Renato Buzio

Diamond-like carbon (DLC) coatings are widely used as protective and self-lubricating surfaces in metal–metal contacts. Their frictional behavior is governed by the formation and evolution of carbon-rich transfer layers (TLs), which can be tailored through functionalization with carbon nanomaterials. Recent studies have shown that graphene sheets (GSs) and nanodiamonds (NDs) act synergistically to achieve ultra-low friction in microrough (~0.2 μm) metal–DLC contacts under dry N₂ at a 1 N load. Here, we probe how this lubrication mechanism evolves with increasing load from 1 to 10 N—corresponding to local contact pressures up to ~11–16 GPa—respectively, in dry N₂ and humid air conditions. Ball-on-disk experiments are performed on an industrial hydrogenated DLC coating sliding against stainless-steel. In dry N₂, GS–ND functionalization yields a low and stable coefficient of friction across the entire load range, reaching a minimum of about 0.05. In humid air, higher friction levels are observed across all loads (CoF ~0.10–0.15), accompanied by oxidation-driven modifications of both wear debris and the counterface contact region, with oxygen content increasing by more than a factor of three compared to dry N₂. Detailed microscopy and spectroscopy analyses indicate that enhanced lubricity in dry N₂ arises from TLs incorporating GSs, NDs, and nanoscroll-like structures, whereas humid air promotes interfacial amorphization and oxidation, leading to load-insensitive friction and boundary lubrication effects through physisorbed water molecules.

A comprehensive understanding of the relationship between defects concentration, optical properties and mechanical behavior of two-dimensional transition metal dichalcogenides (TMDs) is crucial for their integration as active components in micro- and nanomechanical devices. In this study we characterize the nanoscale contact adhesion and friction of WS₂ flakes grown via chemical vapor deposition. We identify two domains named α and β with distinct mechanical properties, which are not apparent in morphological differences but mirror spatial variations of the optoelectronic properties. The α-domains exhibit high photoluminescence (PL) emission, strong Raman response, higher contact adhesion and lower friction, closely resembling the response of pristine WS₂ flakes prepared by mechanical exfoliation. Conversely, the β-domains display very low PL emission, weak Raman response with blueshifted fingerprint peaks, lower adhesion and up to six-fold higher friction. Based on experimental evidence and general arguments, we attribute the mechanical heterogeneity between the α- and β-domains to the differentiated densities of sulfur and tungsten atomic vacancies, which are known to selectively populate the two domains. Our results indicate that the tungsten vacancies in the β-domains not only mediate non-radiative recombination processes but also drive a prominent friction enhancement, either by increasing the amplitude and disorder of the WS₂ potential energy surface or by impacting the stress distribution within the growing flakes. These findings help identify the type of defects and mechanisms that most significantly affect the properties of TMD monolayer flakes prepared by scalable production routes.

Nanofriction experiments typically produce force traces exhibiting atomic stick–slip oscillations, which researchers have traditionally analyzed with ad hoc algorithms. This study successfully unravels the potential of machine learning (ML) to interpret nanofriction force traces and automatically extract Prandtl–Tomlinson (PT) model parameters. A prototypical neural network (NN) perceptron was trained on synthetic force traces generated by simulations across a wide parameter range. Despite its simplicity, this NN successfully analyzed experimental data, marking the first application of a network trained solely on computational data to experimental nanofriction. Challenges encountered in developing the NN model proved to be instructive and revealing. Poor transferability from synthetic to experimental data sets was resolved by incorporating physics-based descriptors into the synthetic training data, without experimental input. Our protocol’s simplicity underscores its proof-of-concept nature, paving the way for advanced approaches. Validation with experimental data, such as graphene-coated AFM tips on 2D materials, highlights the promise of this ML approach for stick–slip nanofriction studies.

Hydrogenated diamond‐like carbon (H‐DLC) coatings are extensively employed in high‐performance tribological applications, yet their frictional behavior in humid environments remains a critical limitation. A scalable and industrially viable strategy based on graphitic nanodiamonds (Gr‐NDs) is here proposed, enabling humidity‐resistant ultralow friction (coefficient of friction <0.1) without requiring controlled atmospheres or complex surface treatments. This approach is distinct from many recent methods involving the deposition of nanoparticles and 2D materials, which often fail under humid conditions. Gr‐NDs leverage their intrinsic core–shell nanostructure to promote the in operando formation of a peculiar graphitic transfer layer (TL), directly enhancing interfacial lubrication. High‐resolution transmission electron microscopy and Raman spectroscopy confirm that these nanostructures are effectively retained within the TL, ensuring superior wear resistance and friction reduction. Furthermore, spectroscopic analysis reveals that moderate tribochemical oxidation stabilizes the TL, extending its durability under realistic operating conditions. This work establishes Gr‐NDs as a disruptive functional additive for H‐DLC coatings, offering a robust, scalable, and environmentally friendly solution for next‐generation tribological systems in demanding mechanical applications.

