Densification, characterization and oxidation studies of novel TiB2+EuB6compounds

Article abstract of DOI:10.1016/j.jallcom.2016.01.216

TiB₂+EuB₆ceramic samples with different EuB₆contents were fabricated using hot -pressing technique. The introduction of EuB₆promoted the sinterability, fracture toughness and oxidation resistance of TiB₂. 98.7% TD achieved by adding 2.5% EuB₆to TiB₂by hot pressing at a relatively low temperature of 1750?°C, 35?MPa, 1h. Formation of complete solid solution of TiB₂+EuB₆observed for all the samples by XRD, EDS and EBSD. Hardness of all samples is measured in the range of 24–27?GPa ～50% higher fracture toughness value of 5.2?MPa?m^1/2was obtained in the 2.5% EuB₆contained sample compared to monolithic TiB₂. Enhancement of oxidation resistance of TiB₂was observed by EuB₆addition due to the formation of EuBO₃and Eu₂O₃. Oxidized cross section of TiB₂+EuB₆sample was measured as 340?μm, which is ～35% less than that of monolithic TiB₂(520?μm), after oxidation at 1400?°C for 8h.

Full text of DOI:10.1016/j.jallcom.2016.01.216

Journal of Alloys and Compounds 670 (2016) 85e95
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Densification, characterization and oxidation studies of novel
TiB þEuB compounds
2
6
a,
*
a
a
b
a
a
T.S.R.Ch. Murthy , J.K. Sonber , B. Vishwanadh , A. Nagaraj , K. Sairam , R.D. Bedse ,
a
J.K. Chakravartty
a
Materials Group, Bhabha Atomic Research Centre, Mumbai, India
Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 December 2015
Received in revised form
TiB þEuB ceramic samples with different EuB6 contents were fabricated using hot -pressing technique.
2
6
The introduction of EuB
6
promoted the sinterability, fracture toughness and oxidation resistance of TiB
2
.
ꢀ
98.7% TD achieved by adding 2.5% EuB
6
to TiB
2
by hot pressing at a relatively low temperature of 1750 C,
14 January 2016
3
5 MPa, 1h. Formation of complete solid solution of TiB
2
þEuB
6
observed for all the samples by XRD, EDS
Accepted 27 January 2016
Available online 30 January 2016
and EBSD. Hardness of all samples is measured in the range of 24e27 GPa ~50% higher fracture toughness
value of 5.2 MPa m1 was obtained in the 2.5% EuB
Enhancement of oxidation resistance of TiB was observed by EuB
EuBO and Eu . Oxidized cross section of TiB
/2
contained sample compared to monolithic TiB
addition due to the formation of
sample was measured as 340
6
2
.
2
6
Keywords:
3
2
O
3
2
þEuB
6
m
m, which is ~35%
TiB
EuB
2
ꢀ
less than that of monolithic TiB
2
(520
m
m), after oxidation at 1400 C for 8h.
6
Hot-pressing
© 2016 Elsevier B.V. All rights reserved.
Hardness
Fracture toughness
Elastic modulus
Oxidation studies
1. Introduction
is to add a suitable sinter additive, such that it lowers the sintering
temperature and improves the fracture toughness [1,6].
Titanium diboride (TiB
2
) is an increasingly important ceramic
A significant research activity has been reported to use several
sinter additives and studied the effect of sinter additive on me-
chanical/physical properties of monolithic TiB [1e7]. In particular,
2
an extensive and critical literature analysis has been reported on
the role of metallic binders (e.g. Ni, Fe, Cu, Co, Ti) in the densifi-
material for engineering applications, owing to the excellent
combination of physicochemical and thermo-mechanical proper-
ties such as high melting temperature, high thermal conductivity,
extreme hardness, high young's modulus, low thermal expansion
coefficient, low density [1,2]. These properties make it ideal for use
as cutting tools, wear resistance materials, coating materials, metal
melting crucibles, impact resistant armors and electrodes [1e3].
2
cation of TiB . More than 99% TD was achieved by liquid phase
sintering [1,7,8]. However, the presence of metallic binder is not
desirable for high temperature structural applications. Therefore,
studies related to the use of non-metallic sinter-additives have also
TiB
2
is also a candidate material for neutron absorber in nuclear
is
reactors owing to the presence of boron [1e4]. However, TiB
2
been pursued. Several ceramic additives such as AlN, MoSi
WSi , TiSi , SiC, Si , CrB , B C, TaC were used for attaining the
densification of TiB with better mechanical properties [1e9]. As
2 2
, CrSi ,
rather limited in real applications due to problems associated with
their consolidation and poor fracture toughness. High melting
point, low self-diffusion coefficient and contamination with oxide
layers on the surface of particles make it difficult to densification
1e5]. These setbacks have provided the driver for considerable
research efforts to improve both the sinterability and toughness.
