NASA/TM--2001-210944

l

The, rmal Processing Effects on the Adhesive

Strength of PS304 High Temperature

Solid Lubricant Coatings

Christopher DellaCorte and Brian J. Edmonds

Glenn Research Center, Cleveland, Ohio

Patricia A. Benoy

St. Louis University, St. Louis, Missouri

May 2001

https://ntrs.nasa.gov/search.jsp?R=20010069194 2018-05-14T14:47:52+00:00Z
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NASA/TM--2001-210944

Thermal Processing Effects on the Adhesive

Strength of PS304 High Temperature

Solid Lubricant Coatings

Christopher DellaCorte and Brian J. Edmonds

Glenn Research Center, Cleveland, Ohio

Patricia A. Benoy

St. Louis University, St. Louis, Missouri

National Aeronautics and

Space Administration

Glenn Research Center

May 2001
This report contains preliminary

findings, subject to revision as

analysis proceeds.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov/GLTRS
THERMAL PROCESSING EFFECTS ON THE ADHESIVE STRENGTH

OF PS304 HIGH TEMPERATURE SOLID LUBRICANT COATINGS

Christopher DellaCorte and Brian J. Edmonds

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Patricia A. Benoy

St. Louis University

St. Louis, Missouri 63103

SUMMARY

In this paper the effects of post deposition heat treatments on the cohesive and adhesive strength properties of PS304, a plasma sprayed nickel-chrome based, high temperature solid lubricant coating deposited on

stainless steel, are studied. Plasma spray deposited coating samples were exposed in air at temperatures from

432 to 650 °C for up to 500 hr to promote residual stress relief, enhance particle to particle bonding and increase coating to substrate bond strength. Coating pull off strength was measured using a commercial adhesion

tester that utilizes 13 mm diameter aluminum pull studs attached to the coating surface with epoxy. Pull off

force was automatically recorded and converted to coating pull off strength. As deposited coating samples

were also tested as a baseline.

The as-deposited (untreated) samples either delaminated at the coating-substrate interface or failed internally (cohesive failure) at about 17 MPa. Samples heat treated at temperatures above 540 °C for 100 hr or at

600 °C or above for more than 24 hr exhibited strengths above 31 MPa, nearly a two fold increase. Coating

failure occurred inside the body of the coating (cohesive failure) for nearly all of the heat-treated samples and

only occasionally at the coating substrate interface (adhesive failure). Metallographic analyses of heat-treated

coatings indicate that the Nickel-Chromium binder in the PS304 appears to have segregated into two phases,

a high nickel matrix phase and a high chromium precipitated phase. Analysis of the precipitates indicates the

presence of silicon, a constituent of a flow enhancing additive in the commercial NiCr powder. The exact nature and structure of the precipitate phase is not known. This mierostructural change is believed to be partially

responsible for the coating strength increase. Diffusion bonding between particles may also be playing a role.

Increasing the heat treatment temperature, exposure time or both accelerate the heat treatment process. Preliminary measurements indicate that the heat treatment also results in a one time, permanent coating thickness increase of about 3 percent. Based upon these results, the incorporation of a heat treatment prior to final

finishing has been incorporated in the application process of this coating technology.

INTRODUCTION

PS304 is a plasma sprayed, high temperature solid lubricant coating developed to reduce friction and wear

of sliding contacts operating from below room temperature to over 750 °C (refs. 1 to 4). PS304 is comprised of

a nickel-chromium (NiCr) matrix (60 wt%) combined with chrome oxide (20 wt%) as a hardening phase and

the solid lubricant additives silver (10 wt%) and barium fluoride/calcium fluoride eutectic (10 wt%). For the

PS304 composite coating, each constituent contributes functionality. For example, silver, a soft metal, provides low temperature lubrication while the fluorides provide high temperature lubrication. The chrome oxide

acts as a thermochemically stable hardening phase. The NiCr matrix bonds the components to each other and

the substrate. Since NiCr bonds well to candidate stainless steel and superalloy substrates the PS304 coating is

routinely applied directly to the surface of parts needing lubrication without the use of intermediate bond coat

layers. Figure 1 shows a typical cross-section photomicrograph of PS304.

