- Email: [email protected]
Thermal expansion behavior of particulate-filled composites II: Multi-reinforcing phases (hybrid composites)
MaterialsScience and Engineering, A 131 ( 1991 ) 145-152
145
Thermal Expansion Behavior of Particulate-filled Composites II: Multi-reinforcing Phases (Hybrid Composites) TAKAKO TAKEI and HIROSHIHATTA
Materialsand Electronic DevicesLaboratory, MitsubishiElectric Corporation. 1-I-57 Miyashimo, Sagamihara229 Hapan) MINORU TAYA l)epartment of Materials Proeessing, Tohoku Universi~',Sendai 980 (Japan) (ReceivedJune 4, 1990:in revised form June 28, 1990)
Abstract
In part 1 of this paper, the effect of filler shape on the thermal expansion behavior of a composite with a single reinforcing phase was studied. In this paper emphasis is" placed upon the effect of using multi-components as the reinforcement (hybrid composite) on the thermal expansion behavior of a short fiber-composite. The hybrid composite system examined consists of a Kerimid matrix and two kinds of reinforcement A L O 3 short fibers and SigN4 whiskers. The former reinforcement represents a large-sized fiber, while the latter plays ~the role of a small-sized fiber. This leads to two advantages: (1) an increase in the volume fraction of the reinforcement, thus resulting in a lower coefficient of thermal expansion (CTE) of the composite, and (2) control of the orientation of these fibers, thus leading to improvement in the thermal expansion behavior of the composite along the thickness direction. I. Introduction
Due to their superior high specific mechanical properties and easiness of fabrications, polymer matrix composites have been used for various applications ranging from aerospace structures to electronic packaging materials. If only one property of a composite was focused on, it would not be difficult to attain a high level of the property by controlling the orientation of the reinforcing fiber. However, this kind of composite design often results in the degradation of other properties. For example, carbon-fiber-reinforced plastics can be designed to possess higher strength along the fiber axis by increasing the volume fraction of the fiber, but often at the cost
of toughness. To prevent the degradation of toughness, Kevlar fiber was used as an additional reinforcement [1, 2]. As this example demonstrates, the multi-reinforcing phase (hybrid composite) has been frequently used to optimize a number of properties and thus to extend the domain of the composite design to what would not be attained by the use of only one kind of reinforcing phase. The thermomechanical properties of a hybrid composite, wherein the major reinforcement was continuous fiber, have been investigated extensively and the effects of the hybridization on various properties have been documented [1-3]. However, relatively little effort has been directed toward hybridized short-fiber composite systems. It appears that no studies have ~een reported todate on the thermal properties of this type of hybrid composite. In part I [4], the coefficient of thermal expansion (CTE) of mono-reinforced composites was discussed. This paper focusses on the effect of the short-fiber hybridization on the thermal expansion behavior. As the reinforcement, two kinds of short fibers of different sizes are used, i.e. the diameter of one type of fiber is larger than the other type. The microstructure of this hybrid composite system is shown schematically in Fig. 1. Figure 1 suggests that the volume fraction of the reinforcement can be increased by filling small-sized fibers B into the matrix phase situated between large-sized fibers A. In a compression-molded composite with just one type of fiber, the fibers tend to lie in a plane perpendicular to the compressive force. H o w ever, in the case of a hybrid composite (Fig. l), small'sized fibers can be oriented along the compressive force even though large-sized fibers Elsevier Sequoia/Printed in The Netherlands
146
2. Experimentalprocedure
still tend to lie in the plane. The control of fiber orientation is thought to improve the thermal expansion behavior along the out-of-plane (thickness) direction, which becomes a critical design parameter in developing new printed circuit boards with lower CTEs along the thickness direction. Thus the composite obtained by the hybridization can be made to be superior to the composites composed of one kind of reinforcement by attaining higher volume fraction and controlling the orientation of the two kinds of fiber. The purpose of this paper is to present experimental results for the effect of hybridization on the composite CTE and its relation to process parameters.
2.1. Sample preparation To lower the CTE of a composite while maintaining good processability, AI203 short fibers (Saffil fiber; ICI) and Si3N 4 whiskers (Tateho Chemical) were chosen for large- and small-sized reinforcements respectively. The room temperature material properties of these short fibers are shown in Table 1, where the aspect ratios are the average values of those measured based on the extracted fibers from the hybrid composite by burning the matrix resin. It should be noted that Kerimid 601 was used as the matrix due to its relatively low CTE, high glass transition temperature (Tg=250°C) and high electric resistivity (1.9x 1015 ~2 cm). The processing route used in the present study consists of two major steps: the paper-making process and the resin impregnation process (Fig. 2). Both processes will be discussed below. The paper-making process shown in Fig. 3 was TABLE 1
Material properties
KerirnM 601 A120~ Young's modulus (GPa)3.50 Poisson's ratio 0.35 CTE (K- ~x 10-6) 50-80 Thermal conductivity 0.23 ( W m - I K-I) Shape -Aspect ratio -Diameter (/~m) --
Fig. 1. Concept of a hybridized short-fiber composite.
300 0.22 8 30.7
Si.~N4 385 0.27 2.5 25.1
Short fiber Whisker 15 17.5 3 0.2-0.5
Methyl AI2/Oa (Short fiber) SizN4 (Whisker)
Hybrid mat
+ Methyl alcohol I Coupling treatment
Making
Compression molding I
Impregnation
Fig. 2. Processing route for a hybrid composite.
[ Stirring
Canvas
I
_ Roller _~
S u s ~ ""'i".:.':.....
~/et hybridma
Paper-makingmachine
~'~"
T R ,NG
~
!
Filter
;ial~eerr[ :Handpress i paper
/ Hot drum // k
j
O HYORAT,ON
Fig. 3. Paper-making process.
