IRON-BEARING SILICON NITRIDE COMPOSITE MATERIALS. MADE FROM POWDERS MILLED IN A PLANETARY MILL. V. N. Antsiferov, 1 V. G. Gilev, t and ...
Refractories and Industrial Ceramics
VoL 38, Nos. 9 - 10, 1997
UDC 666.762.93
IRON-BEARING SILICON NITRIDE COMPOSITE MATERIALS MADE FROM POWDERS MILLED IN A PLANETARY MILL V. N. Antsiferov, 1 V. G. Gilev, t and V. I. Karmanov 2 Translated from Ogneupory i Tekhnicheskaya Keramika, No. 9, pp. 22 - 25, September, 1997. Original article submitted January 14, 1997.
The effect of milling in an AGO-3 centrifugal planetary activator with steel milling balls on the properties of Si3N4 powders, the conditions of sintering, and the phase composition, structure, and properties of composite materials based on them is described. The possibility of and the process conditions for obtaining dense shock-resistant composite materials based on preliminarily milled 13-Si3N4 (SHS) powder and additives that activate the sintering and consist of AIN, A1203, and ZrO2 powders are studied. It is established that in the sintered materials the presence of FeSi formed due to the interaction of Si3N4 and Fe that appears in the milling process does not worsen appreciably the thermomechanical properties of the composite materials.
Mixtures o f Si3N4 powders (preferably with a high content o f (z-Si3N4 [1, 2] and a specific surface o f at least 10 me/g) and sintering additives are used to fabricate dense silicon nitride materials by sintering without pressure. When the sintering additives are AI203 and AIN, the densest sintered materials have the composition o f a sialon [3]. A typical sintering temperature for charges o f the Si3N4 - A1203 - AIN system is 1700 - 1750~ [3, 4]. An available and simple method for dispersing the powder consists in milling it in a mill with steel balls, but this inevitably leads to the presence o f milled iron, assumed to be an extremely harmful impurity in silicon nitride materials [4]. Iron decreases the resistance o f Si3N4 to dissociation and thus increases the loss o f mass in high-temperature sinteting. Washing the iron off with hydrochloric acid is a laborious process. At the same time, dense materials based on Si3N4 with Kic up to 12 MPa. m 1/2 have been reported to be obtained by introducing plastic inclusions o f iron or steel particles; here, the sintering is conducted at a lower temperature (about 1500~ [5]. In addition, the strength o f Si3N4 has been reported to increase at 20 and 1100~ with increase in the content o f iron impurity [2]. In this connection it is o f interest to study the possibility o f preparing dense shock-resistant composite materials based on available Si3N4 powders milled in a planetm-y activator with steel balls.
We studied the effect o f milling in an AGO-3 centrifugal planetary activator with hydrostatic holders on the properties of Si3N4 powder and mixtures based on it, the sintering conditions, and the structure, phase composition, and properties of the final materials. We used Si3N4 powder obtained by the method o f self-propagating high-temperature synthesis (SHS) and consisting entirely of I3-Si3N4 according to data o f an x-ray phase analysis. According to data of[6, 7], Si3N4 (SHS) powder is pure but insufficiently disperse. The mixtures studied consisted o f Si3N4 as the base and A1203 and A1N as the sintering-activating additives in an amount of 5% 3 each. In addition, 30% ZrO 2 was introduced into one o f the mixtures. Method of the experiment. Milling in the AGO-3 was conducted in 2-liter steel drums with milling bodies in the form of steel balls 7 and 9 m m in diameter. The ratio o f the mass of the milling bodies to the mass of the powder was 30 : 1, the mass o f the weighed portion o f powder in one bowl being 100 g. In dry milling the powder stuck to the walls o f the drums, and therefore we resorted to milling in alcohol (80 ml per 100 g o f powder). The milling lasted for 5, 10, and 20 min. The content o f oxygen impurity in the powders was determined from the emission o f CO in melting a sample in a graphite crucible in a helium flow in an A G O M E T installation; the specific surface of the powders was determined by the BET method. Thermal analysis of the Si3N4 powders was conducted with heating of them to 1000~ at a rate o f 5~ in a Q-1500D
I
Research Institute for Problems of Powder Technology and Coatings, Research Center of Powder Materials, Perm, Russia. 2 Institute of Engineering Chemistry of the Russian Academy of Sciences, Perm, Russia.
