Processing of Ceramic and Metal Matrix Composites. Proceedings of the International Symposium on Advances in Processing of Ceramic and Metal Matrix Composites, Halifax, August 20–24, 1989

466 Pages·1989·28.6 MB·English

by  Hamid Mostaghaci (Eds.)

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OF NON-FERROUS METALLURGICAL PROCESSES
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METALLURGICAL INDUSTRIES
Ruddle ACCELERATED COOLING OF ROLLED STEEL
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EXTRACTIVE METALLURGY
Twigge-Molecey PROCESS GAS HANDLING AND CLEANING
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ACTA METALLURGICA
CANADIAN METALLURGICAL QUARTERLY
MINERALS ENGINEERING
SCRIPTA METALLURGICA
PROCEEDINGS OF THE INTERNATIONAL SYMPOSIUM ON
ADVANCES IN PROCESSING OF CERAMIC AND METAL MATRIX
COMPOSITES, HALIFAX, AUGUST 20-24, 1989
Processing of Ceramic and
Metal Matrix Composites
Editor
H. Mostaghaci
Atlantic Research Laboratory
National Research Council
Halifax, Nova Scotia
Symposium organized by the Materials Engineering Section of
The Metallurgical Society of CIM
28th ANNUAL CONFERENCE OP METALLURGISTS OF CIM
28e CONFÉRENCE ANNUELLE DES MÉTALLURGISTES DE L'ICM
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Copyright © 1989 by The Canadian Institute of Mining and Metallurgy
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Foreword
With the growing interest in the use of ceramics as structural components, and the need to use
them in combination with metals (both for joining purposes and as the reinforcement material
for metallic matrices), it has become important to re-evaluate ceramic-metal compatibility and
properties of cermet products. The goal is to combine the advantageous properties of both metals
and ceramics.
The suggestion that a symposium on Advances in Ceramic and Metal Matrix Composites would
be an excellent forum within the Annual Conference of Metallurgists of CIM arose out of discus­
sions with colleagues in the General Organizing Committee of the 28th Annual Conference of
Metallurgists in early 1988. There was no doubt that such a meeting would be both very valuable
and timely. Scientific and technological achievements in the field of both ceramic and metal matrix
composites have become quite considerable in recent years. For a subject of this nature having
both an academic and a strong technological content, close contact with relevant ongoing work
in research institutes, and university, industrial and government laboratories was required.
As a result of this effort, the present book was compiled. It contains the papers presented at the
First International Symposium on Advances in Processing and Application of Ceramic and Metal
Matrix Composites, held in conjunction with the 28th Annual Conference of Metallurgists of CIM,
Halifax, August 20-24,1989. It represents the current achievements of two of the most important
fields in materials research, and serves as an indicator of the interactions between ceramics and
metals.
Now it is time to say thanks to all whose help has made the program and the proceedings a suc­
cess. Many people come to mind. I am very grateful to Dr. Roger Foxall, Director of the Atlantic
Research Laboratory, for his permission to use the facilities, and clerical assistance at ARL, and
to Dr. Stirling Whiteway for invaluable assistance in reviewing the papers. I have also to acknow­
ledge my deep indebtedness to Miss Michelle Lamontagne, ably assisted by Miss Vicky Wagner,
Mrs. Dian Marciniak, Mrs. Debbie MacDonald and Mrs. Beth Garside at the Atlantic Research
Laboratory, for their calm efficiency in dealing with a very large volume of correspondence and
paper-work, and in the typing of many sections of this proceedings. I also extend my thanks to
all the authors, without whom this volume could not have materialized.
Hamid Mostaghaci
Halifax, August 1989
3
Si N whisker synthesis and effect of gas phase composition
3 4
on Si N 0 formation
2 2
Harue Wada
Department of Materials Science and Engineering, University of Michigan,
Ann Arbor, Michigan 48109, U.S.A.
