NO. SA-TR18-1089

DATE 17 September I

FRICTIONAL AND WEAR CHARACTERISTICS OF VARIOUS COATINGS USED (

SMALL CALIBER WEAPON COMPONENTS

Technical Report z

Author ---

George, H. A.

C

DDC-

OCT 2 7 1965"

Best Available Copy

SPRINGFIELD ARMORYS Springfield, Mass.

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REPORT: SA-TRA8-1089

P M J: 17 September 1965

AmcLs CODE: 5016.11.84400.02

FRICTIONA. ANID WEAR CHARACTERISTICS OF VARIOUS COATINGS USED ON

SMALL CALIBER WEAPON COMPONEN'TS

Technical Report

George, H. A.

PROJECT TITLE: In-House Laboratory Initiated Research and DevelopmentFY64

PRO•.: M1-4-50020-01-M1-M6

Preparing Agencv: Springfield Armory, Springfield, Massachusetts

This TECHNICAL REPORT, to the extent known, does not contain anypatentable material, copyrighted and/or copyrightable material, ortrade secrets.

REPORTSA-TR18-1089

ABSTRACT

Friction and wear studies, in which Springfield Armory frictionand wear machine was used, were continued. Information relatingto the frictional and wear properties of coatings was obtainedunder simulated weapon conditions. Various coatiw•s such asphosphate, chromium plate, and hard-anodized aluminum were evalu-ated. Tho' rough porous coatings such as phosphated steel andhard-anodized aluminum generally had high coefficients of frictionwhen unlubricated, but exhibited good wear resLstance when lubri-cated. Chromium plate, when unlubricated, had a lower coefficientoi friction than steel of equal surface roughness. Metallic sur-faces such as chromium plate and hardened steel had limitedantigalling properties Procedure is given, and results are dis-cussed.

I

REPORTSA-TR18-1089

jjCONTENTS

Abstract

SuhJecL

Objective

Summary of Conclusions

Reconmendations

Introduction

Procedure

Results and Discussion

Appendices

A - Figures (11)B - BibliograohyC - Dfstribution

(ii)

REPORTSA-TR18-1089

SURJECT

Investi:•ation oi Frictional and Wear Characteristics of Various CoatingsUsed on Small Caliber Weapon Components.

1BJECTIVE

T,, obtaf.n data on the frictional and wear properties nf various coatingsused in sall caliber weapon systems.

SUMMARY OF CONCLUSIONS

L. Chromium plate slidin.; on chromium plate, with nolubrication, hasa lower coefficient of friction when compared to dry steel on steel of equalsurface roughness.

2. Hard anodic coatings, with no lubrication, have a relatively hihcoefficient of friction (.55), probably because of the rough nature of thesurface.

3. Phosphate coatings appear to be beneficial in reducing the coefficientof friction when they are dry and in contact with a smooth surface.

4. Unprotected steel surfaces have poor wear resistance, especiallyin the prevention of galling.

5. The coefficient of friction for well-lubricated surfaces is primarilydependent upon the nature of the lubricant.

6. Rough porous surfaces, such as those produced by phosphating and.hard anodizing, have better oil-retaining capabilities than smooth metallicsurfaces and, consequently, exhibit better wear resistance, when lubricated,under high loads.

RECOMMENDATIONS

1. The use of phosphate coatings should be continued on small caliberweapon components since the coatings, when properly lubricated, give adequatecoefficients of friction and excellent wear resistance.

2. Chromium plate, when in contact with itself at high loads, exhibitssome ialling tendencies and its use should be avoided.

3. Endurance tests should be conducted on the various combinations ofcoatings to determine the best coating for contact with chromium plate andother coatings at high loads for increased wear resistance.

4. In instances where very low coefficients of friction are required inweapon systems (<.1), a lubricant better than MZL-L-644 oil should be used.

5. Friction tests should be conducted on other oils, greases, and dry

SA-TR,18-1089

RECOMMENDATIONS - Continued

p• film lubricants to determine lubricants that will provide lover coeffof friction than MIL-L-644 oil.

6. Hard-anodized aluminum usually galls or seizes after the coacompletely worn sway; thus, the use of thick, hard-anodized aluminumshould be continued.

