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Michele SCARAGGI

TEMI DI RICERCA

Ball bearing, from WikipediaTribology (from the Greek tribos and logos, study of rubbing) has been defi ned as the science and technology of interacting surfaces in relative motion, and associated practices (Fitch, 1985).

 

Our activities are focussed to resolve critical friction, wear, and lubrication issues involved in mechanical transmissions and bearings (e.g. low friction interactions), sealings, biomedical implants, soft contacts, and alternative energy technologies. We develop:

  • Advanced models of the fundamental mechanisms responsible for friction and wear;
  • Mean field models for the evaluation of the effect of random/deterministic surface texturing on friction (e.g. for bearings and sealings);
  • Fast solvers for elasto-hydrodynamics, mixed lubrication and dry rough contacts;
  • Advanced models and design strategies for bio-inspired surfaces with ad-hoc frictional responce.

Apart from classical knowledge on friction and lubrication mechanisms and modelling (see before), our research activity is focussed on the investigation and development of surfaces whose physical and chemical properties near the confinement, once smartly organized, allow to generate effective contact characteristics (e.g. a reduced friction or leakage, or an increased friction!) independently from the macroscopic characteristics of the contact itself. We therefore believe that a green tribology paradox could be effectively solved by “removing” properties from the lubricant (less additives for greener lubricants adoption, as water) and “adding” those to the surface!

Some recent research outcomes and application can be found below.

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Current research activity:
Some past research outcomes:
 
 
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Rough contact mechanics: theory and numerics

Most surfaces of practical interest have surface roughness over a large range of length scales, typically from cm to nm. The way the interaction (mediated by the roughness) between real surfaces occurs, determines the appereance of a plenty of macroscopic phenomena, including friction, adhesion, contact thermal and electrical resistance, wear, leakage, tc.. Recent multigrid/FFT based approaches help to have a better understanding of such phenomena, as well as to validate and debate on the mean field theory of roughness contact mechanics. The current research is focussed to develop novel and optimized numerical appraoches for the fastest computation of rough contacts.
 

Contact areas for an anisotropic surface with 10 scales roughness.

Contact areas (black) for an anisotropic elastic surface in contact with a rigid substrate. With 10 scales roughness.
 
 

Deforming half space surface over rigid rough substrate

Deforming half space over rigid rough substrate
 
 
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Static and dynamic sealings: multiscale approaches for mixed lubrication

Nowadays bio-inspired research, together with the widely-spreading practice of surface engineering, is showing the many (mainly unexplored) op-portunities offered by the physical-chemical ordered modification of surfaces in order to tailor targeted macroscopic contact characteristics, such as adhesion and friction. Bio-inspired adhesive research is probably the best state of the art example of such research trend. However, investigating the combined effect of, let’s say, quantized roughness and fluid action has not equally attracted the scientific community attention, apart from few experimental investigations and basic theoretical investigations. This may be justified by the complexity of the numerical formulation of the problem, which is expected to not to present an analytical treatment. As a result, the combined effect of lubricant action and single-scale (contact splitter) roughness has not been practically investigated by tribologists’ community. Current research is then focused on novel numerical scheme of soft mixed lubrication, which can be adopted to perform such investigations.
 

Cavitation fingers (black areas) at a micro-structured dynamic seal interface.

Cavitation fingers (black areas) at a micro-structured dynamic seal interface.
 
 
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Hard elasto-hydrodynamics: numerics for transient high-pressure lubrication

Elasto-hydrodynamics is one of the most interesting topics of tribology science!
 

Classical h-EHL contact geometry and lubricant rheology.

Classical Hard-EHL contact geometry and lubricant rheology.
 