One route employed to improve the sinterability/toughness of TiB
2
2
3
N
4
2
4
2
regards the fabrication routes, it has been noted that hot pressing
and pressure-less sintering with the limited effort of using spark
plasma sintering and microwave sintering are employed to densify
[
TiB
TiB
2
[1,10]. Despite significant efforts in material development, bulk
materials have not yet penetrated into commercial market in a
2
2
big way. The bottleneck for such limitation is the processing diffi-
culties, poor mechanical properties in terms of fracture toughness
and moderate oxidation resistance [1,11].
*
Corresponding author.
E-mail address: murthi@barc.gov.in (T.S.R.Ch. Murthy).
http://dx.doi.org/10.1016/j.jallcom.2016.01.216
925-8388/© 2016 Elsevier B.V. All rights reserved.
0

86
T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
Table 1
750
Mechanical and physical properties of TiB
2
[1,5] and EuB
6
[15,17].
Relative Density
Elastic Modulus
700
100
Property
TiB
2
EuB
6
Crystal structure
Density g/cc
Hexagonal
4.52
3225
25e35
5e7
560
700e1000
7.3
60e120
10e40
1100
Cubic
4.99
2580
26.1
3.2
e
650
600
550
500
450
400
Melting point oC
Hardness, GPa
90
80
70
Fracture toughness, MPa$m1
Elastic modulus GPa
Flexural strength
Coefficient of thermal expansion x 10 /K
Thermal conductivity W/m/K
/2
183
6.9
23
6
Electrical resistivity, mU$cm
Oxidation resistance
e
o
C
e
3
50
00
Table 2
3
Details of starting powders of TiB
2
[3] and EuB
6
[17].
TiB
1.1
7.12
0.6
0.5
0.5
TiB
0
1
2
3
4
5
6
7
8
Property/composition
2
EuB
6
Wt. % EuB
6
Mean particle diameter,
Specific surface area, m /g
carbon, wt%
m
m
1.5
e
2
6 2
Fig. 1. Effect of EuB addition on relative density and elastic modulus of TiB - hot
pressed at 1750 C, 1h, 35 MPa.
ꢀ
0.7
0.6
e
Oxygen, wt%
Nitrogen, wt%
Phases identified by XRD
resistant to irradiation swelling [15,18,20]. Crystal structure of EuB
is cubic (CsCl type) in which B octahedra occupies corner sites and
Eu occupies body centered site [21]. EuB has also been reported to
have resistance to oxidation which is due to the formation of Eu
based protective layer [15]. Important properties of TiB and EuB
6
2
EuB
6
6
6
2
O
3
One of the promising applications of TiB
perature structural materials. It can be noted that TiB
excellent choice for applications requiring heat dissipation at high
temperatures due to its high thermal conductivity [1,12]. The
2
includes high tem-
could be an
2
6
2
are presented in Table 1 [1,5,15,17]. As per authors knowledge there
is no literature report on the use of EuB as sinter additive to TiB .
6
2
oxidation resistance of TiB
pressure of oxygen, time of exposure, porosity and sintering addi-
tives. It is reported that oxidation resistance of TiB
composite could be improved by forming a in-situ protective
amorphous fluid borosilicate coatings on the external surfaces
directly facing the oxidizing environment at high temperature
2
is affected by temperature, partial
2
. Experimental
2
þsilicide based
2.1. Starting material
2
In house prepared TiB
2
(D50: 1.1
m
m, Sp.Surface area-7.12 m /g,
(D50: 1.5 m, ‘C’:
.7 wt.%, ‘O’:0.6 wt.%) powders were used as starting materials.
Starting materials (TiB & EuB ) were prepared by carbothermic
reduction of its oxides in the presence of boron carbide. More de-
tails on synthesis of TiB and EuB powder are presented elsewhere
3,17]. Mean particle diameters of starting powders were measured
‘C’: 0.6 wt.%, ‘O’:0.5 wt.%, ‘N’:0.5 wt.%) and EuB
6
m
[
1,8,12,13]. Addition of rare-earth elements could be useful
approach to improve the oxidation resistance of borides at inter-
mediate temperatures in hypersonic air. Addition of LaB to ZrB
improved the oxidation resistance by formation of a La Zr [14].
at the
0
2
6
6
2
2
2 7
O
2
6
EuB
6
addition also improved the oxidation resistance of ZrB
2
[
ꢀ
temperature of 900 C by the formation of protective layer [15].
by laser diffraction method (CILAS PSA 1064L). Table 2 presents the
details of starting powders.
