الاضافات الكيميائية للزيوت الصلبة المستعملة والمكررة

Solid lubricants

مجموعة تكنولاب البهاء جروب

عميد دكتور

بهاء بدر الدين محمود

استشارى كيميائى

01229834104

Solid lubricants are solid materials, which reduce coefficient of friction and wear of rubbing parts preventing direct contact between their surfaces even under high loads.

Forms of solid lubricants

Requirements to solid lubricants properties

Characterization of solid lubricants

Classification of solid lubricants

Forms of solid lubricants

Solid lubricants may be present in the friction area in forms of either dispersed particles or surface films:

Coating (film) of a solid lubricant applied on the part surface.

Composite coating consisting of particles of a solid lubricant dispersed throughout a matrix.

Particles of a solid lubricant dispersed throughout the bulk of the part material (composite material).

Powder of a solid lubricant delivered to the rubbing area (dry lubrication).
Additives in lubricating oils or greases.

Requirements to solid lubricants properties

Low shear strength in the sliding direction.

This property provides low coefficient of friction due to easy shear movement of the lubricant material.

High compression strength in the direction of the load (perpendicular to the sliding direction).

A solid lubricant possessing high compression strength is capable to withstand high loads without sufficient direct contact between the rubbing surfaces.

Good adhesion of the solid lubricant to the substrate surface.

This property provides a presence of the solid lubricant on the part surface even at high shear stresses.

The best combination of the first two properties possess anisotropic materials like graphite, molybdenum disulphide or boron nitride having lamellar crystal structure.

Characterization of solid lubricants

Advantages of solid lubricants:

Ability to work under high loads.

High thermal stability.

Diversity of the application forms

Disadvantages of solid lubricants:

Higher coefficient of friction and wear as compared to hydrodynamic regime.

Low stability of the lubrication film.

Less convenient system of the lubricant delivery to the friction surfaces.

In contrast to solid lubricants fluid lubricants are continuously supplied, filtered and cooled.

Classification of solid lubricants

Inorganic lubricants with lamellar structure.

The crystal lattice of these materials has a layered structure consisting of hexagonal rings forming thin parallel planes.

Within the plane each atom is strongly bonded (covalent bonding to other atoms.

The planes are bonded to each other by weak Van der Waals forces.

The layered structure allows sliding movement of the parallel planes.

Weak bonding between the planes determines low shear strength and lubricating properties of the materials.

The most commonly used inorganic solid lubricants with lamellar structure are graphite, molybdenum disulphide (MoS2) and boron nitride (BN).

Other examples of such materials are sulphides, selenides and tellurides (chalcogenides) of molybdenum, tungsten, niobium, tantalum, titanium (eg. WS2, WS2, MoSe2, TaSe2, TiTe2), monochalcenides (GaS, GaSe, SnSe), chlorides of cadmium, cobalt, lead, cerium, zirconium (eg. CdCl2, CoCl2, PbCl2, CeF3, PbI2) and also some borates (eg. Na2B4O7) and sulfates (Ag2SO4).

Oxides.

Examples: B2O3, MoO2, ZnO, Re2O7, TiO2, CuO-MoO2, NiO-Mo2, PbO-B2O3, CuO-Re2O7.

Soft metals.

Due to their low shear strength and high plasticity some soft metals possess lubrication properties: lead (Pb), tin (Sn), bismuth (Bi), indium (In), cadmium (Cd), silver (Ag).

Soft metals are used in pure form or as alloys, in form of coatings (Lead based engine bearing overlays, Tin based engine bearing overlays) or the second phase in Metal Matrix Composites (Copper based bearing materials, Aluminum based bearing materials).

Coatings from soft metal lubricants are produced by the methods of Electroplating, Vapor deposition and Thermal spraying.

Composites containing soft metal lubricants are prepared by casting or sintering methods.

Soft metal are widely used as solid lubricants in Engine bearing materials.

Organic lubricants with chain structure of the polymeric molecules.

Polytetrafluoroethylene (PTFE) and polychlorofluoroethylene are the typical examples of such materials.

The molecular structure of the materials consist of long-chain molecules parallel to each other.

The bonding strength between the molecules is weak therefore they may slide past one other at low shear stresses.

The strength of the molecules along the chains is high due to strong bonding between the atoms within a molecule.

Such anisotropy of mechanical properties provides good lubrication properties of the materials.

Chain structure lubricants are used in form of coatings (films) applied on the substrates surfaces (Polymer based engine bearing overlays).

Soaps.

Soaps are metals (lithium, calcium, sodium, potassium) salts of fatty acids.

Soaps are prepared by chemical treating of oils and fats by strong alkaline solutions.

A soap molecule is composed of a long non-polar hydrocarbon tale, which is hydrophobic (repelled by water) and the salt polar end, which is hydrophilic (water soluble).