3
Here and below in mass fractions.
354 1083-4877/97/3809-0354518.009 1998 Plenum Publishing Corporation
Composite Materials Made from Powders Milled in a P l a n e t a ~ Mill
co_, %
Ssp, m2/g 16
355
1.8
7 ~
1.5 8
1.4~
"~ 1.0
4 0.6 I
0
5
I
10 z, min
I
15
20
0
5
10 15 z, min
0
5
20
Fig. 1. Dependences of the specific surface Ssp and the oxygen content c02 in Si3N4 (SHS) powder on the milling time -c in a planetary activator; the
10
15 ~, min
20
Fig. 2. Dependence of the technological properties of Si3N4 (SHS) powder on the milling time z in a planetary activator: l ) bulk density; 2 ) density after shaking; 3 ) density of a preform pressed at 200 MPa.
numbers at the curves denote the diameters of the balls, mm.
derivatograph, and x-ray diffraction analysis was conducted using a DRON-4 installation in cobalt and copper radiation. IR spectra of milled Si3N4 were obtained on a Specord M82 IR spectrometer by a standard method in a paste of Vaseline oil. The materials were sintered in a medium of technically pure nitrogen in a nonthrough furnace with a graphite heater in graphite boats. The filled-in mixtures were similar to the sintered mixtures based on Si3N 4 (SHS) powder. The structure of microscopic specimens and the size of the structural components were studied using a Neophot-21 metallographic optical microscope, the chemical composition of the inclusions was estimated semiquantitatively using an MAR-2 x-ray microanalyzer, and the mechanical properties of the sintered materials were determined by indentation [8, 9] using a Vickers hardness meter. The microhardness H V and Kic were measured under a load of 98 N from four indentations and calculated by the formulas KI~
0.203 HVa 2 6"3/2
H V = 463.6.___._~P,
,
(1) (2)
a2
where P is the load, a is half the indentation diagonal, and c is the length of the radial crack. Results and discussion. Milling in a planetary activator for 5 - 20 min provides a powder with the requisite dispersity (Fig. 1). When the milling bodies are balls 7 mm in diameter, the specific surface of the powder attains its maximum value in 5 - 10 min; simultaneously, the oxygen content in the powder increases quite considerably. For milling bodies 9 mm in diameter the specific surface of the powder attains its maximum value in 1 0 - 20 min, but the powder is less contaminated with oxygen. Technological properties of the powders such as the density of the pressings, the density after shaking, and the bulk density improve with increase in the milling time (Fig. 2),
12
Fig. 3. Derivatograms of SisN4 powders before (a) and after (b) milling in a planetary mill for 20 min; the numbers at the curves denote the temperature, ~
which can be explained by enrichment of the powder mixau-e with heavy particles of milled iron. Intense milling changes substantially the physical state of the powder (Fig. 3). Before milling, it exhibits insignificant thermal effects on the DTA curve; no loss of mass was recorded (see Fig. 3a). After prolonged milling (20 rain) the curve behaves as in milling for 5 min, but the thermal effects on the DTA and TG curves are much stronger (see Fig. 3b ). The principal exothermal effect is observed at 3 0 0 - 500~ and is accompanied by an increase in mass. It can be explained by oxidation of the milled iron, which is detectable by an x-ray diffraction analysis. Powders of Si3N 4 have been studied in detail by the methods of IR spectroscopy; the spectra have been interpreted in correspondence with results of x-ray diffraction analysis, and the causes of multiplets have been discussed [6, 7, 10]. Spectra of Si6_zAlzOzNs__, sialons, where z takes values of 0, 0.25, 0.5, and 1, have been investigated by the Fourier method [ 11]. Figure 4 presents absorption spectra of powders milled for different times, and Table 1 presents the frequencies of the absorption bands, their assignment to a type of vibration, and published data on the frequencies for a composition with z = 0.25 [11], which corresponds approximately to the composition studied in our work that contains 5% AI203 and 5% AIN. The spectrum of the initial powder (see Fig. 4 has a wide (structureless) diffusion band in the region of v = 1000 - 700 c m - x (the half-width of the band is
356
V . N . A n t s i f e r o v et al.
