This invention is concerned in general with materials for minimizing the sliding frictional forces which exist between mating rubbing surfaces. It is also concerned with methods of preparing such materials. Usually, either a lubricating film of oil, graphite, or the like or rolling contact is preferred over dry sliding or rubbing contact when two mating surfaces must move relative to and against each other. If the operating conditions for a set of mating surfaces excludes the use of fluid lubricants, such as oil or graphite, or rolling contact, dry sliding contact may be necessary. Such systems are usually referred to as dry lubricated systems. These systems have been used for example in certain parts of turbine engines, such as in the seals of turbine engine rotary regenerator sections, but may also have use in certain types of bearing systems and particularly in various high temperature environments, typically hostile to oil and graphite lubricants and the like, where dry contacting surfaces must slide over each other, for example in Wankel engines. Such systems usually exhibit both high wear and high friction and ordinarily suffer extensive surface damage such as galling. It is a general object of this invention to provide a new material primarily for use in dry lubricated systems and particularly for high temperature use, the material exhibiting desirable friction and wear characteristics.
The prior art seems to have been aware of the fact that various oxides might be used on mating sliding surfaces with some resultant desirable sliding and rubbing characteristics. In the case of certain mating sliding metal pairs such as iron and steel it has been noted that continuous formation and reformation of oxide film occurs at the surfaces when certain elevated temperatures are reached. This action is sometimes accompanied by a decrease in friction but it usually results in extensive surface damage. It has been further noted that any effect which tends to remove the oxide film thus formed, such as a decrease in temperature, scaling or the like, causes an increase in friction. The iron oxides wustite (FeO), magnetite (Fe.sub.3 O.sub.4) and hematite (Fe.sub.2 O.sub.3) have been included among the various oxides studied. For example, a downward trend in the coefficient of friction has been noted for dry steel specimens as a result of the formation of FeO films. This phenomenon has been reported in the Handbook of Mechanical Wear, edited by Lipson and Colwell, published by the University of Michigan Press, 1961, at Chapter 3, Page 41. This same reference reported studies made on pre-formed Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 oxide films 1200 angstroms thick. The conclusion recorded in this reference at page 45 of Chapter 3 is that Fe.sub.3 O.sub.4 can be beneficial in decreasing friction and in preventing surface damage while in comparison Fe.sub.2 O.sub.3 exhibits high friction, excessive welding and a great deal of surface damage. It was, therefore observed at page 55 of this reference that FeO and Fe.sub.3 O.sub.4 may be generally beneficial under dry lubrication conditions while Fe.sub.2 O.sub.3 is generally considered unsuitable for such use. This conclusion is in line with the generally accepted fact that Fe.sub.2 O.sub.3 powder is regarded as an excellent polishing rouge because of its extremely abrasive nature.
U.S. Pat. No. 3,142,894 is of general interest in connection with this invention. The patent issued to S. T. Ross et al. on Aug. 4, 1964 and is an example of a prior art iron oxide turbine seal material. This sintered iron-copper-carbon material is porous and can be oxidized to some depth prior to use to provide a material which forms Fe.sub.3 O.sub.4 and FeO during service.