MEEN 454

What are the causes of static friction?

*Due to high contact pressure over asperity tips, the atoms establish stronger chemical bonds or micro welding *For metallic materials, deformation due to creep may also occur *Capillary forces due to water accumulating from environment *Asperity interlocking (Coulomb Friction)

Why should we care about roughness and other properties of a surface?

*Helps in understanding the mechanisms involved so friction and wear can be effectively reduced *They dictate the type and degree of tribological interactions that take place *Determines real surface area, load distribution, level of friction, etc

What is the Bowden and Tabor Model?

*High pressures at contact spots cause local welding. *Later argued that welding does not have to occur but adhesion is sufficient to account for the friction of metals and ceramics

Disadvantages to AFM

*Incapable of low magnification (wide range) measurements *Sample with significant asperity or roughness cannot be measured *The tip and the sample can be damaged during detection *takes a long time

Atomic Force Microscopy (AFM)

*Key component is a cantilever where an extremely sharp tip is attached *A 3D piezo electric scanner and a laser source in combination with a position sensitive optical detector work together to produce a very precise measure *AFM can work in contact, non contact, and tapping modes

Why is black ice so slippery?

*Layer of liquid on top *smoother surface: water molecules can move around and fill in any surface irregularities *Frictional heating: continuous melting or generation of liquid water

What is friction?

*Resistance to sliding or relative motion *Rely on friction for safety or lack of it for mobility *a source of lost energy *Vibration, sound, static electricity, light, are also directly or indirectly related to friction *coefficient is defined as the ration of friction force over normal force *it is so common yet understood very little

How do we measure or assess surface roughness?

*Stylus profilometry *Atomic force microscopy (AFM) (by far the best in terms of precision/resolution) *optical (light/laser interferometry) measurement profilometry

What does the area of contact depend on?

*Surface texture *material properties *sliding distance *interfacial loading conditions

Assumptions for elastic contact between a long deformable cylinder and a flat surface

*The surfaces are continuous, smooth, and non conforming *Strains are small *Each solid body can be considered as an elastic half space in the proximity of the contact region *The contact is frictionless

What types of surface characterizations are of great importance for a mechanical engineering application?

*Topographical characterization: surface roughness properties favorable for friction and wear performance *Mechanical Characterization: mechanical properties are good for load carrying capacity *Chemical Characterization: Surface chemical composition for protective boundary film formation

Yield criterion for ductile and sometimes brittle materials

*Tresca Maximum Shear stress criterion *Von Mises Shear stress criterion

What are the most prevalent wear mechanisms?

*abrasion *adhesion *fretting *delamination *ploughing *oxidation *corrosion

What is the real area of contact proportional to for cylinders?

*contact load (W) *mechanical properties (E) *geometrical specifications of the cylinders (R)

What depends on surface roughness?

*fatigue crack formation *adhesion of point, coatings, etc. *thermal and electrical conductivity *biocompatibility *light scattering vs reflection *resistance to fluid flow

What is the significance/importance of surface chemistry in tribology?

*reactions with the lubricant and/or additives take place on the contact spots *Chemical nature of these reaction products determines the extent of friction and wear of such lubricated contacts *Reactions produce a protective film on contact spots and is referred to as a boundary film

Optical Profilometer Based

*use a light source instead of a tip or stylus *based on splitting of a beam from a light source into two by a beam splitter *White light interferometry is most commonly used in 3D imaging of surface roughness

Disadvantages of White Light Interferometry

*use is limited on certain objects; very rough and low reflective or transparent surfaces *cannot measure spiky or bumpy parts *sensitive to vibrations

Stress distribution of a static system

According to van mises criterion, maximum contact pressure develops beneath the surface under static or in a friction free contact

Bearing Area Curve (BAC)

Also known as Abbott Firestone Curve is generated to estimate the real contact area of a surface at each height

What consumes energy?

Anything that rolls, rotates, slides, or reciprocate may trigger friction and wear and thus consumes energy

What is Ra or CLA (center line average) ?

Arithmetic average of the absolute values of the profile heights from the mean line through the profile (area under the curves from center line) *CLA is the rectified average (sets peaks from 0 to positive)

Where do two surfaces have real or true contact?

At the high points (protrusions) or asperities

Rpm

Average Maximum Profile Peak Height. Average of the successive values of Rp calculated over the evaluation length

Rz (DIN)

Average peak to valley height: Average of single Rt values over five adjoining sampling lengths

Are roughness and waviness absolute values or some how arbitrary? Why?

