DRY FILM LUBRICANTS / KAL-GARD F.A.

Kal-Gard manufacture a complete range of dry film lubricants for applications where the following conditions exist: extremely high loads, surface temperatures are extreme (-200°C - +800°C), environments are dirty or abrasive, fretting wear, i.e. spline drive in aircraft engines. These coatings are based on various solid lubricants i.e. molybdenum disulphide, graphite, boron nitrite, etc. Other uses for the above products include metal forming, drawing & heading of titanium and other super alloys. Coatings are applied by dip, brush or spray and are generally a dry film thickness of 8 – 12 microns thus will not affect most machine tolerances.

 

Dry Film Lubricants

 

What is a solid or dry film lubricant?

 

A dry film lubricant is precisely what the word implies, namely a lubricant which is dry. Conventional lubricants such as oil or grease are wet and are derived from crude oil or made synthetically and fall in a class of lubrication, which is known as Hydrodynamic. Hydrodynamic lubrication in its ideal state means the separation of 2 metal surfaces by means of a liquid or gas to prevent metal to metal contact. The means of lubrication in a dry film lubricant is by means of boundary lubrication in which metal contact can, and frequently does take place.

 

Type of Lubricant Solids

 

There are many types of lubricating solids such as:

 

  • Molybdenum Disulphide MoS²
  • Graphite
  • Boron Nitrite
  • Tungsten Disulphide WS²
  • Lead
  • PTFE (Teflon)

 

History of Dry Film Lubricants

 

The use of dry film lubricants is relatively new, the first patent was taken out in 1939, and covered the use of a combination of materials including a binder to hold the powders in place. Initially graphite was used as the lubricating pigments but because it can only function in the presence of moisture, it caused corrosion.

 

Molybdenum Disulphide MoS² (MOLY)

 

  • Today most good dry film lubricants use Moly as the major portion of the lubricating solids because of its outstanding performance at both high and low temperatures.
  • MoS² is a by product of molybdenum ore which is found in most parts of the world but mined mainly in the U.S.A., Canada, Chile, Germany and USSR.

 

Basic Action of MoS²

 

MoS² essentially compromises of a layer of Molybdenum atoms sandwiched between 2 layers of sulphur atoms (rather like a piece of bread buttered on both sides). Sulphur atoms are strongly attracted to metallic atoms (those of the molybdenum and iron), but bond between individual sulphur atoms is weak. Friction between 2 metal surfaces – sulphur bond shears. In a coating of 0 – 0001 inch thick can be contained some 4000 sulphur layers in which shear can occur preferentially, (the molecular thickness of MoS² layer lattice is estimated at 0 – 000,000,025 in).


However, this weak bond between sulphur atoms does not entirely explain the remarkable low co-efficient of friction, (0-02 - 0-09) obtained purely with a dry film. Heat is initially generated because of the interference between minute peaks on mating surfaces, as a result of this combination of temperature and pressure the peaks are deformed plastically under the protective MoS² layer which provides a barrier necessary to prevent metal to metal contact. Once this deformation has taken place, the load is carried on the bulk of the surface rather than on a small proportion of high points as during the running in period. The real bearing area is thus increased and the real bearing pressure correspondingly reduced. In addition to this strong bond between the molybdenum and sulphur atoms resist penetration from surface impurities.

 

Performance Capabilities

 

Particle size: the particle size of the MoS² powder plays an important part in the lubrication performance, basically the larger the particle size the better the performance will be at low load and low speed applications and the smaller the particle size, the better the performance at higher loads and speeds.

 

Temperature

 

MoS² can lubricate from - 200°C to 460°C in normal atmosphere after which oxidisation occurs. However, in vacuum conditions the ceiling is 1200°C before oxidisation. Above 540°C MoS² begins to oxidize rapidly and form MoO³. Some of the early work on MoS² contained references to the abrasiveness of MoO³. It has now been proved that when MoS² is deliberately oxidized and the oxide is used in place of the sulphide MoO³ is not abrasive. ASTM Abrasion test D – 1367 shows that MoO³ is less abrasive than many other solid lubricants. The ASTM – 1404 Deleterious Particle test further verifies these results. When a film of MoS² is applied on a metal surface then oxidised it retains its lubricating properties even to a point when the amount of MoS² on the surface has reduced to 0 – 1%.

