NANOGRAPHENE REINFORCED CARBON-CARBON COMPOSITES

Dhruv Bansal1, Selvum Pillay2, and Uday Vaidya3 Department of Materials Science and Engineering

Materials Processing and Applications Development (MPAD) Center University of Alabama at Birmingham (UAB), Birmingham, Alabama 35294

1Doctoral candidate, 2Associate Professor, 3Professor

Abstract Carbon-carbon composites (CCC) have applications in under-the-hood and friction

applications in automobiles where high heat is generated. In this study, CCC was produced by using nanographene platelets (NGP) as nanofillers. Different weight concentration (0.5 wt%, 1.5 wt%, 3 wt%, 5 wt%) NGPs were introduced by spraying the NGPs during the prepreg formation. The nanographene reinforced CCC was characterized for effect of NGP concentration on microstructure, porosity, inter laminar shear strength (ILSS) and flexural strength. It was found that flexure properties and ILSS increased whereas porosity decreased with addition of NGP. At 1.5 wt% NGP CCC, the highest values of ILSS observed was 10 MPa (increased by 22%), flexural strength of 142 MPa (increased by 27%), flexural modulus of 59 GPa (increased by 68%) and porosity of 18% (reduced by 17%) in comparison to neat (without NGP) densified CCC. At low concentration (≤ 1.5 wt%) NGPs filled in the pores, cracks and debonded interface but at concentration higher than 1.5 wt% NGPs lost their effectiveness due to agglomeration.

1. Introduction CCC are known for their unique properties of low density (1.6-18 gcm-3), low coefficient of

thermal expansion, high thermal shock resistance, electric conductivity, high strength, stiffness, wear, fatigue properties at high, non-oxidizing temperatures (30000C). Due to their superior properties they are used for high temperature applications such as nose cones, heat shields, nozzles for rocket re-entry vehicles, disc brakes, diesel engine components, high temperature corrosion resistant fasteners, hot press dies and plates for fuel cell1-5. The properties of CCC can be tailored by proper selection of reinforcement, matrix, processing conditions resulting in desired microstructure.

Processing of CCC is done in four stages - as-cured, carbonization, densification and an optional graphitization step6. In the as-cured stage, the carbon fiber reinforcement is infused with phenolic resin vacuum assisted resin transfer method (VARTM) and the pre-preg obtained is cured by applying pressure and temperature. The cured composite is carbonized in an inert atmosphere at 800-10000C to remove elements expect carbon which forms a carbon-carbon composite. Porosity and cracks are generated during carbonization due to thermal shrinkage and difference in thermal conductivity of carbon fiber reinforcement and matrix. The densification step increases carbon content and eliminates porosity and microcracks by liquid infiltration of a high char yield resin followed by re-carbonization. Repeated densification and carbonization cycles are required to achieve target density and desired properties making the manufacturing of CCC very expensive.

Carbon nanofillers like graphite powder, single-walled carbon nanotubes (SWCNTs), muti-walled carbon nanotubes (MWCNTs) and vapor grown carbon nanofibers (VGCNF) have been introduced for CCC by various researchers. Carbon nanofillers help to tailor electrical, mechanical and thermal properties of CCC, decrease the matrix shrinkage, bridge the cracks formation and increase the carbon content of the carbonized matrix. Kang and Jeong7 introduced micro sized graphite and pitch particles into phenolic resin and reported that the ILSS and flexure properties increase. Yasuda8 found a decrease in the phenolic matrix shrinkage during carbonization with the addition of graphite particles. Ma et al.9 reported an increase in flexural strength and toughness with the incorporation of 5 wt % graphite powder in phenolic matrix due to the decrease in void content after carbonization. Tai et al.10, 11 introduced SWCNT’s and MWCNT’s into phenolic matrix and reported that Young’s modulus increased by 29.7% and the tensile strength increased by 20.3% with the addition of 0.75 wt% and 2 wt% SWCNT respectively. Increase in loss and storage modulus was reported with the incorporation of MWCNTs into the phenolic resin. Manocha12,13 introduced VGCNF into phenolic matrix and reported increased mechanical, thermal and electrical conductivity due to nano crystalline anisotropic orientation at the interface. Dhakate14 was able to achieve 1.8-2.1 gm cm-3 CCC by applying isostatic pressure during carbonization at 1000 0C incorporating VGCNF into pitch followed by heat treatment at 2500 0C without any densification. Jain et al.15 found that VGCNFs provided a bridging mechanism for matrix microcracking and reduce matrix shrinkage during carbonization. They reported the highest ILSS values of ∼40 MPa and 6 MPa at the as-cured and carbonized stages respectively with the addition of 2% VGCNFs.

