Aluminum hydroxides as solid lubricants

#In the quest for reduced friction and wear between rubbing surfaces, several different lubrication methods have been employed. Solid lubricants are often used either alone or in conjunction with liquid lubricants to provide an easily sheared interface between sliding members. One class of compounds that exhibit solid lubricating ability is the lamellar, or layer lattice solids. These compounds contain crystal structures in which the interatomic bonding is significantly weaker in one dimension. This results in a layer structure which is easily sheared in certain directions. The best examples of these types of compounds are graphite and molybdenum disulfide (MoS.sub.2). In some applications, however, the use of graphite or molybdenum disulfide is inappropriate. For instance, chemical incompatabilities between these lubricants, surfaces, and environments may limit their applications. Such as the case when graphite or molybdenum disulfide are used in oxygen containing environments at high temperatures. Also, in some applications carbon and sulfur contamination is undesirable. Further, the use of a heavy metal such as molybdenum may also be impermissible. Thus arises the necessity for a layer lattice solid lubricant which overcomes the above-mentioned drawbacks.

#It is thus an object of the present invention to provide a solid lubricant to reduce frictional coefficients between contacting surfaces such as aluminum oxide surfaces.

#It is a further object of the present invention to produce a solid lubricant for lubricating contacting surfaces at high temperatures.

#The invention will be described in detail with reference to the attached Figures, wherein:

#FIG. 1 illustrates the layer lattice structure of aluminum trihydroxide;

#FIG. 2 illustrates the stacking sequence of two types of aluminum trihydroxide: gibbsite and bayerite;

#There are two classes of aluminum hydroxides as shown in Table 1 below. Aluminum oxide hydroxide [AlO(OH)] is found in two common forms, boehmite and diaspore. Boehmite is a layer lattice compound while diaspore contains strong bonding in all three dimensions. Aluminum trihydroxide [Al(OH).sub.3 ] is commonly found in two forms, gibbsite and bayerite. Both of these forms are layer lattice structures, as shown in FIG. 1, which differ only in their stacking sequence as seen in FIG. 2. In FIG. 1, the solid circles represent aluminum, the small unfilled circles represent hydrogen, and the large unfilled circles represent oxygen. According to FIG. 1, darkened lines represent atomic bonds coming out of the page, dashed lines represent bonds going into the page, and regular lines represent bonds parallel to the plane of the page. Further, in FIG. 1, aluminum atoms (solid circles) are parallel to the plane of the page, atoms represented by unfilled circles are above the plane of the page, and atoms represented by dashed circles are below the plane of the page. The layer lattice hydroxides of aluminum (both aluminum oxide hydroxide-boehmite, and the aluminum trihydroxides-gibbsite and bayerite) possess solid lubricating ability. Similar results are expected for Nordstrandite, another layer lattice trihydroxide of aluminum which differs from gibbsite and bayerite only in its stacking sequence.

#TABLE 1 ______________________________________ Nomenclature for Hydroxides of Aluminum Chemical Nomenclature System Chemical Name Formula Symposium Alcoa ______________________________________ Aluminum Oxide A1O(OH) or Boehmite Alpha Hydroxides (A1.sub.2 O.sub.3.H.sub.2 O) Alumina or Monohydrate (Alumina Diaspore Beta Alumina Monohydrate) Monohydrate Aluminum A1(OH).sub.3 or Gibbsite or Alpha Trihydroxides (A1.sub.2 O.sub.3.3H.sub.2 O) Hydrargillite Alumina or Trihydrate (Alumina Bayerite Beta Alumina Trihydrate) Trihydrate Nordstrandite ______________________________________

#Wear tests were conducted on a four-ball wear tester at 0.23 ms.sup.-1 sliding speed (600 rpm), and loads considered to be in the boundary lubrication regime. Both four-ball and ball-on-three-flat wear test geometries were used. Wear test specimens were 12.67 mm (0.5 inch) diameter polycrystalline alumina balls of 99.5% purity and 97% of theoretical density. Samples of the various powders were added to both unlubricated and water lubricated alumina tests. Friction traces from the unlubricated test series are shown in FIG. 3 for a 5 kg. load and in FIG. 4 for a 2 kg. load and are summarized in FIG. 5. In these tests, boehmite provided a modest decrease in friction and gibbsite gave approximately a 40% drop in friction. A subsequent test on bayerite provided a 40% decrease in friction.