Solid lubricants with a tin sulphide and carbon base

Details Solid lubricants with a tin sulphide and carbon base Solid lubricants with a tin sulphide and carbon base

2000-10-06

2001-10-16

Howard, Jacqueline V. (Department: 1764)

Solid anti-friction devices, materials therefor, lubricant or se

Solid anti-friction device, article or material therefor

Elemental or alloyed metal

Reexamination Certificate

active

06303545

ABSTRACT:

The present invention relates to solid lubricants and solid lubricant combinations based on tin sulfide and carbon, a method for their production as well as their use in friction lining mixtures and friction linings such as brake or clutch linings.
In addition to the classical use as a solid lubricant, in which a lubricating film is formed between two surfaces which are to slide relative to one another, compounds such as molybdenum sulfide have also been used in another tribological field, namely in the manufacture of friction elements such as brake pads, brake shoes or brake- and clutch linings. The purpose of these friction elements is, however, not to avoid, but rather to generate friction. For this reason, the goal in implementing solid lubricants in such friction elements is not the reduction of friction, but rather the stabilization of the frictional course. This stabilization brings with it a reduction of the abrasive processes, having a positive influence on the attrition and fibrational behavior.
In addition to the Molybdenum Sulfide mentioned at the mentioned at the outset, graphite is certainly the most well known special solid lubricant used in friction linings.
However, other further solid lubricants for friction linings have been known for a long time based on graphites, sulfides as well as combinations of these compounds with fluorides and phosphates. A well-known and prevalent solid lubricant is for example lead sulfide. Due to increasing ecological concerns with respect to heavy metals and the associated efforts to cut back their use, lead sulfide is available to an ever restricted degree. Antimony Sulfide is a further representative from the group of metal sulfides, and has found prevalent use in friction linings.
In this context, U.S. Pat. No. 3,965,016 describes the use of antimony sulfides, in particular based on oils and fats, as lubricant additives by which the high-pressure characteristics and the attrition behavior of these materials is drastically improved. Furthermore, a solid lubricant combination is known for example in German patent DE 35 13 031, consisting of graphite, tin sulfide, antimony(III)-sulfide as well as an earth alkali metal phosphate or another inorganic metal phosphate as a solid lubricant. According to DE 35 13 031, friction linings manufactured by using this solid lubricant combination exhibit a relatively low attrition index and a frictional index which fluctuates within a narrow range. Furthermore, it is intended to avoid a forced transmission of liner material onto the brake disks. Seen from an ecological and toxological standpoint, the use of solid lubricants based on antimony sulfide in friction linings is, however, as of recently viesed as questionable. Irrespective of the oxidation state of the metal, the heat generated by the frictional process causes antimony sulfide to react with oxygen in the air to antimony oxide. The latter, however, has recently been suspected of having a carcinogenic effect and for this reason the use of antimony sulfide in friction linings is not recommended.
The goal of the present invention is therefore to provide a solid lubricant as well as a combination of solid lubricants exhibiting comparable or even better tribological characteristics than known solid lubricants of the prior art and combinations derived thereof, yet which are free from ecologically or toxologically critical compounds.
The inventors surprisingly found that such solid lubricants are obtainable by reacting metallic tin in finely dispersed form with sulfur and carbon in the reaction batch under inert gas or under air atmosphere at temperatures of 200 to 1500° C., preferably 800-1200° C. for 0.1 to 6 hours, preferably 40 to 80 minutes, wherein sulfur is used in at least a stoichiometric amount, calculated from sole formation of SnS
2
and carbon is used in an amount of 0.5 to 20 wt. % a relative to the total weight of the reaction batch.
Surprisingly, the method found here makes possible a hitherto unknown alternative for the manufacture of tin sulfides containing predominantly tin(IV)-sulfide.
In the method according to the invention the carbon obviously has the effect of Amoninum chloride or an effect corresponding to that of other known acidic catalysts, which is documented by the formation of large quantities of SnS
2
in the solid lubricant obtained. The carbon remains in the tin sulfide matrix generated as the reaction product and, according to the studies of the inventors, is present in this in a statistically dispersed manner.
The tin sulfides forming the tin sulfide matrix can be represented as a non-charged compound in which internal charges sum to a total external charge of zero of the formula
Sn
x
S
y
as a mixture of tin sulfides of the formulas SnS, Sn
2
S
3
and SnS
2
, wherein the indices in the formula values are taken to be x=1-5 and y=5-10.
The studies performed by the inventors have furthermore shown that the formation of single sulfides is dependent on the exact reaction conditions. Normally, the solid lubricant obtainable according to the inventive process contains about 50-90 wt. % tin(IV)-sultide and 10-50 wt. % other tin sulfides, in particular Sn(II)-sulfide with respect to the total amount of material of the tin sulfides contained therein. Here, the proportion of carbon is about 0.5-20 wt. % relative to the total weight of the solid lubricant.
In the inventive method for production of the solid lubricant, metallic tin in finely dispersed form is first mixed with sulfur and carbon. Here, the sulfur is used in at least a stoichiometric amount calculated for the complete conversion of the tin to SnS
2
. Preferably, a slight stoichiometric excess of sulfur of up to 5%, calculated for the formation of SnS
2
, is used. The carbon is used in an amount of 0.5 to 20 wt. % relative to the total weight of the reaction batch. After mixing, the reaction mixture is reacted under inert gas or under air atmosphere at temperatures of 200° to 1500° C. for 0.1 to 6 hours. After completion of the reaction, the product obtained is allowed to cool and is subsequently ground.
According to a preferred embodiment of the method according to the invention, the carbon is used in an amount of 2-8 wt. % and particularly in an amount of 5 wt. % relative to the weight of the finished reaction mixture.
According to another preferred embodiment of the present invention, the carbon is used in the reaction batch in the form of graphite. The graphite can be of natural as well as synthetic origin. It is, however, also possible to use other modifications of carbon such as for example carbon black.
The tin sulfide reaction product obtainable according to the inventive method can generally be used as a solid lubricant as well as a special additive of friction liner mixtures. In the use as an additive for friction liner mixtures, the solid lubricants containing tin sulfide and graphite are used in an amount of 0.5% to 15 wt. %, preferably in an amount of 5 wt. %. The friction liner mixtures obtained in this way find application primarily in friction liners, preferably in resin-bound friction liners and particularly in friction liners such as clutch or brake liners, The positive influence on the attrition behavior of such friction liners, which positive influence is achieved with the inventive solid lubricant, is comparable to or even better than the effect in this respect of antimony(III)-oxide. Furthermore, advantageous characteristics such as for example low noise generation and a uniformly high friction index are a consequence of the good attrition behavior achieved.
Not only can the solid lubricant obtainable according to the method described here be used as a sole additive to friction liner mixtures but, according to a further modification of the present invention, it can also be used for the production of a solid lubricant combination. The friction liners made using this solid lubricant combination also have an at least equivalent attrition behavior as the friction liners made with the help of combinations of solid lubricant

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