Extensive development work has been conducted and reported on this composite coating. This work

includes compositional tailoring for thermal expansion coefficient control (ref. 4), friction and wear testing

(refs. 1 and 2) and coating performance as a start/stop shaft lubricant for foil air bearings (refs. 5 and 6). During

the course of these studies it was observed that the coating undergoes a one-time dimensional change (coating

thickness increase of a few percent) accompanied by an apparent strength (hardness) increase when exposed

to moderate temperatures (~500 °C) for long periods (-100 hr). Routine metallographic cross sections have

NASA/TM--2001-210944 1
shownevidenceof acomplexinterfacialreactionlayerbetweenthecoatingandsuperalloysubstratesaswell

asanapparent microstructural coarseningof thecoating'smatrix(NiCr)phase.Figure2showssuchchanges

foraPS304coatednickelbasedsuperalloyspecimenexposedto650°Cinairforover700hr.Thesemicrostructuralandinterfacialchangesdonotappeartonegativelyaffectcoatingperformancebutwarrantfurther

study.

Sincecoatingadhesioncanplayacriticalroleincoatingperformanceit hasbeenselectedforfurther

investigationin thecurrentwork.Othershaveobservedsimilarstrengtheningeffectsin NiCrbasedcoatings

followingheatreatmentsbutoffernoexplanation(ref.7).It is believedthatstudyof coatingadhesionpropertiesmayprovideinsightintothephenomenadescribedaboveandhelpestablishprocessingparametersfor

morewidespreadcoatingimplementation. Thefollowingsectionsdescribetheresultsof adhesiontestson

PS304coatingsdepositedonstainlesssteelpriorto andafterheatreatmentsin airatvaryingtemperatures

anddurations.Stainlesssteelis theselectedsubstratebecauseit is alikelycoatingsubstrateandis theshaft

materialbeingusedin anOil-Freeturbocharger usingPS304(refs.8and9).

Theultimategoalsof thisworkareto developabetterunderstandingof theroleof theheattreatmentand

to betteridentifyandunderstandthemechanism(s) in actionduringthermalexposure. It is anticipatedthatthe

resultswill alsobeusedto developapreliminaryheatreatmentprocessforthePS304coatingtofostercommercializationof thiscoatingtechnology.

TESTPROCEDURESANDAPPARATUS

PS304CoatingandDeposition

ThePS304coatingisplasmaspraydepositedusingconventional plasmasprayproceduresdescribedfully

inreferences10and11.Briefly,apowderblendof theconstituentsin PS304(shownintableI) is fedbya

hopperintoanargongasstreamandinjectedintoanargonplasmain aplasmaspraygun.Thehightemperatureintheplasmagunmeltsthepowderandacceleratesit outof thegunnozzle.Themoltenpowderthenimpingesontothesubstratewhereit "splats"andsolidifiesformingathin,discontinuouscoating.Multiplepasses

areappliedto buildupathick(-350pan)coatinglayer.Followingdepositionthecoatingisgroundtoachieve

asmoothsurfacefinishandthedesiredthickness. Thedepositionandsolidificationprocessoccursrapidlyand

oftencreatesresidualstressesinthecoating.Figure1showsarepresentativecross-sectionphotomicrograph of

thePS304coatingdepositedontoastainlesssteelsubstrate. TableII showsselectedpropertiesandthecompositionof thestainlesssteelusedinthiswork.PS304coatingshavealayeredstructurewhereeachlayeristhe

resultof eachcoatingpass.ForPS304,-20lamof coatingis depositedperpassand10to20passesare

appliedtobuildupa250to 350pmthickcoatingshown.Asseenin thephotomicrograph, thePS304coating

is aheterogeneous compositein whicheachconstituent appearsasadistinctphase.