Addition of La
This paper presents the study on the effect of EuB
cation, microstructure and oxidation behavior of TiB
2
O
3
to HfB
2
, altered the oxidation kinetics [16].
on densifi-
. EuB has
6
2
6
2.2. Densification and characterization
high melting point, low thermal expansion coefficient, good ther-
mal conductivity, high hardness, high wear resistance and chemical
For densification, weighed quantities of fine titanium diboride
and europium hexaboride were mixed thoroughly using a motor-
ized mortar and pestle in dry condition for 1h to obtain samples of
stability. It would be a suitable additive toTiB
2
for high temperature
applications [17]. EuB is an excellent neutron absorber material
6
due to high neutron absorption cross section of both boron and
europium [15,17,18]. Neutron absorption cross section of boron and
europium are 759 and 4600 b respectively, which is quite a higher
value [15,17,19]. It is considered a candidate material for control rod
different compositions. The powder mixtures of TiB
TiB , TiB and TiB
2
þ0% EuB
6
,
2
þ2.5wt.% EuB
6
2
þ5wt.% EuB
6
2
þ7.5wt.% EuB
6
were loaded in a high density graphite die (17 mm dia cavity) and
ꢀ
hot pressed at a temperature of 1750 C under a pressure of 35 MPa
application in fast reactors [15,17,20]. The structure of the EuB
quite loose with considerable room to accommodate helium atoms
which generated during neutron absorption) and thus it is
6
is
for 1 h in a high vacuum (0.001 Pa) chamber. The pellets were
ejected from the die after cooling and the density measured by
Archimedes' method in deionized water. Dimensions of densified
(
Table 3
ꢀ
Relative density, Hardness, Fracture toughness and Elastic modulus of TiB
2
þEuB
6
ceramic based materials (hot pressed at 1750 C, 1h, 35 MPa).
EuB
0
6
(%wt.)
Actual density (g/cc)
Relative density (%TD)
Micro hardness (GPa)
Fracture toughness KIC (MPa$m1/2
)
Elastic modulus (GPa)
4.25
4.47
4.36
3.91
94.1
98.7
96.1
85.9
27 ± 1
24 ± 1
26 ± 1
24 ± 1
3.5 ± 0.1
5.2 ± 0.2
4.7 ± 0.1
4.0 ± 0.1
513 ± 21
659 ± 26
546 ± 22
401 ± 16
2
5
7
.5
.0
.5

T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
87
Fig. 2. a) XRD plots of hot pressed monolithic TiB
2
and TiB
2
þ EuB
6
samples (Shows all
ꢀ
peaks are TiB
2
only) - hot pressed at 1750 C, 1h, 35 MPa, b) zoomed portion of (101)
6 2
peak indicates the shift in 2ɵ position with increase of EuB addition in TiB .
pellets are 17 mm diameter and ~6 mm thickness. Relative density
RD) was calculated by determining the volume fraction of porosity
(
Fig. 3. Line scan across different grains of TiB
image, b) plot of number of counts vs. distance.
2
þ 5%EuB
6
sample for Ti, B and Eu a) SEM
of the samples using image analysis technique on SEM images of
the fractured samples. The reported density results are an average
of 5 measurements that are performed on SEM micrographs of
each sample by using Leica materials workstation version 3.6.6
software.
2.3. Oxidation
Densified samples were polished to mirror finish using diamond
powder of various grades from 15 to 0.25
m
m in an auto polisher
Isothermal oxidation studies for monolithic TiB
2
and TiB
2
þEuB
6
ꢀ
(
LaboForce-3, Struers). Micro hardness was measured on the pol-
sample were conducted at a temperature of 1400 C in air up to 8h.
Hot pressed pellet of diameter 17 mm was cut into thin slice of
2 mm thickness. All the surfaces of the cut sample were polished
with emery papers (1/0, 2/0, 3/0, 4/0) and finally with diamond
ished surface at a load of 100 g and dwell time of 10 s. The
indentation fracture toughness (KIC) data were evaluated by crack
length measurement of the crack pattern formed around Vickers
indents (using 10 kg load), adopting the model formulation pro-
posed by Anstis et al. [22], KIC ¼ 0.016 ꢁ (E/H) x P/c , where E is
the Young's modulus, H-Vickers's hardness, P e applied indentation
load, and c e half crack length. Young's modulus (E) of the all
samples was measured by using ultrasonic pulse echo technique
paste up to 1
mm finish. Oxidation tests were conducted in a
1
/2
3/2
resistance heated furnace. In order to avoid oxidation during
heating, the sample was directly inserted into the furnace after the
furnace temperature reached the required temperature. Samples
were placed in an alumina crucible kept into the furnace. Each
sample was carefully weighed before and after exposure to deter-
mine the weight changes during oxidation. Oxide phases formed on
the surface were identified by XRD. The morphology and nature of
(
UT 340, UTEX Scientific Instrument Inc., Canada). 15 MHz normal
beam ultrasonic probe with a sampling rate of 1000 MHz was
employed for velocity measurements. The reported value of hard-
ness fracture toughness and Young's modulus is the average of five
measured values. Polished samples were characterized by X-ray
diffraction (XRD) using PHILIPS PW1830 X-ray diffractometer with
oxide layer was examined by FESEM-EDS. TGA analysis of mono-
ꢀ
lithic TiB
2
and TiB
2
þEuB
6
sample were carried out up to 1200 C in
air with 10 K/min heating rate (NETZSCH STA 409 PC/PG) for
continuous oxidation studies. Usually oxidized cross section sam-
ples are prepared by cutting the oxidized samples using abrasive
diamond cutters, but success rate is very poor due to poor adher-
ence of oxide layers. Special samples were prepared to study the
Ni filtered Cu-Ka radiation for phase identification. Microstructural
characterization of the polished and fractured samples were carried
out by field emission scanning electron microscopy, energy
dispersive spectroscopy (FESEM-EDS) and Electron back scattered
diffraction (EBSD) using Carl Zeiss FESEM.
