Sintering was performed in He for 16 hours at 1200, 1250, and 1300 C. Compacts of as-received Si did not densify during sintering. Milling reduced the average particle size to below 0.5 micrometer and enhanced densification (1.75 g/cc). Leaching milled Si further enhanced densification (1.90 g/cc max.) and decreased structural coarsening. After sintering, the structure of the milled and leached powder compacts appears favorable for the production of reaction bonded silicon nitride

Glascow, T. K.

Herbell, T. P.

Yeh, H. C.

English

NASA Technical Reports Server

3 1176 00169 0396
NASA Technical Memorandum 81602 NASA-TM-8160219810004655
Effect of Milling and Leaching on
the Structure of Sintered Silicon
H. C. Yeh
Cleveland State University
Cleveland, Ohio 44115
and
T. K. Glasgow and T. P. Herbell
Lewis Research Center
Cleveland, Ohio :' ,_._:."" _:"._ _:L!t)::,_ .. ,: _',(
k._ ,. _ . _.-,_ ,7, ,_ - . ::-,', .,._:..,_
, r_ ..... ,, a
,,_,_,,_: . _,_:'_%Prepared for the ........'"" ' ' ,
Annual Meeting of the American Ceramic Society
! Chicago, Illinois, April 28-30, 1980
https://ntrs.nasa.gov/search.jsp?R=19810004655 2020-03-21T14:39:20+00:00Z
EFFECTOF MILLING AND LEACHINGONTHE STRUCTUREOF SINTEREDSILICON
by H. C. Yeh
Cleveland State University
Cleveland, Ohio 44115
and
t
T. K. Glasgow and T. P. Herbell
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
The effects of attrition milling and acid leaching on the sintering
behavior and the resultant structures of two commercial silicon powders were
investigated. Sintering was performed in He for 16 hours at 1200° , 1250° ,
and 1300° C. Compacts of as-received Si did not densify during sintering.
, Millingreducedthe averageparticlesize to below 0.5 _m and enhancedaen-1 t
sification (1.75g/cc). Leachingmilled Si furtherenhanceddensification
(1.90 g/cc max.) and decreasedstructuralcoarsening. After sintering,the
structureof the milled and leachedpowder compacts appearsfavorablefor
the productionof reactionbonded siliconnitride.
INTR(JUUCTIL)N
Sometimesin the productionof reactionbonded siliconnitriae (RBSN)
the compactedsilicon (SiJpowder is sinteredbefore it is nitrided. It is
technicallydesirableto performthese steps independently,because some
" shrinkage ooes occur in the sintering step. Resultant distortions can be
eliminated inexpensively by machining the sintereo bi with conventional
tooling, while after nitriaation Giamond grinding must be employed. Fortu-
nately almost no dimensional change occurs during the nitri_ing step.
It is reasonable to expect that the structure and the properties of
RBSNare dependent on the structure ano uensity of its precursor. While
there have been numerous publications on the subject of RBSN,I few have
diScussed the control of structure of the sintered Si compacts relative to
_ the properties of the final RBSN. A recent paper by H. R. _aumgartner 2
discusses some aspects of the relations between the structure of Si compacts
and the final RBSN. Two eariier papers investigated the effect of nitrogen
permeability through silicon compacts on the degree of nitridation, but the
conclusions were contradictory. 3'4
The objective of this study was to determine the effect of variations
in powder processing on the sintering behavior of bi powder compacts.
Comminution, in the form of wet attrition miiling, and acid ieaching were
used to vary the nature of the Si powders useo. Test bars prepared from two
commercia]Si powders in the as-received,miilea, and mil|eo and ieached
conditionwere sintereain a helium atmosphere. Evaluationof the powders
and the sinteredspecimensincludedchemicalanalysis,density,surface
area, mercuryporosimetry,metallography,and room temperaturebend strength.
EXPERIMENTALPROCEDURES
Two commercialSi powders*,designatedKBI ano UC respectlvely,were
used in this study. Each of the two as-receivedpowderswas wet attrition
milled for 4 hours in a stirredball mill using hardenedsteel balls as
media apd heptanceas a millingfluid. The milled powderswere designated
KBIM and UCM. Attrltionmilling of Si was oescribed in detail in an earlier
paper.5 The milled powderswere furthermodifie_ Dy acid washing (leach-
*Suppliedby Kawecki-berylcoIndustries,P.U. box 14bZ, Reading,PA 196U3
and Union Carbide,Marietta,OH 4575U.
