Ceramic Nanomaterials for High Temperature Applications - Crimson Publishers

Mikhail Gamero*
Department of Materials Science & Engineering, USA
*Corresponding author: Mikhail Gamero, Department of Materials Science & Engineering, Raleigh, NC 27695, USA
Submission: November 02, 2017; Published: February 02, 2018
Ceramic Nanomaterials for High Temperature
Applications
Abstract
Ceramic nanomaterials have exhibited extraordinary characteristics, based in its low density, hardness and when properly engineered, good
corrosion properties in high temperature applications. In the nanoscale world, we can find nanomaterials that do not necessarily apply to the correlation
of strength and density. It has been studied that ceramic nanomaterials have some control boundaries that are critical in high temperature applications;
grain sizes, grain boundaries density, operating conditions (thermal cycles), nano-crystalline structure and large voids (because of the porous nature
of ceramic materials).
In this research paper, it will be discussed some of the most important applications, processing methods and several adhesion methods to metal
substrates with the final purpose to enhance metal properties and mitigate high temperature corrosion. As a quick review, it will be discussed some
of the environmental health & safety (EHS) concerns and some parallel experiments that could be done for reinforcing the investigations done in this
subject.
Introduction
One of the most studied applications is the use of ceramic
nanoparticles as pigments to enhance the material properties and
products life-cycle. This application is more focused to polymers.
The idea is a mitigation of the polymer degradation that can be
observed when exposed to ultraviolet light. It has been observed
that with the proper blending and binder’s ratio, the mechanical
and elevated temperature properties can be improved. Ceramic
pigments have shown to be a good option in the nano-level for high
quality coatings. As reported by Sankar S et al. [1] ceramic pigments
show good impact-resistance for ferrous metals and its application
in corrosive environments as well as high temperature applications
based in the thermal behavior of the coating.
One of the most important applications for this nano-materials,
it’s the direct application to aggressive environments or high
temperature applications like nozzle tips for the inside combustion
parts in the automotive industry or its use in turbomachinery, more
specific, in combustion and hot gas path components of aviation
engines or heavy duty gas turbines for the power business. For
being able to apply this technology, it is important to consider the
adhesion into the metal substrate of the ceramic coating, where
spallation, cracking or not-homogeneous distribution can be
observed when improperly added together.
As reported by Vert R et al. [2] air plasma spraying technique
was used for making a thin film of few micrometers thick, where
it was confirmed by Vickers indentation and tensile tests, the
compatibility of these two different materials (metal substrate with
a ceramic coating).
Processing Methods
There are several processing methods that will be briefly
discussed in this section, some of this are in experimental phase
as nano-powder infiltrated transient eutectic phase (NITE) process,
sol-gel method and carbon nanotubes. As reported by Kuznetsov
NT [3], NITE based in the use of silicon carbide nano-powders
and its mixture of yttrium and aluminum oxides to be subjected
to a temperature of 1800-1950 °C at a pressure of 15-20MPA
(usually referred as hot pressing) so that the melted film favors the
orientation and a good matrix control of the nanoparticles.
Figure 1: Cross section of NITE SiC/SiC [2] other processing
method reported by S.
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How to cite this article: Mikhail G. Ceramic Nanomaterials for High Temperature Applications. Res Dev Material Sci. 3(3). RDMS.000563. 2018.
DOI: 10.31031/RDMS.2018.03.000563
With NITE method, good mechanical characteristics are
exhibited along with stress limit of 400MPA and an elasticity
modulus of 310GPA with good oxidation resistance. Figure 1 shows
a cross section of SiC/SiC NITE micrograph, composite material
suitable for high temperature applications. It can be clearly
distinguished the fiber from the matrix and its interaction.
Sankar et al. [3], was a silicone resin binder developed by high
temperature ceramic pigment nanoparticles with a high hardness
that can be used under high temperature or aggressive environment
applications. The preparation of the ceramic pigments was through
cobalt and alumina compounds by a thermal decomposition with
a limited supply of air or oxygen (calcination) and crushed. After
the raw material process was finished, this concentration was
applied to a mild steel specimen by hand application and dried,
giving a result of a layer coating of 80±5μ. The results revealed that
the system is not degraded up to 600 °C, which makes a suitable
polymer-pigment product that can withstand medium range
temperature and aggressive environments [4].
