Combustion synthesis or self-propagating high temperature synthesis (SHS) is effected by heating a reactant mixture, to above the ignition temperature (Tig) whereupon an exothermic reaction is initiated which produces a maximum or combustion temperature, Tc. These SHS reactions are being used to produce ceramics, intermetallics, and composite materials. One of the major limitations of this process is that relatively high levels of porosity, e.g., 50 percent, remain in the product. Conducting these SHS reactions under adiabatic conditions, the maximum temperature is the adiabatic temperature, Tad, and delta H (Tad) = 0, Tad = Tc. If the reactants or products go through a phase change, the latent heat of transformation needs to be taken into account

Feng, Heng J.

Hunter, Kevin J.

Moore, John J.

Wirth, David G.

English

NASA Technical Reports Server

COMBUSTION SYNTHESIS OF CERAMIC AND METAL-MATRIX
COMPOSITES 9 3" 2 0 2 0:3
John J. Moore, Heng J. Feng, and Kevin J. Hunter
Department of Metallurgical and Materials Engineering
Colorado School of Mines
Golden, Colorado 80401
and
David G. Wirth
Coors Ceramics Company
Golden, Colorado 80401
Introduction
Combustion synthesis or self-propagating high temperature synthesis (SHS) is effected by heating a reactant mixture,
to above the ignition temperature (Tig) whereupon an exothermic reaction is initiated which produces a maximum or
combustion temperature, Tc. These SHS reactions are being used to produce ceramics, intermetallics, and composite
materials.' One of the major limitations of this process is that relatively high levels of porosity, e.g., 50%, remain in the
product. Conducting these SHS reactions under adiabatic conditions, the maximum temperature is the adiabatic
temperature, Tad, and AH (Tad) = 0, Tad = Tc. tf the reactants or products go through a phase change, the latent
heat of transformation needs to be taken into account,
Ttrans Tdd
i.e. A[-l(Tad) = AH(To) _- f CpdT ± AH(t]:ans) + f Cpdt = 0 (6)
T,3 Ttcans
where To is the initial or starting temperature, eg 298 K. Ttrans is the phase transition point of a reactant -&H(trans)
or a product +AH(trans) and AH(trans) is the corresponding heat of transformation. Tad can be determined from
equation (6) or graphically as indicated in Figure 1.
Experimental Reaction Systems
Several model composite systems have been investigated in which an excess amount of AI is used both as a reductant
in the reaction and as the metal component of the ceramic-metal composite? 6
i.e. 3TiO2 + 3C + (4 +x)AI = 3TiC + 2A1203 + xAI (1)
where x was varied between O to 9.2
3TiO 2 + 3B_O3 + (10 + x)AI = 3TiB 2 + 5AI203 + xAI
where x was varied between O to 17.3
(2)
3ZrO_ + 3B=O_ + (10 + x)AI = 3ZrB 2 + 5A1203 + xAI
where x was varied between O and 13.4
(3)
2B_O3 + C + (4 + x)AI = B,C + 2A1_O3 + xAI
where x was varied between O and 3?
The excess amount of liquid AI, created by the combustion synthesis reaction, is allowed to simultaneously infiltrate
the pores produced in the ceramic composite thereby taking advantage of this major limitation in the SHS process, and
hence improve the relative density of the composite produced. Incorporating a ductile metal into a brittle ceramic
composite matrix has considerable potential for substantial improvements in fracture toughness. A variation of reaction
(1) is given in reaction (5) in which the excess AI is used to reduce TiO2 and generate an excess amount of Ti. 6
203
https://ntrs.nasa.gov/search.jsp?R=19930011014 2020-03-17T08:00:17+00:00Z
(3+ x)TiO2+ 3C+ (4+ 4/3x)AI= 3TiC + (2 + 2/3x)AI20 s + xTi
The enthalpy-temperature plots for these five reaction systems have been examined and that for reaction (1) is given
in Figure 1 in which Xr and Xp refer to the respective reactant and product H-T plots associated with the corresponding
excess amount of aluminum used in the reaction. The corresponding melting and boiling points of reactants and
products are indicated in Figure 1 and Table I.
Experimental Procedure
The reactant powders (Table 1) were thoroughly mixed using porcelain ball milling for a minimum of six hours, and
pressed to various green densities with varying values of x, into cylindrical compacts of 0.5 inches (12.7mm) in diameter
and 1 inch (25.4mm) in length. These pellets were dried for 1 hour at 110"C in an oven and were subsequently Ignited
in the propagating mode using a heated tungsten wire in an argon atmosphere as described in an earlier publication. 2
The ignition temperatures, Tig, were determined using a Pt-Pt/10%Rh thermocouple and the combustion temperatures,
