Pressureless sintering of sol–gel alumina matrix composites

May 2000

Materials Letters 43 Ž2000. 281–285 www.elsevier.comrlocatermatlet

Pressureless sintering of sol–gel alumina matrix composites G. Urretavizcaya a,) , J.M. Porto Lopez b, A.L. Cavalieri b a

b

Centro Atomico Bariloche (CAB-CNEA), AÕ. E. Bustillo km 9,5 (8400) S.C. de Bariloche, Rıo ´ ´ Negro, Argentina Instituto de InÕestigaciones en Ciencia y Tecnologıa ´ de Materiales (INTEMA)-(UNMdP-CONICET), J.B. Justo 4302 (7600) Mar del Plata, Argentina Received 26 April 1999; received in revised form 3 December 1999; accepted 3 December 1999

Abstract The effect of the temperature and the conditions of the reducing atmosphere on the sintering behaviour of pressurelesssintered sol–gel alumina matrix composites are studied by scanning electron microscopy ŽSEM., X-ray diffraction ŽXRD., and density measurements. The results are compared with those obtained with fine commercial aluminarSiC W composites, and analyzed by means of a thermodynamic study of the system. A notable improvement in sintering behaviour and a more extensive grain growth as the sintering temperature increases are determined for sol–gel alumina matrix samples. The amount of melt observed in the microstructural analysis of sol–gel alumina matrix materials is lower than that detected in fine alumina matrix samples. The mechanisms of formation of the glassy phase due to the reducing conditions of the atmosphere can operate in both series of samples, but the higher content of impurities in fine alumina matrix materials produces a more extensive liquid formation. The presence of glassy phase, higher starting densities, and lower weight losses during sintering, contribute to the higher final densities of fine alumina matrix composites. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel alumina; Composites; Pressureless sintering; microstructure

1. Main text The extremely reducing atmosphere in pressureless sintering of Al 2 O 3rSiC W materials at high temperature leads to the formation of new phases and compounds. It has been reported that several reactions like aluminium oxide decomposition, formation of carbides, and a liquid phase, can be produced at high temperature and low oxygen partial pressure w1–9x. )

Corresponding author. Tel: q54-2944-445197; fax: q542944-445299. E-mail address: [email protected] ŽG. Urretavizcaya..

In a previous work w10x, the sintering behaviour of composites of a commercial fine alumina matrix, reinforced with silicon carbide whiskers, sintered in a graphite furnace, was analyzed throughout a thermodynamic study of the system. Oxygen partial pressures in equilibrium with CO, CO 2 , and precipitated graphite at a total pressure of 1 atm of about 3 = 10y1 5 and 1 = 10y1 5 atm at 18008C and 17008C, respectively, were estimated. Depending on the arrangement of the samples, in SiC bed or between graphite disks, the oxygen partial pressures are lower or higher than the equilibrium ones, respectively. The formation of a liquid phase observed at 18008C was attributed to the presence of impurities, and also

00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 7 4 - 8

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G. UrretaÕizcaya et al.r Materials Letters 43 (2000) 281–285

