- Email: [email protected]
Polydimethylsiloxane coated carbon fibres for the production of carbon-fibre reinforced carbon
(‘brho~z Vol. 2’). No. 8. pp. Printed in Great Bnuin.
IW-107(1.
(KKK+h2231Yl
1YYl Copyright
0
1991 Pergamon
$3.0(1 + .I10 Press plc
POLYDIMETHYLSILOXANE COATED CARBON FIBRES FOR THE PRODUCTION OF CARBON-FIBRE REINFORCED CARBON K. J. HUTTINGER and G. KREKEL Institut fiir Chemische Technik, Universitat Karlsruhe, Kaiserstr. 12, D-7500 Karlsruhe, Germany (Received 20 August 1990; accepted in revised form 7 December
1990)
Abstract-HT-fibres (Grafil XAS12K) were coated with thin layers of polydimethylsiloxane in order to prevent or diminish the adhesion of the phenolic resin to the fibre surfaces. The physical and chemical changes of the polydimethylsiloxane layer at various stages of curing and thermal treatment were studied using contact angle measurements, scanning electron microscopy, infrared spectroscopy, and ESCA. It is shown by the various methods that the adhesion can, in fact, be prevented up to 600°C or 700°C. Beyond this temperature the polydimethylsiloxane layer is decomposed to silica, which effects a very strong adhesion of the carbonized matrix. This is reflected by an interlaminar shear strength that is 60% higher than that of composites with non-coated fibres. Kev Words-Carbon-fibre she”ar strength.
reinforced
carbon, adhesion,
coating, interlaminar
2. EXPERIMENTAL
1. INTRODUCTION
Carbon-fibre reinforced carbons (CFRC) are frequently made using high-tenacity carbon fibres (HTfibres) and phenolic or similar thermosetting resins as precursors for the carbon matrix. However, the strong adhesion of the resin to the carbon-fibre surface and the significant shrinkage of the resin during curing and carbonisation often create problems if this combination of materials is used. The strong adhesion is indirectly indicated by the stress graphitisation of the non-graphitisable glassy carbon produced by carbonisation of the resin[l]. A less advantageous or even undesired consequence of the strong resin matrix adhesion may be damage to the fibre and even to the matrix[2]. For this reason, it is not surprising that composites produced with high-modulus carbon fibres (HMfibres) insteaad of HT-fibres have superior mechanical properties[3]. HM-fibres possess fewer of the active sites and functional groups that provide a strong adhesion between the HT-fibres and the resin matrix. The influence of surface groups on the mechanical properties of carbon-fibre reinforced carbons was demonstrated by YASUDA, ef al. using HM-fibres with a variety of surface treatments[4]. The objective of the present study was to develop a process for the production of CFRC from HTfibres and phenolic resin, in which the adhesion of the resin is prevented or at least diminished. For this purpose, the HT-fibres were coated with a thin layer of polydimethylsiloxane (PDMS). This polymer has an extremely low surface free energy, no polar groups, and additionally it is thermally very stable. It decomposes with formation of silica only at temperatures above 600°C.
polydimethylsiloxane
2.1 Raw materials HYSOL Grafil XAS12K fibres were used as the HT-fibres. The as-received fibres were surface treated but unsized. Table 1 shows a selection of data of this fibre according to the data sheet of HYSOL Grafil Ltd., England. For model studies, a glassy carbon, Sigradur K, commercially available from SIGRI, with a maximum heat-treatment temperature of llOo”C, was used. Small square plates of lo-mm edge length and a thickness of 1 mm were prepared and cleaned with boiling chloroform, which previously had been dried and distilled. The polydimethylsiloxane used was supplied by SIGMA Chemie GmbH (DPMS-12M). According to the producer, it exhibits a room temperature viscosity of 12.5 . 10-j m* . s-l. The phenolic resin employed was RP226, produced by Bakelite GmbH. This resin is a novolak with about 9 wt% hexamethylenetetramine as the curing agent. The phenolic resin was applied in a solution of 50 wt% in dried and distilled methanol.