Tc, were determined using an Ircon Mirage Two Wavelength Infrared Pyrometer. A video camera was used to record
the propagation of the combustion front. In this way, it was possible to assess the stability of the combustion wave and
also its rate of propagation. The ceramic-metal composites produced were examined using optical and scanning
electron microscopy (SEM) interfaced with an energy dispersive spectroscopy (EDS) facility and also by x-ray diffraction
(XRD). The density of the products was determined using an immersion in water technique.
Results and Discussion
A maximum of 75% theoretical density could be achieved in the composites produced in reactions (1), (2) and (3) while
an expanded or foamed ceramic-metal composite was produced in reaction (4). Reaction (3) also produced a large
volume of AI_O3 whiskers while reaction (2) produced a small amount of AI203 whiskers present in the pores of the
composite. SHS reaction stability diagrams were constructed for each of the reactions (1)-(5) a3 shown in Figure 2-5.
A stable combustion front was observed for each level of excess AI used in reaction (4), i.e. x = O, 1, 2 and 3.
Increasing excess AI and green density eventually resulted in decreased stability of the combustion front and possible
quenching out. This is almost certainly the result of increased heat losses from the SHS reaction on account of
increased thermal conductivity (increased green density) and latent heat of fusion (increased excess AI). There is a
slight increase in thermal conductivity of the green pellet on increasing the excess amount of AI or Ti and increasing
the green density? -eThis increased thermal conductivity could have two opposing effects: (i) the heat from the reaction
front is more efficiently transferred to the reactants ahead of the reaction front and (it) the heat lost from the reactants
ahead of the reaction front is increased. Even though the velocity of the reaction front is considered to be high
[typically 0.2cm s" for reaction (1),2 and (5)' and 2cm s" for reaction (2)? 7cm s" for reaction (3)' and 5cm s" for
reaction (4)], _ it iS more likely that (it) wilt prevail in the small reactant pellet systems used. In each reaction system
studied, except reaction (4), the aluminum becomes liquid (melting point 660"C) before the ignition temperature is
reached, ie. Tig for reaction (1) >900°C? for reaction (2) >850°C/ for reaction (3) >900°C? for reaction (4)
>600°C. e for reaction (5) of >950°C The typical values of Tc achieved in each system were 1600-2000"C for
reaction (1)2 1550-2300 ° C for reaction (2),3 2000_2400oC for reaction (3),' 1850-2050 ° C for reaction (4),5 and 2000-
2300 ° for reaction (5).6
Reaction systems (1), (2) and (5) produced considerable increases in product density with increased excess AI as
expected, eg. from 45% (x = 0) to 75% (x = 9) relative densities. However, unlike reaction (1), the maximum density
in the ceramic-metal product for reactions (2) and (3) were obtained at low to intermediate green densities, while
reaction (4) expanded 400% in length producing a maximum density of 35%. This is most likely due to the high
volatility of B:O3 used in these reactions. The calculated vapor pressures of the reactant and product species in
reactions (1) to (5) (presented in Figure 3) indicate that the combustion temperatures produced in reaction (3) resulted
in considerable volumes of both B203 and AI gases which resulted in AI203 whisker formation from a vapor-liquid-solid
mode of synthesis (Figure 4). AI20 _whiskers were also produced in pores in the TiB2-AI20=-AI composite produced
by reaction (2). However, these whiskers were far less numerous than those produced by reaction (3). Since the Tc
achieved in reaction (2) produces a high vapor pressure (eg. > 1 at) of B203 vapor and only a moderate vapor pressure
of AI gas (ie. from 10.5to 10" at). Both these reaction systems (2) and (3) resulted in improved product densities with
excess AI provided that lower green densities were maintained, i.e., using a compaction pressure of 1.6 x 10 r kg/m 2
or below, The lower Tc value in reaction (4) would result in a low vapor pressure of AI ie. typically 10" to 10.2 at., but
the large vapor pressure of B20_ produced by this Tc (ie. > 1 at) and the larger reaction mass of B203 in reaction (4)
compared with those in reactions (2) and (3) has resulted in a large volume of B203 at 1 atmosphere pressure or higher.
This high pressure, high volume B_O_ gas is able to push or expand the reactant materials ahead of the combustion
204
front in the vertical direction, thereby producing an expanded or foamed ceramic-metal composite which exhibited
