For UG (B.Tech/B.Sc) & PG (M.Tech/M.Sc) Engineering Chemistry | Tribology | Industrial Chemistry
1. Introduction to Lubricants
Lubricants are substances introduced between two moving/sliding surfaces to reduce friction, wear, and heat generation. They play a critical role in machinery efficiency, energy conservation, and equipment longevity.
- Definition: A fluid or semi-fluid material that minimizes frictional resistance between surfaces in relative motion.
- Importance: Reduces energy loss (up to 30% in engines), prevents seizure/welding of surfaces, and acts as coolant/sealant.
- Global Market: ~$150 billion (2023), driven by automotive, industrial, and green tech sectors.
High-yield: Friction causes heat → lubricant forms film → hydrodynamic separation.
2. Functions of Lubricants
- Reduce Friction & Wear: Forms protective film between asperities (surface irregularities).
- Cooling: Dissipates frictional heat via convection.
- Sealing: Prevents ingress of contaminants (dust, water) and leakage of fluids.
- Corrosion Protection: Neutralizes acids, forms barrier against moisture/oxygen.
- Shock Absorption: Cushions impacts in bearings/gears.
- Power Transmission: In hydraulic systems.
- Cleaning: Suspends debris (via dispersants).
3. Classification of Lubricants
3.1 Based on Physical State
| Type | Examples | Applications | Advantages |
|---|---|---|---|
| Liquid | Mineral oils, synthetic oils (PAO, esters), vegetable oils | Engines, turbines, hydraulics | Easy flow, wide temp range |
| Semi-Solid (Greases) | Li/Ca/Na-based greases, Vaseline | Bearings, gears, chassis | Stays in place, water-resistant |
| Solid | Graphite, MoS₂, PTFE (Teflon), BN | High-temp/vacuum (spacecraft, locks) | Extreme conditions, low volatility |
3.2 Based on Origin/Source
- Mineral Oils: From petroleum (paraffinic/naphthenic/aromatic fractions). 90% of market.
- Vegetable/Animal Oils: Castor oil, tallow. Biodegradable but oxidize easily.
- Synthetic Oils: PAO, polyglycols, silicones, esters. Superior stability, temp range (-50°C to 250°C).
3.3 Based on Application
- Automotive: Engine oils (SAE 10W-40), gear oils.
- Industrial: Hydraulic, compressor, turbine oils.
- Specialty: Food-grade (NSF H1), aerospace (MIL-SPEC).
PYQ Focus: Compare mineral vs. synthetic (stability, cost, VI).
4. Properties of Lubricants
| Property | Description | Significance | Test/Scale |
|---|---|---|---|
| Viscosity | Resistance to flow | Determines film thickness; too low → wear, too high → drag | Saybolt Universal Seconds (SUS); ISO VG grades |
| Viscosity Index (VI) | Temp stability of viscosity | High VI (>100) for wide temp range | ASTM D2270 (100 ideal) |
| Flash/Fire Point | Temp at which vapors ignite (flash: brief, fire: sustained) | Safety; >200°C desirable | Cleveland Open Cup (ASTM D92) |
| Cloud/Pour Point | Cloud: wax crystallization; Pour: flow at low temp | Cold-start performance; <0°C | ASTM D97 (pour point) |
| Oiliness | Affinity to metal surfaces | Boundary lubrication | High for low-speed/high-load |
| Emulsification | Water mixing tendency | Low desirable (phase separation) | |
| Acid Value/Neutralization No. | Acidity (mg KOH/g) | <0.5; oxidation indicator | |
| Saponification Value | Ester content (mg KOH/g) | High for vegetable oils | |
| Oxidation Stability | Resistance to air/heat | Prevents sludge/varnish | RPVOT (ASTM D2272) |
Key: VI = (L - U)/(L - H) × 100; L=low VI oil, H=high, U=unknown.
