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Mao, Fang
Publications (10 of 13) Show all publications
Kryshtal, O., Kruk, A., Mao, F., Taher, M., Jansson, U. & Czyrska-Filemonowicz, A. (2019). Microstructure and phase composition of the Ag-Al film wear track: Through-thickness characterization by advanced electron microscopy. Archives of Metallurgy and Materials, 64(1), 251-256
Open this publication in new window or tab >>Microstructure and phase composition of the Ag-Al film wear track: Through-thickness characterization by advanced electron microscopy
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2019 (English)In: Archives of Metallurgy and Materials, ISSN 1733-3490, E-ISSN 2300-1909, Vol. 64, no 1, p. 251-256Article in journal (Refereed) Published
Abstract [en]

Analytical transmission electron microscopy has been applied to characterize the microstructure, phase and chemical composition of the Ag-Al wear track throughout its thickness down to the atomic level. Microscopy findings have been correlated with Ag-Al film tribological properties to understand the effect of the hexagonal solid solution phase on the tribological properties of this film. Ag-25Al (at.%) films have been produced by simultaneous magnetron sputtering of components in Ar atmosphere under 1 mTorr pressure and subjected to pin-on-disc tribological tests. It has been shown that hcp phase with (001) planes aligned parallel to the film surface dominates both in as-deposited and in tribofilm areas of the Ag-Al alloy film. Possible mechanisms of reduced friction in easily oxidized Ag-Al system are discussed and the mechanism based on readily shearing basal planes of the hcp phase is considered as the most probable one.

Place, publisher, year, edition, pages
POLSKA AKAD NAUK, POLISH ACAD SCIENCES, INST METALL & MATER SCI PAS, 2019
Keywords
Ag-Al alloy, TEM, EDX, hexagonal phase, electrical contact
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-381213 (URN)10.24425/amm.2019.126245 (DOI)000461713400036 ()
Available from: 2019-04-09 Created: 2019-04-09 Last updated: 2019-04-09Bibliographically approved
Braceras, I., Ibanez, I., Taher, M., Mao, F., del Barrio, A., Saenz De Urturi, S., . . . Jansson, U. (2018). On the electro-tribological properties and degradation resistance of silver-aluminum coatings. Wear, 414, 202-211
Open this publication in new window or tab >>On the electro-tribological properties and degradation resistance of silver-aluminum coatings
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2018 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 414, p. 202-211Article in journal (Refereed) Published
Abstract [en]

Contact materials in sliding electrical applications must possess low electrical contact resistance, low friction and wear coefficients, and high degradation resistance to the surrounding media. Silver coatings are commonly used in such applications despite their shortcomings. This work has focused on the study of alternative silver-aluminum coatings deposited by PVD. The main findings include the strong dependence of the tribological performance on the concentration of Al and hence the phases present in the coatings. Besides, the wear mechanism was found to be affected by the working media, either on air or insulating oil. Results have shown that for full HCP phase coatings (Ag67Al33), wear rates are lowest, with no adhesive wear and good surface sulphidation resistance, though with some proclivity to oxidation, coupled with a moderate increase in the electrical contact resistance.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2018
Keywords
Ag-Al coatings, Electro-tribology, PVD, Electrical Contact Resistance, H2S test, Oil immersion
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Identifiers
urn:nbn:se:uu:diva-369388 (URN)10.1016/j.wear.2018.08.014 (DOI)000448334700021 ()
Available from: 2019-01-15 Created: 2019-01-15 Last updated: 2019-01-15Bibliographically approved
Taher, M., Mao, F., Berastegui, P., Andersson, A. M. & Jansson, U. (2018). The Influence of Chemical and Phase Composition on Mechanical, Tribological and Electrical Properties of Silver-Aluminum alloys. Tribology International, 119, 680-687
Open this publication in new window or tab >>The Influence of Chemical and Phase Composition on Mechanical, Tribological and Electrical Properties of Silver-Aluminum alloys
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2018 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 119, p. 680-687Article in journal (Refereed) Published
Abstract [en]

