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SINTERING AND MICROSTRUCTURE OF Si3Al3O3N5 PRODUCED FROM KAOLIN
F. van Dijen, R. Metselaar, C. Siskens
To cite this version:
F. van Dijen, R. Metselaar, C. Siskens. SINTERING AND MICROSTRUCTURE OF Si3Al3O3N5 PRODUCED FROM KAOLIN. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-261-C1-265.
�10.1051/jphyscol:1986139�. �jpa-00225568�
SINTERING AND MICROSTRUCTURE OF Si,A1,03N, PRODUCED FROM KAOLIN
F.K. VAN D I J E N , R . METSELAAR a n d C . A . M . SISKENS*
Laboratory for Physical Chemistry, Eindhoven University of Technology, P.O. Box 513, NL-5600 M B Eindhoven, The Netherlands ' ~ n s t i t u t e of Applied Physics TNO-TH, Ceramics Dept. Eindhoven, The Netherlands
~6sum6
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Le f r i t t a g e d'une p u d r e de Si3A1303N5 est discutd c m e une a l t e r - native au f r i t t a g e r e a c t i f plus usuel dZunm6lange de p u d r e s de Si3N4, A1203 e t AlN. C m e l a poudre de Si3A1303N5 e s t prcduite 2 p a r t i r du kaolin, l a pr6sence d ' i q u r e t 6 s dans l e kaolin e s t consid&6e avec attention. Les pro- pri6ti.s f i n a l e s du matdriau f r i t t 6 sont indiqudes.Abstract
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The sintering of Si3A1303N5 powder is discussed as an alternative for the more usual reaction s i n t e r i n g of a mixture of Si3N4, A1203 and AlN powders. As the Si3A1303N5 powder i s produced from kaolin, attention is paid t o i n p i t i e s which are present in the kaolin. Finally properties of the sintered material are given.I
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INTRODUCTIONThe aim of the study i s t o obtain an economic production route f o r sintered, non oxide ceramic a r t i c l e s . Fine non oxide ceramic powders can be obtained by carbo- thermal reclaction of oxides. When minerals are used instead of synthetic oxides t h i s method is supposed t o be very economic. However minerals w i l l contain more impuri- t i e s than synthetic oxides. When the non oxide powders, produced by the carbothennal production method, a r e shaped and pressureless sintered the overall route t o produce ceramic a r t i c l e s seems promising. In t h i s a r t i c l e the authors discuss the sintering of a Si3Alj0 NS powder, which i s produced from the mineral kaolin. A B'-sialon material wlt2 such a high content of A1/0 i s not commercially produced a t the moment.
We w i l l show t h a t t h i s material has a unique combination of properties and therefore i s worth being developed.
I 1
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POWDER PREPARATIONS i AI 0 N was produced from kaolin and carbon black using the c a r b o t h e m l produc- t i 8 n $e$h8d described by Lee and Cutler / l / and van Dijen e t a1 / 2 / . Details of the production method are given in / 2 / . The composition of the kaolin used was ( i n wt.%):
45.6 Si02, 38.6 A1203, 14.0 H20, 0.34 Fe20
,
1.37 TiO2, 0.06 K20, r e s t adsorbed H2O.This corresponds t o a molar rat10 Si:Al=l
.
a0. As confinned by l a t t i c e constant mea- surements the resulting 6'-sialon powder has the composition Si3A1303N5. When the p e l l e t s are taken from the reactor the powder i s strongly agglomerated. Therefore they have t o be milled. The Si3A1303N5 p e l l e t s , with a s p e c i f i c surface area of 8 m2/g ac- cording t o the BET method, were b a l l milled i n a polyethene flask. Alumina b a l l s (95%alumina) were used. The p e l l e t s were wet milled i n propanone f o r 50 hours. Fig. 1 shows the p a r t i c l e s i z e d i s t r i b u t i o n of the powder. A Micromeritics SediGraph was used.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986139
C l - 2 6 2 JOURNAL DE PHYSIQUE
According t o the formula: S = 6/(p dp), one can calculate the specific surface area S from the average particle diameter dp. The density of the powder, p , i s taken as 3.15 g/un3. The x-ray density f o r Si3A1303N5 i s 3.1 but we corrected f o r the impurities TiN and FeSi,. The calculated specific surface area is 1.3 m2/g. The diffe- rence with the measured specific surface area is explained by assuming agglomerates.