cross section of oxidized specimens of monolithic TiB
2
and

88
T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
Fig. 4. Elemental mapping of TiB
2
þ 5%EuB
6
sample for Ti, B and Eu a) SEM image, b) Ti, c) B, d) Eu.
TiB
2
þEuB
6
sample in the present study. 3 mm dia hole was pro-
TiB
crystalline TiB
observed, with increasing the EuB
2
þEuB
6
samples are shown in Fig. 2. It indicates the presence of
peaks in all samples. Slight shift in the peaks were
content more shift towards
vided by using wire cut EDM, before oxidizing the sample. After
oxidation, sample was polished in order to remove the outer oxide
layer on the sample surface. Oxide cross section was obtained at the
interface of hole and sample. The reported value of oxide thickness
is the average of five measured values at different locations of
image of cross section of isothermal oxidized samples.
2
6
lower 2
lithic TiB
EuB
Absence of EuB
q
angle were observed. The major peak position of mono-
ꢀ
2
is at 44.48 , where as for TiB
2
þ5%EuB
6
and TiB
2
þ7.5%
ꢀ
ꢀ
6
samples positions are at 44.46 and 44.40 respectively.
6
and shift in the TiB
2
peaks in XRD patterns of
sample indicates that the TiB
solution.
and EuB form the complete solid
2 6
3
. Results
Fig. 3 presents the BSE image of TiB
2
þ5% EuB
6
. It shows the
3.1. Densification and characterization
presence of different grains of same contrast. In line scan across
different grains shows the presence of more or less uniform dis-
tribution of Ti, Eu and B elements. In order to ensure further,
Hot pressing conditions, actual density and relative densities of
the TiB
presents the effect of EuB
tion of 2.5wt.% EuB resulted in densification of 98.7% TD at a
2
and TiB
2
þEuB
6
samples are presented in Table 3. Fig. 1
elemental mapping was taken over a region of the TiB
sample and presented in Fig. 4. Ti, B and Eu were uniformly pre-
sented all over the region. FESEM-EDS results reconfirm the XRD
2 6
results, i.e. the formation of solid solution of TiB and EuB .
In order to further confirm the presence of single phase in the
sample EBSD analysis was carried out on specially polished
2
þ5% EuB
6
6
addition on densification of TiB . Addi-
2
6
ꢀ
temperature of 1750 C and a pressure of 35 MPa. Samples with 5%
EuB were hot pressed up to 96.1%TD at similar processing condi-
tions. A lower density of 85.9%TD was achieved in TiB
6
2
þ7.5% EuB
6
sample. In case of monolithic TiB
2
, 97.5% TD density was obtained at
TiB
2
þ5% EuB
6
sample. Figs. 5 and 6 present the EBSD images along
ꢀ
a higher hot pressing temperature of 1800 C [5]. In the present
study, the hot pressing temperature was lower by 50 C. Addition of
with analyzed results. Fig. 5a) presents the image showing with
different grains and b) grain size distribution plot. Mean grain
ꢀ
EuB
EuB
6
toTiB
2
may result the formation of solid solution of TiB
2
and
diameter was calculated as 2.29
the distribution of different sized equi-axed grains in the range of
me5 m. These results indicates that, slight particle coarsening
was observed in the hot pressed TiB sample, as starting
þEuB
particle size are in the range of 1e1.5 m particle coarsening could
mm. Image (Fig. 5a) clearly shows
6
. Formation of solid solution was confirmed by XRD and EBSD
analysis of densified pellets, which is discussed in the next
paragraphs.
1
m
m
2
6
XRD pattern of the dense pellet of monolithic TiB
2
and
m

T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
89
of 24e27 GPa. Variation of hardness is not so significant. The
hardness value of TiB sample is remained as 24 GPa,
þ7.5% EuB
even it is having the low density of 86%TD only. It could be due to
the solid solution hardening by formation of TiB solid so-
2
6
2
þEuB
6
lution. On formation of solid solution the parent lattice gets
strained (confirmed by XRD) and results in hardening of material.