ABSTRACT
Single crystal Si N whisker was synthesized by the carbothermal reduction of S1O2 under
3 4
N2 gas flow. The Si N whisker was formed through the VS mechanism. Si N was first
3 4 3 4
formed as a granule, typically a polycrystalline, and then grown as a single crystal
whisker from the {100} plane of the granule along the <210> direction. Lattice parameters
measurements and microanalysis of Si/Al ratio both identified the Si N whisker as β'-
3 4
Sialon with Z value ranging from 0.8 to 1.1. Increasing N2 gas flow rates increased yield
of Si N whisker, but Si N 0 was also formed during the Si N whisker synthesis. The
3 4 2 2 3 4
Si N 0 formation was limited to a bottom area of the charged powders. The formation of
2 2
Si N 0 phase is closely related to the [Psio/PN?-' rat*° anc* P02 *n tne &as P^ase· Stability of
2 2
phases among Si N /Si N 0/SiC is calculated as a guideline to control the Si N 0
3 4 2 2 2 2
formation. To suppress Si N 0, the [Psio/PNol rat*° must De l°wer than that of the phase
2 2
boundary in the Si N /Si N 0 equilibrium, whereas it must be higher than that of the
3 4 2 2
phase boundary in the SiC/Si N 0 equilibrium. The formation of Si N 0 was caused most
2 2 2 2
likely due to a fluctuation in the [psio/PNo^ rat^° *n ^ Sas phase surrounding the lower
area of the charged materials.
INTRODUCTION
Ceramic whiskers, offering the advantages of high melting points, low densities and high
moduli, have become important reinforcing materials in both ceramic-matrix composites
[CMC] and metal-matrix composites [MMC]. A single step process has been developed
for ceramic whiskers synthesis based on the phase stability of the Si-C-N-0 system and
reported [1,2]. In this report, the synthesized single crystal Si N and ß'-sialon whiskers
3 4
are analyzed. Localized formation of Si N 0 phase is discussed based on the [Psio/PNrl
2 2
ratio in the gas phase.
4 CERAMIC AND METAL MATRIX COMPOSITES
EXPERIMENTAL WORK
About 3 grams of silica, carbon and halide (3NaF»AlF3 or NaF) mixture with an atomic
ratio of Si/C/Na = 1/3/1 was charged to a graphite tube inside a horizontal graphite
reaction chamber, which was directly connected to a gas inlet at one end and an outlet at
the other end. The graphite reaction chamber was placed in a mullite tube. A high
purity (N2 + 3% H ) gas mixture was further purified through a Mg(C104>2 column, then
2
introduced directly into the graphite reaction chamber. The gas flow rate was controlled
by a Matheson 602 type flow meter, which had been calibrated by the bubble method
against Ar and (Ar + H2) gases. Four levels of flow rate were applied in this study: 25,
50, 75 and 100 cc/min.
All the reactions were carried out at 1623 K for 10 hours. The temperature was
measured by a Pt/Pt-10% Rh thermocouple calibrated against the melting point of Au.
Upon the completion of reaction, the remaining carbon was evaluated by oxidizing in air
at 973 K to determine approximate reaction yield. For the purpose of comparisons, one
run was carried out with pure Si instead of silica as the starting material.
The reaction products were identified by X-ray diffraction. Pure silicon was used as an
internal standard to determine the lattice constants. The morphology and composition of
whiskers were investigated by SEM (Hitachi-S-520) and TEM (Jeol 2000-FX),
respectively. The growth direction and the composition of whiskers were analyzed by
electron diffraction and micro analysis, respectively. The exhaust was led into a bubbler
containing a saturated PbCl2 solution and the fluorine in the exhaust was collected as
PbCIF precipitate.
In selected runs, the oxygen partial pressure was measured continuously during the
reaction by a Zr02(CaO) solid electrolyte [2]. The exhaust gas in these runs was also
collected at certain time intervals for CO/CO2 analysis by gas chromatography.
RESULTS AND DISCUSSIONS
Synthesis
Whiskers were formed at the original powder bed and around the wall of graphite tube.
Figure 1 shows a typical product before carbon burning. White wool-like whiskers had
formed at the upper part, while the lower part was a mixture of short whiskers and
powders containing approximately 40 wt% of carbon. The letter B shown in Fig. 1 is an
area between them and locates at the lower end of the upper whiskers.
Figure 2 is a TEM micrograph of the whiskers grown at the upper part. These whiskers
are transparent and identified by XRD as ß'-sialon. The diameters of the whiskers are in a
range from 0.5 to 1.2 μπι, and the lengths are on the order of millimeters. The bottom
part comprises short whiskers and powders, and was identified as a mixture of ß'-sialon
and Si N 0. Figure 3 shows the cross section of a whisker. Most of the whiskers have
2 2
rectangular cross sections. The fact that no droplets were ever observed at whisker tips
suggests that the whisker growth was through the gas/solid (VS ) mechanism and the
liquid phase was not involved. Typically, a polycrystalline was found at the root from
which the whisker grew along the <210> direction as shown in Fig. 4. Sometimes a
whisker stemmed from a single crystal, and both the whisker and the single crystal have
the same orientation, with <210> as the growth direction. It can be concluded that S13N4
was first formed as a granule (either polycrystalline or single crystal), and then grew în a
whisker form from the {100} plane of the granule along the <210> direction.
