250
7 Composites
much lower particle concentration (by wt.%) than for microparticles [120]. An interdiffusion annealing treatment at intermediate temperature (500 °C) was found to
improve the oxidation resistance at high temperature (900 to 1000 °C) [206]. The
advantage of the interdiffusion treatment stems from the formation of intermetallic
alloys whose grains withstand to intergranular cracking.
The improvement of the oxidation resistance related to the Al or Cr particle
content of the composite was investigated for Ni–Co [207] and Ni–Cu [204] alloyed
matrices. Alloying Ni with Co increases the incorporation rate of the aluminium
particles because the better wetting behaviour with Co. However, Co is less resistance
to both oxidation and corrosion than Ni, which means that its content in the matrix
was suggested to be 28 wt.% as a maximum [207]. In the case of the Ni–Cu alloys,
the chromium particles effectively hindered the in-depth oxidation to several tens
of micrometre. As the chromium loading of the composite increased, the mixed
CuO–NiO–Cu 2 O layer (i.e., that produced on the particle-free specimens) could not
form but a continuous chromium (III) oxide layer with a thickness of less than 5 µm
formed during the oxidation treatment.
7.2.13 Combination of Various Particles in Electroplated
Dispersion Coatings
The application of different kinds of particles in the electrodeposition of dispersion
coatings may stem from various pursuits. When particle types of similar properties
are used simultaneously, the particle load of the deposit is usually larger than that
achieved with one sort of particles. An example for this case is the codeposition
of Al 2 O 3 and SiO 2 together with a Ni coating [167]. Both sorts of inert ceramic
particles incorporate into the coating in a smaller volume ratio that in the absence of
the other particle, however, the cumulated particle content obtained from the bath
with mixed particles is larger than in the case when either of them was present alone.
Consequently, the hardness and wear behaviour is better with the coating containing
the mixture of the ceramic particles than that containing one particle type only.
The same improvement in the deposit properties can be seen when the two sorts of
nanoparticles belong to different material families, i.e., when ceramic and metallic
particles are applied together [89].
The codeposition of various kinds of particles offers an opportunity to give
different functionalities to a coating by doping it with particles of dissimilar properties. This pursuit can be seen when a PTFE-doped (and consequently softened)
deposit is plated in the presence of TiO 2 particles that improves the wear resistance
of the coating, maintaining at the same time the low friction caused by the PTFE
content [159]. The application of nanoparticles of various functionality has certainly
not been fully exploited so far and various other particle combinations are yet to be
tested.
7 Composites
much lower particle concentration (by wt.%) than for microparticles [120]. An interdiffusion annealing treatment at intermediate temperature (500 °C) was found to
improve the oxidation resistance at high temperature (900 to 1000 °C) [206]. The
advantage of the interdiffusion treatment stems from the formation of intermetallic
alloys whose grains withstand to intergranular cracking.
The improvement of the oxidation resistance related to the Al or Cr particle
content of the composite was investigated for Ni–Co [207] and Ni–Cu [204] alloyed
matrices. Alloying Ni with Co increases the incorporation rate of the aluminium
particles because the better wetting behaviour with Co. However, Co is less resistance
to both oxidation and corrosion than Ni, which means that its content in the matrix
was suggested to be 28 wt.% as a maximum [207]. In the case of the Ni–Cu alloys,
the chromium particles effectively hindered the in-depth oxidation to several tens
of micrometre. As the chromium loading of the composite increased, the mixed
CuO–NiO–Cu 2 O layer (i.e., that produced on the particle-free specimens) could not
form but a continuous chromium (III) oxide layer with a thickness of less than 5 µm
formed during the oxidation treatment.
7.2.13 Combination of Various Particles in Electroplated
Dispersion Coatings
The application of different kinds of particles in the electrodeposition of dispersion
coatings may stem from various pursuits. When particle types of similar properties
are used simultaneously, the particle load of the deposit is usually larger than that
achieved with one sort of particles. An example for this case is the codeposition
of Al 2 O 3 and SiO 2 together with a Ni coating [167]. Both sorts of inert ceramic
particles incorporate into the coating in a smaller volume ratio that in the absence of
the other particle, however, the cumulated particle content obtained from the bath
with mixed particles is larger than in the case when either of them was present alone.
Consequently, the hardness and wear behaviour is better with the coating containing
the mixture of the ceramic particles than that containing one particle type only.
The same improvement in the deposit properties can be seen when the two sorts of
nanoparticles belong to different material families, i.e., when ceramic and metallic
particles are applied together [89].
The codeposition of various kinds of particles offers an opportunity to give
different functionalities to a coating by doping it with particles of dissimilar properties. This pursuit can be seen when a PTFE-doped (and consequently softened)
deposit is plated in the presence of TiO 2 particles that improves the wear resistance
of the coating, maintaining at the same time the low friction caused by the PTFE
content [159]. The application of nanoparticles of various functionality has certainly
not been fully exploited so far and various other particle combinations are yet to be
tested.