| 1. |
- Torres, Hector, et al.
(author)
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Microstructural design of self-lubricating laser claddings for use in high temperature sliding applications
- 2018
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In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 337, s. 24-34
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Journal article (peer-reviewed)abstract
- Nickel-based self-lubricating claddings with the addition of Ag and MoS2 were prepared by means of laser cladding on stainless steel substrates, aiming at their implementation in metal forming applications involving demanding tribological conditions at high temperatures. The novelty of this approach is the addition of MoS2 with the aim to achieve a uniform silver distribution within the resulting cladding by means of an encapsulation mechanism. This prevents it from floating to the surface during the deposition process and thus being subsequently lost during surface preparation. The role of Ag and MoS2 concentration on the encapsulation process is discussed in terms of phase composition and resulting microstructures. The tribological behaviour of the resulting laser claddings was evaluated at up to 600 °C under unidirectional sliding. The encapsulation of Ag leads to outstanding tribological properties while keeping the concentration of used Ag low, thus increasing the economic viability of the claddings. An improvement in terms of both friction and wear was observed for the self-lubricating claddings compared to the nickel-based reference alloy, thus making them good candidates for use in high temperature applications such as hot metal forming.
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| 2. |
- Torres, Hector, et al.
(author)
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Self-lubricating laser claddings for friction control during press hardening of Al-Si-coated boron steel
- 2019
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In: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 269, s. 79-90
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Journal article (peer-reviewed)abstract
- In recent years, the use of Al/Si coatings has become widespread in hot stamping in order to protect the work piece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperature can lead to unstable friction and damage both the tool and the work piece.In the present study, self-lubricating coatings with the addition of silver and MoS2 have been prepared by means of laser cladding deposition, aiming at their use in hot stamping in order to decrease friction and wear. The coatings were evaluated at high temperatures against Al-Si-treated boron steel using two different testing configurations featuring open and closed tribosystems. A significant reduction in friction for the self-lubricating claddings were observed along with decreased material transfer. This could be beneficial for hot stamping applications as it can ensure the stability of the process while preventing surface damage to the work piece. Additionally, closed configuration tribotesting has been found to underestimate friction and wear of the tool/work piece system, thus making it less suited for the lab-scale simulation of hot metal forming compared to open configuration tribometers. This finding has been considered relevant as many references in the available literature still report the use of closed configuration tribometers.
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| 3. |
- Torres, Hector, et al.
(author)
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Self-lubricating laser claddings for reducing friction and wear from room temperature to 600 °C
- 2018
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In: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 408-409, s. 22-33
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Journal article (peer-reviewed)abstract
- In this work, laser cladding has been employed for the preparation of nickel-based self-lubricating coatings featuring the addition of different combinations of soft metal solid lubricants such as Ag and Cu. Transition metal dichalcogenides (WS2, MoS2) were evaluated as precursors for encapsulating and uniformly distributing the soft metals throughout the microstructure. The tribological behaviour of the resulting claddings was evaluated under high temperature reciprocating sliding conditions, including two different counter body geometries that lead to very different ranges of contact pressures during testing. An improved tribological behaviour was observed for the self-lubricating claddings compared to the unmodified nickel-based alloy up to 600 °C, attributed to the presence of silver and the formation of lubricous sulfides during sample preparation due to the thermal degradation of the transition metal dichalcogenides precursors. Additionally, the role of the contact conditions observed when testing the self-lubricating claddings against flat pins instead of spherical counter bodies are discussed in terms of frictional and wear microstructural mechanisms.
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| 4. |
- Torres, Hector, et al.
(author)
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The tribology of Ag/MoS2-based self-lubricating laser claddings for high temperature forming of aluminium alloys
- 2020
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In: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 442-443
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Journal article (peer-reviewed)abstract
- In recent years, the use of aluminium alloys in the automotive industry has gained significant attention due to their specific strength, corrosion resistance and recyclability. However, their forming at high temperature in processes like hot stamping is challenging due to the poor tribological behaviour of aluminium alloys, which is the source of severe adhesive wear and a poor surface quality of the finished product.In an effort to overcome these tribological problems, iron- and nickel-based self-lubricating laser claddings with the addition of solid lubricants such as silver and molybdenum disulfide have been evaluated under conditions representative of hot stamping against the aluminium alloy AA6082. It has been found that self-lubricating claddings decrease friction and counter body wear at high temperatures compared to alloys commonly used in forming tools such as grade 1.2367 steel. Furthermore, nickel-based self-lubricating claddings have shown a better tribological behaviour than their iron-based counterparts, due to the formation of a nickel-based sulfide layer on the counter body. It is thus expected that the implementation of self-lubricating claddings can improve the quality of the final product while reducing the need for added lubricant during the hot stamping of aluminium alloys.
