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.