AD

SWEAR.TR.72.9

A SURVEY OF SOLID WUBRICANT TECHNOLOGY

TECHNICAL REPORT

Peter Martin, Jr.

March 1972 MAY 15 1972

RESEARCH DIRECTORATE-

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMANDRPovmuod by

NATIONAL TECHNICALINFORMATION SERVICE . ..... ..

lprin ltleld, Vai 7215

Approved for public release, distribution unlimited. 9

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DISCLAIMER:

The fifldirsX. of this report are not to be ccnstrued as anofficial Department of the Army position unless so designated byother authorized documenrts.

J'-am

AD

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMAND

TECHNICAL REPORT

SWERR-TR-72-9

A SURVEY OF SOLID LUBRICANT TECHNOLOGY

Peter Martin, Jr.

March 1972

DA IW562604AA91 AMS Code 552D.11.80800.01

Approved for public release, distribution unlimited.

i

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ABSTRACT

New developments in the areas of solid lubricant pow-ders, bonded films, anc self-lubricating composites havebeen reviewed along with new application techniques by theResearch Directorate, Weapons Laboratory at Rock Island.

Recently synthesized lubricant solids include sulfides,selenides and tellurides of heavy metals. Some of these newmaterials have been incorporated into oils, greases, dryfilms, and composites with resulting improved propertiessuch as lower friction coefficients, greater oxidation sta-bility, and chemical stability.

The high temperature and wear life properties of bondedsolid film lubricants have been impruved by use of new or-ganic binder materials such as polyimides, polybenzimidazoles,and polybenzothiazoles. The use of inorganic binders ofsilicates, phosphates, and fluorides has extended the temper-atures to 1200OF with some impairment of performance at roomtemperatures.

New techniques for appi ing solid lubricants are thoseof plasma spraying, electrophoresis, electrodeposition,vacuum deposition, and ion plating.

Self-lubricating composites, recently developed, areof the metal- and plastic-based types. Metal-based compon-ents have higher strength and higher temperature capabilityas compared with that of the plastic composites.

Plastic composites have been improved for higher strengthand heat stability by use of aromatic polyimides. Poly-tetrafluoroethylene-hased composites have been improved inhigh strength properties by use of glass and carbon fibers.

A

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DOCUMENT CONTROL DATA.- R D '(Secmrity daelt~ifcain .1 title.btyo "ebt,. ORd steindlln aflRtfvien mov, be entetd w%.en the9w. l .1.1.1 toI csa tii g3 8 gf

I. ORIGINATING ACTIVITY (Coopieftl EIft" So. REPORT SECURITY CLASSIFICATION

U. S.I Army Weapons Comrand UnclassifiedResearch, Dev. and Eng. DirectorateISGRURock Island, 11linois 61201

S. REPORT TITLE

A SURVEY OF SOLID LUBRICANT TECHNOLOGY (U)

4. DESCRIPTIVE P40TUS (7ypo 4" an1..r d iMcintLV doe@@)

6. AUTI4ORIS) (Pisee nam, Idd.M12 Ifjti.lesot -to"S)

Peter Martin, Jr.

9. REPORT OATZ 70. TOTAL NO. O0 PAGES ?*. NO00 11140s

March 1972 29 38$0. COMTR;CT 114 414ANT NO. so. ORIGINATOR'S REPORT NUMSIIUERISI

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..Oy~afR REPORT NO(S (Aaw 00senaftm& ol1mev "010"CgId

AMS Code 552D.11.80800.01d.

10. DISTRIBUTION STATEMENT

Aporoved for public release, distribution unlimited.

11. SUPPL&MKNTARV NOTES It. SPONSORING MILITARY ACTIVITY

U. S. Army Weapons CommandA

Il- BSRATNew developments in the areas of solid lubricant powders,bonded films, and self-lubricating composites have been reviewed alongwith new application tecnniques by the Research Directorate, WeaponsLaboratory at Rock Island. Recently synthesized lubricant solids in-clude sulfides, selenides and tellurides of heavy metals. Some of thesenew materials have been incorporated Into oils, greases, dry films, andcomposites with resulting improved properties such as lower friction co-efficients, greater oxidation stability, and chemical stability. Thehigh temperature and wear life proper'ties of bonded solid film lubricantshave been improved by use of new organic binder materials such as poly-imides, polybenzimidazoles, and polybenzothiazoles. The use of inorganicbinders of silicates, phosphates, and fluorides has extended the temper-atures to 1200*F with some impairment of performance at room tempera-tures. New techniques for applying solid lubricants are those of plasmaspraying, ele~.trophoresis, electrodeposition, twd vacuum deposition, andion plating. Self-lubricating composites, recently developed, are of thmetal- and plastic-based types. Metal-based components have higherstrength and higher teimperature capability as compared with that of theplastic composites, Plastic composites have been improved for higherstrength and heat stability by use of aromatic polyi.mides. Polytetra-fluoroethylene-based composites have been improved in high strengthproperties by use of glass and carbon fibers. (U) (Martin, Peter, Jr.)l

DDI F"6m .1473 :::r~. .UIN0Unclassified

Unclassified

rLINK A aIK L.N MKev WO RDII

**Li Wt fOLC WT mOLU WI

1. Solid Lubricants

2. Dry Film Lubricants

3. Self Lubricating Composites

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UnclassifiedSecurity Claesiftaitfet