TABLE 1. Results of an Investigation of IR Spectra of Si3N4 (SHS) Powders Band frequency, cm initial 3
Si3N4 milled for, rain
Si3N4
10
1070 1040 1000 - 750
1070 1039 960 946 912 720 . 577 . -
2
1
4~)0
600
1~00
800 v , c n'l
1200
-1
723 650 578 530 497 443
441
20 1070 1039 968 947 916 720 . . 578 . . 441
Assignment
sialon ( z = 0.25) [11] 1036 941 915 718
[3-Si2N4 to a type [7] of vibration 1075 1041 952 917 -
v3(F2) Si3N4 SiON3
v(AIN)
. 576.8
584
441.7
509 447
.
v4(F2)(~ ) Si2ON2
Fig. 4. IRabsorption spectraofSi3N4 (SHS) powders before milling (1)and after 10-min (2) and 20-min (3) milling in a planetary mill; v is the frequency. TABLE 2. Results of Sintering Si3N4 (SHS) with Different Additives Sintering Milling temperature,* time, ~ min 1600
1650
1600
1650
Loss of mass, %
Shrinkage, % over the over the height diameter
Si3N4 (SHS) with an additive of AI203 + AIN 5 8.5 9.5** 3.6 10 11.0 14.5 18.5 15 17.6 20.7 21.2 5 24.5 10 18.9 15 37.5 Si3N4 with an additive o f ZrO2 + AI203 + AIN 5 7.2 11.7 17.3 10 8.1 15.3 20.0 15 11.8 11.9'* 17.3 10 17.0 16.7 20.8 15 25.7 9.8 23.8
Apparent density, g/cm3 2.18 3.29 3.45 1.67 2.88
3.73 3.93 2.83 3.67 3.48
For a l-h hold at the sintering temperature. Fig. 5. Microstructure of a silicon nitride material with an additive of 5% AI203 + 5% AIN sintered at 1600~ with a 5-h hold after milling the mixture in an AGO-3 planetary activator for 10 min; x 500.
about 260 cm-l). T h e s p e c t r u m o f the milled p o w d e r 4 is m o r e structurized; the a b s o r p t i o n b a n d s b e l o w 900 c m - 1 disappear, a n d the h a l f - w i d t h o f the diffusion b a n d decreases to 160 c m - 1 . This b e h a v i o r o f t h e spectra c a n b e associated with the substantial structural d i s o r d e r i n g o f the initial p o w der [12] a n d the increase in the dispersity a n d o r d e r i n g after
Growth, %.
W e studied several s i n t e r i n g t e m p e r a t u r e s for t h e p o w d e r a n d o b t a i n e d the b e s t results at a c o m p a r a t i v e l y l o w t e m p e r a ture (1600~ H o w e v e r , the m i l l i n g t i m e a n d the c o m p o s i t i o n o f the charge turned out to b e quite i m p o r t a n t (Table 2). A d e v i a t i o n from the o p t i m u m m i l l i n g a n d s i n t e r i n g r e g i m e s decreases the shrinkage markedly or increases the loss o f mass, d e c r e a s i n g the density o f the s p e c i m e n s as the final result. T h e h i g h e s t density w a s e x h i b i t e d b y s p e c i m e n s w i t h
the m i l l i n g . It s h o u l d b e n o t e d t h a t in all the spectra studied
5 % A1203 + 5 % A1N (3.3 - 3.45 g / c m 3 ) a n d 3 0 % Z r O 2 +
we o b s e r v e d b a n d s at v equal to 720 a n d 970 c m - t, w h i c h
5 % A1203 + 5 % A I N (3.7 - 3.9 g / c m 3 ). A p e t r o g r a p h i c study s h o w e d that b o t h materials are virtually poreless. I n c l u s i o n s o f a light p h a s e are o b s e r v e d in the structure o f t h e m a t e r i a l a g a i n s t the gray b a c k g r o u n d o f the m a t r i x b a s e d o n Si3N 4
indicates the p r e s e n c e o f A1N a n d SiO f r a g m e n t s [11]. O n the whole, w e can c o n c l u d e that t h e structures o f the p o w d e r s are represented by solid solutions with substitutional defects [12].