PULL/ADHESIONTEST

Toevaluatecoatingstrength,a commercialdhesiontesterisused.Thistesthardware, shownschematicallyinfigure3,consistsof analuminumpullstudwhichisgluedto thecoatingsurface,apneumaticpiston

systemforapplyingthepullof forceandanelectronicpressurecontroller/readout formeasuringandrecording

thepulloffpressure. Theheadof thepullstudis attachedto thecoatingsurfaceusingatwopartepoxyglue

whichismixed,applied,aircuredfor about8 hrthenovencuredat75°Cforanadditional 24hrpriortopull

testing.TheepoxyhaSaratedstrengthof about60MPaunderidealconditionsandtypicallyexceeds12MPa

whenattachedtotheroughplasmaspraycoating.Forreference, plasmasprayedcoatingslikePS304thatexhibitadhesionstrengthgreaterthan4MPaareusuallyconsideredacceptablefor mostribologicalapplications.

Toconductatest,acoatedandgluedspecimenassemblyis placedintothefixtureshownpreviouslyin

figure3.Thecontrolleris startedwhichgraduallypressurizestheloadingpistontherebyincreasingtheapplied

pullforceuntilit exceedsthestrengthof thecoatingortheepoxybond.Whenthepulloffforceexceedsthe

strengththestubdetachesfromthesampleandthetestends.Adigitalpressurereadoutonthecontrollerdisplaysthepeakpressureappliedduringthetest.It is recordedandconvertedtocoatingstrengthinMPausinga

manufacturer's generateddatasheet.Sincenonuniformepoxyapplication, misalignedstudsandsurfaceirregularitiespresentin theas-depositedPS304samplescanresultin datascatteratleastthreerepeattestsaredone

for eachtestcondition.

NASA/TM--2001-210944 2
Thesamplescanfail inoneof threeways.If thecoatingto substrate bond is weakest the stud will pull off

the sample removing the full coating and exposing the stainless steel substrate. If the bond is strong but the

coating cohesive strength is low the stud will pull off the sample and remove some of the coating with failure

occurring within the coating. Finally, if both the coating and its bond to the substrate are strong the stub will

pull off the sample with failure occurring within the epoxy layer. By observing where failure occurs, coating

properties (e.g., bond strength, apparent tensile strength etc.) and the effect of differing heat treatments can be

assessed.

COATING MICROSTRUCTURAL ANALYSES

Selected coating samples were mounted in metallographic potting compound (epoxy) and prepared by

conventional metallographic methods for analyses. Viewing the coating cross sections before and after thermal

processing can lend insight into microstructural changes and their effects on strength. Both optical and Scanning Electron Microscopic (SEM) techniques were employed in this work. For SEM analyses, the samples

were coated with a thin layer of gold to prevent charge build-up that can obscure the images.

RESULTS AND DISCUSSIONS

The PS304 exhibits a pull off strength of 17 MPa in the as-deposited condition with no heat treatment.

Failure occurs within the coating in a cohesive manner suggesting that the bond between the coating and the

substrate is stronger than the coating itself. Coatings exposed to high temperatures for brief periods or moderate

temperatures for long periods exhibit strength around 35 MPa. Table III shows the averaged data for all of the

tests conducted ranging in duration from 24 to 500 hrs at temperatures from 432 to 650 °C. This data is plotted

in figure 4.

The data show that at lower temperatures longer exposure times are needed to achieve the strengthening

effect. This type of behavior is common for diffusion based metallurgical changes (refs. 12 and 13). Abrupt

phase transformations, such as the well known alpha to beta transformation that occurs for Zirconium Oxide,

rapidly occur once a specific temperature threshold is exceeded (ref. 14). For the PS304 coating, the adhesion

data suggests that more gradual changes occur which are enhanced by high temperatures and time. For

instance, after exposure for 500 hr at 482 °C the adhesion strength has only reached ~2/3 of full strength

(~35 MPa). Yet at 650 °C, a mere 24 hr appears more than adequate to fully heat treat the PS304 coating.