[ Handpress I
Plate ( ~
SQUEEZINGOUT OF WATER
DRYING
Hybrid paper l
147
employed to produce short fibers compacted into a paper-like hybrid mat. To gain sufficient strength of the hybrid mat for easy handling during the paper making and composite forming processes, a small amount of two kinds of binders were used: micro-fibrilized cellulose (MFC) and Kymene 557H (water-soluble polyamino epichlorohydrin) supplied by Dic-Hercules Chemical, at 3-6% and 0.8% of reinforcements respectively. These binders were mixed during the stirring process. A typical scanning electron microphotograph of the hybrid mat and the constituents of the original suspension in the paper making process are presented in Fig. 4 and Table 2 respectively. As shown in Table 2, a higher weight fraction of MFC was needed to strengthen the wet mat when the weight fraction of whisker increased. As the mean diameter of Si3N4 whiskers (0.2-0.5 pm) is smaller than that of the pulp that is a standard material of papers, the whiskers tend to pass through the wire gauze during the dehydration process (Fig. 3). To prevent the passage of the whisker through the wire gauze, Cuprammonium Rayon cloth composed of thin fibers was overlapped on the wire gauze.
Even by using the small-sized mesh of the Rayon cloth, we could not completely prevent Si3N4 whiskers from passing through the cloth. Thus the final weight fractions of the Si3N4 whiskers and A1203 short fibers were different from those in the original mixture as indicated in Table 2 (see the columns of the constituents in the original suspension and in the paper). Use of the Rayon cloth, however, made water dehydration difficult as the whisker was stuffed into the gaps in the mesh of the cloth. Thus, a high power vacuum pump was used to facilitate the dehydration. The dehydration process was followed by a drying process (Fig. 3). The second step is to impregnate the hybrid mat with the matrix resin. First, the hybrid mat was soaked in a solution of silane coupling agent (A-1160; Nihon Yunika) in methanol and then impregnated with a mixture (varnish) of Kerimid resin and solvent, N-methyl-2-pyrolidone (NMP), as shown in Fig. 2. The volume fraction of resin was varied by changing the concentration of the Kerimid in the varnish and the pressure during the molding process as shown in Table 3. Three TABLE 3
Hybrid ratio T/and maximum molding pressure
No.
Hybrid ratio (%)
Fig. 4. Scanning electron micrograph IAI:O) : Si)N 4 = 75.56 : 24.44 (wt.%)].
TABLE 2
No.
PS PS PS PS PS PS PS
0 10 20 30 50 70 10(I
of a hybrid
mat
PS 100 PS 7(/
100.0 65.82
PS 50
37.58
PS 30
24.44
PS 20
16.78
PS 10
9.19
PS 0
0.00
Varnish
Molding pressure
(wt.%)
(MPa)
25 25 2(I 25 20 25 20 25 20 15 25 20 15 25
36.0 31.5 34.2 27.0 31.5 15.3 18.0 9.0 14.4 16.2 6.7 13.5 14.4 4.5
Constitution of ingredent materials in original suspension and hybrid paper
Constituents in suspension
Wire gauze
Ab O~ (g)
SisN 4 (g)
K YMENE (g)
MFC (g)
10(1 911 8(1 7(/ 5(I 30 0
(t 111 20 30 5(/ 70 1110
0.1 0.1 0.1 0.1 0.1 0.1 0.1
3 3 3 3 5 5 6
~'Cuprammonium Rayon.
(mesh)
Paper (wt. %) ALOJS(,N4
200 200 200 200 200 200 150 + cloth ~
100.0/00.00 90.81/9.19 83.22/16.78 75.56/24.44 62.42/37.58 34.18/65.82 00.00/100.0
148 kinds of resin concentration in the solvent, 15%, 20% and 25% (varnish weight percentage in Table 3), were used to explore the upper limit of volume fraction Vf of reinforcement in the hybrid composite, because these concentrations were experimentally shown to be near the lower limit at which almost void-free composites can be processed [4]. After drying of the prepreg in an oven at 120 °C for 5 rain, the cooled prepreg was cut into the size of a mold cavity, 70 mm x 16 mm rectangle (lamina), and the lamina (120 sheets of the prepreg) were compression molded at 180 °C for 2 h and post-cured at 200 °C for 48 h. The compression-molding pressure was determined for a given weight fraction ratio of A1203 short fiber to Si3N 4 whiskers such that the maximum pressure is sought as a result of incremental pressurization just before the initiation of fracture of the prepreg sheet during compression molding. Also shown in Table 3 are the maximum compression pressures determined for each hybrid composite system.
measurement of the CTE of composites was carried out only for these two directions: in-plane and out-of-plane directions, by the use of a thermomechanical analyzer (Shimazu) for the temperature range of 20-200°C and at a temperature increase rate of 5 °C min- ~. The composite CTEs under these conditions were found to be almost constant. Therefore, the average values over the entire temperature range were used for the CTEs of the hybrid composites. 3. Results and discussions
3.1. Maximum volume fraction offiber In Fig. 6, the experimentally determined maximum volume fraction of reinforcements (AI203 short fibers and Si3N4 whiskers) of the hybrid composite Vf.max is plotted as a function of the relative weight ratio of AI:O 3 short fibers and Si3N4 whiskers (hybrid ratio) ~/defined by 100(Wfs)
(1)
T]--(Wfa+ Wfs ) 2.2. Measurement The volume fractions of Si3N 4 whiskers and AI203 short fibers were individually determined by the procedure illustrated in Fig. 5, wherein the relative weight fraction of the reinforcements was determined from the measurement of the fluorescent X-ray strength of silicon and aluminum atoms. In hybrid composites, Si3N 4 whiskers and A1203 short fibers are oriented nearly randomly such that the overall properties can be considered almost transversely isotropic. Thus the only two CTEs, i.e. in-plane and out-of-plane directions, are considered to be independent. Hence the
Density of composite material (Pc) i" Weight .......... "i i volume i L ...........