Fig. 8 presents the fracture surfaces of monolithic TiB
EuB sample. The mode of fracture is seen to be intergranular in
both samples. Regular faceted grains are clearly visible. Grains were
2
and TiB
2
þ5%
6
observed slightly bigger in the TiB
monolithic.
2
þEuB
6
sample compared with
Fracture toughness of monolithic TiB
2
and TiB
2
þEuB
6
samples
are presented in Fig. 7. Fracture toughness of monolithic TiB
2
1
/2
sample was measured as 3.5 MPa m . ~50% higher fracture
1
/2
toughness value of 5.2 MPa m was obtained for TiB
sample. For TiB , TiB þ7.5% EuB the fracture toughness
þ5% EuB
values are 4.7 MPa m and 4.0 MPa m respectively. The fracture
toughness values obtained in the TiB sample samples are
þEuB
higher than the monolithic TiB . However, slightly lower fracture
toughness values are recorded for 5% and 7.5% EuB samples,
compared with 2.5% EuB sample. This could be due to the decrease
in relative density. Fig. 9 presents the features of indentation crack
in monolithic TiB and TiB sample samples. Slight crack
þ5%EuB
deflections are mainly observed in monolithic TiB , where as both
crack deflections and bridging mechanisms are observed in the
TiB sample, which explain the good fracture toughness.
þEuB
2
þ2.5% EuB
6
2
6
2
6
1
/2
1/2
2
6
2
6
6
2
2
6
2
2
6
3.3. Oxidation study
The weight gain data obtained during continuous oxidation of
monolithic TiB
Weight gain started for monolithic TiB
2
and TiB
2
þ5%EuB
6
samples are presented in Fig. 10.
ꢀ
2
at ~700 C, whereas for
ꢀ
TiB
2
þEuB
6
sample at ~500 C. Rate of weight gain for TiB
2
þEuB
6
ꢀ
sample is very high upto ~600 C, afterwards weight gain rate
ꢀ
ꢀ
decreased upto 800 C. Above 800 C, both samples show the
similar rate of weight gain upto 1200 C. EuB
oxygen and hence oxidation started early (~500 C) and also
resulted in higher weight gain.
ꢀ
6
has more affinity for
ꢀ
ꢀ
Isothermal oxidation studies were carried out at 1400 C for
monolithic TiB
2
and TiB
2
þ5%EuB
6
samples. Specific weight gain
2
2
Fig. 5. a) Image showing the different sized grains and b) grain size distribution plot of
TiB sample.
þ 5%EuB
after 4h are recorded as 0.244 kg/m and 0.479 kg/m respectively
2
6
for monolithic TiB
2
and TiB
2
þ5%EuB
6
sample. After 8h of oxidation
ꢀ
2
at 1400 C, specific weight gain was calculated as 0.505 kg/m and
2
ꢀ
0
.410 kg/m respectively. After 1200 C, it is expected that evapo-
be happened, while the formation of solid solution. Fig. 6a) present
the orientation of different grains in the TiB sample with
ration of sub oxides of boron. As, the both samples are having the
substantial boron content, it is not worth to calculate the specific
weight gain data. Hence, further oxidation kinetics was not evalu-
ated. However, oxidized surfaces were examined by XRD and
2
þ5% EuB
6
different color coding. This image indicates that there is no pref-
erential texture in the microstructure. Fig. 6b) present the EBSD
image with different phases in different color coding along with
quantification data of phases in tabular format. Entire image shows
FESEM-EDS. Fig. 11 presents the XRD pattern of oxidized surfaces of
ꢀ
monolithic TiB
2
and TiB
2
þEuB
6
samples at 1400 C for 8h. Only TiO
2
the presence of TiB
and hardly any EuB
2
phase (blue (in the web version) color) only
phase (red (in the web version) color). This
peaks were observed on the oxidized surface of monolithic TiB
expected, all B
surface at 1400 C. On other hand, EuBO
identified in addition to TiO on the oxidized surface of TiB
EuB sample. SEM image of oxidized surface along with elemental
mapping and EDS spot analysis are presented in Figs. 12 and 13 for
monolithic TiB and TiB sample respectively. Oxidized sur-
þEuB
face of monolithic TiB (Fig. 12) image clearly shows the presence of
highly textured TiO dendrites (marked on the image) and whole
2
. As
6
2 3
O or sub oxides of boron are evaporated from the
observation confirms the XRD results, i.e. the formation of solid
solution of TiB and EuB . For colors, please refer to the online
ꢀ
3
and Eu
2
O
3
peaks were
þ5%
2
6
2
2
version of the article. However, about 12% zero solutions are
recorded in the sample, this could be due to polish pullouts and
original porosity (~4%) in the sample. Polish pullouts are inevitable
in ceramic samples, while doing the mirror finish polishing.
