Atmospheric Plasma Spray Deposition Research Articles

Evolution of microstructure and thermal conductivity of multifunctional environmental barrier coating systems

Environmental barrier coating (EBC) systems are applied to the surface of silicon-based composites exposed to high temperature combustion gas flow paths in gas turbine engines. They reduce the rate of composite oxidation, its volatilization by reactions with water vapor, and the temperature of the composite as their thermal conductivity decreases. Current EBC systems consist of a silicon bond coat covered by an ytterbium disilicate (Yb2O3⋅2SiO2: YbDS) permeation resistant and low silica activity barrier. When applied by atmospheric-plasma spray deposition, this outer layer contains 10–15% of a secondary ytterbium monosilicate (Yb2O3⋅3SiO2: YbMS) phase. YbDS has a coefficient of thermal expansion (CTE) of 4–5×10−6 °C−1, similar to those of the silicon and the silicon-based composite, but a relatively high thermal conductivity of 5–7 W m−1 K−1. YbMS has a higher steam volatility resistance than YbDS, and it has a much lower thermal conductivity (∼2–2.5 W m−1 K−1) at ambient temperature compared to YbDS, but its higher, highly anisotropic CTE (3–11×10−6 °C−1) results in channel cracking which reduces environmental protection. Here, we use a combination of scanning electron beam and laser-based thermoreflectance methods to spatially map the distribution of the silicate phases and thermal conductivity at ambient temperature in a Si-ytterbium disilicate EBC system exposed to thermal cycling in water vapor. We show that during thermal cycling, diffusion of silica from the thermally grown oxide on the Si bond coat surface to nearby YbMS regions transforms this phase to YbDS, thereby reducing the risk of thermomechanical coating failure but decreasing its effective thermal resistance. We also show that as silica is volatilized at the water vapor–ytterbium silicate interface, YbDS is transformed to YbMS, restoring some of the thermal protection of the coating system lost by its reduction in thickness and the YbMS to YbDS transformation near the bond coat.

Cold spray deposition of Inconel 718 in comparison with atmospheric plasma spray deposition

Inconel 718 is a Ni-based superalloy which has been widely used in aerospace industry. Repairing of Inconel 718 components has been usually carried out using atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF) spray techniques. Cold spray (CS) is a relatively new thermal spray technique, which relies more on powders’ kinetic energy and less on their thermal energy to impact powders onto the substrate, where the powders undergo severe plastic deformation and subsequently adhere to the substrate surface as coatings. In this work, high pressure CS using dry N2 as propellant gas has been deployed to deposit Inconel 718 powders as thick coatings for structural restoration application. The coating’s surface morphology, cross-section microstructure, crystallinity, residual stress and mechanical properties were subsequently characterized and compared with the Inconel 718 coating deposited by APS in order to elaborate the unique features of the Inconel 718 coatings deposited by both spray techniques.

Effect of graphene oxide loading on plasma sprayed alumina-graphene oxide composites for improved anticorrosive and hydrophobic surface

Alumina is a widely used anticorrosive coating material for protection of metal surfaces. The graphene oxide (GO)-alumina composite coatings can enhance the anticorrosion property of carbon steel surfaces used in an industrial scale. In the present work, the spray dried graphene oxide nanoplatelets of 0, 0.5, 1, 1.5 and 2 wt% reinforced alumina (α-Al2O3) were deposited on the low carbon steel substrate by Atmospheric Plasma Spray (APS) technique. The GO-alumina nanocomposite coatings are characterised using XRD, FE-SEM, HR-TEM and Raman Spectroscopy. The electrochemical corrosion behaviour of the coatings on carbon steel has been examined using three electrode electrochemical method in 3.5 wt% NaCl electrolyte. A remarkable improvement in the corrosion resistance (with a corrosion current density of 0.01 nAcm−2) of about six orders with respect to pristine alumina was observed. The wettability tests revealed that, with 1.5 and 2 wt% graphene oxide nanoplatelets addition, the surface coating turned out to be hydrophobic with contact angle of 127°, from hydrophilic with contact angle 36°, which complements the anticorrosion results. The in situ reduction of GO to r-GO by APS deposition and bridging of alumina splats by the GO sheets accounts for observing the superior performance of the composite coatings.

