Synthesis of TiN-Si3N4 Composites from Rutile and ... - doiSerbia

Science of Sintering, 44 (2012) 57-64
________________________________________________________________________
doi: 10.2298/SOS1201057C
UDK 343.256:553.494
Synthesis of TiN-Si 3 N 4 Composites from Rutile and Quartz by
Carbothermal Reduction Nitridation
Kai Chen, Zhaohui Huang *, Minghao Fang, Yan-gai Liu
School of Materials Science and Technology, China University of Geosciences
(Beijing), Beijing 100083, China
Abstract:
TiN-Si 3 N 4 composite powders were prepared by carbothermal reduction nitridation
using rutile and quartz as raw materials. The influence of temperature and carbon addition
on the phase evolution and microstructure of the products were investigated. The equilibrium
phase diagram of Si-C-N-O and Ti-C-N-O system at different temperatures under 0.2 MPa
nitrogen pressure was drew. The results show that the optimum parameters for synthesizing
TiN-Si 3 N 4 by carbothermal reduction nitridation process are carbon addition of
stoichiometric content, temperature of 1873 K for 4 h and nitrogen pressure of 0.2 MPa. The
produced TiN-Si 3 N 4 in this experiment exist in granular and hexagonal columnar shape, and
the average particle size of the synthesized powders is 2 ~ 10 μm.
Keywords: TiN-Si 3 N 4 composites; Carbothermal reduction nitridation; Quartz; Rutile;
Equilibrium phase diagram
1. Introduction
Silicon nitride (Si 3 N 4 ) is a kind of excellent non-oxide ceramic material with high
hardness, good high-temperature strength, thermal and chemical stability. On account of these
desirable properties, silicon nitride based materials have attracted great interests in many
high-temperature structural applications. However, the difficulties in fabricating and
machining of Si 3 N 4 which resulting from its extremely high hardness and low fracture
toughness may limit their broader applications [1]. Introducing the second-phase particles into
the Si 3 N 4 matrix, which can improve the properties of silicon nitride based materials, has been
in the focus of the comprehensive future studies [1, 2]. Titanium nitride (TiN) has been
studied extensively accredit to its outstanding properties such as extreme hardness, good
thermal and electrical conductivity, high melting temperature, etc. It was reported that the
dispersed TiN particles make contributions to the enhancement of wear resistance, fracture
toughness and flexural strength of the Si 3 N 4 matrix, and conductivity as well [3-6].
Methods that have been developed to synthesize TiN-Si 3 N 4 composites including the
chemical vapor deposition method [7], the pyrolysis process [8, 9] and the self-propagating
high-temperature method [10, 11]. In recent years, the microwave plasma synthesis technique
has been developed as a new powerful approach for preparing TiN-Si 3 N 4 composite powders
[12]. However, most of these methodologies are at the expense of high energy consumption
and high cost since that both the achieved raw materials which usually through a complex
high-temperature process and the synthesis reaction need high-priced experimental facilities.
_____________________________
*) Corresponding author: huang118@cugb.edu.cn

