A variety of diamond-like carbon (DLC) coatings was investigated via different experimental techniques to assess their suitability as adhesion promoting interlayers in steel / epoxy-based CFRP hybrid laminates.
We investigated sandwich specimens with a symmetrical layup consisting of unidirectional CFRP (SGL SIGRAPREG® C U230-0/NF-E320/39%) with a thin steel layer (X5CrNi18-10) in the center plane. Four different DLC variants were compared to other types of commonly used surface modifications via short-beam shear (SBS) tests. Among these, high hardness (DLC-HH) and Silicon-doped (DLC:Si) showed apparent interlaminar shear strength values comparable to the monolithic CFRP material and were examined more closely.
Subsequent SBS tests after hot/wet ageing showed a trend from brittle to ductile failure behaviour for DLC-HH and for DLC:Si vice versa. X-ray photoelectron spectroscopy (XPS) measurements indicated a formation of SiOx-precipitations at the DLC:Si surface and thus a depletion of the steel/DLC:Si interface from Si accounting for a weakening of this interface.
To assess the interfacial fracture toughness, commonly used testing procedures like DCB (double cantilever beam), 3-ENF (3-point end-notched flexure) and MMB (mixed-mode beam) test were adapted. The DLC coatings proved efficient to increase fracture toughness significantly, and each variant resulted in characteristic values of the laminate: Both variants exceeded the Mode II values of monolithic CFRP significantly, whereas the Mode I values of monolithic CFRP were exceeded by DLC:Si. In the mixed-mode regime, values of monolithic CFRP were nearly reached by DLC-HH.
Potentiostatic and Potentiodynamic measurements proved the corrosion resistance of the DLC variants: Pitting up to high potential differences could be inhibited and corrosion current densities could be reduced by three magnitudes compared to an uncoated steel surface.
The conducted investigations proved a high potential of DLC-coatings as interlayers for metal / epoxy-based CFRP structures. Their application towards hybrid structures based on thermoplastic matrices is currently under investigation.
|Manuskript||Manuscript als PDF-Datei||Hybrid-2018-Manuscript-ID-28-Monden: "Diamond-Like Carbon (DLC) as Adhesion promoting Interlayer in Steel / Epoxy-based CFRP Hybrid Laminates"||1 MB||Download|