Nanofillers are difficult to disperse in resin and offer processing challenges associated with viscosity. In previous studies16,17, NGPs were compared against VGCNF in phenolic resin. In our prior work, we have made the case for NGPs over VGCNFs for CCC, as illustrated in Figure 1. The dispersion of NGPs has high fluidity at 1.5 wt% and 3 wt% in comparison to VGCNF. This behavior has effect on impregnation and densification of CCC. It was concluded that NGP increase heat of curing, reduce viscosity, increase wetting and carbon content of phenolic resin and thus may be used as nanofillers for CCC.

Figure 1. Fluidity of different concentration dispersions of NGP and VGCNF in neat phenolic resin. The dispersion of

NGPs show high fluidity at 1.5 wt% and 3 wt% in comparison to VGCNF. This behavior has effect on impregnation and densification of CCC.

In the present study, phenolic resin based CCC were fabricated by using NGP as nanofillers. Different weight concentration (0.5wt%, 1.5wt%, 3wt%, 5wt%) NGP were introduced, first by spraying them on desized/surfactant treated 8-harness satin weave carbon fiber. The materials were densification by vacuum liquid re-impregnation. The nanographene reinforced CCC was characterized for effects of NGP concentration on microstructure, porosity, inter laminar shear strength (ILSS) and flexural behavior.

2. Experimental 2.1. Materials

Resole type phenolic resin (GP 486G34) with catalyst (GP 4826C) (Supplier: Georgia Pacific Resins, Inc.) was used as matrix. 8-Harness satin weave carbon fabric (Supplier: U.S Composites) with tow size of 6k and 0.44 mm thickness was used as reinforcement. Trition X-100(t-Octylphenoxypolyethoxyethanol) from Sigma Aldrich was used as a surfactant. Commercial grade acetone was used to desize the fabric. N-N Dimethlyformamide anhydrous 99.8% from Sigma Aldrich used as a dispersing medium. Nanographene platelets (N008-100-P-10) from Angstron Materials, Ohio with 1.4% atomic percentage of oxygen were used as filler having average x-y dimensions less than 10 µm and z dimension between 50-100 nm.

2.2. Methods 2.2.1 NGP sprayed pre-preg

Eight plies of 8-harness carbon satin fabric was desized using acetone by immersing for 15 hours. This was followed by 12.5% v/v surfactant (Triton X-100) treatment in acetone for 24 hours. Surface treated fabrics were dried in air. The required weight of NGP per the desired concentration was dispersed in DMF using Cole Palmer ultrasonic bath (Model 8852-34) for two intervals of 7 minutes each. The dispersion ratio of 0.5 grams of NGP per 75 ml of DMF was maintained. The prepared dispersion was sprayed on the eight fabric layers by means of an air spray gun resulting in a fine uniform layer of NGP on the fabric plies (Figure 2). After spraying, the wet sheets were dried in an air oven for 30 min at 1600C to evaporate the DMF from the plies (boiling point of DMF is 1530C). NGP sprayed plies were used to prepare a pre-preg with the vacuum assisted resin transfer method infusing phenolic resin mixed with catalyst in the ratio 100:8. The composite was left under vacuum for 24 hours to obtain the prepreg.

Figure 2. (a) Sprayed NGP on satin carbon weave ply; (b) SEM image of sprayed NGP’s on carbon fabric

2.2.2 Processing

The prepreg was pressed by applying a pressure of 40 psi and the temperature was ramped from 60 0C to 90 0C at the rate of 10 0C per 2 hours. The material was carbonized in a tube furnace at maximum 8000C for a cycle time of approximately 18 hours. The carbonization step was necessary to remove the volatiles from the phenolic resin. Different concentration dispersions of NGP (0.5 wt%, 1.5 wt%, 3 wt% and 5 wt%) in phenolic were prepared using ultrasonic probe and shear mixer as done in our previous study16. The dispersions were then used to densify the respective NGP concentration carbonized CCC. The densification was done for 24 hours and CCC was cured under 25 mm Hg vacuum at heating rate of 400C to 140 0C with increments of 50 0C / 2 hours for a total of 4 hours.

