DOE/NASA/50306-10

NASA TM-107163

Tribological Evaluation of PS300: A New

Chrome Oxide Based Solid Lubricant

Coating Sliding Against AI203 From

25 to 650 °C

C. DellaCorte

National Aeronautics and Space Administration

Lewis Research Center

and

J.A. Laskowski

Parks College

St. Louis University

Cahokia, Illinois

Work performed for

U.S. DEPARTMENT OF ENERGY

Conservation and Renewable Energy

Office of Vehicle and Engine R&D

Prepared for the

Joint Tribology Conference

cosponsored by the American Society of Mechanical Engineers and

the Society of Tribologists and Lubrication Engineers

San Francisco, California, October 13-17, 1996
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DOE/NASA/50306-10

NASA TM-107163

Tribological Evaluation of PS300: A New Chrome

Oxide Based Solid Lubricant Coating Sliding

Against AI203 From 25 to 650 °C

C. DellaCorte

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

and

J.A. Laskowski

Parks College

St. Louis University

Cahokia, Illinois

Work performed for

U.S. DEPARTMENT OF ENERGY

Conservation and Renewable Energy

Office of Vehicle and Engine R&D

Washington, D.C. 20545

Under Interagency Agreement DE-AI01-91CE50306

Prepared for the

Joint Tribology Conference

cosponsored by the American Society of Mechanical Engineers and

the Society of Tribologists and Lubrication Engineers

San Francisco,California, October 13-17, 1996

Triboiogical Evaluation of PS300: A New Chrome Oxide Based Solid Lubricant Coating Sliding

Against A1203 from 25 to 650 °C

by

C. DellaCorte, NASA LeRC

J.A. Laskowski, Parks College

for Joint Tribology Conference

ABSTRACT

This paper presents the tribological characteristics of A1203 sliding against PS300; a chrome oxide based self lubricating

coating. Al203 pins were slid against PS300 coated superalloy disks in air, under a 4.9N load at velocities of 1 to 8 m/s. At

a sliding velocity of l m/s, friction ranged from 0.6 at 25 °C to 0.2 at 650 °C. Wear factors for the AI203 pins were in the

l0 -7 mm3/N-m range and for the PS300 coating were in the l0 -5 mm3/N-m range. The test results suggest that increased

surface temperature resulting from either frictional heating, generated by increased sliding velocity, or ambient heating

caused a reduction in friction and wear of the sliding couple. Based upon these results, the tested material combination is a

promising candidate for high temperature wear applications.

Key Words

Tribology, Friction, Wear, Ceramics, High temperature, Solid lubricants, Coatings

INTRODUCTION

The need to provide lubrication to ceramics, especially at elevated temperatures is a challenging and ongoing technical

problem (Ref. l ). PS300, a new coating developed at the author's laboratory, has shown promise in lubricating superalloys

at temperatures as high as 800 °C (Ref. 2). In the following paper PS300 is considered as a candidate lubricant for ceramics.

PS300 is a metal bonded-chrome oxide based plasma sprayed coating with silver and fluoride lubricant additions. PS300 is

a follow-on coating to PS200 (Ref. 3), a chrome-carbide based coating, and has been shown to provide comparable friction

and wear properties to PS200 with several key benefits (Ref. 2).
Onebenefitisthatthesubstitutionof chrome oxide for chrome carbide eliminates the need for diamond grinding finishing

processes. Chrome oxide itself has been shown to be a good high temperature lubricant (Refs. 4 and 5). Since it is already

oxidized, it will not degrade upon exposure to air at elevated temperatures. Another benefit is that the raw materials for PS300 are

less costly than those for PS200. In addition, though the reasons are not known, the plasma spray process was found to be less

sensitive to deposition parameters for the new coating and the deposition efficiency was higher compared to the PS200 (Ref. 2).

PS300 has been evaluated in sliding against a variety of materials under low sliding velocity conditions (Ref. 2). The

following paper describes a research program to further develop and evaluate the PS300 coating. In particular, its tribological

performance in sliding contact with the ceramic, aluminum oxide, is investigated over a wide temperature and sliding

velocity spectrum. A goal of the research is to determine if PS300 can be used to lubricate ceramics effictively at high

temperatures up to 650 °C and sliding velocities up to 8 rn/s. Sliding tests were conducted using Al203 pins loaded against

the rotating face of a PS300 coated superalloy disk in a high temperature tribometer.

