Ion-induced adhesion enhancement of Ni films on polyester: 28Si + implantation

Abstract

The adhesion between 30 nm Ni films and poly[ethylene terephthalate] (PET) substrates was examined as a function of the morphological and chemical modifications induced by 28Si + ion mixing through the Ni/PET interfaces. The Ni/PET specimens were held below 100°C and implanted with 55 keV 28Si + at doses ranging from 1 × 10 16 to 1 × 10 17 Si/cm 2. The low dose implantation had no observed influence on the surface topography, but the 1 × 10 17 Si/cm 2 implant did result in increased submicron void formation in the films. Auger electron spectroscopy (AES) and cross-sectional transmission electron microscopy (XTEM) analysis showed that the as-deposited films formed rather discrete interfaces with the PET substrates. After 28Si + ion mixing, those interfaces were graded substantially (up to five times the as-deposited interfacial distance). The extent of interfacial grading increased with the ion dose. The implanted Si reached a maximum concentration of approximately 15 at.% (for the 1 × 10 17 Si/cm 2 implant) in the Ni film and then extended deeply into the PET substrate. X-ray photoelectron spectroscopy (XPS) and electron diffraction analysis showed no detectable interaction between the Ni films and the PET substrates in the as-deposited specimens. However, after 28Si + implantation, the presence of NiSi and SiO bonding was observed at the Ni/PET interface. This bonding was shown to gradually change from primarily NiSi bonding in the Ni-rich regions to primarily Si-0 bonding in the C-rich regions of the substrate. In addition, the concentration of ether-type (COC) bonding in the interfacial region increased after 28Si + implantat compared to the as-received PET and the as-deposited Ni/PET interface. This ion-induced chemical bonding is consistent with NiSiOC complex formation throughout the interfacial region. Adhesion testing, performed using a scratch test in conjunction with AES analysis, showed substantial adhesion increases with the increasing dose of the 28Si + implantation. The as-deposited films were removed at forces less than 1 N. In contrast, the Ni films implanted with 1 × 10 17 Si/cm 2 were only removed at forces of approximately 19 N. This film removal occurred as a result of catastrophic substrate failure. The adhesion increases induced by 28Si + implantation were attributed to a combination of substrate toughening, mechanical interlocking, interfacial grading, and chemical bonding across the interfaces.