I
.J
Weight fraction of reinforcement (Wf) and resin (W r)
Resin ' F ............. I
r i burn-out i ,. .............
vfs = (w,a/p,)/{(wjp~)
where Wfa and Wfs denote the weight fraction of A1203 short fibers and Si3N4 whiskers respectively. "Kerimid %" in Fig. 6 stands for the weight fraction of Kerimid in the varnish at the impregnation process. It should be noted that several data points are missing in Fig. 6: for Kerimid 15% (open triangles) at t/= 0% (A1203 composite) and t/> 30%, and for Kerimid 20% (open squares) at t/= 100% (Si3N 4 composite). These missing data points indicate that the corresponding composites could not be processed due to either excess void formation or due to fracture of the prepreg sheets. The solid line in Fig. 6 indicates
.,
l
Weight fraction of AI203 (Wfa) and Si3N4 (Wfs)
r- .........................
-I
i Fluorescent X-rays ! I i of AI and Si ,•
l
.......................................
i I i
t .......................................
J
[ r l ~ I
L . . . . . . . . . . . . . . . . . . . . . . . . . .~
Volume fraction of , AI203 (Vf,) and SI3N . (Vf,)
+ (w,,/p,) + ( wjpr)}
= Wfs (Pc~P,)
where Pa is the density of AI203 /or the density of Kerimid 601 resin.
short
fibers,
p,
the
density
Fig. 5. Pr•cedure•fdeterminingthev••umefra•ti•ns•fA•2•3(Vfa)sh•rt•bersandSi3N4(Vf`)w•iskers•
of
SiaN 4 whiskers
and
149
the envelope of the maximum Vt attained for the hybrid composites. It is obvious from this envelope curve that V~_ma~ occurs at an intermediate value of r/, i.e. r/= 20%. Thus, it can be concluded that the addition of the small-sized fibers (whiskers) between the large-sized short fibers results in an improvement of the processability of composite materials in terms of increasing the volume fraction of reinforcement. The reason for this improvement is mainly due to the concept illustrated in Fig. 1, i.e. easier high density packing of the composite by using different sized fibers. However, another complementary mechanism must also be considered to explain the phenomena that, at the hybrid ratio of around 10% in Fig. 6, the volume fraction of the A1203 short fiber is 39%, being more than the maximum volume fraction of 34% for the single-phase reinforced composites (open triangles). 3.2. Coefficient of thermal expansion
For all the composites shown in Fig. 6, the measured CTEs along the in-plane ( a,, shown by filled symbols) and the out-of-plane direction (a: shown by open symbols) are plotted as a function of hybrid ratio q in Fig. 7. Also shown in Fig. 7 are the solid curves indicating the lowest CTEs
for two directions. It is clear from Fig. 7 that the lowest CTE occurs at an intermediate value of hybrid ratio q, clearly indicating the hybrid effect. It is also clear from Fig. 7 that remarkable improvement was obtained for the CTE along the out-of-plane direction a:, but a relatively small improvement resulted for that along the in-plane direction a w It should be noted here that although a,y is easy to design, even for monoreinforcement composites, ct_ is hard to control. Thus by hybridization of the reinforcement, it becomes possible to lower a. while maintaining low a,,, although the range of the control is rather limited. Next an attempt is made to compare the measured CTEs of hybrid composites with the predictions based on Eshelby's equivalent inclusion method developed for a misoriented shortfiber composite [4, 5]. This model is best suited for the analysis of complicated microstructures such as misoriented short-fiber composite systems, including hybrid composites. The purpose of this attempt is to estimate the microstructure of the hybrid composites. First, the measured CTEs of the hybrid composite with constant hybrid ratio (y/=9.19%) along the in-plane (filled symbols) and the out-of-
50
7O
O(Oz); • (Oxy):Kerimid 25% [] (Cl.z); • (Oxy): Kerimid 20% 4~(Oz); A(Oxy):Kerimid 15%
45
40
oo
0 O0 0
3! S
o
ooO
0 "X~
I
CD
o
40
Oz
X v
3{
3(2
25
20
O :Kerimid 25% [ ] :Kerimid 20%
i
/4. :Kerimid 15%
|
Oxy
20-
0
t
t
,
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
Fig. 6. Volume fraction V~ of fibers as a function of hybrid ratio q (~1= IOOWt,/(W,~+Wt,)o~)).
0
10
20
30
40
50
60
70
80
90
100
Fig. 7. CTEs of the hybrid composites in the x - y plane (a,, ;, and in the z direction (a:) as a function of hybrid ratio q.
150 I
O(QZ); O(O.xy):Kerimid 25% n(Oz); • (Oxy):Kerimid 20% ~.(Qz); A(Qxy):Kerimid 15%
80 r / - ~ .~. ,,. . . ",~ J- ./ " '~ 70 ~ . . / . ~ / ~ "~,.. ~
.........
AI2O3:2-D Random , SI3N4:2-DRandom AI203:2-D Random
-
Si3N4:3-DRandom
-
\-\
60" •
(13_=9.19) %,.
'
\
c7 ,0 ---'~...............................L.... i_
,o.
"\.,
i i i\
.,\ '~
i
i
10
i
i
t
0
t
10
t
t
20
t
I
30
- ' ~'- Oxy
I
40
50
I
I
60
vf (%) Fig. 8. CTEs of the hybrid composite: in the in-plane (x-y plane) (ax~) and in the out-of-plane (z direction) (a:) as a function of volume fraction Vf of fibers.
plane directions (open symbols) are plotted as a function of the volume fraction of the reinforcements (both Si3N4 whiskers and AI203 short fibers) Vf in Fig. 8. Also shown in Fig. 8 are the predictions based on Eshelby's equivalent inclusion method. The solid and dash-dot lines correspond to the case of A1203 short fibers distributed in a two-dimensionally random manner while Si3N 4 whiskers in a three-dimensionally random manner, and the case of both reinforcements distributed in a two-dimensionally random manner, respectively. By comparing the measured and predicted values of the composite CTEs, one can speculate that the distribution of AI:O3 short fibers is rather three- instead of two-dimensionally random. This is enhanced at higher volume fraction Vy of the reinforcements. This Vf dependency of A1203 short-fiber orientation may be partially seen in Fig. 9 where (a) and (b) denote typical scanning electron micrographs of the composite with the same hybrid ratio r/but with Vf = 35.10% and Vf = 45.07% respectively.