Several coating samples have been exposed for over 700 hr at 650 °C and exhibit strengths around 35 MPa

suggesting that the strengthening effects have a terminus (reach a final value).

Many of the candidate substrates for PS304 are heat treated superalloys and stainless steels. Since a wide

range of thermal exposure temperatures enhance coating strength it is likely that the coating thermal processing can be achieved coincidentally with a substrate heat treatment. In this case, the expense of an additional

separate coating heat treatment could be avoided.

The adhesion data alone suggests diffusion controlled changes are occurring to the PS304 during the thermal processing. The observation that the strengthening occurs much more rapidly above 538 °C further indicates a solid state diffusion controlled effect. The main component in PS304 is NiCr which has a melting point

of 1390 °C (1663 °K). A heat treatment temperature of 538 °C (811 °K) is nearly 0.5 that of NiCr's melting

point. In solid state diffusion, atomic mobility rates increase dramatically when the ambient temperature

exceeds about 40 percent of the absolute melting point (ref. 12). Therefore, it is not surprising to observe what

appears to be diffusion based property (adhesion strength) changes in the coating when exposed to temperatures above about 500 °C.

Post heat treatment metallographic cross sections of the PS304 specimens help elucidate the reasons for

the observed strength increase. Figures 5 and 6 show metallographic images, at various magnifications, of the

PS304 coated stainless steel substrates at selected test temperatures and exposure times. The as deposited

samples exhibit microstructural features typical for thermal spray coatings. The PS304 starts out as a powder

blend of the individual constituents and each particle melts separately in the hot plasma and deforms upon

impact with the substrate creating a layered coating structure made up of "splats" on top of one another. When

examined under an optical microscope the different phases can be readily detected and identified. The NiCr

binder is predominant and bright, the silver is metallic and develops a slight tarnish, the chrome oxide is gray

and the fluorides, when viewed with oblique lighting, appear transparent or slightly opaque. Black, nonreflective regions are porosity, which is typically about 15 percent of the structure.

NASA/TM--2001-210944 3
SinceNiCrisPS304'smajorconstituent itspropertiesarelikelytodominatethecoating'sproperties,

especiallystrength.Therefore, it is anticipatedthatchangestotheNiCrmatrixcanhavesignificanteffectson

thecoatingstrengthproperties. Thesilverandfluoridesarelubricantphases, therefore, anychangesto these

constituents, suchaschemicalreactions,degradation, or decomposition, areanticipatedtohavesignificant

effectstothelubricationpropertiesof thecoating.

Examinationof thecoatingcrosssectionsrevealsthattheonlyreadilyobservedchangedueto thethermal

processingis anapparent phasesegregationor,rather,secondphaseprecipitationisidetheNiCrbinder

regions.WhenPS304depositedontonickelbasedsuperalloysisheatreatedareactionlayerformsattheinterface.ForPS304depositedonstainlesssteel,nointerfacialreactionlayerisobserved. Theappearanceof the

silver,chromeoxideandfluoridephasesremainsunchangedevenafterexposureat650°Cforover100hr.The

NiCrphases, however,developa strikingmicrostructural featurewhichhastheappearanceof darkislandsoftenlockedinsideNiCrparticles.Theseregions,atfirst,wereassumedto besomeoxidephase.However,NiCr

wouldbeexpectedtodevelopanoxidepassivatinglayeronitssurfacewhenexposedto airathightemperaturenotinternaloxidephases. Onthecontrary,theredoesnotappeartobeanyfilmorlayerof oxideonthe

surfaceof theNiCrphases. In addition,theappearanceof the"twophase"NiCrmatrixis uniformthroughout

thecoating.Thatis,its appearanceisthesameatthecoating'soutersurfacewhichisdirectlyexposedto air

duringtheheattreatmentandatthesubstrateinterfacewhichiswellprotectedfromtheatmosphere. These

observationssuggestthatduringtheheatreatmentsometypeof diffusiondrivensecondphaseprecipitation

hasoccurred.

