Page 37 - Handbook of Surface Improvement and Modification

32 Scratch and Mar Resistance

Hysitron TI 950 TriboIndenter nanoindenter has been developed by Bruker as an
automated instrument for nanomechanical and nanotribological characterization tech-
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niques. The high precision instrument permits setting the probe position to within ±10
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nm. The instrument was used for evaluation of automotive clearcoats. It was found that
the instrument was suited to give information about the fundamentals (e.g., chemistry) that
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were responsible for performance.
Figure 2.19 shows results of evaluation of epoxy coating, subjected to nanoindenta-
tion conditions using an ultra micro-hardness tester Shimadzu DUH-202, by atomic force
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microscopy. The scratch resistance is defined as the normal force F per unit of the
n
r
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cross-section of the area of the scratch A (S = F /A ). The normal force, F , comprises
n
s
n
s
two contributions: a minority contribution due to attractive capillary forces (in the nanon-
ewton range) and the majority one, controlled by the voltage applied to the piezo-element
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(the spring constant of the probe and its sensitivity). The pertinent values of both contri-
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butions to the normal force can be obtained from force calibration plots. The shape of
the scratch trace is considered to be a triangle and the scratch area can be obtained by the
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section analysis.
The scratch test of clearcoats containing nanodiamonds was carried out according to
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ASTM-G171. Scratch load was constant at 500 g. Scratching speed was also con-
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stant. After scratching, 5 pictures were taken by optical microscope at 50× magnifica-
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tion. Scratch width was measured from these images and converted to hardness using the
following equation: 13
8F
H W = ------------------ [2.1]
π S ( W ) 2
where:
2
H W hardness based on the scratch width measured in N/mm
F applied load in N
S W scratch width in mm
Pencil hardness, which is a criterion of scratch resistance, was evaluated using Sheen
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720N device based on JIS K 5400 standard.
2.3 EFFECT OF DEGRADATIVE PROCESSES
Similar to any other materials, surfaces of products discussed in this chapter are vulnera-
ble up to a certain extend when exposed to weather factors, such as moisture, temperature,
UV, and others. Extensive discussion of these influences can be found in a monographic
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source. Here we will discuss a few examples of detrimental effects of some wheater fac-
tors.
Effect of environmental conditioning on scratch performance of polymethyl-
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mathacrylate was investigated. The scratch resistance was decreased on initial exposure
to moisture for all three studied grades. In the case of two highly polar PMMA grades, the
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scratch resistance recovered after longer exposure to moisture. It is likely that the mois-
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ture absorbed initially acts as a plasticizer. The moisture absorption continues until satu-
ration and water molecules cluster imparting a degree of lubrication which consequently
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improves scratch resistance.