2.2 Experimental procedures For a coating with PDMS, the samples (fibres and plates of glassy carbon) were impregnated by immersion for two minutes in a solution of, normally, 5 wt% PDMS in chloroform. In a second step, the PDMS on the surface of the samples was cured. For this purpose, the samples were heated in air to 360°C with a rate of 3 K/min (residence time normally 30 minutes). In order to study the decomposition reactions of the cured PDMS coating, some samples were heated to different temperatures in inert gas 1065
1066
K. J. H~?RINGER and G.
Table 1. Properties of the HYSOL Grafil carbon fibre used in the present study Type Producer
: Grafil XAS12K, unsized H/S2 : HYSOL Grafil Ltd., England
Tenacity Young’s modulus Strain to failure
: 4360 MN/m2 : 233 GN/m2 : 1.87 %
(heating rate: 5 K/min). The residence time was always one hour. For the production of unidirectional composites, fibre bundles were impregnated with the solution of the phenolic resin in methanol. After drying, the prepregs were used to manufacture composite specimens. The resin was cured at a pressure of 200 bar according to the temperature program shown in Table 2. Unidirectional composite specimens with an average fibre content of 45 ~01% in the case of the uncoated fibres and 44 ~01% in the case of coated fibres, were obtained. The specimens were carbonised in inert gas at different temperatures. In all cases, the heating rate was 12 Kihr, the residence time one hour, and the cooling rate 24 Kihr. 2.3 Analytical methods Scanning electron micrographs were obtained with a Cambridge Stereoscan S4-10 microscope. The samples were coated with a thin gold film. For infrared spectroscopy a Perkin-Elmer (model 283) IRdevice was used (KBr-technique). A Leybold LHS 10 ESCA device was used to obtain ESCA-spectra from different samples. Contact angle measurements were performed by first placing drops of water on the surface of different samples of glassy carbon. The drops were then photographed, and the contact angles were determined from the pictures. For the determination of the thickness of the coatings on the fibre surfaces the fibres were ashed. The amount of the remaining silica was determined by means of the molybdenum blue method[5]. The short beam test was performed according to DIN 29971 to determine the interlaminar shear strength of the composite specimens. 3.
KREKEL
Figure 1 shows contact angles on cured PDMS layers as a function of the PDMS concentration of the impregnation solutions. A contact angle of about loo”, as compared to 65” for the uncoated glassy carbon, is obtained with a solution of only 1 wt%. This high contact angle of 100” confirms the low surface free energy of the PDMS layer. Above a PDMS concentration of 20 wt% the contact angle strongly increases, but this increase is caused by cracks in the layer which form during the curing treatment. These cracks could be observed in scanning electron micrographs. Equation (1) describes the influence of surface roughness on the contact angle as compared to a smooth surface (no cracks)[6,7]: cos 6, = r * cos 8 0=
contact angle on a smooth surface
0, = contact angle on a rough surface r=
roughness coefficient
(1)
with r = A,IA A, = surface area of the rough surface A = surface area of the smooth surface.
(2)
From eqn (1) follows that the contact angle increases with increasing roughness if the contact angle on the smooth surface is greater than 90”. According to the results of Fig. 1, the fibres were impregnated using PDMS solutions of 2 and 5 wt%, respectively. Additional results by variation of the curing time showed that the maximum contact angle was already reached after 30 minutes curing time. Therefore, this curing time was chosen for the fibre coating. Figure 2 shows IR spectra of the uncured and
RESULTS
3.1 Properties of PDMS layers As already mentioned, PDMS layers on glassy carbon were used for model studies. These model studies comprise determinations of the surface properties and the decomposition of PDMS layers.
Table 2. Temperature profile for curing of the phenolic resin Temperature/C
120
160
180
Hold timeimin
30
120
720
P
6o0
Concentration
20 of the
60 40 Impregnation
80 Solution,
1 wt%
Fig. 1. Contact angle of water on surfaces of glassy carbon as a function of the concentration (PDMS) of the impregnation solution.
Carbon-fibre
PDMS,
uncured
reinforced carbon
(reference)
I
I
4000
1067
I
2000
1000
cm-t
200
Fig. 2. IR-spectra of uncured PDMS, cured PDMS, and silica.
cured PDMS and, for reference, the spectrum of silica, pulverized immediately before the measurement. The most significant change after curing is observed at about 3000 cm-‘. The peaks disappear nearly completely, indicating the loss of methyl groups due to the formation of a Si-0-Si network within the layer. The small peak of the cured PDMS at 3000 cm-’ should result from methyl groups at the surface of the layer, which are responsible for the low surface free energy. The cured PDMS layers on glassy carbon (the concentration of the impregnation solution was 5 wt%) were treated up to 900°C in an inert atmosphere in order to determine the surface free energy change with increasing heat-treatment temperature. Figure 3 shows the contact angles as a function of heattreatment temperature. Surprisingly, the contact an-
lOi,>
z
80
1
OO
200
400
Temperature,
0
600
800
1
1000
‘%
Fig. 3. Contact angle of water on a PDMS-coated surface of glassy carbon as a function of the treatment temperature.
