Surface Modification of Carbon Fibers by Low-Temperature Plasma with Runaway Electrons for Manufacturing PEEK-Based Laminates

Abstract

(1) Background: The paper addresses the effect of carbon fibers (CFs) treatment by low-temperature plasma with runaway electrons on the deformation behavior of the polyetheretherketone (PEEK)-layered composites. (2) Methods: The effect of the interlayer adhesion on the mechanical response of the composites was assessed through the tensile and three-point bending tests. In addition, computer simulations of the three-point bending were carried out with the use of the finite element analysis (FEM) with varying conditions at the “PEEK–CF layers” interface. (3) Results: DRE–plasma treatment during the optimal time of t = 15 min led to formation of a rougher surface and partial desizing of a finishing agent. The shear strength of the layered composites increased by 54%, while the tensile strength and the flexural modulus (at three-point bending) increased by 16% (up to 893 MPa) and by 10% (up to 93 GPa), respectively. (4) Conclusions: The results of the numerical experiments showed that the increase in the stiffness, on the one hand, gave rise to enlarging the flexural modulus; on the other hand, a nonlinear decrease in the strength may occur. For this reason, the intention to maximize the level of the interlayer stiffness can result in lowering the fracture toughness, for example, at manufacturing high-strength composites.

Structure and Deformation Behavior of Polyphenylene Sulfide-Based Laminates Reinforced with Carbon Fiber Tapes Activated by Cold Atmospheric Plasma

“…In a previous paper [ 49 ] by the current authors, it was shown that local electrical breakdowns could occur during the DRE plasma treatment of CFs, causing damage to their surface due to the presence of a technological sizing agent. In the case of PEEK/CF laminates, this phenomenon was accompanied by a decrease in their tensile strength despite an improvement in their interlaminar shear performance.…”

“…However, in addition to improving interlayer (interfacial) adhesion, the cold plasma treatment of CFs could damage their surface, reducing their tensile strength. Moreover, the longer its duration, the more their strength decreased [ 49 ]. For this reason, computer simulation was conducted when the ε τ shear strain in the CF-containing layers was varied.…”

“…According to the obtained results, it can be concluded that the DRE plasma treatment of CFs led to a combined physical–chemical effect: the functionalization of CFs via grafting of the active polar groups on the surface and an increase in their surface roughness [ 49 ]. This combined effect increased interlayer (interfacial) adhesion in the PPS/CF laminates by promoting chemical bonding between fibers and the polymer matrix, as well as mechanical interlocking, enhancing resistance to interlaminar cracking and the total PPS/CF contact surface area.…”

“…However, the physical etching developed upon increasing the treatment time gave rise to damage to individual fibers. Thus, the increase in the chemical component of surface adhesion was, to a certain extent, compensated by the loss of tensile strength [ 49 ]. In the current study, this was proven by the small difference in the ILSS values for the composites treated for 15 and 20 min.…”

“…Initially, the surfaces of CFs were modified in an installation for processing materials with low-temperature plasma with atmospheric discharge with runaway electrons (DRE) [ 49 ]. The procedure was carried out under normal conditions in continuous pulse generation mode with the following characteristics: an output voltage pulse amplitude of 56 kV, a pulse rise duration of 10 ns, and a pulse half-maximum duration of 40 ns.…”