Ag1-xAlx alloys were investigated as potential sliding electrical contact materials. Seven Ag1-xAlx alloys, covering the different phase regions on the Ag-Al phase diagram, were prepared by arc melting. X-ray diffraction (XRD), scanning electron microscopy coupled with X-ray spectroscopy (SEM/EDX), X-ray photoelectron spectroscopy (XPS), nano- and microindentation, and four-point electrical contact resistance measurements were employed to characterize the composition, structure, and physicochemical properties of the alloys. The hardness of Ag1-xAlx alloys increases with Al content. The Ag1-xAlx alloys with hexagonal close-packed (hcp) structure exhibit better tribological properties than pure Ag and other phase compositions. The wear mechanisms change from adhesive, for the alloys with low Al content (<= 20 at. %) to oxidative and abrasive wear for the alloys with high Al content (>= 25 at. %). The Ag1-xAlx alloys with hcp structure exhibit the highest wear resistance. Depth-profile XPS data reveal that the oxide layer grows during the triboprocess and that its thickness increases with number of sliding cycles. Ag/Ag1-xAlx contact pairs exhibit higher contact resistance than the Ag/Ag pair and the contact resistance increases with Al content.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-320229 (URN)10.1016/j.triboint.2017.11.026 (DOI)000424960500066 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2017-04-18 Created: 2017-04-18 Last updated: 2018-04-04Bibliographically approved
Taher, M., Mao, F., Berastegui, P., Andersson, A. M. & Jansson, U. (2018). Tuning tribological, mechanical and electrical properties of Ag-X (X=Al, In, Sn) alloys. Tribology International, 125, 121-127
Open this publication in new window or tab >>Tuning tribological, mechanical and electrical properties of Ag-X (X=Al, In, Sn) alloys
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2018 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 125, p. 121-127Article in journal (Refereed) Published
Abstract [en]

A new design concept for silver based alloys with a hexagonal structure as electrical contact materials with enhanced tribological properties has been investigated. The correlations between the phase composition and the tribological properties have been investigated in the Ag-Al, Ag-In and Ag-Sn systems. In each system, alloys with different chemical compositions were prepared by melting in evacuated ampoules. Characterisation techniques such as: optical microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopies (SEM and TEM) have been used to evaluate the microhardneas, tribological properties and contact resistance of the samples. The phase compositions of the synthesized Ag-Al and Ag-Sn alloys were in agreement with the phase diagrams and the metastable hcp phase was observed in the Ag-In system. The friction coefficients and wear rates of all the hcp-Ag-X (X = Al, In, Sn) alloys were considerably lower than pure Ag or fcc-Ag alloys. This is attributed mainly to easily shearing basal planes in the hcp structure. The Ag-Sn alloys showed high contact resistances, making them less suitable for a sliding electrical contacts. In contrast, the Ag-In alloys showed much lower contact resistance, making them better alternatives for practical applications.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-320227 (URN)10.1016/j.triboint.2018.04.020 (DOI)000435747800014 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage, 38432-1
Available from: 2017-04-18 Created: 2017-04-18 Last updated: 2018-08-27Bibliographically approved
Mao, F. (2017). Synthesis, Characterization, and Evaluation of Ag-based Electrical Contact Materials. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Synthesis, Characterization, and Evaluation of Ag-based Electrical Contact Materials
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ag is a widely used electrical contact material due to its excellent electrical properties. The problems with Ag are that it is soft and has poor tribological properties (high friction and wear in Ag/Ag sliding contacts). For smart grid applications, friction and wear became increasingly important issues to be improved, due to much higher sliding frequency in the harsh operation environment. The aim of this thesis is to explore several different concepts to improve the properties of Ag electrical contacts for smart grid applications.

Bulk Ag-X (X=Al, Sn In) alloys were synthesized by melting of metals. An important result was that the presence of a hcp phase in the alloys significantly reduced friction coefficients and wear rates compared to Ag. This was explained by a sliding-induced reorientation of easy-shearing planes in the hexagonal structure. The Ag-In system showed the best combination of properties for potential use in future contact applications. 

This thesis has also demonstrated the strength of a combinatorial approach as a high-throughput method to rapidly screen Ag-based alloy coatings. It was also used for a rapid identification of optimal deposition parameters for reactive sputtering of a complex AgFeO2 oxide with narrow synthesis window. A new and rapid process was developed to grow low frictional AgI coatings and a novel designed microstructure of nanoporous Ag filled with AgI (n-porous Ag/AgI) using a solution chemical method was also explored. The AgI coatings exhibited low friction coefficient and acceptable contact resistance. However, under very harsh conditions, their lifetime is too short. The initial tribotests showed high friction coefficient of the n-porous Ag/AgI coating, indicating an issue regarding its mechanical integrity.

