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Low-friction and wear-resistant carbon nitride coatings for bearing components grown by magnetron sputtering
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. , 116 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1783
Keyword [en]
Carbon nitride, magnetron sputtering, physical vapor deposition, coatings, bearings, rolling contact fatigue, adhesion, friction, wear, transmission electron microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, coatings tribology, nanotribology
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-132154DOI: 10.3384/diss.diva-132154ISBN: 9789176857007 (Print)OAI: oai:DiVA.org:liu-132154DiVA: diva2:1038549
Public defence
2016-11-18, Planck, Fysikhuset, Campus Valla, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2016-10-19 Created: 2016-10-19 Last updated: 2016-11-09Bibliographically approved
List of papers
1. Low-temperature growth of low friction wear-resistant amorphous carbon nitride thin films by mid-frequency, high power impulse, and direct current magnetron sputtering
Open this publication in new window or tab >>Low-temperature growth of low friction wear-resistant amorphous carbon nitride thin films by mid-frequency, high power impulse, and direct current magnetron sputtering
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2015 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 33, no 5, 05E112Article in journal (Refereed) Published
Abstract [en]

Amorphous carbon nitride (a-CNx) thin films were deposited on steel AISI52100 and Si(001) substrates using mid-frequency magnetron sputtering (MFMS) with an MF bias voltage, high power impulse magnetron sputtering (HiPIMS) with a synchronized HiPIMS bias voltage, and direct current magnetron sputtering (DCMS) with a DC bias voltage. The films were deposited at a low substrate temperature of 150 °C and a N2/Ar flow ratio of 0.16 at the total pressure of 400 mPa. The negative bias voltage (Vs) was varied from 20 V to 120 V in each of the three deposition modes. The microstructure of the films was characterized by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED), while the film morphology was investigated by scanning electron microscopy (SEM). All films possessed amorphous microstructure with clearly developed columns extending throughout the entire film thickness. Layers grown with the lowest substrate bias of 20 V exhibited pronounced intercolumnar porosity, independent of the technique used. Voids closed and dense films formed at Vs ≥ 60 V, Vs ≥ 100 V and Vs = 120 V for MFMS, DCMS and HiPIMS, respectively. X-ray photoelectron spectroscopy (XPS) revealed that the nitrogen-to-carbon ratio, N/C, of the films ranged between 0.2 and 0.24. Elastic recoil detection analysis (ERDA) showed that Ar content varied between 0 and 0.8 at% and increases as a function of Vs for all deposition techniques. All films exhibited compressive residual stress, σ, which depends on the growth method; HiPIMS produces the least stressed films with stress between – 0.4 and – 1.2 GPa for all Vs values, while for CNx films deposited by MFMS σ = – 4.2 GPa. Nanoindentation showed a significant increase in film hardness and reduced elastic modulus with increasing Vs for all techniques. The harder films were produced by MFMS with hardness as high as 25 GPa. Low friction coefficients, between 0.05 and 0.06, were recorded for all films. Furthermore, CNx films produced by MFMS and DCMS at Vs = 100 V and 120 V presented a high wear resistance with wear coefficients of k ≤ 2.3 x 10-5 mm3/Nm.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-118343 (URN)10.1116/1.4923275 (DOI)000361229000012 ()
Note

On the day of the defence date the status of this article was Manuscript.

Available from: 2015-05-27 Created: 2015-05-27 Last updated: 2016-11-09Bibliographically approved
2. Improved adhesion of carbon nitride coatings on steel substrates using metal HiPIMS pretreatments
Open this publication in new window or tab >>Improved adhesion of carbon nitride coatings on steel substrates using metal HiPIMS pretreatments
2016 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 302, 454-462 p.Article in journal (Refereed) Published
Abstract [en]

We investigate the effect of low-temperature metal pretreatments in order to improve the adhesion of CNx coatings on steel substrates, which is crucial for tribological applications. The substrate pretreatments were conducted using five different metal targets: Ti, Zr, Al, Cr, and W, operated in high power impulse magnetron sputtering mode, known to produce significant ionization of the sputtered material flux. The CNx adhesion, as assessed by Rockwell C tests, did not improve upon Ti and Zr pretreatments. This is primarily ascribed to the fact that no interlayer was formed owing to severe re-sputtering due to high fluxes of doubly-ionized metal species in the plasma. A slight improvement in adhesion was observed in the case an Al pretreatment was carried out, while the best results were obtained using Cr and W. Here, 30-s-long pretreatments were sufficient to clean the steel surface and form a metallic interlayer between substrate and coating. Transmission electron microscopy in combination with energy dispersive X-ray spectroscopy revealed that Al, Cr, and W created intermixing zones at the interlayer/substrate and the interlayer/CNx interfaces. The steel surfaces, pretreated using Cr or W, showed the highest work of adhesion with W-adh(Cr) = 1.77 J/m(2) and W-adh(W) = 1.66 J/m(2), respectively. (C) 2016 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2016
Keyword
Adhesion; Steel substrate; Carbon nitride; Coating; HiPIMS; Pretreatment
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:liu:diva-131494 (URN)10.1016/j.surfcoat.2016.06.048 (DOI)000381838400051 ()
Note

Funding Agencies|VINN Excellence Center Functional Nanoscale Materials (FunMat) [2005-02666]; Swedish Foundation for Strategic Research [RMA11-0029]; Carl Trygger Foundation for Scientific Research [CTS 14:431]

Available from: 2016-09-26 Created: 2016-09-23 Last updated: 2016-11-09
3. Rolling contact fatigue of bearing components coated with carbon nitride thin films
Open this publication in new window or tab >>Rolling contact fatigue of bearing components coated with carbon nitride thin films
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2016 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 98, 100-107 p.Article in journal (Refereed) Published
Abstract [en]

Bearing rollers were coated with CNx films using high power impulse magnetron sputtering deposition in order to reduce their rolling-contact fatigue as investigated using a Micro-Pitting Rig tribometer under poly-alpha-olefin lubricated conditions. Coated rollers with a similar to 15 nm thick W adhesion layer to the substrate, exhibit the best performance, presenting mild wear and no fatigue after 700 kcycles. The steady-state friction coefficient was similar to 0.05 for both uncoated and coated rollers. Uncoated rollers show run-in friction in the first 50 kcycles, because of steel-to-steel contact, which is absent for coated rollers. Analytical transmission electron microscopy and X-ray photoelectron spectroscopy show that the presence of a CNx coating prevents steel-to-steel contact of the counterparts, prior to the elastohydrodynamic lubrication, reducing their wear and increasing the lifetime expectancy. (C) 2016 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2016
Keyword
Coating; Pitting; Rolling; Lubrication
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-128933 (URN)10.1016/j.triboint.2016.02.017 (DOI)000374426000011 ()
Note

Funding Agencies|Carl Tryggers Foundation for Scientific Research [CTS 14:431]

Available from: 2016-06-09 Created: 2016-06-07 Last updated: 2016-11-09

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