ScanningElectronMicroscopic(SEM)analyseswereconducted, coupledwithEnergyDispersiveX-Ray

analyses(EDS),toelucidatethecompositionandnatureof thechangesto theNiCr matrix. The results are

suggestive but not conclusive. For comparative purposes, an EDS spot spectrum of a NiCr area in an as deposited coating is shown in figure 7. The unheat treated NiCr, which has an 80/20 wt% composition, exhibits a

nickel to chromium peak height ratio of 6.2. Figure 8 shows EDS spot spectra of the NiCr matrix and its second

phase precipitates. The dark, chromium rich precipitate phases are high in Chromium with smaller peaks of

nickel and silicon. Oxygen and other species (fluorine, carbon barium, calcium etc.) are not detected. The

Ni/Cr ratio for the precipitate phase is about 0.5. EDS spectra of the lighter surrounding NiCr matrix show large

nickel content with a moderate chromium peak. When compared to the EDS spectra of as deposited NiCr, the

heat treated lighter phase appears somewhat Chromium depleted. The Ni/Cr ratio in this case is about 10.2.

This comparison indicates that the Chromium concentration in the precipitated phase is a diffusion migration from the surrounding NiCr phase. The presence of silicon in the spectra is interesting. Silicon is not an

intentional coating constituent. It is present as a ubiquitous additive in the NiCr powder produced commercially. Silicon, in the form of a silicate, is added to the NiCr powder as a powder flow enhancer. It is present at

the 1 percent or less level and is often found in commercial thermal spray powders. The presence of a strong

silicon peak in the precipitated phase suggests the formation of some type of Chromium-Silicon compound,

perhaps a chrome silicide. Although not yet confirmed, coating thickness increases after heat treatment of up

to about 3 percent have been observed and may be linked to the second phase precipitation.

The structure of both the dark precipitated phase and the surrounding NiCr matrix cannot be exactly

determined using SEM,rEDS techniques. Further analytical work using X-Ray diffraction and less complex

(perhaps pure NiCr coatings) samples may be required to determine the true changes taking place during the

heat treatments. It can be speculated, however, that during the heat treatment a chromium rich second phase

precipitates inside the NiCr matrix leading to a strengthening effect. In addition, since the microstructure

appears to be stable following the formation of the second phase, it is expected that the adhesion and strength

properties of the PS304 coating will remain stable over longer periods. In fact, samples exposed to high temperature air for over seven hundred hours display adhesion properties and microstructures virtually indistinguishable from samples exposed for 24 hr at 650 °C.

CONCLUDING REMARKS

The PS304 composite plasma sprayed coating exhibits an increase in adhesion strength of -100 percent

following exposure to temperatures above 500 °C for various lengths of time. The data shows that as the exposure temperature increased the time required to achieve strengthening dramatically decreased suggesting some

type of diffusion controlled metallurgical change is occurring in the coating. Specimens exposed for long time

periods at high temperatures do not experience any further increase in strength properties suggesting some type

of metallurgical change occurs which reaches an equilibrium condition. Post test metallographic cross-sections

coupled with SEM and EDS analyses indicate that a second phase, rich in chromium, precipitates in the NiCr

matrix. The second phase, which appear as dark islands surrounded by a NiCr matrix, are often observed to

contain silicon most likely coming from a silicate flow enhancer found in the NiCr powder feedstock. EDS

NAS A/TM--2001-210944 4
elemental analysishowsthattheNi toCrratioinheattreatedsamplesilowerthanasdepositedsamples

suggestingthatthechromiuminthesecondphaseiscomingfromtheNiCrmatrixandnotthechromeoxide

constituent ofPS304.