>65% uniform porosity (Figure 5) and an expansion of up to 400%. The more readily predicted behavior of product
densities in reactions (1) and (5), ie. increased product densities with increased green density and excess AI, is readily
explained in the absence of any major volatile species. However, reaction (5) was able to produce a semi stable
combustion front up to at least a value of x = 50 due to the fact that the exothermicity of reaction (5) increases with
xTi. The effect of density-driven fluid flow of liquid AI was also evident in these products (Figure 6).
ConcJusJons
Although reaction systems (1) tO (5) use the same concept of synthesizing ceramic-metal composites, the different
physical properties of the reactants and products, eg melting and boiling points, thermal conductivity, and combustion
temperatures achieved ineach system result in different product formations, morphologies, microstructures and relative
densities. The presence and extent of liquid and gaseous species appear to have a significant influence over these
product microstructures and densities. Incorporating an excess amount of metal within the combustion synthesis
reaction in which no significant gaseous phases are generated increased the composite relative density from 45% to
75% without the application of pressure. The prospect of synthesizing reinforcing whiskers in-situ from gaseous
species generated within the combustion synthesis reaction to produce a ceramic-metal composite without the problem
of handling these hazardous materials i_ extremely attractive. The concept of producing stability diagrams has been
introduced for these ceramic-metal combustion synthesis reactions that indicate how the process parameters can
produce stable propagating reaction fronts. Also the effect of green density and excess AI on the composite product
density is dependent on the character and extent of the liquid and gaseous species present at the reaction front. The
concept of producing expanded or foamed ceramic and ceramic-metal composites by utilizing a high volume of a
volatile specie at the reaction front has been introduced. These materials could find considerable applications varying
from catalyst support systems to filters to ultra light weight structural materials. The effect of gravity on density driven
fluid flow and vapor transport is likely to be significant in producing tough, dense ceramic-metal composites with
uniform microstructures and properties.
Acknowledqements
The authors are grateful for partial support of this work from NASA, through the Microgravity Science Division - Code
SN.
References
1. Z.A. Munir, Ceramic Bulletin, 67(2) (1988) 342.
2. H.J. Feng, J.J. Moore, D.G. Wirth, "The Combustion Synthesis of Ceramic-Metal Composites: The TiC-AI203-AI
System", accepted for publication in Met. Trans.
3. H.J. Feng, J.J. Moore, D.G Wirth, "Combustion Synthesis of TiB2-AI203-A! Composite Materials" published in
proceedings of Symposium on Developments in Ceramic and Metal Matrix Composites, TMS Annual Meeting,
San Diego, March 1-5, 1992. Pub by TMS, pp 219-239.
4. HJ. Feng, J.J. Moore, D.G. Wirth, "Combustion Synthesis of Ceramic Metal Composite Materials: The ZrB_-A_O_-
AI System" to be published in proceedings of the International Symposium on Self-Propagating High Temperature
Synthesis (SHS), Alma-Ata, USSR, Sept 23-28, 1991; pub by Soviet Academy of Sciences.
5. H.J. Feng, J.J. Moore, D.G Wirth, "The Combustion Synthesis of B,C-AI=O3-AI Composite Materials" to be
published in proceedings of Symposium on Synthesis and Processing of Ceramics: Scientific Issues, MRS Fall
Meeting, Boston Dec 2-6 1991; pub by MRS.
6. K.J. Hunter, J. J. Moore, D. G. Wirth, Proceedings of 6th International Titanium conference, San Diego, CA, June
28- Aug. 2, 1992, pub. by TMS.
Table I. Physical Properties of Reactants and Products in Reactions (1)-(5)
Material Ti% C
Mpt°C 1830
Bpt=C 3000
SG* 4.26 2.25
Size -44 -44
(i-,m)
":5 pec_tic Gravity
At TiC AI20 B203 TiBz Zr% ZrB= B,C Ti
660 3140 2050 450 2900 2715 3000 2450 1660
2467 4820 2980 1860 5000 3500 3287
2.70 4.93 3.97 2.46 4.50 5.60 6.09 2.52 4.54
-44 -75 -44
205
>.,
vJ
I-,
e_
o_
"4
4O0O
2000
-2000
-4000
t ./__.-,va-,2 /
+,,+,o,1 iJx':"l
,, 1
_+:+ ,+,++:":'_"_'.S'+"°L /
Iooo 2000 3000 +OOO
TBMPERATURE (K)
O.B
03
O,O
0.5 ' • '
0
4
STABLE
g
UNSTABLB
Z
_b
<
5 I0 15 20
EXCESS A1 (Mole)
Figure 1 Enthalpy-temperature plots for reactants (Xr)
and products (Xp) for vanous amounts of excess AI for
Reaction (1)
Figure 2 Effect of excess AI and relative green dunsity
on the stability of the propagating combustion wave for
