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Int. J. Mech. Eng. & Rob. Res. 2014 Aadarsh Mishra, 2014

REDUCTION OF SLIDING WEAR OF ALLOYS BYUSING OXIDES

Aadarsh Mishra1*

*Corresponding Author: Aadarsh Mishra,aadarshm9@gmail.com

Formation of oxide has a significant effect on friction and wear of high temperature alloys duringreciprocating sliding in air. A smooth wear protective oxide layer is developed on the load bearingsurface in these alloys above a certain temperature of 150-250 ºC. However at lower temperaturethe metal-metal contact is reduced due to the oxide debris which in turn reduces the friction andwear rate. Due to transient oxidation of metal surface, removal of such oxide and then reoxidationof exposed metal, the oxide debris is formed. Oxide debris develops a wear protective layerbetween the sliding surfaces. As limited asperity growth occurs before the asperity becomessufficiently large hence the friction during sliding is less. This research paper covers the relationbetween the sliding wear of alloys and the effect of oxides on it.

Keywords: Oxidative wear, Friction, Wear-protective oxides

INTRODUCTIONDue to formation of oxides on metal surfaceswear and friction is reduced. Therefore suchoxides play a significant role reducing thefriction and wear. For instance their applicationcan be in gas turbine engines. For many alloysthere is a well-defined temperature abovewhich the wear oxide protective layers areeffective for reducing the friction and wear.These layers can be deformed plasticallyduring sliding and they also produce a verysmooth surface. The examples include NiOand Cr2O3. In the temperature range of 560-670º iron base alloys have more application

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engineering, Manipal Institute of Technology, Manipal University, Manipal, Karnataka 576104, India.

than cobalt and nickel base alloys.In this paperemphasis has been laid on the role of theseoxides in the wear process.

EXPERIMENTAL METHODSThe wear apparatus consists of a flat discwhich slides in the horizontal motion againstthe spherical specimen under a load of 1.2 kgat a speed of 350 revolutions per minute. Thespecimens were brought in contact and theywere brought to approximately sametemperature in about 10 minutes. When therewas a thermal equilibrium between them thesliding was started. The coefficients of friction

Research Paper

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were recorded continuously during sliding. Thecoefficient of friction and wear rates wererelatively similar for the given alloys. Wearrates of these oxides comes less than 3*10-7

mm3/s. The presence of oxide on the loadbearing regions is also reduced. With respectto time and temperature the wear ratedecreases. The coefficient of friction alsochanges with respect to time. For theproduction of oxides there are two types ofprocesses. In the first one transient oxidationoccurs in the apparent areas of sliding contactand then removal of that oxide occurs. In thesecond mechanism metal debris formationoccurs in the early stages of sliding due towhich fresh areas of metals are exposed tooxidation. Due to oxidation of debris aconsiderable quantity of oxides are produced.Above transition temperature compacteddebris is produced between the slidingsurfaces due to which the wear protectiveoxides are formed. The oxidation mechanismis not significant at lower temperature, i.e., theoxidation is less rapid. The main effect of it isthat the comminution of metal debris occursbefore any significant oxides are produced.Moreover the variety of layers formed can alsobe called Inorganic materials.

First of all we should consider the effect thatthe temperature has on the wear protectiveoxide layer particularly at temperatures above250 ºC. Oxide debris produced at lowertemperature reduces the metal-metal contact.The wear oxide protective layer has not beenfully developed. With development of compactand adherent wear protective oxide layer thesliding time always decreases with increasein temperature. One of the main reasons canbe that at higher temperatures the oxidation

rates are faster. Below transition temperatureno oxide layers are developed. Theexperimental evidence has also concludedthat transition temperature is not thattemperature below which no wear protectiveoxide layers are formed but it can be definedas the temperature below which the wearprotective oxide layer is unstable.

Development of wear protective oxidesurface takes place due to steady oxide statewhich takes place during normal sliding. Thismay not take place at room temperature as atroom temperature the layer may be brokendown and it may happen due to sliding action.Thus we can say that the formation of wearprotective oxide layer may not require anambient/ room temperature but we may getan additional benefit by increasing thetemperature. Low ambient temperature alwayscauses breaking of oxide during sliding.

The mechanical properties of oxide shouldchange with temperature due to which thecoefficient of friction will increase and therebythe ambient temperature will decrease duringsliding which takes place between the wearprotective oxide layers. On decreasing thetemperature shearing forces at slidinginterface will increase causing the breaking ofwear protective oxide layer. Oxides are moreadherent at higher temperature and lessadherent at lower temperatures during sliding.If Ni-Cr alloys are taken then the nickel oxidescale on nickel to form wear protective oxidelayer at room temperature during sliding. Dueto friction and rising of surface flashtemperature there is an increase in the vicinityof contacting oxide asperities which alsocauses the development of wear protectiveoxide layers. However experimentally we can

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say that there is a rise in mean surface flashtemperature during sliding.

The coefficient of friction is relativelyinsensitive to a small rise in ambienttemperature. A wear protective layer is formedat low speed and hence it does not dependon the temperature.

The wear protected oxide layer consists ofcompacted debris. The surface oxide ispostulated on this compacted debris whichproduces a smooth and shiny appearance.Generally the oxides are regarded as brittledue to the properties which they possess inthe normal tensile strength. Minute cracks orflaws develop in the brittle material undernormal tensile condition. These minute cracksact as stress enhancer and cause increase inthe effective tensile strength. Due to theincrease in effective tensile strength a largehydrostatic pressure is produced which in turnpromotes the healing of cracks or flaws.

