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01/20/14

Dry lubricants, bonded coatings and sliding films

Dry lubricants are used wherever the usage of conventional lubricants such as greases, oils or pastes is not possible or is not desirable.

Dry lubricants — the alternative for special application cases

Dry lubricants are used wherever the usage of conventional lubricants such as greases, oils or pastes is not possible or is not desirable. As high-performance lubricants they avoid surface damage. They ensure reliable and maintenance-free continuous lubrication of machine elements subjected to high stress. In addition they allow easy mounting of screw threaded connections.

Dry lubricants can be classified into:

  • Powdery solid lubricants
  • Ceraceous sliding films
  • Solid-content bonded coatings

Solid lubricants (such as MoS2, graphite or PTFE) are substances that, thanks to their structure and properties, are able to form separating films between sur­faces, alone or in combination with other lubricants.

Structure of bonded coatings and sliding films

Bonded coatings are suspensions of solid lubricants – very small-sized particles contained in an inorganic or organic binder. Sliding films, on the other hand, are based on waxes as the separating substance. The choice of binder (usually resins) determines the operating temperature, the surface binding, hardness and abrasion resistance of a bonded coating layer, its chemical stability as well as the application procedure to be used.

To enable distribution of the bonded coating/sliding film a solvent that evaporates during the drying time is added. It dissolves the binder in the bonded coating or respectively the wax in a sliding film. The share of the solvent influences the coating, the viscosity as well as the suitability of the bonded coating/sliding film for specific application methods. Depending on the type of binder, both aromatic and aliphatic hydrocarbons as well as water are used as the solvent.   

Comparison: Dry lubricants — standard lubricants

Purpose Bonded Coating Oil, grease, paste
Vacuum

Very good

Almost impossible

High temperature

Good

Unfavourable, viscosity-specific

High temperature

Very good

Not suitable due to evaporation losses

Low speed

Low influence

Bad

High speed

Limited

Good, hydrodynamic

Combustibility

None in cured film

Often high

Ionising radiation

Good tolerance

Bad tolerance

Environmental risk

Very low

Difficult disposal

Soiling

Low

Creep processes

Relubrication

Not possible

Possible

Solid lubricant MoS2 Graphite PTFE
Colour Black Black Transparent/white
Operating temp. (°C)

-180 to +450

-35 to +600

-180 to +260

Adhesion on metal

+

-

-

Electr. conductivity

- -

+

- -

Chemical stability

-

+ -
Vorteile
  • High load-bearing capacity
  • Low coefficient of friction
  • Prevents stick-slip effects
  • Synergetic effects with MoS2
  • Non-stick properties
  • Synergetic effects with MoS2

Advantages

Moderate

Moderate good

Mode of action of bonded coatings and sliding films

Bonded coatings prevent direct contact of material surfaces in the micro range with surface roughnesses from 5 to 10 µm. The rich supply of solid lubricants in bonded coatings furthermore reduces the risk of initial damage to the material surfaces in tribocontact.
In addition, bonded coatings ensure a working parting film between the coated surfaces from the very begin­ning of the movement at machine parts that reach only low speeds under high loads or execute oscillating movements.
Coating with a sliding film, on the other hand, has the purpose of achieving a consistent coefficient of sliding friction for reliable mounting of screw threaded connections. In addition, bonded coatings / slid­ing films can also be used as corrosion protection, anti-creak and anti-stick coating. Careful selection of suitable solid lubricants, binders and additives allow the development and manufacturing of tailor-made bonded coatings/sliding films for a wide range of industrial applications. Thus, for example, OKS also develops low-solvent and water-dilutable bonded coatings and sliding films in order to fulfil the increas­ing demands in the field of environment and labour protection.

Processing of bonded coatings and sliding films

OKS bonded coatings./.sliding films are suitable for a wide variety of materials such as metals, plastics and elastomers. A decisive factor for the high durability of a bonded coating./.sliding film coating is the prepara­tory treatment of the surfaces and the selection of the application technique. An OKS bonded coating./.slid­ing film is durable under observance of the prepara­tory treatment and coating instructions.
Coating is carried out after thorough degreasing of the metal surfaces either through immersion, spraying or painting. The dry hard-grip layer is between 10 and 20 µm thick. It has good adhesion, resists high pres­sure loads, is resistant to soiling and is characterised by high chemical stability and temperature compat­ibility up to a maximum of 600 °C, depending on the product.
The use of drum and centrifugal systems is advisable for the mass coating of small parts, such as screws, with a sliding film.

Processing of bonded coatings

 

Immersion and centrifugation

 Spraying  Painting

A
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  • Rapid and inexpensive coating of small parts
  • Low consumption
  • No emissions due to closed system
  • Smooth even surface (in particular at automatic application)
  • Thickness of the film coat­ing can be set through viscosity and numbers of spray passes
  • Simple handling
  • For workshop and mounting
  • Coating of low volumes of parts
D
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a
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v
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  • Uneven surfaces
  • Sticking together of the parts
  • No partial coating
  • Expensive machines
  • Extraction equipment required
  • Material loss through overspraying
  • High amount of work
  • Uneven layer

Examples of use
Fitting parts, fastening elements, seals, anchors, springs, positive-locking connections, pivoting bear­ings, threaded spindles, slideways, friction bearings, sliding disks, chain parts, rocker levers, bearing bolts, metal profiles, metal forming, rivets, screws, nuts, switch cams, clamping sleeves, retaining pawls, chipboard screws, hinges, metal fittings, lock parts, shafts, gearwheels.

Bonded coatings are increasingly being used in the commodity goods sector.

Advantages of bonded coating lubrication (depending on type)

  • Dry long lasting lubrication without oil and grease
  • Full lubrication effect even after long standstill periods
  • Extreme pressure resistance
  • Not sensitive to dust, soiling, moisture and chemical influences
  • Allows very low friction values with constant coefficients of friction
  • High temperature resistance within a broad temperature range
  • No evaporation losses and excellent adhesive strength
  • Use in vacuum possible
  • Chemical-physically stable and radiation resistant
  • Effectiveness also at low sliding speeds
  • Run-in aid for motor and gear components
  • Creates emergency running properties
  • Long-term corrosion protection
  • Replacement for environmentally hazardous coatings
  • Highly economical
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