---
title: "Lubricant functions; solid lubricants"
subject: "Engineering Chemistry"
chapter: 8
key: "semester_2/engineering_chemistry:lubricant-functions-solid"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricant-functions-solid
site: BCT Engineering
---

# Lubricant functions; solid lubricants

Engineering Chemistry · Chapter 8: Lubricants and Paints

- Asked in: 2070 Bhadra, 2.5 marks
- Video: [Lubricants || Lubricant Important Functions And Classification ||Lubrication ||Engineering Chemistry](https://www.youtube.com/watch?v=Kuj2X6ncPBc) — Chemistry by Dr. Anjali Ssaxena
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/lubricant-functions-solid
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-8

## Question

Write the important functions of lubricant. What are solid lubricant and specify in which conditions it is used.

## Short answer

Lubricants reduce friction, wear, heat, seal, prevent corrosion, clean, and save energy. Solid lubricants like graphite and MoS₂ have a layered structure and are used at very high temperatures, extreme loads, in vacuum, or where oil would contaminate or cannot stay.

## Answer

#### Important functions of lubricants
1. **Reduces friction** between moving surfaces by forming a thin film that separates them.
2. **Reduces wear and tear**, which increases the life of machine parts.
3. **Acts as a coolant:** carries away heat produced by friction.
4. **Acts as a seal:** prevents leakage of gases, e.g. between piston rings and cylinder.
5. **Prevents corrosion** by coating metal surfaces from air and moisture.
6. **Cleans** by washing away dirt, metal particles and carbon deposits.
7. **Saves energy:** smoother motion means less power loss and higher efficiency; also reduces noise and vibration.

#### Solid lubricants
**Solid lubricants** are solids with a layered structure that slide easily over each other, such as graphite, molybdenum disulfide (MoS₂), talc and soapstone. They are used where oils and greases fail:
- at very high temperatures (graphite up to about 450 °C in air) where oil burns or evaporates;
- under extremely heavy loads and very slow speeds where an oil film would be squeezed out;
- in vacuum or at very low temperature;
- where oil would contaminate the product (food, textile, paper machines) or collect dust;
- as an additive or dry film on parts that cannot be lubricated often.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a lubricant reduce friction?
   A. By bonding the surfaces together
   B. By forming a thin film that separates the surfaces
   C. By increasing surface roughness
   D. By cooling the surfaces to zero
   Answer: B. The film separates moving surfaces. (Key point: Friction)

2. What does a lubricant do between piston rings and cylinder?
   A. Causes gas to leak
   B. Increases wear
   C. Dissolves the gas
   D. Acts as a seal preventing gas leakage
   Answer: D. Lubricant acts as a seal. (Key point: Sealing)

3. What property of graphite and MoS2 makes them solid lubricants?
   A. Very low melting point
   B. Layered structure whose layers slide easily
   C. High solubility in oil
   D. Strong magnetic nature
   Answer: B. Layers slide over each other. (Key point: Solid lubricants)

4. Under which condition is a solid lubricant used?
   A. Light load at high speed
   B. Where regular oiling is easy
   C. At moderate temperature with no contamination worry
   D. Extremely heavy load and slow speed where an oil film would be squeezed out
   Answer: D. Solid lubricants work where oils fail, including heavy load, high temperature and vacuum. (Key point: Use condition)

## Related in Engineering Chemistry

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