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I have been tasked to update and revise where necessary AMS2526, "Molybdenum Disulfide Coating, Thin Lubricating Film, Impingement Applied" first issued in 1979. "AMS" standards are published by the Aerospace Council's Materials Committees of SAE International.

When my draft of the revision was circulated this Spring, there was criticism regarding the tribology testing requirements for this coating. The current requirement for "wear life" determination is "The average of specimen time to failure shall be not less than 15 minutes, determined in accordance with ASTM D2714, Procedure A.", used for "Fales block-on-ring friction and wear" testing.

An objection to this test method was received because:

"1) ASTM D2714-94 does not contain a "Procedure A."

"2) ASTM D2714-94 defines a wear tester with a 10:1 mechanical load lever system for dead weight loading applied to the "bale rod". However, AMS2526 paragraph 3.3.3.1.8 defines a 30:1 mechanical load ratio. Modern versions of this test machine use pneumatic loading paired with a force transducer and digital input of load set points. The defined procedure is outdated and should be updated to specify the required normal force at the test interface, which in this case would be 30, 60, and 90 lbf.

"3) The test does not define what constitutes a failure."

I have no personal experience performing tribological testing and have no access to ASTM D2714 or any other ASTM documents. I am aware that selection of a tribological test method and criterion may be very dependent on the application of the part(s) being coated and tested. By the way, this is a "Periodic" test, to be performed no less than annually.

Dave Himmelblau, Materials and Processes Specialist (Retired)

I have a copy of ASTM D2714. Here is the procedure and calculation/reporting information. Hopefully this helps provide some insight into the "Falex Block-On-Ring" test method:

3. Summary of Test Method

3.1 The test machine is operated using a steel test ring
rotating against a steel test block, the specimen assembly being
partially immersed in the lubricant sample. The velocity of the
test ring is 7.9 m ⁄min 6 0.16 m ⁄min (26 ft ⁄min 6 0.52 ft ⁄min)
which is equivalent to a spindle speed of 72 r ⁄min 6 1 r ⁄min.
The specimens are subjected to 68 kg (150 lb) normal load
applied by 6.8 kg (15 lb) of dead weight on the 10:1 ratio lever
system. Test duration is 5000 cycles.

3.2 Three determinations are made: (1) The friction force
after a certain number of revolutions, (2) the average width of
the wear scar on the stationary block at the end of the test, and
(3) the weight loss for the stationary block at the end of the test.

9. Procedure

9.1 With the revolution counter set at zero, gently place a
6.78 kg (15 lb) load on the bale rod, being very careful to avoid
shock-loading. When the fluid reaches temperature of 43.3 °C
(110 °F) start the machine and bring the speed to 72 r ⁄min.
Record the friction force and the temperature of the liquid at
200, 400, 600, and 4500 revolutions and check the speed at
each of these times. Stop the machine at 5000 revolutions.
Remove the test block and test ring and clean them thoroughly
using solvents selected in 6.3. Remove any excess wear
particles that have accumulated on the side of the scar by
brushing with a camel's hair brush; weigh the test ring and test
block to the nearest 0.1 mg and measure the width of the wear
scar of the test block at three points: in the center and 1 mm
away from each edge as shown in Fig. 3. Four tests are required
to establish a satisfactory average.

10. Calculation and Report

10.1 Report the friction force at 200, 400, 600, and 4500
revolutions, the weight loss for the test block and test ring, and
the average wear scar width. Fig. 3.

10.2 Calculate the coefficient of friction values from the
friction force value as follows:
f = F/W
where:

f = coefficient of friction,
F = measured friction force, kg (lb), and
W = normal load, kg (lb).