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Sintering behavior of Al2O3-TiC composite powderprepared by SHS process

J.H. Lee*, S.K. Ko, C.W. Won

Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, 305-764,

Taejon, Korea

(Refereed)

Received 9 August 2000; accepted 27 November 2000

Abstract

Al2O3-TiC composite powder was prepared by the self-propagating high-temperature synthesis

(SHS) process, using TiO2, Al, and C powders as raw materials. The effects of keeping the TiO

2:Al:C

molar ratio fixed at 3:4:2.7 while varying the carbon sources and cooling time on the products were

studied. The highest combustion temperature and combustion velocity were obtained at activatedcarbon. But, microstructure and mechanical properties of sintered Al

2O3-TiC composite did not show

any significant differences according to the carbon sources. Hot pressing was found to be very

effective in hindering the formation of pore and obtaining a dense sintered body at 1650C. The

sintered body produced by hot-pressing was about 98.8% and 99.2% of the theoretical density for

synthesized and commercial powder respectively. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: A. Ceramics; B. Chemical synthesis; C. X-ray diffraction; D. Mechanical properties

1. Introduction

Self-propagating High Temperature Synthesis is potentially an energy-efficient process tosynthesize many inorganic materials, including intermetallics, ceramics, and ceramic com-posites [13]. Characteristics of the process in the combustion-wave mode are self-generatedhigh temperature (800 to 3500 C), relatively rapid propagating combustion fronts (0.1 to10cm/sec), high rates of heating (up to 106 deg/sec), and thermal gradients (up to 107

* Corresponding author. Fax: 82-42-822-9401.E-mail address: [email protected] (J.H. Lee).

Pergamon Materials Research Bulletin 36 (2001) 989996

0025-5408/01/$ see front matter 2001 Elsevier Science Ltd. All rights reserved.

PI I : S0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 5 7 9 - 7

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deg/cm) at the combustion front. The exact values of temperature, wave velocity, thermal

gradients, and rate of heating are functions of the particular chemical system and experi-

mental parameters. Both solid-solid and gas-solid combustion reactions are used to produce

a variety of advanced technological materials. Al2

O3

-TiC, Al2

O3

-ZrO2

, Si3

N4

and TiC which

have high temperature strength and high thermal shock resistance have been used as

advanced structural materials. Especially, the TiC-Al2

O3

composite used for making abra-

sive tool and wear resistant coating to protect components of oil refining equipment has been

produced by hot pressing TiC and Al2

O3

powder. The preparation of TiC-Al2

O3

composite

powder by SHS process was widely studied in several former researches [47], however

researches on the sintering behavior of the TiC-Al2

O3

composite powder prepared with

various combustion conditions are rare. Hence, in this study, the SHS process was applied

to a system of TiO2/Al/C for the production of TiC-Al2O3 composite. The reaction charac-teristics were discussed in terms of preheating of the pellet for the formation of fine

TiC-Al2

O3

powder when activated charcoal, carbon black and graphite were used as carbon

sources. And the sintering behavior of the TiC-Al2

O3

composite powder synthesized at

different cooling rates and carbon sources were systematically investigated. Finally, the

physical and mechanical properties of the sintered product were compared to those of the

commercial powder.

2. Experimental

The raw materials used in this study were powders of TiO2

(99.3% purity, 0.5 m),

aluminum (99.5% purity, 44m), activated carbon (15 m), carbon black (510 m)

and graphite (38 m). The sintering behavior of these powders was compared with that of

commercial powders, -Al2

O3

(99.99%, 0.4 m, Sumitomo Chemical Co., Japan) and

TiC (99.5%, 2 m, Cerac, USA). Predetermined amounts of the reactants were mixed in

an alumina ball mill for 10 h. The mixed powder was pressed into pellets of 40 mm diameter

with 47% theoretical density and 50 60 mm height. The pellets were placed in a preheating

furnace that was located in a SHS reactor and ignited under argon by a tungsten wireconnected to a power supply. The molar ratio of TiO

2to aluminum powder was fixed at

3.0:4.0 and the carbon content was 2.7 mol which was found to be the optimum molar ratio

in previous research [8]. The temperature was measured with C-type thermocouples (W-

5%Re vs. W-26%Re, 0.5) connected to a data logger (DASTC). The powder thus produced

was analyzed by X-ray diffraction (XRD) to determine its crystal structure and by scanning

electron microscopy (SEM) to determine its microstructure. In the case of the sintering

process, the commercial Al2

O3

powder was mixed with TiC powder in the ratio of 53.2:46.8

by weight and a small amount of about 0.8wt% MgO and 0.5wt% Polyvinylalcohol were

added to the mixture to prevent grain growth and to enhance compactness, respectively. Thereactants were mixed in a sintered Al2

O3

mill for 10h, then the mixed powder was passed

through a sieve. The powder compacts were formed by uniaxially pressing at 180MPa in a

metal mould. When pressureless sintering and hot pressing were performed the sintering

schedule is summarized in Table 1.