 

Pressure

 

Probably the most important characteristic of MoS² is its ability to carry heavy loads. Friction measurements with MoS² have been made with pressures as high as 475,000 PS.1. which is beyond the yield strength of most metals.

 

Coating Thickness

 

These resin bonding lubricating films can be applied by spray, dip or brushing methods. The spray technique usually being the most satisfactory. Coating thickness should range between 5um to 19um, the optimum being about 12um. A film that is too thick will be structurally weak and peel or flake off, with a sliding motion under load. If on the other hand, the film is too thin premature failure may result due to rapture. Several researchers have determined optimum film thickness for resin-bonded films and for high loads the optimum thickness is 7um. For lighter loads the thickness can be substantially increased. However, economics of the coating process should enter into any decision involving the use of thicker films. Also to be considered – if thick films are in use is the generation of wear debris.

 

Wear behaviour

 

The wear behaviour and wear life characteristics of a resin bonded solid lubricant are different from those of other lubricants. Initially, the wear is relatively high, but becomes less with time. This initial high wear rate can be attributed to the loss of loose material from the surface of the film and the compaction of the film by the applied load. As running continues, the film will appear glossy or burnished. It is at this time that the best performance, lowest wear rate and steadiest frictions are obtained.

 

Wear Life

 

Dry film lubricants have a definite wear life which must be taken into consideration. Good limiting figures particularly at loads above 25,000 PSI are 200,000 linear feet of sliding distance between components during their service life, and 40 ft per minute sliding velocity on a continuous basis. Speeds higher than this may be used for short periods but if used for sustained periods heat build up becomes excessive due to no cooling effect.

 

Different Substrates

 

Applying a resin bonded dry film lubricant to different steel and alloys will give different wear life and torque values. It is now possible to offer different type of coatings to obtain the maximum wear life and load carrying performances.

 

ADVANTAGES

 

Loads are extremely high. Liquid lubricants may be completely squeezed out from between interfering surfaces – especially when loads approach the yield points of the surface metals, often causing cold welding.


Speeds are Low. At low sliding velocities the peaks and ridges on opposing surfaces tend to penetrate the lubricant and interfere, causing slip-stick behaviour or cold welding. Here hydrodynamic lubrication fails and boundary lubrication becomes necessary.


Surface Temperatures are extreme. At present, maximum service temperature of liquid lubricants is about 300°C. Recent advances in solid film lubricants have extended coating reliability to above 600°C and below -185°C.


Environments are dirty or abrasive. By attracting troublesome particles liquids or greases often act as grinding paste rather than lubricants.


Fretting is Likely. Vibrations in many types of machine joints can cause fretting wear, even though there is no apparent motion between parts. Classic examples are: splined drives in aircraft engines, sleeve bearings in unbalanced automatic tools.


Mating surfaces are inaccessible to lubrication. Permanent in-place lubrication can be a key performance factor in many complex machines in which liquid lubrication is not feasible. Some examples: buried mechanisms, safe locks, hinges and pivots, flap tracks on aircraft.


Design must be simplified. Cost of support equipment for liquid lubrication can be high, machined oil passages, reservoirs, filter, pumps and tubing.


Users want Re-Lubricate. Equipment which is used by people with no mechanical knowledge or not their own equipment tend to never lubricate their equipment.


Used with liquid Lubricant. Bonded dry film lubricants can be used in conjunction with liquid lubricants but their wear life is reduced in these conditions.


Which surface to Coat? Always coat the member that is constantly presenting a fresh surface area to the point of pressure. For Example: coat the cam, not the cam follower, the shaft not the bearings. This holds frictional temperatures down and distributes wear over a greater area.

 

Solid film Lubricants AVAILABLE from Kal-Gard

 

  • Gun-Kote F.A. MIL-L-8937. Heat cured solid film lubricant used when excellent lubrication and corrosion resistance required.
  • Gun-Kote R.A. MIL-L-46010. Heat cured similar to Gun-Kote F.A.
  • Kal-Gard H.T. Similar to EV but used more on higher temperatures.
  • Kal-Gard A.D. MIL-L-23398B. Solid film lubricant air cured offering good lubrication and corrosion resistance.

 

There is a range of over 100 solid film lubricants available, for more information contact us.