Deepening the understanding of interface-type resistive switching (RS) in metal/oxide heterojunctions is a key step for the development of high-performance memristors and Schottky rectifiers. In this study, we address the role of metallization technique by fabricating prototypical metal/Nb-doped SrTiO₃ (M/NSTO) Schottky contacts via pulsed laser deposition (PLD). Ultrathin Pt and Au electrodes are deposited by PLD onto single-crystal (001)-terminated NSTO substrates and interfacial transport is characterized by conventional macroscale methods and nanoscale Ballistic Electron Emission Microscopy. We show that PLD metallization gives Schottky contacts with highly reversible current-voltage characteristics under cyclic polarization. Room-temperature (RT) transport is governed by thermionic emission with Schottky barrier height ϕ B ( Pt / NSTO ) ∼ 0.71 − 0.75 eV , ϕ B ( Au / NSTO ) ∼ 0.70 − 0.83 eV and ideality factors as small as n ( Pt / NSTO ) ∼ 1.1 and n ( Au / NSTO ) ∼ 1.6 . RS remains almost completely suppressed upon imposing broad variations of the Nb doping and of the external stimuli (polarization bias, working temperature, ambient air exposure). At the nanoscale, we find that both systems display high spatial homogeneity of ϕ B ( ⩽ 50 meV ), which is only partially affected by the NSTO mixed termination ( | ϕ B ( M / SrO ) − ϕ B ( M / Ti O 2 ) | < 35 meV ). Experimental evidences and theoretical arguments—based on a metal-insulator-semiconductor description of the M/NSTO—indicate that the PLD metallization mitigates interfacial layer effects responsible for RS. This occurs thanks to the reduction of the interfacial layer thickness and to the creation of an effective barrier against the permeation of ambient gas species affecting charge trapping and redox reactions. This description allows to rationalize interfacial aging effects, observed upon several-months-exposure to ambient air, in terms of a slow interfacial re-oxidation. Our work contributes to the fundamental understanding of interface-type RS and demonstrates that RT PLD offers a viable platform for the realization of robust, RS-free NSTO-based Schottky contacts.

Solution-processed few-layers graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically-relevant interfaces. Vanishing friction states, i.e. superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes; there is however no clear understanding if superlubricity might persist for larger and morphologically-disordered contacts, as those typically obtained by graphene wet transfer from a liquid dispersion. In this study we address the friction performance of solution-processed graphene flakes by means of colloidal probe Atomic Force Microscopy. We use an additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely graphite and the transition metal dichalcogenides (TMDs) MoS2 and WS2. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally-activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates and sliding regimes.

Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS₂ and WS₂. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.

Colloidal probe atomic force microscopy (AFM) allows us to explore sliding friction phenomena in graphite contacts of nominal lateral size up to hundreds of nanometers. It is known that contact formation involves tribo-induced material transfer of graphite flakes from the graphitic substrate to the colloidal probe. In this context, sliding states with nearly vanishing friction, i.e., superlubricity, may set in. A comprehensive investigation of the transfer layer properties is mandatory to ascertain the origin of superlubricity. Here we explore the friction response of micrometric beads, of different size and pristine surface roughness, sliding on graphite under ambient conditions. We show that such tribosystems undergo a robust transition toward a low-adhesion, low-friction state dominated by mechanical interactions at one dominant tribo-induced nanocontact. Friction force spectroscopy reveals that the nanocontact can be superlubric or dissipative, in fact undergoing a load-driven transition from dissipative stick-slip to continuous superlubric sliding. This behavior is excellently described by the thermally activated, single-asperity Prandtl-Tomlinson model. Our results indicate that upon formation of the transfer layer, friction depends on the energy landscape experienced by the topographically highest tribo-induced nanoasperity. We consistently find larger dissipation when the tribo-induced nanoasperity is slid against surfaces with higher atomic corrugation than graphite, like MoS₂ and WS₂, in prototypical van der Waals layered heterojunctions.