Fig. 9. Scanning electron micrographs of the fracture surface plane) of a hybrid composite at constant hybrid ratio r/ (24.44%): (a) Vt = 35.10%, (b) I/i = 45.07%.
(x-y
The reason for such orientational behavior of the A1203 short fibers is as follows. At the initial stage of the compression-molding process, entangled A1203 short fibers and Si3N4 whiskers interact to exert a compressive force with each other. The compressive force acts on the short fibers as a bending force, leading to the fracture of the short fibers. Apparently, the force applied to the short fibers rotates them and then some of the short fibers fracture resulting in the deviation of the orientation distribution of A1203 short fibers from the original two-dimensional random state to the thickness direction. This effect of the interaction force is obviously more intensive when the volume fraction of short fibers increases. Since Si3N4 whiskers are located in the gaps in the tangled A1203, the Si3N4 whiskers do not support the compressive force. This mechanism does not effectively act on the Si3N4 whiskers. In order to examine the effect of the hybrid ratio ~1, we have replotted the data of axy and a_ as a function of r/for a fixed Vf. For example, in Fig. 8 let us look at the vertical dotted line of Vf = 35%. Along this vertical line, we can find the values of ely and a,. The same kind of procedure
151
V f = 35%
70
.......
• (QZ) (Clxy)
AI203:2-D Random Si3N4:2-D R a n d o m
~
Vf =40%
.......
AI203:2-DRandom Si3N4:2-D Random
• (Qz)
~
AI203:2-D Random
m
AI203:2-D Random Si3N4:3-D R a n d o m
70 -
O(QxY)
Si3N4:3-D Random
m
60
60 Oz
Qz kJ
51
5O
G" I o x
I O X
40
v
30
C~
20
40
30
2~
Clxy lC
I 10
(a)
7C
I 20
i 30
i 40
i 50
I 60
I 70
I 80
I 90
Vf = 45%
AI203:2-D Random Si3N4:2-D R a n d o m
• (Oz)
AI203:2-D Random Si3N4:3-D R a n d o m
@ (Oxy)
I 100
(b)
0
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
6C
5G
az
0 X
4O
3O
2O
10
it/
-" - "
ClxY
i
=
=
t
=
I
I
I
I
I
0
10
20
30
40
50
60
70
80
90
100
Fig. 10. CTEs of the hybrid composite as a function of hybrid ratio r/for a fixed Vt: (a) Vf = 35%, (b) V~= 40%, (c) Vt = 45%.
152 shown in Figs. ll(a) Vf= 37.86%, q = 9.19% and 1 l(b) Vf = 39.61%, r/= 65.82%. If the results of the present study (Fig. 7) are compared with those of part I of this paper [4] (see Fig. 1 1 in part I), one can conclude that the hybrid composite gives rise to a lower c%. and, except for the case of spherical-filler composites, a lower c~= compared with composites with a single reinforcing phase. The processing cost of the hybrid composite, however, is higher than that of the composite with a single reinforcing phase.
4. Conclusions
Fig. 11. Scanning electron micrographs of the fracture surface (x-y plane) of hybrid composites for similar Vt: (a) ~/= 9.19% (V,= 37.86%), (b) r/= 65.82% (V~= 39.61%). was applied at six different ~/values, which correspond to data points in Fig. 7. Thus, the data of a= and c%. are plotted as a function of ~/for a given Vf in Fig. 10(a) Vf = 35%, (b) Vf= 40% and (c) Vf=45%. It follows from Fig. 10 that the measured a_ values are lower than the predication of the dash-dot line and are comparable with the solid line values which were derived by assuming that A1203 short fibers and Si3N 4 whiskers follow two- and three-dimensionally random directions respectively. It appears that the higher r/ becomes, the higher the relative value of the measured a= compared with the corresponding solid line, although at Vf=45% the data points of a_ for higher q values are missing (Fig. 10(c)). Hence, it can be speculated from Figs. 10(a) and 10(b) that for a given Vf, Si3N 4 whiskers at low r/values are more likely to be oriented along the thickness direction, while at higher r/values they tend to lie in the plane. This r/dependency of Si3N4 whisker orientation can be seen by looking at scanning electron photographs
The hybrid composites composed of A120 3 short fibers and Si3N 4 whiskers were fabricated by a combination of the paper-making and the compression-forming process. To clarify the advantages of this type of hybrid composites, attention was focussed on the maximum volume fraction of reinforcements attainable by these processes, and the effects of the microstructure of hybrid composites on the thermal expansion coefficients were studied. The following conclusions can be drawn from the present,study. (1) The maximum volume fraction of reinforcements can be increased by controlling the hybrid ratio. (2) CTEs of the hybrid composites can be decreased compared with the single-reinforcement composite system. This tendency was especially marked for the CTEs along the thickness direction. (3) The effect of hybridization was noticeable even at modest volume fractions of the second reinforcement (small-sized reinforcement).
References
1 K. M. Hardaker and M. O. W. Richardson, Polym. Plast. Technol. Eng., 15(1980) 169. 2 L.N. Phillips, Composites, 8(1976) 7. 3 D. Short and J. Summerscales, Composites, 11 (1979) 10. 4 T. Takei, H. Hatta and M. Taya, Mater. Sci. Eng., AI31 (1991)33. 5 Y. Takao, in J. R. Vinson and M. Taya (eds.), Recent Advances in Composites in United States and Japan] ;ASTM Spec. Tech. Publ., 864 (1985) 685.