gle does not change up to the very high temperature of 600°C. This is a favourable precondition for preventing the resin adhesion. Spreading is observed at 9Oo”C, indicating that the PDMS has completely decomposed to silica. ESCA studies of PDMS layers were performed after curing and heat treatment up to 900°C. Figure 4 shows the spectra in the range of the SL, signals. Peak tips of uncured PDMS and silica, according to [8], are presented for comparison. It follows that the binding energy of the Sizp electrons of the cured PDMS is roughly between those of the uncured PDMS and silica (PDMS/S-coating). At 900°C the binding energy is nearly identical to that of silica. The values resulting from Fig. 4 are summarized in Table 3. These results confirm the previous conclusions about the changes of PDMS caused by curing and further heat treatment. The thickness of the PDMS layers after curing and thermal treatment in inert gas were determined using coated fibres. After curing and thermal treatment up to 5OO”C, the silica content of the fibre bundle (12000 filaments) was found to be 2.7 mgim. After treatment at 900°C this value decreased to 2 mgim. From these amounts of silica the thickness of the layers can be estimated to be about 3 nm (on the basis of cristobalite[9]). 3.2 Carbonisation of the phenolic resin The carbonisation of the cured phenolic resin in inert atmosphere was studied at a heating rate of 12 K/h, which was also used in the carbonisation of the composites. Figure 5 shows the relative mass loss and the relative length shrinkage as a function of heat-treatment temperature. The mass loss occurs between 150” and approximately 6OO”C, but the shrinkage is shifted to higher
K. J.
1068
HOTTINGER
and G. KREKEL
treated at 900°&
110.6 Binding
Energy,
eV
Fig. 4. ESCA spectra over the range of the binding energy of electrons in Si,, orbitals.
temperatures, namely up to 900°C. Nevertheless, 65% at 600°C and nearly 90% at 700°C of the total shrinkage are reached. At 600°C the PDMS layer should be quite effective, and at 700°C it should, at least, be partially effective (see Fig. 3).
The results described have shown that the PDMS layer should actually prevent the adhesion of the resin matrix, at least up to 600°C. For a direct exami~ation of the adhesion phenomenon, fracture cross-sections of unidirectional composites, cured and heat treated up to 9OO”C,were studied by scanning electron microscopy. The Figs. 6(a) and (b) show scanning electron micrographs after treatment of the composite at 250°C (holding time: one hour), The differences between the two micrographs are obvious. Practically no pullout can be observed with the uncoated fibres (Fig. 6a) indicating a strong adhesion of the matrix to the carbon-fibre surface. In contrast, complete pull-out occurred with the coated fibres (Fig. 6b). Figures 7(a) and (b) indicate a different situation after treatment of the composites at 900°C. The fracture cross-sections of the composites with both un-
coated and coated fibres indicate a brittle fracture behavior, but it seems to be even more perfect with the coated fibres. This fracture behavior of the composite with the coated fibre is a direct consequence of the strong adhesion between the fibre and the carbonised resin, due to the silica interface. The differences in adhesion between fibres and matrix also follow from the shrinkage of the composites perpendicular to the fibre orientation. The results are presented in Table 4 for treatment temperatures of 250, 500, and 900°C. It is remarkable that the transverse shrinkage of the composites with the uncoated fibres is significantly higher only up to 500°C. For final testing of the adhesion phenomenon, the interlaminar shear strength of composites with uncoated and coated fibres were measured. The Figs. 8(a) to (c) show force-strain diagrams of the com-
Table 3. Binding energies for electrons in Si,, orbitals far different PDMS coated samples and reference materials (“) from [S]) Binding energylev
Sample PDMS2icoating PDMSYcoating PDMSS/coating (Me,SiO), Silica
treated at 900°C
102.7 102.5 103.6 101.4*1 103.2 to 103.W
Temperature,
%
Fig. 5. Relative mass loss (0) and relative length shrinkage (0) of the cured phenohc resin after carbonisation at vartous temperatures.
Carban-fibre
Fig. 6. Scanning electron micrographs of the cross-section of fractured composites fabricated with uncoated (a) and coated (h) fibres treated at 250°C.
posites
after treatment
1069
reinforced carbun
Fig. 7. Scanning electron micrographs of the cross-section of fractured composites fabricated with uncoated (a) and coated (b) fibres treated at 900°C.
