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Tribology International 43 (2010) 1203–1208

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

E-m

journal homepage: www.elsevier.com/locate/triboint

Elevated-temperature tribology of metallic materials

Peter J. Blau

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6063, USA

a r t i c l e i n f o

Article history:

Received 18 May 2009

Received in revised form

22 December 2009

Accepted 13 January 2010Available online 20 January 2010

Keywords:

Running-in

Friction transitions

Wear transitions

Internal combustion engine

9X/$ - see front matter & 2010 Elsevier Ltd. A

016/j.triboint.2010.01.003

ail address: pvb@ornl.gov

a b s t r a c t

The wear of metals and alloys takes place in many forms, and the type of wear that dominates in each

instance is influenced by the mechanics of contact, material properties, the interfacial temperature, and

the surrounding environment. The control of elevated-temperature friction and wear is important for

applications like internal combustion engines, aerospace propulsion systems, and metalworking

equipment. The progression of interacting, often synergistic processes produces surface deformation,

subsurface damage accumulation, the formation of tribo-layers, and the creation of free particles.

Reaction products, particularly oxides, play a primary role in debris formation and microstructural

evolution. Chemical reactions are known to be influenced by the energetic state of the exposed surfaces,

and that surface energy is in turn affected by localized deformation and fracture. At relatively low

temperatures, work-hardening can occur beneath tribo-contacts, but exposure to high temperatures

can modify the resultant defect density and grain structure to affect the mechanisms of re-oxidation. As

research by others has shown, the rate of wear at elevated temperatures can either be enhanced or

reduced, depending on contact conditions and nature of oxide layer formation. Furthermore,

the thermodynamic driving force for certain chemical reactions, the kinetics of those reactions, and

the microstructure can all affect the response. The role of deformation, oxidation, and tribo-corrosion in

the elevated-temperature tribology of metallic alloys will be exemplified by three examples involving

sliding wear, single-point abrasion, and repetitive impact plus slip.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The elevated-temperature friction and wear behavior of metalsand alloys is an important consideration in the effectiveperformance of certain moving parts in internal combustionengines, bearings in aerospace propulsion systems, cutting tools,and metalworking processes. Sometimes the interfacial tempera-ture derives from external sources of heat, and at other times, asin vehicle brakes, the temperature results from frictional contact.At high temperatures, changes occur in bulk mechanical proper-ties, bulk thermo-physical properties, and surface reactivity.Since there are few viable liquid or solid lubricants that workwell at temperatures upward of 500 1C, a number of elevated-temperature applications for contacting metals depend on theability of the bearing surfaces to self-lubricate based on reactionswith their environments and their ability to form protectiveglazes (tribo-layers) during the contact process [1–3]. Thisdiscussion therefore includes three aspects of elevated-tempera-ture tribology: (a) changes in bulk properties, (b) changes inreactivity, and (c) changes in tribo-layer forming tendency.Furthermore, as will be shown, differences in the dominant type

ll rights reserved.

of wear affect the structure of surfaces that form during repetitivecontact at elevated temperatures.

1.1. Effect of temperature on bulk properties

As the temperature rises, the properties of the load-bearingmaterials and any interposed substances can change. As shown inFig. 1 from collected data, the yield strength of metals and alloysusually tends to decrease at elevated temperatures. One notableexception is an alloy based on the intermetallic compound Ni3Al,the wear characteristics of which have been described elsewhere(e.g., [4–6]). Due to the cross-slip characteristics of the (L12)crystal structure of the ordered g0 phase, some variants of nickelaluminide alloys increase in yield strength up to about 700 1Cbefore it declines. Furthermore, as Fig. 1 shows, the processing ofthe alloy (e.g., cast versus powder-processed) affects its strengthversus temperature behavior. In general, however, based on thedecline in strength of most metals with temperature, one wouldexpect a reduced resistance to deformation and abrasion astemperature increases.

