Modificare la carica, l'adesione e la resistenza all'attrito delle interfacce attraverso alterazioni chimiche di superficie

L’attrito è un fenomeno onnipresente in natura, con un enorme impatto tecnologico ed economico. La tribologia, la scienza dell’attrito, della lubrificazione e dell’usura, si è stabilita negli anni come un autonomo e interdisciplinare campo scientifico. Tuttavia, nonostante i numerosi avanzamenti dell’ultimo secolo, i meccanismi fondamentali che generano l’attrito alla scala microscopica sono ancora poco chiari. Ciò è dovuto principalmente alla complessità dei fenomeni fisici e chimici che avvengono all’interfaccia sepolta durante lo sfregamento, che per di più sono estremamente difficili da monitorare sperimentalmente. Le simulazioni atomistiche possono giocare un ruolo chiave per compiere quest’osservazione in tempo reale con dettaglio alla scala atomica. Le simulazioni ab initio, in particolare, permettono di descrivere accuratamente i processi chimici e fisici in condizioni tribologiche, poiché prendono in considerazione i gradi libertà elettronici. In questo progetto di tesi abbiamo usato metodi ab initio per studiare la relazione tra le modificazioni chimiche di superficie e il comportamento tribologico delle interfacce. Partendo da alcuni casi di studio, abbiamo potuto ricavare considerazioni di carattere fondamentale sui seguenti fenomeni collegati all’attrito: 1) la modificazione dell’adesione, della carica elettronica e delle proprietà di superficie indotta da monostrati adsorbiti; 2) la conversione tribocatalitica di molecole di idrocarburi in nano-ricoprimenti di carbonio; 3) il ruolo degli stati elettronici di superficie nella triboelettrificazione. Riguardo il primo problema, abbiamo applicato la teoria del funzionale densità (DFT) per comprendere come l’adsorbimento di fosforo elementare modifichi le proprietà di superficie del ferro. Attraverso un confronto con altri elementi (N, O, S) abbiamo trovato le caratteristiche che un elemento dovrebbe possedere per ridurre l’attrito adesivo. Abbiamo usato questa comprensione per predire che intercalare selenio in interfacce ferro/ferro può portare a risultati migliori degli elementi più usati, nocivi per l’ambiente, come S e P. Suggeriamo dunque un elemento alternativo per gli additivi lubrificanti. Nella seconda parte della tesi abbiamo studiato l’estrazione tribochimica di grafene dalla dissociazione di metano, con un approccio teorico-sperimentale. L’esperimento d’attrito acciaio su acciaio, insieme ad analisi spettroscopiche, ha mostrato che un composto Ni-VN può promuovere la dissociazione delle molecole di CH4. Le reazioni sono accelerate dalle condizioni tribologiche, portando alla formazione di un nano-ricoprimento di C, composto da grafene, nano-onion, e carbonio disordinato. Usando dinamica molecolare ab initio abbiamo mostrato che il Ni è fondamentale per la dissociazione del CH4, che aumentando la concentrazione si forma una rete di catene di C interconnesse, e che le forze normali e ti taglio inducono una re-ibridizzazione dei legami C-C e la formazione di un fiocco di grafene. Nella terza parte abbiamo studiato con la DFT la relazione tra le proprietà elettroniche di isolanti solidi e polimerici e la loro elettrificazione per contatto. In particolare, abbiamo considerato le proprietà elettroniche della fase ad alta pressione del PTFE, trovando che la configurazione spaziale dei suoi stati elettronici può spiegare la sua capacità di attrarre carica negativa. Infine, abbiamo studiato l’elettrificazione per contatto in un esperimento di attrito tra diamante e silice, mediato dall’emissione di carica. Abbiamo trovato che il trasferimento di carica dalla silice al diamante è associato alla rottura di legami nella silice, e al trasferimento di materiale trasferito dalla silice al diamante, in accordo con gli esperimenti.Friction is a ubiquitous phenomenon in nature, with huge technological and economic impacts. Tribology, the science of friction, lubrication and wear, has become a well-established, autonomous, interdisciplinary scientific field. However, despite the numerous advances made in the last century, many fundamental mechanisms governing friction at the microscopic scale are still unclear. The reason resides in the complexity of the chemical and physical phenomena occurring at the sliding buried interface, which are very difficult to monitor by experiments. Atomistic simulations can play a key role for observing the sliding interface in real time with atomic detail. Ab initio simulations in particular, allow to accurately describe chemical and physical processes in tribological condition because they take into account the electronic degrees of freedom. In this thesis project we used ab initio methods to study the relation between the surface chemical modifications and the tribological behavior of interfaces. Starting from some case studies we derive fundamental insights into the following friction-related phenomena: 1) modification of surface properties, adhesion and electronic charge, induced by adsorbed layers; 2) tribocatalytic conversion of hydrocarbon molecules into carbon nanocoatings; 3) role of surface electronic states in triboelectrification. Concerning the first issue, we applied Density Functional Theory (DFT) to understand how the elemental adsorption of phosphorus modifies iron surface properties. Through a comparison with other elements (N, O, S), we find the features an element should possess to reduce adhesive friction. We then exploit this understanding to predict that selenium intercalation in iron/iron interfaces can outperform the most common elements used in lubricant additives, i.e., S and P, which are harmful for the environment. This study thus suggests a possible alternative element to be used for effective lubricant additives. In the second part of the thesis we studied the tribochemical extraction of graphene from methane dissociation in a combined experimental-theoretical approach. The steel on steel rubbing experiments together with spectroscopic analyses showed that a Ni-VN nanocomposite can prompt a catalytic dissociation of CH4 molecules. The reactions were enhanced by the tribological conditions, leading to the formation of a carbon nano-coating mainly composed by graphene, nano-onions and disordered carbon. By means of ab initio Molecular Dynamics we showed that Ni plays a key role in the catalytic dissociation of CH4. We showed that once the carbon is adsorbed on the Ni surface, and by increasing its concentration an amorphous film of interconnected carbon chains is formed. The shear and normal stresses applied induce the C-C bond rehybridization and the formation of graphene flakes. In the third part of the thesis we studied by means of DFT the relation between the electronic properties of polymeric and solid insulators and their contact electrification. In particular, we considered the electronic properties of the high-pressure phase of Polytetrafluoroethylene (PTFE) and found that PTFE electronic states spatial configuration can explain its capability to attract negative charge. Finally, we studied the contact electrification occurring in a diamond-silica rubbing experiment, mediated by charge emissions. Here we found that the charge transfer from silica to diamond is associated with bond ruptures occurring in silica, and to material transfer from the silica to the diamond surface, in agreement with the experimental observation