4. SUMMARY
at 250,500, and 900°C. With
the uncoated fibres, the typical pseudo-plastic failure due to delamination can be observed, but only at 250 and 500°C. At the same temperatures, an extremely smooth curve was observed for the composites formed by use of coated fibres. After treatment at 900°C both types of composites show a dis-
A process of HT-fibres
was investigated
for ar\ improved
use
for the production of carbon-fibre reinforced carbon with a phenolic resin as the matrix precursor. It is shown that HT-fibres can be coated with a very thin layer of PDMS by impregnation of the fibres with a 5 wt% solution af PDMS in chlo-
tinct brittle fracture behaviour, and it is worthwhile to note that the maximum force is clearly higher with
the coated fibres. The resulting values of the ~nter~amiuar shear strength are presented in TabIe 5. It can be seen that the composite made from the coated fibres and carbonised at 900°C exhibits a 60% higher interlaminar shear strength than that with the uncoated fibres. At lower heat-treatment temperatures the ratio of the values of the interlaminar shear strength is inverse, This result confirms all earlier results and assumptions.
Table 4. Cross-section shrinkage of composites fabricated with uncoated and coated fibres, after carbon&a&n ax different temperatures Carbanisation temperature/T
250
500
900
Cross-section shrinkage of composites with uncoated fibres coated fibres
1.5% 0.2%
10.8% 7.8%
18.5% 36.8%
K. J. HCXTINGER and G. KREKEL
1070
Table 5. Inlerlaminar shear strength and maximum deviation (with 6 to 8 testings) of composites fabricated with uncoated and coated fibres, after carbonisation at different temperatures
8(a)
Carbonisation temperature/‘C - Uncoated
Strain,
mm
1
750,
ILSS [MN/m21 for composites with uncoated fibres coated fibres
Fibre
I
I
8(b)
1
0
2
Strain,
Coated
Uncoated
mm
8(c)
Fibre
Fibre
Strain,
mm
Fig. 8. Force-strain diagrams obtained by the short beam testing of composites fabricated with uncoated and coated fibres. The composites were previously treated at 250°C (a), 500°C (b), and 900°C (cf.
roform. As expected, these layers really have a low surface free energy and a high thermal stability which strongly reduces the adhesion of the phenolic resin to the fibre surface up to advanced carbonisation temperatures. At these temperatures, between 600 and 700°C the carbonisation shrinkage of the phe-
250
500
900
71.4 2 5.5 39.8 -+.3.3
27.2 + 1.6 23.9 t 3.7
15.2 5 5.1 24.1 + 1.9
nolic resin is complete in effect. Beyond these temperatures, the PDMS is completely decomposed to silica. This silica interlayer exhibits a strong adhesion to the fibre surface as well as to the matrix carbon. After carbonisation at 900°C the interlaminar shear *strength of the composites with these fibres is 60% higher than that with the uncoated fibres. Carbon-fibre reinforced carbons are usually applied only after a graphitisation treatment. Such treatments could not be performed. However, no difficulties during a graphitisation treatment are expected since the thickness of the silica interlayer only amounts to 3 nm. It is probable that silica is reduced to silicon monoxide during such a treatment above 2000°C. Intermediately formed silicon carbide by reaction of silicon oxide with carbon should also not be stable. For this reason, the brittleness that is observed after carbonisation should no longer exist after the graphitisation treatment. Acknuwledgement-Financial
&hunk Kohlenstoff-technik acknowledged.
support for this study by GmbH, Gief3en, is gratefully
REFERENCES
1. E. Fitzer and B. Terwiesch, 2. Werkstofftech. 5, 53 (1974). 2. M. A. Malstre, In Ext. Abstr. 14th Biennial Ccmf. Carbon, p. 230. Pennsylvania State University, University Park, PA., (1979). and 919 3. E. Fitzer and W. Htittner, Sprechsaalll3,452 (1980). 4. L. M. Manocha, E. Yasuda, Y. Tanabe, and S. Kimura, Carbon 26, 333 (1988). 5. H. M. Koster, Die chem~che S~likatana~y~e, Springer Vedag, Berlin (1979). 6. R. M. Wenzel, Ind. Eng. Chem. 28,968 (1956). 7. E. Krell, Chem.-Tech. (Leipzig) 16, 591 (1964). 8. Handbook of Photoelectron Spectroscopy, edited by G. E. Muilenbera, Perkin-Elmer, Eden Prairie, MN (1979). Lehrbuch der an9. N. Wiberg, In Hollemann-Wiberg, organ&hen Chemie, Plst-100th edition p. 7531754. W.
de Gruyter, Berlin (1985).