The soap molecules attached to the substrate surface provide good adhesion of the soap lubricant and low coefficient of friction.

Graphite as solid lubricant

Graphite is a crystalline, low density and soft allotrope of carbon.

Graphite is a solid lubricant relating to the class of Inorganic lubricants with lamellar structure, which also includes molybdenum disulphide, boron nitride and some other sulphides, selenides and tellurides (chalcogenides) of molybdenum, tungsten, niobium, tantalum and titanium.

The crystal lattice of graphite consists of hexagonal rings forming thin parallel planes (graphenes).

Each carbon atom is covalently bonded to three other atoms in the plate (the angle between two bonds is 120°).

The graphenes are bonded to each other by weak Van der Waals forces.

The layered structure allows sliding movement of the parallel planes.

Weak bonding between the planes provides low shear strength in the direction of the sliding movement but high compression strength in the direction perpendicular to the sliding movement.

Friction forces cause the graphite particles to orient in the direction, in which the graphenes are parallel to the sliding movement.

The anisotropy of the mechanical properties imparts the combination of low coefficient of friction and high carrying load capacity to graphite.

Graphite forms a lubrication film strongly adhered to the substrate surface.

The lubrication film provides good wear resistance and seizure resistance (compatibility).

Lubricating properties of graphite are highly dependent on the presence of water vapor in the ambient atmosphere.

Water molecules are absorbed on the graphite surface causing further reduction of the bonding between the graphene planes.

Coefficient of friction of graphite in a moist atmosphere is as low as 0.07.

Coefficient of friction of graphite in dry atmosphere or in vacuum reaches 0.5.

Application of graphite as solid lubricant in open air at elevated temperatures is limited to 900°F (482°C).

Higher temperatures cause oxidation of graphite and increase of its coefficient of friction.

Some applications of graphite as solid lubricant:

Additives in lubricating oils

Carbon brushes including copper-graphite and silver-graphite impregnated composites

Components of polymer based composite anti-friction coatings

Second phase particles of metal based composite anti-friction coatings

Molds for continuous casting

Solid lubricant in metal forming

Bronze-graphite composites for sliding bearings

Release coatings and non-sticking refractory linings in foundry

Molybdenum disulfide as solid lubricant

Molybdenum disulfide (MoS2) is a crystalline lamellar structure material contained in the natural mineral Molybdenite.

Molybdenum disulfide is a solid lubricant relating to the class of Inorganic lubricants with lamellar structure, which also includes molybdenum Graphite, boron nitride (BN) and some other sulphides, selenides and tellurides (chalcogenides) of molybdenum, tungsten, niobium, tantalum and titanium.

The crystal lattice of molybdenum disulfide is similar to that of Graphite.

It consists of hexagonal molybdenum planes sandwiched between two hexagonal sulphur planes.

The atoms in the planes are strongly covalently bonded to each other.

The planes are bonded by weak Van der Waals forces.

The layered structure allows sliding movement of the parallel plates.

Weak bonding between the planes provides low shear strength in the direction of the sliding movement but high compression strength in the direction perpendicular to the sliding movement.

Friction forces cause the particles of molybdenum disulfide to orient in the direction, in which the hexagonal layers are parallel to the sliding movement.

The anisotropy of the mechanical properties imparts the combination of low coefficient of friction and high carrying load capacity to molybdenum disulfide.

The sulfur layers of molybdenum disulfide have an affinity for tenacious adherence to the metal substrate atoms therefore a strong lubrication film is formed on the substrate surface.

The lubrication film provides good wear resistance and seizure resistance (compatibility).

In contrast to graphie moist atmosphere is not required for lubrication by molybdenum disulfide.

Therefore it demonstrates low friction in dry atmosphere and in vacuum where its coefficient of friction is even lower than in the presence of water vapor.

Coefficient of friction of molybdenum disulfide is lower than that of graphite and it decreases with increasing load.

At high loads in vacuum it may be as low as 0.03.

Application of molybdenum disulfide in open air at elevated temperatures is limited to 700°F (371°C).

Higher temperatures cause oxidation of MoS2 into the molybdenum trioxide MoO3 and sulfur dioxide SO2. The oxides attract moisture resulting in increase of the coefficient of friction.
In non-ixidizing environment and in vacuum molybdenum disulfide is stable up to 2100°F (1150°C).

Some applications of molybdenum disulfide :

Additives in lubricating oils

Components of polymer based composite anti-friction coatings

Second phase particles of metal based composite anti-friction coatings

Solid lubricant in metal forming

Release coatings and non-sticking refractory linings in foundry

Boron nitride as solid lubricant

Boron nitride may exist in two forms of crystal lattice:

cubic and hexagonal.

Due to its tight diamond-like structure cubic boron nitride is extremely hard.

