Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

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Wear by hard particles - abrasion and erosion

Factors affecting the rate of wear:

Hardness - particles with hardness lower than the surface cause little wear.

Shape– angular particles cause greater wear than rounded particles.

Size – larger particles cause more extensive wear as they carry more kinetic energy

Impact speed (for erosion) – particles with greater speed cause more extensive wear as they carry more kinetic energy.

Impact angle (for erosion) – particles hitting at angles close to perpendicular to the surface cause greater erosion.

Abrasive wear:

The particles are often larger than lubricant film thickness, so contact between the particles and the surface occurs, meaning lubrication does not reduce abrasive wear.

Abrasive wear can arise either from plastic deformation forming a groove in a material or by brittle fracture. In brittle fracture, lateral cracks formed beneath a plastic groove produce chips which are subsequently removed from the surface.

Abrasive wear

Schematics of abrasive wear of (a) ductile material and (b) a material which is brittle.

Materials with high hardness have low toughness (brittle), and visa-versa (ductile), so maximum wear resistance arises through a combination of intermediate values of hardness and toughness.

Wear toughness

Metals (tougher but less hard) suffer abrasive wear by plastic deformation, ceramics (less tough, but harder) by brittle fracture.

Brittle fracture can be modelled through analogy with indentation of brittle materials. If the variables assumed are W (load), H (hardness) and Kc (fracture toughness):

\[Q = A{W^p}{H^q}{K_{\rm{c}}}^{ - r}\]

with Q being the volume wear rate per unit sliding distance for a constant A, and with W, H and Kc raised to powers of p, q and -r respectively.

Models used for predicting wear rates where brittle fracture is involved predict wear rates higher than would be expected from plastic mechanisms.

These models also predict:

  • An increase in wear rate with size of the abrading particles
  • An inverse correlation between fracture toughness (raised to some power) and wear rate, to the extent that fracture toughness is a more important material parameter than hardness
  • A threshold load below which wear by brittle fracture will not occur

Erosive Wear:

Material removal from each impact is very small but the collective damage can be significant.

Variables which a simple model would expect to affect the volume of material, V, removed from an eroding surface of a plastically deforming material are the velocity, U, of the particles and their mass, m, (together in a kinetic energy term) and the hardness, H, of the material being eroded.

Thus, from dimensional analysis, for this simple model, in which indentation-type behaviour is envisaged, we expect

\[V = \frac{{Km{U^2}}}{{2H}}\]

where K is a dimensionless constant. If we define erosion, E, as

\[E = \frac{{{\rm{mass\;\; of \;\;material\;\; removed}}}}{{{\rm{mass\;\; of \;\;erosive\;\; particles \;\;striking\;\; the\;\; surface}}}}\]

it follows that

\[E = \frac{{V\rho }}{m} = \frac{{K\rho {U^2}}}{{2H}}\]

where ρ is the density of the bulk material. This shows the importance of a high hardness of the surface being eroded for wear resistance. A model for erosive wear by brittle fracture would be similar but would show a high fracture toughness being more crucial for wear resistance.

Image result for erosion

An example of erosion, with sand particles wearing away at the rock.

A second example is the Chiltern escarpment in Buckinghamshire in the south of England, a boundary between the hard chalk of the Chiltern Hills and the soft clay of the Vale of Aylesbury. Over geological time periods, the clay has worn away faster than the chalk, so that there is a noticeable slope (escarpment) between the valley and its neighbouring chalk hills.