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CONTENTS

Page

Title Page

Abstract ii

Table of Contents iii

Introduction

Approach 2

Discussion 2

Summary 16

Conclusions and Recommendations 18

Literature Cited 19

Distribution 24

DD Form 1473 (Document Control Data - R&D) 28

A

INTRODUCTION

A literature survey dealing with solid lubricationtechnology has been made by the Research Directorate, Weapons

Laboratory at Rock Island. This technology is approximately20 years Impetus for the growth of this technologyhas come from the lubrication requirements of jet aircraft.During the last 10 years, lubrication requirements forspace applications have provided additional incentive forthe advancement of solid lubrication. The general effortexpended to advance solid lubrication is small when comparedto that expended on liquid lubrication; however, progressmade on solid lubricancs during the last 20 years has be-nsubstantial.

Solid lubricants are usually considered fcr use onlywhere oil and grease lubricants cannot be used. Theseareas include high loads, extreme temperatures, high vacuum,and adverse environments. Solid lubricants, however, canalso be used under moderate conditions in areas commonlylubricated with oils or greases. These lubricants can bein the form of powders and bonded films, or additives inoils and greases, and in the form of an integral part orcomposite.

The use of solid lubricants provides the following ad-vantages: (1) dry airborne dirt does not collect, (2)stability at high temperatures, (3) high load capacities,and (4) application by which simpler design is permitted byelimination of the need for oil seals and pumps.

The use of solid lubricants entails, however, thefollowing disadvantages: (1) the friction coefficient isgenerally higher than that of the fluid lubrication, (2)bonded film coating has a finite wear 1 fe, and (3) this

coating is not applicable to high-speed areas, such asautomotive engines.

However, the outstanding characteristic of solid lubri-cants is the concept of permanent lubrication for the lifeof the system. This feature has definite potential use inthe zrea of weapons in which periodic maintenance of lub-ricating and cleaning are required. The application ofsolid lubricants to weapons would result in reduced main-tenance, reduced wear, dust-free lubrication, and elimin-ation of the "wear in" period.

A

Howc- r, selection of the proper solid lubricantsyttem for a specific application is a complex lubricationengineering task. This effort has been made more difficultbecause of the rapid development of solid lubricationtechnology. Therefore, a systematic survey is needed to-indicate the current state of the art and to provide abasis for the optimui selection of solid lubricants forlutu-e weapon applications.

I • port hts been writte,, in the form of a litera- jturc s, ey and comr.'ises three parts. Part I deals withSol 'joricant powders, Part II involves bonded film solidlubr,cdnts, and Part III concerns information on solidlubricating composites.

ISCUSSION

Part I. Solid Lubricant Powders

Three solid lubricant powders - graphite, molybdenumdisulfide kMoS 2 ), and polytetrafluoroethylene (PTFE) - havebeen given the most attention and application.

Graphite lubricates best in the presence of noisture.This limits its applications since the presence of watercan ca.,se galvanic corrosion when other metals are in con-tact. Molybdenum disulfide oxidizes readily at temperaturesabove 600°F; this limits its use at high temperatures.Polytetrafluoroethylene produces a low friction coating

but cold flows under high loads, and decomposes above 6000F.

The limitations of these materials has stimulated re-search toward the synthesis of other solid lubricants. Theresearch was aimed at the production of materials withlamellar, hexagonal structures similar to molybdenum di-

sulfide. These new materials were sblfides, selenides,and tellurides of heavy metals. These materials were pro-duced by Boes3, Magie , and Devine The new materialswere characterized by having greater oxidation stability,electrical conductivity, and chemical stability than thestandard solid lubricants.

The air stability and coefficient of friction valuesfor the new materials and MoS, are given in Table I.

2

3

TABLE I

NEW SOLID LUBRICANTS

Air Stability Coeff. ofCompound (OF) Friction

Molybdenum disulfide (MoS 2 ) 660 .18

Molybdenum diselenide (MoSe 2 ) 750 .17

Tungsten disulfide (WS2 ) 820 .17

Tungsten diselenide (WSe 2 ) 660 .09

Niobium disulfide (NbS 2 ) 790 .08

Niobium diselenide (NbSe 2 ) 660 .12

Tantalum disulfide (TaS 2 ) 1110 .05

Tanta'um diselenide (TaSe 2 ) 1070 .08

Arsenic thioarsenate (AsAsS 4 ) 750 ----

Arserc thioantimonate (AsSbS4 ) 750

Graphite fluoride (CFx) 750 .05x

For actual applications, these powders may be used inother forms. They can be (a) dusted or burnished and im-pregnate( into the mating surfaces, (b) suspended in acontrolled stream of carrier gas, (c) dispersed in a binderto form a thin film, (d) dispersed in a metal or plasticmatrix (composite), and (e) formulated into oils and greasesas extreme pressure additives.

Magie2 has reported on the work performed with Ag-NbSe 2compacts. The compacts were evaluated for use as commutatorbrushes. These brushes gave considerably better electricalproperties, yet gave wear rates comparable to Ag-MoS, brushes.

