105
Protection and Restoration
MgO ceramics have excellent thermal and mechanical properties with a very
high melting point of 2800°C (Jiang et al., 2017). Magnesium oxide is hygroscopic
and soluble in water, therefore, calcined MgO is used in coatings applications.
Added to a coating, MgO makes it inert toward several types of steel and promotes
self-removability.
Chromium oxide (Cr 2 O 3 ) is applied for enamels as a green pigment, but in hot
forming it is added as a refractory compound. In addition, it improves wettability of
coatings.
Magnesium aluminate spinel (MgAl 2 O 4 or MgО·Аl 2 O 3 ) is known for its high
refractoriness, low thermal expansion, chemical stability, thermal shock resistance,
and corrosion resistance, which make it an attractive refractory material for the steel
and cement industries (Tripathi et al., 2003).
As a rule, refractory ceramic materials, such as aluminosilicate, silica sand, or
chromium-based are introduced to coatings in form of milled powders. They display
chemical inertness and improve heat resistance of enamels.
In order to obtain specific properties of a protected metal surface, some nonoxygen substances are added to protective coatings, such as metals, intermetallics,
carbides, and nitrides.
Among the metals used for protective technological coatings, various powders
can be applied, such as ferroaluminum alloy, aluminum, iron, or titanium. Grain
dimensions of metals are normally below 100 μm. However, metallic powders
require caution and safety measures directly linked to their intensive particle oxidation. For instance, aluminum powder presents a risk due to its high explosibility,
particularly when dispersed in air (Gascoin et al., 2009).
Intermetallic compounds are formed from electropositive and electronegative
metals which chemically bond to form compounds with a specific composition and
crystalline structure (Mattox, 2010). An ordered crystallographic structure of intermetallics is formed when the concentration of alloys exceeds the solubility limit.
Among non-oxide materials for high-temperature applications, molybdenum
disilicide (MoSi 2 ) is widely used to protect Mo-based alloys from oxidation due
to its excellent oxidation resistance at high temperatures and low preparation cost
(Zhang et al., 2018). MoSi 2 is a high-melting-point intermetallic compound noted
for its excellent resistance to oxidation and thermal expansion coefficient closer to
that of metallic substrates comparing to other oxides. Therefore, oxide spallation
from the substrate caused by a large mismatch of thermal expansion coefficient in
thermal cycles can be avoided. It is thought that by further oxygen diffusion into the
substrate following MoSi 2 formation, a thin layer of SiO 2 is formed over the MoSi 2
layer, which protects the underlying metal from further inward oxygen diffusion and
oxidation (Seo et al., 2012).
Boron carbide (B 4 C) is a hard refractory material insoluble in water and chemically inert below 700°C. Above this temperature, oxidation of B 4 C occurs and forms
a B 2 O 3 oxygen barrier. Sun et al. (2019) demonstrate that after boron carbide is converted into B 2 O 3 , it gives rise to secondary coating growth on substrate particles, but
as temperature continues to grow, a drastic evaporation of B 2 O 3 occurs. When boron
carbide is introduced to vitreous coatings, it can be applied for protection of tool
Protection and Restoration
MgO ceramics have excellent thermal and mechanical properties with a very
high melting point of 2800°C (Jiang et al., 2017). Magnesium oxide is hygroscopic
and soluble in water, therefore, calcined MgO is used in coatings applications.
Added to a coating, MgO makes it inert toward several types of steel and promotes
self-removability.
Chromium oxide (Cr 2 O 3 ) is applied for enamels as a green pigment, but in hot
forming it is added as a refractory compound. In addition, it improves wettability of
coatings.
Magnesium aluminate spinel (MgAl 2 O 4 or MgО·Аl 2 O 3 ) is known for its high
refractoriness, low thermal expansion, chemical stability, thermal shock resistance,
and corrosion resistance, which make it an attractive refractory material for the steel
and cement industries (Tripathi et al., 2003).
As a rule, refractory ceramic materials, such as aluminosilicate, silica sand, or
chromium-based are introduced to coatings in form of milled powders. They display
chemical inertness and improve heat resistance of enamels.
In order to obtain specific properties of a protected metal surface, some nonoxygen substances are added to protective coatings, such as metals, intermetallics,
carbides, and nitrides.
Among the metals used for protective technological coatings, various powders
can be applied, such as ferroaluminum alloy, aluminum, iron, or titanium. Grain
dimensions of metals are normally below 100 μm. However, metallic powders
require caution and safety measures directly linked to their intensive particle oxidation. For instance, aluminum powder presents a risk due to its high explosibility,
particularly when dispersed in air (Gascoin et al., 2009).
Intermetallic compounds are formed from electropositive and electronegative
metals which chemically bond to form compounds with a specific composition and
crystalline structure (Mattox, 2010). An ordered crystallographic structure of intermetallics is formed when the concentration of alloys exceeds the solubility limit.
Among non-oxide materials for high-temperature applications, molybdenum
disilicide (MoSi 2 ) is widely used to protect Mo-based alloys from oxidation due
to its excellent oxidation resistance at high temperatures and low preparation cost
(Zhang et al., 2018). MoSi 2 is a high-melting-point intermetallic compound noted
for its excellent resistance to oxidation and thermal expansion coefficient closer to
that of metallic substrates comparing to other oxides. Therefore, oxide spallation
from the substrate caused by a large mismatch of thermal expansion coefficient in
thermal cycles can be avoided. It is thought that by further oxygen diffusion into the
substrate following MoSi 2 formation, a thin layer of SiO 2 is formed over the MoSi 2
layer, which protects the underlying metal from further inward oxygen diffusion and
oxidation (Seo et al., 2012).
Boron carbide (B 4 C) is a hard refractory material insoluble in water and chemically inert below 700°C. Above this temperature, oxidation of B 4 C occurs and forms
a B 2 O 3 oxygen barrier. Sun et al. (2019) demonstrate that after boron carbide is converted into B 2 O 3 , it gives rise to secondary coating growth on substrate particles, but
as temperature continues to grow, a drastic evaporation of B 2 O 3 occurs. When boron
carbide is introduced to vitreous coatings, it can be applied for protection of tool