2.3. Characterization Various physical and mechanical characterization methods were adopted:- (a) Dry weight,

suspended weight and wet weight were measured using Mettler Toledo balance (Model AG204, equipped with immersion density apparatus). Testing and calculations were done according to ASTM C20; (b) ILSS and flexure tests were performed using SATEC T-500 screw driven machine at room temperature using ASTM D2344 and ASTM C393. Samples measuring 24 × 8 × 4mm with span length of 16mm for ILSS and 80 ×10 × 4mm with span length of 64 mm for flexure were loaded at the rate of 1.3mm/min for flexure and at 1mm/min for ILSS testing respectively.

3. Results and discussion 3.1. Microstructure, morphology, porosity and bulk density

The composites were observed under stereomicroscope and field emission scanning electron microscope for changes in microstructure and morphology after the stages of as-cured, carbonization and densification. Figure 3 illustrates the effect of processing stages in neat, 0.5 wt%, 1.5 wt%, 3 wt% and 5wt% NGP reinforced CCC.

Figure 3. Stereomicroscope pictures of different concentration NGP CCC at as-cured, carbonized and densified stage

As illustrated in Figure 4, at the as-cured stage the least porosity of 3.2% was observed at 1.5 wt% NGP followed by 3.9% porosity at 0.5 wt% NGP. Bulk density increased by 6% at 1.5 wt% NGP compared to neat CCC. The images confirmed the decrease in porosity at 0.5wt% NGP and 1.5 wt% NGP. It was also noticed that the size of the pores decreased at 0.5wt% and 1.5wt% NGP as compared to neat as-cured composite. However, as the concentration of NGP increased to 3wt% and 5wt%, the porosity increased to 4.3% and 4.9% respectively.

Decrease in porosity at lower NGP concentration (≤ 1.5wt %) was attributed to filling of pores by NGP as they were less likely to get agglomerated. Figure 5a illustrates NGP filling the pores at 1.5wt% NGP as-cured composite. At concentration greater than 1.5wt% NGP, NGP had tendency to get agglomerated in phenolic resin, as also observed in earlier studies16,17. Agglomeration of NGP may have also created dry areas and increased porosity. Increase in porosity at high carbon nanofiller concentration in phenolic has also been reported by Jain et al15.

Figure 4. Physical characteristics (a) bulk density and (b) porosity measurements of C/C composites infused with

different concentrations of NGP at three stages of manufacturing

During the carbonization stage, the porosity increased for all the cases due to thermal induced matrix shrinkage and microcracking, creating microporosity in the material. Interfacial debonding was also observed due to thermal mismatch. In case of the 0.5wt% and 1.5 wt% NGP carbonized CCC, less matrix cracking was observed compared to neat, 3wt% and 5 wt% NGP carbonized CCC.

This can be explained as follows. The rigid NGP platelets provide the tortuous route for the volatiles to escape resulting in less porosity and increase in char yield21. The least porosity of 19% and highest density of 1.3 g cm-3 were observed in case of the 1.5 wt% carbonized CCC. The behavior was governed by two mechanisms. At lower concentration (≤ 1.5 wt%), NGP were small enough to fill in the pores and slip into the cracks and debonded interface, see Figure 5b. However, at higher concentration (>1.5 wt%) NGP agglomerated and were not small enough to penetrate into cracks and voids. Figure 5b shows agglomerated NGP in the proximity of the surface of the debonded area, where as single NGP sheets penetrate inside to fill the empty space. Figure 5 c,d & e indicate the micro cracks filled by NGP in 3 wt% NGP carbonized CCC. Figure 5f illustrates NGPs between the carbon fiber filaments.

At the densification stage, some of the pores formed during the carbonization were filled. In general, the area of large pores formed during carbonization was reduced. Microstructure of each sample was governed by carbonization process where as densification reduced the damage by filling voids and cracks to some degree. After densification, the porosity decreased to 12.2% and bulk density increased by 6.3% in 1.5 wt% NGP densified CCC as compared to neat CCC. Although more number of densification and carbonization cycles will be required to obtain the desired void and crack free microstructure, due to less porosity in ≤ 1.5 wt% NGP CCC compared to neat CCC, it is expected that the number of required cycles will be less.