Experimental: Materials and Specimen Properties

The detailed compositions, by weight and volume percent, of PS300 and PS200 are given in Table I. PS300 is comprised of

a wear resistant matrix of nickel-chrome bonded chromium oxide, silver, a low temperature lubricant and BaF2/CaF 2 eutectic,

a high temperature lubricant. The PS300 coating is formed by plasma spraying a simple powder blend of the constituents.

Powder particle sizes range from 20 to 150 mm and the plasma spray parameters used to apply the coating are given in

Table II. The coating is characterized using cross section metallography, electron microscopy and x-ray fluorescence (for

bulk composition analysis). Figure 1 shows some representative cross-section micrographs of the coating. The plasma spray

process produces a fairly uniform "splat" type coating with some residual low level porosity. In the optical photomicrographs

shown in Fig. 1, the bright areas are NiCr and Ag; the grey areas are Cr203 and the black areas are voids and BaF21CaF 2.

To prepare a disk sample, a 0.5 mm thick PS300 coating mixture was plasma sprayed onto a grit blasted disk surface which

has been previously coated with a 0.1 mm thick NiCr (80/20) bond coat layer, the resulting coating was then ground to a final

thickness of about 0.3mm. The ground roughness was about 0.Smm rms. 600 grit silicon carbide abrasive paper and water
wasusedtolightlypolishthesurfacetoafinishof0.1to0.2mmrms.

Thepinspecimenswere2.5cmlong,0.95cmdiametercylinderical rodswithbothendsfinishedtoa2.5cmradiusof

curvature. Thepinswerepreparedbydiamondgrindingfrom99.4%purityAl203.Thepinsurfacewaspolishedtolessthan

0.2mmrmssurfaceroughness. Priortotesting,thepinsanddiskswererinsedwithethylalcohol,scrubbedwithapasteof

levigatedAI203andwater,insedwithdeionizedwateranddriedwithlabair.

Experimental: Tribologicai Testing

The specimens were tested in a pin-on-disk test rig described in detail in Ref. 3 and shown in Fig. 2. The pin wears a

51 mm diameter track into the rotating test disk which was inductively heated to the desired test temperature. The air

atmosphere was controlled and maintained at 50% R.H. at 25 °C. Selected test temperatures were 25,500, and 650 °C.

These temperatures represent the anticipated application temperature for foil bearings. The initial test velocity was

370 rpm (1 m/s) and the load was 4.91N. These conditions were chosen to simulate the start-up/shut-down conditions for

foil air bearing applications. Additional tests were conducted at higher sliding velocities (1000 rpm, 2000 rpm and

3000 rpm) to assess the effect of sliding speed on friction and wear. For these tests, the total sliding distance was kept

constant (3.6 km) and equal to 60 minutes of sliding at 370 rpm.

Friction was monitored continuously during the tests which typically last 30 minutes. Wear was measured using optical

microscopy (for pin wear scars) and stylus surface profilometry. For tests conducted at 1 m/s, new pin and disk specimen

pairs were used for each test temperature and typically three repeats were run on each pair. For this condition (1 m/s

velocity) the data uncertainties presented are one standard deviation. For tests conducted at higher velocities, two repeat

tests were conducted on each pin and disk pair tested at each test temperature and velocity. Thus, for these higher velocity

tests, the data set is too small for valid statistical analysis and the data presented represents the average and the range.

RESULTS AND DISCUSSIONS

The test results are summarized in Table III. Friction and wear at 1m/s sliding velocity are shown in Figs. 3 and 4. The

coefficient of friction for the AI203/PS300 sliding pair was high at 25 °C, about 0.62, and decreased significantly to 0.19 at
650°C.Figure5plotsthreetypicaltestrunsandillustratestheuniformityandsmoothness ofthefrictioncoefficient overa

60minutetestinterval.

