Silicon powder was ground in a steel attrition mill under nitrogen. Air exposed powder was compacted, prefired in helium, and reaction sintered in nitrogen-4 v/o hydrogen. For longer grinding times, oxygen content, surface area and compactability of the powder increased; and both alpha/beta ratio and degreee of nitridation during sintering increased. Iron content remained constant

Glasgow, T. K.

Herbell, T. P.

Yeh, H. C.

English

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https://ntrs.nasa.gov/search.jsp?R=19780023293 2020-03-22T03:13:45+00:00Z
DOE4qASA/1040-78/2
NASA TM-78965
(NASA-TA- 78965) EFFECT aF ATTBITIQN MILLING
	 N78-31236
010 THE REACTION SINTERING 07 SILICON t1IT8ICE
(NASA) 21 p HC A02/MT A01
	 CSCL 11G
Unclas
G3/27 30245
EFFECT OF
ATTRITION MILLING
ON THE REACTION
SINTERING OF
SILICON NITRIDE
T. P. Herbell and T. K. Glasgow
National Aeronautics and Space Administration
Lewis Research Center
and
H. C. Yeh
Cleveland State University
Work performed for
U.S. DEPARTMENT OF ENERGY
Office of Conservation and Solar Applications
Division of Transportation Energy Conservation
TECHNICAL PAPER presented at the
Ninetieth Annual Meeting of the
American Ceramic Society
Detroit, Michigan, May 7-12, 1978
NOTICE
This report was prepared to document work sponsored by
the United States Government. Neither the United States
nor its agent. the United States Department of Energy,
nor any Federal employees, nor any of their contractors,
subcontractors or their employees, makes any warranty,
express or implied, or assumes any legal liability or
responsibility for the accuracy, completeness, or useful-
ness of any information, apparatus, product or process
disclosed, or represents that its use would not infringe
privately owned rights.
I
SILICON NITRIDE
T. P. Her eii and T. K. Glasgc ar
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
and
H. C. Yeh
Cleveland State University
Cleveland, Ohio 44114
Prepared for
U. S. Department of Energy
Office of Conservation and Solar Applications
Division of Transportation Energy Conservation
Washington, D. C. 20545
Under Interagency Agreement EC-77-A-31-1040
Eig=ztieth Annual Meeting of the
American Ceramic Society
Detroit, Michigan, May 7-12, 1978
EFFECT OF ATTRITION MILLING ON THE REACTION SINTERING
OF SILICA NITRIDE
by T. P. Heibell, T. R. Glasgow and H. C. Yeh*,t,tt
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
Silicon powder was ground in a steel attrition mill under
nitrogen. Air exposed powder was compacted, prefired in helium,
and reaction sintered in nitrogen-4 v/o hydrogen. For longer
grinding times, oxygen content, surface area and compactability
of the powder increased; and both a/S ratio and degree: of
nitridation during sintering increased. Iron content remained
constant.
*Cleveland State University, Cleveland, Ohio 44114
tPresented at the 80th Annual Meeting, The American Ceramic
Society, Detroit, Michigan, May 10, 1978 (Basic Science
Division, No. 129-B-78)
ttMember, the American Ceramic Society
191'
MN
n
O^
W
r
-2-
INTRODUCTION
Silicon nitride ( i N ) exhibits properties that make it a
desirable material for high temperature engineering applications.
Reaction sintered Si N
	 68N), although lower in strength at
room temperature than hot pressed Si. N .
 (HPSN), has greater
strength above about 1350 °C and is more readily formed into
complex shapes.
Because the formation of RSSN involves the reaction of two
components, silicon metal and nitrogen gas, the rate of reaction
and possibly the completeness of the reaction are expected to be
influenced by the surface area and particle size of the silicon.
This is one reason for using fine starting silicon powder.
Another reason is that the size of the largest of the strength
limiting pores in the final product may be expected to correlate
with the size of the largest silicon particles in the starting
powder (1). Studies have been carried out using commercially
available "fine" silicon powder - 6 m (2) and 4 m (3). However,
much finer powders can be rapidly prepared by attrition milling, a
process in which powder is ground by repeated impact in a stirred
high energy ball mill.
The primary objective of this study was to determine the effect
of attrition milling of silicon powder on nitridation. A secondary
objective was to determine whether a reaction between silicon and
nitrogen could be promoted by milling in a nitrogen atmosphere.
Compacts prepared from as-received and attrition milled
silicon powders were reaction sintered in a nitrogen plus hydrogen
-3-
atmosphere. Comparisons were made on the bus is of chemistry,
density, metallography, and x-ray diffraction.
EXPERIMENTAL PROCEDURE
Fine powder was prepared by grinding as-receiv!.A -325 mesh
silicon powder in a steel attrition mill using 0.6 cm diameter
steel balls a q media. The attrition mill used in this study,
shown schematically in figure 1, was basically similar to a ball
mill but with the energy for comminution supplied by stirring the
balls rather than by tumbling. It was hermetically sealed and
capable of being vacuum leak-checked. In operation it was possible
to load and unload the mill without exposing the mill contents to
air. The media to powder weight ratio was 40:1. Milling was
performed under flowing nitrogen at a slight positive pressure for
periods of 1, 4, and 18 hours. All ground powders were pyrophoric
i.e. self-igniting upon air exposure. Therefore before air handling,
the powders were exposed to air at a controlled rate such that the
temperature and the powder bed did not exceed 200 °C. Exposure
was continued until the powders showed no further tendency for
self-ignition. This procedure limited the oxygen pickup to a lower
value then would have been observed if the powders had been allowed
to self-ignite. In addition, it provided powders that were air