Preliminarymeasurements alsoindicatetlaatavolumeexpansionof afewpercentoccursfollowingheat

treatment. Similarto theadhesionstrength,aftertheheattreatment iscompletenofurthervolumechangeoccursevenafterlongexposureperiods.Theseresultsindicatetheneedforaheatreatmentof thePS304coatingafterplasmasprayingandpriortofinishmachiningtoinsuredimensional stabilityandmaximumadhesion

properties. Furtheresearchiswarrantedto elucidatetheexactnatureofthecoatingchangesthatoccurandto

understandthepossibleffects,if any,of thesubstrateandenvironmentsontheperformanceandenvironmentaldurabilityofPS304.

REFERENCES

1. DellaCorte, C.andEdmonds, B.J.:"PreliminaryEvaluationofPS300:A NewSelf-LubricatingHighTemperatureCompositeCoatingforUseto 800°C,"NASATM-107056, November1995.

2. DellaCorte, C.:"TheEffectof CounterfaceontheTribological Performanceof aHighTemperatureSolid

LubricantCompositefrom25to650°C,"NASATM-107183,Surface. Coatings and Technology, 86-87

(1996), pp. 486492.

3. DellaCorte, C. and Laskowski, J.A.: "Tribological Evaluation of PS300: A New Chrome Oxide Based

Solid Lubricant Coatings Sliding Against AI_O 3 from 25 to 650 °C," NASA TM-107163, Triboiogy Transactions, 1997.

4. DellaCorte, C. and Fellenstein, J.A.: "The Effect of Compositional Tailoring on the Thermal Expansion

and Tribological Properties of PS300: A solid Lubricant Composite Coating," NASA TM-107332, Tribology Transactions, vol. 40 (1997), pp. 639-642.

5. DellaCorte, C.: "The Evaluation of a Modified Chrome Oxide Based High Temperature Solid Lubricant

Coating for Foil Gas Bearings," NASA/TM--1999-208660, Tribology Transactions, vol. 43, pp. 257-262,

April 2000.

6. DellaCorte, C., Valco, M.J., Radii, K.C. and Lukaszewicz, V.L.: "Performance and Durability of High

Temperature Foil Air Beatings for Oil-Free Turbomachinery," NASA/TMII999-209187, Tribology Transactions, October 2000.

7. Walther, G.C.: "Program for Plasma-Sprayed Self-Lubricating Coatings" NASA CR-3163, July 1979.

8. Howard, S.A.: "Rotordynamics and Design Methods of an Oil-Free Turbocharger," Tribology Transactions,

vol. 42, no. 1, 1999, pp. 174-179.

9. Heshmat, H., Walton, J.F., DeilaCorte, C. and Valco, M.J.: "Oil-Free Turbocharger Paves Way to Gas

Turbine Engine Applications," ASME 200(OGT-620.

10. DellaCorte, C. and Sliney, H.E.: "Composition Optimization of Self-Lubricating Chromium Carbide Based

Composite Coatings for Use to 760 °C," ASLE Transactions, vol. 30, no. 1, January 1987, pp. 77-83.

11. DellaCorte, C.: "Experimental Evaluation of Chromium Carbide Based Solid Lubricant Coatings for Use

to 760°, '' NASA CR-180808, August 1987.

12. Shewmon, P.G.: "Diffusion in Solids," McGraw-Hill Book Company, pp. 175-179, 1963.

13. Boyer, H.E. and Gall, T.L.: "Metals Handbook, Desk Edition," chapter 28, pp. 64-78, American Society

for Metals, 1985.

14. Kingery, W.D., Bowen, H.K. and Uhlmann, D.R.: "Introduction to Ceramics," John Wiley and Sons,

Second Edition, 1975.

NASA/TMI2001-210944 5
TABLE I.--PS304 COMPOSITION

Constituent wt% Function

NiCr a 60 binder

Cr203 20 hardener

Ag 10 low temperature lubricant

BaFJCaF., b [10 high temperature lubricant

aNi/Cr ratio is 8/20 by wt%.

bBaF2/CaF2 ratio is 62/38 by wt%.