WSe 2 has been combined in powdered form with galluim-indium alloy by Boes. This work resulted in a compact foruse as self-lubricating members in high speed - high tempera-ture ball bearing systems. This material is of considerableinterest since it resists oxidation at temperatures muchhigher than pure WSe2.

riubbellO, developed compacts with cobalt containingWS,. These materials performed well as separator materialsfoi use in the lubrication of ball bearings. The materialsdeveloped have potential for widespread use in many diff-erent machine element applications such as sleeve andspherical bearings, and gears and tracks.

Two sulfides, AsSbS, and AsAsS,, were prepared and ex-amined as chemical additions for diester lubricating greaseand organic resin solid film lubricants by Devine! Thearsenic sulfides exhibited wear resistance superior toMoS 2 in similar concentrations in the base grease. However,the arsenic sulfides showed no improvement when comparedwith MoS, in resin-bonded solid film.

Fusaro' studied the friction and wear life of burnishedfilms of graphite fluoride. Test comparisons indicatedthat the wear life of CFx films exceeded that of graphiteor MoS . Moisture seems to benefit the wear life of graphitefluori'e, but it is not essential for lubrication.

Additional studies on graphite fluoride have been con-ducted by Petronio'. The solid lubricant was formulatedwith silicate and ep(xy-phenolic bincers, and with diester-based grease. Preliminary studies indicated that qraphitefluoride extended the wear life of bonded Films as comparedwith graphite-bonded films.

4

Part 11. Bonded Solid Lubricants

a. Resin and Ceramic Bonded Films

Bonded or "dry film lubricant" is used todesignate a series of lubricating materials consisting ofa thin film of solid lubricant (MoS 2 , graphite) attachedto tha surface to be lubricated with an adhesive material.Tnis adhesive material may be organic (phenolic, epoxy,polyimide resins) or inorganic (sodium silicate, alu-rinumphosphate) in nature. The organically bonded dry filmlubricants can be subdivided into heat-cured and air-cured,dry film lubricants, and the inorganic films can be sub-divided into nonceramic and ceramic binding. Dry filmlubricants may be applied by the most common methods ofdipping, spraying, and brushing. New methods of applica-tion comprise electrophoretic deposition, plasma-spraying,and vacuum-sputtering. These new methods appear feasible,but are still in the developmental stage.

A list of recently developed dry film lubri-cants, with the method of application, is included in Table II.These new materials are a result of the emphasis on high-temperature applications and improvement of endurance life

In the past, organically banded solid lubri-cants have been temperature-limited by the binder materials.However, new resins for use as binder materials have nowbeen developed that are more thermally stable than MoS 2 ."The use of these new binders has been examined by severaiinvestigators and is snown in the first four filb ofTable II.

A high-temperature resin polyimide (PI) ',asused as the binder or new solid film lubricant designatedas MLR-2. The work, reported by Campbells, showed thatpolyimide-bonded lubricant films exhibited usable wearlife to 700-F.

McConnell' has worked with polybenzo~hiazole(PBT) and has indicated that films (AFSL-26) bonded withthe PBT resin have superior wear performance above 500'F.

Polybenzimidazole (PBI) is another high-temperature resin developed by Hopkins" for use as a bond-ing agent. The new solid film lubricant AFSL-32 has a.nO additive that is beneficial to wear life. This bene-ficial effect by the addition of an oxide has been ex-plained by an oxide interaction hypothesis. In this

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concept, the friction and the wear of bonded films, simpletransfer films, and composites of MoS are decreased byuse or low melting oxides. These o;ides are combined easilyto form desirable eutectics with molybdenum oxides. Theresults o- Hopkins' study and those of other investigationsseem to support the hypothesis for oxide interaction.

All new resins cited above required a heat-curing process; however, a methyl phenyl silicone resin wa.s.elected by Benzing' as a binder for an air-cured solidfilm lubricant. The purpose of this work was to obtain asolid film lubricant that could be applied in the fieldand that would retain performance characteristics compara-ble to those of heat-cured films. The material was desig-

nated as AFSL-41, and could be used at temperatures to 700cF.This material also has the unique capacity to perform wellon titanium surfaces in addition to the more conventionalsteel alloys.

The high temperature inorganic solid lubri-cants are an extension to the resin bonded films. Thesematerials are intended for use at temperatures from 500OFto 1200OF and above. Salt-based or ceramic binders areused to give greater temperature resistance than that ofresins, and usually contain solid lubricants which are morethermally stable than graphite or MoS,. These solid lubri-ca,,ts can be lead oxide, lead sulfide, calcium fluoride,gold, silver, tellurides, and selenides.

The nonceramic binders are usually watersoluble silicates and phosphates which produce a hard coat-ing that tends to be brittle when cured. In general, thesenonceramic binders can be used at temperatures from 300 to1000°F. Hopkins. 2 reports on an inorganic nonceramic suoidfilm lubricant developed for service in space environments.This material contains MoS., graphite, and gold dispersedas the solid lubricants in a sodium silicate binder. Thematerial had good high load-carrying capacity, and thefilms exhibited lower friction coefficients in vacuum thanin air.