145
Thermal Expansion Behavior of Particulate-filled Composites II: Multi-reinforcing Phases (Hybrid Composites) TAKAKO TAKEI and HIROSHIHATTA
Materialsand Electronic DevicesLaboratory, MitsubishiElectric Corporation. 1-I-57 Miyashimo, Sagamihara229 Hapan) MINORU TAYA l)epartment of Materials Proeessing, Tohoku Universi~',Sendai 980 (Japan) (ReceivedJune 4, 1990:in revised form June 28, 1990)
Abstract
In part 1 of this paper, the effect of filler shape on the thermal expansion behavior of a composite with a single reinforcing phase was studied. In this paper emphasis is" placed upon the effect of using multi-components as the reinforcement (hybrid composite) on the thermal expansion behavior of a short fiber-composite. The hybrid composite system examined consists of a Kerimid matrix and two kinds of reinforcement A L O 3 short fibers and SigN4 whiskers. The former reinforcement represents a large-sized fiber, while the latter plays ~the role of a small-sized fiber. This leads to two advantages: (1) an increase in the volume fraction of the reinforcement, thus resulting in a lower coefficient of thermal expansion (CTE) of the composite, and (2) control of the orientation of these fibers, thus leading to improvement in the thermal expansion behavior of the composite along the thickness direction. I. Introduction
Due to their superior high specific mechanical properties and easiness of fabrications, polymer matrix composites have been used for various applications ranging from aerospace structures to electronic packaging materials. If only one property of a composite was focused on, it would not be difficult to attain a high level of the property by controlling the orientation of the reinforcing fiber. However, this kind of composite design often results in the degradation of other properties. For example, carbon-fiber-reinforced plastics can be designed to possess higher strength along the fiber axis by increasing the volume fraction of the fiber, but often at the cost
of toughness. To prevent the degradation of toughness, Kevlar fiber was used as an additional reinforcement [1, 2]. As this example demonstrates, the multi-reinforcing phase (hybrid composite) has been frequently used to optimize a number of properties and thus to extend the domain of the composite design to what would not be attained by the use of only one kind of reinforcing phase. The thermomechanical properties of a hybrid composite, wherein the major reinforcement was continuous fiber, have been investigated extensively and the effects of the hybridization on various properties have been documented [1-3]. However, relatively little effort has been directed toward hybridized short-fiber composite systems. It appears that no studies have ~een reported todate on the thermal properties of this type of hybrid composite. In part I [4], the coefficient of thermal expansion (CTE) of mono-reinforced composites was discussed. This paper focusses on the effect of the short-fiber hybridization on the thermal expansion behavior. As the reinforcement, two kinds of short fibers of different sizes are used, i.e. the diameter of one type of fiber is larger than the other type. The microstructure of this hybrid composite system is shown schematically in Fig. 1. Figure 1 suggests that the volume fraction of the reinforcement can be increased by filling small-sized fibers B into the matrix phase situated between large-sized fibers A. In a compression-molded composite with just one type of fiber, the fibers tend to lie in a plane perpendicular to the compressive force. H o w ever, in the case of a hybrid composite (Fig. l), small'sized fibers can be oriented along the compressive force even though large-sized fibers Elsevier Sequoia/Printed in The Netherlands
146
2. Experimentalprocedure
still tend to lie in the plane. The control of fiber orientation is thought to improve the thermal expansion behavior along the out-of-plane (thickness) direction, which becomes a critical design parameter in developing new printed circuit boards with lower CTEs along the thickness direction. Thus the composite obtained by the hybridization can be made to be superior to the composites composed of one kind of reinforcement by attaining higher volume fraction and controlling the orientation of the two kinds of fiber. The purpose of this paper is to present experimental results for the effect of hybridization on the composite CTE and its relation to process parameters.
2.1. Sample preparation To lower the CTE of a composite while maintaining good processability, AI203 short fibers (Saffil fiber; ICI) and Si3N 4 whiskers (Tateho Chemical) were chosen for large- and small-sized reinforcements respectively. The room temperature material properties of these short fibers are shown in Table 1, where the aspect ratios are the average values of those measured based on the extracted fibers from the hybrid composite by burning the matrix resin. It should be noted that Kerimid 601 was used as the matrix due to its relatively low CTE, high glass transition temperature (Tg=250°C) and high electric resistivity (1.9x 1015 ~2 cm). The processing route used in the present study consists of two major steps: the paper-making process and the resin impregnation process (Fig. 2). Both processes will be discussed below. The paper-making process shown in Fig. 3 was TABLE 1
Material properties
KerirnM 601 A120~ Young's modulus (GPa)3.50 Poisson's ratio 0.35 CTE (K- ~x 10-6) 50-80 Thermal conductivity 0.23 ( W m - I K-I) Shape -Aspect ratio -Diameter (/~m) --
Fig. 1. Concept of a hybridized short-fiber composite.
300 0.22 8 30.7
Si.~N4 385 0.27 2.5 25.1
Short fiber Whisker 15 17.5 3 0.2-0.5
Methyl AI2/Oa (Short fiber) SizN4 (Whisker)
Hybrid mat
+ Methyl alcohol I Coupling treatment
Making
Compression molding I
Impregnation
Fig. 2. Processing route for a hybrid composite.
[ Stirring
Canvas
I
_ Roller _~
S u s ~ ""'i".:.':.....
~/et hybridma
Paper-makingmachine
~'~"
T R ,NG
~
!
Filter
;ial~eerr[ :Handpress i paper
/ Hot drum // k
j
O HYORAT,ON
Fig. 3. Paper-making process.