The modulus of elasticity also changes as a function oftemperature and that affects the elastic behavior of metals. Forexample, Fig. 2 illustrates the calculated maximum elastic contactstress (Hertz stress [7]) for a cylinder 8 mm long and 9.53 mm in

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Fig. 1. Effects of elevated temperature on the yield strength of various alloys

(collected data).

Fig. 2. Decrease in elastic contact stress as a consequence of the effects of

temperature on the elastic modulus for a self-mated Ni-based alloy in a cylinder-

on-flat geometry.

P.J. Blau / Tribology International 43 (2010) 1203–12081204

diameter loaded against a flat plane as a function of temperature.Both cylinder and plane materials were assumed to be composedof a Ni-based exhaust valve alloy called Pyromet 31VTM, whosetemperature-dependent mechanical properties were availablefrom the producer [8]. As the elastic modulus declines with

temperature from ambient to approximately 800 1C, thecalculated maximum contact stress decreases by about 14%.

Thermo-physical properties of substances, such as thecoefficient of thermal expansion (CTE), also change withtemperature. Therefore, the tendency of oxide scales to adhereto the metals on which they grow as temperature changesdepends not only on the specific volume difference betweenmetals and their oxides, but also upon the similarity of their CTEsto those of the substrates upon which they grow. Other factorsinclude the presence of brittle phases that form on and within thesubstrate due to depletion of alloying elements by surfacesegregation or inward diffusion of oxygen from the surface.Therefore, the mechanical properties of metals and their wearresponses are tied to oxidation, sulfidation, and other processesthat alter the chemistry of the region of tribo-contact.

1.2. Effect of temperature on thermodynamics and reaction kinetics

The change in Gibbs free energy (G) is a measure of the phasestability of a certain compound at constant pressure.

DG¼ Gproducts�Greactants ¼�RT ln K ð1Þ

Here, R=the gas constant, T=the absolute temperature, and K isthe ratio of the partial pressures of the products to the partialpressures of the reactants (for more details, see Ref. [9]). Thelower the DG for a certain reaction, the higher its thermodynamicdriving force, and presumably, the more stable is the reactionproduct.

An Ellingham diagram is one way to compare Gibbs freeenergies of formation for various oxidation reactions of metals asa function of temperature [10]. A simplified example of one suchdiagram is shown in Fig. 3. Based on DG considerations, theformation of alumina (Al2O3) is highly stable relative to a numberof other oxidation reactions. For example, chromia (Cr2O3) wouldbe most likely of the possible oxides to form on Fe–Ni–Cr alloys,like stainless steels, because its driving force is greater than thatfor iron oxide or nickel oxide.

Ellingham diagrams are helpful in understanding oxidationtendencies, and are valuable in extractive metallurgy, notablythe steelmaking process. However, they are only a part of thepicture in understanding the nature of tribo-surfaces at hightemperatures. Other factors include microstructural effects andthe kinetics of oxide growth. For example, oxidation kinetics ofexposed clean metals have been observed to follow a few basicrelationships in terms of oxide thickness (x),time (t), and withvarious rate constants (e.g., kx, A, to) [11]:

Linear : x¼ klt ð2Þ

Parabolic : x¼ ð2 kptÞ1=2ð3Þ

Logarithmic : x¼ k log10ðtþtoÞþA ð4Þ

As oxidation progresses, the relationship for the oxidationkinetics can change as well. Chemical reaction rates tend toincrease when temperature increases, but they are also dependenton other factors like the surface area, material microstructure, andthe surrounding environment. As products form on exposedsurfaces, they deplete the reactants in the near-surface zones.Oxygen must either diffuse into the surfaces or reactants mustdiffuse out of the scales to enable the reaction to continue. Defectstructures and grain boundaries can act as short-circuit paths tofacilitate the diffusion of reactants. However, such short-circuitpaths become less important as temperatures rise above approxi-mately two-thirds of the alloy’s melting point. The result of theseinteractive phenomena in multi-component alloys is complex,multi-phase microstructures.

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Table 1Observations of the oxidation of nickel aluminide alloy at various temperatures

[11].

