Solid film lubricants

SOLID FILM LUBRICANTS by Charles J. Beall E/M Engineered

Coating

Solutions,

Peachtree

City, Ga.

Bonded solid film lubricants are unique products, which utilize the special properties of a variety of lubricating solid materials. Solid film lubricants can generally be defined as solid materials with inherent lubricating properties, which are firmly bonded to the surface of the substrate by some methodology. Perhaps a more simplistic definition might be “slippery paint;” however, this analogy does not begin to describe the diversity of products and processes that have evolved over the years. For example, the bonding methodologies include resin bonding, burnishing, sputtering, ion deposition, and mechanical impingement. From a commercial standpoint, over 95% of the materials in use today are resin bonded. Typical formulations for resin-bonded solid film lubricant5 include a lubricating pigment, binder, solvent, and additives to improve corrosion resistance. aesthetics, etc. Formulation considerations, application techniques, typical performance properties. and a few typical applications will be examined in this article. PIGMENT

SELECTION

The primary role of the insoluble solid pigment is generally lubrication. A wide variety of solid materials with inherent lubricating capability is available for use in solid film lubricants. The most commonly used are molybdenum disulfide (MoS,), graphite, and polytetrafluoroethylene (PTFE) (Table I). While these are the most common. materials such as tungsten disulfide, boron nitride. lead oxide. antimony oxide, niobium selenide. lead, tin, silver, iridium. fluorinated ethylene propylene (FEP), petfluoroalkoxy copolymer (PFA). and others are also used. The selection of the proper pigment or pigment combination is generally governed by the performance that the solid film lubricant formulation must deliver. No single formulation can satisfy all of the requirements on a cost-effective basis. Properties that should be considered are coefficient of friction. load carrying capacity, corrosion resistance (susceptibility to galvanic corrosion). and electrical conductivity. Further. one must consider the environment in which the solid film lubricant must perform. These factors include temperature. pre\aure, humidity, oxygen content, radiation, etc. Each of the above solid lubricant material\ has strengths and weaknesses in each area and corresponding compromises must be made. Molybdenum disulfide generally has the highest load carrying capability with a corresponding low coefficient of friction; however, in an oxidative atmosphere in excess of 400°C (750°F) MoS, begins to decompose. Graphite has high temperature capability in an oxidative environment but tends to promote galvanic corrosion and will not function in high vacuum. The fluorinated polymers such as PTFE generally exhibit a low coefficient of friction and are quite aesthetically suitable for formulations with colored pigments; however, the tluorinated polymers cannot sustain high loadmg nor do they have good radiation stability. BINDER

SELECTION

As with the pigments, a wide range of resins is available for use as binders or bonding agents in solid film lubricant formulations. These resins carry names similar to the conventional paint resins and are generally of substantially different molecular weight and crosslinking capability. Nevertheless. the organic bonding agents are divided into two types: (I) air-dried, which include acrylics, alkyds, epoxies, vinyls, and acetates and (2) thermosets, including phenolics, epoxy-phenolics. silicones, epoxies, polyamide-imides, and urethanes. 513

‘able

I. Properties

Molybdenum Graphite

of Common

disultide

Polytetratluoreethylen

Lubricating

pigments Load Carrying Capability

Thermal Stability

(psi)

(7

>lOO,OOO <5o,ooo <6.000

Good (450) Excellent (4200) Fair (-300)

Gray-black Gray-black

(PTPE)

White Typical Particle Size

Molybdenum disulftde Graphite Polytetratluoroethylene

(PTPE)

V~CUUtJl

Moisture

fWJ

Suitability

Sensitivity

2-6 2.5-10

Yes NO

Detrimental

Submicron

Yes

No effect

NeCeSSZUy

Inorganic bonding agents include silicates, phosphates, and ceramics. As with the lubricating pigment selection, the desired physical properties and environment must be considered when selecting the proper product. In addition to the items mentioned above, solvent resistance, corrosion resistance, and cure temperature should also be considered. For example, some of the high temperature curing ceramics may have a deleterious effect on the metallurgical properties of the substrate. In general, the organic air-dty resins are less expensive to apply, are amenable to field application, and, due to the low cure temperature, have no deleterious effects on metallurgical properties; however, the air-dry resin systems generally have much reduced durability, wear life, and solvent resistance. The oven-cured thermoset resins usually have outstanding durability, solvent resistance, and wear life, but like all organ& are subject to radiation damage, vacuum outgassing, limited high temperature applicability, and liquid oxygen incompatibility. The inorganic binder systems generally have outstanding radiation resistance suitable for extremely high vacuum and high temperature and are compatible with liquid oxygen environments; however, these materials have limited wear life and durability and are often difficult to apply. PIGMENT/BINDER

RATIO

A major consideration in the formulation of solid film lubricant products is the pigment/binder ratio or the amount of lubricating solids to resin solids. Again, the performance requirements and environment are of prime importance. Low pigment-to-binder ratio products have higher coefficients of friction, appear glossy, tend to be hard and durable, and have excellent corrosion resistance. High pigment-to-binder ratio products, on the other hand, generally have a low coefficient of friction, a dull, soft, powdery appearance, and less effective corrosion resistance. Thus, the right pigment-to-binder ratio is necessary to achieve optimum performance. SOLVENT

SELECTION

Solvent selection is generally governed by the evaporation rate and solubility of the resin system: however, other factors must also be considered such as flash point, evaporation rate, toxicity, EPA and OSHA regulations, shipping and storage considerations, etc. ADDITIVES variety 514

Finally, additives can be used in bonded solid film lubricant formulations to enhance a of properties such as aesthetics, ease of application, corrosion resistance, etc. Such

Table

II. The Effect

of Pretreatment

on Wear

Life LFW-I Test ASTM D-2714 Cycles to Foilure

PrtWl?UttlWnI

Failure on loading 20900 672,CQO

NCUle

Vapor degrease/Sand blast Vapor degrease/Sand blast/Phosphate

additives inhibitors, typically

include dispersants. anti\ettling agents, wetting agents. flow agents. corroGon and colored pigments or dyes. Generally. these additives are similar to those used in the paint industry for similar purposes.