Another elevated temperature application was documented
by Chichkan AS et al [5], which is basically the use of carbon
nanomaterials for the creation of ceramic nanomaterials. The
preparation of the sample was with the binding of thermochemical
activation (TCA) and hydration (OAO Katalizator). A plasticized
product was made aqueous using nitric acid which was then dried
and compacted to 30mmx2mm pellets. After obtaining the pellets,
it was then subjected to a heat treatment that went from 150 °C to
1200 °C.
The result was a porous structure of CNM-SiO2 of ceramic
membranes that allowed a good control of the porous structure of
ceramic membranes, which could be controlled by composites and
additives. Figure 2 is an electron microscopy of the carbon nano-
filaments CNF-SiO2 composite where it can be appreciated the SiO2
film produced on the CNM surface improving the ceramic matrix
with the formation of mullite phase.
Figure 2: Electron microscopy image of the CNF-SiO2
composite [1].
An excellent processing method for nickel composite coating
was studied by Moustafa EM et al. [6], where it was discussed the
formation of a composite Ni-NC (nanocontainers) for a bi-functional
coating. The idea is based in three fundamental steps, adsorption
of positively charged TiO2-NC on SAM-covered gold coated steel
substrate. Secondly, a metalized NC layer by electroplating of a thin
Ni layer and finally, the conduction of current density in the Ni-NC
composite for showing good adhesion to the substrate. The result
showed an NC finely dispersed in the Ni coating with a high density
incorporated which increased the concentration in the Ni solution.
It was very interesting how the use of gold coated steel electrodes
helped in the stabilization of the SAM showing good adhesion to the
metal substrate. A comparison of Figure 3a, 3b and 3c is for showing
the interface between Au and Ni (Figure 3a), at different TiO2
concentrations (Figure 3b) and in the fabrication with different
steps (Figure 3c). The first step was the adsorption of TiO2-NC and
second, the electroplating for obtaining a 6μm thickness layer.
Figure 3a: TiO2
-Nc concentration 1g/l [6].
Figure 3b: TiO2
-Nc concentration 3g/l [6].
Figure 3c: Coating layer fabricated in two steps [6].

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DOI: 10.31031/RDMS.2018.03.000563
The last processing method that will be discussed is a ceramic
nanomaterial based in a Ti-Si-C-N by sol-gel which was investigated
by Biedunkiewicz A [7]. The process is based in two different stages,
the first one at low temperature, where the resulting product is raw
nc-TiCx material and the second one is at high temperature stage
carbonization where the carbon traces (carbides essentially) are
eliminated of excessive organic compounds.
Figure 4: HRSEM image of Ti-Si-C powder [8].
The second condition enables the purification and limits the
growth of crystallites to ensure the carbonization. After dry-out,
the sol-gel solution was evaluated, showing uniform distribution,
grains of silicon, titanium and carbon were regularly distributed.
The only problem was that a higher oxidation resistance could was
not favorable. Figure 4 shows a distribution of Ti-Si-C homogenous
distribution.
One critical step for the coating use is its adherence to a metal
substrate. In his work, Vert R et al. [2], it was studied the adhesion
of a coating with a plasma process application. The surface was
firstly prepared with a blasting process to make the smooth surface
tougher; the blasting process impacts the surface leaving it under
a compressive stress. The most suitable application of the coating
layer was in two steps, the first one using air plasma spraying (APS)
technique and the second one using suspension plasma spraying
(SPS), which is essentially spraying a ceramic powder into the
surface.
The adhesion of the coating was measured with a hardness test
and by a tensile test. It was noticed that after the air plasma spraying
(APS) application, if the coating layer was bigger than required, the
adhesion was not optimal. Yttria-stabilized Zirconia was sprayed in
the already cleaned surface, during the application of the spraying.
One critical step for the proper adhesion of the ceramic coating to
the metal substrate is a controlled temperature of 200 °C, used in a
preheated surface of 400 °C.
The result showed two different surface that are clearly
observed with the microscope, the first layer was thin and dense,
composed of oxides while the second layer showed a columnar
grain structure with a granular structure. Figure 5a shows the
microstructure and the difference between the two-applied coating
processes. Figure 5b shoes the interface of the SPS and APS layers.