A continuation of this development was thework by Wieser: on an improved solid film consisting ofMoSeý, TaSk, and graphite in an aluminum phosphate binder.Experiments were performed to find out whether the wearlife of the original film can be improved by an overcoatapplied by electrophoretic deoosition. The second coatthat was deposited consisted of MoS. and B:O. Resultshave indicated an improvement in the wear life at 10000F.

7

The in3rganic ceramic bonding agents areglasses that melt when heated. When cooled, these glassesserve as a bonding agent for the dispersed solid lubricant.Although ceramic-bonded materials have higher coefficientof friction and shorter wear life than resin-bonded ma-terials at room temperature, these materials generally ex-ceed the performance of a resin film at higher temperatures(>10001F). However, subjecting ceramic bonded film to highspeeds at room temperature will result in high temperaturesgenerated at the contact zone; this could result in a per-formance comparable to resin-bonded films. One problem inthe use of ceramic-bonded materials is that of the thermalexpansion of the cured coating. This must be matchedclosely with the expansion of the base material to preventfracturing or flaking.

Extensive work by Sliney'" resulted in theestablishment of friction and wear characteristics of fluo-rides as ceramic binders. The films made with these bindersexhibited the following disadvantages: (1) two operationsare required for preparation of the film, (2) high-fusingtemperature is detrimental to many alloys, and (3) filmshad to be applied thick, then ground to achieve desiredfilm thickness.

The study of fluoride-based films was exten-ded by Lavik 1 s. The performance of these films (AFSL-28)indicated that operation in the 750-F to 1450'F range maybe expected to give relatively long wear life at compara-tively high loads. For operation between room temperatureand 750%F, long-term, light-load conditions can be bestachieved with the addition of 10 per cent gold powder tothe films.

In a more recent study by Hopkins, 1 6 prefusedfluorides were combined with MoSz, SbzO3, and polybenzi-midazole. The addition of fluorides enhanced the wear lifeperformance of the original PBI-MoSa films (AFSL-27) in therange from room temperature to 600:F.

b. New Bonding Methods

Solid film lubricants that must be cured fora certain time at elevated temperatures, particularly fusedceramic bonded films, are not feasible for production lineor field application becaus2 3f the time and the facilitiesrequired for curing. In addition the high curing tempera-ture may affect the mechanical properties of the substratemetal or alloy. Therefore, the most desirable processesare those in which films can be applied and cured at thesame time or processes in which deposition of the films

8

results in greater energy of adhesion.

The process with the use of a plasma spray gunfor the application of various solid lubricant films wasdescribed by Hopkins.- This process involved the success-ful application of metal, resin, and ceramic solid lubri-cants. However, the partial thermal degradation of MoS 2to elemental molybdenum and MoO 3 during plasma-sprayingled to further studies by Kremith'? to reduce thermal deg-radation. Kremith's approach involved the use of metalcomposite powders which not only provided thermal protec-tion for the solid lubricant but also decreased materialsegragation during the spray operation. The compositepowders were produced by a special fluidized bed processwhich allowed encapsulation of the solid lubricants withmalleable, metallic coatings such as copper, nickel, orsilver. Additional work by Hopkins's indicated that othercoatings, such as CaF 2 :BaFz combined with MoS, and boro-silicate glass combined with MoS 2 , appear promising asplasma applied deposits.

Other approaches for the development of betteradhesion methods for solid lubricants concerned the use ofelctrophoresis, electrocodeposition and electroplatingtechniques.

In the electrophoretic coating process, a DCpotential is applied dcYoss two electrodes immersed in adispersion of colloidal particles suspended in a nonaqueousmedium, and the particles are further dispersed to one ofthe electrodes at which the charge of the electrode isneutralized, and the particles are deposited. Hopkins, 8

reported that work on electrophoretic deposition resultedin only moderate success. The following problems wereencountered: (a) Depositioan parameters could be estab-lished only by trial and error. (b) Lubricant or bindermay preferentially deposit. %c) Reproducibility of wearlife results was poor. Wieser" indicated that electro-phoretic deposition as an overcoating process providedadditional wear life to {MoSezTaSI.graphite-AlPO.) solidfilms.

Both electrophoretically and electroplatedfilms of MoS2 were studied as solid lubricant film for in-strument bearings by Kirkpatrick. 9 The best performancewas obtained with MoSZ deposited by electroplating.

Electrodepositing of a soft ductile metallicbonding element containing a refractory metal lubricating

9

element was examined by Thornton." 0 The work indicatesthat the Au-Mo film system provides the best traits offriction ana wear.

Vest"1 also used electrodeposition to form aNi-MoS 2 coating on aluminum alloys. Load-carrying capacityon the soft substrate was low and the idhesion of the filmwas not reproducible.

The deposition of highly adherent surfacefilms by vacuum methods has unusual promise. Vacuum de-position methods can be divided into two categories: (a)low energy deposition methods (chemical vapor depositionand flash evaporation) and (b) high energy or plasma de-position methods (d-c, r-f sputtering and ion plating).The efficiency and effectiveness of using variou: high andlow energy deposition methods for the application of solidfilm lubricants was discussed by Spalvins.Z" His observa-tions indicated that the kinetic energy of the impingingmetal atoms has a direct effect on the film formation bywhich, in turn, the strength and the durability of the filmare determined Results show that d-c and r-f sputteringtechniques can be used to apply solid film lubricants. The0endurance life of an MoS d-c sputtered film 2,OOOA thick wasgreater than five times that of a conventional resin-bonded

0solid film 130,OOOA thick in vacuum environment.

