Effect of SiO2 on the Preparation and Properties of ...

Vol. 19 　No. 1

Journal of Wuhan University of Technology - Mater. Sci. Ed.

Mar. 2004

Effect of SiO2 on the Preparation and Properties of Pure Carbon 3 Reaction Bonded Silicon Carbide Ceramics WU Qi2de 　GUO Bing2jian 　YAN Yong2gao 　ZHAO Xiu2jian 　HONG Xiao2lin Wuha n University of Technology

( Received :J une 24 ,2003 ;Accepted ;Nov. 3 ,2003) 　　Abstract : 　Effect of SiO2 content and sintering process on the composition and properties of Pure Carbon Reaction Bonded Silicon Carbide ( PCRBSC) ceramics prepared with C - SiO2 green body by infiltrating silicon was presented . The infiltrating mechanism of C - SiO2 preform was also explored . The experimental results indicate that the shaping pressure increases with the addition of SiO2 to the preform , and the pore size of the body turned finer and distributed in a narrower range , which is beneficial to decreasing the residual silicon content in the sin2 tered materials and to avoiding shock off , thus increasing the conversion rate of SiC. SiO2 was deoxidized by car2 bon at a high temperature and the gaseous SiO and CO produced are the main reason to the crack of the body at an elevated temperature . If the green body is deposited at 1800 ℃ in vacuum before infiltration crack will not be produced in the preform and f ully dense RBSC can be obtained . The ultimate material has the following properties : 3 a density of 3. 05 23. 12 gΠcm , a strength of 580 ±32 MPa and a hardness of ( HRA ) 91 292. 3. Key words : 　reaction bonded silicon carbide ; 　SiO2 ; 　filler ; 　properties

1 　Introduction Reaction2bonded silicon carbides ( RBSC) are fully dense engineering ceramics formed by the bonding togeth2 er of silicon carbide powders with further silicon carbide produced in situ by a chemical reaction between silicon and carbon. As a type of structural ceramics RBSC has many desirable properties and has been widely applied in [1 ,2 ] the field of refractory and machine engineering . In or2 der to cut down the manufacture cost of RBSC ,the method of adopting petroleum cock ,which is the raw material to produce SiC ,as raw material to fabricate the PCRBSC ce2 ramics has been proved feasible. In infiltration process ,the 3 theoretical carbon density of preform is 0. 96gΠcm . In fact , to avoid shocking off in the infiltrating process the carbon density of preform can hardly excess 90 % of the [3 ] 3 theoretical density ,that is to say ,less than 0. 87gΠcm . To prepare a pure carbonaceous body with a porosity over 50 % , there exists a lot of available processes and methods. The body can be prepared by the method of [4 ,5 ] Hucke ,and also we can decrease the shaping pressure while it is difficult to control the construction of the pre2 form. The more effective method is that a kind of filler is incorporated into the carbonaceous raw material to adjust [6 ] the preform density ,porosity and pore size distribution . The effect of SiO2 filler on the microstructure of the porous WU Qi2de ( 武 七 德 ) : Born in 1949 ; Assoc. Prof. ; The Key Laboratory of Silicon Materials Science and Engineering ,Ministry of Eduction ,Wuhan University of Technology ,Wuhan 430070 ,China. 3 This work was financially supported by the Key Research Project of the China ( No. 962A10201207 ) and the Key Research Project of the Wuhan (20001001003)

carbonaceous perform ,the infiltrating process and the ma2 terial properties are presented in this paper.

2 　Experimental The carbonaceous raw material used was petroleum cock heated at 1300 ℃,containing 98. 8wt % carbon and less than 0. 55wt % ash content . The petroleum cock was mined in a gas fluid miner to have d50 of 14. 1μm. The molten silica powder was adopted as the SiO2 filler with SiO2 fraction higher than 99. 8wt % and d50 of 22. 4μm. Carbon and SiO2 were mixed according to the amount in Table 1 and mixed with water for 1h. The green body was prepared with a pressure of 45MPa with PVA as binder. Industry silicon with purity of over 99. 8wt % was used as infiltrate. The sample was put into a graphite die coated with BN and buried by silicon powder. The infiltrating process was conducted in a vacuum electric resistance fur2 nace with pressure at 1230 Pa and the temperature was maintained at 1500 ℃for 1h. The size distribution curve of the powder was deter2 mined with a laser particle size analyzer produced by Malvern film ;the pore size in the preform was tested by an autoscan mercury porosimeter. An optical microscope and SEM were used to detect the microstructure of the prod2 uct . The sample density was determined according to Archimedes’Law ( the preform was heated in vacuum at 1800 ℃ before test ) . The flexural strength of the specimen with the sample size of 3 ×4 ×38 ( mm) was determined by three point method with an American Ceramic Testing system with a loading rate of 0. 5mmΠmin.

