This invention describes systems and methods for implementing bulk metallic glass-based (BMG) macroscale gears with high wear resistance.

This invention creates bulk metallic glasses (BMGs) with selected mechanical properties that are very similar to ceramics, such as high strength and resistance to wear, but without high melting temperatures. Ceramics are high-strength, hard materials that are typically used for their extremely high melting temperatures. Because of their extreme hardness, ceramics are optimal materials for making gears, due to their low wear loss. Unfortunately, ceramics suffer from low fracture toughness (typically <1 MPa·m1/2), and their high melting temperatures prevent them from being cast into net-shaped parts. Ceramic gears, for example, must be ground to a final shape at great expense.

(a) Schematic showing the motivation for developing bulk metallic glass (BMG) gears. Crystalline metals tend to exhibit poor wear while ceramics are brittle. BMGs offer wear resistance similar to ceramics but with up to two orders of magnitude higher toughness. (b) Micrograph showing the teeth of a BMG gear. (c) Two BMG gears in a spur gear test where their performance was shown to be up to three times better than the best steel alloys. (d) Cast BMG gears integrated into a working gearbox. (e) A handful of cast BMG gears demonstrating the ease with which they can be fabricated.
Metal alloys (especially steels) are commonly used as gears and other “wear-resistant” applications (bearings, electronic casings, etc.). Metals can be machined, ground, or forged into wear-resistant parts that have many different properties than ceramics. Metals have higher toughness, lower strength, lower hardness, and lower wear resistance than ceramics. When used in gears, for example, metals commonly break down (galling) during wear.

The problem to be solved is to make metal alloys that combine the benefits of metals and ceramics into a single material, optimally suited for wear-resistant applications. Such an optimal material would (1) have a higher toughness than ceramics, (2) have higher wear-resistance than any metals (approaching ceramics), (3) have low processing temperatures so that net-shaped forming is possible, (4) be machinable, and (5) be robust to extreme environments.

When designed properly, BMGs can have mechanical properties that combine the benefits of ceramics and crystalline metals to make materials with unmatched properties among any other materials for wear resistance. The BMG ceramic-like alloys can be tailored for specific properties, including glass-forming ability, fracture toughness, strength, processing temperature, corrosion resistance, hardness, composition, density, crystallization temperature, and wear resistance.

The basic process for developing BMGs for wear-resistant applications (e.g., gears) is to alter the composition so that the BMG is extremely hard and brittle, yet still has sufficient toughness and glass-forming ability to be cast into an amorphous part without cracking. Extremely brittle BMGs can be fabricated with extreme hardness and strength, but are too brittle to be useful. The concept to be exploited in this application is to make the BMG just barely tough enough to be fabricated into a gear while retaining the lowest wear loss possible.

Moreover, this application also has novelty in the specific BMG alloy compositions that were identified to be wear-resistant. Within a single BMG system, the wear loss can also be improved by annealing, which is known to harden BMGs. One novel reduction to practice in this application is a BMG gear with ceramic-like wear resistance but with the ease of fabrication that comes through the use of BMGs. BMGs have low melting temperatures and can be cast easily and repeatedly into reusable molds. BMG gears may outperform commonly used aerospace gears.

This work was done by Douglas C. Hofmann, Andrew Kennett, and Kobie T. Boykins of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-48689.


NASA Tech Briefs Magazine

This article first appeared in the March, 2015 issue of NASA Tech Briefs Magazine.

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