Metallic films (Ni, Au, Ti, Pb) have been de-posited successfully by ion plating Spalvins4: indicatedthat strong bonding of the film is established because ofthe high energy of the plasma (ionized argon) and the highkinetic energy of the depositing material. Friction experi-ments indicated that, as the ion-plated film wore off, agradual increase occurred in the coefficient of frictioninstead of the sharp rise usually observed Tensile testsshow that the ion-plated film plastically flows with thebulk materia; without peeling or scaling

Part ill. Composite Lubricantb

Since resin-bonded solid lubricants have a finite wearlife and must be replaced or repaired, various means havebeen investigated to lengthen their wear life. Such inves-tigations have resulted in the self-lubricating or composite-lubricant materials In these materials, solid lubricantpowders are incorporated into a basic matrix by vacuum in.-pregnation, sintering, and high- or low-temperature highpressure compaction The basic matrix can be metallic- or

10

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V polymeric-fo-tified with fillers and reinforcement materialsfor strength. Lubrication is accomplished by a transferof solid lubricant from within the structure of the com-posite to the interface between the moving parts. Thistransfer results in the formation of an effective lubri-cating film.

A compiled list of recently developed composite lub-ricants, methods of preparation, and compressive strengthsis contained in Table III. Wear rate data on some of thecomposites listed in Table III are shown plotted in Fig-ure 1. The enclosed areas represent cluster analysis ofdata instead of individual data points.

Extensive work on metal-carbon fiber composites hasbeen accomplished by Giltrow and Lancaster." The carboncomposites containing metals (Pb, Ag, Cu, Ni, Co) haverelatively low wear rates but high coefficient of frictionvalues. The addition of a solid lubricant (MoS2, NbSe2,W~ee) does result in lower coefficient of friction valuesbut slightly higher rates of wear.

Composites based on a metal matrix have been investi-gated by Boes, and the development of self-lubricatingcomposites has been emphasized for use in bearings and gears.These composites were made of two types: (a) a solid lub-ricant dispersed in a metal matrix (Ag, Cu), and (b) PTFEand a solid lubricant dispersed in a metal matrix Ag, Cu).The friction and wear characteristics indicated in Figure 1show that the addition of PTFE reduced the wear rates ofthe original composites. These composites were successfullyused as cage materials in ball bearings at cryogenic tem-peratures, 400°F and high vacuum (lXlO7-Torr).

Hubbell" described the fabrication and testing oflubricating composites for use as ball bearing separators.The hot-pressed solid lubricant compacts were composed ofa solid lubricant dispersed in a tantalum and cobalt-silvermetal matrix. Actual bearing tests conducted showed bear-ing life greater than 6,700 hours in an air environment at3,450 rpm with a 10-pound axial load.

Additional work by Campbell'? was conducted on the de-velopment of a lubricating composite for high-speed, high-temperature bearings. Composites were compounded withMoS, in a matrix of iron and platinum. These compositeswere then machined as retainers for both ball and rollerbearings. They operated successfully in bearing tests at15,000 rpm, 6809F in vacuum (IXO- 5 Torr).

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FIGURE 1COMPARATIVE PERFORMANCE OF VARIOUS

TYPES OF SELF-LUBRICATING COMPOSITES

Weapons Laboratory at Rock Island, Research Directorate.Materials Science and Technology Division 11-'99-1596/AMC-72

Page 13

Qi

A unique solid lubricating composite was developed byBoes 2 which resists oxidation at temperatures up to1500 0 F. These composites were successfully used as retain-ers for ball bearings capable of long-term operation underhigh-speed, high-load, and high-temperature conditions.These composites were formulated from WSe2 - gallium/indiummatrix. Major advances in operating life over the formerstate of the art were achieved by a combination of improve-ments in retainer design, bearing design, and compositelubricating characteristics.

Other high-temperature lubricating composites were de-veloped by Sliney.2 These composites consisted of porousnickel and nickel/chromium vacuum impregnated with moltenfluorides. In air, the maximum useful service temperatureof the nickel composites is about 1lO00F; the correspondingtemperature for the nickel/chromium composites is about1350*F. Higher friction coefficients were obtained withthese porous metals than with coatings of the same fluoridecomposition on dense metals; however, low wear and excel-lent wear life were obtained. The advantages of coatings(lower friction) and of composites (longer wear life) werecombined by application of a thin sintered film of eutecticfluoride to the surfaces of the composites

Nickel/MoSs composites were developed and examined byKauzlarich." These composites show high wear rates, butlow coefficient of friction values. The areas of applica-tion of these composites have not yet been fully developed,nor have their physical and lubricating characteristics beenfully established

A considerable amount of work has been done on the useof plastics as self-lubricating materials. Some of the mostsuccessful and extensive uses have been in the form of in-serts in plain bear'ngs, as reinforced thin sheets on plain

* spherical bearings and as the retainer material for ballbearings. The most notable plastics used are the nylons,acetals, polytetrafluoroethylene, phenolics, and poly-propylene. Fillers and lub-icants incorporated to givethese materials higher strength and lower friction are glassand ceramic fibers, copper, lead, molybdenum disulfide, andgraphite. Discussions on lubricating plastics are givenby Campbell'! and Ziinmerman ?2 along with information onmechanical properties, comparative performance data, andother pertinent information. Composites of polytetra-fluoroethylene (PTFE) containing two types of carbon fiberwere prerared and examined for their wear properties byGiltrow and Lancaster.:-

14

Type I carbon fiber (partially graphite) compositesindicated a lower coefficient of friction and wear ratewhen compared with Type II (nongraphitic) carbon fiber com-posites. However, at higher stresses the reverse situationtakes place.