RESULTS AND DISCUSSIONDuring sliding it is observed that plasticdeformation of oxide and the development ofwear protective oxide layer occurwhentheoxide-oxide asperity contacts aresufficiently small for the hydrostatic pressuredeveloped which causes effective preventionof crack propagation. The hydrostaticpressure has a significant effect on thesurface deformation while the frictionalbehavior depends on the plastic properties.Smooth and shiny surface is forms due to theplastic deformation of oxide asperities. Thenthe formation of adhesive contacts occursbetween the oxide asperities. This can beregarded as one the main reasons for lowcoefficient of friction and wear rate. The

increase in size of asperity junction occurs dueto junction growth which in turn takes placeduring sliding. The main effect of it can involvereduction in contact pressure which in turn canlead to decrease in hydrostatic pressurethereby preventing the brittle failure. Howeverit is very difficult to find the particular point atwhich junction growth terminates. Oxide andoxide coated metal debris is formed duringsliding and the mechanism is known as metaldebris mechanism. If adhesive and cohesiveforces are greater than the frictional forces thenthe substrate alloy remains in a wear track. Theadhesive and cohesive forces areresponsible for binding the debris togetherwhile the frictional forces tend to remove thedebris from the metal surface. Cracking andfragmentation may occur at the junctionswhich will cause the fragmentation of debrisdue to which smaller debris particles areformed. Compaction takes place due to thegeneration of smaller debris particles. Plasticflow of oxide occurs without cracking due toreduction in compacted debris. With respectto the plastic deformation, the thickness ofwear protective oxide is consistent. This wearprotective oxide layer is formed from manyindividual oxide particles. The surface oxidealso originates from many individual oxideparticles. Thus we can say that the thicknessof agglomerated oxide surface layer is thedepth to which the plastic flow has occurred.Since the number of contacting junctions foroxide-oxide sliding is high the load supportedby any one of junctions is quite small. Thusthe rise in surface and flash temperature isalso expected to be much low.

Surface rise in metal-metal contact =Temperature rise in oxide-oxide contacts.

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Oxide layer is a contact of several speciesand on the degree to which it has beencompacted, its ability to absorb and conductheat depends. It is possible to speculate aboutthe material properties if a wear protectiveoxide is deformed plastically and itstribological characteristics are known. It is notpossible to control the nature of wear oxidesformed but we can adjust the oxidationconditions which will favor the growth ofrelatively more ductile oxides. The formationof a wear-protective surface layer for anyparticular oxide occurs more at higher ambienttemperature. Moreover effective lubricants areformed when the surface coatings are mixturesformed from relatively soft oxide like PbO, BaOand CaO. These effective lubricants exhibit lowfriction and wear over a wide range oftemperature. By adding oxides to lubricatinggreases wear rate is directly proportional tothe oxide hardness.The work of Gupta (1974)suggests that two-phase ceramics should betougher and more crack-resistantthan single-phase systems, the presence of particles of aharder phase causinga significant increase inthe energy required for crack growth. Thus, ifawear-protective oxide is to be both tough (toresist surface and subsurfacecracking) and yetamenable to plastic flow, a two-phase systemmay bemore effective than a single-phasesystem. An effective oxide layer is formed dueto a low coefficient of friction as the tensilestress of the asperity junctions’ increase withincreasing the friction force. Brittle fracture andan increased wear rate would be expected ifthe coefficient of friction increasedappreciably. It has been observed thatthecoefficient of friction for sliding oxide surfacesincreases with decrease in temperature (Stottet al., xxxx).

Since the friction is an intrinsic, not anextrinsicvariable therefore coefficient offriction should be low for a wear protectiveoxide layer. Oxide debris should be relativelycompact for rapid formation of wear protectiveoxide layer. This may lead to flat and smoothoxide surface and layer. The contacting oxideasperity junctions should fail fairlyeasily duringsliding for friction and wear between suchsurfacesto remain low. High friction andsevere wear results when the surfaces are veryplastic and form very good adhesive bondsatthe contacting asperities. Therefore the mosteffective oxides have minimum degree ofplasticity.

In other words, Oxide that deformssufficiently to develop the smooth surfaceforms the most effective wear protected oxidelayer as to maintain the low friction and wearthe resulting oxide-oxide contacts break.

CONCLUSIONSmooth and adherent wear-protectiveoxidelayers, resulting in relatively low coefficients offriction and wear rates. Due to transientoxidation and removal of oxides wearprotective oxide layers are formed. Since thesurface is relatively small, oxide debriscompacts to form a layer. Moreover due tolarge hydrostatic pressure healing of cracksin oxide particles takes place during slidingand the junction growth of oxide asperities isrelatively very small.

ACKNOWLEDGMENTI would also like to dedicate this research workto my father late R S Mishra and mother K LMishra.

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REFERNCES1. Glascott J (1982), “The Tribological

Behaviour of the Fe-12Cr Alloys JetheteM152 and Rex 535 from Room-Temperature to 600 °C, Ph.D. Thesis,University of Manchester.

2. Gupta T K (1974), “A Qualitative Modelfor the Development of Tough Ceramics”,Journal of Materials Science, Vol. 9,pp. 1585-1589.

4. Stott F H and Wood G C (1978), “TheInfluence of Oxides on the Friction andWear Alloys”, Tribology International,Vol. 11, No. 4, pp. 211-218.

5. Stott F H, Glascott J and Wood G C(1985), “The Effectiveness of Oxides inReducing Sliding Wear of Alloys”, J. Phys.D: Appl. Phys., Vol. 18, p. 541.