IW-107(1.
(KKK+h2231Yl
1YYl Copyright
0
1991 Pergamon
$3.0(1 + .I10 Press plc
POLYDIMETHYLSILOXANE COATED CARBON FIBRES FOR THE PRODUCTION OF CARBON-FIBRE REINFORCED CARBON K. J. HUTTINGER and G. KREKEL Institut fiir Chemische Technik, Universitat Karlsruhe, Kaiserstr. 12, D-7500 Karlsruhe, Germany (Received 20 August 1990; accepted in revised form 7 December
1990)
Abstract-HT-fibres (Grafil XAS12K) were coated with thin layers of polydimethylsiloxane in order to prevent or diminish the adhesion of the phenolic resin to the fibre surfaces. The physical and chemical changes of the polydimethylsiloxane layer at various stages of curing and thermal treatment were studied using contact angle measurements, scanning electron microscopy, infrared spectroscopy, and ESCA. It is shown by the various methods that the adhesion can, in fact, be prevented up to 600°C or 700°C. Beyond this temperature the polydimethylsiloxane layer is decomposed to silica, which effects a very strong adhesion of the carbonized matrix. This is reflected by an interlaminar shear strength that is 60% higher than that of composites with non-coated fibres. Kev Words-Carbon-fibre she”ar strength.
reinforced
carbon, adhesion,
coating, interlaminar
2. EXPERIMENTAL
1. INTRODUCTION
Carbon-fibre reinforced carbons (CFRC) are frequently made using high-tenacity carbon fibres (HTfibres) and phenolic or similar thermosetting resins as precursors for the carbon matrix. However, the strong adhesion of the resin to the carbon-fibre surface and the significant shrinkage of the resin during curing and carbonisation often create problems if this combination of materials is used. The strong adhesion is indirectly indicated by the stress graphitisation of the non-graphitisable glassy carbon produced by carbonisation of the resin[l]. A less advantageous or even undesired consequence of the strong resin matrix adhesion may be damage to the fibre and even to the matrix[2]. For this reason, it is not surprising that composites produced with high-modulus carbon fibres (HMfibres) insteaad of HT-fibres have superior mechanical properties[3]. HM-fibres possess fewer of the active sites and functional groups that provide a strong adhesion between the HT-fibres and the resin matrix. The influence of surface groups on the mechanical properties of carbon-fibre reinforced carbons was demonstrated by YASUDA, ef al. using HM-fibres with a variety of surface treatments[4]. The objective of the present study was to develop a process for the production of CFRC from HTfibres and phenolic resin, in which the adhesion of the resin is prevented or at least diminished. For this purpose, the HT-fibres were coated with a thin layer of polydimethylsiloxane (PDMS). This polymer has an extremely low surface free energy, no polar groups, and additionally it is thermally very stable. It decomposes with formation of silica only at temperatures above 600°C.
polydimethylsiloxane
2.1 Raw materials HYSOL Grafil XAS12K fibres were used as the HT-fibres. The as-received fibres were surface treated but unsized. Table 1 shows a selection of data of this fibre according to the data sheet of HYSOL Grafil Ltd., England. For model studies, a glassy carbon, Sigradur K, commercially available from SIGRI, with a maximum heat-treatment temperature of llOo”C, was used. Small square plates of lo-mm edge length and a thickness of 1 mm were prepared and cleaned with boiling chloroform, which previously had been dried and distilled. The polydimethylsiloxane used was supplied by SIGMA Chemie GmbH (DPMS-12M). According to the producer, it exhibits a room temperature viscosity of 12.5 . 10-j m* . s-l. The phenolic resin employed was RP226, produced by Bakelite GmbH. This resin is a novolak with about 9 wt% hexamethylenetetramine as the curing agent. The phenolic resin was applied in a solution of 50 wt% in dried and distilled methanol.
