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August 1966 Brief 66-10373 S NASA TECH BRIEF

NASA Tech Briefs are issued to summarize specific innovations derived from the U. S. space program and to encourage their commercial application. Copies are available to the public from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151.

Bearing Alloys with Hexagonal Crystal Structures Provide Improved and Characteristics

The problem: in vacuum and in air and at higher tempera- Catastrophic failure results when tures than previously experienced. surface films fail because of surface speed, load, or How it's done: environment. Space vacuum is a particularly difficult Of the various metals studied during the research environment, for conventional evaporate program, and are more commonly and surface oxide films do not re-form when worn used, and hence, are of the greatest immediate practi- away. The loss of lubricant film, from whatever cause, cal interest. Titanium is well known as a sub- results in metal-to-metal contact, high friction, re- ject to surface welding or galling and otherwise pos- peated surface welding-shear reactions, metal transfer, sessing poor friction properties. Cobalt alloys have and subsequent bearing failure. been used in bearings but usually in alloys with pre- The solution: dominantly cubic structure. Studies have shown that Make the bearings from metals and metal alloys improved friction and wear properties can be obtained with a stabilized hexagonal crystal lattice structure if cobalt and titanium are alloyed in such a way as to and with a maximum C/A ratio (where C = distance stabilize their hexagonal crystal structures over between hexagonal planes; A = distance between ad- greater ranges of temperature and to increase the jacent atoms.) crystal lattice ratio. It is known, for example, that Extensive research into the basic structure of metals cobalt transforms from hexagonal to cubic structure and metal alloys and the influence of crystalline struc- when heated above 750° F. A marked increase in fric- ture parameters on friction and wear has produced tion accompanies this crystal transformation, and data showing large and consistent differences in the wear rate is about 100 greater for cubic cobalt than for shear forces required for cubic and hexagonal crystals. hexagonal cobalt. When welding occurs between two metal surfaces and These studies have shown that the cobalt hexagonal the welds shear, they do so along distinct planes in form can be stabilized over greater temperature ranges the crystals. Shear forces in cubic crystals are com- by certain additions of tungsten and molyb- monly up to 100 times greater than the corresponding denum. Simple binary alloys of titanium with either shear forces in hexagonal crystals. Further, hexagonal tin or aluminum were found to provide the desired crystal alloys shear smoothly at the original surface structural characteristics. Increasing the percentage without changing the surface geometry and allow the of aluminum or tin produced higher crystal lattice bearing to operate much longer. Additional research ratios, greatly reduced friction, and minimized surface showed that the shear force in hexagonal crystals failure tendencies. varies with the relative spacing of the atoms in the Notes: crystal lattice. The studies show that the careful selec- 1. These temperature stabilized hexagonal crystal tion of hexagonal crystal alloys can significantly im- structure alloys are potentially useful in many ap- prove the friction and wear characteristics of bearing plications. They are presently being investigated for (continued overleaf)

This document was prepared under the sponsorship of the National Government assumes any liability resulting from the use of the Aeronautics and Space Administration. Neither the United States information contained in this document, or warrants that such use Government nor any person acting on behalf of the United States will be free from privately owned rights. use in bearings and seals in aircraft, in hydraulic Titanium-Aluminum Alloys in Vacuum," by equipment, and in vacuum installations. Donald H. Buckley and Robert L. Johnson, For the medical field, the hexagonal crystal cobalt- January 1966. A summary of these reports is given molybdenum alloys have been shown to have lower in American Society of Engineers friction and wear properties than the cubic crystal Transactions, volume 9, no. 2, pages 121 to 135, cobalt-molybdenum alloy presently used in arti- April 1966, "Friction and Wear of Hexagonal ficial human hip and elbow joints. Metals and Alloys as Related to Structure and Lat- Further information concerning these alloys is tice Parameters in Vacuum to 10- 10 Millimeter of given in the following NASA reports: TN D-2523, Mercury," by Donald H. Buckley and Robert L. "Marked Influence of on the Johnson. Friction and Wear Characteristics of Cobalt and 4. Inquiries concerning this invention may be di- Cobalt Base Alloys in Vacuum to 10-9 Millimeter rected to: of Mercury. 1—Polycrystalline and Single Crystal Technology Utilization Officer Cobalt," by Donald H. Buckley and Robert L. Lewis Research Center Johnson, December 1964; TN D-2524, "Marked 21000 Brookpark Road Influence of Crystal Structure on the Friction and Cleveland, Ohio 44135 Wear Characteristics of Cobalt and Cobalt Base Reference: B66-10373 Alloys in Vacuum to 10- 9 Millimeter of Mercury. It—Cobalt Alloys," by Donald H. Buckley and Patent status: Robert L. Johnson, December 1964; TN D-2671, This invention is owned by NASA, and a patent

"Influence of Crystal Structure on the Friction and application has been filed. Royalty-free, nonexclusive Wear of Titanium and Titanium Alloys in Vac- licenses for its commercial use will be granted by uum," by Donald H. Buckley, Thomas J. Kucz- NASA. Inquiries concerning license rights should be kowski, and Robert L. Johnson, March 1965; made to NASA, Code GP, Washington, D.C. 20546. TN D-3235, "Friction, Wear, and Adhesion of Source: Robert L. Johnson and Donald H. Buckley (Lewis-320)

Brief 66-10373 Category 03