Page 15 - Handbook of Surface Improvement and Modification

10 Scratch and Mar Resistance

side of the sample (detected by infrared
spectroscopy and molecular weight mea-
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surements). The sample retained good
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transparency after annealing.
The polycarbonate/poly(methyl meth-
acrylate-co-phenyl methacrylate) copoly-
mer blends were miscible at all
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compositions used in the study. With
poly(methyl methacrylate-co-phenyl meth-
acrylate) copolymer content increased, the
scratch resistance of the blend increased
with tensile strength and tensile modulus
remaining unchanged due to a good misci-
bility between both components of the
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blend. The increase in scratch resistance
was a direct result of hardness increase by
Figure 2.9. Illustrated progression of typical scratch the increased content of poly(methyl meth-
deformation features with increasing load for polymers
with various strength and ductility, including acrylate-co-phenyl methacrylate) copoly-
(a) weak and ductile, (b) strong and ductile, (c) weak mer (Figure 2.8).
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and brittle, and (d) strong and brittle polymers. Arrows The inherently rigid nano-particles
indicate the transition between different damage
mechanisms. [Adapted, by permission, from Barr, CJ; such as nano-SiO , nano-Al O , and nano-
2
2
3
Wang, L; Coffey, JK; Daver, F, J. Mater. Sci., 52, 1221- ZrO particles were used to improve the
34, 2017.] 2 16
mechanical properties of coatings. The
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nano-SiO particles are most common because of their low cost. The nano-SiO particles
2
2
are more efficient in increasing the macrohardness and scratch resistance of polyurethane
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coatings than micro-silica particles. Hardness (and scratch resistance) of a deposited
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layer depends on hardness of filler particle. Compared in the Mohs scale, the hardness of
the most common fillers is as follows: titanium dioxide − 6, silica and zirconia − 7, Al O 3
2
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(corundum) − 9, diamond − 10. Combination of silica and corundum nanoparticles gave
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better scratch resistance than silica alone. The deposition of nanoparticles on the surface
improves scratch resistance without altering gloss and transparency of automotive coat-
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ings.
The texture patterns were found to be an effective methods for enhancing the scratch
resistance of polymeric surfaces, especially when they were more sensitive to scratch
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damage due to their relatively low strengths and stiffness. The deformation characteris-
tics of base material, its surface texturing, and the capacity of textures to obscure scratch
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damage influence human perception of damage. Figure 2.9 shows common features of
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scratch damage of polymers varying in strength and ductility as the load increases. Mar
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damage occurs first. It is defined as a small non-recoverable deformation resulting from
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the compression during indentation. There is no material removal and either a shallow
groove is formed or a noticeable change in gloss is observed resulting from the “ironing of
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the surface asperities. At the next stage, at a some critical load, indenter is held back by
increased frictional forces, followed by elastic energy built up and the release of indenter
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once elastic energy is sufficient to overcome static friction. The recurrent behavior of
this kind is known as stick-slip which produces a “fish scale pattern underneath the