2.2 Experimental procedures For a coating with PDMS, the samples (fibres and plates of glassy carbon) were impregnated by immersion for two minutes in a solution of, normally, 5 wt% PDMS in chloroform. In a second step, the PDMS on the surface of the samples was cured. For this purpose, the samples were heated in air to 360°C with a rate of 3 K/min (residence time normally 30 minutes). In order to study the decomposition reactions of the cured PDMS coating, some samples were heated to different temperatures in inert gas 1065
1066
K. J. H~?RINGER and G.
Table 1. Properties of the HYSOL Grafil carbon fibre used in the present study Type Producer
: Grafil XAS12K, unsized H/S2 : HYSOL Grafil Ltd., England
Tenacity Young’s modulus Strain to failure
: 4360 MN/m2 : 233 GN/m2 : 1.87 %
(heating rate: 5 K/min). The residence time was always one hour. For the production of unidirectional composites, fibre bundles were impregnated with the solution of the phenolic resin in methanol. After drying, the prepregs were used to manufacture composite specimens. The resin was cured at a pressure of 200 bar according to the temperature program shown in Table 2. Unidirectional composite specimens with an average fibre content of 45 ~01% in the case of the uncoated fibres and 44 ~01% in the case of coated fibres, were obtained. The specimens were carbonised in inert gas at different temperatures. In all cases, the heating rate was 12 Kihr, the residence time one hour, and the cooling rate 24 Kihr. 2.3 Analytical methods Scanning electron micrographs were obtained with a Cambridge Stereoscan S4-10 microscope. The samples were coated with a thin gold film. For infrared spectroscopy a Perkin-Elmer (model 283) IRdevice was used (KBr-technique). A Leybold LHS 10 ESCA device was used to obtain ESCA-spectra from different samples. Contact angle measurements were performed by first placing drops of water on the surface of different samples of glassy carbon. The drops were then photographed, and the contact angles were determined from the pictures. For the determination of the thickness of the coatings on the fibre surfaces the fibres were ashed. The amount of the remaining silica was determined by means of the molybdenum blue method[5]. The short beam test was performed according to DIN 29971 to determine the interlaminar shear strength of the composite specimens. 3.
KREKEL
Figure 1 shows contact angles on cured PDMS layers as a function of the PDMS concentration of the impregnation solutions. A contact angle of about loo”, as compared to 65” for the uncoated glassy carbon, is obtained with a solution of only 1 wt%. This high contact angle of 100” confirms the low surface free energy of the PDMS layer. Above a PDMS concentration of 20 wt% the contact angle strongly increases, but this increase is caused by cracks in the layer which form during the curing treatment. These cracks could be observed in scanning electron micrographs. Equation (1) describes the influence of surface roughness on the contact angle as compared to a smooth surface (no cracks)[6,7]: cos 6, = r * cos 8 0=
contact angle on a smooth surface
0, = contact angle on a rough surface r=
roughness coefficient
(1)
with r = A,IA A, = surface area of the rough surface A = surface area of the smooth surface.
(2)
From eqn (1) follows that the contact angle increases with increasing roughness if the contact angle on the smooth surface is greater than 90”. According to the results of Fig. 1, the fibres were impregnated using PDMS solutions of 2 and 5 wt%, respectively. Additional results by variation of the curing time showed that the maximum contact angle was already reached after 30 minutes curing time. Therefore, this curing time was chosen for the fibre coating. Figure 2 shows IR spectra of the uncured and
RESULTS
3.1 Properties of PDMS layers As already mentioned, PDMS layers on glassy carbon were used for model studies. These model studies comprise determinations of the surface properties and the decomposition of PDMS layers.
Table 2. Temperature profile for curing of the phenolic resin Temperature/C
120
160
180
Hold timeimin
30
120
720
P
6o0
Concentration
20 of the
60 40 Impregnation
80 Solution,
1 wt%
Fig. 1. Contact angle of water on surfaces of glassy carbon as a function of the concentration (PDMS) of the impregnation solution.
Carbon-fibre
PDMS,
uncured
reinforced carbon
(reference)
I
I
4000
1067
I
2000
1000
cm-t
200
Fig. 2. IR-spectra of uncured PDMS, cured PDMS, and silica.
cured PDMS and, for reference, the spectrum of silica, pulverized immediately before the measurement. The most significant change after curing is observed at about 3000 cm-‘. The peaks disappear nearly completely, indicating the loss of methyl groups due to the formation of a Si-0-Si network within the layer. The small peak of the cured PDMS at 3000 cm-’ should result from methyl groups at the surface of the layer, which are responsible for the low surface free energy. The cured PDMS layers on glassy carbon (the concentration of the impregnation solution was 5 wt%) were treated up to 900°C in an inert atmosphere in order to determine the surface free energy change with increasing heat-treatment temperature. Figure 3 shows the contact angles as a function of heattreatment temperature. Surprisingly, the contact an-
lOi,>
z
80
1
OO
200
400
Temperature,
0
600
800
1
1000
‘%
Fig. 3. Contact angle of water on a PDMS-coated surface of glassy carbon as a function of the treatment temperature.
