The role of oxidation in the wear of alloys

High-temperature sliding wear of metals

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.

The dry sliding wear behavior of Al 2219 alloy and Al 2219/SiCp/Gr hybrid composites are investigated under similar conditions. The composites are fabricated using the liquid metallurgy technique. The dry sliding wear test is carried out for sliding speeds up to 6 m/s and for normal loads up to 60 N using a pin on disc apparatus. It is found that the addition of SiCp and graphite reinforcements increases the wear resistance of the composites. The wear rate decreases with the increase in SiCp reinforcement content. As speed increases, the wear rate decreases initially and then increases. The wear rate increases with the increase in load. Scanning electron microscopy micrographs of the worn surface are used to predict the nature of the wear mechanism. Abrasion is the principle wear mechanism for the composites at low sliding speeds and loads. At higher loads, the wear mechanism changes to delamination.

Dry sliding wear behavior of Al 2219/SiCp-Gr hybrid metal matrix composites

During like-on-like reciprocating sliding in air (amplitude 2.5 mm, load 1.5 kg, speed 500 double traversais per minute), the formation of oxides can have considerable influence on the friction and wear characteristics of high-temperature alloys, such as Jethete M152 and Rex 535. In particular, above a certain transition temperature, between 200 and 300°C for these alloys under these conditions, an adherent, smooth wear-protective oxide layer is developed on the load-bearing surfaces. At lower temperatures, oxide debris reduces the extent of metal-metal contact, thereby reducing the friction and wear rate, but does not eliminate it completely. The oxide debris is produced by two processes; one involves transient oxidation of the metal surfaces, removal of such oxide during each transversal, and reoxidation of the exposed metal; the other involves the formation, fracture, comminution, and oxidation of metal debris particles. At temperatures above the transition temperature, the oxide debris is compacted and comminuted between the sliding surfaces to develop the wear-protective oxide layer. This paper considers the reasons for the effectiveness of such oxides in terms of the influence of the hydrostatic pressures generated on plastic deformation of the very fine oxide particles or asperities in the surface. The resulting friction during sliding is less than during metal-metal contact because only limited asperity junction growth occurs before the asperities become sufficiently large and the hydrostatic pressures sufficiently reduced to allow fracture within the oxide-oxide junctions. The oxide-wear debris produced is recompacted into the surface, resulting in only very low wear rates. It has been shown that the number of asperity-asperity contacts during sliding of wear-protective oxide layers is relatively high, typically 5×103/mm2 of apparent contact area, while the mean surface flash temperature rise is low, typically 2°C. Consideration is given to some of the conditions that favor development of wear-protective oxide layers.

The effectiveness of oxides in reducing sliding wear of alloys

Factors affecting the progressive development of wear-protective oxides on iron-base alloys during sliding at elevated temperatures

The sliding wear of metals in air or oxygen, even at relatively low temperatures, is generally characterized by a period of relatively severe wear, followed by a transition to a period of mild wear, the transition being associated with the generation of sufficient oxide to limit metal–metal interactions. Consideration is given to some of the factors that can influence this oxidation process and it is shown that, in addition to preoxidation, there are three main causes of oxide generation. In situations where surface flash-temperature rises are significant, particularly where the number of load-bearing contacts is few, the formation of oxide on those contacts can be important. If there are many load-bearing contacts, particularly under low-speed conditions and at high ambient temperatures, flash-temperature rises are small and oxidation of the asperity tips may be negligible in comparison with the general oxidation of the entire areas of the specimens. Quantitative models are presented to account for these processes. However, under certain conditions, neither asperity nor general surface oxidation is important in initiating the transition to mild wear; rather, metal debris particles are generated in the severe wear period. As discussed theoretically, these are fractured and refractred during sliding under the loading forces until eventually the particles become so small that they oxidize almost completely spontaneously, even at room temperature. These oxide particles can agglomerate in the wear tracks, there by reducing metal-metal contact and causing a considerable reduction in wear.

J. Glascott

Papers

The effectiveness of oxides in reducing sliding wear of alloys

Factors affecting the progressive development of wear-protective oxides on iron-base alloys during sliding at elevated temperatures

Models for the generation of oxides during sliding wear

The sliding wear of commercial Fe-12%Cr alloys at high temperature

The transition from severe to mild sliding wear for Fe-12%Cr-base alloys at low temperatures