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Protection and Restoration
Suspension plasma spraying (SPS) is able to produce submicron- and nanograined coatings due to application of suspensions instead of dry powders as starting
materials. SPS can be considered an advancement in APS as it enables spraying
of fine feedstock of 100 nm–5 µm diameter (Mahade et al., 2019). Microstructural
features inherently associated with the process, such as a smaller splat size, fine
scale porosity, or low surface roughness enhance wear performance of SPS coatings.
Apart from thermal barrier coatings with improved performance compared to APS
coatings (Kassner et al., 2008; Curry et al., 2014; Bernard et al., 2017), there are
reports on deposition of nanostructured WC–12% Co coatings by the SPS method
(Berghaus et al., 2006), submicron silicon carbide composite coatings avoiding the
decomposition of SiC typically expected with APS (Mubarok and Espallargas, 2015),
as well as pure titanium and chromium carbides wear-resistance coatings (Mahade
et al., 2019). SPS is advantageous for deposition of a functionally graded double
ceramic insulation layer of La 2 Zr 2 O 7 /8YSZ (Wang et al., 2015) or for multilayered
functionally graded coatings with a compact microstructure exhibiting limited presence of pores and other defects, which results in good mechanical properties (Cattini
et al., 2013). It should be added that porosity can vary dependent on different parameters from 5 to 11.6% for YCeSZ powders and from 10 to 27% for YSZ powders
(Sokołowski et al., 2017), while SPS with a < 3 μm powder suspension has generated
the most porous coating of 41 ± 2% open porosity, exhibiting the smallest thermal
conductivity of 0.56 ± 0.1 W/m·K (Ganvir et al., 2015).
The SPS process offers various possibilities to design microstructure of coatings,
for instance (Łatka et al., 2020):
• Dimensions of pores may vary from nanometer through submicrometer
to micrometer size, having various shapes and forms connected or nonconnected networks.
• Morphology may vary from dense and homogeneous through vertically
cracked to fully columnar.
• Deposits may be formed by fully molten splats, sintered particles only, or by
both (the so-called two-zone structures).
• Roughness of obtained resultant coated surface may vary in a very broad
range with Ra values between 1.6 and 14.1 µm.
• Thickness of a coating may range from few micrometers to hundreds of
micrometers.
Thermal spraying with a hybrid powder-suspension feedstock is proposed as a novel
approach to obtain coatings with unusual chemistries and unique microstructures. In
particular, Ganvir et al. (2021) explore benefits of a hybrid powder-suspension system depositing Al 2 O 3 -YSZ ceramic matrix composite (CMC) coatings with unique
multi-length scale microstructures for tribological applications. They demonstrate
that as-deposited CMC coating exhibits significantly improved tribological properties by enhancing wear resistance under scratch, dry sliding ball-on-plate (ca. 36%
decrease), and erosion tests (50% decrease in the erosion wear rate) as compared to
the conventional APS deposited monolithic Al 2 O 3 coating.
Protection and Restoration
Suspension plasma spraying (SPS) is able to produce submicron- and nanograined coatings due to application of suspensions instead of dry powders as starting
materials. SPS can be considered an advancement in APS as it enables spraying
of fine feedstock of 100 nm–5 µm diameter (Mahade et al., 2019). Microstructural
features inherently associated with the process, such as a smaller splat size, fine
scale porosity, or low surface roughness enhance wear performance of SPS coatings.
Apart from thermal barrier coatings with improved performance compared to APS
coatings (Kassner et al., 2008; Curry et al., 2014; Bernard et al., 2017), there are
reports on deposition of nanostructured WC–12% Co coatings by the SPS method
(Berghaus et al., 2006), submicron silicon carbide composite coatings avoiding the
decomposition of SiC typically expected with APS (Mubarok and Espallargas, 2015),
as well as pure titanium and chromium carbides wear-resistance coatings (Mahade
et al., 2019). SPS is advantageous for deposition of a functionally graded double
ceramic insulation layer of La 2 Zr 2 O 7 /8YSZ (Wang et al., 2015) or for multilayered
functionally graded coatings with a compact microstructure exhibiting limited presence of pores and other defects, which results in good mechanical properties (Cattini
et al., 2013). It should be added that porosity can vary dependent on different parameters from 5 to 11.6% for YCeSZ powders and from 10 to 27% for YSZ powders
(Sokołowski et al., 2017), while SPS with a < 3 μm powder suspension has generated
the most porous coating of 41 ± 2% open porosity, exhibiting the smallest thermal
conductivity of 0.56 ± 0.1 W/m·K (Ganvir et al., 2015).
The SPS process offers various possibilities to design microstructure of coatings,
for instance (Łatka et al., 2020):
• Dimensions of pores may vary from nanometer through submicrometer
to micrometer size, having various shapes and forms connected or nonconnected networks.
• Morphology may vary from dense and homogeneous through vertically
cracked to fully columnar.
• Deposits may be formed by fully molten splats, sintered particles only, or by
both (the so-called two-zone structures).
• Roughness of obtained resultant coated surface may vary in a very broad
range with Ra values between 1.6 and 14.1 µm.
• Thickness of a coating may range from few micrometers to hundreds of
micrometers.
Thermal spraying with a hybrid powder-suspension feedstock is proposed as a novel
approach to obtain coatings with unusual chemistries and unique microstructures. In
particular, Ganvir et al. (2021) explore benefits of a hybrid powder-suspension system depositing Al 2 O 3 -YSZ ceramic matrix composite (CMC) coatings with unique
multi-length scale microstructures for tribological applications. They demonstrate
that as-deposited CMC coating exhibits significantly improved tribological properties by enhancing wear resistance under scratch, dry sliding ball-on-plate (ca. 36%
decrease), and erosion tests (50% decrease in the erosion wear rate) as compared to
the conventional APS deposited monolithic Al 2 O 3 coating.