J. Cent. South Univ. Technol. (2009) 16: 0725-0729

DOI: 10.1007/s11771-009-0120-5

Preparation of β-Sialon/ZrN bonded corundum composites from zircon by nitridation reaction sintering process

MA Bei-yue(马北越), YU Jing-kun(于景坤), LIU Tao(刘 涛), YAN Zheng-guo(颜正国)

(School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China)

Abstract: β-Sialon/ZrN bonded corundum composites were synthesized using fused white corundum, alumina micro powder, zircon and carbon black by nitridation reaction sintering process. Phase composition and microstructure of the synthesized composites were investigated by X-ray powder diffraction and scanning electronic microscope, and the formation process of the composites was discussed. The results show that the composites with different compositions can be obtained by controlling the heating temperature and contents of zircon and carbon black. The proper temperature to synthesize the composites is 1 773 K.

Key words: β-Sialon; ZrN; zircon; corundum; composites; nitridation reaction sintering process

1 Introduction

With the development of modern metallurgy technologies especially continuous casting and secondary refining technology, the contact time between high temperature molten steel and some metallurgical containers such as steel ladle and tundish has become much longer than before [1], so it is difficult to fulfill the demand of metallurgy technology process for some monolithic refractories. In recent years, some composite refractories especially oxide-nonoxide composites have developed rapidly, and have become new generation refractories.

β-Sialon (Si_6-zAl_zO_zN_8-z, 0＜z＜4.2) corresponds to a solid solution of Al and O atoms in the crystal lattice of β-Si₃N₄. The usual synthesis process is based on sintering mixtures of Al₂O₃, Si₃N₄, SiO₂ and AlN. It was reported that β-Sialon could be synthesized from natural alumino-silicate minerals like clay and pyrophyllite by carbothermal reduction and nitridation process [2-4]. β-Sialon ceramic has been regarded as a promising engineering material because of its excellent properties such as high strength and toughness as well as chemical stability at high temperature, and has been found many applications to engineering ceramics, cutting tools and refractories [5-7].

β-Sialon bonded corundum composites prepared by using β-Sialon as bonding phase in the corundum matrix material have many outstanding properties such as high strength, good thermal shock resistance and slag corrosion resistance, which makes the composites used in ceramic cup of blast furnace successfully [8-9]. Since then, β-Sialon bonded corundum composites have been attracted much attention [10-13]. In recent years, some workers have focused on preparation of β-Sialon bonded corundum composites by reaction sintering process, using natural minerals like clay [10] and bauxite [8], which can prepare the excellent composites, decrease the cost of the composites and provide a new route for comprehensive application of natural raw materials. So far, there have been a few reports on synthesis of β-Sialon/ZrN bonded corundum composites from zircon.

In the former work [14], ZrO₂/SiC composite powder was synthesized at 1 873 K for 4 h from zircon and carbon black by carbothermal reduction reaction under argon atmosphere. In the present work, the β-Sialon/ZrN bonded corundum composites were synthesized by nitridation reaction sintering process, using fused white corundum, alumina micro powder, zircon and carbon black. The effects of starting materials composition and heating temperature on synthesis of β-Sialon/ZrN bonded corundum composites were investigated, and the formation process of the composites was discussed.

2 Experimental

2.1 Starting materials

Fused white corundum (mesh size of 0-1 mm), alumina micro powder (mesh size≤44 μm, w(Al₂O₃)＞99.0%), zircon (mesh size≤44 μm), carbon black (mesh size≤30 μm, w(C)＞98.0%) and phenolic resin were used as the starting materials. The chemical compositions of fused white corundum and zircon are listed in Table 1.

Table 1 Chemical compositions of fused corundum and zircon (mass fraction, %)

In addition, the volume fraction of nitrogen gas (N₂) was 99.99%.

2.2 Preparation of samples

According to reaction (1), the mass ratio of carbon black to zircon was 0.3. The starting materials were weighed in terms of the desired mass ratios as shown in Table 2, mixed fully, pressed at 100 MPa to the samples with d 20 mm×10 mm and dried at 523 K fully. The prepared samples were put in an atmosphere-controlled tubular furnace with nitrogen gas flow of 0.6 L/min, and heated at 1 673, 1 723, 1 773 and 1 823 K for 6 h, respectively. After the desired reaction temperature reached, the system was cooled to room temperature in air.