6
2
2
6
2
2
3
.2. Mechanical properties and fractography
surface is not evenly covered. Some holes (marked on the image)
were observed on the oxidized surface, it may be resulted from the
evaporation of sub oxides of boron. Elemental mapping of the re-
gion shows presence of rich in Ti and O some traces of B. Spot
analysis was carried out on dendrites and glassy phase region
Variation in micro hardness, fracture toughness and elastic
modulus of monolithic TiB
in Table 3. Effect of EuB
presented in Figs. 1 and 7. Hardness of all samples are in the range
2
and TiB
2
þEuB
6
samples are presented
6
addition on mechanical properties is
2
(Fig. 12). Dendrites were confirmed as TiO . Oxidized surface of

90
T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
Fig. 6. EBSD analysis of TiB
given in table. For colors, please refer to the online version of the article
2
þ 5%EuB
6
2 6
sample a) different orientation of grains with different color coding, b) color mapping of different phases of TiB and EuB , quantitative data
TiB
consisting of Ti, O, Eu and B (Fig. 13).
FESEM image of cross section of isothermal oxidized (1400 C,
2
þEuB
6
sample image shows the more continuous oxide layer
presented in Fig. 14 and Fig. 15 respectively. Total oxide cross sec-
tion of monolithic TiB was measured as 520 m, where as for
TiB sample is only of 340 m. Overview of the entire
cross section of oxidized sample shows that more uniform width of
the oxide layer was observed over the entire region for both sam-
2
m
ꢀ
2
þ 5%EuB
6
m
8
h) samples of monolithic TiB
2
and TiB
2
þ 5%EuB
6
sample are
ples. At higher magnifications, oxide layer of monoltich TiB
(Fig. 14) looks like loosely packed with some porosity. Whereas for
TiB sample, oxide layer is completely covered the entire
region without any porosity (Fig. 15), which indicates that formed
oxide layer is not allowing to ingress the oxygen through it. This
2
3
0
7
Hardness
2
þ 5%EuB
6
Fracture toughness
28
6
5
4
3
2
data infers that addition of EuB
oxidation resistance.
6 2
to TiB , helped to improve the
26
4. Discussion
24
Owing to its strong covalent bonding and low self-diffusion
coefficient, monolithic TiB
densification and reported to be densified at or above 2000
2
ceramic is always hard to achieve full
ꢀ
C
22
[
1,23,24]. The introduction of appropriate second phase particle is
proved to be an efficient method to improve the sinterability.
Wettability and dissolution of TiB in the transient liquid phase
2
20
have been widely identified as critical factors when sintering with
metallic additives [1,23]. From the perspective of high temperature
applications, the presence of metallic binder is not desirable: the
low melting point of either sintering liquid or metallic additives
leads to incipient fusion and consequent degradation of high
0
1
2
3
4
5
6
7
8
Wt. % EuB
6
Fig. 7. Effect of EuB
at 1750 C, 1h, 35 MPa.
6
addition on hardness and fracture toughness of TiB
2
- hot pressed
ꢀ

T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
91
104.0
103.5
103.0
102.5
102.0
TiB +5% EuB
2
6
101.5
101.0
100.5
100.0
TiB
2
99.5
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
o
Temperature, C
Fig. 10. Weight gain data of continuous oxidation in TGA for monolithic TiB
2
and
ꢀ
TiB
2
þ 5%EuB
6
sample in air up to 1200 C at heating rate of 10 K/min.
sintering [1,8,11,12]. When CrB
densification was seen due to the higher mobility of CrB
mation of solid solution with TiB [7]. Sonber et al. reported the
improvement in densification of ZrB , when used the EuB as ad-
ditive by formation of solid solution [15]. Similar observations were
also made in the present study that addition of 2.5% EuB helped in
densification by forming a solid solution with TiB . This is a com-
plete solid state sintering, hence further addition of (beyond 5%)
EuB has not helped much to improve the densification. However,
complete solid solution formation was confirmed in all the
TiB samples by XRD, EDS and EBSD.
þEuB
Usually addition of second phase (sinter additive) will enhance
2
is added to TiB
2
, improvement in
2
and for-
2
2
6
6
2
Fig. 8. Fractography of monolithic TiB
intergranular mode of fracture).
2
(a) and TiB
2
þ 5%EuB
6
sample (b) (Shows
6
temperature properties [1]. Various non-metallic additives (espe-
cially ceramic based) have been used for attaining densification of
2
6
TiB
2
with retention of high temperature properties [1]. Especially
the fracture toughness due to the compressive stresses at the
interface of matrix and second phase. Origin for compressive
stresses is due to the difference in coefficient of thermal expansion.