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K.Chen et al. /Science of Sintering, 44 (2012) 57-64
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It is well known that powders of carbonitrides and nitrides can be economically produced by
carbothermal reduction nitridation (CRN) processes. CRN has attracted great interests as the
most cost-efficient method to synthesize Si 3 N 4 based materials from natural minerals [13, 14].
The purpose of this paper is to obtain the cost-effective synthesis of TiN-Si 3 N 4
composite powders by using cheaper starting material and synthesizing method. In this study,
rutile and quartz were used as the raw materials for the synthesis of TiN-Si 3 N 4 composites by
CRN process. The influence of temperature and carbon addition on the phase evolution and
the microstructure of the products were investigated. The equilibrium phase diagram of Si-C-
N-O and Ti-C-N-O system at different temperatures under 0.2 MPa nitrogen pressure was
drew, and the optimum experimental conditions for synthesizing TiN-Si 3 N 4 composite
powders were obtained.
2. Experimental Procedure
The raw starting materials are as follows: rutile powder (grain size < 0.5 μm, purity 95
wt %), quartz powder (grain size < 38 μm, chemical composition (wt %): SiO 2 : 97.8, Al 2 O 3 :
0.63, Fe 2 O 3 : 0.33, CaO: 0.08). Carbon black powder (grain size < 25 μm, carbon content: 99
wt %) and N 2 (purity 99.99%) were employed as reducer and reactive gas, respectively.
Quartz powder and rutile powder with a weight ratio of 90 : 10 were mixed, then carbon black
with mass additions (wt %) of 39.0, 42.9, 58.5 and 78.0 were added into the mixtures. The
mixtures were mixed and milled with Al 2 O 3 balls in anhydrous alcohol at 200 rpm for 6 h,
then dried at 353 K. And the powder mixture samples with different carbon additions were
marked as S1, S2, S3 and S4, respectively. Finally, the mixed powders were placed in a BNcoated
graphite crucible, and CRN reactions were carried out in a tube furnace at different
temperatures (1723 K, 1773 K, 1823 K and 1873 K) for 4 h under 0.2 MPa nitrogen pressure.
The samples were slowly cooled down to room temperature. Then the samples were sintered
at 973 K for 1 h to have a decarburization process.
The crystalline phases were determined by X-ray diffraction (XRD; D8 Advance
diffractometer, Germany), using Cu Kα 1 radiation (λ = 1.5406 Å) with a scanning rate of 8°
min -1 . The particle size and microstructure of the final products were examined by scanning
electron microscopy (SEM; JEM-6460LV microscope, Japan) and energy dispersive
spectroscopic (EDS; Oxford Inca X-sight, UK).
3. Results and Discussion
3.1 Thermodynamic considerations
In this experimental condition, the synthesis process of TiN-Si 3 N 4 composite powder
by CRN reaction is very complex and involves many chemical reactions. The final products
of the nitridation reaction of rutile are TiN and TiC, though several intermediate crystalline
phases can be obtained in the CRN process. The overall reactions during the CRN process of
rutile are as follows:
TiO 2
(s) + 3C(s) = TiC(s) + 2CO(g)
(1)
1
TiO2 (s) + 2C(s) + N2(g)
= TiN(s) + 2CO(g)
(2)
2
1
TiC(s)
+ N
2
(g) = TiN(s) + C(s)
(3)
2
From the results of the investigations of the solid intermediate reaction products during
the forming process of TiN, the reaction sequence TiO 2 →Ti 4 O 7 → Ti 3 O 5 → Ti 2 O 3 →

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Ti(N,O) → TiN was established [15]. However, there is some controversy on the reaction
mechanism of TiC formation in the CRN reactions. Some authors claim that this is due to
solid-solid mechanism by the direct reaction of TiO 2 with C mixture particles, while others
are in favor of the Boudeward vapor-solid approach by the reaction of TiO 2 with CO [16, 17].
With the change of the raw materials composition, the reaction products maybe
involve Si 2 N 2 O, Si 3 N 4 and SiC. The chemical reactions are likely to occur during the
synthesis process are as follows:
SiO 2
(s) + 3C(s) = SiC(s) + 2CO(g)
(4)
SiO (s) + 6C(s) + 2N (g) = Si N (s) 6CO(g)
(5)
3
2 2
3 4
+
2SiO
2
(s) 3C(s) + N
2
(g) = Si
2N
2O(s)
+
+ 3CO(g)
(6)
1
3
1 1
Si2 N2O(s)
+ C(s) = SiC(s) + N2(g)
+ CO(g)
2
2
2 2
(7)
3
1 3
3
Si2 N2O(s)
+ N2(g)
+ C(s) = Si3N4(s)
+ CO(g)
2
2 2
2
(8)
1
2
Si3 N4(s)
+ C(s) = SiC(s) + N2(g)
3
3
(9)
It is generally accepted that the CRN reactions (1) and (2) of quartz occurs through
step by step mechanism, where SiO acts as a gaseous intermediate [17, 18]. SiO can be
formed by solid-solid reaction (10) and vapor-solid reaction (11).
SiO 2
(s) + C(s) = SiO(g) + CO(g)
(10)
SiO
2
(s) + CO(g) = SiO(g) + CO2
(g)
(11)
Then, Si 3 N 4 can be produced according to the carbonitriding reactions (12) and (13).
Under normal conditions, α-Si 3 N 4 was produced by CRN process. But the eutectic reaction
occurred due to the effect of the liquid phases of TiO 2 and Fe 2 O 3 , and led to the formation of
β-Si 3 N 4 instead of α-Si 3 N 4 [19].
2
1
SiO(g)
+ N
2
(g) + C(s) = Si3N
4
(s) + CO(g)
(12)
3
3
2
1
SiO(g)
+ N
2
(g) + CO(g) = Si3N
4
(s) + CO
2
(g)
(13)
3
3
The formation reaction of SiC occurs via the vapor-solid approach or by the gas phase
reaction:
Si (g) + 2C(s) = SiC(s) + CO(g)
(14)
SiO(g)
+ 3CO(g) = SiC(s) + 2CO2
(g)
(15)
Fig.1 shows the equilibrium phase diagram of the stable solid phases in Si-C-N-O and
Ti-C-N-O system at different temperatures under 0.2 MPa nitrogen pressure. It is constructed
1 P
by calculating the relationship between and ln(
CO
RT ) of reactions (1) ~ (9). It is seen in
T
θ
P
Fig.1 that Si 2 N 2 O exists as a stable phase at 1406 K to 1876 K when the P CO was 0.01 MPa.
Besides, Si 3 N 4 keeps stable under a maximum permissible P CO of 1.78×10 -3 MPa at 1794K.
This indicates that Si 3 N 4 can coexist with TiN under a smaller P CO condition. Therefore, the
TiN-Si 3 N 4 composites can be obtained by optimizing the reaction conditions such as reaction
temperature, atmosphere and gas partial pressure. In this experiment, increasing the velocity
of the flow of nitrogen was conducted at the temperatures above 1273 K to reduce the partial
pressure of CO.