It has poor lubrication properties and is used in cutting and abrasive tools as a diamond substitute.

Hexagonal boron nitride (HBN) is a solid lubricant relating to the class of Inorganic lubricants with lamellar structure, which also includes molybdenum disulphide, graphite and some other sulphides, selenides and tellurides (chalcogenides) of molybdenum, tungsten, niobium, tantalum and titanium.

The crystal lattice of hexagonal boron nitride consists of hexagonal rings forming thin parallel planes.

Atoms of boron (B) and nitrogen (N) are covalently bonded to other atoms in the plane with the angle 120 ° between two bonds (each boron atom is bonded to three nitrogen atoms and each nitrogen atom is bonded to three boron atoms).

The planes are bonded to each other by weak Van der Waals forces.

The layered structure allows sliding movement of the parallel planes.

Weak bonding between the planes provides low shear strength in the direction of the sliding movement but high compression strength in the direction perpendicular to the sliding movement.

Friction forces cause the particles of boron nitride to orient in the direction, in which the planes are parallel to the sliding movement.

The anisotropy of the mechanical properties imparts the combination of low coefficient of friction and high carrying load capacity to boron nitride.

Boron nitride forms a lubrication film strongly adhered to the substrate surface.

The lubrication film provides good wear resistance and seizure resistance (compatibility).

Similar to molibdenum disulfide moist atmosphere is not required for lubrication by boron nitride.

It demonstrates low friction in dry atmosphere and in vacuum.

Coefficient of friction of boron nitride is within the range 0.1-0.7, which is similar to that of graphite and molybdenum disulfide.

Impurities (eg. boron oxide) exert adverse effect on the lubrication properties of boron nitride.

Boron nitride is chemically inert substance. It is non-reactive to most acids, alkalis, solvents and non-wetted by molten aluminum, magnesium, molten salts and glass.

The main advantage of boron nitride as compared to graphite and molybdenum disulfide is its thermal stability.

Hexagonal boron nitride retains its lubrication properties up to 5000°F (2760°C) in inert or reducing environment and up to 1600°F (870°C) in oxidizing atmosphere.

Boron nitride has high thermal conductivity.

Some applications of hexagonal boron nitride:

Additives in lubricating oils

Components of polymer based composite anti-friction coatings

Second phase particles of metal based composite anti-friction coatings

Solid lubricant in metal forming

Release coatings and non-sticking refractory linings in foundry

Sintered ceramic parts for high temperature applications

Polytetrafluoroethylene (PTFE) as solid lubricant

Polytetrafluoroethylene (PTFE) is a crystalline polymer consisting of long parallel macro-molecules, each of them is made of a carbon chain surrounded by fluorine atoms forming a linear repeating structure -CF2-CF2-CF2-.

The monomer molecules (tetrafluoroethylene) have the structure F2C=CF2.

The fluorine atoms covering the carbon chain protect it from chemical activity.

They also repeal other PTFE molecules and molecules of other substances resulting in high chemical resistance and exceptional non-sticking properties of polytetrafluoroethylene.

Due to very weak bonding between neighboring PTFE molecules the may slide easily along each other at low shear stresses similar the hexagonal planes of solid lubricants with lamellar structure (graphite, molybdenum disulphide and boron nitride (BN)).

PTFE is capable to withstand relatively high compression strength in the direction perpendicular to the sliding movement resulting in good load carrying capacity.

Polytetrafluoroethylene is a solid lubricant relating to the class of organic lubricants with chain structure of the polymeric molecules.

Coefficient of friction of PTFE is lowest of all solid lubricants.

It varies within the range 0.02 - 0.1.

Due to non-stick properties of polytetrafluoroethylene there is very small difference between the static and dynamic coefficients of friction.

Coefficient of friction of PTFE does not depend on the environment.

It is as low in vacuum as in oxidizing, non-oxidizing moist and dry atmospheres.

Application of polytetrafluoroethylene in open air at elevated temperatures is limited to 500°F (260°C).

The main disadvantages of PTFE:

low melting point

low themal conductivity

relatively low load carrying capacity (as compared to boron nitride, molybdenum disulfide and graphite)

Because of the disadvantages polytetrafluoroethylene is used in light low low speed applications.

As a solid lubricant PTFE is used in the following forms:

As a friction modifier added to lubrication oils

PTFE particles dispersed throughout sintered bronze matrix (sliding bearings)

Components of polymer based composite anti-friction coatings

Second phase particles of metal based composite anti-friction coatings

Polymer Matrix Composites with PTFE matrix reinforced by glass fibers or carbon fibers

(better mechanical properties, higher thermal conductivity, better wear resistance)
Polymer matrix composites reinforced by fibers of carbon, glass or steel interwoven with PTFE fibers