Materials Science & Engineering A 750 (2019) 132–140
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Materials Science & Engineering A
journal homepage: www.elsevier.com/locate/msea
Effect of nano-sized sintering additives on microstructure and mechanical
properties of Si3N4 ceramics
Anil Kumara, Aditya Gokhaleb, Sudarsan Ghosha, Sivanandam Aravindana,
a
b
T
⁎
Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi, India
Department of Materials Science & Engineering, Indian Institute of Technology Delhi, New Delhi, India
ARTICLE INFO
ABSTRACT
Keywords:
Si3N4
Nano-sized RE2O3
Microstructure
Nanohardness
Fracture toughness
Nano-sized sintering additives such as MgO, Al2O3, Y2O3 and La2O3 are added to process Si3N4 based ceramics.
The effects of such sintering additives on the phase transformation, mechanical properties and microstructure of
silicon nitride were investigated. XRD analysis of sintered silicon nitride with these sintering additives have
shown the complete transformation of α-Si3N4 into β-Si3N4. The silicon nitride ceramics sintered with nano-sized
additives (4MgO-4Al2O3–5La2O3) have exhibited the highest fracture toughness. This is not only due to the weak
glass|grain interface but also due to the formation of high aspect ratio grains (β-Si3N4). Superior mechanical
properties of Si3N4 can be obtained by way of adding small and large RE3+ radii RE2O3 (Y2O3 and La2O3)
together than the addition of single RE2O3.
1. Introduction
Among the structural ceramic materials, silicon nitride (Si3N4)
based ceramics are widely studied because of their applications at room
as well as elevated temperature environments. The tailored microstructure in accordance with the properties required can be achieved
easily by liquid phase sintering, and this is the main advantage of silicon nitride ceramics [1]. Silicon nitride has excellent chemical, mechanical, thermal and tribological properties. Owing to these properties, silicon nitride based ceramics are widely used in heat exchangers,
gas turbine engines, turbocharger rotors, automotive engine components and microwave devices [2–6]. However, microstructural development and densification are very difficult in Si3N4 because of the
strong covalent bonding between Si and N, and the low self-diffusivity
of Si (DSi ≈ 0.5 × 10–19 m2s−1 at 1400 °C) and N (DN ≈ 6.8 ×10−10
m2 s−1 at 1400 °C) in bulk and at the grain boundaries of Si3N4 [7,8].
Sintering additives are added for the fabrication of silicon nitride based
ceramics, which forms the low-temperature eutectic liquid phase
through the chemical reaction between the sintering additives and the
nascent SiO2 present on the surface of silicon nitride particles [9,10].
The addition of both oxide (Al2O3, MgO and RE2O3) and non-oxide
(AlN, YF, YbF3, CaF2, and FeSi2) sintering additives plays a vital role in
densification and microstructure development. For example, dense silicon nitride with a fine grain is required for wear resistance, whereas
matrix with elongated reinforcing grain is desirable for improved
⁎
toughness [11]. Satet et al. [10] examined the effect of MgO + RE2O3
(RE = La, Sm, Yb, Sc, Y, Lu) addition on the mechanical properties and
grain growth anisotropy of sintered silicon nitride. It is highlighted that
the increase in the ionic radius of RE3+ has resulted in enhanced
fracture toughness. This is not only attributed to the increased aspect
ratio of β-Si3N4 grains but also to the changed crack growth mechanism
from transgranular to intergranular. MgO doped silicon nitride with
Gd2O3, La2O3 and La2O3-Lu2O3 combination has exhibited higher
toughness and strength when compared with MgO + (Lu2O3 and Y2O3).
This is due to low viscosity oxynitride phase generation in intergranular
films (IGFs) or two-grain junctions and formation of larger aspect ratio
grains [11,12]. Liu et al. [13] fabricated silicon nitride using 2 wt%
MgO and 5 wt% Y2O3 through hot pressing at 1800 °C and reported an
increase in thermal conductivity, hardness, bending strength and fracture toughness. The enhancement in these properties are attributed to
the fine microstructure with more elongated β-Si3N4 grains. Han et al.
