| 1. |
- Bakoglidis, Konstantinos D., 1985-
(author)
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Low Friction and Wear Resistant Carbon Nitride Thin Films for Rolling Components Grown by Magnetron Sputtering
- 2015
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Licentiate thesis (other academic/artistic)abstract
- The scope of this licentiate thesis is the investigation of carbon based thin films suitable for rolling components, especially roller bearings. Carbon and carbon nitride are materials with advantageous tribological properties and high resiliency. Such materials are required in order to withstand the demanding conditions of bearing operation, such as high loads and corrosive environments. A fundamental condition for coated bearings is that the deposition temperature must be striktly limited. Thus, carbon nitride (CNx) thin films were synthesized here at low temperature of 150 oC by different reactive magnetron sputtering techniques, which are mid-frequency magnetron sputtering (MFMS), direct current magnetron sputtering (DCMS), and high power impulse magnetron sputtering (HiPIMS). While DCMS is a very well studied technique for carbon based films, MFMS and HiPIMS are relatively new sputtering techniques for carbon, and especially CNx depositions. Using different magnetron sputtering techniques, different ionization conditions prevail in the chamber during each process and influence the obtained film properties at a great extent. It was found that bias duty cycles and the amount of working gas ions are key parameters and affect the morphology and microstructure as well as the mechanical response of the films. Moreover, different bias voltages, from 20 V up to 120 V were applied during the processes in order to investigate the changes that the different ion energies induce in the film structure.The structural, mechanical and tribological properties of CNx films are also presented in this licentiate thesis. The morphology of CNx films strongly depends on both the deposition technique and ion energy. The special configuration of MFMS mode produces highly homogeneous and dense films even from low applied bias voltages, while in HiPIMS mode high bias voltages above 100 V must be applied in order to produce films with similar structural characteristics. DCMS is also proven as a good technique for homogeneous and dense films. Low bias voltages do not favor homogeneous structures, thus at 20 V all techniques produced films with columnar structures with intercolumnar voids. High bias voltages influence the N incorporation in the films, with the appearance of re-sputtering of N-containing species and a promotion of sp2 bonding configurations with increasing ion energy. Nevertheless, the different deposition mode influences the sp2 content in different ways, with only MFMS showing a clear increase of sp2 content with increasing bias voltage and HiPIMS showing relatively constant sp2 content. The morphology and microstructure of the CNx films affects their mechanical response, with higher ion energies producing harder films. A dependency of hardness and elastic modulus with increasing ion energy was obtained, where for all deposition modes, hardness and elastic modulus increase linearly with increasing bias voltage. Films with hardness as high as 25 GPa were synthesized by MFMS at 120 V , while the softer film yielded a hardness of 7 GPa and was deposited by HiPIMS at 20 V . The elastic recovery of the films differs with increasing ion energies, presenting a correlation with the C sp2 bond content. The highest elastic recovery of 90% was extracted for the film deposited by MFMS at 120 V and is a value similar to the elastic recovery obtained for FL-CNx films. All films developed compressive residual stresses, depending also on the ion energies and the deposition mode used. It is demonstrated that the induced stresses in the films increase when denser and more homogeneous film morphologies are obtained and with higher Ar intercalcation. Low friction coefficients were obtained for all films between 0.05 and 0.07, although the deposition conditions are not detrimental for the development of friction coefficient. The wear resistance of the films was found to be dependent on the morphology and to some extent on the microstructure of the films. Harder, denser, and more homogeneous films have higher wear resistance. Especially, CNx films deposited by MFMS at 120 V present no wear.The tribological characteristics of the surface of the films were also investigated at nanoscale by a new reciprocal wear test. In this wear test, the recording of the track profile is performed in between consecutive test cycles, eliminating also thermal drift. The very low wear of the films deposited by MFMS at 100 V and 120 V revealed that during the wear test a phase transformation on the surface may take place, possibly graphitization. It is also demonstrated the way that the surface characteristics, such as asperities and roughness affects the tribological measurements. Attention is also turned to the presence of large asperities on the film surface and the way they affect the obtained average friction coefficient and tribological measured data.
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| 2. |
- Hänninen, Tuomas
(author)
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Silicon Oxynitride Thin Films Grown by Reactive HiPIMS
- 2015
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Licentiate thesis (other academic/artistic)abstract
- Amorphous silicon oxynitride (SiOxNy) thin films were grown by reactive high power impulse magnetron sputtering from a pure silicon target in Ar/N2O plasmas. The elemental composition of the films was shown to depend on the target surface conditions during the film deposition, as well as on the reactive gas flow rate. When the target was sputtered under poisoned surface conditions, the film composition was predominantly silicon oxide, whereas films deposited in the transition regime between poisoned and metallic target surface conditions showed higher nitrogen concentrations, as measured by X-ray photoelectron spectroscopy (XPS) and elastic recoil detection analysis (ERDA). The different target surface conditions were identified based on the evolution of the target current waveforms upon variation of the deposition parameters. The average electron temperatures during the peak target current were determined by Langmuir probe measurements, to assist with the explanation of the observed target current behavior and target poisoning characteristics.The chemical composition of the films was shown to range from silicon-rich to effectively stoichiometric silicon oxynitrides, where no Si–Si contributions were found in the XPS Si 2p core level spectra. The film optical properties, the refractive index n and the extinction coefficient k, were shown to depend on the film chemical bonding, with the effectively stoichiometric films displaying optical properties falling between those of SiO2 and Si3N4.