Thefrictionat25°CismuchhigherwhenPS300is slidingagainstAI203ratherthanInconelX-750.Asreported

previouslyunderthesametestconditions(Ref.2),thefrictioncoefficientforPS300versusInconelX-750wasabout

0.3.TheincreasedfrictionwhenslidingagainstAI203maybeduetothepoorwetabilityofsilver(Ref.6),whichis

thelowtemperaturelubricantinPS300,totheA1203counterfaceinhibitingthedevelopment ofasufficientlubricating

transferfilm.

Atelevatedtemperatures, however, thefrictionwassignificantlyreduced. Thismaybedueto thelubricatingeffectof

PS300'shightemperaturelubricantadditive,BaF2/CaF2eutectic.Theeutectichasbeenshowntobeaneffectivesolid

lubricantabove400°C(Ref.7).ThefrictionreductionwithtemperaturemayalsobeaffectedbythepresenceoftheCr203

whichisanintrinsicsolidlubricantabove500°C(Refs.4and6).Fromanengineeringstandpoint, however, lowfriction

(<0.30)atfulloperatingtemperatures, foragasturbinesealforexample, isofsignificant valuesincemostmachineryuns

atlowtemperatures onlyduringbriefstart-upperiods.

Slidingvelocityhasaninterestingeffectonfrictionwhichappearstobecomplimentaryto theeffectof temperature.

Figure6showsthefrictionasafunctionofslidingvelocityforallthreetestemperatures. Adiscussed, at25°Cthefriction

at1ntisishigh,approximately 0.6.Thisvalueisreducedtoabout0.3at8.1m/s.Atelevatedtemperatures, theeffectof

velocityisverysmall.Thisresponsetovelocitysuggeststhatfrictionalheatingoftheslidingsurfacescontributestoreduced

frictionasdoesambient heatingofthesurfaces.

ThewearofboththeAI203andthePS300,showninFigs.7and8,exhibitsthesametrendwithtemperatureandvelocityas

thetrendobservedforfriction.Reference3defineslowwearas10-7mm3/N-m, moderatewearas10-5to10-6mm3/N-mand

highwearasgreaterthan10-4mm3/N-m. Atlowspeed(1m/s),thewearfactorsforthePS300coatingrangefrom10-4

mm3/N-mto 10-6mm3/N-masthetesttemperatureincreasedfrom25to650°C.Athighslidingvelocitiescoatingwear

alsodecreasedwithincreasingambienttemperaturebutthetrendwaslessdramatic. Similarly,pinwearfactorsexhibiteda
largetemperaturedependence at1and2.7m/sslidingvelocity,butathigherspeedsthetemperaturedpendence wasnot

uniformallyobservedorassignificant.

InprevioustestingofPS300against thenickelbasedsuperalloyInconelX-750theoppositerendswithambient temperature

wereobserved(Ref.2).Atroomtemperature, thefrictionwas0.23andat650°Cit increasedto0.31.Similarly,coating

wearincreasedanorderofmagnitudefrom7x10-5to7x10-4mm3/N-mwhenthetestemperaturewasincreasedfrom25

to650°Cunderthesameslidingconditions. Clearly,thetribological propertiesaresensitivetothecounterfacematerial.

Thisphenomenamaybeduetotransferfilmdevelopment andhasbeenreferredtoastribological compatibility(Ref.8).It

hasbeenshowninworkwithcarbidebasedcoatingsusingthesamelubricants(Ag+BaF2/CaF2) thattransferofthesolid

lubricantadditivesplayakeyroleinreducingfrictionandwear(Ref.3).SincenoblemetalslikeAuandAg donot

spontaneously wetceramics(e.g.A1203), roomtemperatureprformanceislikelytobepoor(Refs.6and9).However, as

theambienttemperatureisincreased, thefluoridesoftenandareabletodevelopalubricoustransferfilmontheA1203

counterfacesincefluoridesdowetoxideceramics(Ref.10).

EDSanalysisofthewornAI203pinscorroborates thishypothesis. Littleornodetectabletransferofsilveroccursduring

roomtemperaturesliding.Noticeablex-raysignalsfromthelubricantsBa,CaandAgare,however, detectedontribological

specimensfromhightemperaturetests(seeFig.9).

Theseresultsuggest thatroomtemperatureprformanceoftheAI203/PS300 slidingcouplemaybeimprovedbymaking

thecounterfaceacompositeof metalandceramic.

