Tribology and Wear

FUNDAMENTALS OF SURFACE INTERACTION

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS1. Surface Roughness

• Surface Roughness:

• Departure of the surface shape from ideal form

• The ratio of the true overall area to the nominal projected area

• Slop of a profile taken along a prescribed line

• The distance between high points and low points on the surface

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS• Analysis of roughness

• Surface profile

• Mean line: Equal area of the profile lie above and below it.

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS• Quantification of the surface profile OR Measurement of roughness

• Average Roughness or Ra: Average of a set of individual measurements of a surfaces peaks andvalleys

• y: Height of the profile above or below the mean line at a distance x from the origin

• L:The overall length of the profile

Ra=(|y1|+|y2|+…|yn|)/n)

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS

• RMS Roughness or Rq: Root mean square average of the profile height deviations from themean line.

• Ra and Rq are similar for many surfaces

• Rq is more sensitive to variations happen in the profile

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS

Table from ISO 1302:1992

Typical roughness values for engineering surfaces

Ra (µm)

Milling

Drawing /Extrusion

Grinding

1-6

1-3

0.1-2

Polishing 0.1-0.4

0.05-0.4Lapping

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS

• R Or maximum valley depthv

• R Or minimum valley depthp

• R , R Or maximum height of the profile = maximum valley depth - minimum valley deptht y

• Ra and Rq No information on the shapes and spacings of the surface irregularities

• R Or Skewnesssk

• R Or Kurtosisku

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS• How measure surface roughness?

a) Stylus Profilometer

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS

a) Stylus Profilometer

• Pyramidal or conical diamond tip

• Minimum angel of 60°

• Tip radii of 1-2.5 µm

• Special tip: 0.1 µm

• Vertical resolution: 0.1 µm

• Horizontal resolution limited size of the tip and

some smoothing of the profile

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS

b) Optical Interferometer:

A non-contact optical technique for determination of 3D surface profile

• Vertical resolution: 0.1 nm

• Horizontal resolution: ~ micron scale

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS

c) Atomic Force Microscope (AFM)

• Similar to stylus profilometer with higher resolution on atomic and nano-meter scales

• Both contact and non-contact modes

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS• Surface roughness is of importance to surface interactions; it affects the contact stress,

adhesion and friction.

• For example, when two extremely smooth surface are in contact, friction could be veryhigh due to the atmospheric pressure.

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS2. Chemical Properties of the Surface

• Surface (Oxide) Film: Many materials can react with environment and form surface filmsthat may strongly influence their tribological properties.

• Example 1: Cast iron contains graphite,which can be spread across surface duringmachining to form a solid lubricant film. Graphite has a low friction coefficient

• Example 2: In TiNi-%3 Fe: Fe rusts which decreases COF on the surface.

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS• Example 3:Al alloys and stainless steel form passive films over their surfaces which is

beneficial to their resistance to corrosive wear.

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS2. Chemical Properties of the Surface

• Adsorbed Films

• Example 1: Humidity > Water film (~ 10^-7 mm thick):Affects friction and wear

• Example 2: Greasy or oily film from the atmosphere: Change friction and wear

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FACTORSTHAT INFLUENCE SURFACEINTERACTIONS3. Metallurgical Properties of the Surface

• Properties of engineering finished surface are usually quite different from those ofcorresponding bulk materials due to the existence of surface defects and residual stress

• This could make tribological properties very different from that expected.

FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Surface machining: Surface machining may create cracks on thesurface

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• Surface hardness: Hardness of the surface could be higheras

30-50% or even few times than the bulk metal due to

greater number of dislocations or smaller grains

Hardness distribution in the cutting zone for 3115 steel. Some regions in

the built-up edge is as much as three times higher than the bulk metal.

FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Internal stress in the surface

• Texture:

• Distribution of crystallographic orientation

• Preferred orientation or random?

• Strong effect of texture on properties

•Texturing during grinding and polishing leading

to changes in wear and friction behavior

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Phase transformation:

• Phase transformations may result in very different properties

• Deformation induced martensite in Fe-Ni alloy:

• Increased hardness

• Residual compressive stress

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

4. Compatibility in Materials Pairs

• The interaction between two rubbing surfaces it strongly influenced by their

compatibility.

• Some materials like each other and this affinity results in a great amount of adhesion

between their surfaces.

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A

B

A

B

γA

γB

γAB

γA:Surface Energy ofA

γB: Surface Energy ofB

γAB:Interface Energy ofAB

FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• What is the energy we need to separate A from Bnow?