gle does not change up to the very high temperature of 600°C. This is a favourable precondition for preventing the resin adhesion. Spreading is observed at 9Oo”C, indicating that the PDMS has completely decomposed to silica. ESCA studies of PDMS layers were performed after curing and heat treatment up to 900°C. Figure 4 shows the spectra in the range of the SL, signals. Peak tips of uncured PDMS and silica, according to [8], are presented for comparison. It follows that the binding energy of the Sizp electrons of the cured PDMS is roughly between those of the uncured PDMS and silica (PDMS/S-coating). At 900°C the binding energy is nearly identical to that of silica. The values resulting from Fig. 4 are summarized in Table 3. These results confirm the previous conclusions about the changes of PDMS caused by curing and further heat treatment. The thickness of the PDMS layers after curing and thermal treatment in inert gas were determined using coated fibres. After curing and thermal treatment up to 5OO”C, the silica content of the fibre bundle (12000 filaments) was found to be 2.7 mgim. After treatment at 900°C this value decreased to 2 mgim. From these amounts of silica the thickness of the layers can be estimated to be about 3 nm (on the basis of cristobalite[9]). 3.2 Carbonisation of the phenolic resin The carbonisation of the cured phenolic resin in inert atmosphere was studied at a heating rate of 12 K/h, which was also used in the carbonisation of the composites. Figure 5 shows the relative mass loss and the relative length shrinkage as a function of heat-treatment temperature. The mass loss occurs between 150” and approximately 6OO”C, but the shrinkage is shifted to higher
K. J.
1068
HOTTINGER
and G. KREKEL
treated at 900°&
110.6 Binding
Energy,
eV
Fig. 4. ESCA spectra over the range of the binding energy of electrons in Si,, orbitals.
temperatures, namely up to 900°C. Nevertheless, 65% at 600°C and nearly 90% at 700°C of the total shrinkage are reached. At 600°C the PDMS layer should be quite effective, and at 700°C it should, at least, be partially effective (see Fig. 3).
The results described have shown that the PDMS layer should actually prevent the adhesion of the resin matrix, at least up to 600°C. For a direct exami~ation of the adhesion phenomenon, fracture cross-sections of unidirectional composites, cured and heat treated up to 9OO”C,were studied by scanning electron microscopy. The Figs. 6(a) and (b) show scanning electron micrographs after treatment of the composite at 250°C (holding time: one hour), The differences between the two micrographs are obvious. Practically no pullout can be observed with the uncoated fibres (Fig. 6a) indicating a strong adhesion of the matrix to the carbon-fibre surface. In contrast, complete pull-out occurred with the coated fibres (Fig. 6b). Figures 7(a) and (b) indicate a different situation after treatment of the composites at 900°C. The fracture cross-sections of the composites with both un-
coated and coated fibres indicate a brittle fracture behavior, but it seems to be even more perfect with the coated fibres. This fracture behavior of the composite with the coated fibre is a direct consequence of the strong adhesion between the fibre and the carbonised resin, due to the silica interface. The differences in adhesion between fibres and matrix also follow from the shrinkage of the composites perpendicular to the fibre orientation. The results are presented in Table 4 for treatment temperatures of 250, 500, and 900°C. It is remarkable that the transverse shrinkage of the composites with the uncoated fibres is significantly higher only up to 500°C. For final testing of the adhesion phenomenon, the interlaminar shear strength of composites with uncoated and coated fibres were measured. The Figs. 8(a) to (c) show force-strain diagrams of the com-
Table 3. Binding energies for electrons in Si,, orbitals far different PDMS coated samples and reference materials (“) from [S]) Binding energylev
Sample PDMS2icoating PDMSYcoating PDMSS/coating (Me,SiO), Silica
treated at 900°C
102.7 102.5 103.6 101.4*1 103.2 to 103.W
Temperature,
%
Fig. 5. Relative mass loss (0) and relative length shrinkage (0) of the cured phenohc resin after carbonisation at vartous temperatures.
Carban-fibre
Fig. 6. Scanning electron micrographs of the cross-section of fractured composites fabricated with uncoated (a) and coated (h) fibres treated at 250°C.
posites
after treatment
1069
reinforced carbun
Fig. 7. Scanning electron micrographs of the cross-section of fractured composites fabricated with uncoated (a) and coated (b) fibres treated at 900°C.