" ~-2-

RzPOwr

SA-TRI8-1089

1. INTRODUCTION

Friction and wear studies were continued at the Sprin:jfieid Armory.Previous work reported in SA-TR18-1084 1 was expanded to tncl,•de variouscoatins used on small caliber weapons today. The purpose of this investigationwas to obtain intormation relatint to the frictional and wear properties ofcoatin'ls under simulated weapon conditions.

A friction and wear machine designed and built at Springfield Armory wasemployed in the investigation. The machine is unique in that it offers areciprocating type of motion while prnviding a plaae-to-plane type of contactbetween specimens durin• the sliding action. The machine is designed tooperate at speeds fron. 600 to 2000 rpm, under loads from 32.1 pounds to417.1 pounds, and strokes up to approximately 2.5 inches.

The previous study involved the factors influencin4 the friction ofphosphate coatings. This report includes the study of the frictinnal andwear properties of various coatings such as chromium plate and hard-anodizedaluminum.

2. PROCEDURE

a. Preparation of Test Specimens

(1) Machining.

The 4340 steel blocks used in the testing of chromium plateand phosphate coating were heat-treated and quenched to afinal hardness uf Rockwell C 45-50. The 6061-T6 aluminumblocks used in the testing of the hard-anodized coating had

hardneases of Brinell 83 to 86. Final surface grinding producedsurfaces that were parallel within .0002 inch and gavesurface roughness values of approximately six microinches(rma) as measured on a profilometer.

(2) Surface Preparation and Metal Finishing

(a) Chromium 3pecimens.

Prior to plating, the steel specimens were alkaline-cleaned and given a 15-second reverse etch. The platingwas done in a stardard chromium bath of 250 armo perliter at a temperature of 130 0 F. An average thicknessof .0005 inch was obtained in 30 minutes. The throwingpower of chromium plating was considered in that ananode of the same size as the wearing area of the largeblock was used. Some buildup at the edges occurred;

1. Appendix B - Bibliography, Listing No. I-3-

REPORTSA-TRIS- io8q

2. PRXEDURF Cont inurad

howeve r, It wan not impo-rtAnt sit'ce i L wnq not inwith the smaller bilock. vii. pintin,, of thr- smn!rI.required use of rohhprm around tfu. ,,d',-*. Pr,,I irmIIriction testIn,, revwaled that the. rhror.himtr- pin9t.,!we'r,, nlot devoid () of ii ditp in some' arr'ns. For rhith-e' smptc i sei were jioli.4h1ed Aftvr platai .,., prducisurfaces with ro..ihness reAdingsq of three to loutInches (rms).

(b) Steel Specimens.

Preliminary testLn-, oi the .it!cl si,-cimens r,'.iealecgalling tendencies at the minimum load of cre trac,.This problem was corrected to some de-rec by polisksamples. The surface roughness vaiucs prior to te!were three to tour microinches (rms).

(c) Phosphat:d Specimens

Prior to phosphating, the steel specimens were abrablasted with steel :grit (Number 120). The blocks w

then phosphated for 30 minutes at 2u50F. in a standmanganese phosphate solution with the following contotal acid 55-65 points, free acid 10-11 points, an.1 to .3 points. Prior to phosphatin;, the surfaceness was 63 microinches (rms); it was not significachanged after the phosphating.

(d) Hard-Anodized Specimens.

The aluminum specimens were hard-anodized accordingMHC process. The surface preparation consisted of I

blast. The 15 per cent sulfuric acid electrolyte wsat 280F. and the process was accomplished at a curreof .2 amps per square inch. An average thickness ofinch was obtained after one hoar. The surface roughof the hard-anodized specimens was 45 to 55 microinc(rMs).

b. Friction Testing

(1) Static Testing.

A simulated static coefficient was obtained by hand-operaof the mcvable block at a very slow rate. The maximum deon the recorder was considered to be the static frictionThe load was increased by adding weights to the weight tr

4-4'

REPORT

SA-TR18-1089

2. PROCEDURE - Continued

in one-pound increments, up to 10 pounds. The friction forceobtained for each load was taken when the recorder deflectionhad reached a constant value for each cycle of movement. Thiswas considered the break-in point for the coating.

A dry and a lubricated trial were conducted for each coating.All oile-' coatings were lubricated with MIL-L-644 oil. Thecoatings had a slight excess of oil since the blocks werealigned in the dry state arl lubrication followed correctalignment. Hydrodynamic lubrication was probably approachedduring the static testing.