WAB = γA + γB - γAB

• If both A and B are the same material(A=B):

WAB = 2 γA

• If A and B are different (dissimilarmaterials):

WAB = γA + γB - γAB < γA +γB

• Usually γAB = ½ to ¼ (γA + γB )

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Materials like each other more (more compatible): WAB is larger

WAB = Cm (γA + γB)

• Cm: Compatibility parameter. Varies between 0 to1

• Identical metals: Cm = 1

• Compatible metals: Cm 0.5

• Partially compatible metals: Cm ~ 0.3

• Partially incompatible metals: Cm ~ 0.2

• Incompatible metals: Cm 0.1

•The Surface Forces Apparatus (SFA) can measure inter-molecular

potentials by direct force measurement.

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Compatibility between in a pair of material:Adhesion force between their surfaces

• Adhesion force can be measured using AFM (Atomic ForceMicroscope)

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Compatibility could be estimated using binary phase diagrams

• More solubility More compatible More adhesion force

• Need more friction? Choose compatible materials.

• Need less friction? Choose incompatible materials.

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• Deducing compatibility from phase diagrams

could be complex.To make it easy,compatibility

charts have been developed.

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

5. Effects of Crystal Structure

• Crystal structure strongly affects mechanical properties of materials.

• Crystal structure also influences adhesion and thus surface interactions.

• Greater number of slip systems More adhesion

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

5. Effects of Crystal Structure

• Crystal structure strongly affects mechanical properties of materials.

• Crystal structure also influences adhesion and thus surface interactions.

• Greater number of slip systems More adhesion

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

• In a HCP structure:

• A larger air gap between two surfaces in contact Less adhesion Less friction

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

6. Effects of Texture or crystal orientation

• An example:

• For FCC crystals, (111) plane has a lower surface energy than (100) plane.Thus,(111) is more

stable than (100), meaning that it is reluctant to react with a counter-face, compared to (100)

plane.

• 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐸𝑛𝑒𝑟𝑔𝑦 ∝ 𝐵𝑟𝑜𝑘𝑒𝑛 𝐵𝑜𝑛𝑑 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑟𝑜𝑘𝑒𝑛 𝑏𝑜𝑛𝑑𝑠

𝑈𝑛𝑖𝑡 𝑎𝑟𝑒𝑎

• BBD (111) < BBD (100)

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7. Mechanical Properties of Bulk Materials

• Mechanical properties of a material are of particular importance to its tribological

properties

• Surface mechanical properties are closely related to mechanical properties of the bulk

material.

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FACTORS THAT INFLUENCE SURFACE

INTERACTIONS

WHAT WE LEARNT?

Many factors (internal/intrinsic and external) influences surface interactions.

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MECHANICAL ASPECTS OF SURFACE INTERACTION

• Although there are many factors influencing wear, the eventual removal of material from

surface is mainly caused by the mechanical force.

• It is therefore of importance to analyze the mechanical interaction between two moving

surfaces in contact and understand how surface failure occurs.

• The contact area is dependent on the applied force and mechanical properties of the

materials.

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MECHANICAL ASPECTS OF SURFACE INTERACTION

Mechanisms for Surface Failure under Contact Stress

• Surface failure experiences two main steps:nucleation and propagation of cracks or voids

under contact stress.

• Crack nucleation at inclusions

•

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MECHANICAL ASPECTS OF SURFACE INTERACTION

• Crack nucleation at grain boundary (GB)

• Because of lattice irregularity and impurity segregation, grain boundary is a favorablesite

for crack nucleation when dislocation pile up at GB under stress.The resultant increase

in local stress may cause GB cracking.

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MECHANICAL ASPECTS OF SURFACE INTERACTION

• Cracking at lattice defects

• During material processing, defects such as voids and micro-cracks could be generated,

e.g., micro-cracks caused by thermal stress during casting and void/pores in sintered

materials.At the defects, cracking proceeds easily because of the stressconcentration

in the vicinity of the defects.

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MECHANICAL ASPECTS OF SURFACE INTERACTION

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MECHANICAL ASPECTS OF SURFACE INTERACTION

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• At the tip of a microcrack,stress is considerably higher, and the stress concentration

makes the material easier to fail under external force.

• Wear resistance can be improved by reducing lattice defects, GB segregation of impurity,

and enhancing interfacial bonding strength.

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MECHANICALASPECTS OF SURFACE

INTERACTION

• Crack Propagation

• The presence of microscopic flaws or cracks makes the material to have a significantly

lower fracture strength.

• An applied stress may be amplified or concentrated at the tip of a flaw.

• The maximum stress occurs at the crack tip

• : The nominal applied tensile stress

• : The radius of curvature of the crack tip

• : The length of a surface crack, or half of the length of an internal crack

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MECHANICALASPECTS OF SURFACE

INTERACTION

• The critical stress required for crack propagation

• E modulus of elasticity

• specific surface energy

• plastic work required for crack extension

• c one-half the length of an internal crack

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