[14] employed hot pressing to fabricate silicon nitride with or without
oscillatory pressure using Al2O3 and Y2O3 as sintering additives. The
hardness, fracture toughness, elastic modulus and flexural strength of
oscillatory pressure assisted hot pressed ceramics sintered at 1770 °C
were 16.04 ± 0.21 GPa, 12.8 ± 0.4 MPa m1/2, 307 ± 6 GPa and
1348 ± 31 MPa respectively. Hardness, fracture toughness, elastic
modulus and flexural strength for hot pressed (without oscillatory
pressure) silicon nitride ceramics were 15.46 ± 0.39 GPa,
10.5 ± 0.1 MPa m1/2, 304 ± 4 GPa and 1128 ± 39 MPa respectively.
Corresponding author.
E-mail address: aravindan@mech.iitd.ac.in (S. Aravindan).
https://doi.org/10.1016/j.msea.2019.02.020
Received 15 October 2018; Received in revised form 29 December 2018; Accepted 6 February 2019
Available online 07 February 2019
0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 750 (2019) 132–140
A. Kumar, et al.
The application of oscillatory pressure reduced the defects and enhanced the densification, and this contributed to the improvement of
fracture toughness and strength. Jeong et al. [8] investigated the effect
of nanocomposite particles and powder mixing technique on the spark
plasma sintering of submicron sized particles of α-Si3N4. The nano-size
5 wt% Y2O3 and 2 wt% MgO were added as sintering additives using
mechanical treatment (MT). For the same composition, the Si3N4
ceramics sintered at 1600 °C with nanocomposite particles prepared by
MT exhibited higher fracture toughness and density compared to the
mixture prepared by ball milling. The higher fracture toughness and
density of the mechanically treated specimens were attributed to the
homogeneous mixture of starting powders and elongated β-Si3N4
grains.
Dense silicon nitride ceramics with excellent thermomechanical
properties can be easily produced by commercial methods such as hot
pressing (HP), spark plasma sintering (SPS), hot isostatic pressing (HIP)
and gas pressure sintering (GPS). The major problems associated with
these processing methods are high fabrication cost, small size and relatively simple shapes. In order to achieve low fabrication cost, large
size and complex shape with improved thermomechanical properties,
pressureless sintering is an ideal fabrication method for the advanced
structural ceramics [4,15]. The addition of 0.2 wt% FeSi2 with 3 wt%
Al2O3 and 9 wt% Y2O3 in pressureless sintered silicon nitride at 1780 °C
for 2 h improved the fracture toughness (KIC) and flexural strength [4].
The fracture toughness, thermal conductivity and flexural strength of
pressureless sintered silicon nitride (fired at 1800 °C for 2 h in N2 atmosphere) specimens were 10.3 MPa m1/2, ~ 60 W/mK and
924 ± 54 MPa respectively with the addition of 25% Si and ZrO2
+ Gd2O3 + MgO as sintering additives in starting powder [15]. Penas
et al. [16] developed high-density Si3N4 using pressureless sintering by
adding 8 wt% Y2O3 and 1.5 wt% Al2O3 as sintering additives at a sintering temperature of 1800 °C. The study highlighted that the mechanical or thermo-mechanical properties of pressureless sintered Si3N4
were better than hot pressed Si3N4 under the same sintering conditions.
A finer microstructure with homogeneous and elongated β-Si3N4 grains
was achieved for the pressureless sintered specimens as compared to
hot pressed samples.
The choice of sintering additives and process parameters certainly
can influence the grain growth and phase transformation, which in turn
tailors the microstructure leading to varying mechanical properties
[11]. The microstructure of sintered ceramics can be regulated by the
addition of β-Si3N4 seeds in the starting powder. β-Si3N4 seeds in αSi3N4 powder act as nucleation sites for the growth of β-Si3N4 grains.