The use of graphene as a solid lubricant in sliding electrical contacts was investigated as well. The results show that graphene is an excellent solid lubricant in Ag-based contacts. Furthermore, the lubricating effect was found to be dependent on chemical composition of the counter surface. As an alternative lubricant, graphene oxide is cheaper and easier to produce. Preliminary tests with graphene oxide showed a similar frictional behavior as graphene suggesting a potential use of this material as lubricant in Ag contacts.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 98
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1517
Keywords
electrical contact, bulk, coating, Ag-based alloys, Ag-based delafossite, AgI, graphene, graphene oxide, combinatorial material science, dc magnetron sputtering, friction, wear, hardness, contact resistance
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-320235 (URN)978-91-554-9915-0 (ISBN)
Public defence
2017-06-08, Room 2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-05-18 Created: 2017-04-18 Last updated: 2017-06-07
Mao, F., Nyberg, T., Thersleff, T., Andersson, A. & Jansson, U. (2016). Combinatorial magnetron sputtering of AgFeO2 thin films with the delafossite structure. Materials & design, 91, 132-142
Open this publication in new window or tab >>Combinatorial magnetron sputtering of AgFeO2 thin films with the delafossite structure
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2016 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 91, p. 132-142Article in journal (Refereed) Published
Abstract [en]

The main objective of this study is to demonstrate the strength of the combinatorial approach to rapidly and effectively identify suitable process parameters for the synthesis of AgFeO2 filmswith layered delafossite structure. (00l)- textured delafossite AgFeO2 thin films have been successfully deposited for the first time without post-annealing by magnetron sputtering from elemental silver and iron targets in a reactive Ar-O-2 atmosphere. Gradient filmswith a wide composition range were deposited on singlewafers and subsequent screenings of phase- and chemical compositions were employed to optimize process parameters. The optimum deposition temperature for single-phase AgFeO2 growth was 450 degrees C using a Ag target powered at 15 W with a pulsing frequency of 150 kHz and a Fe target powered at constant 120 W at a total pressure of 4 mTorr and a O-2 partial pressure of 0.8 mTorr. Selected films were studied with scanning electron microcopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The optical band gap for the indirect transition in the AgFeO2 film was determined to 1.7 +/- 0.1 eV, and the band gap for the direct transition was 2.5 +/- 0.1 eV. The film showed insulating electrical properties.

Keywords
Combinatorial sputtering, Delafossite, Thin film, AgFeO2
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-274897 (URN)10.1016/j.matdes.2015.11.092 (DOI)000367235400016 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy StorageKnut and Alice Wallenberg FoundationSwedish Research Council
Available from: 2016-02-02 Created: 2016-01-26 Last updated: 2018-03-21Bibliographically approved
Mao, F., Taher, M., Kryshtal, O., Kruk, A., Czyrska-Filemonowicz, A., Ottosson, M., . . . Jansson, U. (2016). Combinatorial Study of Gradient Ag-Al Thin Films: Microstructure, Phase Formation, Mechanical and Electrical Properties. ACS Applied Materials and Interfaces, 8(44), 30635-30643
Open this publication in new window or tab >>Combinatorial Study of Gradient Ag-Al Thin Films: Microstructure, Phase Formation, Mechanical and Electrical Properties
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2016 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 44, p. 30635-30643Article in journal (Refereed) Published
Abstract [en]

A combinatorial approach is applied to rapidly deposit and screen Ag-Al thin films-to evaluate the mechanical, tribological, and electrical properties as a function of chemical composition. Ag-Al thin films with large continuous composition gradients (6-60 atom % Al) were deposited by a custom-designed combinatorial magnetron sputtering system. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning and transmission electron microscopy (SEM and TEM), X-ray photoelectron spectroscopy (XPS), nanoindentation, and four-point electrical resistance screening were employed to characterize the chemical composition, structure, and physical properties of the films in a time-efficient way. For low Al contents (<13 atom %), a highly (111)-textured fcc phase was formed. At higher Al contents, a (002)-textured hcp solid solution phase was formed followed by a fcc phase in the most At-rich regions. No indication of a mu phase was observed. The Ag-Al films with fcc-Ag matrix is prone to adhesive material transfer leading to a high friction coefficient (>1) and adhesive wear, similar to the behavior of pure Ag. In contrast, the hexagonal solid solution phase (from ca. 15 atom %Al) exhibited dramatically reduced friction coefficients (about 15% of that of the fcc phase) and dramatically reduced adhesive wear when tested against the pure Ag counter surface. The increase in contact resistance of the Ag Al films is limited to only 50% higher than a pure Ag reference sample at the low friction and low wear region (19-27 atom %). This suggests that a hcp Ag Al alloy can have a potential use in sliding electrical contact applications and in the future will replace pure Ag in specific electromechanical applications.