M i l l wear was about 2%. To compensate for the mill wear the sialon powder i s produced with a s l i g h t nitrogen excess. This is achieved by using a carbon black: kaolin mass r a t i o of 0.26 instead of the $heoretical 0.24 r a t i o . As a r e s u l t some 15R phase can be detected i n the powder by XRD.
In t h i s way the composition of the milled powder f a l l s within the region of 6'-sialon solid solutions and the formation of glassy phases in the sintered product i s preven- ted (apart from those due t o sintering aids). However, impurities which originate from the kaolin s t a r t i n g powder do lead t o impurities like TiN and FeSix i n the sialon powder.
CUMULATIVE MASS PERCENT
100 50 10 5 1 0.5 0.1
EQUIVALENT SPHERICAL DIAMETER, pm
Fig. 1
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Particle s i z e distribution of a Si3A1303N5 powder a f t e r 50 hours b a l l milling.I11
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SINTERING OF Sifl-klg3N5 ACCORDING TO THE LITERATUREUsually 6'-sialon i s produced by mixing powders, for instance Si3N4, A1203 and A l N or Si3N4 and polytypes. Often sintering aids are added. The powder mixture is then shaped, often by i s o s t a t i c pressing, into a t a b l e t fonn. Next t h i s t a b l e t is sin- tered and reacted f o r 1/2 t o 2 hours a t about 1700 t o 18000C. The cooling and hea- ting of the sample usually takes half an hour. This reaction sintering technique is similar t o that of Si3N4. Descriptions of t h i s technique are given by several authors /3,4,5/. Sintering of B'-sialon powder i t s e l f instead of reaction sintering is scarcely described, however Mitomo e t a1 published some work on t h i s subject /6/.
Whether pure 6'-sialon powder s i n t e r s without a liquid phase a t the grain boundaries is not clear, see Boskovic /3/ andMitomo /6/. Such a liquid phase gives enhanced sintering rates and can be provided by the use of stable oxides which form low melting eutectics. Usually oxides of high melting points are used. The resulting glassy phase a t the grain boundaries influences the mechanical properties of the product i n a negative way /7/. Sometimes the glassy phase between the grain bounda- r i e s can be crystallised a t the t r i p l e points /8/.
Oxides which function as sintering aids are, f o r example: MgO, CaO, Y203, CeOZ, La203, Sc203, Be0 etcetera. CaO i s usually rejected due t o its bad influence on high
Si3A1303N5 powder o r without sintering aid was dry pressed i n t o a t a b l e t of 13 mm diameter and a weightof 1 gram. The pressure was 3000 kg/cm2. After i s o s t a t i c a l l y pressing with a pressure of 4000 kg/cm2 the density of the t a b l e t was 1.8 g/cm3.
During the sintering of the p e l l e t i n an alumina crucible weight loss occurs due t o the reaction: Si3A1303N5 + 3Si0 + 3AlN + NZ. The SiO pressure as a function of the temperature can becalculated, assuming that the t o t a l pressure is 1 atmosphere.
Data from Turkdogan /9/ and Daner e t a1 /-l 0/ was used. See table 1. A calculation shows that t h i s SiO pressure is similar t o that caused by the reaction:
Table 1 - Calculated SiO pressure as a function of the temperature f o r the reaction Si3A1303N5 + 3Si0 + 3A1N + NZ.
Temperature (K) PSiO (am)
Si3N4 + Al O3 + 3Si0 + 2AlN + NZ. So t h i s mixture as well as Si3A1303N5 powder can be use2 t o generate an SiO pressure in a pawder bed.
Weight loss can beprevented by a high nitrogen pressure, an SiO pressure o r a high sintering r a t e as there w i l l be a competition between sintering and SiO loss. An atmosphere containing SiO can be obtained by use of the powder bed technique /11/.