5. Additives in Lubricants
Additives (5-30% by vol.) enhance base oil performance. Composition: Base oil (70-99%) + Additives.
| Type | Function | Examples | Chemistry |
|---|---|---|---|
| Anti-Wear (AW) | Forms protective film | ZDDP (zinc dialkyldithiophosphate) | Organophosphorus/sulfur compounds |
| Extreme Pressure (EP) | High-load protection | Chlorinated paraffins, sulfurized olefins | Reactive with metals → sulfides/chlorides |
| Antioxidants | Prevents oxidation | Phenolics, amines | Free radical scavengers |
| Detergents/Dispersants | Cleans/suspends sludge | Calcium/Mg sulfonates, succinimides | Neutralize acids, polar groups |
| Viscosity Index Improvers | Temp-stable viscosity | Polymethacrylates, olefins | Polymers expand/contract with temp |
| Pour Point Depressants | Low-temp flow | Polymethacrylates | Inhibit wax crystallization |
| Anti-Foam | Reduces foaming | Silicones, polyacrylates | Surface tension modifiers |
| Corrosion Inhibitors | Protects metals | Benzotriazoles | Adsorb on surfaces |
PG Focus: ZDDP mechanism – forms Zn/Fe phosphate glass-like film.
6. Mechanisms of Lubrication
- Hydrodynamic (Thick Film): Full fluid film (>1000 Å); no surface contact. High speed, low load (e.g., journal bearings).
- Boundary (Thin Film): Partial contact; lubricant clings to surfaces. Low speed, high load (e.g., start-stop).
- Extreme Pressure (EP): Chemical reaction forms sacrificial layer (e.g., sulfides). Very high load (e.g., gears).
- Elastohydrodynamic (EHL): Elastic deformation + hydrodynamic pressure (e.g., ball bearings).
Stribeck Curve: Friction vs. viscosity/speed/load – plots regimes.
7. Testing & Evaluation
- Viscosity: Redwood/Capillary viscometer (ASTM D445).
- VI: ASTM D2270.
- Flash Point: Pensky-Martens (ASTM D93).
- Four-Ball Test: Wear/EP (ASTM D4172).
- Timken OK Load: EP for gears.
- FTIR/Spectroscopy: Additive depletion (PG level).
8. Applications & Selection Criteria
| Application | Lubricant Type | Key Criteria |
|---|---|---|
| IC Engines | Mineral/Synthetic multigrade (SAE 5W-30) | High VI, low volatility, detergency |
| Gears | EP gear oils (GL-4/5) | High load, anti-wear |
| Bearings | Greases (NLGI 2) | Water resistance, consistency |
| Hydraulics | Anti-wear hydraulic oils (HLP) | Filterability, rust inhibition |
| High-Temp | Synthetics + Solid (MoS₂) | Thermal stability >200°C |
Selection Factors: Load, speed, temp, environment (wet/dusty), cost.
API SN for engines; NLGI grades for greases (0-6 consistency).
9. Environmental & Future Trends
- Bio-Lubricants: Vegetable-based (e.g., soy oil); biodegradable, low toxicity.
- Green Additives: P-free AW (e.g., ashless), CHON-based (no S/Zn).
- Nanotechnology: Nano-additives (e.g., inorganic fullerenes) for reduced friction.
- Recycling: Re-refining used oils; EU REACH compliance.
- EV Shift: Low-viscosity oils for e-motors; solid lubricants in batteries.
PG: ILs (ionic liquids) as eco-friendly lubricants – high thermal stability, low volatility.
Summary Table
| Aspect | Key Points |
|---|---|
| Functions | Friction reduction, cooling, sealing, corrosion protection |
| Classification | Liquid (oils), Semi-solid (greases), Solid (MoS₂) |
| Properties | High VI, Flash >200°C, Low pour point |
| Additives | ZDDP (AW), VI improvers, Antioxidants |
| Mechanisms | Hydrodynamic, Boundary, EP |
| Trends | Bio-lubricants, Nano-additives, Sustainability |
Read also Lubricants PYQs MCQs for CSIR/NET | GATE | B.Tech. | M.Tech.