Effect of Surface Roughness on Corrosion Resistance and Mechanical Properties of Diamond Like Carbon (DLC) Coating System

Open Access
- Author:
- El Tantawy, Mohamed
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 10, 2020
- Committee Members:
- Douglas Edward Wolfe, Thesis Advisor/Co-Advisor
Hojong Kim, Committee Member
Elzbieta Sikora, Thesis Advisor/Co-Advisor
John C Mauro, Program Head/Chair - Keywords:
- DLC
Corrosion resistance
Friction resistance
Wear resistance
Diamond
Graphite - Abstract:
- Diamond like carbon (DLC) coatings are widely known to have extraordinary chemical and mechanical properties. These properties come from the combination between the graphitic and diamond characteristics that represented chemically by sp2 and sp3 hybridization, respectively. The graphitic content yields coating properties of high lubrication and low friction coefficient while the diamond content gives it its high hardness. In addition to the improvement of the mechanical properties, DLC coatings may act as a diffusion barrier which influence the corrosion protection of metallic structures. However, DLC coatings need to be specially designed to avoid the pin holes that exist from most physical vapor deposition processes. The desired DLC coating system is designed to have amorphous carbon in order to avoid grain boundaries that exist with crystalline forms of carbon which often act as corrosion pathways. The DLC coating system was also designed to have an interlayer of titanium which enhances the diffusion barrier properties by disrupting pin holes from the inner to outer DLC layers. The DLC coating system was deposited using direct current (DC) sputtering technique at 200 ˚C as the deposition temperature to avoid forming crystalline carbon which generally forms at higher temperatures. The crystal structures in the DLC coating system were evaluated using X – ray diffraction (XRD) while, the graphitic and diamond configurations were assessed using Raman spectroscopy and X – ray photoelectron spectroscopy (XPS). Raman spectroscopy measurements revealed an overview for the graphitic content in the coating system. The ratio between the intensity of the disordered peak (I(D)) to the graphite peak (I(G)) after subtraction of T peak intensity indicates the degree of disorder from the graphitic configuration in the DLC film. The currently deposited DLC coatings composed of high degree of disorder including a probability of graphenic clusters, polycrystalline graphite and ordered graphene and low probability of sp3 carbon configuration. However, these crystal structures were not observed by X – ray diffraction. X – ray photoelectron spectroscopy spectra indicated that the top 10nm layer of the coating was rich in oxygen, and is confirmed by a broadening in XPS C 1st spectra which indicates the existence of carbon in the oxidized form. Also, the first derivative of X – ray excited Auger Electron Spectroscopy (XAES C KLL) was analyzed and compared with results from P. Me´rels’ study. The D value (the energy difference in eV between the major negative-going and positive-going excursion in the first derivative of the XAES spectra) in P. Me´rels’ research for diamond was around 14.0 eV and 22.1 eV for graphite. While, the D value for the tested DLC – Ti coating system appears to be around 20.0 eV and 22.0 eV for highly oriented pyrolytic graphite (HOPG) as graphite reference material (very close to the graphite value in P. Me´rel study). X – ray diffraction (XRD) used to investigate the crystalline structures exist in the DLC – Ti coating system. The diffraction pattern of the coating system does not show existence of crystalline graphite and diamond. Also, the XRD pattern confirms the presence of hexagonal-close-packed (hcp) crystalline titanium which also imaged by using scanning electron microscopy. The corrosion resistance of the DLC – Ti coating system was investigated by the means of cyclic potentiodynamic polarization (CPDP) and potentiodynamic polarization testing in synthetic sea water. CPDP results show that the DLC – Ti coating system reveals high resistance against the localized pitting corrosion and crevice corrosion. Also, the surface roughness effect on corrosion resistance was studied for uncoated and DLC – Ti coated 420 stainless steel samples. CPDP curves for uncoated samples show that as the surface roughness value increases, the pitting potential decreases to more active potential hence the pitting corrosion increase. While, the CPDP curves for DLC – Ti coating system reveal that the coating system act as a good diffusion barrier against the aggressive attacking chloride ions regardless of the surface roughness values of the DLC coating. On the other hand, the corrosion current density values (calculated from Tafel extrapolation of a straight line in the cathodic and anodic regions in potentiodynamic polarization curves) are higher for coated DLC – Ti specimens than the uncoated 420 stainless steel. The mechanical properties of DLC – Ti coating system were studied in terms of hardness (by the means of nanoindentation), friction and wear resistance (by using tribometer facility). The load – displacement curves of nanoindentation tests of the DLC – Ti coating system show a mean hardness of 4.55 GPa and 78.12 GPa as a mean elastic modulus, the mean contact depth was approximately 142 nm. The surface roughness value of the smoothest DLC – Ti coating system was 10 nm which is considered to be high. The surface roughness of the coating has a scattering effect on the measured values of hardness and residual elastic modulus so 16 indents were performed on a 15 µm by 15 µm square area with a fixed 5 µm spacing between each indent to reduce this scattering effect. Tribological evaluation was performed for uncoated 420 stainless steel samples in dry and wet environment using 6 mm and 4 mm spinning radius for 7 surface roughness values from 0.065 µm to 0.003 µm, the same was performed for DLC – Ti coating system as a function of 6 surface roughness values ranging from 0.061 µm to 0.010 µm. The results were used to assess the friction resistance and the wear resistance of the DLC – Ti coating system. For uncoated samples, in dry environment, the mean friction coefficient values decrease as the surface roughness value decreases for both 4 mm and 6 mm spinning diameter. In wet environment, the friction coefficient values show the same trend as a function of surface roughness in both 4 mm and 6 mm spinning radius. For DLC – Ti coating system, in dry environment, for 6 mm and 4 mm spinning radius, the friction coefficient values show identical trend as a function of surface roughness. No distinct relationship between the surface roughness of the coating system and the friction coefficient was observed in this study. In wet environment, the friction coefficient values show the same trend as a function of surface roughness in both 4 mm and 6 mm spinning radius. Finally, the friction coefficient values of uncoated 420 stainless steel and DLC – Ti coated samples show that the coating system reveals high friction resistance in comparison with the uncoated samples which is desirable for the wear resistance. The wear resistance of the DLC – Ti coating system on 420 stainless-steel substrates was investigated in terms of wear volume. The wear volume is considered to be a cylindrical ellipsoid where the volume was calculated based on wear path geometry. The wear resistance efficiency of the coating system was represented as the added percent improvement. For 6 mm spinning radius, the DLC – Ti coating system shows higher percent of improvement in dry conditions (99.7%) in comparison with the wet conditions (81.7%). The same was in case of 4 mm spinning radius, the DLC – Ti coating system provides a higher percent of improvement in dry conditions (99.8%) in comparison with the wet conditions (87.3%). Also, the wear volume calculations did not show a specific trend as a function of surface roughness for uncoated 420 stainless steel and DLC – Ti coated samples in all conditions.