This enhances the transformation of α- Si3N4 into β- Si3N4 grains during
liquid phase sintering. Therefore, the obtained microstructure has a
preponderance of high aspect ratio of β- Si3N4 grains, which improves
the mechanical properties especially, fracture resistance [17–19]. When
β- Si3N4 seeds were added to starting powder, the fracture toughness of
gas pressure sintered (at 1850 °C for 6 h under 0.9 MPa N2 pressure)
Si3N4 was improved from 6.3 to 8.7 MPa m1/2 [20]. Silicon nitride
ceramics sintered with 5 wt% of β -Si3N4 seeds significantly improved α
→ β phase transformation (from 42% to 84% β -Si3N4) and fracture
toughness as compared to silicon nitride sintered without seeds [21].
The effect of micron sized sintering additives (commonly used Al2O3,
MgO, and Y2O3) on the mechanical properties and microstructure have
been extensively studied for both pressureless and pressure assisted
sintering methods [1,3,8,11,15,20,21]. Till date, only a few studies
have focused on the mechanical properties and microstructure of micron sized α-Si3N4 and β-Si3N4 containing nanopowder based sintering
additives [8,22]. In the present work, dense silicon nitride ceramics
have been prepared from commercially available Si3N4 powder by
compounding use of nanosized rare-earth oxides (Y2O3 and La2O3)
sintering additives using pressureless sintering. The effect of nanosized
sintering additives (MgO, Al2O3, Y2O3 and La2O3) on the densification,
microstructure, phase transformation, mechanical properties and crack
propagation mechanism of silicon nitride ceramics have been
systematically investigated.
In the present work, the described method for fabrication of dense
silicon nitride ceramics using pressureless sintering would be an alternative for developing the advanced ceramics with enhanced thermomechanical properties. The addition of nanosized MgO, Al2O3, Y2O3
and La2O3 in Si3N4 produced a finer microstructure with improved
mechanical properties.
2. Experimental procedure
2.1. Material and methods
Fine micron size α-Si3N4 powder (SN-E10, β / (α + β) < 5 wt%, d50
= 0.5 µm, SSA = 10.9 m2/g, UBE industries, Japan) was used as a raw
material. The raw α-Si3N4 powder was mixed with nano sintering additives, i.e. Al2O3 (de-agglomerated, 50 nm, Allied High Tech Products,
Inc. CA), MgO (99.95% pure, 50 nm), Y2O3 (99.99% pure, 30–45 nm)
and La2O3 (99.99% pure, 40–60 nm). The β- Si3N4 powder (99.6% pure,
d50 = 0.8 µm, 90–95% β - Si3N4, US Research Nanomaterials, Inc. USA)
was mixed with the starting powder for enhancing the growth of βSi3N4. The α-Si3N4, β-Si3N4, MgO, Al2O3, Y2O3 and La2O3 powders with
defined compositions were ball milled for 6 h in isopropanol (2Propanol). The composition of Si3N4-based ceramics with different
sintering additives used for fabrication is presented in Table 1.
Polyvinyl alcohol (PVA) (0.9 wt%) was added to the powder mixture as a binding agent [23]. After mixing and drying, the homogeneous
powder mixture was sieved through a 100-mesh screen and uni-axially
pressed at 125 MPa for 2 min. The dewaxing (removal of PVA) of green
compacts was done at 600 °C for 1 h in a muffle furnace. Finally, the
dewaxed compacts were embedded into powder bed of α-Si3N4 in a
high-density graphite crucible. A high-temperature graphite furnace
with Eurotherm 2404 controller and pyrometer (IMPAC, Germany) was
used to sinter the green compacts at 1700 °C for 2 h under 0.1 MPa N2
pressure. The compacts were manually heated to 800 °C, and after that,
a heating rate of 10 °C/min till 1700 °C was controlled by the Eurotherm controller. After holding the specimens at 1700 °C for 2 h, a
cooling rate of 10 °C/min was used till 800 °C and then furnace cooling
to ambient temperature was accomplished.