APPLICATION

OF

RESIN-BONDED

PRODUCTS

Once the product has been selected for the desired application. the final step is to assure that the product is properly applied. Generally speaking, 808 of failures of solid film lubricants are due to poor sul-face pretreatment or other misapplication. To demonstrate the importance of proper pretreatment. the example in Table II was performed. Three identical Timken TS4148 test races were coated with a commercially a\nilahle phenolic-bonded molybdenum disulfide/graphite-containing product. Coating thicknesses. cure times, and temperatures for all three races were identical. The tests were conducted on an LFW-I test machine (ASTM D 2714) at 72 rpm and 630.lb load, which corresponds to approximately 100,000 psi. The only difference between the specimens was the surface pretreatment. The first specimen was vapor degreased and coated with solid film lubricant. The second was degreased and grit blasted with 200.mesh aluminum oxide before coating. The third wa\ degreased, grit blasted. and phosphated with a manganese phosphate conforming to DOD-P-16232 before coating. Table II summarizes the results. Clearly. the preferred pretreatment of degreasing, sand blasting, and phosphating provides the necessary base to achieve the ultimate performance of the solid film lubricant. While the example in Table II is specific for hearing steel in a particular test, similar results are obtained with a variety of \uhstrates, pretreatments. and solid film lubricants encountered in real applications.

METHODS

OF APPLICATION

The methods of applying resin-bonded solid film lubricants are basically identical to those of other paintlike materials; however. resin-bonded solid film lubricants generally need a very tightly controlled film thickness. Unlike conventional paints, the normal recommended thickness is in the range 0.0002~0.0005 in. for optimum wear and lubrication performance: however. other performance factors can require compromise. For example. a thicknc\s of 0.0005-0.0020 in. is preferred for optimum corrosion resistance. The major factors that govern the selection of method of application of solid film lubricants include the number of parts, size of parts. types of parts, film thickness. tolerance. and functional areas to he coated including masking and overspray allowances. Once these parameters have been defined, it can be determined whether the coating should he applied by conventmnal-spray equipment, electrostatic spray. dipping. roll coating, brush coating. etc. Generally, spray application with multiple coats provides the best lubrication. Performance properties of typical bonded solid film lubricants are listed in Table III. While this table is by no means inclusive of all products commercially available. it can provide insight into properties that can he expected.

516

2

260°C -220°C IV% 2,500 lb gauge Falex

2wc -22O~C 1vTa 2,500 lb gauge Falex 1,000 lb gauge Falex ~450 min 43.l very good

N/A N/A N/A X X X N/A

N/A N/A X X X X Fair

l,CitOlb gauge Falex >60 min co.1 Good

Epoxy Spray 204°C

1,WO lb gauge Falex 2.60 min 43.1 Fair

2.500 lb gauge Falex

371°C -157°C IV%

N/A N/A L X X X N/A

MOSd Graphite Silicone Spray 260°C

MoSd Metallic oxide

Phenolic Spray 149°C

MoS,

MIL-L-46010

PTFE

I .ooO lb gauge FZi1e.X >lZOmin co. I GOOd

LFW-I >120,ooo 4.1 Excellent

2,500 lb gauge Falex

176°C -220°C N/A

N/A N/A N/A X X X N/A

Spray Ambient

MoS,

MIL-L-23398

150 lb

I50 lb LFW- I

260°C -220°C N/A

N/A N/A N/A X X X N/A

Spray 204°C

Phenolic

Organic

L1X. compatible; L. low; N/A nci applicable. bFaler tests arc all designed for English units anndarc. thcrcfore. mponed as such.

Test Time Coefficient of friction Corrosion resistance

Wear life” Load

Binder Application CUR Compatibility LOX 02 Rocket fuel Jet fuel Hydrocarbons Solvents Radiation Operating Temp. Air (high) Air (low) Vacuum Load capacityb Force test

Composition Lubricant

MIL-L-8937

Thermo Set

lhbie 111.Performance Properties of Qpical Solid Film Lubricants

CO.1 -

-

-

mwe Falex

2,500 lb

204°C - 185°C N/A

N/A N/A N/A L L L -

Aerosol Ambient

Graphite

MoSi

MIL-L-46009

Air Dry

Falex >2 min co.1 -

50

-

-

I$00 lb gauge Falex >70 min
I ,093”C - 240°C lo-%

X X X X X X VG

149°C

Graphite

AMS2525A

649°C -24OYI I &?a

X X L X X X VG

Graphite Silicate Spray 204°C

MoS2/

MIL-L-81329

Inorganic

Falex >5 min CO.1

50

-

IV’Pd

400°C -220°C

X X X X X X VG

Impingement 149°C

-

MoS2

AMS2526A