246
7 Composites
also increases. Other options include magnetite [175, 176, 182] and cobalt [175]. For
the case of magnetite incorporation, the coercivity is of marginal importance but the
increase in magnetization is the goal of the deposit optimization. Composites like
Ni–Co–P(BaFe 12 O 19 ) belong to the family of the so-called soft/hard composites in
which the hard magnetic particles enhance the remanence of the matrix by pinning
the matrix magnetization.
The concept of the application of the homogeneous external magnetic field in the
MCE process for enhancing the coercivity is as follows. The orientation of the particles as magnetic dipoles to the direction of the external field leads to a deposit in which
magnetic particles are “frozen” with a fixed magnetization direction. This direction
defines the magnetization direction of the entire deposit since the hard-magnetic
particles interact with each other and also with the matrix when it is magnetic. This
aligning mechanism works for particles that are large enough to have a fixed magnetization direction. In contrast, composites with small enough nanoparticles exhibiting
superparamagnetism have little effect on the magnetization of the composite deposit
[181].
However, the impact of the external magnetic field proved to be much more
complicated than representing a simple orientation effect for the particles. The application of a permanent magnet behind the substrate generates an inhomogeneous
magnetic field that attracts the particles, whose accumulation may hinder the onset
of the deposition process. In contrast, after the start of the plating process, the same
attraction force speeds up the particle incorporation [178]. A similar field gradient
effect was also demonstrated with an electromagnet if the field was perpendicular to
the substrate and with a field gradient pointing from the bulk solution towards the
substrate surface [175]. Interestingly, the field parallel to the substrate may lead to
the decrease of the particle incorporation, too. This was explained by the Lorentz
force that is comparable to the adhesion force between the small particle and the
substrate. Since the Lorentz force moves the particles parallel to the surface, it can
inhibit the particle incorporation by breaking the relatively weak particle/substrate
interaction in the first stage of its immobilization [175].
7.2.11 Role of the Incorporation of Inert Particles
in the Corrosion and Oxidation Behaviour of Metals
In the MMCs with inert particles, the change in the corrosion rate of the metal matrix
as a result of the particle incorporation is mostly of physical origin, and hence, is
unrelated to the chemical properties of the particles themselves. In this respect, the
impact of the particle on the corrosion rate is partly similar to the phenomenon of
dispersion hardening. In general, the corrosion rate of MMCs is substantially smaller
than that of the metal matrix deposited without the filling particles. As for any other
effect of physical origin, this was also exemplified for a large variety of codeposited
particle such as Al 2 O 3 [164, 165, 188], AlN [136], SiC [82, 87, 103, 104, 124, 129,
7 Composites
also increases. Other options include magnetite [175, 176, 182] and cobalt [175]. For
the case of magnetite incorporation, the coercivity is of marginal importance but the
increase in magnetization is the goal of the deposit optimization. Composites like
Ni–Co–P(BaFe 12 O 19 ) belong to the family of the so-called soft/hard composites in
which the hard magnetic particles enhance the remanence of the matrix by pinning
the matrix magnetization.
The concept of the application of the homogeneous external magnetic field in the
MCE process for enhancing the coercivity is as follows. The orientation of the particles as magnetic dipoles to the direction of the external field leads to a deposit in which
magnetic particles are “frozen” with a fixed magnetization direction. This direction
defines the magnetization direction of the entire deposit since the hard-magnetic
particles interact with each other and also with the matrix when it is magnetic. This
aligning mechanism works for particles that are large enough to have a fixed magnetization direction. In contrast, composites with small enough nanoparticles exhibiting
superparamagnetism have little effect on the magnetization of the composite deposit
[181].
However, the impact of the external magnetic field proved to be much more
complicated than representing a simple orientation effect for the particles. The application of a permanent magnet behind the substrate generates an inhomogeneous
magnetic field that attracts the particles, whose accumulation may hinder the onset
of the deposition process. In contrast, after the start of the plating process, the same
attraction force speeds up the particle incorporation [178]. A similar field gradient
effect was also demonstrated with an electromagnet if the field was perpendicular to
the substrate and with a field gradient pointing from the bulk solution towards the
substrate surface [175]. Interestingly, the field parallel to the substrate may lead to
the decrease of the particle incorporation, too. This was explained by the Lorentz
force that is comparable to the adhesion force between the small particle and the
substrate. Since the Lorentz force moves the particles parallel to the surface, it can
inhibit the particle incorporation by breaking the relatively weak particle/substrate
interaction in the first stage of its immobilization [175].
7.2.11 Role of the Incorporation of Inert Particles
in the Corrosion and Oxidation Behaviour of Metals
In the MMCs with inert particles, the change in the corrosion rate of the metal matrix
as a result of the particle incorporation is mostly of physical origin, and hence, is
unrelated to the chemical properties of the particles themselves. In this respect, the
impact of the particle on the corrosion rate is partly similar to the phenomenon of
dispersion hardening. In general, the corrosion rate of MMCs is substantially smaller
than that of the metal matrix deposited without the filling particles. As for any other
effect of physical origin, this was also exemplified for a large variety of codeposited
particle such as Al 2 O 3 [164, 165, 188], AlN [136], SiC [82, 87, 103, 104, 124, 129,