(Fig. 5). T h e inclusions are 5 - 8 ~tm in size for t h e first c o m 4 The form of the spectrum is close to that described in [11].
p o s i t i o n and a b o u t 2 p m for t h e second. I n b o t h c o m p o s i t i o n s
Composite Materials Made f r o m P o w d e r s Milled in a P l a n e t a r y Mill TABLE 3. Results of a Study on the MAR-2 Microanalyzer Phase component Light grains Gray matrix
TABLE 4. Mechanical Properties of Si3N4 (SHS) with Different Additives
Ratio of the intensities of iron x-my radiation from structuralcomponents of the material and from a standard, %, for different process parameters* 10/1 10/5 15/1 15/5 55 18
51 24
38 19
357
62 21
* The duration of milling in the AGO-3 mill (min) is given in the numerator, the duration of sintering at 1600~ (h) is given in the denominator.
Duration of milling, of sintering rain at 1600~ h
HV, GPa
K~c' IVIPa 9m2/2
Grain size of the light phase, ~.m
Si3N4 (SHS) with an additive of AI203 + AIN 1 6.88 + 0.65 4.57 __.0.40 5 8.82_+0.32 5.17_+0.59 1 6.59 __+0.58 4.88 _+0.27 5 6.32 _+0.41 5.27 -+0.47 Si3N4 (SHS) with an addit•e of ZrO2 + A1203 + AIN 5 1 9.32_+0.57 5.96_+0.51 10 1 8.78 _+0.20 6.40 + 0.32 10 10 15 [5
8.6 5.8 5.3 7.9 2.3 1.8
the inclusions are m u c h smaller in size than the iron inclusions described in [5]. According to data o f an x-ray diffraction analysis the principal phases in both materials that formed after sintering
REFERENCES
are [3-Si3N 4 and FeSi. The ZrO2 additive is retained in the material after sintering too; just as after milling it is present in two modifications, namely, monoclinic and tetragonal (or cubic). The light phase in the microspecimens seems to be FeSi; its presence in sintered specimens is determined b y the x-ray method. Results of a microscopic x-ray spectral analysis o f microspecimens with an additive o f 5% A1,O 3 + 5% AIN on the MAR-2 confirm this conclusion. Table 3 presents ratios o f the intensity o f signals o f x-ray radiation o f Fe from different structural components to the intensity o f the signal from a standard. It can be seen that the intensity o f the signal from the light phase corresponds to inclusions with an SiFe composition. Thus, in contrast to the data o f [13] we observed the formation o f silicides in the sintering o f mixtures o f a similar composition with additives o f powder o f iron and alloys based on it. The difference in the results can be associated with the higher sintering temperatures and the dispersity and activity o f the particles o f iron and Si3N 4 formed in the process of high-energy milling in a planetary activator. It is obvious that FeSi is a more brittle inclusion than the plastic particles of iron and stainless steel used earlier for reinforcement [5]. The properties o f the obtained materials presented in Table 4 are comparatively high. K k increases with the duration o f the milling, whereas H V decreases. A n increase i n the hold time at 1600~ increases Kic insignificantly. Thus, the use o f high-energy milling in an A G O - 3 centrifugal planetary activator with hydrostatic holders affects substantially the properties o f the Si3N 4 powder, the sintering conditions, and the structure and properties o f the materials obtained on the basis o f it.
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