[ Handpress I
Plate ( ~
SQUEEZINGOUT OF WATER
DRYING
Hybrid paper l
147
employed to produce short fibers compacted into a paper-like hybrid mat. To gain sufficient strength of the hybrid mat for easy handling during the paper making and composite forming processes, a small amount of two kinds of binders were used: micro-fibrilized cellulose (MFC) and Kymene 557H (water-soluble polyamino epichlorohydrin) supplied by Dic-Hercules Chemical, at 3-6% and 0.8% of reinforcements respectively. These binders were mixed during the stirring process. A typical scanning electron microphotograph of the hybrid mat and the constituents of the original suspension in the paper making process are presented in Fig. 4 and Table 2 respectively. As shown in Table 2, a higher weight fraction of MFC was needed to strengthen the wet mat when the weight fraction of whisker increased. As the mean diameter of Si3N4 whiskers (0.2-0.5 pm) is smaller than that of the pulp that is a standard material of papers, the whiskers tend to pass through the wire gauze during the dehydration process (Fig. 3). To prevent the passage of the whisker through the wire gauze, Cuprammonium Rayon cloth composed of thin fibers was overlapped on the wire gauze.
Even by using the small-sized mesh of the Rayon cloth, we could not completely prevent Si3N4 whiskers from passing through the cloth. Thus the final weight fractions of the Si3N4 whiskers and A1203 short fibers were different from those in the original mixture as indicated in Table 2 (see the columns of the constituents in the original suspension and in the paper). Use of the Rayon cloth, however, made water dehydration difficult as the whisker was stuffed into the gaps in the mesh of the cloth. Thus, a high power vacuum pump was used to facilitate the dehydration. The dehydration process was followed by a drying process (Fig. 3). The second step is to impregnate the hybrid mat with the matrix resin. First, the hybrid mat was soaked in a solution of silane coupling agent (A-1160; Nihon Yunika) in methanol and then impregnated with a mixture (varnish) of Kerimid resin and solvent, N-methyl-2-pyrolidone (NMP), as shown in Fig. 2. The volume fraction of resin was varied by changing the concentration of the Kerimid in the varnish and the pressure during the molding process as shown in Table 3. Three TABLE 3
Hybrid ratio T/and maximum molding pressure
No.
Hybrid ratio (%)
Fig. 4. Scanning electron micrograph IAI:O) : Si)N 4 = 75.56 : 24.44 (wt.%)].
TABLE 2
No.
PS PS PS PS PS PS PS
0 10 20 30 50 70 10(I
of a hybrid
mat
PS 100 PS 7(/
100.0 65.82
PS 50
37.58
PS 30
24.44
PS 20
16.78
PS 10
9.19
PS 0
0.00
Varnish
Molding pressure
(wt.%)
(MPa)
25 25 2(I 25 20 25 20 25 20 15 25 20 15 25
36.0 31.5 34.2 27.0 31.5 15.3 18.0 9.0 14.4 16.2 6.7 13.5 14.4 4.5
Constitution of ingredent materials in original suspension and hybrid paper
Constituents in suspension
Wire gauze
Ab O~ (g)
SisN 4 (g)
K YMENE (g)
MFC (g)
10(1 911 8(1 7(/ 5(I 30 0
(t 111 20 30 5(/ 70 1110
0.1 0.1 0.1 0.1 0.1 0.1 0.1
3 3 3 3 5 5 6
~'Cuprammonium Rayon.
(mesh)
Paper (wt. %) ALOJS(,N4
200 200 200 200 200 200 150 + cloth ~
100.0/00.00 90.81/9.19 83.22/16.78 75.56/24.44 62.42/37.58 34.18/65.82 00.00/100.0
148 kinds of resin concentration in the solvent, 15%, 20% and 25% (varnish weight percentage in Table 3), were used to explore the upper limit of volume fraction Vf of reinforcement in the hybrid composite, because these concentrations were experimentally shown to be near the lower limit at which almost void-free composites can be processed [4]. After drying of the prepreg in an oven at 120 °C for 5 rain, the cooled prepreg was cut into the size of a mold cavity, 70 mm x 16 mm rectangle (lamina), and the lamina (120 sheets of the prepreg) were compression molded at 180 °C for 2 h and post-cured at 200 °C for 48 h. The compression-molding pressure was determined for a given weight fraction ratio of A1203 short fiber to Si3N 4 whiskers such that the maximum pressure is sought as a result of incremental pressurization just before the initiation of fracture of the prepreg sheet during compression molding. Also shown in Table 3 are the maximum compression pressures determined for each hybrid composite system.
measurement of the CTE of composites was carried out only for these two directions: in-plane and out-of-plane directions, by the use of a thermomechanical analyzer (Shimazu) for the temperature range of 20-200°C and at a temperature increase rate of 5 °C min- ~. The composite CTEs under these conditions were found to be almost constant. Therefore, the average values over the entire temperature range were used for the CTEs of the hybrid composites. 3. Results and discussions
3.1. Maximum volume fraction offiber In Fig. 6, the experimentally determined maximum volume fraction of reinforcements (AI203 short fibers and Si3N4 whiskers) of the hybrid composite Vf.max is plotted as a function of the relative weight ratio of AI:O 3 short fibers and Si3N4 whiskers (hybrid ratio) ~/defined by 100(Wfs)
(1)
T]--(Wfa+ Wfs ) 2.2. Measurement The volume fractions of Si3N 4 whiskers and AI203 short fibers were individually determined by the procedure illustrated in Fig. 5, wherein the relative weight fraction of the reinforcements was determined from the measurement of the fluorescent X-ray strength of silicon and aluminum atoms. In hybrid composites, Si3N 4 whiskers and A1203 short fibers are oriented nearly randomly such that the overall properties can be considered almost transversely isotropic. Thus the only two CTEs, i.e. in-plane and out-of-plane directions, are considered to be independent. Hence the
Density of composite material (Pc) i" Weight .......... "i i volume i L ...........
