TEM investigation of MoSeC films

Transition metal dichalcogenides (TMD) are widely used as self-lubricating material either as oil additive or directly as thin films. Magnetron sputtering is a deposition method allowing depositing such films with high density and adhesion. However, their spread use in practical applications is still hindered since their excellent sliding properties are deteriorated in the presence of humidity and under high contact pressures. MoSe2, one of the members of TMD family recently studied, has been co-sputtered with carbon in order to improve the mechanical and tribological properties when compared to pure MoSe2 film

Materials incompatibility as a major cause of hip prostheses rejection

The development of new multifunctional coatings to apply on medical biomaterials continues to be required, since materials commonly used in hip prostheses still presenting failures. Multifunctionality is the result of a synergy, on the nanoscale level, of good corrosion, mechanical and tribological properties. Additionally, a biomaterial must always be biocompatible. Besides these properties, the major challenge would be to get a material that also has antimicrobial activity. In this context, the development of advanced materials with the ability to present these properties is being regarded as a strategy to prevent the colonization of implant and biofilm formation by bacteria. So, in this review, the attention is focused on the description of the funda- mental points of the natural synovial joint, since, its mechanical and tribological characteristics are the main causes that lead to the necessity of its replacement by an implant. Moreover, a contextualization was also performed on the hip replacement surgery and the biomaterials used, with a focus on their mechanical and tribological properties. Finally, it is explained the need of surface modification and the potential of TiCN coatings doped with silver.This research is sponsored by FEDER funds through the program COMPETE - Programa Operacional Factores de Competitividade - and by national funds through FCT - Fundacao para a Ciencia e a Tecnologia - in the framework of the Strategic Projects PEST-C/FIS/UI607/2011, PEST-C/EME/UI0285/2011, PTDC/CTM/102853/2008, and FCT - Fundacao para a Ciencia e a Tecnologia through the grant SFRH / BD / 67022 / 2009.The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the project "BioInd - Biotechnology and Bioengineering for improved Industrial and Agro-Food processes", REF. NORTE-07-0124-FEDER-000028 Co-funded by the Programa Operacional Regional do Norte (ON.2 - O Novo Norte), QREN, FEDER

Friction force microscopy analysis of self-adaptive W-S-C coatings:nanoscale friction and wear

Transition metal dichalcogenides (TMD) are increasingly popular due to unique structural and mechanical properties. They belong, together with graphene and similar 2D materials, to a small family of solid lubricants with potential to produce ultralow friction state. At the macroscale, low friction stems from the ability to form well-oriented films on the sliding surface (typically up to 10 nm thick), with the TMD basal planes aligned parallel to the surface. In this study, we quantitatively evaluate tribological properties of three sputtered tungsten–sulfur–carbon (W–S–C) coatings at a nanoscale using friction force microscopy. In particular, we investigate possible formation of well-ordered tungsten disulfide (WS2) layers on the coating surface. The coefficient of friction decreased with increasing load independently of coating composition or mechanical properties. In contrast, hard coatings with high tungsten carbide content were more resistant to wear. We successfully identified a WS2 tribolayer at the sliding interface, which peeled off as ultrathin flakes and attached to AFM tip. Nanoscale tribological behavior of WSC coatings replicates deviation of Amonton’s law observed in macroscale testing and strongly suggests that the tribolayer is formed almost immediately after the start of sliding

Dense nanocrystalline W alloys: Enhancement of hardness and thermal stability by Al addition

Nanocrystalline W100-xAlx (x up to 20 at.%) powders obtained by mechanical alloying have been consolidated by spark plasma sintering (SPS). Alloying W with Al significantly improves the sinterability, allowing the fabrication of dense samples. The consolidation process keeps the nanocrystalline microstructure substantially unaffected, which contributes to the hardness of the final W-Al alloys. The room-temperature nano-and micro-hardness of dense W80Al20 alloy sintered at 1100 degrees C are as high as 18 and 14 GPa, respectively. These values are significantly higher than those previously reported for pure coarse-or submicron-grained W and may be ultimately ascribed to the grain boundary segregation of Al

Evolution of structural, mechanical and tribological properties of Ni-P/MWCNT coatings as a function of annealing temperature

