The role of oxidation in the wear of alloys

Sliding wear can be influenced significantly by heat, either frictional or externally applied, since it can facilitate oxidation of the contacting metal or alloy surfaces. This can result in a decrease in wear rate, usually associated with a change from metallic debris to oxide debris. The present paper reviews some of the models developed to account for the generation of oxide during sliding and the effects of such oxides on the rates of wear. Emphasis is placed on high-speed unidirectional sliding, where frictional heat can lead to surface temperatures that are sufficiently high to result in relatively thick oxides on the contacting surfaces, and low-speed reciprocating sliding, where frictional heat is low but externally applied heat can lead to oxidation of the surfaces. Under the former conditions, wear is caused by spallation of oxide from the contacting asperities; this occurs when the oxide attains a critical thickness, leading to mild-oxidational wear. At very high speeds, surface temperatures may be high enough for the oxide to melt, leading to severe-oxidational wear. Under low speed, reciprocating sliding conditions, oxide and oxidized metal debris can be retained and compacted onto the contacting surfaces, giving wear protection. Here, increased temperature facilitates the generation of oxide debris and assists in compaction of the debris to give the wear-protective layers. At low temperatures, such layers consist mainly of loosely-compacted particles; at higher temperatures, typically >250 degrees C, smooth 'glaze' surfaces develop on top of solidly compacted particle layers, giving even more effective protection against wear damage. (C) 1998 Elsevier Science Ltd. All rights reserved.