CONCLUDINGREMARKS

ThefrictionandwearofPS300wasevaluatedinslidingagainstAI203overawidetemperaturerange.Frictionandwear

werenoticeablyhigheratroomtemperaturethanat650°C.Athigherslidingvelocities, frictionandwearemainedlow

overtheentiretesttemperaturerangeexhibitingverylittlesensitivitywithtemperature. Onthebasisoftheseresultsit is

apparent thatambient orselfgeneratedsurfacetemperatures maybeplayingaroleinreducingfrictionandwear.
Analysesof thewornpinsurfacessuggest thatthehighertemperatures promotethedevelopment ofalubricousfluoride

transferfilm.Atlowspeedandlowtemperature, theslidingcouplemustrelyonthelowtemperaturelubricant,silver,

which;perhapsduetoisinabilitytowetoxideceramics, fallstoprovideadequatelubrication.

Nonetheless, AI203 is a practical engineering counterface for the PS300 coatings. Wear during sliding against AI203 was an

order of magnitude less than for the PS300 coating sliding against a superalloy. In addition, high temperature friction was

observed to he significantly lower. Many intended applications for this type of solid lubricant coating (e.g. foil bearings,

turbine seals) operate almost exclusively at high sliding speeds and elevated temperatures, making room temperature, low

speed friction performance a less ctitical issue.

REFERENCES

1. Benoy, EA. and DellaCorte, C. , "Au/Cr Sputter Coating for the Protection of Al203 During Sliding at High

Temperatures", NASA TM-106853, March, 1995.

2. DellaCorte, C. and Edmonds, B.J., "Preliminary Evaluation of PS300: A New Self-Lubricating High Temperature

Composite Coating for Use to 800 °C", NASA TM-107056, November 1995.

3. DeUaCorte, C. and Sliney_H.E._``C_mpositi_n_ptimizati_n_fSelf-LubricatingChromium-CarbideBasedC_mposite

Coatings for Use to 760 °C", ASLE Transactions, Vol. 30, 1, pp 77-83, 1987.

4. Liu, G.H., Gras, R., and Blovet, J., "Characterization of Composite Tribological Coatings: Composition, Microstructure

and Mechanical Properties", Surface Coatings and Technology, 58 (1993) pp. 199-203.

5. Liu, G. H., Rubbevalloire, E, and Blovet, J., "Improvement in Tribological Properties of Chrome Oxide Coating at

High Temperature by Solid Lubricants", Wear, 160 (1993) pp. 181-189.

6. Erck, R.A., Erdemir, A., and Fenske, G.R., "Effect of Adhesion on Tribological Properties of Ag-Coated Alumina",

Surface and Coatings Technology, 43-44, pp. 577-578, 1990.8. H.E. Sliney, T.N. Strom and G.E Allen, "Fluoride Solid

Lubricants for Extreme Temperatures and Corrosive Environments", ASLE Transactions, 8 (1965)

pp. 307-322.

7. Sliney, H.E., Strom, T.N., and Allen, G.E, "Fluoride Solid Lubricants for Extreme Temperatures and Corrosive

Environments", ASLE Transactions, 8 (1965) pp. 307-322.
8. Fukuda, K., Sato, Y., Sato, T., and Veki, M., "Wear Properties of SiC-Whisker Reinforced Ceramics Against Bearing

Steel" in Tribology of Composite Materials, ASM International, Metals Park, Ohio, 1990 pp. 323-328.

9. DellaCorte, C., Pepper, S.V. and Honecy, ES., "Tribological Properties of Ag/Ti Films on A1203 Ceramic Substrates",

Surface Coatings Technology, Vol. 52, pp. 31-37, 1992.

10. Levin, E.M., Robbins, C.R., and McMurdic, H.E, Phase Diagrams for Ceramists #1673, pp. 470, American Ceramic

Society, 1984.

TABLE I.--COMPOSITION BY WEIGHT AND VOLUME PERCENT OF PS300 AND PS200

Coatin_ Densit),

Designation P, _/cm 3

PS300 5.81

PS200 6.75

*By wt% contains 80 Cr20 3, 16Ni, 4Cr.

