening mechanism by phase
transformation is more relevant when using CeO2 as a
dopant in substitution for the traditional Y2O3 (Y-TZP)
[35,36]. Therefore, the shielding zone at the tip of the crack
in Ce-TZP is higher than in Y-TZP, thus inducing greater
https://doi.org/10.1016/j.jmrt.2021.11.141
https://doi.org/10.1016/j.jmrt.2021.11.141
Fig. 7 e Grain size distribution analysis of: a) Ce-TZP/Al2O3 doped with 10 wt.% Y2O3; b) Ce-TZP/Al2O3 doped with 10 wt.%
RE2O3; c) Al2O3 grains.
j o u r n a l o f ma t e r i a l s r e s e a r c h a nd t e c h no l o g y 2 0 2 2 ; 1 6 : 4 5 1e4 6 0458
crack locking and more effective reduction of crack
propagation.
The Al2O3 grains present in the matrix are considered a
secondary toughening mechanism that generates residual
stress around the ZrO2matrix. Thus, during crack propagation
in biphasic regions of the composite, higher fracture energies
are necessary for the crack to continue propagating. In addi-
tion, the presence of a third phase in greater quantity, when
using RE2O3, favours the appearance of residual thermal ten-
sions in the material. As Al2O3 is preferentially found in triple
junctions, spontaneous crack locking is obtained considering
that, as the cracks are intergranular, they are located along the
grain boundaries. Theoretically, residual thermal stresses in
Table 4 e Properties of the ceramic composites doped
with Y2O3 or RE2O3.
Sample Relative
density
(%)
Vickers
Hardness
HV98N
(GPa)
Fracture
toughness
KIC 98 N
(MPa.m1/2)
(Ce,Y) - TZP/Al2O3 95.2 ± 0,7 11.3 ± 1.4 8.6 ± 0.7
Ce-TZP/Al2O3/10%Y2O3 93.8 ± 1.2 9.8 ± 1.2 8.5 ± 0.9
Ce-TZP/Al2O3/10%RE2O3 94.5 ± 1.7 10.1 ± 1.7 7.1 ± 0.4
multiphase materials can be calculated using the mathemat-
ical approximation given in the Equations (6)e(8) [37,38]:
sb ¼ Eb , ða�abÞ,DT (6)
sm ¼ Em , ða�amÞ,DT (7)
a¼ abCbEb þ acCcEc þ amCmEm
CbEb þ CcEc þ CmEm (8)
Considering the Young's modulus of the ZrO2, Al2O3 and
Y3Al5O12 phases as 190 GPa, 390 GPa and 300 GPa, respectively,
and their respective thermal expansion coefficients (a) as
10.5 � 10�6 K�1, 8.5 � 10�6 K�1, 6.2 � 10�6 K�1, the residual
stresses in each phase for the ATZ-Y2O3 composite are esti-
mated to be sZrO2 ¼ �182.1 MPa, sAl2O3 ¼ 776.6 MPa, and
sY3Al5O12 ¼ 1615 MPa, whereas the ATZ-Re2O3 composite pre-
sents calculated residual stresses of sZrO2 ¼ �257,8 MPa,
sAl2O3 ¼ 621.3 MPa, and sY3Al5O12 ¼ 1495 MPa.
Reflections of these thermal stresses distributed among
the phases present in the composites contribute to generate
stress fields that act to hinder crack propagation in a stable
way. This reduced crack propagation, in combination with the
shielding zones around the major phase of ZrO2-(t) grains (Ce-
TZP), hinder crack growth and provide the material with high
fracture toughness.
https://doi.org/10.1016/j.jmrt.2021.11.141
https://doi.org/10.1016/j.jmrt.2021.11.141
Fig. 8 e a) Example of Vickers indentation (magnification 1000£); b) emphasis on the propagation of cracks and their
behaviour (magnification near to 17,000£).
j o u r n a l o f m a t e r i a l s r e s e a r c h and t e c hno l o g y 2 0 2 2 ; 1 6 : 4 5 1e4 6 0 459
4. Conclusions
Themixed yttrium and rare-earth oxide (RE2O3) obtained from
mineral xenotime is a solid solution of yttria and other rare-
earth oxides with partial similarity to high purity Y2O3.
Within the experimental limits investigated in this manu-
script, it was observed that the use of this oxide (RE2O3) as a
dopant in ZrO2eAl2O3eCeO2 systems induced the preferential
formation of Y3Al5O12 during sintering, partially consuming
Al2O3 from the initial chemical composition, while the use of
Y2O3 it tends to form cubic-ZrO2 while maintaining the orig-
inal Al2O3 contents. The results indicated that the addition of
this oxide (10 wt.%) to a Ce-TZP/15%Al2O3 powder mixture
resulted in ceramic composite with high hardness and frac-
ture toughness values, in the order of 10.1 ± 1,7 GPa and
7.1 ± 1.7 MPa m1/2, respectively, containing a complex micro-
structure formed by ZrO2(t), ZrO2(c), Al2O3, and Y3Al5O12. This
microstructural configuration allows the material to resist
well to crack propagation, because different toughening
mechanisms act simultaneously during crack growth. These
results are similar to those of composites obtained with
addition of Y2O3 to this ceramic composite and demonstrate
the feasibility of using RE2O3 to replace high purity commer-
cial Y2O3.
Declaration of Competing Interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
Dr. Eduardo de Sousa Lima would like to thank National
Council for Scientific and Technological Development e CNPq
(project nº 312161/2020e4) for the financial support received.
Dr. Claudinei dos Santos would like to thank FAPERJ (grant nº
E-26/202.997/2017) and National Council for Scientific and
Technological Development e CNPq (project nº 311119/
2017e4) for the financial support received. The authors are
grateful to Dr. Jos�e Eduardo Amarante and CBPF (Brazilian
Centre for Research in Physics) for the SEM/EDS analysis
(Laborat�orio de Nanoestruturas - LabNano, CBPF-RJ, Brazil)
and to Prof. Dr. Sebasti~ao Ribeiro (USP-EEL, Brazil) for sup-
plying the RE2O3 powder.
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https://doi.org/10.1016/j.jmrt.2021.11.141
https://doi.org/10.1016/j.jmrt.2021.11.141
	Development and characterization of alumina-toughened zirconia (ATZ) ceramic composites doped with a beneficiated rare-eart ...
	1. Introduction
	2. Methods
	2.1. Materials
	2.2. Processing
	2.3. Characterization
	3. Results and discussion
	3.1. Characterization of the starting powders
	3.2. Characterization of the sintered specimens
	3.3. Mechanical properties
	4. Conclusions
	Declaration of Competing Interest
	Acknowledgments
	References