Tribology and Wear...Tribology and Wear FUNDAMENTALS OF SURFACE INTERACTION 2 FACTORSTHAT INFLUENCE...
Transcript of Tribology and Wear...Tribology and Wear FUNDAMENTALS OF SURFACE INTERACTION 2 FACTORSTHAT INFLUENCE...
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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• 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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• 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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• 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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