Figure 5. SEM images of CCC infused with NGP at different stages

3.2 Inter Laminar Shear Strength (ILSS) and Flexure Testing

ILSS and flexure testing of the CCC with different concentrations of NGPs was conducted. A general trend of increase in properties at all three stages (as-cured, carbonization and densification) with NGP addition was observed (Figure 6).

At the as-cured stage the highest interlaminar shear strength observed was 36.4 MPa with 0.5 wt% NGP addition an increase of 36.5% compared to neat composite. Flexure modulus increased by 45% to 61GPa in 1.5 wt% NGP composite and flexure strength increased by 8.8 % to 548 MPa in 5 wt% as-cured composite. This increase could be attributed to the benefits of NGPs which translate to increased properties of the as-cured composites.

After the carbonization, ILSS and flexure properties of all the CCC samples decreased due to increase in porosity, transverse thermal cracking and shrinkage of matrix. The flexure and ILSS behavior could be explained by the microstructure observed. As the porosity reduced at ≤ 1.5 wt% NGP, flexure and ILSS increased. As the porosity started to increase at concentration > 1.5 wt% the ILSS and flexure properties plateaued and then started to decrease.

Figure 6. Flexural modulus - strength (a,b) and ILSS (c) measurements of C/C composites infused

with different concentrations of nanographene platelets after as-cured, carbonization and densification stage.

The highest properties were observed at 1.5 wt% NGP, flexure strength of 94 MPa, flexure modulus of 41.8 GPa and ILSS of 7.33 MPa. Though there was a reduction in ILSS and flexure properties after carbonization, it was observed that with addition of NGP, higher inter laminar and flexure strength were retained in the CCC.

Flexure and ILSS properties after densification increased from the corresponding carbonized composites. The increase was more with addition of nanographene platelets with maximum ILSS and flexure properties observed at 1.5 wt%. The increase could be attributed to the filling of pores generated during carbonization by the char generated from carbonization of phenolic resin or by NGP. The highest ILSS observed was 10.5 MPa (increase of 22% compared to neat CCC), flexure strength of 142.4 MPa (increase of 27%) and flexural modulus of 59.2 GPa increase of 68%).

In summary, the ILSS and flexure properties at all three stages (as-cured, carbonized and densified) increased with addition of NGP due to them filling the pores and increasing the carbon content in the CCC. At higher concentration > 1.5 wt% NGP though increased the ILSS and flexure properties of the CCC compared to neat CCC but were not as effective as CCC at concentration ≤ 1.5 wt% NGP due to agglomeration.

4. Conclusions Carbon-Carbon composites (CCC) were fabricated by introducing nanographene platelets

(NGP) as nanofillers by spraying technique. NGP were introduced in concentration of 0.5 wt%, 1.5 wt%, 3 wt% and 5wt% to see the effect of concentration on microstructure, flexure properties, inter laminar shear strength and damping properties of CCC. The following observations were made:

• At 1.5 wt% NGP densified CCC, highest ILSS observed was 10.5 MPa (Increased by 22%), flexure strength of 142.4 MPa (increased by 27%), flexural modulus of 59.2 GPa (Increased by 68%) and porosity of 18.8% (reduced by 17.5%) compared to neat densified CCC. The increase in mechanical properties was attributed to NGP filling the pores generated after carbonization which led to increase in stiffness and density.

• At low concentration (≤ 1.5 wt%) NGP filled in the pores, cracks and debonded interface but at concentration higher than 1.5 wt% NGP lost their effectiveness due to agglomeration. This led to plateauing of the properties after 1.5 wt% followed by decrease on further NGP addition.

• Due to improved mechanical properties observed in NGP reinforced CCC compared to neat CCC, the required number of densification-carbonization cycles for desired density/properties CCC will be reduced.

5. Acknowledgement The support from NSF Experimental Program to Stimulate Competitive Research

(EPSCoR)-Alabama Center for Nanostructured Materials (ACNM) is gratefully acknowledged.

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