TABLE I1.--13-8 Mo STAINLESS STEEL COMPOSITION

Propert?

Composition, wt%

Density, g/cc

Ultimate tensile strength, MPa

Yield strength, MPa

Elastic modulus, GPa

Thermal expansion coefficient/°C

Thermal conductivity, W/m-k

Heat capaciQ', J/g-°C

Value

Fe-76, Cr-12.5, Ni-8.0, Mo-2.5, AI-I.0

7.8

1480

1415

221

11.0

14

0.46

TABLE III.--ADHESION DATA SUMMARY FOR PS304

DEPOSITED ON 13.8 MO STAINLESS STEEL

Heat treatment

Temperature. Time, hr

°C

None as-deposited coating

482 75

482 150

482 250'

482 500

538 I00

538 150

538 200

5i8 500

iii ii

593 25

593 50

593 100

593 150

593 200

650 25

650 50

650 100

650 150

650 200

Adhesion strength, MPa, ksi

(17.2_+1.2) 25.1_+4.3

( 17.7 +_.03) 26.4_+ 1.6

(22.8 _+1.1) 33.9±1.6

(20.2_+1.3) 30.0+_2.0

] (22.8 +9.0) 34.0+_3.0

(20.2 +_3.4) 30.0+.5.0

(32.5.-.-_.3) 48.3-+4.9

(30.2 +__'_.4) 45.0+__5.0

(36.6+1.4) 54.5+__2.0

(28.8 +-8. I) 42.9:t:1.2

(35.9-+0.7) 53.5__+1.0

(37.4+1.0) 55.6_+1.5

(34.4_+1.4) 51.2-+.2.0

(37.5 _+1.2) 55.8+ 1.5

(33.3 -+0.7) 49.5+ 1.0

(31.2 :t:0.7) 46.5± 1.0

(33.9-+1.5) 50.5.-k2.0

(32.4_+1.8) 48.2+2.2

(36.5 _+1.2) 54.3+ 1.7 Y

NASA/TM--2001-210944 6
Figure 1 ._ross-section photomicrograph of an asdeposited PS304 coating on a 13-8 Mo stainless

steel substrate,

........... '...................... 0.25 mm

Figure 2.--Cross-section photomicrographs of PS304

coating deposited on a nickel based superalloy substrate after exposure in air at 650 °C for 700 hr. Note

interfacial reaction layer and second phase precipitates in NiCr matrix.

NASA/TM--2001-210944 7
rAluminum pull stud

t- ) /

t

) /

/

) / i

) / / /- Epoxy adhesive layer

_) l//

i / _- PS 304 coating

_uu_u\u_u_u__/J _" A //--Stainless steel substrate

Pull-test specimen

Compressed air line

©

Substrate --_ Pulling force

Pi_itlI/////////tlI/J

sembly --_ _ I_ #

assembly --_ ._

Reaction plate \\

_-Adhesion pull-test

specimen assembly

(upside down)

CD-01-81442

Pull-test control unit

and readout

Figure 3.--Adhesion pull-test specimen assembly and system.

NASA/TM--2001-210944 8
40

¢0

a.

30

ct-

._o 20

10

a

C_ C3

C3

O As-deposited

/k 482 °C

[] 538 °C

E3 593 °C

<_ 650 °C

[]

Z_

A /k

[] /k

I I I I I I I

0 50 100 150 200 300 400 500

Heat treatment exposure time, hr

Figure 4.--Adhesion strength versus exposure time at various temperatures

for PS304 deposited on 13-8 Mo stainless.

Figure 5._Cross-section photomicrographs of PS304

exposed in air for 150 hours. (a) 593 °C. (lo) 650 °C.

NASA/TM--2001-210944 9
i

_, _%_ __ L,!_-

Figure 6.--Cross-section photomicrographs of PS304

after exposure in air at 650 °C for 50 hr.