Keywords
Ag-Al alloy, combinatorial approach, low friction, adhesive wear, hexagonal phase, electrical contact
National Category
Materials Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-310756 (URN)10.1021/acsami.6b10659 (DOI)000387737200089 ()
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage, 38432-1Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research
Available from: 2016-12-20 Created: 2016-12-19 Last updated: 2017-11-29Bibliographically approved
Meng, Q., Malinovskis, P., Nedfors, N., Mao, F., Andersson, M., Sun, Y. & Jansson, U. (2015). Characterization of amorphous Zr-Si-C thin films deposited by DC magnetron sputtering. Surface & Coatings Technology, 261, 227-234
Open this publication in new window or tab >>Characterization of amorphous Zr-Si-C thin films deposited by DC magnetron sputtering
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2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 261, p. 227-234Article in journal (Refereed) Published
Abstract [en]

Zr-x(SiyC1-y)(1-x) films with different Si/C atomic ratios and Zr contents were deposited using non-reactive dc-magnetron co-sputtering. All films exhibited an X-ray amorphous structure with a complex distribution of chemical bonds. The presence of Zr in the films reduced the amount of C-C and Si-C bonds but favored the formation of Zr-C and Zr-Si bonds. The mechanical and electrical properties were dependent on the bond distribution in the amorphous structure and a linear relationship between film hardness and the relative amount of Si-C bonds was observed. The addition of Zr in films also gave rise to an increase in metallic character resulting in a lower electrical resistivity. Analysis of the tribofilm showed that a low friction coefficient was favored by the formation of a lubricating a-C layer and that the formation of zirconium and silicon oxides in the more Zr-rich films has a detrimental effect on the tribological performance. (C) 2014 Elsevier B.V. All rights reserved.

Keywords
Zirconium silicon carbide, Chemical bond structure, Mechanical properties, Tribofilm, Electrical resistivity
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:uu:diva-245365 (URN)10.1016/j.surfcoat.2014.11.024 (DOI)000348255500031 ()
Available from: 2015-02-26 Created: 2015-02-26 Last updated: 2017-12-04Bibliographically approved
Mao, F., Wiklund, U., Andersson, A. M. & Jansson, U. (2015). Graphene as a lubricant on Ag for electrical contact applications. Journal of Materials Science, 50(19), 6518-6525
Open this publication in new window or tab >>Graphene as a lubricant on Ag for electrical contact applications
2015 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 50, no 19, p. 6518-6525Article in journal (Refereed) Published
Abstract [en]

The potential of graphene as a solid lubricant in sliding Ag-based electrical contacts has been investigated. Graphene was easily and quickly deposited by evaporating a few droplets of a commercial graphene solution in air. The addition of graphene reduced the friction coefficient in an Ag/Ag contact with a factor of similar to 10. The lubricating effect was maintained for more than 150,000 cycles in a pin-on-disk test at 1 N. A reduction in friction coefficient was also observed with other counter surfaces such as steel and W but the life time was strongly dependent on the materials combination. Ag/Ag contacts exhibited a significantly longer life time than steel/Ag and W/Ag contacts. The trend was explained by an increased affinity for metal-carbon bond formation.

National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-260595 (URN)10.1007/s10853-015-9212-9 (DOI)000358149600028 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research
Available from: 2015-08-25 Created: 2015-08-21 Last updated: 2017-12-04Bibliographically approved
Meng, Q. N., Wen, M., Mao, F., Nedfors, N., Jansson, U. & Zheng, W. T. (2013). Deposition and characterization of reactive magnetron sputtered zirconium carbide films. Surface & Coatings Technology, 232, 876-883
Open this publication in new window or tab >>Deposition and characterization of reactive magnetron sputtered zirconium carbide films
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2013 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 232, p. 876-883Article in journal (Refereed) Published
Abstract [en]

Zirconium carbide films have been deposited on silicon (100) substrates using direct current magnetron reactive sputtering using CH4 as a carbon source. The films exhibit a typical nanocomposite structure consisting of nanocrystalline ZrCx (nc-ZrC) grains embedded in a matrix of amorphous carbon (a-C) at low carbon content. Almost no crystalline phase can be found for carbon contents above 86 at.%. The mechanical, tribological and electrical properties of the films showed a significant dependency on the amount of the a-C in the nanocomposite structure. A larger amount of a-C gives rise to reduced hardness and higher resistivity of the film. However, both friction coefficient and wear resistance are improved by increasing the content of the surplus a-C. The influence of binding state of excess a-C phase on the properties has also been investigated. A larger sp(2)/sp(3) ratio was beneficial to relax the stress and improve the electrical properties. The Zr-based films exhibited lower friction coefficients than nanocomposites films based on e.g. Ti suggesting a potential application for this material in sliding contacts.

Keywords
Zirconium carbide, Mechanical properties, Friction and wear, Electrical resistivity, Magnetron sputtering
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-214005 (URN)10.1016/j.surfcoat.2013.06.116 (DOI)000327691300113 ()
Available from: 2014-01-06 Created: 2014-01-06 Last updated: 2017-12-06Bibliographically approved
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