The results for the tensile bond strength were between 12-24
MPa, depending on the thickness of each individual layer and the
thickness of them both. Vickers indentation test confirmed that
the failure of the duplex coating occurrs in the APS coating, closer
to SPS-APS interface. The remark of this methodology showed an
excellent adhesion of the insulating coating to the metal surface.
In other words, the first coating serves as a bond coating between
the metal substrate and the thicker coating layer. The coating of the
second layer is important because the thicker the layer, the colder
the metal substrate surface but the it is also hotter the ceramic
coating surface. It is important to find the proper balance because a
hotter ceramic surface impacts the life of the part since it is suitable
to spallation and cracking.
Figure 5a: YSZ microstructure duplex-coat [2].
Figure 5b: Interface between SPS and APS layers [2].
Environmental, Health and Safety Concerns
The use of nanomaterials has increased enormously during the
last years. Although we are not discussing a biological or disease
treatment application in this research, it is important to consider
that the EHS concerns increase with a large use but it has not been
completely studied.
The use of crystalline silica (like the one used as a matrix
for several of the examples of ceramic composites) can raise the
question of medical implications. As reported by Johnson SM [8],
it has been studied the concern of toxicity for the exposure of this
materials and it has pointed in two critical directions, inhalation

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DOI: 10.31031/RDMS.2018.03.000563
studies show pulmonary and cardiovascular symptoms which arise
the question of its implications for long-term exposition use.
The oxides can cause inflammation because of the accumulation
and engineered nanoparticles can cause the blood pressure to
become unstable, impacting the myocardial infarction or stroke.
Before the use of this nanomaterials at a big scale, it shall be further
investigated the EHS implications, and its awareness to engineers,
manufacturers and consumers.
Proposal
TaC-SiC ultrahigh temperature composite was analyzed
by Yan Lu et al. [9]. The main goal was the use of cross-linking
nanocomposites prepared by the blend of the precursors in an inert
atmosphere (Argon). The precursor was obtained by the blending
of poly-carbosilane (PCS) and poly-tantaloxane (PT) for obtaining a
metal-ligand interaction for the polymer.
After annealing of 1000-1800 °C, it was obtained a
homogeneously distributed sample with traces of free carbon. The
annealing precursors at 1600 °C gave a ceramic nanocomposite
with promising application of high temperature and aggressive
environment conditions [10-12].
This same ceramic composite TaC-SiC synthesis can be applied
to some other compositions consisting of a group IV transition
metals, in other words, the use of nitrides, or borides can be used
for ultrahigh temperature ceramics as it was TaC-SiC [13].
A finely dispersed network of borides or nitrides could be
use along a silica-based matrix. This could be verified with some
other experiments using SiC and Si3N4 as the matrix former
complimented with nitrides, carbides of borides for the ceramic
composite high temperature material [14-16].
Conclusions
We have discussed several technologies that have been
developed for enhancing the high temperature properties of a
metal substrate, and more specifically, improving the oxidation
resistance. It was further reviewed techniques like NITE and APS/
SPS multilayer coatings, the use of ceramic pigments and more
importantly, the adhesion of the ceramic coating to the metal
surface and some of the most critical elements to consider during
the process (like coating thickness and surface preparation).
Some other critical points discussed were the size of grains and
the importance of maintaining the proper conditions like voltage
(for Au electrode methodology) and the room temperature during
spray nozzle technique process. Additionally, it was discussed
several tests of how to characterize the product, like Vickers
indentation, tensile tests, coating thickness and to determine if the
crack or spallation observed might be related with the procedure
followed.
It was discussed how this nanomaterial technologies need
further investigation to better understand the problems related to
environment, health and safety because of the nature of their scale
size and exposure.
There is no question that ceramic coatings are an excellent
option for enhancing the high temperature properties and are a
potential solution for the corrosion mitigation, which is observed
at those aggressive operating conditions. They have been used in
combustion elements, hot gas path components (like buckets or
nozzles) and even for tip nozzle inside automotive engines [17,18].
Producing homogenous, uniform, dense and crack-free coatings
is suitable only with well-controlled manufacturing process.
Nanomaterials open an enormous number of options when we talk
about different oxides or multilayer coatings.
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