106
Remanufacturing and Advanced Machining
steels during heat treatment in order to improve their surface hardness and adhesion
between a coating and substrate.
Titanium carbide (ТіС) exhibits high melting point, hardness, mechanical
strength, thermal conductivity, and corrosion resistance (Grebenok et al., 2016).
Moreover, it is wettable by nickel, chromium, cobalt, and iron, forming cermets
with them. In turn, silicon carbide (SiC) is highly resistant to oxidation in a oxidating environment, since during passive oxidation it liberates SiO 2 that forms a dense
layer which acts as an anti-oxidation protective coating for the surface of SiC (Roy
et al., 2014).
Boron nitride (BN) is a promising interphase material with better oxidation resistance than pyrocarbon. Furthermore, after oxidation, boron and its compounds give
a B 2 O 3 glassy phase with a low melting point of 450°C, turning into an efficient
protective layer (Willemin et al., 2017).
Additional materials can be introduced in order to achieve required performance
and functionality of technological coatings. For instance, to obtain proper suspension, clay is introduced as an additive when a mixture is milled, for example, from
frit, water, and chromium oxide. The suspending ability of clay depends mainly
on its dispersion and is not determined by its composition, while heat resistance of
coatings depends on composition and fire resistance of a clay. Its main compound is
kaolinite, but bentonite clays are also used, where the basis is the mineral montmorillonite Аl 2 O 3 ·4SiО 2 ·nН 2 О. Bentonites possess a remarkably high suspending ability
and swell strongly in water.
Clay is added to most technological coatings, except for organosilicate ones containing organosilicon varnishes and organic solvents like acetone or toluene. To
obtain stable suspension, water is commonly used which must be free from mechanical contaminations and impurities. In order to improve wettability, some surfactants are added during the milling or immediately after. Binders are introduced to
enhance mechanical strength of coatings.
2.2.3 comPosiTion of coaTings
With regard to chemical and physical composition of a coating, the following groups
can be distinguished (Lavrentiev and Lavrentiev, 2015):
1. Glass-like coatings or enamels
2. Glass-ceramics
3. Metallic glasses
4. Organosilica
5. Protective grease
6. Metals
7. Coating systems
The above classification indicates the nature of main components responsible for
protection of a metal surface during heating. Each group can be divided into subgroups dependent on proportions of the major components.
Remanufacturing and Advanced Machining
steels during heat treatment in order to improve their surface hardness and adhesion
between a coating and substrate.
Titanium carbide (ТіС) exhibits high melting point, hardness, mechanical
strength, thermal conductivity, and corrosion resistance (Grebenok et al., 2016).
Moreover, it is wettable by nickel, chromium, cobalt, and iron, forming cermets
with them. In turn, silicon carbide (SiC) is highly resistant to oxidation in a oxidating environment, since during passive oxidation it liberates SiO 2 that forms a dense
layer which acts as an anti-oxidation protective coating for the surface of SiC (Roy
et al., 2014).
Boron nitride (BN) is a promising interphase material with better oxidation resistance than pyrocarbon. Furthermore, after oxidation, boron and its compounds give
a B 2 O 3 glassy phase with a low melting point of 450°C, turning into an efficient
protective layer (Willemin et al., 2017).
Additional materials can be introduced in order to achieve required performance
and functionality of technological coatings. For instance, to obtain proper suspension, clay is introduced as an additive when a mixture is milled, for example, from
frit, water, and chromium oxide. The suspending ability of clay depends mainly
on its dispersion and is not determined by its composition, while heat resistance of
coatings depends on composition and fire resistance of a clay. Its main compound is
kaolinite, but bentonite clays are also used, where the basis is the mineral montmorillonite Аl 2 O 3 ·4SiО 2 ·nН 2 О. Bentonites possess a remarkably high suspending ability
and swell strongly in water.
Clay is added to most technological coatings, except for organosilicate ones containing organosilicon varnishes and organic solvents like acetone or toluene. To
obtain stable suspension, water is commonly used which must be free from mechanical contaminations and impurities. In order to improve wettability, some surfactants are added during the milling or immediately after. Binders are introduced to
enhance mechanical strength of coatings.
2.2.3 comPosiTion of coaTings
With regard to chemical and physical composition of a coating, the following groups
can be distinguished (Lavrentiev and Lavrentiev, 2015):
1. Glass-like coatings or enamels
2. Glass-ceramics
3. Metallic glasses
4. Organosilica
5. Protective grease
6. Metals
7. Coating systems
The above classification indicates the nature of main components responsible for
protection of a metal surface during heating. Each group can be divided into subgroups dependent on proportions of the major components.