Structural and Corrosion Properties of Hydrogenated Amorphous Carbon Films with Silicon-Based Interlayer

Abstract

The main objective of this work is to improve the mechanical properties and corrosion resistance of chromium-plated and NAB substrates by a-C:H coating using the RF-PECVD method. For the chromium-plated substrate, the silicon-based interlayers were prepared using the DCMS method. The a-C:H films with silicon-based interlayers were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, field emission-secondary electron microscopy, nanoindentation, micro-scratching, and electrochemical corrosion measurements in terms of structure, morphology, mechanical and adhesive properties, and corrosion resistance. The surface morphology shows very homogeneous, smooth and dense microstructures without microparticle defects. The cross-sectional morphology between the a-C:H films and the interlayers shows a clear boundary at the interface without delamination and cracking, indicating good adhesive properties. The average thickness of the a-C:H films and the silicon-based interlayers was determined to be 317±12.99 and 306±14.23 nm, respectively. The Raman spectra of the a-C:H films clearly show the presence of D and G peaks at 1,413–1,417 cm-1 and 1,562–1,569 cm-1 with the ID/IG of about 1.26, indicating the typical structure of the amorphous carbon films. The XPS results show that the bonding of C=C sp2 and C–C sp3 hybridizations with the carbon content of all samples is about 51.26±0.22 and 30.11±2.52%, respectively. The a-C:H films with a-Si:H interlayer exhibit the lowest corrosion current density, which is about 36 times lower than that of the uncoated chromium-plated substrate. Moreover, the hardness increases from 8.48 GPa for the uncoated substrate to 20.98 GPa for the a-C:H/a-Si:H sample. The mixing with hydrogen gas could reduce the residual oxygen during the deposition process, which could reduce the Si–O and Cr–O bonding and improve the adhesion between the a-Si:H interlayer and the a-C:H film and between the chromium-plated substrate and the a-Si:H interlayer. Therefore, based on the adhesive, hardness and corrosive properties, the a-C:H film with the a-Si:H interlayer can be very useful to meet the multifunctional applications of the chromium plated products.

For the NAB substrate, the multilayer DLC films were fabricated with the a-Si film as the adhesive layer. The a-Si interlayer was prepared by the DCMS method. A combination of DCMS and RF-PECVD was used to deposit the Si-doped DLC film. The a-Si interlayer has a high density of up to 93%, which is a good quality for the adhesive layer. The DLC and Si-doped DLC films with density of 1.98 and 1.86 g/cm3 were used as hard and soft layers, respectively. The multilayer design was investigated in terms of deposition period or stack (1, 3, and 5 stacks) with the same thickness. The surface morphology of the multilayer DLC exhibits a dense and smooth surface with an average total thickness of ~1,373±6.15 nm. The XPS and Raman spectroscopy results indicate that the silicon atoms are not bonded to the carbon atoms to form a Si–C bond, leading to an increase in graphite disorder in a Si-doped DLC structure. The a-Si interlayer (5.43±0.35 GPa) has a higher hardness than the NAB substrate (3.71±0.34 GPa). All multilayer coatings have favorable adhesion strength on NAB with Lc3 value higher than 12 N. The multilayer DLC film with 5 stacks (15.57±0.51 GPa) has about 4.2 times higher hardness and about 70 times lower corrosion current density (0.02 uA/cm2) compared to the uncoated NAB substrate. These good mechanical and corrosive properties can be explained by the thickness term of the top DLC layer and the number of interfaces of the coating. The protective coating with multiple DLC layers can positively influence the excellent mechanical properties and corrosion resistance of NAB.

-