6ZrSiO₄(s)+3Al₂O₃(s)+27C(s)+8N₂(g)=6ZrN(s)+2Si₃Al₃O₃N₅(s)+27CO(g)      (1)

Table 2 Compositions of samples (mass fraction, %)

1) Mass ratio of carbon black to zircon was 0.3.

2.3 Characterization of samples

The phase compositions of the synthesized samples were analyzed by X-ray powder diffraction (XRD) with Cu K_α radiation at 30 kV and 30 mA, and the micro- structures were observed by scanning electronic microscope (SEM) attached with energy diffraction spectrum (EDS).

3 Results and discussion

3.1 Thermodynamic analysis

During the synthesis process of the β-Sialon(z=3)/ ZrN bonded corundum composites, the following reactions are given as:

ZrSiO₄(s)=ZrO₂(s)+SiO₂(s)                    (2)

3Al₂O₃(s)+6SiO₂(s)+15C(s)+5N₂(g)=

2Si₃Al₃O₃N₅(s)+15CO(g)              (3)

2ZrO₂(s)+4C(s)+N₂(g)=2ZrN(s)+4CO(g)         (4)

Al₂O₃(s)+3C(s)+N₂(g)=2AlN(s)+3CO(g)         (5)

3Al₂O₃(s)+6ZrSiO₄(s)+15C(s)+5N₂(g)=

6ZrO₂(s)+2Si₃Al₃O₃N₅(s)+15CO(g)      (6)

3Al₂O₃(s)+6ZrSiO₄(s)+27C(s)+8N₂(g)=

6ZrN(s)+2Si₃Al₃O₃N₅(s)+27CO(g)       (7)

4Al₂O₃(s)+6ZrSiO₄(s)+30C(s)+9N₂(g)=

2AlN(s)+6ZrN(s)+2Si₃Al₃O₃N₅(s)+30CO(g) (8)

According to the correlated thermodynamic data [15], the standard Gibbs free energies for reactions (3)-(5), the expression formulas between partial pressure of CO gas (p(CO)) at p(N₂)=0.1 MPa and temperature can be obtained respectively as follows.

                  (9)

                    (10)

                    (11)

Fig.1 shows the equilibrium relationship of the condensed phases in Al₂O₃-ZrO₂-SiO₂-C-N₂ system plotted by the thermodynamic data as shown in formulas (9)-(11), which indicates that when p(CO) in the reacting furnace remains constant, with the increase of the heating temperature, C in sample will react with SiO₂ and N₂ to form Si₃Al₃O₃N₅ (reaction (3)), and the stability domain of the condensed phases changes from Al₂O₃(s)+ZrO₂(s)+SiO₂(s)+C(s) to Al₂O₃(s)+ZrO₂(s)+ Si₃Al₃O₃N₅(s)+C(s) and AlN(s)+ZrN(s)+Si₃Al₃O₃N₅(s)+ C(s). For example, when p(CO) is 0.1 MPa, the initial temperatures to form Si₃Al₃O₃N₅, ZrN and AlN are about 1 791, 1 898 and 1 961 K, respectively. In addition, when the temperature remains constant, with the decrease of the partial pressure of CO gas, the stability domain will change from Al₂O₃(s)+ZrO₂(s)+SiO₂(s)+C(s) to Al₂O₃(s)+ ZrO₂(s)+Si₃Al₃O₃N₅(s)+C(s) and AlN(s)+ZrN(s)+Si₃Al₃- O₃N₅(s)+C(s).

3.2 Phase composition and microstructure

Fig.2 shows the XRD patterns of the samples heated at 1 673-1 823 K for 6 h, with the mass fraction of zircon and carbon black of 10% (sample S10), 30% (sample S30)

Fig.1 Equilibrium relationship of condensed phases in Al₂O₃- ZrO₂-SiO₂-C-N₂ system: (a) 3Al₂O₃(s)+6SiO₂(s)+15C(s)+ 5N₂(g)=2Si₃Al₃O₃N₅(s)+15CO(g); (b) 2ZrO₂(s)+4C(s)+ N₂(g)=2ZrN(s)+4CO(g); (c) Al₂O₃(s)+3C(s)+N₂(g)= 2AlN(s)+3CO(g)

and 50% (sample S50), respectively. It indicates that the crystalline phases of the samples involve O′-Sialon (Si_2-xAl_xO_1+xN_2-x, x=0.17), β-Sialon(z=3), ZrO₂, ZrN, Al₂O₃ and Al₄Si₃C₆. When the heating temperature remains constant, with the increase of the contents of zircon and carbon black, the diffraction peak intensities

of β-Sialon and ZrN strengthen gradually. However, the intensities of O′-Sialon and ZrO₂ weaken. Meanwhile, the Al₂O₃ phase nearly remains constant. It can also be seen that Al₄Si₃C₆ forms at 1 823 K. The reason for this is that SiO₂ and Al₂O₃ in the sample react with C to form SiC and Al₄C₃, respectively.