(2) Dypnamic Testian.

Dynamic testing of the dry coatings was not possible. Thehigh friction forces and vibrations from the motor tended totilt the blocks out of position and galling occurred. Dynamictests were run on the lubricated coatings. Loads were increaseby applying weights in one-pound increments to the weight tray.The break-in point for each loAd usually occurred within 10 or15 seconds. The maximum deflection, representing the maximumfriction force for one cycle of movement, was taken at thebreak-in point, where the maximum deflections had reached asteady value.

Dynamic endurance tests were conducted for the followingspecimens: steel-on-steel, chromium-on-chromium, hard-anodizedaluminum on hard-anodized aluminum, and ohosphate-on-phosphate.The blocks were well-lubricated prior to testing and a constantload was applied. Initial testing under a load of 87.1 poundsdisclosed that only the steel-on-steel blocks would fail withina period of one hour. It was decided to increase the load to307.1 pounds to get aai earlier indication of the relative wear

aed resistance of the various materials.

c. Calculation of ".he C.efficient of Friction

The coefficient of friction is determined by dividing the frictionforce by the normal force. The normal force equals the load applied to theblocks. The load consists of two parts: the minimum load with no weightsattached, and the additional load developed by the attachment of weights.The minimum load consisted of the weight of the upper friction block attachedto its holder, and the reaction of the weight of the lever arm and weighttray applied at the point where friction occurs. This minimum load wasdetermined to be 32.1 pounds.

The lever arm acts as a second-class lever with a mechanical advantal

~-

REPORTSA-TR ISH-1! 89

of 11 to one. I'hus, an increase ol one pound on the weLrIhr tray resulan Increase in the load of II pounds.

'he frictioti force is caLculated from strain measurements obtairthe use of strain .n•.es attached to the recil;rocntlnp rod, which drivemoving triction Il.cL. i'he strain ;:.ages were callhrated under tensilecompressi'e Ioods to •.ive one millimeter recorder deflection Ior a 1 IH

pound l oad.

An Inalvsis oL tCe design of the friction machine revealed thatm3xinvur deflection should he Iivided hv a factor of four to ;ive the atriction torce between the surfaces ot the specimen blocks. The maximtlection tor one cycle of movement represents two frictional forces asas the total o( the frictional forces acting in compression and tensioThe two friction forces are developed at the upper and lower surfacesmiddle block where contact is made with the two stationary blocks. Thdifters by the weight of the middle block at the two contact points, bthis difference was neilected since the middle block weighs only 1/4 pThe following: equation represents the method of calculating the static,icienc of friction:

D 5/4UA . 32.1 + 11W

Us :Static coefficient of friction

D : Maximum recorder deflection

W : Load applied to weight tray

In reciprocating motion, forces due to acceleration or inertialare present because velocity is not constant. The rotary motion of th,is changed to rectilinear motion by a crank and connecting rod mechaniThe force due to acceleration was determined easily when the machine wierated with the middle block only and the friction forces were eliminaThe acceleration forces were dependent upon the length of stroke and tlber oi revolutions per minute of the motor. A short stroke and a low Ispeed were used so that the effect of acceleration could be minimized.values selected for all dynamic tests were a one-inch stroke and 600 r;Under these conditions, the maxim=.m recorder deflection for one cycle ,ment was found to be only one millimeter.

The following equation represents the method of calculating thecoefficient of friction:

(D-l) 5/4D 32.1 + I1W

ALD :Dynamic coefficient of friction

D : ",aximtum recorder deflection

W : Load applied to weight tray

.6-

REPORTSA-TR 18-1089

When high friction forces were present, it was necessary to change theattenuator on the amplifier from the I scale to the 4 scale. This, in as-sence, cut the deflection fourfold and increased the calibration factor to20 pounds per millimeter.

3. RESULTS AND DISCUSSION

a. Wear Resistance

Wear can be defined as the deterioriation caused by use and can usuall)be classified into two main categories: normal wear and destructive wear.Normal wear refers to the wear that is to be expected, i.e., the loss of materisfrom thI working surfaces when two materials are in contact during a slidingaction. Normal wear can be beneficial at times, since the surface roughnesscan be improved by the leveling of local projections on the surfaces.