A Combined Experimental and Theoretical Study on the Mechanisms Behind Tribocharging Phenomenon and the Influence of Triboemission

This work describes recent research into the mechanisms behind tribocharging and the influence of triboemission. The term tribocharging is a type of contact-induced electrification and refers to the transfer of charge between rubbing components. The term triboemission, on the other hand, refers to emission of electrons, ions and photons generated when surfaces are rubbed together. The understanding of tribocharging is of wide interest for several industrial applications and in particular the combination of tribocharging and triboemission may be important in lubricated contacts in the formation of boundary lubricant films. We report the use of a unique vacuum measurement system that enables to measure surface charge variations while simultaneously recording triboemission events during the sliding of a diamond tip on silica specimens. Results show for the first time that tribocharging and triboemission behavior are linked and depend on the surface wear. The contribution of contact-induced electrification to the charging of the surface is then described by means of density functional theory (DFT). Results give insight into the transfer of charge from the SiO2 amorphous surface (silica) to the C(111) surface (diamond ) and into the variation of charging during simulated sliding contact

Ideal adhesive and shear strengths of solid interfaces: A high throughput ab initio approach

We release a computational protocol to calculate two intrinsic tribological properties of solid interfaces from first principles, namely the adhesion energy, γ and the ideal interfacial shear strength, τ. These properties, which correspond to the energy required to separate two surfaces from contact and to the static friction force per unit area, respectively, are ruled by physical/chemical interactions between the surfaces in contact. First principles calculations based on Density Functional Theory (DFT) can accurately describe surface-surface interactions, offering the possibility to characterize the adhesive and shear strengths of materials in silico. We implemented the computational protocol as an AiiDA workflow (WF) that allows to obtain the γ and τ figures of merits in a high throughput manner. The software we produced uses a simple input file and most computational parameters determined automatically. To our best knowledge, this is the first time a high throughput approach has been used in tribology

First-principles insights into the structural and electronic properties of polytetrafluoroethylene in its high-pressure phase (form III)

Polytetrafluoroethylene (PTFE), commercially known as Teflon, is one the most effective insulating polymers for a wide range of applications because of its peculiar electronic, mechanical, and thermal properties. Several studies have attempted to elucidate the structural and electronic properties of PTFE; however, some important aspects of its structural and electronic characteristics are still under debate. To shed light on these fundamental features, we have employed a first-principles approach to optimize the two coexisting PTFE structures (monoclinic and orthorhombic) at high pressure by using the characteristic zigzag planar chain configuration. Our electronic analysis of the optimized structures shows charge transfer from carbons to fluorines, supporting the PTFE electronegative character. In addition, band structure calculations show that the band gap is estimated to be around 5 eV, which correlates with previous studies. Moreover, the analysis of the valence and conduction states reveals an intrachain and an interchain character of the charge distribution, suggesting additional insights into the PTFE electronic properties