In more recent works by Giltrow and Lancaster, 3 4 thefriction wear properties of thermosetting resins reinfor-ced with Type I and Type II carbon fibers were studied.In general, the Type I carbon fiber composites show a lowercoefficient of friction and wear rate values than Type IIcarbon fiber composites. The wear rates of Type II carbonfiber composites are dependent upon the counterface materialwhereas Type I carbon fiber composites are not. The wearrates of Type I carbon fiber are dependent upon the surfaceroughness, whereas Type II carbon fiber composites are not.

In more recent work by Devine," 5 the lubricating prop-erties of a new therm%,<lastic based on aromatic polyimideswere investigated. The polyimides are heat stable to 700OFand have good mechanical and electrical insulating proper-ties. The graphite-filled polyimide composites that weredeveloped had a lower wear rate and a lower friction co-efficient than the unfilled polymer. However, the mechanicalstrength is lower than the unfilled polymer. The use ofthis composite in ball bearings results in a significantincrease in p-rf-rmance life.

Several commercially available polyimide compositescontaining PTFE and MoSz were studied by Jones. 3 6 Thefriction and wear behavior of these composites was studiedunder sliding conditions, with the use of light loads inboth air and vacuum. Polyimide-filled PTFE exhibited lowerfriction coefficients than MoSz-filled polyimides, whereasthe lowest ;,ear occurred with composites containing MoS 2 -.

Halliwell" has conducted tests with PTFE reinforcedwith metallic filament windings. The purpose was to de-velop a piston seal fur high pressure air compressors.The filament winding technique resulted in a superior re-inforcing matrix when compared with randomly dispersedparticles and fibers previouily report. Possible reliablecompressor operation could be expected at 5000 psi withsuch seals in lieu of conventional split rings for periodsbeyond 1000 hours.

.Vest" reports on tests in which PTFE-MoS 2 retainersare used in B542 size bearings for spacecraft components.Wer 'ife in excess of 12,000 hours was obtained at highvacu-.,i (l0-1 to l0" Torr).

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SUMMARY

New developments in the areas of solid lubricant pow-ders, bonded films, and self-lubricating composites havebeen reviewed along with some new applications.

Generally, the research on new lubricating powdershas involved the mechanism of lubrication and the incor-poration of these new lubricants into a bonded film orcomposite. Solid lubricant powders in gas streams havepromise, particularly for extreme high-temperature oper-ation or for use in other environmental extremes in whichfluid or bonded fi ;s would deteriorate.

The bonded solid film lubricants remain the most widelyused dry lubrication systems in military and civilian appli-cations. The emphasis on improved wear life and better high-temperature performance continues to be the main objectiveof research. The synergistic effect of various oxides suchas SbZO, for improving wear life has been supported by manyinvestigators. The development of organic resins such aspolyimide, polybenzimidazole, and polybenzothiazole forbonding films has resulted in better high-temperature per-formance. The success of these films points out the needfor keeping abreast of new high-temperature organic ma-terials developed to guarantee thq rapid use of these ma-terials in bonded f'lms.

The development of inorganic bonded films has not beenso rapid as that of organic films. The use of silicates,phosphates, and fluorides has extended the use of solidfilm lubricants to temperatures above 1200^F. The use ofthese lubricants is limited, however, to the range of 500*Fto 1200'F because they do not perform so well as the resin-bonded films at lower temperatures. In addition, the highcuring or fusing temperatures of these lubricants is detri-mental to many alloys Therefore, inorganic bonded filmsneed better low temperature performance, longer wear life,and improved application techniques.

Work on new processes of application has been empha-sized because the method of application greatly influencesthe performance of bonded films. The plasma-sprayingprocess has the most potential for widespread use. Thisprocess offers a means of (a) applying selid film lubri-cants on a production-llne basis, (b) applying widelyv..rying types of fibers, and (c) applying high-temperature(ceramic-bonded) solid-film lubricants without seriouslyaffecting the physical properties of alloys.

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The electrophoretic deposition process has resultedin only moderate success; more work needs to be done onestablishing the deposition parameters. The process shouldbe considered as a method of enriching or supplementingfilms applied by other methods.

Vacuum deposition methods of application by use ofsputtering and ion-plating techniques offer unique methodswith unusual promise. The combined conditions of an ex-tremely clean substrate, high-velocity application of coat-ing material, and ionization of tnat material providesqualities needed for highly adherent films. Vacuum-depositedfilms appear to have better wear life in vacuum environments.However, these films are limited to light loads. Therefore,much needs to be resolved before these techniques can beseriously considered for production use.