By definition the roughness and waviness are defined by specific wavelengths. Roughness has short wavelengths and waviness has larger wavelengths. The wavelengths are to some extent arbitrary and hence their values are as well.

How does surface roughness quantification help us with contact mechanics?

Friction and wear occur at the surface and the first points to engage in this process are topmost asperities. Surface roughness describes the distribution of surface features such as asperities

What is friction caused by?

Friction is caused by the physical/chemical interactions at the contact interface *physical/chemcial bonds may occur on contact spots *relative motion requires breaking of such bonds *intensity or strength of bonding determines the level of friction

How does contact area and location of maximum shear stress change if we change the contact between two cylinders from two cylinders rolling onto each other to a cylinder placed inside the other cylinder?

Increase in contact area radius which means higher contact area and lower depth for max shear stress

Where is energy used?

Industrial Sector Transportation Sector (largest) Residential Sector commercial Sector (smallest)

Rku

Kurtosis: measure of the peakedness of the profile shape when compared to an ideal Gaussian surface. It is a positive value

Classification of Solid Lubricants

Lamellar Solids (Graphite) Soft Metals (Ag) Mixed Oxides (CuO) Single Oxides (B2O3)

What is waviness?

Large wavelength irregularities of the profile that represent periodic undulation of the surface profile *This type of irregularity is intermediate in wavelength and periodicity

What of the frictional laws is not as universal?

Law 3: the sliding velocity for some dry contacts and especially for lubricated contacts can have a significant effect on the resistance to sliding or motion

Valleys

Local minima on the surface profile

What are some emerging technologies for friction reduction in passenger cars?

Low viscosity fluids Ionic Liquids Low friction tire design

Rv

Maximum Profile Valley Depth is the distance between the deepest valley of the profile and the mean line within the evaluation length

Rp

Maximum Profile peak Height. Distance between the highest point of the profile and the mean line

Are Rq and Ra sufficient parameters to describe a surface?

No. Significant amount of information such as slopes, shapes and size of asperities or their frequency of occurrence is lost when a complex profile is expressed in terms of amplitude parameters.

Rt

Peak to valley height: separation of highest peak and lowest valley

Asperity

Peaks or local maxima on the surface profile

Ploughing Mechanism

Ploughing of hard asperities through a softer surface (ploughing friction). The harder materials asperities plow through the softer material and generate plastic deformation.

What is stick slip phenomenon?

Process by which static and dynamic friction oscillate

What is the advantage of statistical analysis of surface profile?

Real area of contact is a key parameter in tribological analysis of any contact

What is wear?

Removal of material from the surface as a result of motion

What formation of cracks are produced in brittle materials with radial stress?

Ring cracks: In brittle materials, such as soda lime glass, this stress is responsible for the ring cracks that emerge under contact

What is Rq (sigma)

Root mean square of the profile height measured from the mean line

Rsk

Skewness: measure of the symmetry of the profile shape about the mean line *can be zero, negative, or positive. It is a dimensionless parameter

What is Roughness?

Small scale irregularities of the surface *it has the shortest wavelength and highest periodicity

What is a solid lubricant?

Solid material with inherent/intrinsic lubricating properties. Firmly bonded to the surface to reduce friction and wear by shearing easily during sliding. Can be naturally occurring or man made

What are the two types of friction?

Static friction: most often higher than dynamic friction. The initial tangential resistance force that needs to be overcome before the motion starts Dynamic Friction: Is the tangential force that needs to be applied to sustain the motion of a sliding body. Dry friction or lubricated/fluid friction

What is the real area of contact? (Ar)

Sum of all the contact spots (asperities) constitutes the real area of contact. Only a few percentage of the nominal or apparent contact area (Aa)

What is surface roughness?

Surface Roughness is a vertical deviation of peaks or valleys from a mean horizontal line

What is the purpose of quantifying surface topography/roughness?

Surface topography of the contacting bodies affects their tribological interactions.

Rz (ISO)

Ten point height: Separation of average of five highest peaks and five lowest valleys within single sampling length

What is error form?

The deviation between the surface profile of the produced part and the intended ideal shape * This type of irregularity has the largest wavelength and lowest periodicity

How do solid lubricants work?

The layers of the crystal structures are far apart and the forces that bond them are weak. When sliding, these layers transfer and bond to the sliding surface with allows them to slide over one another with relative ease

T/F Combination of skewness and kurtosis could be used to analyze special surfaces

True

T/F Friction can lead to high wear

True

T/F Friction is a system's property not a material's property?