*NiCr-Cr203

80 (8o.3)

Constituent, wt% (voi%)

+Ni-Co-Cr 3C2 Ag

lO (5.5)

80 (77.1) lO (6.4)

BaF2/CaF 2

10 (14.2)

10 (16.5)

+By wt% contains 54 Cr3Cr 2, 28Ni, 12Co, 2Mo, 2AI, 1B, 1 Si.

TABLE II.--PLASMA SPRAY PARAMETERS

[Used to app! I

Parameter

Current

Volta[e

Standoff distance

Argon

Arc _as flow rate

Powder flow rate

Powder gas flow rate

PS300 coatings]

Value

600 amps

30-32 volts

8-10 cm

35 sl/min

=1 kshar

0.4 m 3/hr

Temperature,

°C

25

25

25

25

500

500

500

500

650

650

650

650

TABLE re.--FRICTION AND WEAR DATA SUMMARY

(4.9N load, air atmosphere)

rpm ix Kpin, Kdisk,

(velocity), mm3/N-m mm3/N-m

rn]s

376 (1)

1000 (2.7)

2000 (5.4)

3000 (8.1)

376 (1.0)

1000 (2.7)

2000 (5.4)

3000 (8.1)

0.62._+0.06

0.52_+0.02

0.34_+0.02

0.31_-+0.01

0.32_+0.07

0.335:0.02

0.25_+0.01

0.28_+0.03

0.19-2-0.02

0.19-20.02

0.28_-+0.02

0.26--+0.01

3.3+7.1 xl0 -_

1.3_+0.9x 10 -s

6.9+__3.1×10 -7

1A_+0.1 xl0 -7

2.6+1.9x10 -7

2,7+1.9 xl0 7

8.1+7.1 xl0 -7

5.6+1.6x10 -6

2.1+1.3x10 -7

0.85_+0.35×10-7

6.3+1.5x10 -7

9.1 _.3 xl0 4

2.3-t-0.4 xlO -_

6.9+-3.5 xlO-4

4_5+_3.0xlO-5

1.8-+0.5 xlO -5

2.5+1.0xi0 -_

3.2+1.1 xlO

1.81-0.4 xlO-5

4.5 +__2.3x lO-S

7.8+_5.3 xlO -6

1.3_'2,'0.2xlO -5

2.8 _+0.2x 10-5

2.7 + 1.8 x 10 -5

376 (1.0)

1000 (2.7)

2000 (5.4)

3000 (8.1)

Number

repeats

7

2

2

2

7

2

2

3
Figure 1 .--Cross-sectional optical micrographs of

PS300 showing plasma sprayed composite coating

structure. Bright areas are NiCr and Ag; gray areas

are Cr203 and black areas are porosity and

BaF2/CaF 2.
Gimbal

assembly.-_

\

Support \

bearing

spindle "7

t /

! ;r

i

I_ Elect'rical

I_ insulation

I

Wear

track --_ F Rider

Friction _ _

force _ --_l

Applied _ed

isk_ load

Direction

of rotation

Friction

specimens

heating coil

Coolant

water \

\ window

. _ L-ThermocouDle

\

L. Coaxial electrical

feed through

Figure 2.mlnduction heated pin-on-disk rig.

1.0

=L 0.8

c"

.__

._o 0.6

¢:

O

(3

,- 0.4

O

,m

,F 0.2

0.0

25 500 650

Test temperature, °C

Figure 3.mFriction coefficient for AI203 pins sliding

against PS300 at I m/s, 4.9°N load, in air.

9
,E

_10-6 4---

¢J

10-7 --

h.

O

._c

a.

10-8

I

!

I

25

t

i-_" m

500 650

Test temperature, °C

-- 10-4

E

z

E

E

-- 10-5 ._

a)

0

.__

a

10-7

Figure 4.--Specimen wear for AI203 pins sliding against

PS300 at I m/s, 4.9-N load, in air.

1.0

0.9

.e 0.8

0

,_

O.7

0

o 0.6

C

•- 0.5

0.4

o_ 0.3

0.2

0.1

0.0

m

Temperature,

°C

25

5O0

650

ill __ - .....

I I I

0 20 40 60

Time, min

Figure 5.mTypical friction vs. sliding time traces for

AI20 3 pins sliding against PS300 at I m/s.