2.2. Characterization
The bulk density of the sintered specimens was measured using
helium pycnometer (ULTRAPYC 1200e). The sintered specimens were
ground using a resin bonded diamond wheel (SD54R75B1/3) and polished using the metal bonded diamond disc and diamond slurries
(9 µm, 6 µm, 3 µm and 1 µm). The microstructure of the polished samples after etching in conc. HF for 2 min, was observed in scanning
electron microscopy (Zeiss, EVO MA10). X-ray elemental mapping was
done using Energy-dispersive X-ray spectroscopy (EDAX, Bruker's). The
phase analysis of the sintered ceramics was carried out by X-ray diffraction (Ultima IV, Rigaku) using the Cu Kα radiation. Vickers indentation (Z2.5, Zwick Roell) was used to measure the Vickers’ hardness at a load of 49 N with a dwell time of 15 s (a minimum of 8
indentations were selected for the measurement). The indentation
fracture toughness (KΙC) was evaluated by Vickers indentation crack
length method proposed by using Antis et al. [24].
Table 1
The composition of Si3N4 with different sintering additives (wt%).
133
Symbol
α-Si3N4
β-Si3N4
MgO
Al2O3
La2O3
Y2O3
4Mg4Al
4Mg4Al5Y
4Mg4Al5La
4Mg4Al2.5Y2.5La
89
84
84
84
3
3
3
3
4
4
4
4
4
4
4
4
–
–
5
2.5
–
5
–
2.5
Materials Science & Engineering A 750 (2019) 132–140
A. Kumar, et al.
3. Results and discussion
3.1. Microstructure
The nano-size sintering additives have high specific surface area and
surface energy. More number of additives (due to their nonmetric size)
as seen in Fig. 1 surrounds the α-Si3N4 and β-Si3N4 particles in mixing.
During sintering, the joining area between the sintering additives and
the Si3N4 particles got increased as compared to their micron counter
parts [8,25].
The growth in β-Si3N4 grains along the length (c-axis) is diffusion
controlled and as fast as compared to the growth of diametrical prism
faces (a-axis). The β-Si3N4 grain growth and α-Si3N4 to β-Si3N4 transformation depends upon supersaturation, solubility, rate of dissolution,
wetting and viscosity. All these can be varied by changing the α-Si3N4
to β-Si3N4 powder ratio in starting powder, altering sintering additives
type, size and size distribution of the sintering additives because this
lead to change in chemical composition and chemistry of liquid phase
[26,27]. Fig. 2 reveals the microstructure of the sintered specimens
after etching in concentrated HF for 2 min. The addition of β-Si3N4
powder and homogeneous mixing of nano-sized sintering additives in
starting powder not only enhanced the phase transformation but also
the anisotropic growth of β-Si3N4 grains. This produced a specific interlocking microstructure of β-Si3N4 grains.
The microstructure of Si3N4 produced by the addition of MgO and
Al2O3 sintering additives are characterized by small and thin β-Si3N4
grains. The addition of RE2O3 changed the chemical composition and
chemistry of the oxynitride liquid, which enhanced the growth of βSi3N4 grains. The temperature required for the initiation and completion of α-Si3N4 to β-Si3N4 phase transformation is strongly influenced
by the addition of rare earth (RE2O3) oxides because it changes the
viscosity of oxynitride liquid. The viscosity of oxynitride liquid tends to
decrease with increasing in the ionic radius of RE3+ as per Lofaj et al.
[28]. The addition of Y2O3 (4Mg4Al5Y composition) produced high
Fig. 1. Schematic representation of green compact with nano-size sintering
additives and sintered silicon nitride.
K C = 0.016
E
H
0.5
P
…
c1.5
(1)
where E is Young's modulus in GPa (measured using nano-indentation
P-h curves), H is micro-hardness in GPa (measured using Vickers’ indentation), c is crack length in m and P is indentation load in N.
