CLS Tribology & Wear Prevention Techniques 2 — Questions and Answers
Question 1: Which wear mechanism involves the transfer of material from one surface to another due to adhesion at asperity contacts?
- Abrasive wear
- Adhesive wear (Correct answer)
- Erosive wear
- Fatigue wear
Correct answer: Adhesive wear
Adhesive wear occurs when asperities from opposing surfaces weld together and material is torn from one surface and transferred to the other.
Question 2: The Archard wear equation relates wear volume to which primary factors?
- Load, hardness, and sliding distance (Correct answer)
- Velocity, temperature, and viscosity
- Surface roughness, lubricity, and contact area
- Yield strength, friction coefficient, and speed
Correct answer: Load, hardness, and sliding distance
Archard's equation states that wear volume is proportional to the applied load and sliding distance, and inversely proportional to the hardness of the softer material.
Question 3: What is the primary purpose of an extreme pressure (EP) additive in a lubricant?
- Increase viscosity at elevated temperatures
- Form a protective sacrificial layer at metal-to-metal contact zones (Correct answer)
- Reduce oxidation of the base oil
- Prevent rust and corrosion on metal surfaces
Correct answer: Form a protective sacrificial layer at metal-to-metal contact zones
EP additives react chemically with metal surfaces under high contact pressure and temperature to form low-shear-strength films that prevent welding and severe wear.
Question 4: Fretting wear is most likely to occur under which condition?
- High-speed sliding with hydrodynamic lubrication
- Small-amplitude oscillatory motion between clamped surfaces (Correct answer)
- Continuous unidirectional rotation under full film conditions
- Impingement of abrasive particles carried in a fluid stream
Correct answer: Small-amplitude oscillatory motion between clamped surfaces
Fretting wear is caused by the micro-motion (typically 1–300 µm amplitude) between surfaces that are nominally clamped together, leading to oxidative debris and surface damage.
Question 5: Which parameter best describes the severity of contact conditions when evaluating lubrication regime selection?
- Saybolt Universal Viscosity (SUV)
- Specific film thickness (Lambda ratio, Λ) (Correct answer)
- Flash point of the lubricant
- Base oil saturates content
Correct answer: Specific film thickness (Lambda ratio, Λ)
The Lambda ratio (film thickness divided by composite surface roughness) determines whether a contact operates in full-film, mixed, or boundary lubrication regimes.
Question 6: Cavitation erosion in lubricated systems is caused by:
- Fatigue cracking from repeated Hertzian stress cycles
- Collapse of vapor bubbles near metal surfaces generating micro-jets (Correct answer)
- Chemical dissolution of the metal by acidic lubricant degradation products
- Abrasive cutting by hard particles entrained in the lubricant
Correct answer: Collapse of vapor bubbles near metal surfaces generating micro-jets
When vapor or gas bubbles collapse near a metal surface, the implosion generates localized high-pressure micro-jets and shock waves that pit and erode the material.
Question 7: In a Stribeck curve, the transition from mixed to hydrodynamic lubrication is characterized by:
- A sharp increase in friction coefficient as speed rises
- A minimum in the friction coefficient followed by a gradual rise (Correct answer)
- Constant friction across the full speed range
- A sudden drop in friction once a critical load is exceeded
Correct answer: A minimum in the friction coefficient followed by a gradual rise
As speed increases along the Stribeck curve, the friction coefficient decreases through boundary and mixed regimes, reaches a minimum at the onset of full-film lubrication, then rises slightly with hydrodynamic drag.
CLS Tribology & Wear Prevention Techniques 2
Which wear mechanism involves the transfer of material from one surface to another due to adhesion at asperity contacts?