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Fig.1 Equilibrium phase diagram of Si-C-N-O and Ti-C-N-O system.
3.2 Effect of the reaction temperature
Fig. 2 shows the results of XRD measurements on sample S2 after heat-treatment at
different temperatures for 4 h. It can be seen that cristobalite, β-SiC, Ti(C,N) and Si 2 N 2 O were
formed in the products when the reaction temperature increased to 1723 K. With the increase
of the reaction temperature, the diffraction peak intensity of Si 2 N 2 O increased gradually,
while the intensity of cristobalite dropped. At temperatures above 1773 K, an increasing
number of Si 2 N 2 O, β-SiC and TiN were observable. β-Si 3 N 4 was obtained at 1823 K in the
products and became the predominant crystalline phase, while the diffraction peak intensity of
β-SiC phase was weaker than that of others and the cristobalite phase was barely discernible.
Fig.2 XRD patterns of S2 sintered at different temperatures.
It is inferred that β-SiC and Si 2 N 2 O reacted and formed β-Si 3 N 4 following as the
carbonitriding reactions (4) ~ (6), respectively. The product synthesized at 1823 K includes β-

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Si 3 N 4 , Si 2 N 2 O, β-SiC and TiN. The β-SiC phase vanished in the sample obtained at 1873 K,
differing from the results of the corresponding samples derived from the carbon addition. β-
Si 3 N 4 entirely replaced Si 2 N 2 O and the major products formed at 1873 K were β-Si 3 N 4 and
TiN. In addition, the diffraction peak intensity of β-Si 3 N 4 or TiN is higher than that obtained
at 1723 K, 1773 K or 1823 K. This indicates that increasing the reaction temperature can
promote the formation of β-Si 3 N 4 and TiN. The optimum temperature for the synthesis of high
TiN-Si 3 N 4 composites was 1873 K for 4 h.
The SEM micrographs and the EDS patterns of the products of sample S2 sintered at
different temperatures are shown in Fig. 3. Fig. 3(a) shows the SEM micrographs of sample
S2 heated at 1823 K. The average particle size of the globular agglomerates is 1 ~ 5 μm.
Besides, many holes in the middle of the agglomerated grains as an open hollow structure are
apparent in the samples. It is believed that these holes were resulted from the release of
gaseous reduction product such as CO and SiO during the further reduction. EDS analysis
results in Fig. 3(c) and XRD pattern of sample S2 heated at 1823 K in Fig. 2 indicate that the
globular agglomerates as shown in Fig. 3(a) are composed of β-Si 3 N 4 , Si 2 N 2 O and TiN. Fig.
3(b) shows the short columnar or hexagonal morphology of β-Si 3 N 4 synthesized at 1873 K. It
can also be observed from Fig. 3(d) that titanium was detected in the area of the short
columnar grain. This indicates that a small amount of TiN was formed in the products of
sample S2. And it is proposed that solid solutions between TiN and Si 3 N 4 were formed.
Fig.3 SEM micrographs and EDS patterns of the products of S2 sintered at different
temperatures: (a) 1823 K; (b) 1873 K; (c) and (d) result of EDS analyses of the area marked
by +1 in (a) and the area marked by +2 in (b).
3.2 Effect of the carbon addition
In Fig.4 the samples with various carbon black additions were heated at 1873 K for 4 h.
It is shown that the predominant crystalline phases of the products in samples S1 and S2 were
β-Si 3 N 4 and TiN, while those of the products in samples S3 and S4 were β-SiC and Ti(C,N).
The diffraction peak intensities of β-Si 3 N 4 phase decreased and vanished with the carbon
addition increasing to more than 58.5 wt %. This indicates that excess carbon can accelerate
the formation of SiC and TiC following as the reaction (6) and the reverse reaction of reaction