I
.J
Weight fraction of reinforcement (Wf) and resin (W r)
Resin ' F ............. I
r i burn-out i ,. .............
vfs = (w,a/p,)/{(wjp~)
where Wfa and Wfs denote the weight fraction of A1203 short fibers and Si3N4 whiskers respectively. "Kerimid %" in Fig. 6 stands for the weight fraction of Kerimid in the varnish at the impregnation process. It should be noted that several data points are missing in Fig. 6: for Kerimid 15% (open triangles) at t/= 0% (A1203 composite) and t/> 30%, and for Kerimid 20% (open squares) at t/= 100% (Si3N 4 composite). These missing data points indicate that the corresponding composites could not be processed due to either excess void formation or due to fracture of the prepreg sheets. The solid line in Fig. 6 indicates
.,
l
Weight fraction of AI203 (Wfa) and Si3N4 (Wfs)
r- .........................
-I
i Fluorescent X-rays ! I i of AI and Si ,•
l
.......................................
i I i
t .......................................
J
[ r l ~ I
L . . . . . . . . . . . . . . . . . . . . . . . . . .~
Volume fraction of , AI203 (Vf,) and SI3N . (Vf,)
+ (w,,/p,) + ( wjpr)}
= Wfs (Pc~P,)
where Pa is the density of AI203 /or the density of Kerimid 601 resin.
short
fibers,
p,
the
density
Fig. 5. Pr•cedure•fdeterminingthev••umefra•ti•ns•fA•2•3(Vfa)sh•rt•bersandSi3N4(Vf`)w•iskers•
of
SiaN 4 whiskers
and
149
the envelope of the maximum Vt attained for the hybrid composites. It is obvious from this envelope curve that V~_ma~ occurs at an intermediate value of r/, i.e. r/= 20%. Thus, it can be concluded that the addition of the small-sized fibers (whiskers) between the large-sized short fibers results in an improvement of the processability of composite materials in terms of increasing the volume fraction of reinforcement. The reason for this improvement is mainly due to the concept illustrated in Fig. 1, i.e. easier high density packing of the composite by using different sized fibers. However, another complementary mechanism must also be considered to explain the phenomena that, at the hybrid ratio of around 10% in Fig. 6, the volume fraction of the A1203 short fiber is 39%, being more than the maximum volume fraction of 34% for the single-phase reinforced composites (open triangles). 3.2. Coefficient of thermal expansion
For all the composites shown in Fig. 6, the measured CTEs along the in-plane ( a,, shown by filled symbols) and the out-of-plane direction (a: shown by open symbols) are plotted as a function of hybrid ratio q in Fig. 7. Also shown in Fig. 7 are the solid curves indicating the lowest CTEs
for two directions. It is clear from Fig. 7 that the lowest CTE occurs at an intermediate value of hybrid ratio q, clearly indicating the hybrid effect. It is also clear from Fig. 7 that remarkable improvement was obtained for the CTE along the out-of-plane direction a:, but a relatively small improvement resulted for that along the in-plane direction a w It should be noted here that although a,y is easy to design, even for monoreinforcement composites, ct_ is hard to control. Thus by hybridization of the reinforcement, it becomes possible to lower a. while maintaining low a,,, although the range of the control is rather limited. Next an attempt is made to compare the measured CTEs of hybrid composites with the predictions based on Eshelby's equivalent inclusion method developed for a misoriented shortfiber composite [4, 5]. This model is best suited for the analysis of complicated microstructures such as misoriented short-fiber composite systems, including hybrid composites. The purpose of this attempt is to estimate the microstructure of the hybrid composites. First, the measured CTEs of the hybrid composite with constant hybrid ratio (y/=9.19%) along the in-plane (filled symbols) and the out-of-
50
7O
O(Oz); • (Oxy):Kerimid 25% [] (Cl.z); • (Oxy): Kerimid 20% 4~(Oz); A(Oxy):Kerimid 15%
45
40
oo
0 O0 0
3! S
o
ooO
0 "X~
I
CD
o
40
Oz
X v
3{
3(2
25
20
O :Kerimid 25% [ ] :Kerimid 20%
i
/4. :Kerimid 15%
|
Oxy
20-
0
t
t
,
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
Fig. 6. Volume fraction V~ of fibers as a function of hybrid ratio q (~1= IOOWt,/(W,~+Wt,)o~)).
0
10
20
30
40
50
60
70
80
90
100
Fig. 7. CTEs of the hybrid composites in the x - y plane (a,, ;, and in the z direction (a:) as a function of hybrid ratio q.
150 I
O(QZ); O(O.xy):Kerimid 25% n(Oz); • (Oxy):Kerimid 20% ~.(Qz); A(Qxy):Kerimid 15%
80 r / - ~ .~. ,,. . . ",~ J- ./ " '~ 70 ~ . . / . ~ / ~ "~,.. ~
.........
AI2O3:2-D Random , SI3N4:2-DRandom AI203:2-D Random
-
Si3N4:3-DRandom
-
\-\
60" •
(13_=9.19) %,.
'
\
c7 ,0 ---'~...............................L.... i_
,o.
"\.,
i i i\
.,\ '~
i
i
10
i
i
t
0
t
10
t
t
20
t
I
30
- ' ~'- Oxy
I
40
50
I
I
60
vf (%) Fig. 8. CTEs of the hybrid composite: in the in-plane (x-y plane) (ax~) and in the out-of-plane (z direction) (a:) as a function of volume fraction Vf of fibers.
plane directions (open symbols) are plotted as a function of the volume fraction of the reinforcements (both Si3N4 whiskers and AI203 short fibers) Vf in Fig. 8. Also shown in Fig. 8 are the predictions based on Eshelby's equivalent inclusion method. The solid and dash-dot lines correspond to the case of A1203 short fibers distributed in a two-dimensionally random manner while Si3N 4 whiskers in a three-dimensionally random manner, and the case of both reinforcements distributed in a two-dimensionally random manner, respectively. By comparing the measured and predicted values of the composite CTEs, one can speculate that the distribution of AI:O3 short fibers is rather three- instead of two-dimensionally random. This is enhanced at higher volume fraction Vy of the reinforcements. This Vf dependency of A1203 short-fiber orientation may be partially seen in Fig. 9 where (a) and (b) denote typical scanning electron micrographs of the composite with the same hybrid ratio r/but with Vf = 35.10% and Vf = 45.07% respectively.