Destructive wear refers to the transfer of metal from one surface toanother by welding under sliding conditions. The welding Is caused by greatlocalized pressure in which sufficient temperature is generated to weld thesurfaces together. Terms such as galling, scuffing, scratching, Ind scoringusually are used to express varying degrees of damage by welding.

In this investigation, wear resistance will refer to the ability toprevent galling and other types of severe surface damage. A criterion inthis investigation for good wear resistance is a near constant relationshipbetween load and the coefficient of friction. A sharp increase in the coefficieiof friction would strongly indicate that destructive wear is occurring.

The plot of the static coefficient of friction versus load for the un-lubricated steel specimens (Fig. 1) reveals a tendency for the coefficient torise with increasing load. At lower loads this increase in the coefficientappears to be caused by some slight destructive wear at the surfaces. Thiswear was probably caused by light scuffing or minor welding between the steelspecimens. At a higher load the wear was such more severe as definite gallingoccurred and there was a sharp increase in the coefficient of friction.

The dry, chromium-plated specimens exhibited much better wear resistancdthan steel. The plot of the static coefficient of friction versus load (Fig. 2)showed that there was a• approximately constant relationship. At the conclusionof the static test, the chromium-plated ipecimens showed no visual damage as thegalling tendency of chromium was much loes than that of steel.

The static friction tests for dry, hard-anodic coatings and dry phosphatcoatings revealed that little or no destructive wear occurred. The coefficientsof friction were higher than for either the steel or chromium-plated specimens,but the coefficients remained reasonably constant with an increase in load (Figs.3 and 4).

2. Appendix 3 - Bibliography, Listing No. 23. Appendix B - Bibliography, Listing No. 3

o en-7-

RE.PO1RT

SA-TR I$- 10i8Q

.3. RFIS'1..TS AND I)TSOIISS ION ContIntied

Two signir I cant pointi are noticeable front thh, rpel lts of the wt.s

involviikn ctmbinations of two dillerent matprials. Th,, reqgilrct nf to|ts I

dry steel in coitAct with either chromium plate or hnrfl-nroddiied Aluminum

that some destructive wear occurs at the steel siurfac,.. In Figures I and

static coetticietit ot Irtction increased gradually as the load was Incresaq

Figure 7, however, when, a dry, phosphate-coated specimen Is in cortact wit

steel specimetn, the static coe;ficient of friction remained constant with

increase in lond. Phosphate coatings, when in contact with a smooth iteol

appear to reduce significantly the possibilities or gallinlg.

The results for the endurance tests on the variouq surfaces can b

in Figure 11. The endurance tests consisted of a dynamic test at 60f rpm

a relativeLy high load (307.1 pounds). The poor wear resistance of steel

substantiated by the fact that the specimens were severely galled after apj

2-1/2 minutes or 1500 cycles. The chromium-plated samples underv.ent sever,

aoter approximately six minutes. The specimens were well-lubricated prior

testing. During the dynamic test, sufficient lubricant was retained on the

surtace of the movable block, but the amount of lubricant on the upper sur:

gradually diminished as the test progressed. The failure attributed to go"

usually occurred at the upper surface because of this lack of lubrication.

The endurance test for phosphate coatings and hard-anodized coati

(Fig. 11) reveals that the wear resistance of both coatings is better than

of chromium. The primary factor influencing this result is believed to be

by the surface roughness of the individual coatings. The hard-anodized cos

and the phosphate coating are porous and have the ability to absorb and ret

lubricants, while the chromium-plated specimens had been polished and had I

ability to retain oil.

It is also interesting to note from Figure 11 that the failure of

steel and chromium blocks was very abrupt. It appeared that the lack of lu

tion in areas led to rapid overheating and, consequently, galling occurred

instantaneously. The coefficient of friction versus the number of cycles c

for the hard anodic and phosphate coatings shows, in time, a gradual incres

the coefficient caused by the simultaneous loss of lubricant and wearing aw

the coating. The sharp increase in the coefficient for hard-anodized alumi

after approximately 11,000 cycles represents the point at which the bard-an

coating was completely removed and bare aluminum was exposed. It appears t

inorganic finishes, such as hard anodic and phosphate coatings, have better

galling characteristics, in general, as compared to metallic surfaces.

b. Coefficient of Friction

Hardness is considered to be a significant factor influencing the

resistance of a material. This is becausq surfaces undergo compressive str

while wearing conditions are present, and since hardness tests are tsually

pressive, hardness can be employed as a significant index of merit.

