Coefficient of friction and damage to contact area during the early stages of fretting 1: Glass, copper, or steel against copper Page: 5 of 24
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NACA TN 3011
Oxide films were formed on copper by heating clean specimens in air
under different time-temperature combinations. Films of Cu20 (500-1000 A
thick as determined by interference colors) were formed by heating copper
100 minutes at 1500 C. Films of CuO with a thickness greater than 1500 A
(several orders of interference colors to gray-black) were formed by
heating copper 5 minutes at 3900 to 4000 C. The composition of the films
and the specimen cleanliness were confirmed by electron diffraction.
Compacts of oxide powders were made by pressing and sintering chem-
ically pure Cu20 and CuO powders. Details of the process are given in
the appendix. c
Experimental procedure. - For all sets of specimens, the procedure
was the same and the run was repeated with new specimens three or more
times. The freshly cleaned specimens were mounted without delay in the
specimen holders of the apparatus and the load applied. The cover was put
in place and dry air was started flowing through the enclosure. When the
relative humidity of the escaping air had dropped to 10 percent, the recip-
rocating action was started. In the case of glass specimens, microscopic
observations during fretting were correlated with the friction tracing.
Fretting runs in the presence of hexadecane were made by applying
to the contact area, with a glass rod, a drop of hexadecane taken fresh
from the end of the percolating column. Surface tension held the liquid
around the contact area.
The normal load was measured at the end of each fretting run by
pulling upward on the flat specimen, (by means of weights, pan, pulley,
and cord assembly) with just enough force to separate the specimens.
Coefficient of friction (ratio of friction force to measured normal
load) was plotted against the number of fretting cycles for each run.
The length of most runs was 300 cycles, which was enough for the friction
coefficient i to reach an essentially constant value. For unidirec-
tional sliding experiments, the procedure was the same except that the
run was limited to one sliding pass of approximately 1/4 inch.
Chemical spot tests were used for identification of small quantities
of debris. Details of the procedure are given in the appendix. Electron
diffraction was used to determine the chemical composition of surfaces
before the runs and also to identify debris when possible.
Preliminary experiments showed that the values of j and the
extent of damage are influenced by several factors. The coefficient of
friction for any given run drops 0.05 when the relative humidity is
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Godfrey, Douglas & Bailey, John M. Coefficient of friction and damage to contact area during the early stages of fretting 1: Glass, copper, or steel against copper, report, September 1953; (https://digital.library.unt.edu/ark:/67531/metadc56786/m1/5/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.