990 J.H. Lee et al. / Materials Research Bulletin 36 (2001) 989996

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Fig. 1. SEM photographs of products combustion synthesized varying with preheating temperature at (a) activated

carbon (b) carbon black (c) graphite.

Table 1

Schedule of pressureless sintering and hot pressing of Al2

O3

-TiC

Temp (C) Heating rate

(C/min)

Atmosphere Pressure

(MPa)

Holding time

(Min)

Pressure less Sintering RT1200 20 Vac. (102 torr) 102 torr 012001200 0 102 torr 3012001850 5 Ar 0.1 018901890 0 0.1 106018901200 10 0.1 0

Hot pressing RT400 10 Vac. (102 torr) 0 04001200 20 025 0

12001200 0 25 1012001650 30 2536 0

16501650 0 36 30

991J.H. Lee et al. / Materials Research Bulletin 36 (2001) 989 996

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3. Results and discussion

Fig. 1 shows SEM photographs of the combustion synthesized products with varyingpreheating temperatures and different carbon sources. The products were irregular in shapewith 520 m in size and did not show any significant difference in morphology. However,the particle size of TiC synthesized at room temperature increased to 2 m and thisincreased to even 510 m at 600C of preheating temperature. Especially, TiC particlessynthesized with carbon black and graphite at 300C of preheating were much larger thanthat synthesized with activated carbon even though the combustion temperatures were nearlythe same. The combustion temperatures of the sample synthesized with activated carbon at

Fig. 2. Effect of the preheating temperature on the cooling time.

Table 2

Measured combustion temperatures of the reactants

Carbon source

Preheating temp. (C)

Activated carbon Carbon black Graphite

25 1841 1731 1732300 1957 1841 1869600 2115 1987 1957


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600 C of preheating were observed to be 128158 C higher than that with the other carbonsources as shown in Table 2. However considerable difference in the particle size of TiCcould not be observed as shown in Fig. 1. Also, the cooling time to 827 C (below this value,the particle growth does not take place [8]) was nearly the same when the preheatingtemperature was equally applied, see Fig. 2. Hence, it is possible to conclude that the mostimportant factor in determining particle growth is the cooling time. Jo et al. reported that theparticle size of MoSi

2increased as the preheating temperature increased due to decreasing of

undercooling [9]. From these results, Al2O3 particle is formed at combustion front and itsparticle size is influenced by neither combustion temperature nor cooling time, whereas theparticle size of TiC is greatly influenced by the cooling time even though it is not associatedwith the combustion temperature.

Fig. 3 shows the variation of the particle size and the specific surface area with millingtime. The mean particle size of the composite powder decreased as the milling time increasedand became 2 m after 50 hrs of milling time. In order to get the particle size similar to thecommercial one, the synthesized composite powder was milled for 100 hrs.

To survey the sintering characteristics of the prepared TiC-Al2

O3

composite powder,

pressureless sintering and hot-pressing methods were applied. Fig. 4 shows the variation ofthe density of the Al

2O

3-46.8wt%TiC composite pressureless sintered with temperature and

time. The density increased to 95.2%TD as the sintering temperature increased up to 1890C, and then it decreased. This sintering temperature was somewhat higher than the previ-ously reported value, 18701875 C which was reported by Kim and Lee [10] and Cutler

Fig. 3. Variation of the particle size and the specific surface area with milling time.


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et al [11] in the Al2

O3

-30wt%TiC system. This is because the combustion synthesized

composite powder contains higher amount of TiC powder, 46.8 wt%. The density increasedas the sintering time increased up to 20 min, and then it decreased. The density decrease withthe over sintering time has been explained that the gas such as Al

2O, CO and TiO formed

by the chemical reaction between Al2

O3

and TiC causes the pores in the composite [10].Table 3 shows the physical and mechanical properties of the pressureless sintered Al

2O3

-

Fig. 4. Variation of the density of Al2O3-46.8wt%TiC composites pressureless sintered with temperature and

time. (sintering time and temperature were fixed to 20min and 1890C respectively, carbon source : graphite,

cooling time : 200sec).

Table 3

Properties comparisons of the pressureless sintered Al2O3-46.8 wt% TiC composites with carbon sources

(temp.: 1890C, time: 20min)

Carbon sourceProperties Density(%TD) Hardness(Gpa) Bending Strength(MPa) Fracture Toughness(MPa m1/2)

Activated Carbon 95.5 17.4 470 57 4.3 0.4Carbon Black 94.8 17.3 460 64 4.5 0.4Graphite 95.2 17.5 468 43 4.9 0.2


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46.8wt%TiC composites with carbon sources. It was found that the overall properties wereuniform irrespective of the carbon sources. These results were attributed to the similarmicrostructural properties of the combustion synthesized products with carbon sources.