The nanoindentation test was performed using ASMEC‫׳‬s Universal
Nanomechanical Tester (Bautzner Landstraße 45, Germany). A 184 nm
tip radius Berkovich diamond indenter was used to make an indent at
100 mN load at loading and unloading rate of 1 mN/s. The nanoindenter was calibrated on a silica sample for the measurement of
Young's modulus and nanohardness. A quasi-continuous stiffness measurement (QCSM) technique was used for the indentation.
Fig. 2. The microstructure of polished and etched samples sintered with nano-sized sintering additives.
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A. Kumar, et al.
Fig. 3. Elemental mapping of sintered ceramics with different composition, and distribution of elements.
viscosity glass phase, which reduced the rate of grain growth along caxis and led to coarser grains. The La2O3 addition (4Mg4Al5La composition) generated low viscosity glass phase, which enhanced the mass
transport and dissolution of small grains in liquid phase by Oswaldripening mechanism. The microstructure of 4Mg4Al5La composition is
characterized by preponderance of finer β-Si3N4 grains.
The α → β-Si3N4 phase transformation during sintering occurs by (і)
dissolution of the α phase and saturation of the oxynitride liquid phase
and re-precipitation out of large Si3N4 grains (іі) Si and N ions transport
through oxynitride liquid phase, and (ііі) attachment and growth onto
existing β-Si3N4 grains or particles [9,11]. At high temperature
(> 1400 °C), the liquid phase produced by the addition of nano-sized
RE2O3 particles with nano-sized Al2O3 and MgO, increased the wettability and wrapping of α-Si3N4 particles which enhanced the dissolution rate of α-Si3N4 particles via solution–reprecipitation process. The
enhanced dissolution rate and more nucleation sites by addition of βSi3N4 particles have provided additional time for β-Si3N4 grains to increase their size and number. The small RE3+ radius RE2O3 (RE3+ = Y
= 0.89 Å) have higher stability of RE3+ in oxynitride glass and also
promotes the attachment of Si and N to the prismatic plane surface
because of high cationic field strength (CFS) of RE3+ produced larger
diameter β-Si3N4 grains (coarser) compared to large RE3+ radius RE2O3
(RE3+ = La = 1.06 Å).
The elemental mapping of sintered Si3N4 ceramics is shown in
Fig. 3(a-d). All the samples exhibit a uniform distribution of Si and N in
the matrix. These elemental mappings also show some traces of
sintering additives in the matrix which may be below the detection
limit of XRD [19].
3.2. Phase transformation
The XRD patterns of the starting powder (α-Si3N4 powder with α
content ≥ 95%) and the sintered Si3N4 ceramics with nano-sized sintering additives are shown in Fig. 4(a) and (b). XRD analysis of sintered
Si3N4 with different nano-size sintering additives showed the complete
transformation of α-Si3N4 into β-Si3N4 without the existence of α-Si3N4
and any other phases of sintering additives.
At sintering temperature, the sintering additives reacted with αSi3N4 particles and the nascent SiO2 present on α-Si3N4 particles and
formed a low-temperature eutectic phase, which crystallized at grain
boundaries on cooling [29]. The transformation of α-Si3N4 to β-Si3N4
takes place via dissolution-diffusion-precipitation process and this depends upon the viscosity and chemical composition of oxynitride liquid
phase, which is governed by the sintering additives [3,14].
The addition of nano-size sintering additives has enhanced the
dissolution rate of α-Si3N4 particles in the oxynitride liquid, which has,
in turn, resulted in more precipitation of β-Si3N4 grains according to the
following equation
[
Si3 N4] + [SiO2] + [Mx Oy ]
[
Si3N4] + [M
Si
O
N]…
(2)
Where MxOy is metal oxide used as sintering additive. In this work,
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A. Kumar, et al.