O

o

x

Ni

Cr

gr_ Ni Cr i

I

0 1 2 3 4 5 6 7

Energy, keV

Ni

8 9 10

Figure 7.---EDS spectra of NiCr matrix phase of PS304 as deposited

coating. Ni/Cr peak height ratio is _--6.2.

NASA/TM--200 i-210944 10
C

0

X

Cr

Ni

N

0 1 2 3 4 5 6 7

Energy, keV

Figure 8.--EDS spectra of dark chromium rich precipitate inside NiCr

matrix phase of PS304 exposed at 650 °C for 150 hours. Ni/Cr peak

height ratio Is -_ 0.5.

Ni

8 9 10

"E

O

O

Ni

Ni

/ t " Ni

0 2 3 4 5 6 7 8 9 10

Energy, keV

Figure 9.reEDS spectra of NiCr phase surrounding dark chromium rich

precipitate from PS304 exposed at 650 °C for 150 hours. Note apparent

depletion of chromium content. Ni/Cr peak height ratio is _- 10.6.

NASA/TM--2001-210944 11
REPORT DOCUMENTATION PAGE FormApproved

OMB No. 0704-0188

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1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

May 2001 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Thermal Processing Effects on the Adhesive Strength of

PS304 High Temperature Solid Lubricant Coatings

6. AUTHOR(S)

Christopher DellaCorte, Brian J. Edmonds, and Patricia A. Benoy

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546- 0001

WU-708-18-13-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12798

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210944

11. SUPPLEMENTARY NOTES

Christopher DellaCorte and Brian J. Edmonds, NASA Glenn Research Center; and Patricia A. Benoy, St. Louis University,

St. Louis, Missouri 63103. Responsible person, Christopher DellaCorte, organization code 5960, 216--433-6056.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 23 Distribution: Nonstandard

Available electronically at http. ://gltrs._c.nasa.gov/GLTRS

This _aublication is available from the NASA Center for AeroS_)ace Information, 301-621--0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

In this paper the effects of post deposition heat treatments on the cohesive and adhesive strength properties of PS304, a plasma sprayed nickel-chrome

based, high temperature solid lubricant coating deposited on stainless steel, are studied. Plasma spray deposited coating samples were exposed in air at

temperatures from 432 to 650 °C for up to 500 hr to promote residual stress relief, enhance particle to particle bonding and increase coating to substrate

bond strength. Coating pull off strength was measured using a commercial adhesion tester that utilizes 13 mm diameter aluminum pull studs attached to

the coating surface with epoxy. Pull off force was automatically recorded and converted to coating pull off strength. As deposited coating samples were

also tested as a baseline. The as-deposited (untreated) samples either delaminated at the coating-substrate interface or failed internally (cohesive failure)

at about 17 MPa. Samples heat treated at temperatures above 540 °C for 100 hr or at 600 °C or above for more than 24 hr exhibited strengths above

31 MPa, nearly a two fold increase. Coating failure occurred inside the body of the coating (cohesive failure) for nearly all of the heat-treated samples

and only occasionally at the coating substrate interface (adhesive failure). Metallographic analyses of heat-treated coatings indicate that the NickelChromium binder in the PS304 appears to have segregated into two phases, a high nickel matrix phase and a high chromium precipitated phase. Analysis

of the precipitates indicates the presence of silicon, a constituent of a flow enhancing additive in the commercial NiCr powder. The exact nature and

structure of the precipitate phase is not laaown. This microstructural change is believed to he partially responsible for the coating strength increase.

Diffusion bonding between particles may also be playing a role. Increasing the heat treatment temperature, exposure time or both accelerate the heat

treatment process. Preliminary measurements indicate that the heat treatment also results in a one time, permanent coating thickness increase of about

3 percent. Based upon these results, the incorporation of a heat treatment prior to final finishing has been incorporated in the application process of this

coating technology.

14. SUBJECT TERMS

Coatings; Lubrication; Solid lubricants

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

i9. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

17

16. I_RICE CODE

20. LIMITATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102