10
1.0

0.8

0.6

::L

0.4

0.2

0.0

Temperature,

oC

A 25

[] 500

O 650

I I I I

376 1000 2000 3000

Sliding velocity, rpm

Figure 6.mFriction vs. sliding velocity.

10-4

,E 0-5 z 1

"5. 10-7

10-8

Temperature,

oc

A 25

[] 500

O 650

1 I I I

376 1000 2000 3000

Sliding velocity, rpm

Figure 7.mPin wear factor vs. sliding velocity.

10-3

J_

.-=lO-S

=T

10-7

Temperature,

°C

A 25

[] 500

O 650

- _::_ _

I I I

376 1000 2000

Sliding velocity, rpm

3OOO

Figure 8._Disk wear factor vs. sliding velocity.

11
.=¢-

O

tJ

x

1O0 000

50 000

(b) AI

C 0

I I

0 1 2

Cr

! I I I I I I

3 4 5 6 7 8 9

Energy, keY

Figure 9.--(a) SEM micrograph of AI203 pin surface after sliding against PS300 at 650 °C, 1 m/s.

(b) Corresponding EDS X-ray spectrum of AI203 wear surface showing lubricant (Ba, Ca, Ag)

peaks, 20 kV accelerating energy.

12

Form Aopmv_/

REPORT DOCUMENTATION PAGE OMaNo.o;,o_olu

Pu_cmpoNinO_ _ t_ ma_on ol_onT_ is_VmRd to awaOeI hourpa'mpoom,_c_O _ Um__ m.._,mg.._m_T=m, .Hmcmneu_ dm m,

[:mde Hlglmay. Sulto 1204, Ad_ VA _ and to the Offk:o o_Mana_cn_tt and Budget. Papenm_ RoducllonProject(0704-0 88), Wasmn0mn,

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

June 1996

4. Tm_ AND SIJB'ITILE

Tribological Evaluation of PS300: A Now Chrome Oxide Based Solid Lubricant

Coaling Sliding Against A1203 From 25 to 650 ¢ C

e. AUTtK_S)

C. DcllaCorte and J.A. Laskowski

7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS{ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORINGAGENCY NAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

Technical Memcrandum

& FUNDING NUIgBERS

WU-505--63--5A

8. PERFORMING OFIGANIZATION

REPORT NUMBER

E-10113

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASATM-107163

DOE/NASA/50306-10

11. SUPP_HT#J:IY NOTES

F'mal Report. Prepared tmdea interagency agn_mcat DD-A101-91CF-50306. C. DdlaCom_e, NASA Lewis Research C.ent_, J.A. Laskowski, Pad_

College, St. Louis University, Cahokia, ]]Ynoh. Prepmed for the Joint Tribology Coherence cosponsored by _hc American Society of M_

Engineen and the Society of Tr_ologists and Lubrication Engineen, San _, California, Octo_r 13--17, 1996. Responn'ble pers_ C.

DellaCort%o_anizadon code 5140, (216) 433--605_

12L DIS'ITIIBUTION/AVAILABlUTY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - Unlimited

Subject Category 23

This publication is available from the NASA Center for AezoSpace Infommlion, (301) 621-0390.

13. ABSTRACT(M_xknum2O0word.)

This paper presents the tribologic_l characteristics of AI203 sliding against PS300; a chrome oxide based self lubricating

coating. AI203 pins were slid against PS300 coated superalloy disks in air, under a 4.9N load at velocities of I to 8 m/s.

At a sliding velocity of 1 m/s, friction ranged from 0.6 at 25 °C to 0.2 at 650 °C. Wear factors for the AI203 pins was in

the 10-7 mm3/N-m range and for the PS300 coating was in the 10 -5 mm3/N-m range. The test results suggest that

increased surface temperature resulting from either frictional heating, generated by increased sliding velocity, or ambient

heating caused a reduction in friction and wear of the sliding couple. Based upon these results, the tested material

combination is a promising candidate for high temperature wear applications.

14. SUBJECT TERMS

Tribology; Solid lubricants; Ceramics; Coatings; High temperature

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20. UMITATION OF A_:*IKACl"

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Presodbed by ANSI Std. Z39-18

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o__z

_ __ <o|. _.

_=c __ _o0_"o=_-"

e_ z_ _ _

I _o m

o

Z __

o

3