Vol. 19 　No. 1 　　　　　　WU Qi2de et al : Effect of SiO2 on the Preparation and Properties of. . . Table 1 　Composition of green body Sample Code

Filler contentΠwt %

Shaping pressureΠMPa

Carbon density of preformΠ(gΠcm3 )

A B C D E

12 14 14 16 18

45 45 30 45 45

0. 90 0. 87 0. 84 0. 84 0. 81

3 　Results and Discussion 3. 1 　Effect of SiO2 on the microstructure of the pre2 form Fig. 1 shows the relationship between the filler amount and the carbon density of preform. It can be seen from Fig. 1 that the carbon density of preform decreased when the SiO2 filler amount increased ,and the two values have a well linear relation ship . When compared with the preform containing starch as filler ,the perform can reach the same density with only two thirds of the starch cont2 ent ’s SiO2 filler. It is because SiO2 reacted with C at high temperature ,producing the gaseous SiO and CO that made the carbon amount decrease further. Fig. 2 shows the relationship between the shaping pressure and the carbon density of preform. When the pre2

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form is fully made up of carbonaceous raw material , the shaping pressure must be lower and the preform density changes in a large range as the shaping pressure is altered slightly. However , the green body can be made under higher pressure when 14wt % SiO2 is incorporated in the raw material and the carbon density decreases as the SiO2 content increases. When the shaping pressure is over 33 MPa the preform density appears unchanged ,which is be2 nefcial for accurate control of the carbon density of pre2 form. Moreover , the average pore size decreases and its distribution turned narrower ( Fig. 3) under higher shaping pressure ,that is beneficial to decreasing the residual sili2 con fraction in the material and to obtaining high perfor2 mance materials. 3. 2 　Infiltration mechanism of liquid silicon for the preform containing SiO2 and C Silicon cannot be wetted on SiO2 at a high tempera2 ture while both SiC and C can be wetted on silicon , with an contact angle of 35°and 0°respectively. So it appears that the preform containing SiO2 and C may not be infil2 trated or sintered at first for the above reason ,but SiO2 re2 acts with C according to the following formula at 900 ℃: SiO2 ( s) + 3C ( s) = SiC ( s) + 2CO ( g)

Table 2 　Reactions ,gibbs free energies and equilibrium constants of Si —C —O system[7 ]

Reactions (1) SiO2 (s) + C(s) = SiO (g) + CO(g) (2) SiO2 (g) = SiO (g) + 1Π2O2 (g) (3) SiO(g) + 2C(s) = SiC(s) + CO(g) (4) SiO(g) + 3CO(g) = SiC(s) + 2CO2 (g) (5) CO2 (g) + C(s) = 3CO(g)

ΔG° 688354 - 344 T 802718 - 258. 2 T - 132770 + 33. 9 T - 465568 + 379 T 167738 - 172 T

K1180

K1350

K1450

1. 67 ×10 - 7 4. 26 ×10 - 16 1005. 47 8. 71 ×10 - 4 900. 26

6. 52 ×10 - 5 4. 49 ×10 - 13 317. 98 1. 54 ×10 - 5 3854. 8

1. 26 ×10 - 3 1. 42 ×10 - 11 179. 64 2. 08 ×10 - 6 7931. 18

Fig. 1 　Carbon density of green body versus the SiO2 content

Fig. 2 　Carbon density of green body versus shaping

Fig. 3 　Pore size distribution under different shaping

Fig. 4 　SEM of the sample fracture surface with fSi

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Journal of Wuhan University of Technology - Mater. Sci. Ed. 　　　　　　　　　　Mar. 2004

　　With the SiO2 converting into SiC and CO given out , liquid Si can infiltrate into the preform. Therefore , it is feasible that the silica can theoretically be used as raw material for PCRBSC. All preforms containing 14wt % SiO2 cracked or even exploded without exception during infiltrating in vacuum with a heating rate of 425 ℃Πmin and maintained at 1500 ℃for 2h ,and free silicon line can be seen on the sample fracture surface in SEM image ( Fig. 4) . Yet2Ming Chiang et al . believed that the overwhelming thermal stress owing to the fast reaction rate is the main cause to the crack or explosion of the samples and these can be avoided by decreasing the reaction rate between Si and [8 ] C . There exist a lot of reactions between SiO2 and C at high temperature ,whose free energy and equilibrium con2 stant are listed in Table 2. These data indicate that reac2 tion three is the major one to produce SiC ,whose reactants come from reaction one and reaction two. The equilibrium constant of reaction two is very low even at high tempera2 ture ,so SiO is mainly supplied by reaction one. Reaction four whose outcome is CO2 mainly occurs at low tempera2 ture while SiO is difficult to be produced at this tempera2 ture. So there is little chance for reaction four to produce SiC. Reaction one and reaction three is the major reaction to produce SiO ,and reaction three are successive to reac2 tions one ,so the release of the gaseous impurities is con2 trolled by reaction one. The equilibrium constant for reaction one increases as the temperature increases ,but the value is fair low even at 1450 ℃. So reaction one cannot be finished when the temperature increases up to the molten point of silicon ( 1410 ℃) with a heating rate of 5 ℃Πmin. If the liquid silicon infiltrates into the preform at this moment ,the re2 action to form SiC will take place immediately from sur2 face to inside of the preform : Si ( s) + C ( s) = SiC ( s) 　- 66. 88kJΠmol It is an intensively exothermal reaction ,which makes the temperature of the preform be about 1900 ℃ [9 ] instantly , and the equilibrium constant of reaction one