4. SUMMARY
at 250,500, and 900°C. With
the uncoated fibres, the typical pseudo-plastic failure due to delamination can be observed, but only at 250 and 500°C. At the same temperatures, an extremely smooth curve was observed for the composites formed by use of coated fibres. After treatment at 900°C both types of composites show a dis-
A process of HT-fibres
was investigated
for ar\ improved
use
for the production of carbon-fibre reinforced carbon with a phenolic resin as the matrix precursor. It is shown that HT-fibres can be coated with a very thin layer of PDMS by impregnation of the fibres with a 5 wt% solution af PDMS in chlo-
tinct brittle fracture behaviour, and it is worthwhile to note that the maximum force is clearly higher with
the coated fibres. The resulting values of the ~nter~amiuar shear strength are presented in TabIe 5. It can be seen that the composite made from the coated fibres and carbonised at 900°C exhibits a 60% higher interlaminar shear strength than that with the uncoated fibres. At lower heat-treatment temperatures the ratio of the values of the interlaminar shear strength is inverse, This result confirms all earlier results and assumptions.
Table 4. Cross-section shrinkage of composites fabricated with uncoated and coated fibres, after carbon&a&n ax different temperatures Carbanisation temperature/T
250
500
900
Cross-section shrinkage of composites with uncoated fibres coated fibres
1.5% 0.2%
10.8% 7.8%
18.5% 36.8%
K. J. HCXTINGER and G. KREKEL
1070
Table 5. Inlerlaminar shear strength and maximum deviation (with 6 to 8 testings) of composites fabricated with uncoated and coated fibres, after carbonisation at different temperatures
8(a)
Carbonisation temperature/‘C - Uncoated
Strain,
mm
1
750,
ILSS [MN/m21 for composites with uncoated fibres coated fibres
Fibre
I
I
8(b)
1
0
2
Strain,
Coated
Uncoated
mm
8(c)
Fibre
Fibre
Strain,
mm
Fig. 8. Force-strain diagrams obtained by the short beam testing of composites fabricated with uncoated and coated fibres. The composites were previously treated at 250°C (a), 500°C (b), and 900°C (cf.
roform. As expected, these layers really have a low surface free energy and a high thermal stability which strongly reduces the adhesion of the phenolic resin to the fibre surface up to advanced carbonisation temperatures. At these temperatures, between 600 and 700°C the carbonisation shrinkage of the phe-
250
500
900
71.4 2 5.5 39.8 -+.3.3
27.2 + 1.6 23.9 t 3.7
15.2 5 5.1 24.1 + 1.9
nolic resin is complete in effect. Beyond these temperatures, the PDMS is completely decomposed to silica. This silica interlayer exhibits a strong adhesion to the fibre surface as well as to the matrix carbon. After carbonisation at 900°C the interlaminar shear *strength of the composites with these fibres is 60% higher than that with the uncoated fibres. Carbon-fibre reinforced carbons are usually applied only after a graphitisation treatment. Such treatments could not be performed. However, no difficulties during a graphitisation treatment are expected since the thickness of the silica interlayer only amounts to 3 nm. It is probable that silica is reduced to silicon monoxide during such a treatment above 2000°C. Intermediately formed silicon carbide by reaction of silicon oxide with carbon should also not be stable. For this reason, the brittleness that is observed after carbonisation should no longer exist after the graphitisation treatment. Acknuwledgement-Financial
&hunk Kohlenstoff-technik acknowledged.
support for this study by GmbH, Gief3en, is gratefully
REFERENCES
1. E. Fitzer and B. Terwiesch, 2. Werkstofftech. 5, 53 (1974). 2. M. A. Malstre, In Ext. Abstr. 14th Biennial Ccmf. Carbon, p. 230. Pennsylvania State University, University Park, PA., (1979). and 919 3. E. Fitzer and W. Htittner, Sprechsaalll3,452 (1980). 4. L. M. Manocha, E. Yasuda, Y. Tanabe, and S. Kimura, Carbon 26, 333 (1988). 5. H. M. Koster, Die chem~che S~likatana~y~e, Springer Vedag, Berlin (1979). 6. R. M. Wenzel, Ind. Eng. Chem. 28,968 (1956). 7. E. Krell, Chem.-Tech. (Leipzig) 16, 591 (1964). 8. Handbook of Photoelectron Spectroscopy, edited by G. E. Muilenbera, Perkin-Elmer, Eden Prairie, MN (1979). Lehrbuch der an9. N. Wiberg, In Hollemann-Wiberg, organ&hen Chemie, Plst-100th edition p. 7531754. W.
de Gruyter, Berlin (1985).