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| 5. |
- Torres, Hector, et al.
(author)
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Tribological behaviour of MoS2-based self-lubricating laser cladding for use in high temperature applications
- 2018
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In: Tribology International. - : Elsevier. - 0301-679X .- 1879-2464. ; 126, s. 153-165
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Journal article (peer-reviewed)abstract
- Many high temperature (HT) forming processes require the use of solid lubricants in order to control friction and reduce wear. In an attempt to eliminate the need for solid lubrication in high temperature sliding applications, nickel-based self-lubricating coatings with the addition of Ag and MoS2 were prepared by means of laser cladding on stainless steel substrates.The behaviour of the resulting laser claddings was thoroughly evaluated up to 600 °C, including the oxidation behaviour and reciprocating tribotesting using different counter body geometries (ball and flat pin). The self-lubricating coatings showed lower friction than the unmodified reference alloy at all tested temperatures, in addition to a significant microstructural stability after prolonged exposure at high temperatures. The addition of solid lubricants to the claddings was also found to be beneficial in terms of the counter body wear at HT, as no material loss could be measured for the bearing balls after testing at 600 °C against the self-lubricating claddings, despite the significant softening experienced by AISI 52100 bearing steel at HT.
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| 6. |
- Torres, Hector, et al.
(author)
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Tribological behaviour of self-lubricating materials at high temperatures
- 2018
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In: International Materials Reviews. - : Taylor & Francis. - 0950-6608 .- 1743-2804. ; 63:5, s. 309-340
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Journal article (peer-reviewed)abstract
- Self-lubricating materials are becoming more widespread in fields like metal forming or power generation due to the inability to use conventional lubricants in high-temperature (HT) applications. In an effort to summarise the progress done in this field, a detailed literature review has been carried out, ranging from micron-thickness thin films to hardfacings and bulk materials, and classified by the reported solid lubricants. Moreover, the most-cited deposition techniques have been reviewed for each lubricant class in addition to their advantages and limitations. HT friction and wear data for self-lubricating materials have also been examined in order to identify effective lubrication ranges and general trends in their tribological behaviour, which is expected to be useful for researchers interested in this field. Finally, several apparent research gaps have been described, with suggestions for new experimental work that could lead to the development of new high-temperature self-lubricating materials.
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| 7. |
- Torres, Hector, et al.
(author)
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Tribological performance of iron- and nickel-base self-lubricating claddings containing metal sulfides at high temperature
- 2022
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In: Friction. - : Springer. - 2223-7690 .- 2223-7704. ; 10:12, s. 2069-2085
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Journal article (peer-reviewed)abstract
- Iron-based coatings with the incorporation of solid lubricants have been prepared by means of laser cladding, in an effort to control friction and decrease tool wear at high temperatures during metal forming applications. The choice of a Fe-based powder has been considered advantageous, as it can lead to decreased costs compared to nickel-based claddings previously studied by the authors, in addition to having a lower environmental impact. In particular, the incorporation of transition metal dichalcogenides such as MoS2 as precursors leads to the encapsulation of silver in Fe-based self-lubricating claddings, resulting in a uniform distribution of the soft metal across the thickness of the coating. Subsequent tribological evaluation of the claddings at high temperatures shows that the addition of lubricious compounds leads to lower friction at room temperature and significantly decreased wear up to 600 °C compared to the unmodified iron-based reference alloy, although higher than similar self-lubricating Ni-based claddings. In order to cast light into these observed differences, the corresponding microstructures, phase composition, and self-lubricating mechanisms have been studied and compared for Fe- and Ni-based claddings having both of them the addition of silver and MoS2. The results suggest a key role of the formation of protective tribolayers on the counter body during high temperature sliding contact. Additional simulation of the phase evolution during solidification reveals that the formation of different chromium- and nickel-based metal sulfides in Fe- and Ni-claddings during laser cladding by the decomposition of MoS2 plays a key role in determining their tribological behaviour at high temperatures.
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