Conclusions

CHAPTER VI Conclusions and suggestions

6.1 Conclusions

The conclusion was divided into two main parts for chromium-plated and NAB substrates. In the first part, a-Si, a-Si:N, a-Si:H, and a-SixCy:H films were used as interlayers to improve the adhesion on the a-C:H film deposited on the chromium- plated substrate. The deposition rate of all interlayers prepared by the DCMS method has different values depending on the gas source, which were used to estimate the deposition time at the same film thickness. The a-C:H films were deposited using the RF-PECVD method under the same conditions. The average thickness of the a-C:H films and the silicon-based interlayers were controlled as 317±12.99 and 306±14.23 nm, respectively. 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. For the a- C:H film deposited on a chromium-plated substrate without an interlayer, delamination from the substrates evident. For the a-Si and a-Si:H interlayers, it can be seen that the interlayers have similar grain sizes and these layers appear to merge with the a-C:H layer at the interface.

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 of Raman shift with a relative intensity ID/IG of about 1.26, indicating the typical structure of the amorphous carbon films. These peaks are associated with the vibration of A1g symmetry breathing mode of sp² atoms only in the aromatic rings for disorder of graphite (D peak) and with the vibration of E2g stretching mode of sp² atoms in both aromatic rings and chains for graphite (G peak). In addition, the X peak was found at about 1,222−1,225 cm^–1, which probably originates from the vibrations of a pentagonal atomic ring in the carbon nanostructures. The XPS results show that the bonding of C−sp² and C−sp³ hybridizations with the carbon content of all samples is about 51.26±0.22 and

161 30.11±2.52%, respectively. Moreover, C−O and C=O bonds derived from residual oxygen and surface oxidation are also found in the a-C:H structure. Therefore, it can be concluded that the a-C:H film prepared in this part with a density of about 2.15 g/cm³ agrees very well under the same deposition conditions.

For all silicon-based interlayers, the XPS results indicate the presence of silicon dioxide and suboxides, mainly due to the adsorption process during the measurement. The low hardness of all the interlayers with density in the range of 1.68−2.05 g/cm³ shows that the a-C:H film is very important for improving the mechanical properties of the chromium-plated substrate. In the a-SixCy:H interlayer, the carbon dopant can be bonded to the silicon atoms and form Si−C, which has a negative effect on the adhesive properties. This behaviour is probably due to the low dangling bond and the high residual stress of the interlayer. The a-C:H/a-Si:H sample exhibited higher Lc2 and Lc3 (5.67±0.28 and 15.15±1.16 N) than the other silicon- based interlayers, indicating higher resistance to adhesion failure between the coating and the interlayer, and higher adhesion strength of the interlayer with a substrate, which correlates with the highest hardness of about 20.98±0.63 GPa that is 2.5 times higher than the substrate (8.30±0.48 GPa). In addition, it exhibits the lowest corrosion current density with the lowest porosity, which is about 36 times lower than that of the uncoated chromium-plated substrate. This result indicates that a-Si and a-Si:H are a good supporting interlayer for the formation of a-C:H film. The mixing with hydrogen gas for the a-Si:H interlayer could reduce the residual oxygen, which could reduce the Si−O bonding and improve the adhesion between the interlayer and the a- C:H film. It can be concluded that the protective a-C:H coating with an a-Si:H interlayer has excellent potential to significantly improve the corrosion resistance and hardness properties of the film, which can extend the durability and service life of materials used in abrasive and corrosive environments.

In the second part, the multilayer DLC films were developed and fabricated using the a-Si film as an adhesive layer to improve the mechanical and corrosive resistance of NAB. The a-Si interlayer was prepared using the DCMS method. The DLC film was deposited using the RF-PECVD deposition method. The DCMS and RF-PECVD method were used the Si-doped DLC film. The a-Si interlayer has a high density of up to 93% of the calculated density, which is a good quality for the

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adhesive layer. The DLC and of Si-doped DLC films with densities 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 stack, 3 stacks and 5 stacks) with a controlled film thickness of about 1.3 m. The XPS results of the a-Si interlayer show the silicon peak (Si2p and Si2s) and the oxygen-rich peak originated from the silicon precursor during the deposition process and the residual oxygen during the surface exposure to air, respectively. The Raman spectra of DLC and Si-doped DLC films show three peaks of X, D, and G in the range of 1,222 and 1,241 cm^-1, 1,417 and 1,422 cm^-1 and 1,566 and 1,577 cm^-1. The ID/IG ratio of the Si-doped DLC film is higher than that of the DLC film due to the increase in disordered sp² clusters caused by the incorporation of silicon atoms into the amorphous carbon structure. From the results of XPS and Raman spectroscopy, it can be concluded that the silicon atoms are not bonded with the carbon atoms to form Si−C bond, which leads to an increase in graphite disorder in a Si-doped DLC structure.

The surface morphology of the multilayer DLC exhibits a dense and smooth surface with an overall average thickness estimated to be 1,373±6.15 nm. The cross- sectional image of the a-Si layer shows the smooth and compact morphologies with small columnar grains, while the Si-doped DLC and DLC layers show the smooth and dense surface microstructures of the amorphous structure, corresponding to the difference in film density. The a-Si interlayer shows higher hardness than NAB, which is a good support for the multilayer formation of NAB. The hardness of DLC multilayers decreased with increasing stacking from 1 stack to 5 stacks. The change in hardness of DLC multilayers can also be explained by the thickness of the top DLC layer. The a-Si adhesive layer and multilayer design show good adhesion strength of the film on NAB for higher thickness above 1 m. The shift of the potentiodynamic polarization curves of the multilayer coating toward a more positive region compared to the uncoated NAB indicates the good corrosion resistance. Moreover, using higher number of stacks can significantly reduce the corrosion current density, which can be explained by the lateral propagation of the corrosion product in the plane of the interface.

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