Text preview for : 5991-4865EN Evaluation of Bearing Materials Using Nano-Scale Wear Testing - Application Note c201408 part of
| Home
Keysight Technologies
Evaluation of Bearing Materials Using
Nano-Scale Wear Testing
Application Note
Introduction
The most common materials for bearing fabrication are metals, such as low-carbon steel, stainless
steel, chrome steel and high-speed steel. Polymeric materials are alternative candidates due to
their self-lubrication ability, high impact durability, high corrosion resistance, low specific gravity,
and high melting temperature. Polymers have therefore received widespread attention as new
tribological materials for dry, aqueous and corrosive conditions. Among these polymeric materials,
polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) and also their composites are often used
in tribological applications.
In fans used for cooling of computers and other electronics, it has been found that one of the main
contributors to failure is degradation of the miniature ball bearings [1], with deterioration of the
lubricant as the primary failure mechanism in these applications. Due to the criticality of ball bearings,
diagnosis and prognosis of these failures have been of interest to the industry. There are several
techniques to detect faults in ball bearings. In 2011 Oh, et al, found a correlation between acoustic
emission (AE) features and bearing degradation in computer cooling fans [2]. More recently, in 2013
Kumar, et al [3] determined the failure mechanisms of polymeric bearings using the analysis of vibration,
speed and acoustic emission data, together with characterization of the worn bearing surfaces and
measurements of friction.
The bearing materials tested in this work were previously evaluated [3] at a rotational speed of 4800rpm
in a test fixture which supports a load of approximately 1.4N. Acoustic emissions from these bearings
during the initial stages of the operation of the bearings were monitored to compare the performance
of these materials. These prior results are shown in Figure 1. Throughout most of the AE test, and
especially near the end, the steel bearings exhibited the fewest AE events, followed by the PEEK and the
PTFE bearings. Thus, from the AE test, we would rank the materials in order of performance (from best
to worst) as: bearing steel, PEEK, PTFE.
However, AE testing requires a long time. Thus, the aim of the present work is to establish a rapid
assessment tool for bearing materials, where indications of the performance of the material can be
known in hours, rather than days. We hypothesize that the results of nano-indentation and nano-wear
testing are related directly to the results of AE testing.
Abstract
Self-lubricating polymeric materials are attractive candidates to be used as bearing materials in
lightly loaded applications. In this study, miniature ball bearings made of steel, polytetrafluoroethylene
(PTFE) reinforced with graphite, and polyether ether ketone (PEEK) are evaluated by means of nano-
indentation and nano-wear tests. As quantified by the volume of the wear track, bearing steel has
the best tribological performance, followed by the PEEK and PTFE. The volume of the wear track
correlates with acoustic emissions measured during life testing. Other measurements available from
a nano-wear test may indicate in-product performance; these include pile-up and the production of
wear debris.
Carlos Morillo, Ranjith-Kumar Sreenilayam-Raveendran, Michael H. Azarian and Michael Pecht
CALCE (Center for Advanced Life Cycle Engineering) University of Maryland, College Park,
MD 20742 USA
03 | Keysight | Evaluation of Bearing Materials Using NanoScale Wear Testing - Application Note
Experimental Procedure
Samples Wear Testing
Three different materials were selected for testing: bearing steel, The test method "G-Series Pass and Return Wear Test" used to
PTFE-graphite composite, and PEEK. These are the same samples perform one multi-pass, constant-load wear test on each sample.
used in a previous study [3]. Samples were cut and mounted in The chronology of a single test was as follows: The indenter profiled
epoxy, then ground and polished to a mirror-like finish. the original surface along the length of the anticipated wear test,
then returned to its starting position (profile length = 120m,
Equipment profile force = 50N). The indenter then performed a beginning
profile (10m, 50N), increased the applied force to the wear load
For all testing, the Keysight Technologies, Inc. G200 NanoIndenter and performed the first wear pass (100m, 20mN), then performed
was used, having an XP head with a Berkovich tip (20nm diameter), an ending profile (10m, 50N), and finally returned to its starting
and the continuous stiffness measurement (CSM) option. position to complete the first wear cycle. The beginning and ending
profiles for each wear pass were used for leveling. Ten wear cycles
Indentation Testing were performed in the same way (profile, wear, profile). After the
wear cycles, the indenter performed a final profile along the entire
The test method "G Series CSM Standard Hardness, Modulus and
length of the wear test (120m, 50N), then performed a final
Tip Cal method" was used for the indentation tests. As the indenter
cross-profile of the wear track (100m, 50N) at its midpoint.
was pressed into the material, a small oscillation (45Hz, 2nm) was
Each wear test returned measurements of scratch width, depth,
imposed in order to measure elastic modulus and hardness as a
deformation area, and pile-up. Post-test imaging of the wear
continuous function of penetration depth. Ten indentations were
tracks by scanning-electron microscope gave further qualitative
performed on each material to a peak depth of 1000nm. We used
information about each material. One wear test was performed on
data in the range of 900