111
Protection and Restoration
Four important groups of nano-coatings can be distinguished (Shourgeshty
et al., 2016):
1. Nano-grade coatings
2. Superlattice and multilayer coatings
3. Thin-film coatings
4. Nanocomposite coatings (Shourgeshty et al., 2016)
A distinctive family of ceramic materials has come to be known as ultra-hightemperature ceramics (UHTCs). It includes ceramic borides, carbides and nitrides
of early transition metals such as Zr, Hf, Nb, and Ta, especially hafnium diboride
and zirconium diboride-based compositions, characterized by high melting points,
chemical inertness and relatively good oxidation resistance in extreme environments
(Gasch et al., 2005). Fahrenholtz and Hilmas (2017) point out that UHTCs are often
defined as compounds that have melting points above 3000°C or, in the most pragmatic way, calling UHTCs ceramic materials that can be used for extended times at
temperatures above 1650°C. However, none of these definitions captures the wide
range of extreme conditions in which UHTCs may be used. The strong covalent
bonds between the transition metals and B, C, or N produce UHTCs not only with
melting temperature, but also with high hardness and stiffness, as well as higher electrical and thermal conductivities than that of oxide ceramics. The authors emphasize that the above-mentioned intriguing combination of metal-like and ceramic-like
properties allows UHTCs to survive extreme temperatures, heat fluxes, radiation
levels, mechanical loads, chemical reactivities, and other conditions that are beyond
the capabilities of existing structural materials (Fahrenholtz and Hilmas, 2017).
Gnanavelbabu et al. (2021) note that by adding ceramic reinforcements, it is possible to enhance corrosion resistance of a magnesium alloy and concentrate on UHTC
reinforcements. They analyze Mg-AZ91D matrix composites reinforced with 5 wt%
of TaC, HfC, TiN, TiB2, and TiC particles. The authors demonstrate that the addition of reinforcing UHTC particles increases density and decreases porosity of the
obtained composites, as well as raises hardness of the composites as compared to
pure Mg-AZ91D material. They also find that higher corrosion resistance could be
obtained due to formation of a dense and stable anti-corrosion protective layer and
reduction of β-Mg17Al12 phase.
Considering some aspects of heat transfer in interactions between components
with a sharp leading edge and high-enthalpy high-speed flows of dissociated air,
Simonenko et al. (2013) name some characteristics of the UHTC materials which
would make them promising for use in hypersonic flight vehicles:
1. Besides high melting points and phase stability in a wide temperature
range, the materials must have comparatively high oxidation resistance, in
particular, in reactions with atomic oxygen.
2. The material should have a high thermal conductivity that must ensure heat
removal from strongly overheated regions, which is especially important
for samples with a sharp leading edge, since local overheatings to ~2500°C
Protection and Restoration
Four important groups of nano-coatings can be distinguished (Shourgeshty
et al., 2016):
1. Nano-grade coatings
2. Superlattice and multilayer coatings
3. Thin-film coatings
4. Nanocomposite coatings (Shourgeshty et al., 2016)
A distinctive family of ceramic materials has come to be known as ultra-hightemperature ceramics (UHTCs). It includes ceramic borides, carbides and nitrides
of early transition metals such as Zr, Hf, Nb, and Ta, especially hafnium diboride
and zirconium diboride-based compositions, characterized by high melting points,
chemical inertness and relatively good oxidation resistance in extreme environments
(Gasch et al., 2005). Fahrenholtz and Hilmas (2017) point out that UHTCs are often
defined as compounds that have melting points above 3000°C or, in the most pragmatic way, calling UHTCs ceramic materials that can be used for extended times at
temperatures above 1650°C. However, none of these definitions captures the wide
range of extreme conditions in which UHTCs may be used. The strong covalent
bonds between the transition metals and B, C, or N produce UHTCs not only with
melting temperature, but also with high hardness and stiffness, as well as higher electrical and thermal conductivities than that of oxide ceramics. The authors emphasize that the above-mentioned intriguing combination of metal-like and ceramic-like
properties allows UHTCs to survive extreme temperatures, heat fluxes, radiation
levels, mechanical loads, chemical reactivities, and other conditions that are beyond
the capabilities of existing structural materials (Fahrenholtz and Hilmas, 2017).
Gnanavelbabu et al. (2021) note that by adding ceramic reinforcements, it is possible to enhance corrosion resistance of a magnesium alloy and concentrate on UHTC
reinforcements. They analyze Mg-AZ91D matrix composites reinforced with 5 wt%
of TaC, HfC, TiN, TiB2, and TiC particles. The authors demonstrate that the addition of reinforcing UHTC particles increases density and decreases porosity of the
obtained composites, as well as raises hardness of the composites as compared to
pure Mg-AZ91D material. They also find that higher corrosion resistance could be
obtained due to formation of a dense and stable anti-corrosion protective layer and
reduction of β-Mg17Al12 phase.
Considering some aspects of heat transfer in interactions between components
with a sharp leading edge and high-enthalpy high-speed flows of dissociated air,
Simonenko et al. (2013) name some characteristics of the UHTC materials which
would make them promising for use in hypersonic flight vehicles:
1. Besides high melting points and phase stability in a wide temperature
range, the materials must have comparatively high oxidation resistance, in
particular, in reactions with atomic oxygen.
2. The material should have a high thermal conductivity that must ensure heat
removal from strongly overheated regions, which is especially important
for samples with a sharp leading edge, since local overheatings to ~2500°C