Fig. 9. Scanning electron micrographs of the fracture surface plane) of a hybrid composite at constant hybrid ratio r/ (24.44%): (a) Vt = 35.10%, (b) I/i = 45.07%.
(x-y
The reason for such orientational behavior of the A1203 short fibers is as follows. At the initial stage of the compression-molding process, entangled A1203 short fibers and Si3N4 whiskers interact to exert a compressive force with each other. The compressive force acts on the short fibers as a bending force, leading to the fracture of the short fibers. Apparently, the force applied to the short fibers rotates them and then some of the short fibers fracture resulting in the deviation of the orientation distribution of A1203 short fibers from the original two-dimensional random state to the thickness direction. This effect of the interaction force is obviously more intensive when the volume fraction of short fibers increases. Since Si3N4 whiskers are located in the gaps in the tangled A1203, the Si3N4 whiskers do not support the compressive force. This mechanism does not effectively act on the Si3N4 whiskers. In order to examine the effect of the hybrid ratio ~1, we have replotted the data of axy and a_ as a function of r/for a fixed Vf. For example, in Fig. 8 let us look at the vertical dotted line of Vf = 35%. Along this vertical line, we can find the values of ely and a,. The same kind of procedure
151
V f = 35%
70
.......
• (QZ) (Clxy)
AI203:2-D Random Si3N4:2-D R a n d o m
~
Vf =40%
.......
AI203:2-DRandom Si3N4:2-D Random
• (Qz)
~
AI203:2-D Random
m
AI203:2-D Random Si3N4:3-D R a n d o m
70 -
O(QxY)
Si3N4:3-D Random
m
60
60 Oz
Qz kJ
51
5O
G" I o x
I O X
40
v
30
C~
20
40
30
2~
Clxy lC
I 10
(a)
7C
I 20
i 30
i 40
i 50
I 60
I 70
I 80
I 90
Vf = 45%
AI203:2-D Random Si3N4:2-D R a n d o m
• (Oz)
AI203:2-D Random Si3N4:3-D R a n d o m
@ (Oxy)
I 100
(b)
0
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
6C
5G
az
0 X
4O
3O
2O
10
it/
-" - "
ClxY
i
=
=
t
=
I
I
I
I
I
0
10
20
30
40
50
60
70
80
90
100
Fig. 10. CTEs of the hybrid composite as a function of hybrid ratio r/for a fixed Vt: (a) Vf = 35%, (b) V~= 40%, (c) Vt = 45%.
152 shown in Figs. ll(a) Vf= 37.86%, q = 9.19% and 1 l(b) Vf = 39.61%, r/= 65.82%. If the results of the present study (Fig. 7) are compared with those of part I of this paper [4] (see Fig. 1 1 in part I), one can conclude that the hybrid composite gives rise to a lower c%. and, except for the case of spherical-filler composites, a lower c~= compared with composites with a single reinforcing phase. The processing cost of the hybrid composite, however, is higher than that of the composite with a single reinforcing phase.
4. Conclusions
Fig. 11. Scanning electron micrographs of the fracture surface (x-y plane) of hybrid composites for similar Vt: (a) ~/= 9.19% (V,= 37.86%), (b) r/= 65.82% (V~= 39.61%). was applied at six different ~/values, which correspond to data points in Fig. 7. Thus, the data of a= and c%. are plotted as a function of ~/for a given Vf in Fig. 10(a) Vf = 35%, (b) Vf= 40% and (c) Vf=45%. It follows from Fig. 10 that the measured a_ values are lower than the predication of the dash-dot line and are comparable with the solid line values which were derived by assuming that A1203 short fibers and Si3N 4 whiskers follow two- and three-dimensionally random directions respectively. It appears that the higher r/ becomes, the higher the relative value of the measured a= compared with the corresponding solid line, although at Vf=45% the data points of a_ for higher q values are missing (Fig. 10(c)). Hence, it can be speculated from Figs. 10(a) and 10(b) that for a given Vf, Si3N 4 whiskers at low r/values are more likely to be oriented along the thickness direction, while at higher r/values they tend to lie in the plane. This r/dependency of Si3N4 whisker orientation can be seen by looking at scanning electron photographs
The hybrid composites composed of A120 3 short fibers and Si3N 4 whiskers were fabricated by a combination of the paper-making and the compression-forming process. To clarify the advantages of this type of hybrid composites, attention was focussed on the maximum volume fraction of reinforcements attainable by these processes, and the effects of the microstructure of hybrid composites on the thermal expansion coefficients were studied. The following conclusions can be drawn from the present,study. (1) The maximum volume fraction of reinforcements can be increased by controlling the hybrid ratio. (2) CTEs of the hybrid composites can be decreased compared with the single-reinforcement composite system. This tendency was especially marked for the CTEs along the thickness direction. (3) The effect of hybridization was noticeable even at modest volume fractions of the second reinforcement (small-sized reinforcement).
References
1 K. M. Hardaker and M. O. W. Richardson, Polym. Plast. Technol. Eng., 15(1980) 169. 2 L.N. Phillips, Composites, 8(1976) 7. 3 D. Short and J. Summerscales, Composites, 11 (1979) 10. 4 T. Takei, H. Hatta and M. Taya, Mater. Sci. Eng., AI31 (1991)33. 5 Y. Takao, in J. R. Vinson and M. Taya (eds.), Recent Advances in Composites in United States and Japan] ;ASTM Spec. Tech. Publ., 864 (1985) 685.