When is sticking (shear) friction tau = m k used instead of Coulomb friction?

At high pressure in hot working such as forging and hot rolling

At light loads in lubricated cold work

Only in machining of soft metals

Only with solid lubricants at room temperature

Crater wear on carbide cutting tools is mainly due to

Diffusion of atoms at high temperature

Abrasive ploughing by hard particles

Corrosive attack in humid media

Which is NOT a listed function of a lubricant in manufacturing?

Raise the friction to improve grip of the work

Carry away heat and wash away chips

Prevent welding and improve surface finish

Protect work and die from corrosion

How does the viscosity of a liquid lubricant vary with temperature?

Decreases roughly exponentially as temperature rises

Increases roughly exponentially as temperature rises

Practice · 6 marks

Tribology in manufacturing: friction, wear, lubrication

Short answer

Tribology studies friction, wear and lubrication. Friction may be sliding (Coulomb) or sticking; wear types include adhesive, abrasive, diffusion, fatigue and corrosive. Lubricants reduce friction/wear, cool and flush chips; liquid viscosity decreases with temperature, so hot working often uses solid lubricants or glass.

What is tribology? Explain the types of friction and wear found in metal-forming and machining, and the functions and types of lubricants used in manufacturing. Mention how viscosity of a liquid lubricant varies with temperature.

Tribology is the science of interacting surfaces in relative motion: it covers friction, wear and lubrication. In manufacturing it controls forming loads, tool life, surface finish and energy use.

Friction

Sliding (Coulomb) friction: $F = \mu N$ , valid at light loads (cold working with good lubrication, $\mu = 0.05$ - $0.2$ ).
Sticking (shear) friction: at high pressure in hot working the real contact area equals the apparent area, so the friction stress is a fraction of the shear yield stress, $\tau = m k$ ( $m$ = friction factor from 0 to 1). Used in forging and hot rolling.
Friction raises forming force, causes non-uniform flow and heats the tool.

Wear

Type	Cause	Seen in
Adhesive	Welding and tearing of asperities	Tool rake face, built-up edge
Abrasive	Hard particles ploughing the surface	Flank wear of tools, dies
Diffusion	Atoms migrate at high temperature	Crater wear in carbide tools
Fatigue / thermal	Cyclic stress or temperature	Hot forging dies
Corrosive	Chemical attack	Dies in humid or acid media

Functions of lubricants

Separate the surfaces and reduce friction and wear.
Carry away heat (coolants) and wash away chips.
Improve surface finish and prevent welding.
Protect the work and die from corrosion.

Types of lubricants

Liquids: mineral oils, soluble (emulsion) oils, synthetic fluids, with extreme-pressure (EP) additives.
Solids: graphite, molybdenum disulphide, soap, glass (for hot extrusion).
Gases and mists: compressed air, aerosol of oil.
Regimes: hydrodynamic (full film), boundary (thin molecular film) and mixed.

Viscosity and temperature

Viscosity $\eta$ (shear stress divided by shear rate, Pa.s) decreases as temperature rises, approximately exponentially. A lubricant for hot working must therefore keep an adequate film at the die temperature, which is why solid lubricants or glass are used above about 700 degree C. Viscosity rises with pressure, which helps film formation in rolling and drawing.

✓ Answer checked by Claude Opus 5.5 (high effort)

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