to two mechanisms that can operate in the conditions of the sintering atmosphere: Ža. formation of a melt with the composition of the eutectic of the binary Al 2 O 3 –Al 4 C 3 system; and Žb. formation of a liquid simultaneous with the Al 4 SiC 4 formation by reaction of Al 2 OC with SiC. In the first step, Al 2 OC is formed by reaction of Al 2 O 3 and Al 4 C 3 . The loss of material measured at 18008C was associated with Al 2 O 3 and Al 2 OC decomposition, and the reaction between Al 2 O 3 and SiC. At 17008C, a small amount of melt was observed. This was attributed to impurities and to a minor contribution of liquid arising from the reaction between Al 2 OC and SiC. A permanent melt with the eutectic composition cannot be produced at this temperature. The aim of this work is to study the sintering behaviour of sol–gel alumina matrix composites reinforced with silicon carbide whiskers and pressureless-sintered in a low oxygen partial pressure atmosphere, in comparison with commercial fine alumina matrix composites. Al 2 O 3 and Al 2 O 3rSiC whisker samples were prepared using a-alumina and commercial whiskers ŽTateho SCW-1; 35 mm in length and 0.5 mm in diameter.. Two materials were used as matrices: alumina obtained by the sol–gel process, and commercial fine a-alumina ŽReynolds RC-HP DBM, 0.35 mm.. The sol–gel Al 2 O 3 was obtained by hydrolysis of aluminium isopropoxide ŽAIP. in excess of water at room temperature, with AlŽNO 3 . 3 P 9H 2 O as peptizing agent. The peptization was made at 908C. The product was dried and calcined at 5008C for 1 h in order to decompose the boehmite into gamma alumina keeping a high specific surface area and avoiding the formation of hard agglomerates by effect of the temperature w11x. The chemical analysis of sol–gel alumina powder calcined at 5008C, as received fine alumina and SiC whiskers are given in Table 1. The sol–gel alumina powder was attrition milled in isopropanol with alumina media at 1045 rpm during 40 h. After milling, no significant contamination ŽFe 2 O 3 - 0.06 wt.%. was determined. In the attrition-milling step, the gel was seeded by incorporation of 2 wt.% of a-Al 2 O 3 particles Ž0.35 mm. to microstructural control during the thermal transformation to alpha phase w12x. This transformation was carried out by thermal treatment at 10008C for 1 h. Porous agglomerates Ž) 1 mm. of

Table 1 Chemical analyses Žwt.%. of sol–gel Al 2 O 3 after thermal treatment at 5008C, as received fine Al 2 O 3 and SiC whiskers Compound

Sol–gel Al 2 O 3

Fine Al 2 O 3

SiC W

SiO 2 Fe 2 O 3 MgO Na 2 O TiO 2 CaO K 2O Al 2 O 3 SiC W.L. Ž10008C.

0.092 0.030 0.007 0.009 – – – 99.662 a – 0.200

0.170 0.020 0.047 0.060 0.001 0.020 0.017 99.235a – 0.430

0.100 0.080 0.030 0.008 0.008 0.200 0.004 0.230 99.340 a –

a

Values calculated by difference.

rounded submicronic particles of uniform size lower than 1 mm were obtained. Green compacts with 0, 5, 10, and 15 vol.% of SiC whiskers were prepared by slip casting of aqueous suspensions of Al 2 O 3 and SiC W and then dried at room temperature and precalcined at 9508C for 90 min. The samples were impregnated with AlŽNO 3 . 3 in order to improve both the green densities and the sintering behaviour of the compacts w13x. After each step of impregnation, the material incorporated by this process was transformed into g-alumina by thermal treatment. Pressureless sintering was done in a graphite-element resistance furnace ŽASTRO Group 1000., under He atmosphere, at 17008C and 18008C for 220 min. The samples were arranged between graphite disks into the furnace. After sintering, the X-ray diffraction ŽXRD. pattern of the final product indicates that g-Al 2 O 3 arising from impregnation transformed into a-Al 2 O 3 . The densities were measured by the Archimedes method in water and the relative density values were calculated using the theoretical densities of a-Al 2 O 3 Ž3.98 grcm3 ., g-Al 2 O 3 Ž3.50 grcm3 ., and SiC Ž3.20 grcm3 . for precalcined samples, while theoretical densities of a-Al 2 O 3 and SiC were used for sintered samples. The developed microstructures were examined by scanning electron microscopy ŽSEM. ŽPhilips 505. on fracture surfaces. A quantitative determination of the amount of silicate glass in the sintered specimens was made by leaching. Powdered samples were treated with 20% HF Ž1 g powderr10 ml HF. at room temperature for

