THE INFLUENCE OF MICROFILLER ADDITION ON THE FLEXURAL AND IMPACT BEHAVIOR OF CARBON FIBER REINFORCED PHENOLIC

Abstract

In this study, the effects of microfiller addition on flexural and dynamic impact behavior of carbon fiber reinforced phenolic matrix composites were investigated. The composite materials were produced using 2D woven PAN-based carbon fibers and two variants of phenolic resins (HRJ-15881 and SP-6877). The resins have the same phenol and solid content but differ in their viscosities and HCHO (formaldehyde) content. Colloidal silica and silicon carbide (SiC) microparticles were used as fillers and were added to the phenolic matrix in four weight fractions; namely 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.%. Thermogravimetric analysis (TGA) was used for thermal analysis of the phenolic resins. The flexural properties were determined using a three-point bending test, while the dynamic impact properties were determined using the Split Hopkinson pressure bar (SHPB). The damage evolutions under both loading conditions were investigated using optical and scanning electron microscopy. Thermal analysis results indicated that a cross-linking reaction occurred in both phenolic resins in the temperature range 110-130 °C and decomposition of the resin to form amorphous carbon began around 450 °C. The reinforcement of HRJ-15881 and SP-6877 phenolic resins with carbon fibers increased their flexural strength by 508% and 909%, respectively. The flexural strength of the carbon fiber reinforced phenolic (CFRP) composites further increased with the addition of SiC particles up to 1 wt.% SiC, but decreased with further increase in the amount of SiC particles added. On the other hand, the flexural modulus of the composites generally decreased with the addition of SiC microfiller. Both the flexural strength and flexural modulus of the composites did not improve with the addition of colloidal silica microfiller. The decrease in flexural properties was caused by the agglomeration of the microfillers at higher filler content, with colloidal silica particles exhibiting more tendency for agglomeration than SiC particles. Microscopic evaluation of the fractured specimens revealed fiber breakage, matrix cracking, and delamination under flexural loading. The tendency for cracking and fragmentation worsened at a microfiller loading of 1.5 wt.% and above. Two impact momenta were applied in the dynamic impact testing of the fabricated CFRP: 15 kg m/s and 28 kg m/s. At an impact momentum of 15 kg m/s, the dynamic impact strength increased with SiC addition for HRJ-15881 resin up to 1.5 wt.% and up to 0.5 wt.% for SP-6877 resin. At an impact momentum of 28 kg m/s, the dynamic impact strength increased at all SiC addition for SP-6877 resin, and up to 0.5 wt.% for HRJ-15881 resin. However, the impact strength deteriorated with colloidal silica addition for both phenolic resins and at both impact momentums. For CFRP with and without microfiller addition, no specimen fragmentation occurred at the impact momentum of 15 kg m/s. However, the CFRP without microfillers failed at an impact momentum of 28 kg m/s. Failure was observed to occur only in CFRP containing not more than 1 wt.% SiC addition. For composites containing colloidal silica addition, failure only occurred at a higher filler content of 2 wt.%. In all cases, with and without microfiller, the damage modes under the impact loading consist of fiber bundle rupture, matrix cracking, and delamination. XRD analysis results suggested that the microfillers were intercalated in the phenolic matrix of the CFRP. The addition of the SiC microfiller improved the crystallinity of both neat (unreinforced) phenolics and CFRP, irrespective of the phenolic resin used. However, with colloidal silica addition, improvement in crystallinity was only obtained for the HRJ-1881 resin, and CFRP made with this resin. For the SP-6877 resin, there was a decrease in the crystallinity of the phenolic and the CFRP made with the colloidal silica.