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OXIDATIVE WEAR

Oxidative wear is the wear of dry unlubricated metals in thepresence of air or oxygen. Atmospheric oxygen changes thefriction coefficients and wear rates of dry sliding metals.

x a ve wear was oun w en c anges n e c em ca compos onof wear debris generated. When the load and sliding speed were high enough to increase thefrictional contact temperature to several hundred degrees Celsius, the

wear debris changed from metallic iron to iron oxides. Oxidative or mild wear shows a moderate and stable coefficient of friction of about 0.3 -0.6 compared to much larger fluctuating valuesfor severe wear.

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Oxidative wear can be found in cases when a high process temperaturecauses rapid oxidation and the formation of thick oxide films.

The hole piercer used in the hot drawing of tubes is a good example of oxidative wear . A multilayer cap of scale and deformed metal accumulatesat the tip of the piercer and the thickness of the scale (thick oxide film) canbe as high as 0.1 [mm].

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Kinetics of Oxide Film Growth on Metals at Highand Low TemperaturesOxide films are present on almost all metals and will form on any cleanmetal surface exposed to oxygen even at cryogenic temperatures.

The oxidation rate of metals is dependent on temperature as isexpected of a chemical reaction.

At low or ambient temperatures , e.g. 20C, the oxidation of metal isinitially rapid and is immediately followed by the passivation of thesurface which limits the oxide film thickness.

For example , the limiting film thickness can be as low as 2 [nm] (about5 atom layers) for steels when the temperature is below 200C.

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If the temperature of steel is increased to 500C, almost unlimitedoxidation occurs which results in a very thick oxide film, e.g. in therange of 1 - 10 [m]. The distinction between these two forms of oxidation is illustrated inFigure

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The difference in oxidation kinetics results from much more rapidmovement of oxygen or metal ions across the oxide film at hightemperatures, when solid state diffusion is sufficient to ensureadequate transport of the metal or oxide ions necessary for acontinuous film growth.

At low temperatures , the oxide films are form rapidly and effectively suppress a esive wear. I a system operates un er mi oxi ationaconditions wear is greatly reduced.

At high temperatures , however, oxidation resembles corrosion in itshigh rate of reaction and can become a direct cause of increased wear.

This rapid oxidation at high temperatures forms the base of oxidative wear.

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The high temperatures can either be imposed externally or can be due tohigh frictional heating at high speeds and loads as illustrated in Figure

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Oxidative Wear at High Sliding Speeds At sliding speeds above 1 [m/s] the surface flash temperatures can be ashigh as several hundred degrees Celsius and if the load is low enoughto permit mild wear, oxide films several micrometers thick can build upon the worn surface.

Under these conditions the oxidation proceeds very rapidly, especially .

enough to physically separate the wearing surfaces, it is reasoned thatthe oxidative wear which occurs must be due to the formation andremoval of these thick oxide layers.

At high speed, the thin oxide films present on unworn steel surfaces are

rapidly destroyed and the friction and wear rates increase, initiating aperiod of severe wear. When each oxide layer reaches a critical thickness, it becomes too weakto withstand the load and frictional shear stress and is removed duringthe sliding.

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Sequence of events associated with the formation and removal of oxide layers

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At high sliding speeds any asperity of a surface is subjected to a randomsequence of short periods of high temperature oxidation when contact is made with asperities of an opposing surface as shown

Periodic rapid oxidation between the asperities in dry high-speed slidingcontact; d is average diameter of thick oxide patches

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Oxidative Wear at Low Sliding Speeds When a steel surface is exposed to air and subjected to low speedsliding wear the initial thin (about 2 [nm]) films are rapidly worn away and a period of severe or adhesive wear results. At low sliding speedsbelow 1 [m/s] frictional temperature rises are not high enough to causerapid oxidation at the asperity tips.

The fractured oxides and oxidized metallic wear particles compact toform oxide islands on the worn surface. The area of these islandsincreases with the sliding distance and there is progressive reduction inthe coefficient of friction.

The top surface of the islands is smooth and consists of plastically deformed fine oxide debris and underneath this top layer there ismixture of much larger oxide and oxidized particles.

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Oxidative Wear at High Temperature and Stress

When the temperature is progressively increased from close to ambientto several hundred degrees Celsius, oxidative wear of a metal becomesmore intense.

The time necessary for the development of the wear protective,,

particles and the thickness of the oxide film are dramatically increased. This is associated with the increase in oxidation rate at highertemperatures.

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Usually the fractured fine metallic debris which remains on the wornsurface is oxidized and compacted into a glaze. As the glaze spreadsover the worn surface the wear process becomes mild.

A practical example of this form of wear is found in gas turbinecomponents where thermal cycling causes slow periodic movementsbetween contacting surfaces.

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Oxidative Wear at Low Temperature

Applications

Oxidative wear persists even at very low temperature applications, e.g.Rocket turbo pumps contain rolling bearings which operate underimmersion in liquid oxygen or liquefied hydrocarbon gases.

A comparative laboratory wear tests of ball bearings in liquid oxygen.

The examination of worn bearings from space shuttle turbo pumpsindicated that excessive damage was due to adhesive wear/shearpeeling following the breakdown of the oxide scale formed on the ballsand rings.

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Transition Between Oxidative and Adhesive

Wear

Sharp transitions, referred to as T1 and T2, between oxidative (mild)and adhesive (severe) wear are observed in metal-to-metal dry slidingcontacts. The transition loads depend on the material properties of the

sliding surfaces and their relative velocity.

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Means of Controlling Corrosive and OxidativeWear

Corrosion inhibitors based on strong adsorption to the corrodingsurface can be effective in controlling corrosive wear, however, they may interfere with the adsorption lubrication of the sliding contact or

else with an adjacent contact lubricated by the same oil or processfluid. Other types of inhibitors, e.g. those which involve the formation of apassive layer, are usually ineffective.