Figure 5 shows a typical X-ray energy spectrum of a ß'-sialon whisker. The Si K and Al
a
K peaks are clearly seen but no sodium peaks were detected. The Si/Al ratio was
a
determined by using the equation, Cgi/CAl = KSi-Al^Si/lAl, where C is element wt%, I is
characteristic X-ray intensity and K is Cliff-Lorimer factor. A K . i=1.002 was used in
SiA
CERAMIC AND METAL MATRIX COMPOSITES 5
I The upper area
The lower area
L^ \_
«#K -1
Figure 1. Reaction products before carbon removal.
0»B urn
Figure 2. TEM micrograph of ßf-sialon Figure 3. SEM micrograph of cross
upper whiskers. section of ß'-sialon whisker.
6 CERAMIC AND METAL MATRIX COMPOSITES
Figure 4. SEM micrograph of £?-sialon Figure 6. SEM micrograph of of-SißNA
whisker with polycrystalline whiskers .
root.
LT= 400 SECS
SIALON B SPECTRUM 3
Figure 5. Typical X-ray
spectrum from
A'-sialon whisker,
N 0
1.220 1.720 2.220
ENERGY keV
CERAMIC AND METAL MATRIX COMPOSITES 7
this study. Based on the measured Si/Al atomic ratio, the Z value of ß'-sialon (Si^.
zAlzOzNg_z) was determined to be in the range from 0.8 to 1.1. The lattice parameters of
ß'-sialon are closely related to the Z value, as demonstrated by Jack [3] and Hohnke and
Tien [4]. Our results are in reasonable agreement with these studies.
With [3NaF*AlF3l as the molten bath, ß'-sialon was the main product; however, when
[NaF] substituted for [3NaF«AlF3], a-Si N became the dominant phase. The ot-Si N
3 4 3 4
whisker is easily distinguished by the ribbon-like morphology, as shown in Fig. 6, with
about 1 μηι in width and less than 0.1 μιη in thickness. The growth direction of the oc-
Si N whisker is <211>. The same morphology of a-Si N whiskers was also reported by
3 4 3 4
other studies [5,6].
Reaction Mechanism
As discussed previously, the whisker formation is most likely through the VS mechanism.
There are three possible reaction routes that could be responsible for the S13N4 whisker
formation:
3SiO(g) + 3CO(g) + 2N(g) = Si N (ß) + 3C0 (g) (1)
2 3 4 2
âG° = -1437.972 + 0.942T, kJ/mole of Si N (1-1)
1 3 4
3SiO(g) + 3C(s) + 2N(g) = Si N (ß) + 3CO(g) (2)
2 3 4
AG° = -952.356 + 0.438T, kJ/mole of Si N (2-1)
2 3 4
3SiF (g) + 2N (g) = Si N (ß) + 6F (g) (3)
4 2 3 4 2
ÛG°3= 3920.19 + 0.018T, kJ/mole of Si N (3-1)
3 4
All the standard free energies of formation used in this study were obtained from JANAF
Tables [71, except that of ß-Si N and Si N 0(s), which were from the same source as
3 4 2 2
described in Ref. [1]. The contribution from Eq. (3) is considered to be insignificant for the
following reasons: (1) 4G°3 =3,949 kJ at 1623 K which is much higher than those of Eq. (1)
and Eq. (2), 91 kJ and -241 kJ, respectively, and (2) the fluorine recovery as the PbCIF
precipitate was always about 40% despite the fact that the Si yield changed as reaction
conditions were changed. However, the evolution of SiF from the melt, even small, may
4
enhance the evolution of SiO(g). The necessary presence of solid carbon at the reaction
site, i.e., the whisker tip, rules out the possibility of the gas-solid reaction of Eq. (2) as the
major reaction, especially when the whiskers were as long as millimeters; furthermore, a-
Si N whiskers have been found on an alumina substrate where solid carbon is absent.
