Browsing by Author "Evaristo, Manuel"
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- Deciphering the mechanical strengthening mechanism: Soft metal doping in ceramic matrices: A case study of TiN-Ag filmsPublication . Luan, Jing; Kong, Fanlin; Xu, Junhua; Fernandes, Filipe; Evaristo, Manuel; Dong, Songtao; Cavaleiro, Albano; Ju, Hongbo; Fernandes, FilipeSoft metals have been widely added into ceramic-based films for fully meeting the demanding requirements of green tribological applications. However, the resulting considerable increase of the mechanical strength by adding a soft metal below 5 at.%, which reversed the rule-of-mixture, was still not fully revealed. In this paper, a case study of TiN-Ag films was carried out to investigate the strengthening mechanism induced by adding soft metal in TiN-Ag composite/multilayered films deposited by magnetron sputtering. The results showed that dual-phases of fcc-TiN and fcc-Ag co-existed in the composite films with the Ag particles embedded in the matrix. In some areas of the Ag particles, with a size below 4 nm, epitaxial growth with the TiN template was detected, which obliged the lattice to be distorted and shrunken. Consequently, both hardness and elastic modulus were enhanced from 21 and 236 GPa, for the reference TiN film, to 26 and 323 GPa for the TiN-Ag composite film with 2.4 at.% Ag. The possibility of having the epitaxial growth of Ag within TiN were also confirmed by designing a TiN/Ag multilayered film with an Ag layer thickness of ∼3 nm.
- Design and magnetron sputtering of nanomultilayered W2N/Ag-SiNx films: Microstructural insights and optimized self-lubricant properties from room temperature to 500 ◦CPublication . Luan, Jing; Kong, Fanlin; Evaristo, Manuel; Fernandes, Filipe; Zhou, Yazhou; Cavaleiro, Albano; Ju, Hongbo; Fernandes, FilipeNovel multilayered films were engineered by integrating W2N and Ag-SiNx layers in a multilayer structure to obtain improved hardness and tribological properties. The films were fabricated by alternating magnetron sputtering, depositing 40 nm layers of W2N with varying thickness of Ag-SiNx layers varying in thickness from 4 to 20 nm. The effect of the increase thickness of the Ag-SiNx layers in the films microstructure and tribological properties were accessed. Tribological experiments were conducted at room temperature (RT), 500 °C, and RT-500 °C cycling conditions. The results revealed the production of a multilayered structure comprising single fcc-W2N layers interspersed with dual-phase layers consisting of fcc-Ag and amorphous SiNx phases. Tribological results indicated an improvement in the tribological performance with increase thickness of the Ag-SiNx layer up to 12 nm. The tribo-synergistic/combined action of both W2N and Ag-SiNx layers, along with the presence of layered lubricant tribo-phases of WO3 and Ag2WO4, showcased the pivot role in reducing friction and enhancing wear resistance. The optimized multilayered film, featuring a 12 nm Ag-SiNx layer, demonstrated exceptional tribological properties under temperature-cycling from RT to 500 °C.
- Exploring tribological characteristics of ZrN-MoSN composite films fabricated via RF magnetron sputtering: Insights from microstructure and performance analysisPublication . Luan, Jing; Lu, Hongying; Xu, Junhua; Fernandes, Filipe; Fernandes, Filipe; Evaristo, Manuel; Ma, Bingyang; Xie, Fuxiang; Cavaleiro, Albano; Ju, HongboAchieving the stringent demands of sustainable tribological industrial applications poses a significant challenge, particular in optimizing the self-lubricant performance of nitride-based films. This paper tackled this challenge by designing and depositing a series of ZrN-MoSN composite films with varying (Mo + S)/Zr ratios, employing RF magnetron sputtering, aimed to enhance the tribological properties through utilizing the high loading capacity of the ZrN matrix and the exceptional self-lubricating attributes of Mo-S-N additives. After conducting thorough investigations on the microstructure, and tribological properties, the results revealed that the dense columnar structured ZrN-MoSN composite films displayed a polycrystalline composition comprising fcc-ZrN and hcp-MoS2 phases, intertwined with amorphous phases of Mo(SN)x and MoS2(N2). (Mo + S)/Zr ratios below 1.08 exhibited a minor impact on the room temperature (RT) tribological properties, while higher ratios led to degradation on RT average friction coefficient (COF) and wear rate (WR). However, the synergistic effect of ZrN matrix and the tribo-phases of layered MoO3 and hard ZrO2 contributed to the significant enhanced 500 °C tribological properties, particularly with an optimized (Mo + S)/Zr ratio of 0.43.
