Failure mechanisms of nitrogen-doped diamond-like carbon films: Exfoliation or localized attack behaviour of films under hydrochemistry evolution

Abstract

Nitrogen-doped diamond-like carbon (N-DLC) films with different nitrogen content were prepared on the surface of 2024 aluminium alloy using plasma-enhanced chemical vapour deposition technology. The failure mechanisms of the N-DLC films during seawater exposure have been investigated. The results demonstrated that nitrogen doping significantly affected the failure behaviour of the DLC films in seawater. Extensive exfoliation of the DLC film was found without the addition of nitrogen. However, when the nitrogen flow rate is 10 sccm and 15 sccm, significant localized corrosion of the DLC film and the aluminium alloy substrate occurred. The difference in the thermodynamic process of corrosion of the N-DLC films was mainly due to the changes in pH value on the local solution caused by the formation and hydrolysis of NH4+. In the DLC film without nitrogen additive, large oxide particles were formed at the film-substrate interface due to the acidification of the solution. Meanwhile, the internal stress of the DLC film was high under this condition. The film was easily exfoliated and ultimately lost its protective effect on the substrate. The corrosion products could not grow sustainably due to the variable pH value when excess nitrogen was added to the DLC film. No extensive exfoliation of the DLC films with 10 sccm and 15 sccm nitrogen addition has occurred, but obvious corrosion channels were formed in the substrate because of the localized corrosion. The evolution mechanism of oxidation behaviour in the presence of variations in local hydrochemistry is also discussed.