G. UrretaÕizcaya et al.r Materials Letters 43 (2000) 281–285

15 min with agitation. Powders were filtered, washed, and dried. After firing at 9008C for 1 h, the weight losses were measured. Precalcined relative densities of sol–gel alumina and sol–gel alumina matrix composites and final relative densities of the same samples sintered at 17008C and 18008C are plotted in Fig. 1. The relative density values obtained with the fine alumina based materials w10x are also included. The final densities of both the fine alumina and the sol–gel alumina composites are lower than the density of the Al 2 O 3 material sintered in air Ž99.5%.. This effect is attributed to the decomposition of Al 2 O 3 and Al 2 OC, and the reaction between Al 2 O 3 and SiC giving gaseous products, associated to weight losses ŽTable 2., thermodynamically possible in the reducing atmospheres used in this work. Additionally, the densities of precalcined samples have a marked effect on sintered materials: the sol–gel alumina matrix composites achieve lower densities than the samples prepared with fine alumina due to the low densities of their precalcined samples Žbetween 65% and 70%. for all whisker contents. The density of the samples prepared by slip casting is influenced

Fig. 1. Relative densities of precalcined and sintered sol–gel Al 2 O 3 -matrix and fine Al 2 O 3 -matrix samples as a function of whiskers content.

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Table 2 Weight losses Ž%. after pressureless sintering at 18008C SiC content Žvol.%.

Sol–gel Al 2 O 3 matrix

Fine Al 2 O 3 matrix

0 5 10 15

4.9 6.6 8.6 12.4

4.6 5.3 6.4 7.3

by the characteristics of the starting materials, the presence of porous agglomerates in the sol–gel alumina powder, and the presence of the whiskers that make compaction difficult. The influence of these two factors is considered responsible for the relatively low green densities. Also, the diminution of final density as the whisker content increases is explained by the low starting densities in addition to the harmful effect of whiskers on sintering, which interfere with shrinkage by crosslinking among themselves.

Fig. 2. SEM micrographs of sol–gel Al 2 O 3 samples pressurelesssintered at 17008C Ža. and 18008C Žb..

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On the other hand, all the samples with sol–gel alumina matrix show an improvement in sintering behaviour as the sintering temperature increases, which has relatively little influence on the densities of fine alumina based materials. Fig. 2a,b shows SEM micrographs of alumina samples sintered at 17008C and 18008C, respectively. At the two temperatures studied, samples present intergranular fracture, and a more extensive grain growth is observed after sintering at the higher temperature. A bimodal grain size distribution is also developed, and the more frequent grain sizes are 4–6 and 1 mm at 17008C, and 10–15 and 2–3 mm at 18008C. Microstructures of 5 and 15 vol.% SiC W rAl 2 O 3 composites are shown in Fig. 3a–d. A continuous decrease in the mean grain size with increasing whisker content was observed. Additionally, when the samples were treated at 18008C, a small amount of liquid was formed, as we can observe in detail in Fig. 4, where the whiskers are stuck to the Al 2 O 3 grains.

Fig. 4. SEM micrographs of 15 vol.% SiC W rsol–gel Al 2 O 3 sample pressureless-sintered at 18008C.

The formation of the glassy phase can be explained as a consequence of the highly reducing atmosphere in addition to the presence of metallic impurities in raw materials. In conditions of very low oxygen partial pressure, two mechanisms are considered adequate w10x to explain the presence of a melt: formation of a melt with the composition of the

Fig. 3. SEM micrographs of 5 and 15 vol.% SiC W rsol–gel Al 2 O 3 samples pressureless-sintered at 17008C Ža and c. and 18008C Žb and d..

