Enhancing the Resistance Welding of GF/PEI Composites via a PECVD-Deposited HMDSO Coating on a Heating Element

Resistance welding of thermoplastic composites critically depends on the quality of the metal-polymer interface formed between the heating element and the polymer matrix. However, the intrinsic chemical incompatibility between metallic heating elements and high-performance thermoplastics often limits joint strength and fracture resistance. In this study, we investigate the use of plasma-engineered siloxane coatings to tailor the interfacial properties of AISI 304 stainless-steel heating elements and enhance the performance of resistance-welded glass fiber/poly-(ether imide) (GF/PEI) joints. Thin films based on hexamethyldisiloxane (HMDSO) were deposited by plasma-enhanced chemical vapor deposition (PECVD), followed by oxygen plasma post-treatment to promote surface functionalization via the formation of polar groups. The modified interfaces were characterized by contact angle measurements, FTIR, XPS, and atomic force microscopy, revealing a transition from hydrophobic to highly polar surfaces driven by the incorporation of oxygen-containing species. Mechanical performance was evaluated by single lap shear strength (LSS) testing combined with detailed fractographic analyses using optical microscopy and scanning electron microscopy. The plasma-engineered interfaces exhibited an increase of approximately 48% in lap shear strength compared to untreated joints, accompanied by a clear transition in fracture mode from interfacial failure to intralaminar and mesh wire rupture mechanisms. These results demonstrate that plasma-deposited siloxane coatings provide an effective strategy to bridge the chemical gap in metal-polymer interactions, offering a versatile route for the design of high-performance welded interfaces in structural composite applications.