Mechanical wear?

 Mechanical wear

Mechanical wear is the gradual removal, deformation, or damage of material from a solid surface due to mechanical action such as friction, sliding, rolling, impact, or repeated loading between contacting surfaces.

It commonly occurs in machine parts like bearings, gears, cutting tools, and engine components, and leads to loss of efficiency, dimensional changes, or eventual failure.

Types of Wear

1. Adhesive Wear (Micro-Welding Theory)

What really happens:

• Clean metal surfaces touch → atoms form bonds (“cold welding”)
• Sliding breaks junctions
• Material tears from weaker surface

Key factors:

• High load
• Low lubrication
• Similar metals (same material increases adhesion)

Result:

• Surface transfer + roughening + galling

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2. Abrasive Wear (Cutting Mechanism)

Two mechanisms:

(a) Two-body abrasion

Hard asperity acts like a cutting tool
→ grooves form on soft surface

(b) Three-body abrasion

Loose particles roll/slide between surfaces
→ repeated scratching

Governing idea:

Hardness difference controls severity:

Hard particle > soft surface → cutting occurs

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3. Surface Fatigue Wear (Crack Growth Mechanism)

Happens under repeated stress cycles:

1. Subsurface shear stress develops
2. Micro-cracks initiate below surface
3. Cracks propagate to surface
4. Material breaks off → spalling/pitting

Very important in:

• Bearings
• Gear teeth

Key concept:

Even if stress < yield strength → failure still occurs due to fatigue

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4. Corrosive Wear (Tribocorrosion)

Combined process:

• Chemical reaction (oxidation, corrosion)
• Mechanical removal of protective film

Cycle:

1. Oxide layer forms
2. Sliding removes oxide
3. Fresh metal exposed
4. Re-oxidation occurs

Result:

Wear accelerates drastically compared to pure corrosion or wear alone

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5. Erosive Wear (Momentum Transfer Mechanism)

Caused by particle impact:

• Solid particles (sand, dust)
• Liquid droplets (rain erosion)
• Gas-solid mixtures

Mechanism depends on angle:

• Low angle → cutting action
• High angle → deformation + crater formation

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6. Fretting Wear (Micro-Oscillation Damage)

Conditions:

• Very small amplitude motion (microns to mm)
• High normal load

Mechanism:

• Micro-slip at interface
• Oxide debris forms
• Debris acts as abrasive → accelerates damage

Common in:

• Bolted joints
• Press fits
• Vibrating assemblies

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7. Impact Wear (Shock Loading Mechanism)

Occurs when:

• Repeated high-energy impacts deform surface plastically

Mechanism:

• Local yielding
• Crack formation
• Fragment detachment

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Wear Rate (Engineering Model)

A common empirical model is Archard’s Wear Law:

$$W = \frac{K \cdot L \cdot P}{H}$$

Where:

• W = wear volume
• K = wear coefficient (material + lubrication dependent)
• L = sliding distance
• P = load
• H = hardness of softer material

Key insight:

• ↑ Load → ↑ wear
• ↑ Hardness → ↓ wear
• ↑ Sliding → ↑ wear

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Factors Affecting Wear (Very Important)

Material factors:

• Hardness (most important)
• Toughness
• Microstructure
• Surface coatings

Operating factors:

• Load
• Speed
• Temperature
• Lubrication

Environmental factors:

• Dust/particles
• Moisture
• Corrosive gases