196,033 research outputs found
Microstructural and nanomechanical characterisation of Ni-Ti(-Cu) shape memory alloy thin films for tribology
No full textProtective and functional coatings have been undergoing development for decades and further improvement to their mechanical and tribological properties are more and more challenging. Nowadays most research is aimed at improving the tribological behaviour of hard and functional coatings through the optimisation of their microstructure on the nanoscale. Over time significant breakthroughs have been achieved, however improving material performance is becoming harder. Combining different layers is another possible way of improving the tribological performance of functional coatings. The use of bonding layers can mitigate the differences in mechanical and thermal properties between coating and substrate and might change the behaviour of the coating system in a tribological scenario. As a consequence, the nature of this bonding interlayer plays an important role in the response of the coatings to the complex stress conditions taking place in a tribological system. Among the possible interlayer candidates a layer with the capability of accommodating large deformation, thus protecting the substrate from plastic deformation as well as improving the adhesion of the top layer to the substrate, could represent a suitable choice. One of the potential classes of materials satisfying the above requirements are the Ni-Ti based alloys which are known to exhibit superelastic properties also when sputter deposited. In this study we first focus our investigation on the characterisation of sputter-deposited Ni-Ti based thin films. In particular, the Ni-Ti system is doped by a third element, Cu through which mechanical and microstructural properties can be changed without detrimental effects on the typical functional properties of Ni-Ti alloys. The effects of Cu, in the range 0 – 20 at.%, and of the post-deposition heat treatments, with particular regard to annealing temperature, on mechanical and microstructural properties of sputter-deposited Ni-Ti(-Cu) thin films are investigated by nanoindentation, X-ray diffraction and transmission electron microscopy. Based on the objective of using Ni-Ti(-Cu) thin films as the interlayer in tribological coatings, some of the Ni-Ti(-Cu) films are selected and integrated in a bilayer design. Among the tribological coatings self-lubricant W-S-C coatings are known for their excellent non-Amonton frictional behaviour with friction decreasing with increasing contact pressure. The low friction of W-S-C coatings is associated with the formation of a WS2 tribolayer on the sliding surface. When the basal plane of the WS2 tribolayer is aligned with the sliding direction, the friction coefficient drops to very low values owing to the weak bonding between chalcogenide atomic planes. The formation of the low-shear surface layer is directly related to contact pressure; therefore, a W-S-C coating is an ideal functional layer with which to study the effect of Ni-Ti(-Cu) interlayers on sliding properties. W-S-C/Ni-Ti(-Cu) bilayer coatings are fabricated following a three-step process consisting of deposition and annealing of the Ni-Ti(-Cu) layers and subsequent deposition of the top functional layer. Mechanical and nano scratch behaviour of these bilayers is investigated in order to study the functional role of different Ni-Ti(-Cu) interlayers on the response of the bilayers to nanoindentation and nano-scratch tests.The tribological performance of W-S-C single layer and selected W-S-C/Ni-Ti(-Cu) bilayers are investigated by sliding tests in humid air under different test conditions in order to assess the potential beneficial effects of the interlayer on the tribological properties of W-S-C. Correlation between the tribological properties measured and microstructural changes induced by sliding is achieved by investigating the tested coatings by focused ion beam and transmission electron microscopy. Chemical changes on the sliding surfaces are also investigated by Raman spectroscopy in order to highlight possible differences in the tribolayer formation under different test conditions and for different Ni-Ti(-Cu) interlayers. The study is also aimed at understanding how the stress-induced martensitic transformation is activated in the interlayers during sliding.<br/
Structural and mechanical properties of irradiated multilayer nanocomposites
No full textRadiation damage processes in ion-irradiated metals have been thoroughly studied in the last decade revealing complexity and multiscale nature of material damage. Conversely, very little attention has been paid regarding gamma rays damage, which, in reactor pressure vessels, was found to be comparable to that produced by fast neutrons. Nanoscale structural control of nuclear materials through design of interfaces is a promising way of limiting radiation damage. Here we report two case studies regarding: (i) the role of interfaces and of He-ion radiation doses on the structural and mechanical properties of a sputter-deposited Cu/W multilayer, and (ii) the role of interface density distribution on the structural and mechanical properties of gamma-irradiated Zr/Nb multilayers. Transmission electron microscopy and X-ray diffraction were employed to investigate radiation damage, while mechanical properties were explored by nanoindentation. We propose correlations between radiation experiments and materials properties for each case
Ni-Ti-Cu shape memory alloy interlayers supporting low-friction W-S-C coatings
No full textThis work is aimed at assessing the capability of Ni-Ti(-Cu) interlayers, integrated in a bilayer design (tribological top layer/Ni-Ti(-Cu) layer/substrate), to improve the resistance against adhesion damage and the tribological performance of W-S-C self-lubricant coatings, when these bilayers are subjected to different sliding conditions. Interesting differences on adhesion and tribological performance are observed in relation to the grain size of the interlayer materia
Microstructural evolution of nanometric Ti(NiCu)2 precipitates in annealed Ni–Ti–Cu thin films