3 4
Therefore, Eq. (1) is the most probable reaction for the S13N4 whiskers formation. For the
crystal grown from the vapor phase, it is generally believed that the growth form is
closely related to the supersaturation of gaseous components. In general, a lower
supersaturation favors the formation of whiskers, whereas a higher supersaturation ratio
would results in powders. Since the gas-phase reaction of Eq. (1) is the route for Si N
3 4
formation, the growth form of Si N is closely related to the partial pressures of N , SiO
3 4 2
and CO/CO2.
Figure 7 is a SEM micrograph of whiskers grown in the area B of Fig. 1. Some whiskers
have knuckles. A SEM/EDX analysis showed that both whiskers and knuckles are ß'-
sialon. The knuckles were polycrystalline, while the whisker portions between knuckles
were single crystals. It seems that the anisotropic whisker growth and the isotropic
polycrystalline growth have occurred alternately in these particular whiskers. The fact
that these whiskers were found only in the area B, suggests that the formation of these
whiskers is location-related. The gas composition is expected to fluctuate during the
8 CERAMIC AND METAL MATRIX COMPOSITES
formation of whiskers in this area, and it was probably responsible for the change of
growth form, i.e., from whisker to knuckle and vice versa.
The importance of SiO gas for S13N4 whisker formation was clearly demonstrated by
replacing Si0 witn Si as the starting material: no whiskers were formed and the carbon
2
consumption was only one-third of that in the Si0 cases of the same reaction conditions.
2
The major morphology of the product was the hexagonal particulate of a mixture of α-, β-
S13N4 and Si which was unreacted remaining Si.
The formation of ß'-sialon can be expressed as the following reaction [8],
(6-Z)SiO(g) + (4-272JN(g) + (6-2Z)CO(g) + ZAl(g)
2
=Si( -Z)AlzO N(8-Z) + (6-2Z)C0 (g) (4)
6 Z 2
Effect of iB.jQ/P-N1 Ratio on Phase Stability
s 2
As shown in Fig 1, a localized formation of silicon oxynitride was observed even under
low oxygen partial pressures. The formation of Si N 0 phase was limited to the lower
2 2
area of charged materials. The upper area, on the other hand, showed wool yarn-like ß-
S13N4 or sialon whiskers. Formation of Si N 0 is observed not only in the whisker
2 2
synthesis but also in other Si N ceramic processings, such as reaction bonding and
3 4
sintering. When a =l, ß-Si N and Si N 0 are in equilibrium from 1400 to 1650 K under
c 3 4 2 2
p = 0.1 MPa and p = 10"21 -10"22 MPa range[l]. The ß-Si N phase becomes a stable
N2 02 3 4
phase with lower oxygen partial pressures, whereas Si N 0 is a stable phase with higher
2 2
oxygen partials pressures [1]. There were two possibilities that causes the Si N 0
2 2
formation in the lower area in the present study: (1) the flowing nitrogen gas might have
had less accessibility to the lower area than the upper area, or (2) SiO partial pressure
was higher in the lower area compared to that in the upper area. In either case, the
[p /pN l rati° might not be the same in these two areas.
Si0 2
Effect of the [Psio/P^ ratio on tne Pnase stability was calculated for ß-Si N , Si N 0 and
3 4 2 2
ß-SiC phase, while Si0 was not included because it is not an equilibrium phase when ß-
2
S13N4 is a stable phase [1,2]. The main components of the gas phase assumed as N, 0 , SiO
2 2
and CO/C0 . The standard states of activities are pure solids components for all the solid
2
phases, and 1 atm pure gases for all the gases and temperature is 1623 K.
ß-SJ3N^/Si N 0 Equilibrium
2 2
Since the CO/C0 ratio has been found very close to its equilibrium value in the whisker
2
synthesis [21, an overall reaction of ß-SiN formation from SiO(g) can be expressed as Eq.
3 4
(5) by combining the following two reactions, 3SiO (g) + 3CO (g) + 2N (g) = Si N (ß) +
2 3 4
3 C0 (g) and C0 (g) + C = 2CO (g),
2 2
3 SiO (g) + 2N (g) + 3C = SiN (ß) + 3 CO (g) (5)
2 3 4
log K = log { a p 3 / p 3 p 2 a 3 } = 49,747/T - 22.88
5 Si3N4 co Si0 N2 c
where p 's and a 's are the partial pressures and the activities of X, respectively.
x x
When S13N4 is a stable solid phase, a§j N =1, then p can be obtained from Eq. (5) as,
3 4 Si0
log p = -1/3 log K - 2/3 log p + log p - log a (5-1)
Sjo 5 N2 co c