2
ing) to proGuce two additional powders designated K_IML and UCML. The ef-
fect of leaching as-received powder was not determined. Leaching was _one
using an !:1 volume ratio of concentrated HL1 anu Uistilled water. 7U grams
of milled powder were stirred into 770 ml of the acid solution. After
° 24 hours of leaching with intermittent stirring, the powder was recovered by
filtering (S a S No. 565 analytical filter paper). The residue was rinseo
L
with distilledwater until no acid was detected by pH paper. After air dry-
ing, the powderwas sievedthrough a 70 mesh sieve.
Iron (FeJ,carbon (C) ano oxygen (0) contents of each as-received,as-
milled, and as-milledand leachedpowder were determinedby chemicalanaly-
sis. Powder surfaceareas were oeterminedby the three point BET (Brunauer,
Emmett,and Teller}technique.
The as-receivedKBI and UC powderscould not be compactedby uniaxial
pressing. However,cy]indricalbars about 1.3 cm diametercou]d be made by
hydrostaticpressing in a rubber sleeve at 350 MPa. Forconvenienceot
evaluationand testing,these cylindricalbars were cut into rectangular
bars approximately0.3 by 1 by 5 cm using a diamonusaw prior to sinterin9.
The attrition-milledand the milled and leachedpowderswere easily die
pressed into bars of approximately0.3 by 0.9 by 7.5 cm at 60 MPa. These
bars were subsequentlyrepressedhyurostaticallyat 350 MPa to increase
their strength.
in order to minimize the weight loss the bars uuring sintering,samples
were buried in looseK_I Si powders in a high purity Al203 boat and then
were loaded in an Al203 tube furnacewith provisionsfor a flowinyheli-
um atmosphere. Two bars of each powder were put in the same boat anu sin-
tered at the same time. A more detaileodescriptionof the sinteringfur-
6
nace and procedurescan be found in an earlierpubliGation.
Bars were sintered for 16 hours at 12UO°, 125Uo and 1300° C. Tempera-
ture was increased from ambient to the sinteriny temperature at a linear
rate over a period of 4 hours.
Weights and dimensions of the bars were measured before and after each
s i nteri ng run. Dens ity was ca lcu 1ated from these measurements. Two bend
test specimens were cut from each sintered bar. Room temperature tour-pOint
bend strength was determined for all with their surfaces in the as-sintered
condition samples. Light microscopy, mercury intrusion porosimetry analy-
si s, and chemical analyses were jJertormed on most samp"les before and after
sintering. Samples for light microscopy were infiltrated with epoxy resin
prior to polishing.
Reported densities and BET surface areas are an average of two deter-
minations, strepyths are an averaye of four. One specimen was used for each
mercury intrustion porosimetry and chemical analysis.
RE~ULTS AND UlSCUSSlON
To show the effects of milling and milling ana leaching, Table 1 lists
the Fe, C, 0 contents and Table 11 lists the BET specific areas of the Si
powders under investigation. For convenience of comparison, also listed in
Table 11 are the particle sizes calculated from the corresponaing measured
BET surface areas assuming uniform spherical particle shape. It is apparent
that milling orastically increased the impurity contents and the specific
surface areas of the as-received powders; and leaching of the rllilled powders
significantly reauced iron content, but only slightly altered the surface
area. The varia~ions in impurity contents and surface areas lor equivalent
particle sizes) will be correlated with the sintering behaviors of the pow-
ders and the structures of the sintered materials to be presented in the
following aiscussion.