handleable. Exposed powders were analyzed for oxygen carbon, nitrogen,
and iron content. And their specific surface areas were determined
by the 3-point BET technique.
-4-
The silicon powders were cold foLmed without the use of binders
into disks 1.3 cm in diameter and 0.5 cm high by pressing at 170
MPa in a single acting die followed by isostatic pressing at 500 MPa.
Sample compacts were fractured and examined at this point by
scanning electron microscopy (SEM).
Compacts of as-received and of ground silicon powders were
presintered for four hours at 1150 °C in a flowing atmosphere of
purified helium. Weights and densities were determined after
this step.
For nitriding, the compacts were supported by 0.5 cm diameter
RSSN spheres in a high purity Al203 boat and placed in the central
15 cm of a tube furnace with temperature variation of +2 °C over
this length. The furnace was equipped with a 9.0 cm diameter high
purity Al 0 tube and silicon carbide heating elements. The
furnace was sealed, pressurized, and leak checked; then a slow flow
of purified nitrogen with 4% hydrogen was established. Heating
to 1390 °C (20 °C below the melting point of pure Si) was conducted
over a period of 140 hours according to the nitriding schedule
selected from the literature (4) and shown in figure 2. Notice that
this nitridation program consisted primarily of a 70 hour hold at
1150 °C followed by a slow continuous heating to 1390 °C without
holding, thus the time at highest temperature was minimal.
After nitridation the samples were weighed and their densities
determined by vacuum mercury displacement. Nitrided samples were
also examined by light microscopy and x-ray diffraction. Relative
-5-
peak intensities of the phases detected were measured from the
diffraction patterns.
RESULTS AND DISCUSSION
Chemical analysis and surface area of the as-received and of
the ground silicon powders (after controlled air exposure) are
shown in Table I. With increasing grinding time there was a
significant increase in oxygen content, a moderate increase in
nitrogen content, and essentially no change in either carbon or
iron content. During grinding all of the steel hardware became
coated with an adherent shell of silicon. The presence of this
coating limited the contact between the silicon powder and the steel
hardware, thus preventing iron contamination.
Surface area of the silicon powder was increased from 3.0 m2g 1
for as-received material to 23.3 m 2g 1 for material ground 18 hours.
The surface area increased rapidly during the first hour of
grinding and more gradually thereafter. All of the ground powders
were pyrophoric prior to the controlled stir exposure. Figure 3
shows the effect of attrition milling on the morphology of the
silicon powder as observed by scanning electron microscopy of
fractured presintered compacts. The as-received powder contained
a considerable volume fraction of large particles along with a
relatively small amount of fines. After only one hour of grinding
the majority of the large particles had been reduced to a very fine
size. Continued milling for 4 and 18 hours further reduced both the
number and size of the large particles. All of the ground powders
-6-
were characterized by the presence of large agglomerates of vsry
fine particles.
As received silicon powder is difficult to compact without the
use of binders; i.e., the compacts are very fragile and friable.
The ground powders used in this study, however, produced well defined
compacts which could be readily handled.
As shown in Table II, however, under the compaction conditions
employed the powder ground 18 hours (finest powder) compacted to a
lower density - (1570 kgm 3) than did either the as-received
(1720 kgm 3 ) or the powders ground 1 and 4 hours (1700 to 1740 kgm 3).
Nitrided density was higher for compacts prepared from ground
powders - 2520 to 2560 kgm 3 - than for compacts prepared from
as-received powder - 2120 kgm 3 . F;gure 4, a plot of surface area
versus weight gain illustrates a consistent arts significant increase
in percent weight gain during nitriding as surface area increase.
However, the weight gain did not increase linearly with increasing
surface area. This indicated that some of the grinding was redundant
as may to understood from the following discussion.
Atkinson et al (5) have scudied the nitridation of pure silicon
powder and proposed a model involving three sequential steps: 1) Si3N4
nuclei form on particle surfaces, 2) the nuclei grow laterally to
eventually cover the whole surface (the thickness of the Si 3N4 coating
being a function of the temperature and pressure during nitridation)
and finally, 3) when the entire surface is covered by Si 3N4 nitridation
essentially stops. Although this model was formulated fz,-m observatcns
V-7-
of nitri3ation of iron-free silicon powder, the same mechanism
appear to be applicable to this work.
Based on the model described above, all the silicon particles
of a size as small as approximately twice the thickness of the
}
	 Si3g4 layer (critical size) would nitride completely. Particles
larger than the critical size will not nitride completely, but will
have an interior of unreacted silicon surrounded by a SiA shell.
Any particles smaller than the critical size will also nitride
completely, but will have been ground unnecessarily fine. Thus
increased specific surface area may not result in proportionate
increases in amount of nitride formation.
Figure 5 shows the relationship between initial powder surface
area and density of both presintered and nitrided compacts. The
importance of the fine powder size (higher surface area) in the
achievement of more complete nitridation and associated, higher
percent we..ght gain, is apparent.
Typical light micrographs of the four nitrided materials are
shown in figure 6. Residual or unnitrided silicon appears as the
