The porosity of the slip cast Si3N4 is similar to that of pressed Si3N4 formed at 2500 kg/sq cm. The porosity of cast Si oxynitride is equivalent to that of samples stressed at 10,000 kg/sq cm. Crucibles formed from these materials by slip casting have high thermal shock and corrosion resistance

Dobysh, A. V.

Guzman, I. Y.

English

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NASA TM-77495NASA TECHNICAL MEMORANDUM
(NASA-TM- 77495)
 STUDY OF ZHE PAUSE STRUCTURE
	 N85-13048
OF CERAMICS PREFASED EY THE S11E CASTING
!METHOD (National Aeronautics and Space
Administration) 7 p hC 102IME A01 CSCI, 11B
	 Unclas
G3/27 24500
STUDY OF THE PORE STRUCTURE OF CERAMICS
PREPARED BY THE SLIP CASTING METHOD
I. Ya. Guzman and A. V. Dovbysh
Translation of "Issledovaniye porovoy struktruy
keramiki poluchennoyy metodom shlikernoeo lit'va"
Tr. ivbsk. Knim. Teknnol. Inst. , 1976, Vol. 92, pp 70-72.
- 7
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON D. C. 20546 	 JULY 1984
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The porosity of	 the slip cast Si 3 N 4 is simila
-
to that of pressed Si 33N 4	formed at 2500 kg/cm	 .
The porosity of cast 5i oxynitride is equivflent
to that of samples	 stressed at	 10,000 kg/cm	 .
Crucibles	 formed from these materials by	 slid,
casting have high thermal shock and corrosion resistance.
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STUDY OF THE PORE STRICTURE OF CERAMICS PREPARED BY THE SLIP CASTING METHOD
I. Ya. Guzman, A. V. Dovbysh
Monophase materials made of silicon nitride and oxynitride, as /70*
well as composition materials based on silicon carbide with nitride
bonds possess a complex of valuable properties, primarily high
corrosion resistance in relation to melts of various salts and
nonferrous metals [1-3]. The corrosion resistance of the products
is significantly affected not only by the value of the porosity, but
also by the structure of the products, in connection with which it
is necessary to evaluate the pore structure of samples obtained by
the slip casting method, which is promising for making products of
a complex profile, and particularly thin-walled products.
The initial materials used for making the slips were silicon of
grade KR-0 (GOST 2169-43), quartz sand from the Tashlinskiy deposit,
and green silicon carbide from the Zaporozhskiy Plant. All the materials
were subjected to dry vibration grinding for various times to a specific
surface on the order of 5-8 m2/g, determined by the method of low-
temperature nitrogen adsorption (Table 1).
The slip casting was performed in dry gypsum molds covered with /71
a protective film of sodium alginate, in an optimal sphere of pH 8-10.
Firing is done on a muffle type device in a medium of industrial
nitrogen according to the conditions recommended in works [4-5] with
reduction of the holding time at final temperature to 1-2 hours for
the thin-walled samples.
* Numbers in margins indicate foreign pagination.
1
TABLE 1.	 Specific surface of powders
Duration of Specifi	 surface
Material	 vibropulveriza- area, m /g
tions hrs.
Silicon	 4 5.2
Mixture Si-SiO2
(60:40)	 5 6.3
Mixture SiC-Si	 3-4* 8.2
(60:40)	 0.5
Mixture SiC-Si-Si0	 3-4-2* 7.9
(70:18:12)	 2	 0.5	 -
* Given in the numerator is the time for pulverization of the in-
gredient components, respectively; in the denominator -- time for
combined pulverization of the mixture.
The properties of the castings and fired	 products subjected to
the study of pore structure are presented in Table 2.
TABLE 2.	 Properties of raw and fired ceramics
Slip indicators
	