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(9), respectively. Therefore, the raw materials composition has a vital effect on phase
composition of the products. The optimum carbon addition for synthesizing TiN-Si 3 N 4
composite powders by CRN process is the stoichiometric content.
Fig.4 XRD patterns of the products of the samples (S1~S4) sintered at 1873 K.
Fig.5 shows the SEM micrographs of the products of samples S1 ~ S4 sintered at 1873
K for 4 h. Corresponding to the phase composition of the sample S1 sintered at 1873 K, the
integral hexagonal β-Si 3 N 4 grains with dimensions of 2 ~ 10 μm were seen in Fig. 5(a). SEM
micrographs and EDS analyses of the products show that the globular agglomerates are
composed of the short hexagonal β-Si 3 N 4 particles and the irregular shape TiN grains. A
similar microstructure picture was obtained from the products of sample S2 (Fig. 5(b)). The
products consist of granular and short columnar materials, and the typical pronounced grain
growth of the hexagonal prisms can be observed. Fig. 5(c) and Fig. 5(d) show the products of
samples S3 and S4 reacted at 1873 K both contained particles of two different morphologies.
Fig.5. SEM micrographs of the products of the samples sintered at 1873 K: (a) S1; (b) S2; (c)
S3; (d) S4.

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The major part of the products had rod-like structure β-SiC of 0.1 ~ 0.4 μm in diameter and
2 ~ 10 μm in length, and the remainder consisted of granular agglomerates with dimensions of
0.1 ~ 1 μm. The formation and growth of rod-like β-SiC can be explained in term of the
vapor-solid (VS) mechanism [20]. It also can be found that more rod-like grains were
obtained in the product of sample S4 than sample S3. This indicated that the increase of
carbon addition promoted the formation of β-SiC.
4. Conclusion
A low cost preparation method of the TiN-Si 3 N 4 composite powders were obtained
from rutile and quartz. The optimum parameters for synthesizing TiN-Si 3 N 4 by CRN process
are carbon addition of stoichiometric content, temperature of 1873 K for 4 h and nitrogen
pressure of 0.2 MPa. The produced TiN-Si 3 N 4 in this experiment exist in granular and
hexagonal columnar shape, and the average particle size of the synthesized powders is 2 ~ 10
μm.
Acknowledgements
The authors greatly appreciate the National Natural Science Foundation of China for
financial support (Grant Nos. 51032007 and 50972134). We also thank the Fundamental
Research Funds for the Central Universities (Grant Nos. 2010ZD12 and 2011PY0174) and
the New Star Technology Plan of Beijing (Grant No. 2007A080). Z.H. Huang also thanks the
Gledden Visiting Senior Fellowship from University of Western Australia in 2010.
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Садржај: Прахови композита TiN-Si 3 N 4 припремљени су карботермалном редукционом
нитридацијом користећи рутил и кварц као почетне материјале. Испитиван је утицај
температуте и додатка угљеника на фазни састав и микроструктуру продуката.
Нацртани су фазни састави система Si-C-N-O и Ti-C-N-О на различитим
температурама под притиском азота од 0,2 MPa. Резултати су показали да су
оптимални параметри синтезе TiN-Si 3 N 4 употребом карботермалне редукционе
нитридације, додатак угљеника у стохиометријском односу, температура од 1873 K
током 4 h и притисак гаса од 0,2 MPa. Синтетисани TiN-Si 3 N 4 у овом експерименту
постоји у грануларном и хексагоналном облику, и средња величина честица је
2 ~ 10 μm.
Кључне речи: TiN-Si 3 N 4 композити; карботермална редукциона нитридација; кварц;
рутил; фазни дијаграм