Investigations on the effect of plasma treatment on the adhesive bonding behavior of ultra high modulus K13C2U/cyanate ester fiber composites

Surface preparation of ultra-high modulus fiber composites for adhesive bonding is challenging due to the subsurface damages that can occur during a standard abrasion operation that could weaken the laminate cohesion. Therefore, it is highly desirable to utilize a non-contact method that can clean and activate the composite surface without exerting an external force to eliminate potential damage. This investigation focuses on the use of atmospheric plasma treatment (APT) for the surface preparation of unidirectional ultra-high modulus, composites for adhesive bonding. The laminates were manufactured using Mitsubishi K13C2U pitch-based carbon fiber with Tencate’s RS-3C cyanate-ester resin and bonded with Henkel 9394 epoxy. Surface analysis of plasma-treated samples revealed a significant reduction of surface contamination and increased chemical modification of the matrix material resulting in improved wetting and the formation of chemical groups conducive to bonding. The plasma treatment also produced a minor increase in surface roughening, well below levels expected to contribute to mechanical interlocking. APT epoxy-bonded lap-shear specimens (LSS) exhibited a 50% increase in strength as compared to mechanically abraded samples. It was also shown that solvent rinsing after APT removed a highly-oxidized, poorly bonded carbonate component from the treated resin surface as evidenced by XPS. This surface ash is observed at fairly high concentrations after APT of cyanate ester resin composites limiting functional group contributions. The removal of this ash resulted in increases in LSS of 25% as compared to unrinsed APT surfaces. Lap shear strengths and GIC delamination resistance tests showed improvements over abrasion surface treated samples. Image analysis of the fracture faces for both surface treatments showed that abrasion treated surfaces resulted in sub-surface damage to the fibers that translated into brittle fracture through weak, highly aligned layers within these graphitic fibers. On the other hand, plasma treatment was successfully applied to minimize subsurface damage and resulted in crack propagation along with the more tortuous fiber-matrix interface. The degree of the improvements was ultimately limited by the interlaminar strength of these composites.