BN powder i s added by many authors t o prevent sintering of the bed. According t o Jack /12/ packing the sample in pure BN is also effective. In t h i s work Si3A1303N5 powder was used t o generate an SiO pressure.
In kaolins CaO and MgO are comon impurities. Therefore, in our sintering studies we investigated the influence of 1 wt% additions of these oxides. Na and K eva- porate during processing / l 3/ and a r e therefore not of interest. The oxides Fe203 and TiO? in our kaolin lead t o the formation of FeSi, and TiN i n the sialon powder.
Figure 2 shows that these impurities are present as inclusions i n the sintered pellets. Therefore it. i s not likely that they a c t as sintering aids. When the
Fig. 2
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Photomicrograph of a sintered sialon sample, showing pores and FeSi, inclusions.
C l - 2 6 4 JOURNAL DE PHYSIQUE
inclusions are small and dispersed homogeneously i n the matrix they also do not deteriorate the properties of the f i n a l product /14/. Apart from CaO and MgO we also investigated the influence of 1 wt% of Y2O3, CeO2 and La203 on the sintering properties. In l i t e r a t u r e often higher concentrations of Y2O3 (e.g. 5 wt%)
are mentioned. However, since our sialon powder is supposed t o be cheap, the use of such large amounts of an expensive sintering aid was not considered.
The heating and the cooling rates of the samples was 1750C per hour. The results for two sintering times and two sintering temperatures are presented i n table 2.
The weight loss was l e s s than 3 w t % . The theoretical density for S i A1303N5 is 3.10 g/cm3. k e t o impurities and sintering aids a somewhat higher i e m i t y f o r samples without porosity might be expected.
Table 2 shows that Si3A1303N5 s i n t e r s , although f u l l density i s not obtained. I t is also shown t h a t M@ is not a very effective sintering aid, which is i n accordance with the results of Lowell /IS/.
Table 2
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The influence of sintering time, temperature and sintering aid on the sintering of a Si3A1303N5 powder. The apparent density i s given i n g/cm3.Sintering aid 15 hour 20 hour 15 hour 20 hour
(1 wt%) 1675OC 1675'~ 1 7OO0C 1700°c
none 2.4 2.6 2.8 2.5
M@ 2.7 2.8 2.8 2.8
CaO 2.9 3.0 2.9 3.0
Y2°3 3.0 2.9 2.9 3.1
La20g 2.9 3.0 2.8 3.0
Ce02 2.9 2.9 3.1 3.0
Compared with the density measured on samples which were sintered f o r 15 hours a t 167S°C, an increase i n density f o r the other samples i s expected. However, often a decrease of the density is observed. This is due t o loss of SiO, which results either i n a decrease of the sintering r a t e , or i n the development of a surface layer on the sample. This surface layer may be short i n SiO, i n which case it consists mainly of the 15R phase. [dowever, t h i s layer may also be rich i n SiO. This extra SiO originates from the bed. An example of the l a t t e r case was observed on the samples which were sintered with Y203.
Sintering can be improved by the use of larger amounts of sintering aids. I t can also be improved by further milling of the powder, which not only decreases the s i z e of the agglomerates but also increases the oxygen content of the powder due t o wear losses of the alumina balls.
When these results are compared with those of Lee and a t l e r / l / , who found that Si3A1303N5 powders made from kaolin were easily sintered, it seems probable that t h e i r powder contained a higher oxygen content and or a higher calcium content.
Properties of samples which were sintered f o r 20 hours a t 1 7 0 0 ~ ~ were measured.
1 w t % Ce02 was used as a sintering aid. The powder was made from a carbon black- kaolin mixture with a r a t i o of 0.25. The powder was milled for 48 hours which yielded an average p a r t i c l e s i z e of 1 .S pm. The density was 3.16 g/m3 as measured by ArchimedesT method. The s i z e of the pores and of the FeSix and TiN inclusions was l e s s than 10 vm. The average c r y s t a l l i t e s i z e was estimated t o be about 5 um.