white phase in the micrographs. Most obvious is the decreased
residual silicon content with increased grinding time and the
fineness of the residual porosity in the 18 hour ground material.
A few large silicon particles as well as agglomerates of smaller
silicon particles are still present even in the samples prepared 	 I
from the 18 hour ground material. There was no appreciable difference 	
i
in appearance between the surface and interior of any of the nitrided 	 3
i
compacts which is a fair indication that nitrogen was able to get into
-8-
the interior at all times. The largest pores observed were associated
with partially unreacted silicon particles as shown in figure 7, a
sample prepared from ground 18 hours. The pores are the black phase
in this figure. X-ray phase analysis results are listed in Table III.
Only silicon, alpha Si3N4 , and Beta Si3N4
 were detected by x-ray dif-
fraction. The trend of decrease in relative amount of silicon (or
the increase in nitride formation) as the grinding was increased is
consistent with the weight gain and light micrographic results already
presented. Of particular interest is the increase in alpha/beta-Si3N4
ratio with grinding time. There are at least two possible explanations
for this.
Amato et al (6) and Jennings and Richman (7) showed that the
increasing alpha/beta-Si 3N4
 ratio during nitridation was related to
the increasing oxygen content of the powder. In this investigation
the oxygen content of air exposed powder increased with grinding time
(Table II); therefore it could be expected that the alpha/beta-Si3N4
ratio observed in this investigation would also increase with grinding
time. Amato et al (ref. 6) attribute this correlation between
alpha-Si 3N4 being an oxynitride (Sill.5 N15 00.3) (8) containing more
oxygen in its lattice than beta-Si 3N4.
The finer particle size derived from extended grinding time may
also contribute to increased alpha/beta ratio in the following
manner. Jennings and Richman (7) have pointed out that lower
reaction sintering temperatures favor the formation of alpha phase.
Smaller silicon particles should be completely reacted at lower
-9-
temperatures. And the nitriding schedule used allowed tier: for
reaction at low temperatures. So increased grinding time may
result in increased alpha/beta ratio both by increasing the
oxygen content and by reducing the silicon particle size.
CONCLUSIONS
The results of this study on the nitriding of attrition
milled silicon powder lead to the following conclusions:
The surface area of silicon powder can be greatly increased
by dry grinding in a steel attrition mill without iron or carbon
contamination.
Using a nitriding cycle in which the melting point of silicon
was not exceeded, it was possible to achieve considerable nitride
formation in a compact prepared from the finest ground powder.
Similar compacts made from powders containing larger amounts of
unground material contained considerably more unreacted silicon.
The grinding conditions utilized did not eliminate all of the
large particles present in the starting silicon powder.
The alpha to beta-Si 3N4 ratio of nitrided compacts increased
with increased grinding time of the silicon powder. This mey be
attributed to either (or both) an increased oxygen content or a
decreased particle size of the ground silicor. powder.
The attrition milling approach appears to offer potential for
industrial application in the preparation of reaction sintered
silicon nitride. In the final analysis, however, modulus of rupture
strengths must be determined. And we are now in the process of
obtaining this data.
M t
-10-
REFERENCES
1. A. G. Evans, and R. W. Davidge, "The Strength and Oxidation of
Reaction-Sintered Silicon Nitride," J. Mater. Sci., 5 314-325
(19/O;.
2. B. F. Jones, and M. W. Lindley, "Strength, Density, and Nitrogen
Weight Gain Relationships for React+on-Sintered Silicon Nitride
prepared from Fine Silicon Powders," Powder Metall. Int. 8 (1)
32-34 (1970.
3. H. R. Baumgartner, "Effect of Silicon Microstructure. Upon Nitridation
and Strength of Rea-tion Bonded Silicon Nitride," Proceedings -
DAPA * ,IAVSEA-Garrett/Airesearch Ceramic gas Turbine Engine Demon-
stration Program Review, Castine, Maine, August 1977 (To be pub-
lished)
4. J. A. Mangels, pp. 569-574 in Nitrogen Ceramics. Edited by F. L.
Riley.	 lhoff, Netherlands, 1977.
5. A. Atkir.con, A. J. Moulson, and E. W. Roberts, "Nitridation of High
%urity Silicon," J. Am. Ceram. Soc., 59 (7-8) 285-289 (1976).
6. I. Amato, D. Martorana, and M. Ross:, "The Nitriding of Silicon
Powder Compacts," Powder Metall., 18 (36) 339-348 (1975).
7. H. M. Jennings, and M. Richman, "Structure, Formation Mechanisms
and Kinetics of Reaction-Bonded Silicon Nitride," J. Mater. Sci.,
11 2087-2098 (1976).
8. S. Wild, P. Grieveson, and K. H. Jack, pp. 385-396 in Special Ceram-
ics, Vol. 5. Edited by P. Popper. British Ceramic Research Asso-
ciation, London, 1972.
TABLE I. - CHEMICAL ANALYSIS AND SURFACE AREA OF SILICON POWDERS
Material Oxygen, Carbon, Nitrogen, Iron, N Surface
wt % wt 2 wt x wt % area,
m2/g
As-received 0.60 0.03 0.004 0.62 3.0
1 hr grind 2.29 0.04 0.017 0.59 11.5
4 hr grind 2.57 0.04 0.028 0.58 14.5
18 hr grind 3.67 0.04 0.037 0.55 23.3
TABLE II. - DENSITY AND WEIGHT GAIN OF SILICON COMPACTS
Material Sintered Nitrided :!ight gain
density, density, on nitriding,
kg/m 3 kg/m 3 %
As-received 1.72x103 2.12x10-3 26
1 hr grind 1.74 2.56 45
4 hr grind 1.70 2.56 50
18 hr grind 1.57 2.52 58
1 hr, 1150 C, helium.
TABLE III. - RELATIVE X-RAY PEAK INTENSITIES IN
NITRIDED SILICON COMPACTS
Material Amount of phase present
Residual
Si aSi3N4 SSi3N4 a/0 ratio
As-received 77 13 10 1.3
Ground 1 hr 68 23 9 2.6
'Ground 4 hr 40 44 16 2.8
Ground 18 hr 14 72
t
14 1	 5.1
ORWAL PAGE
in PWIt OJALM
N2
ills
Attrition mill
Figure L - Attrition mill (schematic).
1600
1400	 SILICON MELTING POINT
1200C
1000k
s
	