True

T/F Higher values of (Y/E) ratio is desirable for contacts that are required to carry high loads without yielding

True

T/F Surface roughness can significantly impact friction, wear, lubrication, sealing, vibration and other mechanical functions

True

T/F Wear can occur due to physical and/or chemical interactions at the interface

True

T/F creep occurs more readily at elevated temperatures, so the friction may also be higher at higher temperatures under certain cases

True

T/F: most often the softer side tears out and the friction force would depend on the bulk shear strength of that material

True

T/F Topography (and roughness) may have a strong influence on materials performance

True: also influence friction and wear as well as retention of lubricant on contact surface

T/F Both 2D and 3D measurements can be misleading?

True: using of multiple techniques to visualize a surface is the best practice

Stress distribution in a sliding case

When in sliding, the location of maximum contact pressure moves towards the surface (if so high, can create surface cracks)

Where do we see/find Tribology?

Wherever there is an interacting surface in relative motion

What is Tribology?

means 'rubbing or to rub' Scientific Definition: The science and technology of interacting surfaces in relative motion and of the practices related thereto

Friction Coefficient

ration of friction force to normal force

How do you define 'tribological surface/contact'?

A tribological system (contact) occurs on asperities which are the contacting bodies surrounded by the environment (air, liquid, lubricant)

What is a non conforming surface?

Dissimilar profiles are said to be non conforming: ball bearings and roller bearings

Hysteresis Mechanism of Friction

During relative motion, the viscoelastic material deforms under pressure but as the pressure is lifted they recover. In metals it is very small but in rolling bearings it is very important.

What happens during the polishing of metals, alloys, or glasses?

A structurally changed surface layer forms referred to as the beilby layer

What are the three laws of friction?

1. Friction force is proportional to the normal load 2. Friction force is independent of the apparent are of contact 3. Friction force is independent of the sliding velocity **under lubrication (or wet) contact or rolling contact, these laws may not apply

How can you tell if skewness is zero, positive, or negative?

Check the symmetry of the profile around the mean line. If a profile is symmetric about the mean line, then the Rsk is zero, if height of peaks are relatively higher than depth of valleys ,then it would be positive. If deep valleys exist then it would be negative.

What are some of the smoothest surfaces?

Cleaved mica, natural diamond, graphene

What is a conforming surface?

Conforming surface if the surfaces of the two bodies 'fit' exactly or even closely together without deformation. Ex: journal and flat slider bearings

What can be considered the first model on friction mechanism?

Coulomb Model: it assumes that during the rise phase of a surface a certain amount of work is required, which is recovered during the descend phase. Fails to account for the energy dissipation that occurs due to frictional heat

What is controlled roughness?

Creating a certain roughness using honing or laser surface texturing) common in piton bore or liners in engines

Quantification of Surface Topography

Error of form Waviness roughness Surface profile = error of form + waviness + rougness

T/F As load increases, less contact spots come into contact and the contact size of existing spots decreases

False

T/F Friction coefficient is independent on the operating or experimental conditions?

False: Friction is dependent on... *normal load *sliding velocity *surface finish *sliding materials *operating temperature *humidity

T/F Rq is more applicable compared to Ra

False: Ra is more applicable compared to Rq because Ra is an averaged value but Rq highlights the outlier points further away from the mean line

T/F The radius of a tip on a contact stylus does not change the accuracy of roughness measurements

False: Sharper tips give better readings but are vulnerable to chipping or breaking

T/F Silver is not a preferred solid lubricant

False: Silver is a preferred solid lubricant for bearings due to their excellent thermal and electrical conductivity

T/F Tribological response of a system is independent on the interactions of the sliding surfaces

False: Tribological response of a system is strongly dependent on the interactions of the sliding surfaces. Such interactions are determined by surface roughness and near surface properties (mechanical hardness, chemical composition, and topographical profile)

T/F Room temperature cannot be considered elevated temperatures for certain metals and alloys

False: room temperature could be considered elevated temperatures

T/F Some engineering surfaces could be geometrically exact or precise

False: there are always machining or manufacturing flaws. There are always surface geometrical specifications such as roughness

Where are solid lubricants used?

Fasteners Bearings Gears Pins couplings

What percentage of global energy output is contributed to friction and wear?

Friction and wear consumes nearly 23% of global energy output

For a surface with a distinctive lay what should be the direction of surface scan? parallel or perpendicular?

for an isotropic surface, the direction is generally not important. For an anisotropic surface, perpendicular to lay of the surface

What provides a very smooth surface finish?

honing or laser surface texturing

Lay

is the principle direction of the predominant surface profile which is generally determined by production method