The composite lubricants offer the most promise of

current solid lubricant technology since many more appli-cations are possible with composites. For example, the com-posites can be machined or formed to various configurations.In addition, with the use of composite lubricants, the prob-lem of finite wear life of bonded films can be solved byprovision of a larger reservoir supply of lubricant.

The metal-based composites have the advantage of higherstrength and higher temperature capability when comparedwith the plastic composites. However, the metal composite.sstill do not haye the overall strength when compared withthat of bearing materials. So the research emphasis hasbeen on the improvement of mechanical prop3rties throughthe study of such parameters as pressing pressures, pressingtemperatures, and die materials. In addition, more infor-mation is needed on design properties such as tensile andcompressive strength for selection purposes.

The recently developed plastic composites in whichpolyimides are used have high strength and are heat stableup to 700*F. The use of graphite and molybdenum disulfide,as solid lubricant additives has improved the wear life ofthe plastic composites, but has reduced the mechanicalstrength of these composites. The strength of PTFE-basedcomposites ha3 been considerably improved by use of fila-ment-winding techniques with glass and carbon fibers.

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CONCLUSIONS AND RECOMMENDATIONS

The successful use of solid lubricants is dependent uponthe technique of application and the design of application.Therefore, a need exists for more component test data onproperties of the new materials being developed.

This work will lead to a group of recommended lubri-cating composite systems. As a continuing study, the mostapplicable areas for composite lubrirant use on small armsshould be determined.

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18

LITERATURE CITED

1. Boes, D. J., "New Solid Lubricants: Preparation,Properties and Potentials," Paper presentedat 20th Annual ASLE Meeting, Detroit, Michigan,May 4-7, 1965, Preprint 65AM5C3.

2. Magie, P. M., "A Review of the Properties and Poten-tials of the New Heavy Metal Dt.rivative SolidLubricants," Lubrication Engineering, Vol. 22,No. 7, pp. 262-269, July 1966.

3. Devine, M. J., Cerini, J. P., Chappell, W. H., andSoulen, J. R., "New Sulfide Addition Agents forLubricant Materials," ASLE Transactions, Vol.11, No. 4, pp. 283-289, October 1968.

4. Boes, D. J., and Chanberlain, B., "Chemical Inter-actions Involved in the Formation of Oxidation-Resistant Solid Lubri,.ant Composites," ASLETransactions, Vol. 11, No. 4, pp. 131-13•TOctober 1968.

5. Hubbell, R. D., McConnell, B. D., and Van Wyk, J. W.,"Development of Solid Lubricant Compacts for UseIn Ball Bearing Separators," Lubrication.Enineering, Vol. 25, No. 1, pp. 31-39, January

6. Fusaro, R. L., and Sliney, H, E., "Graphite Fluoride

(CF x~n A New Solid Lubricant," ASLE Trans-

actions, Vol. 13, No. 1, pp. 56-65, January 1970.

7. Gisser, H., Petronio, M., and Shapiro, A., "GraphiteFluoride as a Solid Lubricant," ASLE Proceedin -

International Conference on Solid Lubrication 17American Society of Lubrication Engineers, ParkRidge, Illinois, 1971, pp. 217-221.

8. Campbell, M., and Hopkins, V., "Development of Poly-imide Bonded Slid Lubricants," LubricationEngineerin, Vol. 23, No. 7, pp. 288-294,July 1967.

at

19

LITERATURE CITED

•. McConnell, B. D., "Synthesis of PolybenzothiazoleBonded Solid Lubricating Films," Proceedings ofthe AFIL-MRI Conference on Solid Lubricants,September 9-11, 1969, AFML-TR-70-127, pp. 166-193, July 1970.

10. Hopkins, V., Hubbell, R. D., and Lavik, M. T.,"Improved High-Temperature Solid Film Lubricants,"AFML-TR-71-61, Part I, April 1971.

11. Benzing, R. J., and McConnell, R. D., "Wear Behaviorof an Air-Drying Methyl Phenyl Silicone BondedMolybdenum Disulfide Lubricating Film,"AFML-TR-70-179, October 1970.

12. Hopkins, V. and Gaddis, D., "Friction of Solid FilmLubricants Being Developed for Use in SpaceEnvironments," Lubricating Engineering, Vol. 21No. 2, pp. 52-58, February 1965.

13. Wieser, L. E., "Development of a Double-Deposited1000F Solid Lubricant," AFML-TR-70-177,October 1970.

14. Sliney, H. S., "Lubricating Properties of Some BondedFluoride and Oxide Coatings for Temperatures to15000F," NASA TND-478, October 1960.

15. Lavik, M. T., "The Friction and Wear of Thin, Fused,Fluoride Films," Proceedings of the AFML-MPlConference on Solid Lubricants, Sept 7-tAiinl1969, AFML-TR-70-127, pp. 212-238, July 1970.

16. Hopkins, V., Hubbell, R., and Kremith, R., "PlasmaSpraying, A New Method of Applying Solid FilmLubricants," Lubrication Engineering, Vol. 24,No. 2, pp. 72-b0, February 1968.

17. Kremith, R. D., Rosenberry, J. W., and Hopkins, V.,"Solid Lub-icant Coatings Applied by PlasmaSpray," American Ceramic Society Bulletin,Vol. 47, No. 9, pp. 813-818, September 1968.