Collaborative research project Hardalt (Grant No. 606110) funded by the EU's seventh framework programme

Control of energy dissipation in sliding low-dimensional materials

Frictional forces acting during the relative motion of nanosurfaces are the cause of energy loss and wear which limit an efficient assembly and yield of atomic-scale devices. In this research, we investigate the microscopic origin of the dissipative processes as a result of the frictional response, with the aim to control them in a subtle way. We recast the study of friction in terms of phonon modes of the system at the equilibrium, with no need to resort to dynamics simulations. As a case study, we here consider layer sliding in transition metal dichalcogenides thin films. We find that the population of specific atomic orbitals and the relative contribution of the atomic type to selected system vibrations are the crucial quantities which determine the frictional response in tribological conditions. A reduced amount of energy dissipation is found when the bond character is more ionic and the layer sliding is realized by a faster motion of the chalcogen atoms. The individuated relevant parameters governing the energy dissipation can be used as descriptors in high-throughput calculations or machine learning engines to screen databases of frictional materials. The presented framework is general and can be promptly extended to the design of tribological materials with targeted frictional response, irrespective of the chemistry and atomic topology.</p

Stress-induced martensitic transformation in Ni–Ti(–Cu) interlayers controlling stress distribution in functional coatings during sliding

The 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/

Structural, mechanical and functional properties of irradiated multilayer nanocomposites

A common goal for materials employed in nuclear environments is to exhibit the highest radiation tolerance. The lifetimes of current and even more of future reactors are largely determined by materials issues such as embrittlement and swelling. In the process of energy production via fission and fusion, structural materials are subject to substantial radiation damage, which appears in the form of point defects and their agglomeration to form dislocation loops and vacancy clusters. Combination of vacancy clusters with transmutation products such as helium (He) promotes the formation of He bubbles. These bubbles cause swelling, embrittlement and dimensional instabilities in structural metals, which represent a real challenge for application of metals in nuclear industry. It is well known that surfaces, grain boundaries and heterointerfaces are good sinks for radiation-induced point defects and traps for implanted He. Composite materials with a high interface density distribution showed enhanced radiation tolerance compared to conventional single phase metals. In spite of this beneficial effect, the role of He bubbles on the mechanical properties and structural integrity of nanostructured materials is still to be understood. This study is aimed at evaluating and correlating the effects of He bubbles formation with structural and mechanical properties of nanomaterials with high interface density distributions such as nanoscale metallic multilayers. With this aim, Cu/W multilayers were deposited by magnetron sputtering and subjected to He ion implantation (1 MeV) with two different fluences (1.1 and 3.2 ×1016 cm-2) and incident angles. Structure of pristine and irradiated multilayers was investigated by XRD and FIB/TEM analyses, while mechanical properties changes were evaluated by nanoindentation, through which possible deformation mechanisms in multilayers with He bubble-decorated interfaces were also investigated. By combining calculated He concentration profiles, throughout the multilayer thickness and TEM images, it is found that in low He concentrations regions, bubbles formed mostly along interfaces, while more homogeneously distributed bubbles were found in Cu layers and along columnar grain boundaries in higher He concentrations regions. It is suggested 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 mostly attributed to He storage induced changes in residual stresses, and columnar grain boundary sliding facilitated by He bubbles. <br/

Volume and pressure of helium bubbles inside liquid Pb16Li. A molecular dynamics study

The behaviour of helium impurities inside metals has been well studied in the last 30 years, however, little attention has been devoted to helium atoms inside liquid metals. Here we have investigated the nucleation and coalescence processes of helium atoms inside liquid eutectic lithium-lead alloys using atomistic simulations. Several key findings regarding He bubbles inside liquid PbLi eutectic are presented. The radius versus the number of atoms has been calculated in the temperature range 600-1000 K. The trend can be fitted and likely extrapolated to larger bubbles (micrometer size). The value of thermal expansion of He bubbles is given as well and compared to the thermal expansion of bulk He. The pressure inside He bubbles has been calculated as a function of bubble size. Finally, the importance of accurate interatomic potentials for the He-metal interaction is discussed.</p