Synthesis of TiN-Si 3 N 4 Composites from Rutile and Quartz by Carbothermal Reduction Nitridation

TiN-Si3N4 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-Si3N4 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-Si3N4 in this experiment exist in granular and hexagonal columnar shape, and the average particle size of the synthesized powders is 2 ~ 10 μm.


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][4][5][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.
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.

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 = 1.5406Å) 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).

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: 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 → 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: 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).
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 2 ) g ( SiO The formation reaction of SiC occurs via the vapor-solid approach or by the gas phase reaction:  9).It is seen in 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.

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 (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.
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 by calculating the relationship between

Fig. 1
Fig.1 Equilibrium phase diagram of Si-C-N-O and Ti-C-N-O system.

Fig. 2
Fig.2shows 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. 3
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).

Fig. 5
Fig.5shows 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.