Fig. 6. Variation of Young's modulus with displacement in nanoindentation of
Si3N4 ceramics with different compositions.
3.3. Mechanical properties
The highest density of 3.17 g cm−3 was achieved for 4Mg4Al5Y
composition, whereas a density of 3.12 g cm−3 was obtained for the
4Mg4Al composition. The density obtained in the present case is
comparable to spark plasma sintered Si3N4 [30]. The addition of nanosized sintering additives (Al2O3, MgO, La2O3 and Y2O3) enhanced the
densification of Si3N4 ceramics. This can be attributed to the complete
wetting and wrapping of silicon nitride grains and grain boundary
phases by sintering additives during sintering. The oxynitride liquid
formed by the reaction of sintering additives, SiO2 and fine Si3N4 particles enhanced the densification by particle rearrangement followed by
dissolution-diffusion-precipitation process. The combination of particle
rearrangement and dissolution-diffusion-precipitation process resulted
in densification and complete phase transformation [31].
The micro-hardness of Si3N4 (4Mg4Al composition) is 15.5 ( ± 0.3)
GPa. The addition of 5 wt% nano-sized Y2O3 in the starting powder has
achieved the highest hardness of 16.7 ( ± 0.2) GPa. Fig. 5 represents
the load – displacement of the sintered ceramics with the different
additive compositions. At constant load, the indentation depth varied
with composition because of the hardness of the sintered sample. The
Young's modulus has been measured with respect to the indentation
depth (Fig. 6). Mostly researchers have used Oliver and Pharr method
for calculating Young's modulus (the slope of the unloading curve)
[32,33]; however, in the present work QCSM technique was employed
to have the accurate measurement of Young's modulus, and nanohardness.
The nano-sized (MgO, Al2O3, and Y2O3) additives doped sample
resulted in the highest nanohardness of 21.0 ( ± 2.2) GPa. The combination of MgO, Al2O3, and Y2O3 accelerated the densification and
phase transformation, which has resulted in increased hardness and
Young's modulus. Due to the less absorption of small RE3+ radius
RE2O3 at the prismatic surface of Si3N4 surface and high cationic field
strength (CFS) of RE3+, the viscosity of oxynitride glassy phases is
higher, and bonding of oxynitride glassy phases within the intergranular films (IGFs) and neighbouring grains is strong and highly
dense [10–12,34]. The small RE3+ radius RE2O3 (RE3+ = Y = 0.89 Å)
composition showed highest hardness compared to other compositions
because of the highest CFS of Y3+. Similar trend of hardness change
with RE3+ ionic radius was observed by Tatarko et al. [34] for monolithic and nanocomposite of Si3N4 where the hardness value decreased
with an increasing RE3+ ionic radius or decreasing CFS.
Fig. 7 shows the Vickers indent made on 4Mg4Al5Y bearing composition to evaluate the fracture toughness. Cracks were evident originating from the apex of the indent, and the crack length for each
samples were measured. The crack length found out to be
65.47 ± 0.66 µm,
54.92 ± 0.71 µm,
52.63 ± 0.41 µm
and
54.88 ± 0.44 µm respectively for 4Mg4Al, 4Mg4Al5Y, 4Mg4Al5La and
4Mg4Al2.5Y2.5La bearing compositions respectively. Using Eq. (1), the
fracture
toughness
obtained
were
5.43 ± 0.11 MPa m1/2,
Fig. 4. The XRD pattern of (a) raw α-Si3N4 powder with SEM and (b) sintered
Si3N4 ceramics with different composition.
Fig. 5. Load (P) versus displacement (h) curves using Berkovich diamond indentations at 100 mN for different compositions.
MxOy signifies the nano-sized MgO, Al2O3, Y2O3 and La2O3. The enhanced dissolution rate, higher atom diffusion rate and more precipitation of β-Si3N4 grains resulted in complete transformation of αSi3N4 to β-Si3N4.
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A. Kumar, et al.