Moreover, in the temperature range of 1 673-1 823 K, for sample S10, the crystalline phases are composed of β-Sialon, ZrO₂, ZrN and Al₂O₃, but there is no O′-Sialon. It can be deduced that SiO₂ in the sample can be nitridized completely to form Si₃N₄, and Si₃N₄ reacts with Al₂O₃ to form β-Sialon. For sample S30 heated at  1 673-1 723 K, the crystalline phases involve O′-Sialon. Meanwhile, O′-Sialon and β-Sialon appear in the XRD pattern at 1 723 K. Compared with the diffraction peak intensity of O′-Sialon at 1 673 K, the intensity of O′-Sialon at 1 723 K is lower. This reveals that O′-Sialon can be converted into β-Sialon with increasing the temperature. When the temperature reaches 1 773-1 823 K, O′-Sialon vanishes completely, and the crystalline phases involve β-Sialon, ZrO₂, ZrN and Al₂O₃. For sample S50 heated at 1 673-1 723 K, the diffraction peak intensity of O′-Sialon is higher than that of sample S30. In the temperature range of 1 773-1 823 K, there  is no O′-Sialon in the sample, which indicates       that O′-Sialon can be converted into β-Sialon at high

Fig.2 XRD patterns of samples heated at different temperatures for 6 h: (a) T=1 673 K; (b) T=1 723 K; (c) T=1 773 K; (d) T=1 823 K

temperature. Hence, increasing temperature favors the formation of Si₃N₄ and the preparation of β-Sialon/ZrN bonded corundum composites.

It was reported that the β-Sialon powder was synthesized from clay by carbothermal reduction and nitridation reaction, and then the β-Sialon bonded corundum composites were prepared by pressureless sintering process [10]. The composites sintered at      1 873 K had still two distinguishable stable crystalline phases involving corundum and β-Sialon, and there were no any other phases like O′-Sialon and X-Sialon. In this work, O′-Sialon can be observed in the samples synthesized at temperatures below 1 773 K. However, it vanishes completely at temperatures above 1 773 K. Therefore, during the preparation of β-Sialon/ZrN bonded corundum composites, O′-Sialon is an intermediate phase.

Fig.3 shows the SEM photographs and EDS pattern of sample S30 heated at 1 773 K for 6 h.

It can be found that the surface of the sample is smooth. EDS pattern (Fig.3(c)) shows that zone 1 belongs to Zr-Si-Al-O-N material. From XRD pattern, the sample obtained at 1 773 K involves β-Sialon, ZrN and Al₂O₃ (corundum particle, zone 2 in Fig.3(b)). So it can be concluded that the particle as shown in zone 1 is the composite body of β-Sialon and ZrN. In Ref.[16] the β-Sialon bonded corundum composites were prepared from bauxite by reaction sintering process under nitrogen atmosphere at 1 773-1 823 K for 5 h, and the SEM photographs show that the β-Sialon matrix and the corundum particles are bonded to a compact body, and β-Sialon crystalline grain mostly exists as columnar and interlaced. In this work, compact microstructure can be obtained (Fig.3(a)), β-Sialon and ZrN form a composite body which is granular (Fig.3(b)).

3.3 Analysis of formation process

The overall reaction equation of synthesizing β-Sialon(z=3)/ZrN bonded corundum composites using zircon, alumina micro powder, carbon black and fused corundum can be expressed by

3Al₂O₃(s)+6ZrSiO₄(s)+27C(s)+8N₂(g)=6ZrN(s)+2Si₃Al₃O₃N₅(s)+27CO(g)      (12)

The formation process of β-Sialon (z=3) is given by reactions (13)-(16). At high temperature, ZrSiO₄ can be decomposed to form ZrO₂ and SiO₂ (reaction (13)), and SiO₂ reacts with C and N₂ to form Si₃N₄ (reactions (14)-(16)), and finally the solid solution (β-Sialon) can be obtained.

