Appeal No. 1999-2131 Page 9
Application No. 08/971,504
high-friction operation, and except for the
stellites, these alloys exhibit a rapid
reduction in yield strength at temperatures
above about 5000 F. Because such surface
temperatures are not uncommon in bit thrust
bearings, stellites have been the structural
inlay material of choice for journal surfaces
(column 2, lines 5-22).... The combination of
an aluminide coating and a superalloy substrate
material to form a sliding bearing element in a
roller cutter drill bit provides increased
bearing performance and consistency under high-
PV operating conditions encountered in rock bit
service. The mechanical behavior of this
coating/substrate combination extends the range
of bit bearing operating temperatures, provides
improved bearing recoverability from load and
friction spikes, and allows the use of floating
element bearing designs in higher-stress,
higher-velocity applications. This is achieved
through the combination of improved high-
temperature mechanical properties of the
substrate along with the tribological
characteristics of the aluminide coating.
Aluminide coatings are particularly adapted for
use with nickel-base superalloys such as Inconel
718, which may be formed from strip, powders, or
machined or cast to produce thin bushing or
washer elements which are subsequently
diffusion-coated to a thickness of preferably
about 0.005 inch, and thermally treated to a
yield strength of more than about 140,000 psi.
(column 3, lines 58-68 and column 4, lines 1-
11).
Thus, Drake teaches an aluminide coating on a superalloy
bearing element that has a yield strength of more than about
140,000 psi.
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