· Definition: The removal of material from two surfaces under load, due to the sliding motion between them.1
· Like friction, wear is independent of the contact area between the two surfaces, and proportional to the force pressing the surfaces together.
· However, importantly, the amount of wear is also proportional the sliding distance travelled by the surfaces.
· NB. Corrosive wear is an exception to these rules being both independent of the loading between surfaces and sliding distance.
· Reducing wear is key to the survivorship of natural and prosthetic joints.
Mechanisms of wear
- Wear can be broadly categorised into:
- MECHANICAL – dependent on loading and sliding distance:
- CHEMICAL – independent of loading and sliding distance:
- Asperities from the two surfaces under load adhere to each other forming bonds (cold welding). As the surfaces slide over each other, some of the adhered asperities from one surface will shear off, remaining stuck to the other surface.
- This usually occurs between two surfaces that have similar molecular structures, as bonds form easily between them.
- Example: in metal-on-metal hip bearings.
Fig 2a. two asperities bond to each other.
Fig 2b. as the materials pass over each other the bonded asperity shears off its original surface and now becomes part of the other surface.
- Asperities from the harder surface literally knock off the asperities from the softer surface, as they slide.
- This usually occurs between two materials of different hardness.
- Example: between a metal femoral head and polyethylene liner.
Figure 2c. and 2d. These show how the asperities of the harder bearing surface break off the softer asperities of the surface below.
- Repeated cyclical loading of one surface, at loads greater than the fatigue strength, leads to small cracks forming under the surface. Through repeated loading these cracks propagate, eventually joining together, and the loose material comes away from the surface.
- Example: delamination of the polyethylene in total knee replacements (TKRs)
Figure 2e. Here loads much lower than that needed for abrasive wear, after repeated cycles, gradually cause cracks to form under the surface. These cracks eventually coalesce and fragments of the surface break off.
- Hard particles travelling in fluid interposed between the two surfaces remove some of the surface as they collide into it.
Figure 2f. The presence of third body particles carried in the lubricant fluid can lead to erosion of the bearing surfaces.
- Material is removed via a chemical reaction.
- In crevice corrosion small crevices between surfaces (e.g. between trunion and femoral head) trap fluid which becomes stagnant. Fluid at the apex of the crevice quickly uses up its O2, where fluid at the entrance to the crevice remains well oxygenated. This oxygen gradient leads to the formation of an anode (area of metal giving up electrons and forming positive metal ions) in the area of low O2 and a cathode (area of metal receiving electrons) in the high O2 environment. The positive metal ions in the fluid lower pH leading to acidic fluid formation and thus dissolution of the material.
- Example: the crevice formed between the trunion and femoral head can lead to corrosive trunion wear.
Figure 2g. The difference in O2 tension between the apex of the cravice and the entrance to the crevice leads to the formation of an anode and cathode. As the anode gives off electrons (e-) to the cathode, pH at the apex decreases, leading to acidic erosion of the surfaces.