Coefficient of friction and damage to contact area during the early stages of fretting 2: steel, iron, iron oxide, and glass combinations Page: 4 of 27
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MATERIALS AND PROCEDURE
Materials. - Three different steels were chosen as representing
common engineering materials:
(1) The convex specimens for all steel fretting runs were 1/2-inch-
diameter commercial steel balls containing approximately 1 percent
carbon and having a case hardness of Rockwell C-62.
(2) Some flat specimens were made of a drill-rod stock containing
approximately 1 percent carbon and were either hardened to Rockwell
C-60 or annealed to less than Rockwell B-80.
(3) Other flat specimens were made of an unhardened tool steel
containing approximately 1 percent carbon and 1.45 to 1.55 percent
manganese and having a hardness of Rockwell B-90.
The pure iron specimens were made from Puron stock (99.97 percent
Fe) hardened to Rockwell B-82 by cold working; this material was cho-
sen for simplicity, homogeneity, and established friction characteristics.
The glass specimens were microscope slides. Chemically pure
powders of ferric oxide Fe203 and ferrosoferric oxide Fe304 were ob-
tained commercially.
Specimen preparation. - All specimens except glass were abraded
on 2/0 emery paper to give a uniform surface finish of 10-20 micro-
inches root mean square. Consistent and thorough cleaning of speci-
mens to remove the last trace of grease was important for reproduci-
bility. Freedom from grease was indicated by high initial values of
coefficient of friction L. Details of the cleaning procedures for
metal and glass are given in appendix A.
The oxide specimens were machined from compacts made by pressing
and sintering pure Fe203 and Fe304 powders under controlled conditions
(appendix B).
Experimental procedure. - The cleaned specimens were mounted in
the specimen holders of the apparatus and the toad applied by adding
weights. The cover of the Lucite box 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 reciprocating
action was started. In the case of glass flat specimens, microscopic
observations (X75 to X200) of the fretting action as it occured were
correlated with the friction tracing. For metal-metal combinations,
the surfaces were separated and examined microscopically after runs
lasting 1/2, 1, 5, 10, 20, 50, 100, 200, 300 or 400 cycles. New
specimens were used for each run.NACA TN 3144
3
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Bailey, John M. & Godfrey, Douglas. Coefficient of friction and damage to contact area during the early stages of fretting 2: steel, iron, iron oxide, and glass combinations, report, April 1954; (https://digital.library.unt.edu/ark:/67531/metadc56989/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.