Temperature

range (1C)

Microstructural condition

450–800 Overall, the NiO outer layer grows by outward diffusion of

Ni

500–600 Mixed layer [NiO, Ni2Al2O4 and Al2O3] forms below the NiO

outer layer

700–800 Inner layer is Al2O3 with some metallic Ni. Cr additions of

8 wt% can add Cr2O3 to the NiO outer layer and also reduces

inward O diffusion

4900 Al2O3 inner layer, NiAl2O4 (spinel) outer layer

Fig. 3. Simplified Ellingham diagram indicating the relative free energies for

various metal oxidation reactions.

P.J. Blau / Tribology International 43 (2010) 1203–1208 1205

Consider, for example, the formation of oxidation products incertain nickel aluminide alloys. Table 1 indicates the variousphases that have been observed to form in the near-surfaceregions during exposure of a Ni3Al-based alloy to oxidizingconditions at different temperature ranges [12]. At comparableexposure times, they range from predominantly NiO topredominantly Al2O3, depending on the temperature, but micro-structures reveal that the near-surface layers are dominated byreaction products of various types.

As oxides form, their adhesion to the substrate becomesimportant. The Pilling-Bedworth Ratio (PBR) depicts thedifference between the volume per metal ion in an oxideand the volume per atom in the related metal (see Ref. [11],pp. 118–119). If there is a large volume difference, stresses canbuild up in the oxide scales, leading to cracks, a loss of adhesion,and increased propensity for scales to be removed by mechanicalcontact. When the PBR exceeds 1, there is expected to be a

compressive stress in the oxide, and when it is less than 1, atensile stress. For example, the PBR for aluminum oxide onaluminum is 1.28, and than that for Fe3O4 on a-Fe is 2.10. Astemperatures increase or decrease, differences in thermal expan-sion for the scales and substrates can also cause scales to detach.

When one adds the complications of different kinds of mechan-ical interactions, such as repetitive impact, abrasion, or sliding, to thestatic evolution of oxide scales on metals, the picture becomes morecomplicated. At the same time as the various processes oftribochemistry are taking place, material is being displaced orremoved, and new near-surface defects are being created.

1.3. Tribo-layer formation

In the context of this paper, a tribo-layer is any distinctmaterial that forms in an interface as a direct result of mechanicalcontact. For example, it could be a highly-deformed layer at thenear surface of a metal subjected to tangential shears duringsliding; it could be a mechanically-mixed layer of metal and oxidethat results from fretting or repetitive impact; or it could be adeposit of accumulated wear particles that become trappedwithin an interface. Applications like car and truck brakes dependon the formation of tribo-layers on contact surfaces to control andstabilize friction. Mechanically-mixed layers can form duringsliding and, as Fu et al. [13] have shown by elegant computermodeling, the contacting solids can mix in a turbulent manner,producing stringers and dissociated islands of one materialsurrounded by another of different composition.

Stott, Wood, and their colleagues have investigated theinteraction between sliding wear and high-temperature oxidationin superalloys [1–3]. They described the role of tribo-layers called‘glazes’ that form on sliding surfaces during frictional contact.Glazes can temporarily protect the surfaces from further contactdamage. If they happen to wear off, new glazes can be formed totake their place. This progression of formation, loss, and reforma-tion can result in short-term friction or wear transients. Thetemperature experienced in sliding interfaces is the sum of thetemperature of the surroundings plus that generated by frictionalcontact. Therefore, the response of metals and alloys to elevatedtemperature sliding needs to consider these two contributions, aswell as the effects they may have on the tribological evolution ofglazes. For example, substrates that are locally softened bythe superimposed effects of environmental temperature andfrictional heating, can more easily mix into glazes to createmarble-cake-like structures with swirled, alternating layers ofoxide and metal.

It is also worthwhile to consider how different forms ofwear – sliding, abrasive, and impact for example – affect the tribo-corrosion behavior of metals at elevated temperatures.

