Introduction
Enhanced surface durability is desired for new bearing materials,
including corrosion resistant materials. Development of advanced
bearing materials is generally focused on material microstructure
for rolling contact fatigue resistance. The challenge is to
develop surface durability attributes that complement fatigue
resistance attributes.
One can categorize surface deterioration into three basic
modes: wear, scuffing and fatigue. The term “adhesive
wear” is commonly used when failed surfaces appear to
have undergone plastic flow due to local “adhesion”
at the interface. When attempting surface fatigue simulation
tests with advanced bearing materials having corrosion resistant
properties, local adhesive events have been found to prevent
operation when the EHD film thickness (h) is small relative
to surface roughness height (?). If the chemistry of the material
does not allow the formation of surface films from reactions
with the oil, adhesive wear can supersede surface deterioration
due to surface initiated fatigue. In addition, material properties
that affect plastic flow, like hardness, seem to influence
the onset of adhesive wear. The mechanisms that control the
ability of a surface to handle high normal and tangential
stress and to recover subsequent to local damaging events
are a mystery. Testing for these mechanisms and associated
surface durability attributes is essential for material development
and assurance of performance in service.
It was found that surface failure by adhesive wear is initiated
at microscopic sites of insufficient surface film lubrication
or at sites of debris encounters. With limited chemical reactivity
between lubricating oil and some corrosion resistant materials,
local adhesion events, which are not able to recover, propagate
into broad patches of adhesive wear damage. With sufficient
sliding velocity and contact stress, adhesive wear can transition
into a major scuffing event. A scuffing event is characterized
by a rapid rise in friction and temperature. These tribological
features, as measured with an adhesive wear test method, correlate
with experience in full-scale bearing tests.
At the heart of surface durability is material compatibility
with lubricating oil chemistry to form surface films, which
prevent local adhesion. The adhesive wear test method described
below progressively increases the degree of asperity encounter
at the interface under rolling/sliding conditions. The test
method invokes tribological interactions, which are measured
in terms of friction (traction), gentle polishing wear of
surface features, adhesive wear events and scuffing. The test
protocol described below is an attempt to simulate the adhesive
wear mechanisms that believed to occur in rolling element
bearings.
WAM Test Machine and Test Specimens
The Wedeven Associates, Inc. machine, WAM8,
is a highly flexible tribology test machine for rolling element
bearing and other machine element simulations. Rolling element
/ raceway simulations are conducted with ball and disc specimens.
Test protocols are programmed to provide a combination of
normal stress and tangential motions to invoke surface deterioration
modes of wear, scuffing and fatigue.
Ball and disc specimens are usually supplied by a bearing
vendor. Final finishing is conducted by Wedeven Associates,
Inc. Surface preparation consists of fine grinding, followed
by abrasive lapping to a surface finish of 2 µ-inch, Ra. The
lapping operation is done with specimen rotation about its
centerline to create a circumferential lay in the direction
of rolling motion. The finishing method consists of several
lapping stages to obtain a consistent surface texture. Care
is taken to avoid microscopic bends and folds (“leafing”)
in the surface texture. The final finish is similar to a finely
honed bearing raceway.
Test balls are usually 1.125-inch (28.58 mm) diameter. The
selection of ball size and quality are determined by availability.
The test ball specimens, which are production quality, have
much better surface finish than the disc specimen.
Adhesive Wear Test Method
The original adhesive wear test protocol was developed under
various projects for Pratt & Whitney (PO Nos. F772352,
F777459 and F791674). Highlights of the test method are given
below.
| Maximum Hertzian contact stress: |
1.95 GPa (282 ksi) |
| Entraining (rolling) velocity, Ue: |
10.16 m/sec (400 in/sec to 50 in/sec)
Ue reduced in four stages
of 180 seconds each |
| Contact slip: |
15%, subsequent tests 30% & 50% until failure
If ashesive wear occurs at 15% slip, 8% slip is run
% slip = (Ub- Ud)/
½(Ub+ Ud)
x 100
where
Ub = surface velocity
of ball
Ud
= surface velocity of disc |
| Test oil: |
MIL-PRF-23699 (Mobil Jet II) |
| Test temperature: |
200°C (392°F) |
An adhesive wear test series for a material generally consists
of three tests: one each at 15% slip, 30% slip and 50% slip.
If an adhesive wear event occurs at 15% slip, the test is
repeated at 8% slip. Generally, two test series are run for
each material pair. At the end of each test, the running tracks
on the ball and disc specimens are photographed at 100x. Traction
coefficient is plotted over time for each test. The surfaces
of ball and disc specimens are documented with photomicrographs,
which are stored as digital images.
At 200 °C the disc surface can become lightly discolored
with oxide or surface films from the oil. To avoid the influence
of accumulated surface chemistry on the disc, the test track
on the disc specimen is refinished after each test.
Traction Test Plots
Typical traction test plots are shown in Figure 2. Initial
operation at an entraining velocity of 10.16 m/sec gives an
EHD film thickness to surface roughness ratio (h/?) close
to 1.0 and a traction coefficient on the order of 0.02. Incremental
reductions in entraining velocity increase the traction coefficient
due to thinner EHD films and greater surface interaction.
A gradual decrease in traction coefficient is attributed to
polishing wear of surface features.
An adhesive wear event is identified by a rapid excursion
in traction coefficient. While the traction may recover after
a few seconds, the local surface damage that occurs during
the adhesive wear event is permanent. The local adhesive wear
damage becomes a vulnerable site for surface initiated fatigue.
Ranking Adhesive Wear Resistance
A system of ranking adhesive wear performance has been developed
to characterize the overall lubricating performance of various
material pairs and lubricants. Performance is based on measured
behavior reflecting the ability of the material pair to accommodate
low h/? and high slip operation without adhesive wear and
with traction behavior reflecting good lubrication at the
interface. A point system is used to give weighting factors
for tribology attributes that we feel are important for high-speed
rolling element bearings. The criteria and weighting factors
for performance ranking are shown in Table 1.
Performance is judged relative to a baseline material pair
and lubricant of M50/M50 and Mobil Jet II. Mobil Jet II is
classified as a Standard (STD) MIL-PRF-23699 oil. The lubricating
ability of Mobil Jet II ranks high among typical jet engine
oils. The material pair of M50/M50 gives excellent performance
for adhesive wear resistance. These baseline materials, which
survive the test protocol with no significant surface damage,
achieve a score 50 with the performance criteria discussed
above. Other materials and lubricants can be ranked relative
to the baseline materials, as shown in Figure 3.
Discussion
Some material pairs and lubricants may experience adhesive
wear problems with this test protocol under low h/? conditions,
particularly in the presence of high slide-to-roll ratios.
A deficiency in adhesive wear compared to the baseline materials
does not preclude the successful utilization of certain materials
and lubricants. The vulnerability of a tribo-system to adhesive
wear can be at least partially offset by careful run-in where
the presence of chemically active additives and improvement
in surface topography condition the surface for durability.
However, sudden “events” like skidding or debris
rollover, may again make the surfaces vulnerable to adhesive
wear. If start/stop cycles are not severe with respect to
micro-slip and contact stress, and if near full-film EHD conditions
are prevalent during operation, a material pair deficient
in adhesive wear may operate satisfactorily. On the other
hand, a good performance ranking in this test does not assure
performance in service. Successful performance under the selected
test conditions of the test protocol does not guarantee success
under other conditions. Assessment of the tribological parameters
for the particular application, along with simulation testing,
is recommended for applications with challenging operating
conditions. As a minimum, the test protocol provides a good
screening test for materials and lubricants.
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