CLS Tribology & Wear Prevention Techniques 3 — Questions and Answers
Question 1: Which type of surface fatigue wear produces shallow, saucer-shaped pits on gear tooth flanks or rolling element bearing raceways?
- Spalling
- Pitting (Correct answer)
- Delamination
- Micropitting
Correct answer: Pitting
Pitting is a subsurface-initiated fatigue process producing relatively large, shallow pits that are typically visible to the naked eye on gear and bearing surfaces.
Question 2: What does the term 'tribochemical wear' refer to?
- Wear due to electrochemical galvanic action between dissimilar metals
- Wear resulting from chemical reactions between the lubricant, environment, and surface at tribological contacts (Correct answer)
- Wear caused by thermal softening of surfaces during high-speed sliding
- Wear produced by impact loading from abrasive particles in the lubricant
Correct answer: Wear resulting from chemical reactions between the lubricant, environment, and surface at tribological contacts
Tribochemical wear involves chemical reactions accelerated by frictional heating and nascent surface activity, producing reaction films or corrosive wear products at the contact.
Question 3: A gear set experiences scuffing. Which lubrication property is most critical to prevent this failure mode?
- High viscosity index (VI)
- High load-carrying capacity and anti-scuff film formation (Correct answer)
- Low pour point
- High oxidation stability
Correct answer: High load-carrying capacity and anti-scuff film formation
Scuffing is a form of adhesive wear triggered by breakdown of the lubricant film; EP additives that rapidly form shear films under high flash temperature prevent metal welding and tearing.
Question 4: Which property of a solid lubricant makes molybdenum disulfide (MoS₂) effective in reducing friction?
- High melting point allowing it to remain solid at elevated temperatures
- Lamellar crystal structure with weak van der Waals bonds between layers (Correct answer)
- High thermal conductivity that dissipates frictional heat rapidly
- Chemical inertness preventing reaction with metal surfaces
Correct answer: Lamellar crystal structure with weak van der Waals bonds between layers
MoS₂ has a hexagonal lamellar structure; the sulfide layers slide easily over each other due to weak interlayer forces, providing low shear resistance and excellent dry lubrication.
Question 5: When a bearing operates in the 'elasto-hydrodynamic lubrication' (EHL) regime, what unique phenomenon occurs?
- The lubricant film collapses and boundary additives carry the load
- Elastic deformation of the contact surfaces and pressure-viscosity increase of the lubricant together sustain a fluid film (Correct answer)
- The bearing surface hardens due to repeated stress cycles
- Viscosity decreases to zero at the contact center due to shear heating
Correct answer: Elastic deformation of the contact surfaces and pressure-viscosity increase of the lubricant together sustain a fluid film
In EHL, the high contact pressure causes significant elastic flattening of surfaces and a dramatic increase in lubricant viscosity (piezoviscous effect), enabling a thin but effective separating film.
Question 6: Surface hardening treatments such as carburizing and nitriding primarily reduce which type of wear?
- Corrosive wear from acidic lubricant degradation
- Abrasive and adhesive wear by increasing surface hardness (Correct answer)
- Erosive wear from liquid droplet impingement
- Fretting wear in oscillating joints
Correct answer: Abrasive and adhesive wear by increasing surface hardness
Hardening the surface increases resistance to plastic deformation at asperity contacts, directly reducing material removal in both abrasive cutting and adhesive junction growth.
Question 7: What is the significance of the 'wear coefficient' (K) in Archard's equation?
- It quantifies the lubricant's anti-wear additive concentration
- It is a dimensionless measure of the probability that a given asperity contact produces a wear particle (Correct answer)
- It represents the ratio of kinetic to static friction at the contact
- It describes the rate of oxidative degradation of the lubricant film
Correct answer: It is a dimensionless measure of the probability that a given asperity contact produces a wear particle
K in Archard's equation (W = KPL/H) is a dimensionless wear coefficient that encapsulates material, lubricant, and surface condition factors affecting how efficiently contacts generate debris.
CLS Tribology & Wear Prevention Techniques 3
Which type of surface fatigue wear produces shallow, saucer-shaped pits on gear tooth flanks or rolling element bearing raceways?