In the case of pressureless sintering, the sintered body of the composite powder with 100sec of cooling time has higher porosity than that of the powder with 200 sec of cooling time,also, relatively poor distribution of TiC particles. Table 4 shows the physical and mechanicalproperties of sintered Al

2O3

-46.8wt%TiC composites prepared by combustion synthesis withcooling time. The composite of synthesized powder with 200 sec of cooling time exhibitssuperior density, hardness, bending strength and fracture toughness than that of the powderwith 100 sec of cooling time. This is because Ti did not completely react with carbon, and

the carbon existed as free carbon in the product owing to the relatively short reaction time;then TiC

0.50.95formed instead of the desirable composition, TiC

0.9with the mole ratio of

Ti:C3.0:2.7 at the cooling time of 100 sec. Hence, it is necessary to decrease the coolingrate by preheating and increasing the pellet diameter, when the ceramic composite powderis synthesized by SHS process, in order to give sufficient reaction time.

In the case of the presureless sintered Al2

O3

-46.8wt%TiC composites, many pores wereobserved in the matrix, and no significant differences were observed between commercialand synthesized composites. However, the pores were virtually removed by the appliedpressure of 36 MPa during sintering even at a relatively low temperature, 1650C, and finer

microstructure was observed in the combustion synthesized composite than the commercialone. It can be concluded that the combustion synthesized Al

2O3

-46.8wt%TiC composite

Table 5

Properties comparisons of Al2

O3

-46.8wt%TiC composites prepared by combustion synthesis and mixed

commercial powder

Density

(%TD)

Hardness

(Gpa)

Bending Strength

(MPa)

Fracture Toughness

(MPa m1/2)

Pressureless Combustion synthesized

(200 sec of cooling time)

95.2 17.5 468 43 4.9 0.2

Commercial 95.1 16.9 420 54 5.2 0.3Hot-press Combustion synthesized

(200 sec of cooling time)

98.8 20.5 775 40 4.2 0.3

Commercial 99.2 20.9 810 70 4.7 0.4

Table 4

Comparison of the properties of Al2

O3

-46.8 wt% TiC composites prepared by combustion synthesis varying

with cooling time

Properties

Cooling time(s)

Density

(%TD)

Hardness

(Gpa)

Bending Strength

(MPa)

Fracture Toughness

(MPa m1/2)

Pressureless 100 91.2 14.8 381 23 4.3 0.2200 95.2 17.5 486 43 4.9 0.2

Hot press 100 98.1 19.6 666 50 4.1 0.3200 98.8 20.5 775 40 4.2 0.3


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powder has comparable sintering characteristics to the commercial powder from the resultsdepicted in Table 5. Actually, the characteristics of pressureless sintered composites were

superior in the combustion synthesized powder, whereas relatively poor sintering character-istics were observed in the hot pressed sample. However, the differences are negligible andwe believe that the SHS process is one of the promising methods of ceramic compositepowder production.

4. Conclusion

Al2

O3

-TiC composite powder was successfully synthesized by SHS process. The cooling

time affected largely grain size of titanium carbide. However, the grain size of alumina wasnot affected by cooling time. When the cooling time is 100 sec or 200 sec, the grain size oftitanium carbide was about 2 m o r 510 m, respectively. The highest combustiontemperature and combustion velocity were obtained with activated carbon. But, the micro-structure and mechanical properties of sintered Al

2O3

-TiC composite did not show anysignificant differences depending upon the carbon source. The overall physical and mechan-ical properties are comparable to commercial powder.

References

[1] Z.A. Munir, Ceramic Bulletin 67 (1988) 342.

[2] M. Ouabdesselam, Z.A. Munir, J. Mat. Sci. 22 (1987) 1799.

[3] A.P. Hardt, P.V. Phung, Combustion and Flame 21 (1973) 77.

[4] H.J. Feng, J.J. Moore, D.G. Wirth, Met. Trans. 23A (1992) 2373.

[5] H.J. Feng, J.J. Moore, J. Mater. Synth. Proc. 2 (1994) 71.

[6] H.J. Feng, J.J. Moore, Met. Trans. 26B (1995) 265.

[7] H.C. Yi, H.J. Feng, J.J. Moore, A. Petric, J.Y. Guigne, Int. J. SHS 5 (1996) 39.

[8] S.K. Ko, K.H. Min, H.S. Kim, C.W. Won, J. Kor. Inst. Met. Mater. 36 (1998) 2111.

[9] S.W. Jo, G.W. Lee, J.T. Moon, Y.S. Kim, J. Kor. Inst. Met. Mater. 33 (1995) 107.

[10] Y.W. Kim, J.G. Lee, J. Am. Ceram. Soc. 72 (1989) 1333.[11] R.A. Cutler, A.C. Hurford, A.V. Virkar, Mater. Sci. Eng. 105A (1998) 183.


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