‘Si’ contained additives like silicon nitride, transition metal silicides
are proved to be effective sinter additives in order to reduce the
sintering temperature by liquid phase sintering and/or activated
Fig. 11. XRD data of isothermal oxidized surfaces of monolithic TiB
samples at 1400 C, 8h.
2
and TiB
2
þ 5%EuB
6
ꢀ
Fig. 9. Crack propagation pattern of a) monolithic TiB
2
and b) TiB
2
þ 5%EuB
6
sample.

92
T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
ꢀ
Fig. 12. a) FESEM image, Elemental mapping (b-Ti, c-O, d-B) and spot analysis (e-spectrum 1, f-spectrum 2) of isothermal oxidized surfaces of monolithic TiB2 sample at 1400 C, 8h.
In some reports, observed the crack deflections, branching and
bridging mechanisms due to the presence of second phases. Usu-
ally, in ceramics above cited mechanisms are responsible for the
improvement of fracture toughness. Fracture toughness of mono-
2
The oxidation of TiB composites can be minimized either by
providing resistance to the diffusion of oxygen ion into the material
or to the diffusion of the base ceramic forming element ions
through the oxide to the oxide-air interface. It is interesting to note
that oxidation resistance of composite is not only depends on the
type of sinter additive but also on the presence of quantity. In our
earlier studies, found that with increasing sinter additive (transi-
tion silicides) content from 2.5 to 5 or 10%, nature of oxidation
1
/2
lithic TiB
2
was report in the range of 3e4 MPa m [1]. By using
non-metallic additives, slightly improved fracture toughness of
~
6 MPa m1 was reported [1]. As expected, further improvement of
/2
1
/2
fracture toughness up to 10 MPa m
was reported by adding
ꢀ
metallic additives due to plastic deformation of second phase [1].
changes from linear to parabolic or cubic [6e10] up to 1000 C.
1
/2
~
50% higher fracture toughness value of 5.2 MPa m was obtained
in the present study by adding 2.5% EuB to TiB . Crack deflection,
branching and bridging mechanisms were mainly responsible for
the improved fracture toughness value of the TiB sam-
Boron/silicon based protective oxide layers (borosilicate/glassy
ꢀ
6
2
phases) are only stable up to 1200 C [1,6e10]. Beyond this tem-
perature, additives based on rare earth elements such as La, Eu, Ce
are attributed to improve the oxidation resistance of ultrahigh
2
þ 5%EuB
6
ples. The improvement of fracture toughness is could be due to the
increase of density and therefore Young's modulus.
temperature ceramics (ZrB
2
, HfB
2
) [14e16] by forming a oxide
, Eu [14e16]. In the present
products such as La Zr , LaBO
2
2
O
7
3
2 3
O

T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
93
Fig. 13. a) FESEM image, Elemental mapping (b-Ti, c-O, d-Eu, e-B) and spot analysis (f-spectrum 3) of isothermal oxidized surfaces of TiB2 þ 5%EuB6 sample at 1400 ꢀC, 8h.
preliminary oxidation study also identified the oxide products of
stabilizing the layer and making it protective by formation of solid
solution between TiO and Eu . Following possible oxidation
reactions are given below:
ꢀ
EuBO
3
and Eu
2
O
3
after oxidation at 1400 C for 8h. These oxide
2
2 3
O
layers appear to be continuous and crack free.
The hypothesis behind the enhancement of oxidation resistance
of TiB
sibility is the formation of very thin protective layer consisting of
EuBO and Eu . EuB has been reported to have good oxidation
resistance which is due to the formation of Eu based protective
layer [15]. Another possibility is the formation of protective layer by
combined effect of Eu and TiO . TiO layer alone is not protective
and results in linear weight gain but Eu may be probably
2
by EuB
6
addition is presented in following lines. One pos-
2/5TiB
2
þ O
2
/ 2/5TiO
2
þ 2/5B
/ 2/21Eu
þ 12/21B
þ 2/21B
2
O
3
(1)
(2)
(3)
3
2
O
3
6
4/21EuB
6
6
þ O
þ O
2
2
2
O
3
2 3
O
2 3
O
2/21EuB
/2/21 EuBO
3
2 3
O
2
O
3
2
2
2
O
3
As the thermodynamic data is not available for EueB system,

94
T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
ꢀ
Fig. 14. a) FESEM image of cross section of isothermal oxidized sample of monolithic TiB2 sample at 1400 C, 8h (Oxide layer thicknesse520
mm) b) & c) are at higher magnification.
Fig. 15. a) FESEM image of cross section of isothermal oxidized sample of TiB2 þ 5%EuB6 sample at 1400 ꢀC, 8h (Oxide layer thicknesse340
mm) b) & c) are at higher magnification.