- Influence of the alloying elements on the tribological performance of DLC coatings in different sliding conditionsPublication . Evaristo, Manuel; Fernandes, Filipe; Cavaleiro, AlbanoReduction of friction and wear at the sliding surfaces are the main concern in several applications. From an extensive list of low friction hard coatings, DLC type is one of the most studied and develop for achieving low wear and friction. The addition of an alloying element is extensively used, since it can change significantly the coatings performance. Therefore, in this research the tribological behaviour of DLC coatings alloyed with different elements was evaluated, since no works comparing the tribological properties of DLC films with approximately the same alloying elements concentration are reported in the literature. The coatings were deposited by physical vapor deposition and alloyed with: tungsten (W), silver (Ag), silicon (Si), silicon and oxygen (SiO), with concentrations of metallic elements between 10 and 15 at. % and a pure DLC coating was also deposited for comparison purposes. The coatings hardness varied between 23 GPa for the pure DLC and 11 GPa for the alloyed with Si and O. The tribological behaviour of the coatings was evaluated by pin-on-disk against 10 mm 100Cr6 steel balls, at room temperature (RT) and 100 ◦C. The best wear resistance was achieved with the pure DLC and DLC-W for RT tests. At RT the lowest friction was obtained with the DLCSi and DLCSiO coatings. Despite of the low COF values, those coatings displayed the highest specific wear rate values among all the films. This is attributed to the presence of hard SiC particles stick at the ball counterpart which promotes severe abrasion of the coatings surface. The increase of the temperature to 100 ◦C increases significantly the friction coefficient for the DLC and DLC-W coatings, whilst, for DLC-Ag, DLC-Si and DLC-SiO coatings a slight decrease is noticed. At this temperature DLCAg coating is the more performing due to the formation of a tick tribolayer rich in Ag adhered at the counterpart surface. With this work it was concluded that friction and wear are not directly related, and the third bodies formed on the sliding surfaces have major influence on the tribological performance of the system.
- Nano-multilayered ZrN‒Ag/Mo‒S‒N film design for stable anti-frictional performance at a wide range of temperaturesPublication . Ju, Hongbo; Luan, Jing; Xu, Junhua; Cavaleiro, Albano; Evaristo, Manuel; Fernandes, Filipe; Fernandes, FilipeA multilayer film, composed by ZrN-Ag (20 nm) and Mo-S-N (10 nm) layers, combining the intrinsic lubricant characteristics of each layer was deposited using DC magnetron sputtering system, to promote lubrication in a wide-range of temperatures. The results showed that the ZrN-Ag/Mo-S-N multilayer film exhibited a sharp interface between the different layers. A face-centered cubic (fcc) dual-phases of ZrN and Ag co-existed in the ZrN-Ag layers, whilst the Mo-S-N layers displayed a mixture of hexagonal close-packed MoS2 (hcp-MoS2) nano-particles and an amorphous phase. The multilayer film exhibited excellent room temperature (RT) triblogical behavior, as compared to the individual monolayer film, due to the combination of a relative high hardness with the low friction properties of both layers. The reorientation of MoS2 parallel to the sliding direction also contributed to the enhanced anti-frictional performance at RT. At 400 °C, the reorientation of MoS2 as well as the formation of MoO3 phase were responsible for the lubrication, whilst the hard t-ZrO2 phase promoted abrasion and, consequently, led to increasing wear rate. At 600 °C, the Ag2MoO4 double-metal oxide was the responsible for the low friction and wear-resistance; furthermore, the observed transformation from t-ZrO2 to m-ZrO2, could also have contributed to the better tribological performance.
- The Influence of H Content on the Properties of a-C(W):H CoatingsPublication . Evaristo, Manuel; Fernandes, Filipe; Jeynes, Chris; Cavaleiro, AlbanoDiamond-like-carbon “DLC” coatings can be deposited in many different ways, giving a large range of material properties suitable for many different types of applications. Hydrogen content significantly influences the mechanical properties and the tribological behavior of DLC coatings, but its determination requires techniques that are not available in many research centers. Thus, it is important to find alternative indirect techniques, such as Raman spectroscopy or nanoindentation (hardness measurements), which can give comparative and indicative values of the H contents in the coatings, particularly when depositions with a reactive gas flow are being studied. In this work, “DLC” composite coatings with varying H content were deposited via Physical Vapor Deposition (PVD) magnetron sputtering in a reactive atmosphere (Ar + CH4). An Ion Beam Analysis was used to determine the elemental depth profile across the coating thickness (giving both average C:W:H ratios and film density when combined with profilometer measurements of film thickness). The hardness was evaluated with nanoindentation, and a decrease from 16 to 6 GPa (and a decrease in the film density by a factor of two) with an increasing CH4 flow was observed. Then, the hardness and Raman results were correlated with the H content in the coatings, showing that these indirect methods can be used to find if there are variations in the H content with the increase in the CH4 flow. Finally, the adhesion and tribological performance of the coatings were evaluated. No significant differences were found in the adhesion as a function of the H content. The tribological properties presented a slight improving trend with the increase in the H content with a decrease in the wear rate and friction.