Fig. 5 　XRD pattern of green body after pretreatment

increases largely at high temperature and the volume of the gas such as CO will expand greatly. A great stress will occur inside the preform when the gas is released quickly. At the same time ,the preform is surrounded by RBSC ma2 terial with a great strength on the surface. When the stress inside the preform excesses the strength of the material , the sample cracks. In addition to SiO2 ,impurities that can release gas2 eous outcomes at high temperature include S ,Al2 O3 , etc . in the ash of the petroleum coke. Reaction of Al2 O3 takes place at higher temperature than that of SiO2 . Almost all reactions that can release gas will turn sharpen because a great deal of heat is produced when Si reacts with C to form SiC ,which is the main reason why PCRBSC material is easier to crack than traditional RBSC. To overcome the problem , the preform is preheated in vacuum at 1800 ℃ for 2h to get rid of the gas ,and the XRD analysis indi2 cates that SiO2 in the preform is completely converted into SiC ( Fig. 5) . The preheated preform will not crack when liquid silicon infiltrates in vacuum. In our research ,to compare with the vacuum infiltra2 tion ,the sample was also sintered at 1950 ℃for 2h in the fluid atmosphere of argon. The sample did not crack , which indicates that the sinter mechanism in argon atmo2 sphere is different from in vacuum. When wetted by liquid silicon in argon atmosphere ,the surface of the preform is surrounded by a sheet of RBSC material with a thickness of about 1mm immediately ,but the infiltrating rate will be hold back by the pressure of the gas in the core of the preform. Thus the infiltrating rate is controlled by the rate of the gas overflowing from the preform ,then the reaction time between Si and C will be lengthened greatly and the exothermal temperature of the body will decrease. The gas2 eous production is eliminated in the form of gas bulb in the beginning of the reaction. The capillary full of liquid silicon will turn discontinuous , and the gas will diffuse through the liquid silicon in the following stage of the re2 action. The sintering process turns gentle , therefore , the preform does not crack.

Fig. 6 　XRD pattern of PCRBSC

Vol. 19 　No. 1 　　　　　　WU Qi2de et al : Effect of SiO2 on the Preparation and Properties of. . .

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Fig. 7 　Optical microscopic of PCRBSC and RBSC( a) RBSC ( b) PCRBSC

3. 3 　Effect of SiO2 on the properties of PCRBSC material Fig. 6 shows the XRD pattern of PCRBSC material . Unlike the traditional RBSC material ( contain SiC and 14wt %C in the green body) , SiC in PCRBSC material only consists of β2SiC and has very low fraction of f Si and 3 f C and a higher density up to 3. 1023. 12gΠcm . Fig. 7 shows the optical microscopic of the PCRBSC and RBSC material sintered under the same condition. In the PCRBSC material prepared by the infiltrating of liquid silicon into the preform containing SiO2 and C , the free silicon distributes evenly and all the granules are fine. And it has a higher fraction of SiC ,owing to the narrow pore size distribution in PCRBSC preform. When the solid C converts into SiC ,its volume will expand by 2. 2 times. The expansion effect decreases the pore volume and in2 creases silicon carbide fraction on the one hand and other hand it makes the capillary narrower and even blocked. When local carbon density is higher than the average val2 ue , the core part of the carbonaceous preform cannot be reacted. It is believable that the theoretical capillary struc2 ture should provide the routeway for the liquid silicon in2 filtration and be totally filled up by the newly formed sili2 con carbide. So a method should be adopted to eliminate the coarser capillary to decrease the residual silicon cont2 ent in the materials , and the capillary should be wide enough to avoid shocking off to decrease the residual car2 bon in the materials ,which is the key to prepare the high2 er performance PCRBSC materials. The SiO2 filler was in2 corporated in the raw material in this research to make the pore size finer and its distribution narrower ,and this is vi2 tal to avoid the shocking off and decrease the residual sili2 con. We obtained a PCRBSC materials with a density of 3 3. 12gΠcm and a bending strength of 580 ±32MPa.

4 　Conclusions The SiO2 was incorporated into the pure carbon2

aceous raw material to adjust the preform carbon density accurately and obtain a green body with finer pore size and narrower pore size distribution. SiO2 reacted with C in higher temperature and produced SiO and CO gas ,which is the main cause to the crack of the sample. The reaction of the carbon and silicon is strong exothermic reaction , which accelerates the gas production formation. The sinter will be completed successfully when the green body pre2 fires at 1800 ℃ to eliminate the impurity which can re2 lease gas. SiO2 filler can improve the structure of preform and decrease the content of free silicon and carbon ,which is the key to prepare the higher performance PCRBSC ma2 terials.

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