Fig. 7. The micro-indent on 4Mg4Al5Y sample with indent diagonal length from the apex of the indent.
Fig. 8. Schematic representation of (a) transgranular and (b) intergranular crack propagation mechanism.
7.43 ± 0.15 MPa m1/2, 8.00 ± 0.09 MPa m1/2 and 7.48 ± 0.02 MPa
m1/2 respectively for 4Mg4Al, 4Mg4Al5Y, 4Mg4Al5La and 4Mg4Al2.5Y2.5La bearing compositions respectively. The 4Mg4Al5La composition exhibited higher fraction of crack deflection at grain boundaries and interlocking of grains, which results in high toughness.
The fracture toughness of silicon nitride ceramics is dependent on
the crack deflection at the glass|grain interface. The aspect ratio of
grains and interfacial strength between grains and the surrounding
phase are directly influenced by the presence of specific RE2O3, which
have a substantial impact on the crack propagation mechanism. The
interfacial (glass|grain interface) strength in large size RE3+ cation
(La3+) is weak because of the higher coverage of the Si3N4 grains and a
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A. Kumar, et al.
Fig. 9. Crack propagation behaviour of Si3N4 ceramics sintered with different compositions (a) 4Mg4Al5Y (b) 4Mg4Al (c) 4Mg4Al5La and (d) 4Mg4Al2.5Y2.5La.
Table 2
Density and mechanical properties of sintered Si3N4 ceramics with different compositions (average value ± standard deviation).
Sample
4Mg4Al
4Mg4Al5Y
4Mg4Al5La
4Mg4Al2.5Y2.5La
Density (g/cm3)
3.12
3.17
3.15
3.16
Hardness (GPa)
Microhardness
Nanohardness
15.5
16.7
15.9
16.4
17.7
21.0
19.3
20.6
±
±
±
±
0.3
0.2
0.4
0.5
±
±
±
±
1.9
2.2
1.3
1.8
higher tendency to bond with nitrogen on the prism plane of Si3N4 [35].
The addition of La2O3 produced the high aspect ratio grains with
weaker interfacial strength, and this has increased the fracture toughness but simultaneously has decreased some of the mechanical properties [10,36]. In case of Y2O3 (4Mg4Al5Y composition) the interfacial
bonding is strong because of high CFS so crack deflection should not
occur at the glass|grain interface. The change of high CFS RE3+ (Y2O3)
to low CFS RE3+ (La2O3) results in significant reduction in the interfacial strength and this lead to onset enhancement of frictional energy
dispersion mechanism and increased fracture toughness. The large
Young's modulus (GPa)
Fracture toughness (KIC) (MPa m1/2)
208.4
248.9
242.8
246.1
5.43
7.43
8.00
7.48
±
±
±
±
13.3
5.20
11.6
14.4
±
±
±
±
0.11
0.15
0.09
0.02
debonding length and frequent pull-out events were observed by
Satet al el. [35] in case of sintered ceramic with larger RE3+. The crack
propagation mechanism for strong (Y2O3) and weak (La2O3) glass|grain
interface is schematically shown in Fig. 8(a-b). However, the mechanical properties of the silicon nitride ceramics sintered with nano-sized
sintering additives were higher than the ceramics sintered with the
micron sized sintering additives [30,37].
The microstructure reveals the elongated grains in each composition. However, the final mechanical properties are governed by the
glass|grain interface. The addition of nano-sized Y2O3 produced strong
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A. Kumar, et al.
Table 3
Hardness, fracture toughness of pressureless sintered Si3N4 using different micron sized sintering additives.
Additives
Sintering temperature (°C)
Holding time
(min.)
Phase present
Hardness (GPa)
Fracture toughness (MPa.m1/2)
Ref.
