A Conceptual Strategy toward High-Reliability Metal–Thermoplastic Hybrid Structures Based on a Covalent-Bonding Mechanism

Abstract

Metal−thermoplastic hybrid structures have proven their effectiveness to achieve lightweight design concepts in both primary and secondary structural components of advanced aircraft. However, the drastic differences in physical and chemical properties between metal and thermoplastic make it challenging to fabricate high-reliability hybrid structures. Here, a simple and universal strategy to obtain strong hybrid structures thermoplastics is reported by regulating the bonding behavior at metal/ thermoplastic interfaces. To achieve such, we first researched and uncovered the bonding mechanism at metal/thermoplastic interfaces by experimental methods and density functional theory (DFT) calculations. The results suggest that the interfacial covalency, which is formed due to the interfacial reaction between high-electronegativity elements of thermoplastics and metallic elements at metal surfaces, dominates the interfacial bonding interaction of metal−thermoplastic hybrid structures. The differences in electronegativity and atomic size between bonding atoms influence the covalent-bond strength and finally control the interfacial reliability of hybrid structures. Based on our covalent-bonding mechanism, the carboxyl functional group (COOH) is specifically grafted on polyetheretherketone (PEEK) by plasma polymerization to increase the density and strength of interfacial covalency and thus fabricate high-reliability hybrid structures between PEEK and A6061-T6 aluminum alloy. Current work provides an in-depth understanding of the bonding mechanism at metal−thermoplastics interfaces, which opens a fascinating direction toward highreliability metal−thermoplastic hybrid structures.

Heteroatom Introduction to Reconstruct Interfacial Chemical Structures for High-Reliability CFRTP/A6061-T6 Hybrid Structures

“…The current work used simple and economic plasma polymerization to introduce heteroatoms onto the CFRTP surface. According to the published covalency bond mechanism of metal–thermoplastic hybrid structures − and considering the environmental protection concept, a green reaction atmosphere of distilled H 2 O is selected to graft the hydroxyl group on CFRTP, as shown in Figure a. The introduced heteroatom of oxygen is expected to form high-strength covalencies with 6061 and thus enhance the joint reliability of CFRTP/6061 hybrid structures.…”

“…It is necessary to point out that the interfacial interatomic interaction with CFRTP/metal hybrid structures would be developed as long as the welding heat temperature is high enough to melt the interfacial CFRTP . It meant that the interatomic interaction between CFRTP and 6061 would be formed within the current PVD simulative samples. , An additional sample where CFRTP is finished by Al 2 O 3 film with a thickness of 20 nm (CFRTP-20) is prepared for comparison.…”

“…FSLW processing was performed using a friction stir welding device with a no-probe cylindrical rotation tool with a shoulder diameter of 13 mm. 33 By observing the joint morphologies, the FSLW parameters of rotational speed, fixed tool plunge depths, and dwell time were selected to be 1400 rpm, 0.8 mm, and 6 s, respectively. After FSLW, the cross-sections below weld-tool indentation of CFRTP/6061 and CFRTP-HI/6061 hybrid structures were mechanically ground and then observed by scanning electron microscopy (SEM, S-3700N, Hitachi) and scanning transmission electron microscopy (TEM, JOEL 2100f, JOEL Ltd.).…”

“…The unrevealed bonding behavior at the CFRTP/metal interfaces makes it challenging to provide a fundamental principle for interfacial design concepts toward high-reliability hybrid structures. Very recently, our research reported that the high-electronegativity and low-atomic size elements within thermoplastics play an important role in interfacial chemical bonds within the CFRTP/metal hybrid structures . During the welding process, the orbitals between these elements and metallic elements of the oxidation layer at the metal surfaces would be strongly hybridized to form interfacial covalencies that determine the joint reliability of the CFRTP/metal hybrid structures.…”

“…Very recently, our research reported that the high-electronegativity and low-atomic size elements within thermoplastics play an important role in interfacial chemical bonds within the CFRTP/metal hybrid structures. 33 During the welding process, the orbitals between these elements and metallic elements of the oxidation layer at the metal surfaces would be strongly hybridized to form interfacial covalencies that determine the joint reliability of the CFRTP/metal hybrid structures. The universal covalent bond theory provides a fascinating principle of designing and introducing heteroatoms into interfacial microstructures to reconstruct chemical structures and induce high-strength covalent bonds at CFRTP/metal interfaces and thus to manufacture highreliability hybrid structures.…”