No full textCompeting mechanisms on the strength of ion-irradiated Zr/Nb nanoscale multilayers: interface strength versus radiation hardening
No full textThe structural stability and mechanical properties of sputter-deposited Zr/Nb nanoscale multilayers subjected to Si-ion irradiation were investigated in relation to the individual layer thickness. The interface density distribution played a major role on the nature and amount of accumulated radiation damage. The multilayer with a smaller periodicity experienced a significantly higher atomic-scale disorder and radiation hardening compared to the multilayer with thicker individual layers. In the latter case, an enhanced radiation damage tolerance was achieved due to the balance between competing deformation mechanisms.</p
Ad Concordatvm Henrici V. Et Callisti II. De Investitvris Episcoporvm Et Abbatvm / Praeside Io. Gvilielmo Hoffmanno ... D. X. Octob. MDCCXXXIX. In Avditorio Maiori Dispvtabit M. Gottlieb VVernsdorffivs Vitembergensis
Get PDFAD CONCORDATVM HENRICI V. ET CALLISTI II. DE INVESTITVRIS EPISCOPORVM ET ABBATVM / PRAESIDE IO. GVILIELMO HOFFMANNO ... D. X. OCTOB. MDCCXXXIX. IN AVDITORIO MAIORI DISPVTABIT M. GOTTLIEB VVERNSDORFFIVS VITEMBERGENSIS
Ad Concordatvm Henrici V. Et Callisti II. De Investitvris Episcoporvm Et Abbatvm / Praeside Io. Gvilielmo Hoffmanno ... D. X. Octob. MDCCXXXIX. In Avditorio Maiori Dispvtabit M. Gottlieb VVernsdorffivs Vitembergensis (1)
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Ad Concordatum Henrici V. Et Callisti II. (3)
Synopsis Dissertationis (67
In situ TEM observations on the structural evolution of a nanocrystalline W-Ti alloy at elevated temperatures
No full textThe thermal stability and nanoscale structural evolution at elevated temperatures of a sputter deposited W-Ti alloy thin film were studied by a combination of ex situ and in situ techniques. XRD, FIB, SEM-EDX and STEM-EDX were used to characterise the film annealed ex situ in vacuum at 1373 K for 48 h. In situ TEM heating experiments were conducted at various temperatures up to 923 K to capture transitional phenomena occurring in the alloy upon heating and cooling. At a microscopic level, the alloy annealed at 1373 K for 48 h transformed from a single-phase β-(WTi) solid solution into a two-phase alloy consisting of Ti-rich grains in equilibrium with Ti-depleted β-(WTi) solid solution grains. In situ TEM observations revealed initial Ti segregations along columnar grain boundaries at T ∼ 423–573 K, followed by Ti-rich clusters formation in the grains interior at T ∼ 573–773 K. The microstructure observed at 923 K remained stable upon cooling to room temperature and consisted of Ti-rich segregations along the columnar grain boundaries and of alternate Ti-rich and Ti-depleted nanoscale domains in the grains interior, which formed a stable dual-phase nanocrystalline structure.</p
Effects of Cu and of annealing temperature on the microstructural and mechanical properties of sputter deposited Ni-Ti thin films
No full textEffects of Cu and of annealing temperature on the microstructural and mechanical properties of sputter deposited Ni-Ti thin film
Bubbles formation in helium ion irradiated Cu/W multilayer nanocomposites: effects on structure and mechanical properties
Get PDFThis study investigates the effects of He bubbles on structural and mechanical properties of sputter-deposited Cu/W multilayers. A multilayer with a periodicity of 10 nm was deposited and subjected to helium ion irradiation with two different fluences. He bubbles formed mostly in Cu layers and their distribution was affected by He concentration and radiation damage. According to SRIM calculations, in low He concentration regions bubbles formed mostly along interfaces, while more homogeneously distributed bubbles were found in Cu layers and along columnar grain boundaries in higher He concentration regions. We suggest that the capability of interfaces to annihilate point defects is weakened by the He bubbles shielding effect. Nanoindentation tests revealed a hardness decrease amounting to ?0.5 and ?1 GPa for low and high fluences, respectively. The observed softening effect is attributed to He storage-induced changes in residual stresses and columnar grain boundary/interfacial sliding facilitated by He bubbles
Stress-induced martensitic transformation in Ni–Ti(–Cu) interlayers controlling stress distribution in functional coatings during sliding
No full textThe stress-induced martensitic transformation occurring in sputter-deposited Ni48.1Ti51.9 and Ni43.4Ti49.6Cu7 interlayers, integrated in a W-S-C/Ni–Ti(–Cu) bilayer design, was investigated by transmission electron microscopy, after these bilayers were subjected to different sliding conditions. Martensitic bands across the interlayers were formed depending on the sliding direction with their shape and distribution a function primarily of both applied normal load and grain size.The Ni48.1Ti51.9 interlayer (lateral grain size of ?3 ?m) showed well oriented and ordered martensitic bands extended through the interlayer thickness under low load (5 N). At a higher load (18 N) the growth of these bands was limited by the stabilised martensite formed as a consequence of the high compressive stress, at the interface with the substrate.The Ni43.4Ti49.6Cu7 interlayer (lateral grain size of ?650 nm) exhibited no significant evidence of stabilised martensite under different loading conditions. The martensitic transformation was limited by the smaller grain size and most of the stress was relaxed by elastic and, to some extent, pseudo-elastic deformation of the austenitic phase. Grain boundaries were found to stop the growth of martensitic bands, thus limiting the activation of the martensitic transformation into the neighbouring grains during sliding.The grain refinement caused a change in the capability of the interlayer to relax shear and compressive stresses. Such a change was found to affect the formation of the WS2-rich tribolayer on the W-S-C sliding surface, and consequently the shear stress transmitted down throughout the bilayers thickness. Accordingly, different levels of deformation were observed on the top layer.<br/
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