Page 39 - Handbook of Surface Improvement and Modification

34 Scratch and Mar Resistance

Figure 2.25. Scratching maps of polyester and
polyphthalamide coating after ESD and HD fluidized
bed: 800 μm tip radius indenter. TP − thermoplastic
coating, TS − transparent polyester. [Adapted, by per-
mission, from Barletta, M; Gisario, A; Trovalusci, F;
Vesco, S, Prog. Org. Coat., 76, 244-56, 2013.]

spray coating deforms at very low load
(onset of scratch visibility at 7.7 N) and,
55
then, delaminates at very high load. The
fluidized bed coating has the scratch visi-
55
bility at 10.7 N. At higher load, it presents
a range of damages and failure events
which vary from the plowing at intermedi-
ate load (10-15 N) to cracking and failure at
55 Figure 2.26. Increased rigidity and stress-absorbing
higher load (>16 N). Figure 2.26 shows action of coatings with different thickness: (a) low thick-
the effect of thickness on rigidity and ness; and (b) high thickness. [Adapted, by permission,
55
stress-absorbing capability. The thinner from Barletta, M; Gisario, A; Trovalusci, F; Vesco, S,
Prog. Org. Coat., 76, 244-56, 2013.]
coating has a reduced capacity to absorb
55
the stress (Figure 2.26a). Accordingly, some breakages can occur at the very outermost
55
layer of the coating. The thicker polyester coatings may absorb the residual stress field
55
more readily (Figure 2.26b).
Precursors are hydrolyzed in the presence of water and nanocrystalline or amorphous
56
nanoscale solid particles are formed in a liquid phase (sol obtained). The subsequent
condensation results in gelation with a solid network formation having pores filled with
56
liquid (gel). The sol can be immediately coated on a workpiece and subsequently dried
56
resulting in a three-dimensional network. Hybrid nanocomposite coatings can be gener-
ated by external addition of nanoparticles or by in-situ generation of nanoparticles in the
56
hybrid matrix. The commonly used precursors include metal salts, oxides, hydroxides,
complexes, alkoxides, and amines, out of which alkoxides are more common in usage. 56
The 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, and
vinyltrimethoxysilane are the most extensively studied and commercially successful
56
organically modified alkoxides with polymerizable organic functional groups. The criti-
cal coating thickness is the thickness beyond which the coatings are not able to retain the