ZrSiO₄(s)=ZrO₂(s)+SiO₂(s)                   (13)

SiO₂(s)+C(s)=SiO(g)+CO(g)                  (14)

3SiO(g)+3C(s)+2N₂(g)=Si₃N₄(s)+3CO(g)        (15)

Fig.3 SEM photographs of sample S30 heated at 1 773 K for  6 h and EDS pattern: (a) Secondary electron image; (b) Back- scattered electron image; (c) EDS pattern of zone 1 in Fig.3(b)

3SiO₂(s)+6C(s)+2N₂(g)=Si₃N₄(s)+6CO(g)       (16)

The samples heated at 1 673-1 723 K (Figs.2(a) and (b)) include O′-Sialon that vanishes completely at      1 773-1 823 K (Figs.2(c) and (d)). It can be deduced that the following reactions favor the formation and vanishment of O′-Sialon (x=0.17) as well as the conversion of it into β-Sialon.

2SiO₂(s)+3C(s)+N₂(g)=Si₂N₂O(s)+3CO(g)       (17)

1.83Si₂N₂O(s)+0.17Al₂O₃(s)=2Si_1.83Al_0.17O_1.17N_1.83(s)

               (18)

Si_1.83Al_0.17O_1.17N_1.83(s)+1.33Al₂O₃(s)+3.33C(s)+

1.25N₂(g)=0.50Si₃Al₃O₃N₅(s)+0.33SiO(g)+

3.33CO(g)                              (19)

SiO₂ in sample reacts with C and N₂ to form Si₂N₂O (reaction (17)), and it reacts with Al₂O₃ to form O′-Sialon (x=0.17) (reaction (18)). As shown in reaction (19), with the increase of temperature, O′-Sialon reacts with Al₂O₃ and C to form β-Sialon (z=3).

In the temperature range of 1 773-1 823 K, ZrO₂ is converted into ZrN gradually.

2ZrO₂(s)+4C(s)+N₂(g)=2ZrN(s)+4CO(g)        (20)

In this experiment, the formation process of β-Sialon(z=3)/ZrN bonded corundum composites can be summarized as follows.

(1) When the temperature is below 1 773 K, SiO₂, one of the decomposition product from zircon, reacts with C to form Si₂N₂O under nitrogen atmosphere, and Si₂N₂O reacts with Al₂O₃ to form O′-Sialon. During the formation process of O′-Sialon, the other decomposition product (ZrO₂) is nitridized to form ZrN.

(2) When the temperature reaches 1 773 K, the formation of β-Sialon includes two aspects. On one hand, SiO₂ in the sample reacts with C and N₂ to form Si₃N₄, β-Sialon can be obtained directly from Al₂O₃ and Si₃N₄, namely, β-Sialon is a solid solution of Al₂O₃ in the crystal lattice of Si₃N₄. On the other hand, the formed O′-Sialon as step (1) reacts with Al₂O₃, C and N₂ to form β-Sialon. During the formation process of O′-Sialon and β-Sialon, ZrO₂ is nitridized to form ZrN.

(3) β-Sialon/ZrN matrix formed as steps (1) and (2) can be distributed regularly around corundum particles. It is stable in thermodynamics. Therefore, β-Sialon/ZrN bonded corundum composites can be prepared.

4 Conclusions

(1) By using zircon, carbon black, alumina micro powder and fused corundum as the starting materials, the composites with different compositions can be obtained by controlling the heating temperature and the contents of zircon and carbon black.

(2) The proper temperature to synthesize the β-Sialon/ZrN bonded corundum composites is 1 773 K.

(3) The formation process of the β-Sialon(z=3)/ZrN bonded corundum composites includes the decomposition of zircon, the formation of Si₂N₂O and Si₃N₄, the formation of O′-Sialon and β-Sialon and the nitridation process of ZrO₂.

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(Edited by YANG You-ping)

Foundation item: Project(50274021) supported by the National Natural Science Foundation of China and Baoshan Iron and Steel Co., Ltd.

Received date: 2008-10-13; Accepted date: 2009-03-10

Corresponding author: YU Jing-kun, Professor; Tel: +86-24-83681576; E-mail: jingkunyu@yahoo.com