High-Energy Electron-Irradiated Fluorinated Ethylene Polypropylene Copolymer Coatings on Al Substrates for Enhanced Metal Adhesion and Protection

Abstract

Fluorocarbon polymers (FPs) are ideal coatings for sliding parts in automotive, medical, or semiconductor manufacturing equipment owing to their excellent nonadhesiveness, low friction, and high heat and chemical resistance. FPs are used as coatings of slide bearings, oil pumps, sliding packing in automobiles, robot housing in biomedical fields, and the inner surface of the exhaust duct in semiconductor manufacturing equipment. However, the nonadhesiveness of FPs hinders their adhesion to metals, preventing their application. Recently, high-energy electron irradiation during the coating process above the melting point of FPs and under anoxic conditions has been proven to enhance the adhesion of FPs to metal substrates. However, the mechanism underlying this phenomenon remains unveiled because the investigation of the buried interface is difficult. Herein, we fabricated samples of fluorinated ethylene–polypropylene (FEP) coating an Al substrate using a unique approach based on semiconductor manufacturing processes to clarify the morphology and chemical interactions of the Al–FEP interface. Time-of-flight secondary ion mass spectrometry showed that the uncoated Al surface comprised 0.5 nm-thick Al hydroxides. Scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy showed that a 1000 keV electron irradiation during the coating process caused a significant diffusion of F atoms from the FEP layer to Al. Soft X-ray photoelectron spectroscopy (XPS) clearly indicated that the O atoms in the native Al2O3 layer were substituted by F atoms. Hard XPS showed the appearance of strong C–O–Al bonds caused by the irradiation. These results stem from the irradiation-induced cleavage of the C–F bonds in the FEP coating and O–Al–O bonds in the native Al2O3, facilitating the diffusion of F atoms and the formation of C–O–Al bonds, improving the adhesion between the FEP coating and Al substrate. Thus, high-energy electron irradiation modified the nanostructure at the Al–FEP interface through the FEP layer.