2. Effects of elevated temperature on different types of wear

Three examples of elevated-temperature tribology of metalsare provided. The first one involves the sliding wear of nickel-aluminum alloys, the second exemplifies the reformation ofoxides after single-point abrasion damage, and the third describesthe formation of tribo-layers due to repetitive impact inconjunction with minor slip.

2.1. Sliding friction and wear of nickel aluminide (Ni3Al) against

alumina

As Fig. 1 indicates, certain nickel aluminide (Ni3Al)-based alloysexhibit increasing yield strength above room temperature until

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P.J. Blau / Tribology International 43 (2010) 1203–12081206

they reach relatively high temperatures. Likewise, they have ratherinteresting elevated-temperature wear behavior as well (e.g., [4–6]).In a series of pin on disk tests using an alumina ceramic sphere as aslider, it was shown that the nickel aluminide wear rate decreasedby nearly a thousand times at 650 1C compared to room temperature(see Fig. 4). Interestingly, the wear rate of the counterface (pin) rosesignificantly over the same temperature range. At roomtemperature, the disk wore at a normalized rate of about 250times that of the pin, but at the high temperature, the pin woreabout 9 times faster than the disk. The measured kinetic frictioncoefficient (m) for this sliding couple in air remained atapproximately 0.62 irrespective of temperature. This suggests thatthe frictional work was about the same at all test temperatures, butthe dissipated energy in the system was partitioned differently as afunction of temperature. It is interesting that despite the fact thatthe ceramic wore more, any effects of ceramic wear debris were notobserved. That is, the nickel aluminide disks continued to wear lessas temperature increased despite the increase in pin wear debris.

The reason why disk wear decreased at 650 1C can be associatedwith the increased yield strength of the nickel aluminide alloy athigh temperature. Cross-slip becomes easier in the ordered g0 phase

-4.0

-4.5

-5.0

-5.5

-6.0

-6.5

-7.0

-7.5

-8.0

log

WE

AR

RAT

E (m

m3 /

N-m

)

0 100 200 300 400 500 600 700T (°C)

1.0 N, 0.1 m/s, 10000 cyc

ALUMINA PINIC 74M DISK

Fig. 4. Log of the linearized wear rate of an alumina sphere (pin) sliding on a disk

of Ni3Al alloy IC-74M as a function of temperature in air. The relative rate of wear

reversed as temperature increases.

Table 2Effects of scratching on oxide Re-formation on Fe-, Ni-, and Co-based alloys.a

Concentration of the element within the re-oxidized scratch groove

Element Fe alloy (composition in wt%: 75 Fe, 10.9 Ni,

11.4 Cr, 1.61 Ti, 0.9 Mo, with minor Co, Mn,

Cu, Al, C, N, S, P, O)

Ni alloy (compo

20.2 Cr, 2.5 Ti,

Cu, C, N, S, P, O

O Significantly enriched Somewhat enri

Cr Mottled deposit: significantly enriched at

some places, but depleted at others

Significantly en

Ni Significantly depleted Significantly de

Fe Mottled deposit: significantly enriched at

some places, but depleted at others

Slightly deplete

Al c Thin areas of en

bottom, but sig

scales outside o

Ti Mottled deposit: slightly enriched at some

places, but depleted at others

Significantly en

a The Fe-based alloy was a stainless steel (Custom 465TM), the Ni-based alloy wasb Energy dispersive X-ray mapping of ka radiation on a tapered cross-section of thc Not detected. Not a major alloying element in this material.

as temperature increases, leading to a greater potential for work-hardening during deformation. It is well-known that polycrystallineceramics like alumina and silicon nitride wear more as temperatureincreases above room temperature [14,15]. Temperatures above100–200 1C tend to drive off moisture that lubricates surfaces andhelp to blunt cracks. The wear rate of ceramics like alumina andsilicon nitride tends to maximize around 400 1C. At much highertemperatures (say 4700–800 1C), the wear rate improves becauseglassy layers and debris deposits tend to cushion and protectsurfaces. Therefore, the wear of nickel aluminide improves while thewear of the alumina degrades. A third factor in explaining thisbehavior concerns the difference in oxidation products that form onnickel aluminide at different temperatures (see Table 1). Inconclusion, no single factor explains the behavior in Fig. 3, but thetribosystem and both members of the sliding pair must beconsidered to understand it.