Reaction (1).
O.g
0.8
O,7
_ G.tl
0,5
* STABLE
• 11
m
Z
0 5 |O 15 20
E.XCESS AI X [Mole)
0.8
0.7
D.6
0.5
UNSTABLE •
------------------__ SEMI-STABLE
•STABLE
2 4 8 B
EXCESS A1 (Mole)
,tO
Figure 3 Effect of excess AI and relative green density
on the stability of the porpagating combustion wave for
Reaction (2).
Figure 4 Effect of excess AI and relativegreen density
on the stability of the propagating combustion wave for
Reaction (3),
206
TO
85
eo
m
o 55
5O
• i
l
l
Stable
A x •
A x •
Seml-StLble
A x 4
• D _, x •
(significant cracklns)
i I i i i i l I i
10 20 30 40
EXC_ Ti (mole)
Figure 5 Effect of excess Ti and relative green
density on the stability of the propagating
combustion wave for Reaction (5).
5O
w B
._ O
-10
3-15
5OO
o
're rma_ far m_ (B)
_ ra_ tar _tJkm (4)
I I T_ m,_, f_ rectum (2)
. TiO z
.,////
1500 2500 3500
Figure 6 The effect of temperature on the calculated
vapor pressues of species present In Reaction (1) to
(4).
Figure 7 AI20_ whisker formation during Reaction (3).
207
(a)
Figure 8 (a) Expanded or foamed ceramic and ceramic-metal composites produced by reaction (4)
comparison of reacter (left) with unreacted (right) green pellet; (b) scannir_g electron photomicrograph of (a)
for x = O.
Figure 9 Optical photomicrograph of TiC-AI_Q-9At composite, indicating gravi',y-driven fluid flow of the liquid
AI to the bottom half of the coposite.
208
COMMENTS
Question (Prof. Bcrnard J. Matkowsky, Northwestern University): You referred to cxperiments in the former
Soviet Union, in which the sample produced by S.H.S. is significantly expanded (i.e., its porosity is
significantly increased). I presume you are referring to the experimcnts of A. Shteinbcrg. First, in his
experiments the increase in porosity was due to gas being liberated, rathcr than consumcd, in the reaction.
Was this true in your experiments as well? Second, he observed that the degree of increase in porosity was
differcnt if the sample was arrangcd (1) horizontally or (2) vertically in normal gravity, and (3) in
microgravity. I understand that you have not yet done the experiments in microgravity, but did you observe
similar differenccs between horizontal and vertical arrangements in normal gravity.
Finally, you described the propagation of the combustion wave as bcing eithcr (1) stable, by which you mean
that it propagated with a planar front with a constant velocity, or (2) unstable, by which you mean that
propagation ceased. However, in addition to the stable mode of propagation, thcre have been observations
of pulsating propagation, in which a planar front propagates with an oscillatory velocity, and spinning
propagation, in which a hot spot (or spots) moves in a helical fashion. Did you observe either of these
modes of propagation?
Answer: The increase in porosity was due to gaseous species both liberatcd and consumed. For example, in
combustion synthesis reactions iw¢olving B203, which has a boiling point slightly in excess of 1800 degrees C,
the conabustion temperature was well above this point, and, therefore, gcneratcd B203 gas of I atmosphere
pressure. At the same time, the B203 gas was also consumed in the reaction to produce compounds such as
TiB., with TiBO, reactant or B:C with graphite as the reactant, in certain combustion svnthesis reactions
that-we studicd,-the combustio_ Icmpcrature was sufficient to produce a significant volatility (i.e., partial
pressure of aluminum gas). In this respect, the aluminum gas and the B20 3 gas reacted to produce A120 3
whiskers. In our expcrimcnts, the very large levels of porosity ( e.g., greater than or equal to 75%) were
produced when thcre was only one major gaseous species produced (i.e., B20 3 gas).
Yes, I was referring to Professor Alex Shtcinbcrg's work in Russia. Most of our experimcnts were conducted
in which the pellet was ignited at the bottom of the cylindrical pcllct and the major expansion occurred in
the vertical direction against gravity.
We defined a stable combustion front as one which propagated a steady rate down through the pcllct,
wherein an unstable conabustion front was one which eventually quenched out. Our tlcfinilion of a scmi-
slablc combustion front was one th:lt slowed down, almost extinguished, and Ihcn pickcd up speed again,
probably going through two or three of thcsc cycles. In one or two occasions wc did observe a spinning
mode of propagation, which wc defined as semi-stable front.
209

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Combustion synthesis of ceramic and metal-matrix composites

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