4
The densities of silicon specimens, green and after sintering in helium
for 16 hours at 1200°, 125Uo, and 1300° C, are shown in a block diagram in
Fiy.1. In the diagram the specimens are groupea into two families in
accordance with the starting powder type. In each family, M stands for
specimens maae from mi llea powders, and ML stands for specimens made from
milled and leached powders. Each row corresponds to a given sintering tem-
perature marked on the left tace of the block and the as compacted state
(uns;ntered) labeled IIGreen. 1I The densities shown in Fig. 1 range from a
minimum of 1.35 to a maximum of 1.90 glcc, corresponding to from 58 to
82 percent theoretical density of silicon (2.33 g/cc). The 1.90 y/cc speci-
men has the potential of forming a 3.15 glcc Si3N4 body corresponding to
99 percent of the theoretical density of B-Si 3N4 (3.18 g/cc); this is
based on the theoretical weight gain value, 66 percent, for a complete con-
version of 5i ~o Si3~4' A more generally observea weight gain is 60
percent; a corresponding final density would be 3.04 glee or 95.6 percent at
theoretical. The ability of such dense sintered silicon samples to be
tully nitrided is currently under investigation.
As-received K~I and UC powders did not exhibit appreciable densifica-
tion. This is in agreement with the predicted behavior of 5i powder with a
particle size larger than about 0.5 micron.? The milled ana the millea
and leached KBI and UC powders (designated K~IM, K~IML, UCM and UCML), all
have a particle size in the submicron range (Table II). These all showea a
continued increase in density with sintering temperature as might be ex-
pected for Si powders less than 0.5 ulII (Jiameter. More interesting to note
is the higher degree of densification with increasing temlJerature of KBIML
and UCML specimens than of their K81M an(J UCM counterparts.
5
One of the reasons tor the CGlllllIOn practice of sinteriny Si powder com-
pacts prior to nitriding is to increase their strength for ease of handlin~
during machlning into a desired shape. The average bend strengths of the
sintered bars are plotted in a olock diagram, Fig. 2. The measurea
strengths of the sintered bars ret"lect the bond (or neck) formation between
particles. The formation of necks between adjaCent particles during sinter-
ing of covalently bonaed materials coula be caused by mass transport through
evaporation-condensation and surface diffusion mechanisms, which do not re-
sult in aensitication, as well as by volume and grain bounuary diffusion
mechanisms, which do lead to densification. 7 The phenomenon of forming
necks without densification is clearly reflected by the steady increase in
strength with temperature in both the K1H and UC samples, Fig. 2, which did
not show appreciable densification, Fig. 1. In contrast milled and milled
and leached specimens showed increases in both strength and density, Figs. 1
and 2. Leaching appearea to enhance both densification and strength devel-
opment.
The changes in sintered bar strength and density were reflected also in
the porosirnetry measurements. For the purpose of comparison the character-
istic pore size, defined as that pore size corresponding to 50 volume per-
cent mercury penetration, was used. This characteristic pore size is
plotted as a function of sintering temperature in Fig. 3. Note that the
pore sizes of the four nli J lea and the four mi l·led and leached powder COITl-
pacts were almost identical at room temperature (the green state). The pore
sizes of these four samples increased, with some spreau, from about U.12 to
about 0.22 urn after sintering at 1200° C. The pore size continued to in-
cr~ase with sintering temperature at ditferent rates, UCM having the highest
and KBIML having the lowest. We believe it is significant that the two
6
leached powder con~acts (K~lML and UCML) exhioited less pore growth than the
unleached counterparts (KBlf"1 and UCfvl). This lower pore growth exhibited by
the leached powder, relative to the unleached counterpart, is consistent
with the higher degree of densification of the leached powder results pre-
sented earlier in Fig. 1. Another interesting observation on the four finer
powders (KBIM, KBIML, UCM and UCML) is that the degree ot pore growth of the
samples increased with the Fe content (Table I). The significance of this
parallel trend between Fe content and degree of pore growth requires more
detailed experimental lnvestigation before a definite correlation can be
made.
Pore size was also measured on two KBI powder compacts and all the UC
powder compacts. The trend of an increase in pore size with sintering tem-
perature is also evident in Fig. 3.
The effects of milling and of milling and leaching on the micro-
structures of the sintered samples are shown in ~g. 4, Which compares the
microstructures of UC, UCM, and UCML after sintering at 1300° C for
16 hours. Simil ar Illl crostructura 1 beha vi or was also observed among KBI,
KBIM and KBIML samples. The variation in observed pore structures was in
agreement with the paras imetry reSLJ lts presented in Fig. 3. The retent ion
of fine pore and particle size in the leached pOWders, even after sintering
for 16 hours at 1300° C, was consistent with the increased degree of shrink-
age observed for leached powders.
