Molybdenum Disulphide in Greases

The indications are therefore that the incorporation of molybdenum disulphide in greases will usually give an improvement in the load-carrying performance with little or no adverse effect on other grease properties at concentrations up to 10%. 

The use of oils containing molybdenum disulphide to solve lubrication problems was mentioned previously. Similar use of greases containing molybdenum disulphide can be even more effective, because the benefits of molybdenum disulphide in greases are not offset by any serious disadvantages. 

Organic compounds are less resistant to nuclear radiation, and conventional oils and greases are therefore inferior to molybdenum disulphide in this respect. The same applies to the organic polymers which are often used as binders in bonded films. Inorganic binders should be used in preference to organic binders where whole-life radiation doses higher than lO R are predicted. 

The use of dispersions in polyglycol is a special case, because the purpose is not simply to deposit a uniform film of molybdenum disulphide by evaporating an inert carrier. The technique is in fact more comparable with the incorporation of molybdenum disulphide in an oil or grease, and will be described in Chapter 13. 

While molybdenum disulphide lubrication is adversely affected by liquids, liquid lubrication can be improved by molybdenum disulphide. There is in fact a considerable industry based on the use of molybdenum disulphide in liquids, in dispersions, lubricating oils, greases, anti-seizes and pastes. There is an important contradiction in the fact that in many situations the lubricating action of molybdenum disulphide is either partly or completed destroyed by the presence of liquids, whereas in other situations it can provide useful lubrication benefits in a liquid medium. A great deal of effort has been applied in attempts to establish the mechanism and the conditions by which lubrication by molybdenum disulphide can occur in the presence of a liquid. 

Pastes can be made with the same carrier liquids by simply increasing the concentration of powder to achieve a paste-like semi-solid consistency. More stable pastes are obtained by using a carrier which is inherently semi-solid, such as a grease, petrolatum (soft petroleum wax) or a semi-fluid polymer. The concentration of powder in a paste or a dispersion may be anything between 35 and 75%, depending on the application. The British military specification Def Stan 80-81/1 requires not less than 50% of molybdenum disulphide in a mineral oil grease for an anti-seize and anti-scuffing compound for use up to 250 C. This is probably a fairly typical level for anti-seize use. 

Nevertheless, there were reliable reports of corrosion occurring with molybdenum disulphide in films and in greases. Several such reports arose from the US Army , originating with a salt fog test of a missile launcher in which all parts coated with solid film lubricant rusted badly. Subsequent reports described galvanic corrosion of various metals with molybdenum disulphide in moist atmospheres. 

The degree of improvement in a few years is clearly indicated by the marked reduction in the number of publications about corrosion after 1976. With the elimination of graphite and of excessive ball-milling, and the incorporation of corrosion inhibitors, it should now be possible to use molybdenum disulphide in bonded films, composites, greases or anti-seizes with confidence that no corrosion problem will arise, although commonsence precautions should always be taken ... 

In terms of volume, the most important area of application of molybdenum disulphide lubrication is now the automotive field. A major part of this volume consists of molybdenum disulphide greases, and these applications are discussed in more detail in Chapter 13. There is little doubt that their use has made a significant contribution to the extended chassis lubrication intervals in vehicles. 

An alternative method of improving wettability was first described by Groszek , and has been described in Section 7.6. This is the production of oleophilic molybdenum disulphide by ball milling in oil. The product is readily wetted by organic liquids, and can in fact be used as a grease thickener. Groszek described dispersions in several different organic liquids and it is clear that the improved wettability leads to the formation of much more stable dispersions. 

Reinforced nylon containing molybdenum disulphide is very commonly used for gears in low-stress situations such as windscreen wipers and car door window winders. It gives good wear and impact resistance and quiet operation, and is particularly useful where oil or grease lubrication is marginal or intermittent. 

There is no doubt that the presence of a liquid affects the ability of molybdenum disulphide to form a good adherent film on a bearing surface. It is often not possible to observe a deposited film on surfaces which have been lubricated by oils or greases containing molybdenum disulphide. Black et al found that steel balls which had been "rumbled" for 8 days at low load in a 10% dispersion of molybdenum... 

When molybdenum disulphide is added to a grease, the effect on the flow properties depends critically on the concentration of molybdenum disulphide. Where the quantity added is high enough to cause a major change in the flow properties, the result is a product which no longer behaves as a grease, but behaves like a paste. Such materials are generally used as anti-seize compounds rather than lubricants, and they will be described in Section 13.4. 

The best performance found in the Climax Molybdenum Company test programme was for a lithium hydroxystearate soap-thickened grease with a conventional EP additive and 10% by weight of molybdenum disulphide with a nominal particle size of 7.0 jum. This grease had a Timken OK Load of 23 lbs (10.5 kg), Falex load capacity of 1450 lbs (658 kg). Mean Hertz Load of 90.5 and Weld Load of 630 kg. 

There is a general tendency for wear to be reduced in molybdenum disulphide greases, but the improvement is not as great nor as consistent as for load-carrying capacity. Friction is also generally lower. The effects on physical and chemical properties of greases are more complicated, and addition of molybdenum disulphide has been shown to affect the stability of the gel structure, the oxidation resistance and the corrosion resistance. 

Table 13.7 Increase in Load-Carrying Capacity of a Di-Ester Grease With Molybdenum Disulphide Content (Data from Ref. 173) (Mean Hertz Load figures obtained by ASTM D-2596 the IP239 method will give results between 10% and 20% higher)...

Molybdenum disulphide has been used in greases based on other oils, as well as mineral oils and esters, but probably the only other common type is silicone. Silicone greases are usable to over 250°C, but they are poor lubricants under boundary lubrication conditions. Addition of molybdenum disulphide gives a useful... 

The Caterpillar Company has also been quoted as specifying a multipurpose (usually NLGI No 2) molybdenum disulphide grease for its heavy duty products from about 1973. Such greases have also been reported to be more widely used in offroad vehicles, and equipment in the construction industry . In both cases the molybdenum disulphide concentration is between 3% and 5%. 

Heavily-loaded rolling bearings in air to 200 C Molybdenum disulphide grease... 

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