Name: Luiz Carlos Bragatto Junior
Type: MSc dissertation
Publication date: 23/05/2016

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Examining board:

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André Paulo Tschiptschin External Examiner *
Cherlio Scandian Internal Examiner *

Summary: Cobalt base superalloys are used in many industrial applications due to its
characteristic of maintaining its mechanical properties at high temperatures. Three
different cobalt superalloys were produced by cast process with chemical compositions
close to those of commercial alloys Stellite 6, Triballoy T-400, and an alloy Co-Cr-Fe.
To better understand the mechanical and tribological behavior of these superalloys,
scratch tests were performed using a rockwell C indenter with two different
configurations: a single pass with different normal loads, 5 to 20N, and the scratch
speed, 0.01 mm / s to 1 mm / s; and one with several passes, 1 to 15 passes,
maintaining a constant scratching speed of 0.1 mm / s and loads, from 5 to 100N. In
these tests were analyzed the scratch hardness of each material, the fab factor, which
evaluates the ratio of the area of the material displaced to the side and the area of
scratch groove, the degree of penetration, which is a ratio between the depth and the
width of the scratch, and additionally were evaluated the wear rates. The abraded
surface was analyzed by 3D profilometry and scanning electron microscope image to
evaluate the dominant wear mechanism and the wear rate of each material. In all the
three alloys the predominant micromechanisms was microploghing. In the Co-Cr-Fe
alloy, the predominant mechanism was ductile at all settings of the tests. Already in
tests with Stellite 6 and Triballoy T-400 alloys, cobalt matrix showed ductile behavior
and hard phases, carbides and laves phase showed brittle behavior, and were
fractured. In the alloys Stellite 6 and Triballoy T-400, tests with several passes was
observed plastic deformation of the matrix of cobalt which covered the carbides and
laves phases, respectively, and also a mechanical mixing due to excessive fracture of
carbides and laves phases that are mixed with cobalt matrix. The alloy Co-Cr-Fe
showed a lower performance for resistance to wear, and Stellite 6 and Triballoy T-400
alloys showed similar behavior and better wear resistance.
Keywords: Cobalt superalloys, scratch test, wear mechanisms, wear resistance.

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