2.2. Effects of single-point scratch damage on the reformation of

oxide layers on a nickel-based alloy

Sliding wear tends to shear surface materials off rather thangenerate third-body tribo-layers by smearing and mechanicalmixing. Abrasion is a cutting process that exposes the underlyingmaterial to the environment rather than cover it up with a third-body layer. Therefore, the effects of tribo-oxidation may bedifferent for abrasive damage. This effect can be illustrated bysome recent work in which alloys based on Fe, Ni, and Co werefirst oxidized in air at 850 1C for two hours, then scratched and re-oxidized for four more hours in air at the same temperature. Aftertaper-polishing through the scratched areas, energy dispersiveX-ray analysis revealed differences in the elemental distributionon the 6-hour oxidized surfaces and on the scratches that wereproduced and then re-oxidized. Additional results of that studyhave been published elsewhere [16,17], but the key findings arequalitatively summarized in Table 2.

Table 2 data indicate that the re-oxidation of single-pointabrasion damage produces selective enrichment of certainelements in mechanically-damaged areas while other elementsare depleted. In particular, note that the elemental Cr composi-tions in the Fe-, Ni-, and Co-based alloys have a relative ratio ofabout 1:2:3. Normally, the higher the Cr content, the better theoxidation resistance it provides to the alloy [16,17]; however,since some of the Cr in the Co-based alloy is tied up in the bulkalloy as chrome carbides, it does not afford quite as much

relative to that of the oxide that grew on the non-scratched areab

sition in wt%: 0.96 Fe, 75 Ni,

1.4 Al, with minor Mo, Si, Mn,

)

Co alloy (composition in wt%: 57.5 Co, 2.84

Fe, 2.7 Ni, 29.6 Cr, 4.27 W, 0.58 Si, 1.45 Mn,

1.4 Al, 1.0 C, with minor Mo, S, P, O)

ched Somewhat enriched

riched Little difference in elemental distribution

on or off the scratch

pleted c

d Slightly depleted

richment near the scratch

nificantly enriched on the

f the scratch

c

riched c

Pyromet 80ATM, and the Co-based alloys was Stellite 6BTM.

e scratch grooves.

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P.J. Blau / Tribology International 43 (2010) 1203–1208 1207

protection as might be expected if it were free to migrate to thesurface and form a continuous chromia scale.

2.3. Effects of repetitive impact plus slip on the development of

tribo-layers on a nickel-based alloy

A high-temperature, repetitive impact testing apparatus(HTRI) was constructed to study the effects of impact, slip,oxidation, and temperature on candidate diesel engine exhaustvalve alloys. The construction and features of the HTRI arediscussed elsewhere [18], but the basic concept involves avertically oscillating shaft on which are fixed two cylinderswhose axes are inclined at 45 degrees to the direction ofoscillation. These cylinders repetitively contact a pair of blockswith rounded corners. This produces in-effect a crossed-cylinderscontact geometry (see Fig. 5). As the cylinders contact the blocksand come to rest under the applied normal force (typically20–30 N), they slip a few tens of micrometers, imparting acombination of crushing and shear to the tribo-layers that form

Fig. 5. Cylinders-on-rounded blocks HTRI test geometry. D=cylinder diameter

(9.53 mm typ.) and r=block corner radius (3 mm typ.). The cylinders are tilted at

45 degrees to the shaft axis.

Fig. 6. Tribo-layers formed on a Ni-alloy block after 20,000 impacts from a Co-

alloy cylinder at 800 1C. A Knoop indentation was made in the layer to indicate the

manner in which cracks propagate in the heterogeneous tribo-layer after the block

cooled to room temperature. (back scattered electron image at 10 kV accelerating

voltage).

during elevated temperature testing. This results in the formationof a heterogeneous brittle layer.