5 wt% TiO2 + 5 wt% MgO
6 wt% Y2O3 + 9 wt% Al2O3
6 wt% Y2O3 + 2 wt% Al2O3
9 wt% Y2O3 + 3 wt% Al2O3
5 wt% Y2O3 + 5 wt% MgO + 2 wt% Al2O3
9 wt% Al2O3 + 6 wt% Y2O3
7.4 wt% ZrO2 + 4.4 wt% MgO + 3.5 wt%
Y2O3
12.5 wt% LiYO2
3 wt% MgO + 1.5 wt% Al2O3 + 3.5 wt%
SiO2
1780
1650
1780 (gel-casting)
1780
1700 (microwave)
1700
1800
120
60
80
120
10
120
240
β-Si3N4, TiC0.3N0.7
–
β-Si3N4, Al2Y4O9
–
α, β-Si3N4
β-Si3N4
β-Si3N4, Y2Si3O3N4
15.06 ± 0.14
1380 ± 15 HV0.5
–
13.6 ± 0.2
14.92 ± 0.2
16.7 ± 0.1
16.5 ± 1.0
5.1
6
6.5
7.01 ± 0.1
6.44 ± 0.02
3.6 ± 0.2
7.0 ± 0.5
[39]
[33]
[40]
[4]
[41]
[37]
[42]
1600
1780
120
180
β-Si3N4, Y5Si3O12N
β-Si3N4
13.0 ± 0.3
14.2 ± 1.0
6.1 ± 0.2
6.4 ± 0.5
[43]
[44]
glass|grain interface, which leads to transgranular crack propagation
mechanism (broken elongated grains and low crack deflection at the
interface) as evident in Fig. 9(a). Similar mechanism for crack propagation can be proposed for 4Mg4Al bearing composition as revealed
from Fig. 9(b).
On the other hand, the addition of nano-sized La2O3 produced weak
glass|grain interface with intergranular crack propagation mechanism
(high crack deflection at the interface and more grain bridges) as shown
in Fig. 9(c), which results in increased energy for crack growth, and so
the fracture toughness. The addition of 2.5 wt% of nano-sized Y2O3 and
2.5 wt% of nano-sized La2O3 led to an improved hardness, Young's
modulus and fracture toughness because of the inter-transgranular
crack propagation mechanism (Fig. 9d). The compounding use of RE2O3
with small and large RE3+ radius significantly modified the grain
boundary phases by lowering down the viscosity, improved homogenization, increased diffusion, and inter-transgranular crack propagation mechanism [12,38]. The homogenous and low viscosity grain
boundary phase lead to improved mechanical properties compared to
singular rare earth oxide (RE2O3) doped ceramic. The density, hardness,
Young's modulus, and fracture toughness for all the compositions are
summarized in Table 2.
The effect of micron sized sintering additives (amount, and type)
along with the sintering conditions on phase transformation, hardness
and fracture toughness using pressureless sintering is listed in Table 3.
Compared to these micron-sized additives, the nano-sized additives
improved the mechanical properties substantially. Thus, the used nanosized sintering additives are perfectly suitable for improving the mechanical properties of silicon nitride ceramics.
3) The addition of nano-sized Y2O3 and La2O3 produced strong and
weak glass|grain interface respectively. The fracture toughness of
4Mg4Al2.5Y2.5La bearing composition Si3N4 was higher than that
of 4Mg4Al and 4Mg4Al5Y. The density, microhardness, nanohardness Young's modulus and fracture toughness of 4Mg4Al2.5Y2.5La
was 3.16 g cm− 3, 16.4 ( ± 0.5) GPa, 20.6 ( ± 1.8) GPa, 246.1
( ± 14.4) GPa, and 7.48 ( ± 0.02) MPa m1/2 respectively.
4) The compounding use of RE2O3 with small and large RE3+ radii
significantly modified the grain boundary phase by lowering its
viscosity, which leads to improved mechanical properties compared
to singular RE2O3 doped Si3N4 ceramics.
Acknowledgement
The authors gratefully acknowledge the financial support provided
by the Department of Science and Technology (India) (Project No.
RP3452) for this research work.
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