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Lubricants and their classification

Short answer

A lubricant is a substance introduced between moving surfaces to reduce friction and wear. They are classified as liquid (animal/vegetable, mineral, synthetic oils), semi‑solid (calcium, sodium, lithium soap greases) and solid (graphite, MoS₂, talc).

What are lubricants? How are they classified?

A lubricant is a substance introduced between two moving or sliding surfaces to reduce friction and wear between them. It reduces friction, wear and heat.

Classification

Lubricants
 ├── Liquid   (oils)
 │     ├── Animal/vegetable oils: castor, olive, tallow
 │     ├── Mineral (petroleum) oils
 │     ├── Synthetic oils: silicones, polyglycols
 │     └── Blended oils (oil + additives)
 ├── Semi-solid  (greases): Ca, Na, Li soap greases
 └── Solid: graphite, MoS₂, talc, soapstone
  • Liquid lubricants form a fluid film; mineral oils are the most common as they are cheap and stable.
  • Greases are oils thickened with metallic soap.
  • Solid lubricants have layered structures and are used at high temperatures and loads.

✓ Answer checked by Claude Opus 5.5 (high effort)

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