Fig. 6 exemplifies a wear scar for the case of Co-based alloyStellite 6BTM cylinders impacting upon the corners of blocks ofNi-based Pyromet 80ATM. After 20,000 impacts, a tenacious,compacted tribo-layer had formed on the blocks. A 300 g-f Knoopindentation was placed on the cooled wear scar to show howcracks can propagate along the boundaries of swirls in themechanically-mixed layer (Fig. 6). In the image, using energydispersive X-ray spectroscopy, the lighter-appearing areas areNi- and O-rich with some minor Cr. The darker gray swirls arenearly free of Ni, but as X-ray compositional mapping indicates(Fig. 7), they are enriched in Cr. Note in particular that the edgesof the compacted layers were enriched in Cr, and this suggeststhat the initially-formed chromium oxide was gradually pushedto the edges of the contact region. The material that formed laterwas depleted in Cr relative to the surrounding oxide, beyond theedges of the scar. A cross-section of the wear surface from asimilar experiment on Pyromet 80ATM but impacted by a Pyromet

Fig. 7. Chromium enrichment occurred near the edges of the tribo-layer generated

on the block specimen indicating a gradual displacement of the initial oxide

deposit by Cr-depleted debris. The field of view is 843mm (horizontal). (Cr karadiation, 10 kV.)

Fig. 8. Polished cross-section of a mixed layer of oxide and metal at the surface of

a Pyromet 80A block specimen. The surface was repetitively impacted (with slip)

by a Pyromet 31V exhaust valve for 20,000 cycles at 800 1C (in air). Interpenetrat-

ing tongues of deformed metal and oxides can be observed, particularly toward

the left of the field of view.

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P.J. Blau / Tribology International 43 (2010) 1203–12081208

31V diesel engine exhaust valve instead, is shown in Fig. 8. Itdisplays oxides of various shading and interpenetrating materialinclined in the direction of relative slip.

As indicated by the foregoing examples, the factors thatcontribute to the elevated-temperature tribological behavior ofmetals and alloys vary depending on the type of wear involved,the material composition, and the nature of prior surfaceexposure. In light of the variability in temperatures andenvironments experienced by engines and other kinds ofmachinery in the normal course of operation, the field ofelevated-temperature tribology pushes the limits of interdisci-plinary understanding. There remains much to explore in thischallenging area because the occurrence of transient tribologi-cal phenomena seems to be more the rule than the exception insuch practical systems.

3. Summary

Understanding the elevated-temperature tribology of metalsunder oxidizing environments involves an interdisciplinaryapproach. Non-steady-state, time-dependent processes can affectthe progression of surface damage. The following constitute themajor points of this review:

(1)

As temperature increases for metals in tribo-contact in air,changes in their mechanical properties, coupled with the roleof oxidation can change the partitioning of frictional workinto wear and surface damage. The same magnitude of frictionforce can result in different manifestations of wear dependingon the temperature.

(2)

The formation of oxidation products is affected not only bythe thermodynamic driving force, but by the kinetics ofreaction, the diffusion of reactants to and from the surface,and the evolution of species below the initially-formedsurface scales.

(3)

At least three factors affect the response of metallic interfacesat elevated temperatures: (a) the mechanical properties of thereaction products on the surface, (b) the tendency to formstable tribo-layers, and (c) the resistance of the bulk metalsbelow the oxides and tribo-layers to deformation and fracture.

(4)

Wear scars generated by experiments with single pointabrasion, sliding contact, and repetitive impact with slip allsuggest that the species that form on mechanically-damagedsurfaces are different in composition from those that form inthe absence of mechanical contact, but the geometry contactcan also affect their elemental distribution.

Acknowledgements

A portion of this work was sponsored by the US Department ofEnergy, Office of Vehicle Technologies, under contract DE-AC05-00OR22725 with UT-Battelle LLC, Oak Ridge, Tennessee.

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