Wear

• Wear is the loss of material from contacting surfaces in relative motion

• Various types of wear• Defined by their mechanism of material

removal• 1.5% GDP can be saved by the control of

wear

Corrosive wear

• Reaction of a surface during wear due to its chemical reaction with the environment.

• Clean metal surface reacts to give contaminant film, which is then removed removed by sliding to expose more clean metal.

• Debris particles produced by wear are often oxides, which can cause three-body abrasive wear.

• Corrosive wear can sometimes be beneficial in that it prevents metal-to-metal contact.

Common corrodents in corrosive wear

• Oxygen• Water• Atmospheric sulphur dioxide• Sea water• Organic acids• Process liquids and gases

Oxidative wear

• Subset of corrosive wear• Reaction of wearing surface with oxygen in

the air to form an oxide layer• Flash temperatures depend particularly on

sliding velocity e.g. 1 m/s gives 700oC for steel

• Oxidation-scrape-reoxidation• Oxide growth follows Arrhenius equation

Possible methods to reduce corrosive wear

• Identify nature of corrosive reaction• Eliminate or reduce corrosive substances e.g.

driers to remove water, partial inerting to reduce oxygen.

• Reduce temperature• Change surface material e.g. if mild abrasion,

painting or epoxy primer is possible remedy e.g. if erosion, elastomer (rubber) is possible remedy.

• Reduce sliding speed, load or presence of abrasives

Adhesive wear

• Common type and mechanism• Caused by 2 sliding surfaces sticking together• Pieces of softer surface pulled out of surface to

form debris or transferred patches on harder countersurface

• Archard wear equation: Q = KW/H where Q is wear rate, K wear coefficient, W load and H hardness.

Adhesive wear

Understanding adhesive wear

• The wear coefficient in Archard equation may be considered as the probability that a wear particle will be formed by an asperity-asperity encounter.

• This is related to the tendency of the two sliding materials, A and B, to stick or adhere to each other. Those that have a high tendency to adhere, will have a high K.

• Tendency to adhere relates to solubility and phase diagrams. If B is soluble in A, then it will tend to adhere and have a high adhesive wear rate.

• The next slide gives a guide based on phase diagrams on how to minimize adhesive wear by material selection.

Mutual solubility of sliding couples

• The next slide gives examples the solubilities of metal sliding couples.

• Compatibility in this context means the two metals are mutually soluble and therefore have a high tendency for adhesion.

• The last column gives the measured wear coefficient and shows that mutually soluble materials tend to incur high wear rates.

• Caution required as other factors can have an influence e.g. surface films.

Possible methods to reduce adhesive wear

• Use insoluble materials and avoid like-on-like.• If cannot avoid like-on-like (e.g. gears), then raise

hardness (e.g. carburizing).• Apply an insoluble materials as a coating e.g.

thermally sprayed alumina, heat treated electrolessnickel.

• Use surface texture to break up adhesions or junctions.

• Operating conditions e.g. speed, load, lubrication.

Abrasive wear

• Softer surface has grooves parallel to the sliding direction

• Harder asperities ploughing out or deforming the surface of softer material

• Two-body abrasive wear• Three-body abrasive wear

Wear equation for abrasive wear

• Asperity or hard particle as cone of semiangle θsliding under load W across softer material of hardness H

• If Q is the volume material removed per unit sliding distance, then Q = KW/H where K = wear coefficient of softer surface

• K is proportional to 1/tan θ and so small θ values (sharp particles or asperities) give large wear coefficients K and wear rates Q.

Possible methods to reduce abrasive wear

• Use hard materials e.g. alumina HV ~ 1500• Use hard materials as coatings as less susceptible

to brittle fracture• Use cermets as less brittle than ceramics e.g. hard

tungsten carbide in ductile cobalt matrix WC-Co• Use smooth surfaces by grinding and polishing as

these have low θ e.g. Ra<0.5µm• Give protection to the surface most exposed to

wear or the most expensive/difficult to replace