Page 28 - Handbook of Surface Improvement and Modification

2.2 Additives used 23

Figure 2.18. Scratch failure modes for hard coating on compliant substrates. [Adapted, by permission, from
Chen, Z; Wu, LYL, Scratch damage resistance of silica-based sol-gel coatings on polymeric substrates.
Tribology of Polymeric Nanocomposites. Elsevier, 2013.]
coating becomes harder and stiffer, which
36
lowers its ability to absorb energy. The
beneficial effect observed in practice with
increased colloidal silica content could be
explained by the difference in porosity. 36
Coatings with more colloidal silica had a
lower residual porosity due to the filling of
the pores and the chemical bonding of the
silica nanoparticles with the sol-gel matrix.
The number of pores decreased as did their
36
size. Considering that the pores in brittle
coatings act as flaws causing stress concen-
tration; the larger the flaw size, the lower
the coating fracture resistance and the more
Figure 2.19. The effect of normal force on surface
damage of silica-containing epoxy coatings. [Adapted, the pores the lower resistance to scratch-
36
by permission, from Spirkova, M; Slouf, M; Blahova, ing.
O; Farkacova, T; Benesova, J, J. Appl. Polym. Sci., 102, The size, shape, and concentration of
5763-74, 2006.]
colloidal silica particles in epoxy coatings
37
and their effect on scratch resistance have been studied. The highest surface hardness
(measured by nanoindentation, pendulum test, and the scratch resistance) was measured
o
37
for materials with the glass transition temperature close to 20 C. The addition of 20
37
wt% of silica nanoparticles was required to increase wear and scratch resistances. Figure
2.19 shows the effect of progressive increase of normal force during scratch testing on sur-
37
face damage as measured by atomic force microscopy.
Colloidal nano-silica particles were used to improve the scratch and mar resistance
38
of waterborne epoxy coatings. The nanosilica particles were modified with 3-glycidoxy-