20

LITERATURE CITED

18. Hopkins, V., Hubbell, R. D., and Lavik, M. T.,"Improved High Temperature Solid Film Lubricants,'AFML-TR-67-223, Part II, p. 121, February 1969.

19. Kirkpatrick, D. L., and Young, W. C., "Solid Lubri-cants for Instrument Bearings," Proceedings ofthe AFML-MRI Conference on Solid Lubricants,Sept 9-11, 1969. AFML-TR-70-127, pp. 407-427,July 1970.

20. Thornton, H. R., and Wolanski, Z. R., "Lubricationwith Electrodeposited Films of Silver-Rheniumand Gold-Molybdenum," Lubrication Engiraerinq,Vol. 23, No. 7, pp. 271-277, July 1967.

21. Vest, C. E., and Bazzarre, F. D., "Co-DepositedNickel-Molybdenum Disulfide," Metal Finishing,Vol. 65, No. 11, November '1967.

22. Spalvins, T., "Energetics in Vacuum Deposition Mnthodsfor Depositing Solid Film Lubricants," Lubri-cation Engineering, Vol. 25, No. 11, pp.73--441,October 1969.

23. Spalvins, T., "Bonding of Metal Lubricant Films byIon Plating," presented at the ASME/ASLE Lubri-cation Conference, Cincinnati, Onio, Oct 13-15,1970, Preprint No. 70LC-16.

24. Giltrow, J. P., and Lancaster, J. K., "Friction andWear Properties of Carbon-Fibre-ReinforcedMetals," Wear, Vol. 12, No. 2, pp. 91-105,August 196T.-

25. Boes, D. J., and Bowen, P. H., "Friction-Wear Char-acteristics of Self-Lubricating CompositesDeveloped for Vacuum Service," ASLE Transactions,Vol. 6, pp. 192-200 (1963).

26. Hubbell, R. D., McConnell, B. G., and Van Wyk, J. W.,"Development of Solid Lubricant Compacts forUse in Ball Bearing Separators," LubricationEngineerin , Vol. 25, No. 1, pp. 31T39,January 1969.

S-21

LITERATURE CITED

27. Campbell, M. E., and Van Wyk, J. W., "Developmentand Evaluation of Lubricant Composite Materials,"Lubrication Engineering, Vol. 20, No. 12, pp.463-469, December' 16T.

28. Boes, 0. J., Cunningham, J. S., and Chasman, M. Ri ,"The Solid Lubrication of Ball Bearings UnderHi 4n Speed - High Load Conditions From -225 to+16000F," Lubrication Engineering, Vol. 27,No. 5, pp. 150-159, May 1971.

29. Sliney, H. E., "Self Lubricating Composites of PorousNickel and Nickel Chromium Alloy Impregnatedwith Barium Fluoride - Calcium Fluoride Eutectic,"presented at 21st ASLE Annual Meeting, Pittsburgh,Pennsylvania, May 2-5, 1966, Freprint 66AM-lCZ.

30. Kauzlarich, J. J., "X-Ray Analysis of MoS 2 - Ni Com-posites," Proceedings of the AFML-MRl Conferenceon Solid Lubricants, September 9-1l1 1969, 'AFML-TR-70-127, pp. 337-339, July 1970.

3l. Campbell, t4. E,, Loser, J. B., and Sneegas, E.,"Soild Lubricants," Technology Survey, M!ationalAeronautics and Space Administration, pp. 33-49,Report SP-5059, May 1966.

32. -,irrimerman, P. A., "Lubrication in Outer Space - ASurvey of Literature Including the Aspects ofFriction and Wear," Masters Degree Thesis forMS in Department of Meýchanical Engineering atUniversity of Iowa, pp. 321-378, June 1964.

33. Giltrow, J. P., and Lancaster, J K, "The Frictionand Wear of Carbon Fiber-Reinforced PTFE,"Proceedings of the AFML-MRI Conference on SolidLubricants, September 9-11, 1969, AFIL-TR-70-I27,pp. 3-06-331, July 1970.

34. Giltrow, J. P., and Lancaster, J. K., "The Role ofthe Counterface in the Friction and Wear ofCarbon Fibre Reinforced Thermosetting Resins,"Wear, Vol. 16, No. 5, pp. 359-374, November 1970.

22

-V - 4...........

LITERATURE CITED

35. Devine, M. J., and Kroll, A. E., "Aromatic PolyimideCompositions for Solid Lubrication," LubricationEngineering, Vol. 20, No. 6, pp. 225-230, June19•64.

36. Jones, J. R., and Gardos, M. N., "Friction and WearCharacteristic5 of Lubricative Composites in Airand in Vacuum," Lubricatio' Engineering, Vol. 27,No. 2, pp. 47) February 1971.

37. Halliwell, H., Ward, J. R., Skruch, H. J., and Thomas,G. L., "An Application of Self Lubricated Com-posite Materials," Lubrication Enoineering,Vol. 23, No. 7, pp. 278-287, July 1 9 67.

38. Vest, C.,E., and Bowden, W. W., "Evaluation of SpaceLubricants Under Oscillatory and Slow SpeedRotary Motion," L.ubrication Engineering, Vol. 24,No. 4, pp. 163-167, Anril 1968.

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