REPORTSA-TR18-1089

3. RESULTS AND DISCUSSION - Continued

Hardness also might be expected to have some influence on the coefficientof friction because of its O'ise association with wear resistance. The hardnessvalues for chromium plate and hard anodic coatings are generally considered to berelatively high, although the values vary considerably and are sometimes difficultto obtain. The hardness of bright chromium plate is equivalent to 950 to 1050Vickers. Microhardness tests on hard-anodized surfaces have shown hardness valuesof 530 VPN for a coating produced by the Martin hardcoat process on 6061-T6 alloy.

A comparison of the static coefficients of friction between the dry chromium plate and hard-anodized surfaces showed that the chromium on chromium platehad a much lower coefficient of friction. The static coefficient for chromiumplate averaged .16 while the static coefficient for hard-anodized aluminum on hardanodized aluuinum averaged .55 (Figures 2 and 3). The difference in the coeffi-cients appears to be$a function of the surface roughness rather than the hardnessof the respective samples. The surface roughness of the chromium plate was threeto four microinches while the hard anodic coatings were 45 to 55 microinches (rms)The coefficients of friction for hard anodic coatings have been quoted to be below.15, but these values are for hard anodic surfaces which have been lapped or honedafter anodizing to improve the surface finish.

The chromium-plated and steel specimens had approximately the saw surfac,finish. The static coefficient of friction for steel was always higher than thatfor chromium, even at the lightest loads where no galling occurr-ed for the steel.The minimum coefficient for steel was .20 at a load of 32.1 pounds.

Very high static coefficients of friction were obtained when phosphate-coated blocks were in contact with each other. The high coefficient was primarilycaused by the rough nature of the grit blast pretreatment. The dry phosphatecoating might retard galling tendencies; however, it did not improve the lubricitybetween the rough surfaces.

The dry phosphate coatings in contact with smooth surfaces, such as thechromium plate and bare steel samples, had a much lower coefficient. The staticcoefficient of friction was less than .2 (Figs. 7 and 8). The phosphate coatingin contact with the smooth surface during the break-in period prevented metal-to-metal contact and appeared to have some beneficial lubricating qualities. Thedry phosphate coating in contact with the rough haid-anodized surface did notimprove the static coefficient of friction (Fig. 10).

c. Lubrication

The lubricant used throughout this investigation was MIL-L-644 oil. Thisgeneral purpose preservative and lubricating oil is suitable for use in lubricatiotand corrosion protection of small caliber weapons.

-9.

REPORT

SA-TR18-1089

3. RESULTS AND DISCUSSION - Continued

The coefficients of friction of the various surfaces, when lubridid not vary to any extent from coating to coating. The only exception tooccurrence was in the case of the steel-on-steel specimens, when the stati,coefficient reached values as high as .18. The static coefficient for allother coatinAs was usually in the range of from .13 to .15, while thedynamic coefficient usually was in the range of from .12 to .15.

The results indicated that during the simulated static test, theslight excess of oil present produced hydrodynamic lubrication in which nonietal-to-metal contact was present. In the case of steel on steel, hydro-dynamic lubrication was probably not completely present since the smoothsurfaces had no cavities to retain oil. The lubricated chromium specimens,although they were equally as smooth, did not have a higher coefficient offriction.

During the dynamic testing, the load was increased approximatelyevery 30 seconds. Hydrodynamic lubrication was not present when the hitherloads were applied. This was evident because the samples were polished andscratched to some extent. The dynamic coefficient of friction, however, diappear to be significantly affected by the change in type of lubrication.The coefficient in some cases was slightly higher at the higher loads. Thiincrease in the coefficicnt occurred in random fashion and could not beassociated with any particular coating or coatings. It is believed to beprimarily caused by vibrational effects at some loads in which the weights*to the lever arm were not held stable.

The static and dynamic coefficients of friction for well-lubricattsurfaces strongly indicate that the coefficients of friction are primarilydependent upon the nature of the lubricant, while the coating has little orno influence upon the coefficient. The minimum coefficient of frictionprovided by MIL-L-644 oil appears to be approximately .12. Although nosystematic study involving another lubricant was made, friction tests of adry film lubricant on hard anodized aluminum showed that the static coefficiwas reduced to approximately .09.