T.S.R.Ch. Murthy et al. / Journal of Alloys and Compounds 670 (2016) 85e95
carbothermal synthesis of TiB
95
could not calculate the free energy data for the formation of above
reactions (2) & (3). Free energy data for the reaction (1) are re-
ported elsewhere [9]. The weight gain of TiB sample is more
þEuB
than that of TiB (Fig. 10), but the overall oxide layer thickness is
þEuB sample during isothermal oxidation at
has more affinity for oxygen than TiB , hence
2
assisted by extended high-energy milling,
Powder Technol. 275 (2015) 69e76.
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[
2
6
materials, Metals Mater. Process. 16 (1) (2004) 39e52.
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2
2
2
less in case of TiB
400 C for 8h. EuB
2
6
ꢀ
1
6
2
[
[
[
[
the higher weight gain was observed during continuous oxidation.
2 2
strength properties of TiB with TiSi sinter-aid, J. Eur. Ceram. Soc. 29 (10)
(2009) 2119e2128.
ꢀ
But during isothermal oxidation at 1400 C, for prolonged period of
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A.K. Suri, Effect of CrB addition on densification, properties and oxidation
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densification of TiB ceramic by the spark plasma sintering process with Ti as
8
h resulted a formation of protective oxide layer in TiB
2
þEuB
6
2
sample, which not allowed to further ingress of oxygen ions
through it. As a result oxide layer thickness was ~35% less than that
2
2
of monolithic TiB
. Conclusion
TiB ceramic novel samples with different EuB
þEuB
2
observed in TiB
2
þEuB
6
sample.
a sintering aid, Scr. Mater. 66 (3e4) (2012) 167e170.
9] T.S.R.Ch. Murthy, J.K. Sonber, C. Subramanian, R.C. Hubli, A.K. Suri, Densifica-
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Refract. Met. Hard Mater 33 (2012) 10e21.
10] A. Turan, F.C. Sahin, G. Goller, O. Yucel, Spark plasma sintering of monolithic
5
2
þ WSi
2
composite, Int. J.
[
2
6
6
contents
TiB2 ceramics, J. Ceram. Process. Res. 15 (6) (2014) 464e468.
were fabricated using hot-pressing technique. The microstructure,
mechanical properties and oxidation studies were carried out in
detail. The following conclusions were drawn from the results:
[11] G.B. Raju, K. Biswas, B. Basu, Microstructural characterization and isothermal
oxidation behavior of hot-pressed TiB -10wt.% TiSi composites, Scr. Mater.
1 (2009) 104e107.
12] T.S.R.Ch. Murthy, J.K. Sonber, C. Subramanian, R.C. Hubli, N. Krishnamurthy,
A.K. Suri, Densification and oxidation behavior of a novel TiB e MoSi e CrB
composite, Int. J. Refract. Met. Hard Mater. 36 (2013) 243e253.
2
2
6
[
[
[
2
2
2
(
6
1) Addition of 2.5wt.%EuB was found to be favoring the
13] T.S.R.Ch. Murthy, J.K. Sonber, C. Subramanian, R.C. Hubli, N. Krishnamurthy,
densification and resulting the maximum component den-
sity of ~98.7% TD.
A.K. Suri, Densification, characterization and oxidation studies of (TiCr)
B
2
þ20%MoSi
14] F. Monteverde, D. Alfano, R. Savino, Effects of LaB
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Mater. 35 (2012) 96e101.
2
, Int. J. Refract. Met. Hard Mater. 37 (2013) 12e28.
6
addition on arc-jet con-
(
(
2) EuB
TiB
6
was observed to form a complete solid solution with
2
2
.
3) Hardness of all samples is measured in the range of
4e27 GPa. Fractography indicate the mode of fracture is
intergranular for TiB sample.
þEuB
4) Maximum fracture toughness of 5.2 MPa m was obtained,
2
6
2
2
6
[
16] D.D. Jayaseelan, E.Z. Solvas, C.M. Camey, Z. Katz, P. Brown, W.E. Lee, Micro-
1
/2
(
(
structure and evaluation of HfB2 based ceramic during oxidation at 1600-
o
2
000 C, Adv. Appl. Ceram. 114 (5) (2015) 277e295.
when 2.5wt.% EuB
6
was added with TiB
2
.
ꢀ
[17] J.K. Sonber, T.S.R.Ch. Murthy, C. Subramanian, R.C. Hubli, A.K. Suri, Synthesis,
5) At 1400 C, the oxide layer thickness of TiB
2
þEuB
m, which is ~35% less than that of
m). This enhancement in oxidation
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6
sample
densification and characterization of EuB
(2013) 67e72.
[18] T. Fujita, M. Suzuki, Y. Ishikawa, Specific heat of EuB
6
, Int. J. Refract. Met. Hard Mater. 38
was measured as 340
monolithic TiB (520
m
m
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