Page 36 - Handbook of Surface Improvement and Modification

2.2 Methods of testing 31

Nano-indenters with a horizontal tip movement capability are frequently used in
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scratch testing. The tips are pyramidal or conical. The pyramidal tip has sharp corners,
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resulting in significant stress concentration and a complex stress distribution. A conical
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tip is better due to its circular projection. Considering that the conical tip is small, the
contact stress is extremely high resulting in a rapid failure which makes it difficult to
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observe transitions in deformation behavior. For this reason, expensive analysis equip-
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ment is usually required.
A practical methodology was developed and standardized by ASTM (D7027-13) and
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ISO (19252:2008). The test involves application of a progressive normal load at a con-
stant rate using a 1 mm diameter spherical
tip (simulates the surface scratching with a
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car key). The geometry of tip permits
more effective examination of scratch
behavior than with pyramid or conical
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indenters. The results of the test simulate
real-life surface damage at a similar length
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scale. Because the applied load is known,
the applied stress can be approximated with
finite element methods, providing the cor-
responding mechanistic information of the
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scratch process. The methodology
employs a variety of microscopy tools for
detailed description of deformation mecha-
Figure 2.23. Scratch tester. [Adapted, by permission, nisms and an assessment of scratch visibil-
from Sangermano, M; Messori, M, Macromol. Mater.
Eng., 295, 603-12, 2010.] ity using a commercially available software
package called Automatic Scratch Visual-
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ization. The software uses physiological parameters to simulate the human eye perception
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of a scratch. The Scratch 5 is a research grade polymer, coatings, films, and bulk materi-
als tribology instrument, capable of exploring scratch, mar, and other surface properties in
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a rigorous and meaningful way. It replicates a variety of industry standards (e.g., 5-Fin-
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ger, Erichsen cross-hatch).
Figure 2.23 shows a scratch tester which was used in evaluation of scratch resistance
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enhancement of polymer coatings. A scratch experiment is performed in three stages: an
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original profile, a scratch segment, and a residual profile. The penetration depth of the
indenter is estimated by comparing the indenter displacement normal to the surface dur-
ing scratching with the altitude of the original surface, at each position along the scratch
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length. Roughness and slope of the surface are taken into account in the calculation of
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the indenter penetration. Two different critical loads are defined which correspond to
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failure and detachment of the coating. The fracture events can be visible on both the
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microscope view and the penetration curves.
The ISO standard suggests conducting the mar test as follows: The paint or coating is
applied onto flat panels, dried/cured, and subjected to the mar resistance determination by
pushing the panels beneath a curved (loop-shaped or ring-shaped) stylus which is mounted
in such a manner that it presses down on the surface of the test panel at an angle of 45°. 49
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The load is increased in steps until the coating is marred.