Raw goods indicators Fired products
apparent	 apparent qualitative apparent apparent
Initial material	 slip	 density,	 porosity, phase dens ty, porosity,
pH	 g/cm	 % composition g/cm %
Si	 8.1	 1.63	 30.1 Si3N4 2.38 24.1
Mixture
Si-SiO2 (60.40)	 9.85	 1.50	 35.0 S12ON2 2.20 19.0
Mixture
SiC-Si (60:40)	 9.9	 1.82	 33.5 Sic-Si3N4 2.19 28.0
Mixture
Sic-Si-Si0	 9.45	 1.96	 30.9 Sic-Si ON2 2.20 27.8(70:18:12)2 2
The pore distribution by size was evaluated by the method of
mercury porometry according to the methodology of work [6]. The
results of the pore structure determination for the fired ceramics
obtained by slip casting are presented in Figures 1, 2 and in Table 3.
2
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Figure 1.	 Differential porograms Figure 2. Differential porograms
of samples: 1 - silicon nitride; of compositional ceramics based
2 - silicone oxynitride on silicon carbide: 1 - with
nitride; 2 - with oxynitride
bonding.
An analysis of the differential porograms of the samples showed that
in practically all the samples there is an absence of large pores (larger
than one mm). The main mass of pores corresponds to a size of 0.5 - 0.01 Rm. /72
The samples made of silicon nitride and oxynitride have a similar pore
structure, with pores 0.1 - O.Ol,fm in size comprising 83-85% (See Table 3).
TABLE 3. ,Distribution of pores by size in fired samples of monophasal
and composition ceramics
Pore radius,	 m SiA SiON2 SiC-Si8N4 Sic-Si2ON2
Greater than 1 0.6 1.7 1.8 1.1	 '4
1-0.5 0.6 1.0 1.1 0.7
0.5-0.1 13.8 14.3 71.3 2.5
0.1-0.01 85.0 83.0 25.8 95.7
Smaller than 0.01
3
Despite this fact, the samples made of composition ceramic differed in
their pore structure. In comparing the differential porograms of the
samples made of silicon carbide with nitride and oxynitride bonding, we
may conclude that the former have a coarser porosity and the main mass
of the pores is within the margins of 0.5 - 0.1 ,{m, while in the latter
it is within the margins of 0.1 - 0.01 N m (see Figure 2). The different
pore structures of the samples may be explained by the character of inter-
action of the reaction bonding with the silicon carbide filler. Works
[7.8] discuss the chemical interaction of silicon oxynitride with the
grains of silicon carbide, partially surrounded by an oxide film, unlike
silicon nitride, for which no such interaction is found. This chemical
interaction prevents the formation of larger pores along the boundaries
of the bonding and the filler.
Let us compare the pore structure of samples made of monophase ceramic
obtained by slip casting with the pore structure of samples obtained by
pressing at various specific pressures from powders of identical dispersity.
The cast samples made of silicon nitride were similar in their pore
structure to the pressed samples at a specific pressure of 2500 kG/cm2.
The oxynitride samples obtained by casting were similar to the same ones
obtained by pressing at specific pressure of 10,000 kG/cm2.
We must note that crucibles made of the indicated materials and
subjected to tests for heat resistance .(at 950°C in an air medium) and
for corrosion resistance in relation to melted cryolite simultaneously,
after 15 melts showed no visually apparent changes.
Thus;. the slip casting method from silicon and its mixtures with
I
4
subsequent reaction baking may be used to obtain products on the basis
of nitride, and silicone oxynitride, as well as silicon catbide with
nitrice bonds. These products do not yield by fineness of their pore
structure to products obtained by pressing at rather high specific pressures.
LITERATURE
1. Popper P., Rudlesden S. N. TRANS. BRIT. CERAM. SOC., 1961, ?Jol 60,
No 9, 1603-1623.
2. Yasinskaya G. A. "Khimiya i fiaika -iitridov," [Chemistry and Physics
of Nitrides], Kiev, "Naukova dumka", 1968, 178-180 p.
3. Peskin W. L. Modern Metals, 1961, Vol 11, No 7, 32, 34, 36, 38, 40,
42 pp.
4. Tumakova Ye. I. Abstract of Cand. Diss. MKhTI im. D. I. Mendeleyev,
1970, 24 p.
5. Zabruskova T. N. Abstract of Cand. Diss., MKhTI im , D. I. Mendeleyev,
1971, 24 p.
6. Plachenkov T. G., "Rtutnayi purometricheskaya ustanovka," [Mercury
Porometric Installation], LTI im. Lensoveta, 1968, 26 p.
7. Washburn M. E., Love R. W. J. AMER. CERAM. SOC., 1962, Vol 45, No 7,
447-449 pp.
8. Washburn M. E. REFRACTORIES J., 1963, Vol 39, No 10, 412-416 pp.
5


Study of the pore structure of ceramics prepared by the slip casting method

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