G. UrretaÕizcaya et al.r Materials Letters 43 (2000) 281–285

eutectic point of the binary Al 2 O 3 –Al 4 C 3 system, and reaction between SiC and Al 2 OC, giving Al 4 SiC 4 and a liquid. The amount of melt observed in samples of sol– gel alumina matrix is notably lower than that detected in fine alumina matrix samples. This is in agreement with the results of the quantitative analysis of the glass performed by HF leaching on 15 vol.% SiC composites sintered at 18008C: 0.7% and 3.2% for sol–gel and fine Al 2 O 3 matrix, respectively. This difference can be explained bearing in mind the lower content of impurities in sol–gel alumina powder Ž0.138 wt.%. than in fine alumina powder Ž0.335 wt.%. shown in Table 1. In the last sample, there is a more extensive liquid formation due to the contribution of the impurities, in addition to the melt formed by reactions between the compounds of the system in the particular conditions of the atmosphere. A silicate glass originated by the presence of 0.25 wt.% of impurities has been reported w14x. Thus, it appears that the contribution of impurities to melt formation in the fine alumina matrix is more significant than in the sol–gel one. This fact, together with the low starting densities of sol–gel alumina matrix samples, justify their lower final densities. Moreover, another fact could contribute to these low densities. The Al 2 O 3 decomposition and the partial dissociation of Al 2 OC at high temperatures, together with the reaction of SiC and Al 2 O 3 at low oxygen partial pressures giving the following gaseous products: SiC Žs. q Al 2 O 3Žs. m SiOŽ g . q COŽ g . q Al 2 OŽ g . are responsible for the weight losses produced during sintering as proposed by Oscroft and Thompson w7x in agreement with Misra’s study w2x. Sol–gel alumina materials exhibit higher weight losses Žattributed to the reactions between components of the system. than fine alumina samples ŽTable 2.. Considering the above results, we can conclude that a control of the degree of densification and the amount of the glassy phase in the alumina matrix composites is possible by adjusting the characteristics of the starting material and the sintering temperature, the latter contributing to determining the reducing conditions of the atmosphere. Microstructures with very low glassy phase content are ob-

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tained using sol–gel alumina powder in the fabrication of the composite materials. This low amount of liquid phase, the low starting densities, the weight losses attributed to compound decomposition and reactions giving gaseous products, and the harmful effect of whiskers on sintering, are the most important facts in hindering complete densification. On the other hand, high final densities are achieved if fine alumina powders are used. This fact is attributed to the high starting densities and to a liquid-phase assisted mechanism of sintering.

Acknowledgements Helpful discussions with Dr. Angel Caballero Cuesta, from Instituto de Ceramica y Vidrio ŽICV., ´ Madrid, are gratefully acknowledged. The authors want to thank Ann Borsinger for the English revision.

References w1x M.I. Nieto, P. Miranzo, S. de Aza, J.S. Moya, J. Ceram. Soc. Jpn. 100 Ž1992. 459. w2x A.K. Misra, J. Am. Ceram. Soc. 74 Ž1991. 345. w3x A. Gadalla, M. Elmasry, P. Kongachuicay, J. Mater. Res. 7 Ž1992. 2585. w4x P. Lefort, D. Tetard, P. Tristant, J. Eur. Ceram. Soc. 12 Ž1993. 123. w5x L.M. Foster, G. Long, M.S. Hunter, J. Am. Ceram. Soc. 39 Ž1956. 1. w6x Y. Larrere, ` B. Willer, J.M. Lihrmann, M. Daire, Rev. Int. Hautes Temp. Refract. Fr. 21 Ž1984. 3. w7x R.J. Oscroft, D.P. Thompson, J. Am. Ceram. Soc 75 Ž1992. 224. w8x J.M. Lihrmann, T. Zambetakis, M. Daire, J. Am. Ceram. Soc. 72 Ž1989. 1704. w9x T. Sata, T. Sasamoto, in: Structure and Properties of MgO and Al 2 O 3 Ceramics, W.E. Kingery ŽEd.., Adv. Ceram. Vol. 10 Am. Ceram. Soc, Columbus, 1984, p. 541. w10x G. Urretavizcaya, J.M. Porto Lopez, L.A. Cavalieri, J. Eur. Ceram. Soc. 17 Ž1997. 1555. w11x G. Urretavizcaya, A.L. Cavalieri, J.M. Porto Lopez, I. Sobrados, J. Sanz, J. Mater. Synth. Process. 6 Ž1998. 1. w12x G. Urretavizcaya, J.M. Porto Lopez, Mater. Res. Bull. 27 Ž1992. 375. w13x G. Urretavizcaya, A.L. Cavalieri, J.M. Porto Lopez, Ceram. Int. 21 Ž1995. 97. w14x C.A. Bateman, S.J. Bennison, M.P. Harmer, J. Am. Ceram. Soc. 72 Ž1989. 1241.