CONCLUSION
Attrition milling, as a powder preparation procedure, enhanced the den-
sification during sintering of two commercial silicon pOWders. Acid leach-
ing further enhanced the densification of the milled powders. Increasing
sintering temperature increased the density of milled and milled and leached
7
powder compacts but compacts made from as-received powders coarsened without
densification. Atter sintering the milled and leached silicon material ex-
hibited a uniformly fine structure with densities up to l.~U g/ec, which may
be beneificial in subsequent nitridation.
RE FERENCES
1. O. 1<. Messier, and ttl. t'l. Murphy; IIAn Annotated Bibliography on Silicon
Nitride for Structural Applications,1I MCIC-77-29, Metals and Ceramics
Information Center, 1977.
2. H. R. Baumgartner; "Effect of Silicon Microstructure upon Nitridation and
Strength of Reaction Bonded Silicon Nitride," pp. 273-284 in
Proceedings of the 1977 OARPA/NAVSEA Ceramic Gas Turbine Demonstration
Engine Program Review. Edited by J. W. Fairbanks and R. W. Rice.
Battelle Columbus Laboratories, Columbus, Ohio, 1978. Metals and
Ceramics lnfonnation Center report MCIC-78-36.
3. A. Atkinson, P. J. Leatt and A. J. Moulson; "The Role of Nitrogen Flow
into the Nitrioing Compact in the Production of Reaction-Sintered
Silicon Nitride," pp. 253-274 in Ceramics for Turbines and Other
High-Temperature Engineering Applications, Proc. no. 22. Edited oy O.
J. Godfrey. British Ceramic Society, Stoke-On-Trent (England), 1973.
4. 1. Amato, O. Martorana, and M. Rossi; "The Nitriding of Silicon Powder
Compacts," Powder Metallo, 18, (36) 339-348 (1975).
5. T. P. Herbell, T. K. Glasgow, and N. J. Shaw; "Reaction Bonded Silicon
Nitride Prepared from wet Attrition-Milled Silicon," NASA TM-81428,
1980.
8
6. N. J. Shaw, and T. K. Glasgow;"_rmation of Porous SurfaceLayers in
Reaction BondedSilicon NitrideDuringProcessing,"NASA TM-81493,1979.
7, A. L. Stuijts; "Basicand PracticalAspects in SinteringNitrogenCeram
ics," pp. 331-350in NitrogenCeramics. Eaitedby F. L. Riley.
Noordnoff,Leyden, 1977.
9
TABLEi. - Fe, 0 ANOC CONTENTSIN Si PUWbERS,
WEIGHTPERCENT
KBI* KBl_i KBIML UC UCM UCML
0.36 3.53 1.28 O.b6 4.93 1.75
0 .13 2.96 3.3u 17 4.11 4.36
C .085 .77 .57 .3 .7 .42
*KBI : as-received Kawecki-Berylco 3i powder.
KBIM : milled KBI.
KBIML : milled and leached KBI.
UC = as-received Union Carbide Si powder.
UCM: milled UC.
UCML= mi]led and leached UC.
TABLE II. - SPECIFICSURFACEAREA AND PARTICLESIZE
OF Si POWDER
KBI* KBIM KBIML UC UCM UCML
BET surface 0.5 11.42 13.04 2.99 12.33 8.28
area, M21g
Particle 5.20 .22 .2U .90 .21 .30
size, um**
*KBI : as-received Kawecki-Beryleo Si powder.
KBIM = milled KBI.
KBIML = milled anu leacheaKBI.
UC = as-receivedUnion CarbideSi powder.
UCM = milled UC.
UCML: milled ana leachea UC.
**Equivalent spherical particle size calculateo from bET
surface area.
I0
2.0_
I.90
• ___"1.5
SINTERING",.[.J I I I
TEMP.,°C _ _ .;
KAWECKI-BERYLCO UNIONCARBIDE
KBI-as-received;M -milled
UC-as-received;ML -milledandleached
FigureI.-DensityofSipowdercompacts,greenandafterslntering
for16hours.
