Friction

Friction

Resistance opposing the relative motion or intended motion between two contacting surfaces

Direction of friction force

Always acts opposite to the direction of motion or attempted motion

Role of friction in manufacturing

Strongly affects efficiency, wear, energy loss, and process reliability

Tribology

Study of friction, wear, and lubrication between interacting surfaces

Classical friction

Friction behavior where force is proportional to normal force and independent of contact area

Coefficient of friction (µ)

Ratio of friction force to normal force

Friction force equation

Ff = µFN

Normal force

Force acting perpendicular to the contact surface

Law of classical friction (1)

Friction force is proportional to the normal force

Law of classical friction (2)

Friction force is independent of the apparent contact area

Apparent area of contact

Visible area where two surfaces seem to touch

Real area of contact

Actual microscopic contact area at asperity tips

Asperities

Microscopic peaks and valleys on a surface

Relative size of contact areas

Real contact area is much smaller than apparent contact area

Location of real contact

Occurs mainly at the tops of asperities

Cause of friction force

Shearing of junctions at real contact areas

Asperity deformation

Plastic deformation occurs at asperity tips under load

Metallic bonding at asperities

Intimate bonding forms between contacting asperities under pressure

Effect of increasing normal force

Increases real area of contact via plastic deformation

Yield stress role in friction

Asperities deform until stress equals yield strength of softer material

Yield stress relation

σy = FN / AR

Why friction ∝ normal force

Increased normal force increases real contact area proportionally

Why friction is independent of apparent area

Larger apparent area lowers pressure, keeping real contact area roughly constant

Classical friction assumption

Friction depends on normal force, not apparent contact area

Seizure

Extreme friction condition where real contact area equals apparent contact area

Onset of seizure

Occurs at very high normal loads

Real vs apparent area in seizure

AR ≈ AA

Friction behavior in seizure

Friction force is proportional to apparent contact area

Normal force effect in seizure

Friction becomes independent of normal force

Difference between classical friction and seizure

Classical: Ff ∝ FN, Seizure: Ff ∝ apparent area

Seizure in manufacturing

Common in machining due to extremely high pressures

Normal stress in cutting

Can reach values up to ~4 GPa at tool–chip interface

Tool–chip interface condition

Partial or full seizure occurs during cutting

Effect of seizure on friction law

Classical friction law no longer applies

Evidence of seizure in machining

Chip sticks to tool rake face with no visible gap

Sticking region

Zone of complete contact and seizure at tool–chip interface

Shear stress in sticking region

Constant and independent of normal stress

Sliding region

Zone beyond sticking region where partial contact occurs

Friction behavior in sliding region

Coefficient of friction remains constant

Contact ratio in sliding region

Real contact area is less than apparent contact area

Shear stress trend in sliding region

Decreases as normal stress decreases

Stress distribution in cutting

Highest at tool–chip interface, decreases along contact length

Key takeaway on friction

Classical friction works at low loads, seizure dominates at extreme pressures