 
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Soft wet contacts, biological contacts: from experimental micro-EHL to mean field theory

Compliant contacts, most commonly known as soft contacts, are quite common in nature (e.g. cartilage lubrication, eye-eyelid contact) and technology (e.g. tires, rubber sealings, adhesives). It has long been stated that the friction and fluid leakage characteristics of wet soft contacts is strongly related, among others, to the local (asperity scaled) interactions occurring at the contact interface. In the case of randomly rough surfaces, the basic understanding on the role played by the asperity-asperity and fluid-asperity interactions, occurring over a (often) wide range of roughness length-scales, has been largely investigated and debated in the very recent scientific literature. Given the (usual) fractal nature of random roughness, a number of interesting phenomena have been highlighted, e.g. the viscous-hydroplaning, the viscous flattening, the fluid-induced roughness anisotropic deformation, the local and global fluid entrapment, and many others. The current research is focused on their quantitative evaluation, with particular emphasis on bio-mimetic solutions.
 

Roughness anisotropic viscous flattening.

Roughness anisotropic viscous flattening.
 
 

Initial roughness and roughness deformed by the fluid action.

Initial roughness and roughness deformed by the fluid action.
 
 

Frictional micro-EHL.

Frictional micro-EHL.
 
 
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Man made roughness and (micro-)texture hydrodynamics: bearings lubrication

The research is focussed to the investigation of near-optimum texturing for iso-viscous rigid applications (e.g. plain bearings).
 

Schematic of a tipical tapered bearing geometry.

Schematic of a tipical tapered bearing geometry.
 
 
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Role of lubricant rheology in hard-EHL squeeze

We analyze the influence of di fferent fluid rheologies on the high loaded normal approach of elastic balls, which is of utmost importance in gears, bearings and continuously variable transmissions. The analyzed lubricant rheologies are 1)Newtonian (linearly viscous), 2)Maxwell (linear viscous - linear elastic) and 3)Rabinowitsch (non-linear viscous, shear thinning) constitutive laws. For the Newtonian fluid, we show that the spatial pressure distribution is characterized by an annular (sharp) pressure peak, which first appears in the external region of the contact domain and after moves toward the center of the pin with rapidly decreasing speed. This high pressure fi eld determines the formation of a high viscosity oil dimple in the center of contact. The lifetime of this pressurized oil dimple, which corresponds to the time required by the lubricant to be expelled from the conjunction, actively determines the friction and wear characteristics at the interface. In the case of Maxwell rheology we show that the pressure field is exactly the same as in the Newtonian case but with a deep reduction in the annular pressure peak value, which explains the non-failure behavior of such contacts; thus we find that the Maxwell rheology enables a more realistic prediction of high loaded lubricated contacts (for lubricants not exhibiting limiting shear stress or shear thinning). The latter case is investigated with a Rabinowitsch constitutive law. We show that if the shear stress threshold, which characterizes the transition from the linear viscous to the non-linear viscous lubricant behavior, is sufficiently small the annular pressure peak may even disappear. In this case the squeeze process occurs faster (shorter lifetime), the fi lm thickness distribution is reduced and the lubricant may not be able to avoid direct asperity-asperity contact between the two approaching surfaces. The lubrication models is applied to the investigation of the pure squeeze motion at the pin-pulley interface in continuously variable transmissions (CVTs).
 

Newtonian and Maxwell lubricant pressure field. Comparison.

The oil pressure field for Newtonian and Maxwell fluid film. The oil viscoelasticity intervenes locally to smooth the annular pressure spike.
 
 
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The transition from boundary to hydrodynamic lubrication in soft contact

We consider the contact between elastically soft solids with randomly rough surfaces in sliding contact in a fluid, which is assumed to be Newtonian with constant (pressure-independent) viscosity. We discuss the nature of the transition from boundary lubrication at low sliding velocity, where direct solid-solid contact occurs, to hydrodynamic lubrication at high sliding velocity, where the solids are separated by a thin fluid film. We consider both hydrophilic and hydrophobic systems, and cylinder-on-flat and sphere-on-flat sliding configurations. We show that for elastically soft solids such as rubber, including cavitation or not result in nearly the same friction.
 

An asperity contact region observed at a given magnification

An asperity contact region observed at a given magnifi cation.
 
 
 

Fluid and asperity contact pressure surfaces.

Fluid and asperity contact pressure surfaces.
 
