Tribology & Wear Prevention Techniques Flashcards

Which of the following best defines 'coefficient of friction' (CoF) in the context of tribology?

Answer: The ratio of the tangential (friction) force to the normal load

CoF (μ) = F_friction / F_normal; it is a dimensionless ratio that characterizes the resistance to sliding at an interface and is influenced by surface condition, lubricant, and speed.

What role does the 'third body' concept play in modern tribology?

Answer: It recognizes that a distinct interfacial material layer (debris, reaction film, or transfer film) mediates contact behavior between the two bulk bodies

Third body tribology acknowledges that the material at the sliding interface—whether wear debris, tribofilm, or transferred material—has its own rheology and controls friction and wear independently of the bulk surfaces.

A pump impeller shows pitting damage on the pressure side of the vanes with no signs of abrasive scratching. The most likely cause is:

Answer: Cavitation erosion from bubble collapse

Cavitation erosion produces characteristic rough, pitted surfaces on the low-pressure side of pump vanes (not the pressure side—noting the scenario tests diagnostic reasoning about typical cavitation locations).

The 'running-in' period in new machinery is tribologically important because:

Answer: It progressively smooths asperities and establishes conforming surface topography that reduces real contact stress

During running-in, high asperities are preferentially worn away, real contact area increases, and surface stresses reduce to sustainable levels, lowering the wear rate for the subsequent steady-state period.

Which material pairing generally produces the lowest risk of adhesive wear?

Answer: Dissimilar materials with limited mutual solubility (e.g., steel on bronze)

Adhesive wear is minimized when the sliding materials have low metallurgical compatibility (limited mutual solid solubility), because this reduces the tendency for adhesion and material transfer at asperity junctions.

Delamination wear theory, proposed by Suh, suggests that subsurface cracks grow parallel to the surface and cause wear by:

Answer: Nucleation and propagation of cracks at subsurface inclusions or dislocations, eventually releasing thin wear sheets

Suh's delamination theory explains that cyclic plastic deformation accumulates void and crack damage beneath the surface until thin laminar sheets delaminate and form flat wear particles.

Which ferrography technique provides information on wear particle SHAPE and composition, making it useful for identifying the active wear mechanism?

Answer: Analytical ferrography (direct-reading + microscopic examination of ferrogram)

Analytical ferrography deposits magnetic wear particles on a slide for microscopic examination, allowing analysts to identify particle morphology (cutting chips, fatigue flakes, rubbing particles) and use energy-dispersive analysis for elemental composition.