Fabrication and Characterisation of DLC-graphene Nanocomposite Coatings for Tribological Application

This work presents the development and characterisation of single and multilayer Diamond-Like Carbon (DLC)-graphene nanoplatelet (GNP) nanocomposite coatings. This study opens up a new challenge in the fabrication of carbon composites using DLC and GNP with enhanced mechanical and tribological properties. The purpose of the composite is to exploit the advantages of the excellent mechanical and tribological properties of graphene that have been reported by many works.
The objectives of this thesis are to develop a method to fabricate DLC and GNP nanocomposite coatings, to prepare the nanocomposite coatings and to investigate their physical, mechanical and tribological properties. The fabrication of DLC-GNP nanocomposite coatings was carried out using the combination of spin coating of GNP and DLC deposition using PECVD. The two types of DLC-GNP that have been prepared are single-layer and multilayer.
The surface morphology and microstructure of DLC-GNP was characterised using optical microscopy and Scanning Electron Microscopy (SEM). Focused Ion Beam (FIB) SEM was used to observe the layers in the composite and measure the thickness of the multilayer DLC-GNP nanocomposite coating. The coating comprises the interlayer, spin-coated GNP and DLC film.
This study shows that an optimised post-treatment is required to substantially improve the adhesion strength of spin-coated GNP and thus that of the whole nanocomposite coating. It was observed that columnar structure was generated in-situ during a wear tests on coatings post-treated for more than 180 minutes. The results were unintentionally found after three hours of sliding test. The columnar structure contributed to the significant reduction of the coefficient of friction (CoF) to 0.06, and the wear rate compared to other samples.
According to Raman spectroscopy analysis, both single and multilayer DLC- GNP nanocomposite coatings have typical spectra similar to that of pure DLC. However, DLC-GNP has a broad range of ID/IG ratio values compared to pure DLC due to the dispersion of spin-coated GNP. The observation though cross-section FIB also proved that DLC film covered the spin-coated GNP by creating a bonding layer during DLC film deposition.
The multilayer DLC-GNP demonstrated major improvements in adhesion strength of almost doubling the value obtained by single-layer DLC-GNP. The wear resistance also increased remarkably which can be related to the enhancement of adhesion strength. It is proposed that the GNP in the composite is released during the running-in period and acted as a slider between the counterpart and coating.