We s h a l l briefly discuss some properties of these ceria doped sialons and compare these with l i t e r a t u r e data. We measured a Rockwell hardness 92 Hra, which indicates a higher value than for Si3N4 in agreement with Lumby /16/, but i n disagreement with Gauckler e t a1 /17/. According t o Gauckler e t a1 /17/ and Glandus and Boch /18/ a Youngs modulus of 225 GPa can be expected, which i s close t o our observation of 230 GP~: (Using a resonance technique a Poisson r a t i o of 10. 285 was obtained. From the indentation method /19/ we found a K value of 5 MPa m Z , which i s higher than the value reported by Gauckler e t a1 /17jf. The same holds for our value of the bend strength of 550 MJ?a as obtained bv using the diametral compression method. These data
-F- Measured by Dr. G. de With, Philips Research Laboratories, Eindhoven
sible after further sintering studies. The room temperature electrical resistivity exceeds 3. 1012 am, in accordance with literature /20/.
It sk uld be remarked that the thermal conductivity is also rather low, about 7 Eh
?
/17,21/ and therefore the thermal shock resistance will be lower than of Si3N4.V1 CONCLUSIONS
Si3N303N5 powder can be sintered to about 90% of theoretical density without sintering aids. When the powder is made from kaolin CaO is the only impurity which acts as an effective sintering aid. The other impurities which are usually present in a kaolin hardly influence the sintering behaviour.
Despite the presence of impurities the Si3A1303N5 material is not a second class material.
Most striking properties of the material are its low thermal conductivity and its low Youngs modulus compared with those of Si3N4.
REFERENCES
/l/ Lee J.G., Cutler I.B., Am. Ceram. Soc. Wlll 58 (1979) 869.
/2/ Van Dijen F.K., Metselaar R., Siskens c.A.M.TJ. Am. Ceram. Soc.
68
(1985) 16./3/ Boskovic S., Gauckler L.J., Petzow G., Tien T.Y., Powder Metallurgy Int.
fi
(1979) 169.
/4/ Briggs J., Mat. Res. Bull. 12 (1977) 1047.
/5/ Lumby R.J., Butler E., LewisM.H., Progress in Nitrogen Ceramics, Riley F.L., ed., Martinus Nijhoff Publishers, Boston (1983) 683.
/6/ Mitomo M., Shiogai T., Yoshimatsu H., TsutsLrmi M,, Yogyo-Kyokai Shi
93
(1985)69.
/ 7 / Greil P., Weiss J., Progress in Nitrogen Ceramics, Riley F.L., ed., Martinus
Nijhoff Publishers, Boston, (1 983) 359.
/8/ Greil P., Bressiani J.C., Petzow G., International Symposium on Ceramic Components for Engines, Hakone, October 1 7-21
,
(1 983).
/9/ Turkdogan E.T., Physical Chemistry of High Temperature Technology, Academic Press, New York, (1 980)
.
/10/ Mrner P., Gauckler L.J., Krieg H., Lukas H.L., Petzow G., Weiss J., Calphad 3 (1979) 241.
71 l/ Pompe R., Carlsson R., Progress in Nitrogen Ceramics, Riley F.L., ed., Martinus Nij hoff Publishers, Boston (1983) 219.
/12/ Jack K.H., Wilson W.I., U.S. Patent, Nr. 3, 991, 166, November 9, (1976) /13/ Wirika R., Corn. Am. Ceram. Soc.
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(1984) C-232./14/ Lange F.F., J. Materials for Energy Systems,
5
(1984) 107./l 5/ Lowell R.F., unpublished.
/16/ Lumby R.J., J. Mat. Sci. Lett. 2 (1983) 345.
/17/ Gauckler L.J., Prietzel S., ~o&mer G., Petzow G., Nitrogen Ceramics, Riley F.L., ed., Noordhoff, Leiden, (1977) 529.
/18/ landu us J.c., Boch P., ~itrogen Ceramics, Riley F.L., ed., Noordhoff, Leiden, (1977) 515.
/19/ ~iihara K., Morena R., Hasselman D.P.H., J. Mat. Sci. Lett. 1 (1982) 13.
/20/ ~uwabara M., ~ubota Y., Tsukidate T., J. Mat. Sci. Lett. 2 0983) 299.
/21/ Inomata