I00... 102
93
_. 80 ._
"" 41}
LI
SINTERING
TEMP.,°C _-,,._--_ _ _,-
KAWECKI-BERYLCO UNIONCARBIDE
KBI - as-received; M - milled
UC - as-received; ML- milledandleached
Figure2. - Bendstrengthsof silicon samples interedfor 16hours
at 1200,1250,or 1300o C.
(1300o Co7.3_m}
0_ C,8.06pm)(1300o1.2--
KBI
uc
l.(--
E--L °_ --
N
cJ ,_--
(.3
< .4--
(.J
°_--
I I I I0 RT 1200 1250 1300
SINTERINGTEMPERATURE,°C
Figure3. - Characteristicporesizeversussinteringtempera-
ture (16hr.) UC-as-receivedunioncarbideSi powder;
UCM- milledUC; UCML= milledandleachedUC; KBI -
as-receivedKawecki-BerylcoSi powder;KBIM- m_lledKBI;
* KBIML- milledandleachedKBI.
(a) AS -RECEI VED UC 5ILI CON P = 1. 67 g/cm3. (bl AITRITION MILLED UC SILICON lUCMl
p • 1. 71 g/cm3.
(c) AITRITI ON MI LLED AND LEACHED UC 51 LI CON
lUCM) p = 1. 74 g/cm3•
Figure 4. - Effect of milling and milling and leaching on UC (union carbide) Si Powder compacts sintered for 16 hours at 13000C(M= milled; ML= milled
and leachedl.
1. Report No. 2. Government AccessionNo. 3. Recipient's CatalogNo.
NASA TM-81602
4. Title and Subtitle 5. Report Date
EFFECT OF MILLING AND LEACHING ON THE STRUCTURE
OF SINTERED SILICON 6. PerformingOrganizationCode
7. Autl3or(s) 8. Performing Organization Report No.
H. C. Yeh, T. K. Glasgow and T. P. Herbell E-591
]0. Work Unit No.
9. Performing Organization Name and Address
NationalAeronauticsand Space Administration
Lewis Research Center 11. Contract or Grant No.
Cleveland , Ohio 44135
13. Type of Report and Period Covered
12. Sponsoring AgencyName and Address Technical Memorandum
NationalAeronauticsand Space Administration
Washington,D.C. 20546 14. SponsoringA encyCode
15. Supplementary Notes
H. C. Yeh, Cleveland State University, Cleveland, Ohio 44115, T. K. Glasgow, and
T. P. Herbell, NASA Lewis Research Center, Cleveland, Ohio. Prepared for the Annual
Meeting of the American Ceramic Society, Chicago, Illinois, April 28-30, 1980.
16. Abstract
The effects of attrition milling and acid leaching on the sintering behavior and the resultant
structures of two commercial silicon powders were investigated. Sintering was performed in
He for 16 hours at 1200°, 1250 °, and 1300 ° C. Compacts of as-received Si did not densify
during sintering. Milling reduced the average particle size to below 0.5 /_m and enhanced
densification (1.75 g/cc). Leaching milled Si further enhanced densification (1.90 g/cc max. )
and decreased structural coarsening, After sintering, the structure of the milled and leached
powder compacts appears favorable for the production of reaction bonded silicon nitride.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Silicon;Siliconitride;Sintering; Unclassified- unlimited
Milling;Leaching;Nitridation STAR Category 27
i 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price"
Unclassified Unclassified
* ForsalebytheNationalTechnicalInformationService.Springfield.Virginia22161
Trade names or manufacturer'snames are use_ in tnis report for identifica-
tion only. This usage eoes not constitutean officialendorsement,either
expressedor implied,by the NationalAeronauticsand Space Administration.
National Aeronautics and SPECIAL FOURTH CLASS MAIL Postage and Fee_ Paid
Space Administration BOOK NationalAeronauticsandSpace Adminittration
NASA-451
Washington, D,C.
20546
Of ! icl,31 Business
Penalty for Private Use, $300
x
LIBRARY
NASA L
LANGLEY RESEARCH CENTER
HAMPTON, VA 23365
N_r_ A if Undeliverable (Section 158
POSTMASTER: Postal Manual) 1)o Not Return


Effect of milling and leaching on the structure of sintered silicon

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server