,. ~-10o

"* REPORTAPPENDICES SA-TR18-1089

A - Figures

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REPORTSA-TRI8-1089

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BIBLIOGRAPHY

I." George, M. A., Factors Influencing Friction of Phosphate Coatings,SA-TRI8-1084 (April 1964)

2. Almen, J. 0., 'Surface Deterioration of Gear Teeth" Mechanical Wear

A.S.N. (January 1950) p 230

3. Almen, J. 0. p 267

4 Avery, H. S. "Testing for Wear Resistance" Surface Protection AgainstWear and Corrosion A.S.M. (1954) p 4

5. Van Horn, H. S. Metal Finishing (1952) 50, No. 6, p 110-2

-23-

r¶

UNCLASSIFIEDSecurty Classification

DOCUMENT CONTROL DATA - R&D(Secutfty classificatmon of ite. bodyO of ebaet t a" indexing annotation must be entered wh.en the overall report is claasilied)

J I ORIC91NATING ACTIVITY (Corporate author) 24. NCPOOT SECURITY CLA ICATIONFSpringfield Armory, Springfield, Massachusetts UNCL~ASSFE

3. REPORT TITLEFRICTIONAL AND WEAR CHARACTERISTICS OF VARIOUS CCATINGS USED ON SMALL CALIBERWEAPON COMPNTS

4. DESCRIPTIVE NOTES (T ype of reportand inchui*ve date@)

Technical ReportS AUTHOR(S) (Lest name. firit name, Initial)

George, M. A.

4. REPORT DATE 70. TOTAL NO. OF PA699 b. No. or cps

17 September 1965 28ia. CONTRACT OR GRANT NO, N/A #a. ORIGIiATOR$S REPORT NUMBER(S)

'. PROjCT No. MI-4-50020-01-M1-M6 SA-TR18-1089

c AMCMS CODE 5016.11.84400.02 9b O•W'NERR.oNS) (no., see,,,,,. ,sy be e..,eed

d. None10 AVAILASILITY/LIMITATION NOTICES Qualified requesters may obtain copies of this report

4 from the Defense Documentation Center, Cameron Station, Alexandria, Virginia 2231Other requesters may purchase copies of this report from the Clearinghouse, Depar

ament of Comnerce. Sprinafield. VirginlA 22151-11. SUPPLEMENTARY NOTES 12. SPONSORINO MILITARY ACTIVITY

None U. S. Army Materiel Command

13, ABSTRACT

Friction and wear studies, in which Springfield Armory frict!onand wear machine was used, were continued. Information relatingto the frictional and wear properties of coatings was obtainedunder simulated weapon conditions. Various coatings such "phosphate, chromium plate, and hard-anodized aluminum were evalu-ated. The rough porous coatings such as phosphated steel andhard-anodized aluminum generally had high coefficients of frictionwhen unlubricated, but exhibited good wear resistance when lubri-cated. Chromium plate, when unlubvicatad, had a lower coefficientof friction than steel of equal surface roughness. Metallic sur-faces such as chromium plate and hardened steel had limitedantigalling properties. Procedure is given, and results are dis-cussed.

"wood

_______Uf4I..IA~Lf LW __U

Secunity ClaS16fcaiiton

1.KEY INRO L.IkK A S

Wear

F

2. ]Protectivetreatments

finishing

INSTRUCTIONS

I. ORIGINATING ACTIVITY: Enter the name and address 10 AVAIL ABILIT Y/LIMITATION NOTICES: EnterOf tecontractor, tubcontracttr, prant..,DeatetoDe Rainonfrhrds mnton fterprohr

atthe t rreporranzt.o ( *t~ore author) issuing imposed by security classification, using standard at

2a. REPORT SEUIIYCASFCTO:Etrteofer (1DQaiieeeuser.a baioisoall security classification of the report. Indicate whether rpr rmDC"Restricted Data' Is included. Marking is to be In accord- D.

oncewithappoprite scurty rgulaion . (2) S "PoLeignAR a noun Emen an isemfor adtional2b.~~~~~~~~~~~a 014PAuoai onidginseiedayDo tory reotrt by- Ci o uhrx

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