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| 3. |
- Bakoglidis, Konstantinos
(author)
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Low-friction and wear-resistant carbon nitride coatings for bearing components grown by magnetron sputtering
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The scope of this thesis is the investigation of magnetron sputtered carbon nitride coatings suitable for roller bearing components. The research field of tribology of bearings focuses on minimizing friction between components by improving the lubricants. The development of lubricants is, however, expensive and involves environmentally deleterious chemical byproducts. A solution to avoid such harmful conditions, reduce the processing cost, and more importantly, minimize the friction, is to apply a low-friction and wear-resistant coating on the surface of the bearing. The deposition of such coatings on components can substantially increase their lifetime, reduce the maintenance costs, and eventually increase the reliability of the machinery.Carbon nitride (CNx) coatings have high resiliency and can withstand the demanding conditions of bearing operation. The morphology of CNx coatings is highly affected by applying a negative substrate bias voltage. At high bias (100-120 V ), the coatings become denser and more homogeneous with decreased porosity, resulting in more wear-resistant materials. I also found that the duty cycle of the applied bias affects the layer morphology. Less homogeneous films are produced using lower duty cycles (i.e., in high power impulse magnetron sputtering, HiPIMS) for a specific value of bias voltage. Thus, changing bias voltage, we can manipulate the structure of CNx and design layers, depending on the requirements of the bearing application.My results show that denser films yield higher hardness and wear-resistance, but also higher compressive stress, which is a disadvantage for the coating-substrate adhesion. In order to obtain improved adhesion on bearing steel, we developed an in-situ surface treatment, prior to the CNx deposition, which also surpasses the limitations set by the properties of each material. The steel substrates are successfully pretreated using W or Cr ions originating from a HiPIMS source. Plasma ions are accelerated to the substrates with energies of 900 eV , due to the application of a synchronized high bias voltage, which clean effectively the substrate surface from residual contaminants and strengthen the interfacial bonding.CNx-coated rollers are tested in rolling operation and show the absence of run-in period in all lubrication regimes. This is a big advantage for applications which rotate under boundary lubrication (BL). The coated rollers yield friction coefficients in the range of 0:020 and 0:025 in elastohydrodynamic (EHDL) and hydrodynamic (HDL) lubrication regimes, being lower than the friction coefficients of 0:026-0:052, exhibited by the uncoated rollers. Here, friction decreases steadily with increasing number of cycles, due to the presence of CNx in the contact. In BL, CNx-coated rollers present an increased friction coefficient of 0:052, but the wear is much lower than in the case of uncoated rollers. All rollers are covered with CNx in the wear tracks after the tests, avoiding failures and presenting low abrasive wear. The obtained tribological performance of the CNx-coated rollers in rolling is overall improved compared to the established operation of uncoated rollers. Thus, CNx layers can function as low-friction and wear-resistant coatings protecting the steel components in several roller bearing applications, such as in gearboxes and wheels in automotive, aerospace, marine, and turbine industry.
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| 4. |
- Schmidt, Susann, 1981-
(author)
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Carbon Nitride and Carbon Fluoride Thin Films Prepared by HiPIMS
- 2013
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Doctoral thesis (other academic/artistic)abstract
- The present thesis focuses on carbon based thin films prepared by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). Carbon nitride (CNx: 0 < x < 0.20) as well as carbon fluoride (CFx: 0.16 < x < 0.35) thin films were synthesized in an industrial deposition chamber by reactive magnetron sputtering of graphite in Ne/N2, Ar/N2, Kr/N2, Ar/CF4, and Ar/C4F8 ambients. In order to increase the understanding of the deposition processes of C in the corresponding reactive gas mixture plasmas, ion mass spectroscopy was carried out. A detailed evaluation of target current and target voltage waveforms was performed when graphite was sputtered in HiPIMS mode. First principle calculations targeting the growth of CFx thin films revealed most probable film forming species as well as CFx film structure defining defects. In order to set different process parameters into relation with thin film properties, the synthesized carbon based thin films were characterized with regards to their chemical composition, chemical bonding, and microstructure. A further aspect was the thin film characterization for possible applications. For this, mainly nanoindentation and contact angle measurements were performed. Theoretical calculations and the results from the characterization of the deposition processes were successfully related to the thin film properties.The reactive graphite/N2/inert gas HiPIMS discharge yielded high ion energies as well as elevated C+ and N+ abundances. Under such conditions, amorphous CNx thin films with hardnesses of up to 40 GPa were deposited. Elastic, fullerene like CNx thin films, on the other hand, were deposited at increased substrate temperatures in HiPIMS discharges exhibiting moderate ion energies. Here, a pulse assisted chemical sputtering at the target and the substrate was found to support the formation of a fullerene-like microstructure.CFx thin films were found to have surface energies equivalent to super-hydrophobic materials for x > 0.26 while such films were polymeric in nature accounting for hardnesses below 1 GPa. Whereas, an amorphous structure for carbon-based films with fluorine contents ranging between 16 % and 23 % was observed. For those films, the hardness increased with decreasing fluorine content and ranged between 16 GPa and 4 GPa. The HiPIMS process in fluorinecontaining atmosphere was found to be a powerful tool in order to change the surface properties of carbon based thin films.
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