 
 

Friction coefficient for PDMS-PDMS interaction. The green line corresponds to the predicted Couette friction.

Friction coefficient for PDMS-PDMS interaction. The green line corresponds to the predicted Couette friction, while the other curves have been obtained by Bongaerts et al., 2007.
 
 
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Mixed lubrication theory in soft contacts: The case of lip sealings.

We consider the contact between a soft rough sealing lip and a smooth rigid rotating shaft. We model the asperity-asperity and asperity-fluid interactions with a deterministic or a statistical approach depending on length scale at which the contact region is observed. Indeed, the roughness at large length scales, which mainly determines the fluid flow at the interface, is deterministically included in the model while the roughness at short-wavelengths, which strongly contributes only to the friction, is included by means of a homogenization process. This contact scheme allows to correctly capture the shear-induced deformation of the roughness asperities occurring in soft mixed lubrication contacts.
 

A schematic of a typical lip seal construction.

A schematic of a typical lip seal construction.
 
 
 

Flux lines at the contact interface (red curves are). The velocity field is shown in the vector form (black arrows) and in module (white-blue color gradient, where blue color is used for the higher values).

Flux lines at the contact interface (red curves are). The velocity fi eld is shown in the vector form (black arrows) and in module (white-blue color gradient, where blue color is used for the higher values).
 
 
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The transition from hydrodynamic to mixed lubrication in high loaded squeeze contacts.

We analyze the high loaded strongly non-stationary squeeze process of an oil fi lm sandwiched between an elastic spherical ball and a rigid rough substrate. We show that the coupling between the elastic properties of the contacting solids, the oil rheology, the surface roughness and the applied load determines a wide range of lubrication conditions from fully elasto-hydrodynamic to mixed and even boundary lubrication. In particular we fi nd that increasing (decreasing) the surface roughness (the applied normal load) speeds up the squeeze process, anticipates and shrinks the time interval during which the transition to mixed lubricated conditions occurs. On the contrary, the initial separation between the approaching bodies only marginally aff ects the transition time. We also observe that, in mixed-lubricated conditions, the highest asperity-asperity contact pressure occurs in the annular region where the separation between solids takes its minimum value. One then conclude that surface damage and wear should nucleate in the outer region of the contact
 

A film of lubricant squeezed between a smooth elastic sphere and a rough rigid substrate.

A film of lubricant squeezed between a smooth elastic sphere and a rough rigid substrate.
 
 
 

The typical spatial distribution of fluid pressure, solid-solid contact pressure and interfacial separation for mixed lubrication squeeze contacts.

The typical spatial distribution of fluid pressure, solid-solid contact pressure and interfacial separation for mixed lubrication squeeze contacts. Observe that in the gray area across the minimum value of separation, where the solid-solid pressure takes its maximum value, the solid-solid contact spots may coalesce and obstruct the fluid passage. The oil then may not be squeezed out and remain entrapped between the two solids.
 
 
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Adhesive contact of rough surfaces.

We have employed a numerical procedure to analyze the adhesive contact between a soft elastic layer and a rough rigid substrate. The solution of the problem, which belongs to the class of the free boundary problems, is obtained by calculating the Green's function which links the pressure distribution to the normal displacements at the interface. The problem is then formulated in the form of a Fredholm integral equation of the first kind with a logarithmic kernel, and the boundaries of the contact area are calculated by requiring that the energy of the system is stationary. The methodology has been employed to study the adhesive contact between an elastic semi-infi nite solid and a randomly rough rigid profi le with a self-affine fractal geometry. We show that, even in presence of adhesion, the true contact area still linearly depends on the applied load. The numerical results are then critically compared with the prediction of an extended version of the Persson's contact mechanics theory, able to handle anisotropic surfaces, as 1D interfaces.
 

The logarithm of the probability as function of pressure and contact magnification.

The logarithm of the probability as function of pressure and contact magnification. Points are numerical predictions whereas dashed lines are Persson's results. We observe that the tail of the probability distribution at large values of pressure follows exactly a Gaussian distribution.
 
 
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