244
7 Composites
In the cases of the hard particles, the improved wear behaviour is not related to any
lubrication effect caused by the redistribution of the particle material. Although the
decrease of the friction coefficient as a result of the particle incorporation was detected
for a variety of coatings and particles [128, 144, 164–166], the diminished wear loss
is rather related to the increase of the hardness of the coating [167]. A significant
decrease in the wear factor can be seen even for cases when the particle incorporation
leads to an increase in the friction coefficient [108]. It was the common experience that
the abrasion-induced weight loss has the minimum where the hardness exhibits the
maximum [81, 122, 144, 164, 166, 168–170], and similar coincidence can be seen for
the maximum hardness and the minimum of the friction coefficient [159]. The latter
fact clearly indicates that, beside the surface effect of the particles themselves, the
structural features of the deposit also have a fundamental role in the wear behaviour
of the composite coatings.
7.2.9 Hydrophobic Dispersion Coatings
Hydrophobicity is a typical feature of a composite in which not only the presence
of the particles matters (like it was shown for hardening), but the physico-chemical
properties of the dispersed particles are of high importance. Namely, the hydrophobic
nature of a composite surface stems from the hydrophobicity of the particles incorporated; therefore, it cannot be achieved with any arbitrary type of particle but with
purposefully chosen ones only. In this respect, hydrophobic dispersion coatings differ
from dispersion-hardened ones in which the particle-induced structural features had
the major role in the functionality of the coatings. More details on the theory of
hydrophobicity will be given in Chap. 8 where roughness-induced hydrophobicity
will also be dealt with.
Although the hydrophobic properties of the composites strongly stem from the
hydrophobic properties of the incorporated particles themselves, the particle concentration at which superhydrophobicity is set in varies much with the quality of the
particles. For PTFE dispersions, a particle content around 70 vol.% was reported
to lead to a sufficiently high water contact angle [147, 158]. Since PTFE is a soft
material, the hardness of these coatings is much lower than that of the metal matrix,
even though it is nanograined due to the particle incorporation. A great advantage of
the PTFE filling in the metal matrix is that the hydrophobicity of the surface can be
retained after sufficient wear damage since the self-lubricating PTFE maintains the
hydrophobic nature of the surface. This is because the PTFE content of the coating
is smeared out along the surface upon wear, hence increasing the PTFE-coated areas
as compared to the undamaged surface.
Concerning the hydrophobic properties of MMCs with hard particles, CeO 2 -filled
composites were also of high particle content, ranging between 55 and 97 vol.%
[139]. Much lower particle content was enough to achieve superhydrophobicity with
ITO and WS 2 particles (about 1.8 [171] and 3.5 [160] w.%, respectively). In the
hydrophobic dispersion coatings with low-concentration hard particles, the change
7 Composites
In the cases of the hard particles, the improved wear behaviour is not related to any
lubrication effect caused by the redistribution of the particle material. Although the
decrease of the friction coefficient as a result of the particle incorporation was detected
for a variety of coatings and particles [128, 144, 164–166], the diminished wear loss
is rather related to the increase of the hardness of the coating [167]. A significant
decrease in the wear factor can be seen even for cases when the particle incorporation
leads to an increase in the friction coefficient [108]. It was the common experience that
the abrasion-induced weight loss has the minimum where the hardness exhibits the
maximum [81, 122, 144, 164, 166, 168–170], and similar coincidence can be seen for
the maximum hardness and the minimum of the friction coefficient [159]. The latter
fact clearly indicates that, beside the surface effect of the particles themselves, the
structural features of the deposit also have a fundamental role in the wear behaviour
of the composite coatings.
7.2.9 Hydrophobic Dispersion Coatings
Hydrophobicity is a typical feature of a composite in which not only the presence
of the particles matters (like it was shown for hardening), but the physico-chemical
properties of the dispersed particles are of high importance. Namely, the hydrophobic
nature of a composite surface stems from the hydrophobicity of the particles incorporated; therefore, it cannot be achieved with any arbitrary type of particle but with
purposefully chosen ones only. In this respect, hydrophobic dispersion coatings differ
from dispersion-hardened ones in which the particle-induced structural features had
the major role in the functionality of the coatings. More details on the theory of
hydrophobicity will be given in Chap. 8 where roughness-induced hydrophobicity
will also be dealt with.
Although the hydrophobic properties of the composites strongly stem from the
hydrophobic properties of the incorporated particles themselves, the particle concentration at which superhydrophobicity is set in varies much with the quality of the
particles. For PTFE dispersions, a particle content around 70 vol.% was reported
to lead to a sufficiently high water contact angle [147, 158]. Since PTFE is a soft
material, the hardness of these coatings is much lower than that of the metal matrix,
even though it is nanograined due to the particle incorporation. A great advantage of
the PTFE filling in the metal matrix is that the hydrophobicity of the surface can be
retained after sufficient wear damage since the self-lubricating PTFE maintains the
hydrophobic nature of the surface. This is because the PTFE content of the coating
is smeared out along the surface upon wear, hence increasing the PTFE-coated areas
as compared to the undamaged surface.
Concerning the hydrophobic properties of MMCs with hard particles, CeO 2 -filled
composites were also of high particle content, ranging between 55 and 97 vol.%
[139]. Much lower particle content was enough to achieve superhydrophobicity with
ITO and WS 2 particles (about 1.8 [171] and 3.5 [160] w.%, respectively). In the
hydrophobic dispersion coatings with low-concentration hard particles, the change