Rrf	 I	 I	 I	 I	 I	 I	 I
	
0	 20	 40	 60	 so	 100	 120	 1611
TIME, hr	 CS- 7e-1227
Figure 2. - Nitriding schedule.
V
E
R60
F.
t
u
a
2
Q
zv
AS RECEIVED	 MILLED 1 hr	 ===r
Figure 3. - As received and attrition milled silicon powder; SEM; X1D ODD.
70F MAX POSSIBLE WEIGHT GAIN
0
SURFACE AREA, mzlq 	 ('.F_ 
Figure 4. - Nitridation weight gain as a function of silicon powder
surface area for compacts prepared from ^s-received and ground
powders.
i	 I
1
q NIfRIDED 110 hr IN NITROCEN-4%HYDROGEN
u PRESINTTRED 4 h AT 115& C IN HEIIUM
Z null 3
2.
23
^..	 WT GAIN
E
21	 SBS50%	 WT GAIN
YL+	 WTI IN
^	 26ri	 it
1.9 WT GAIN
1.
1.5	 1	 1	 1	 1	 I
0	 S	 10	 15	 20	 a
cS _, r _.	 SPECIFIC SURFACE AREA. Mn q
Figure 5. - Relation of presintered and of nitrided
compact densities to specific surface area of
starting powders. d!
-PACE IS
PMR avMM
-l.
AS RECEIVED	 ~
mym
GROUND I hr	 AD P Mgym
r<-	
- GROUND d hr 	 m y m	 GROUND 16 hr 	 40 jjm
Fi qure e, - Microstrucit a of nitrided compacts prepared from "as received" and
from attrition milled sillcon R*wder.
Figure 7. - Porosity associated with unreacted silicon present in nitrified
sample prepared from silicon powder ground 18 hours; X250.


Effect of attrition milling on the reaction sintering of silicon nitride

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