Atmospheric plasma sprayed 7%-YSZ thick thermal barrier coatings with controlled segmentation crack densities and its thermal cycling behavior

Yttria-stabilized zirconia (YSZ)-coatings are deposited on Ni-based superalloy IN738 by atmospheric plasma spraying (APS). For the first time, controlled segmentation crack densities are manually developed in the coatings, even after the APS deposition. This method allows to user to control segmentation densities as well as cracks depth, which could be designed as per coating thickness and required application. Thermal cycling test shows promising strain tolerance behavior for the segmented coatings, whereas coating without segmentation could not sustain even for its first thermal cycle period. Further, microstructural studies reveal that a very thin layer of TGO was formed and obvious no coating failure or spallation was observed after thermal cycling test at 1150°C for 500 cycles.

The Effect of Argon Controlled Pre-Oxidation on TGO Formation of NiCoCrAlYTa/YSZ

Thermally grown oxide (TGO) plays important roles in thermal barrier coating system (TBC) for high temperature application such as in aircraft gas turbine engine blades (GTE). The TGO formed between the bond coat and topcoat interface can increase oxidation resistance to creep of GTE blades by minimizing oxygen diffusion into the metal substrate. In this research a NiCoCrAlYTa metallic bond coat was deposited on Inconel 625 substrate using two methods of deposition namely; high velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS). After coating process, both types of samples underwent pre-oxidation in argon furnace for 12-24 hours at 1000 °C. Results showed that the TGO formation for samples in which the bond coat deposited via HVOF method produced much thinner and continuous TGO formation compared to APS deposition. This TGO characteristic is very useful to lengthen the lifetime of the metals substrate.

Three-dimensional electrode coatings for hydrogen production manufactured by combined atmospheric and suspension plasma spray

Nickel-based electrode coatings for hydrogen production by alkaline water electrolysis were prepared using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS) methods. To enhance the active surface area of the electrodes and obtain various levels of surface roughness, three-dimensional fin arrays were fabricated by masking the substrates with mesh screens during APS deposition. Subsequently, SPS top coatings were deposited on the fin-structured electrodes. The electrodes were then characterized by studying their surface microstructure, topography, roughness and electrochemical performance. The highest electrocatalytic activity with the exchange current density of 1.8 × 10− 3 A/cm2 and overpotential (η250) of − 339 ± 15 mV was obtained for the electrode coated by the combined APS–SPS method using the mesh screen with the smaller opening size. The high activity of this electrode is mainly attributed to the unique surface structure formed by deposition of particles through the fine mesh screen and impacting at different angles on the fin geometry.

Molten silicate reactions with plasma sprayed ytterbium silicate coatings

Abstract The reactions between molten calcium aluminum magnesium silicates (CMAS) at 1300 °C and atmospheric plasma spray (APS) deposited environmental barrier coatings on SiC substrates have been investigated. The tri-layer coatings comprised a silicon bond coat protected by a layer of mullite and either Yb 2 SiO 5 (ytterbium monosilicate, YbMS) or Yb 2 Si 2 O 7 (ytterbium disilicate, YbDS) as the topcoat. The APS deposition process resulted in two-phase top coats; the YbMS coating contained Yb 2 O 3 regions in a matrix of Yb 2 SiO 5 while the YbDS coating contained Yb 2 SiO 5 in a matrix of Yb 2 Si 2 O 7 . Exposure of both coatings to a model CMAS resulted in dissolution of the topcoat accompanied by a rapid rise in the concentration of Yb in the melt, and formation of the same Ca 2 Yb 8 (SiO 4 ) 6 O 2 apatite reaction product phase. The thickness of the apatite layer initially varied with (time) 1/4 , but transitioned to approximately parabolic kinetics after 5–10 h of CMAS exposure. The reaction mechanism on the YbMS layer was consistent with recent observations on Y 2 SiO 5 , wherein molten CMAS transport to the undissolved silicate was controlled by diffusion through thin amorphous films at the apatite grain boundaries. The reaction mechanism for the YbDS layer was more complex, and involved preferential reaction with the YbSiO 5 rich regions, resulting in a reaction zone that contained CMAS, the apatite reaction compound and undissolved Yb 2 Si 2 O 7 . The coating composition and microstructure significantly influenced the mechanism and rate at which the YbDS top coat was consumed by the reaction.

Evaluation of Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition by Laser AE Method

Vertical cracks can be generated in the top coat of thermal barrier coatings (TBCs) deposited by atmospheric plasma spraying (APS). Since they are known to improve the durability of TBCs such as in the case of dense vertically cracked TBC, clarification of the mechanism and the criteria of cracking are very important. In this study, generation of such vertical cracks was monitored during APS process by laser acoustic emission (AE) method, which is an in situ, non-contact, and non-destructive technique. Temperature was also monitored inside and on the surface of a specimen during APS process for estimation of the temperature field in the top coat. Results of the AE and temperature monitoring were combined to evaluate the relationship between cracking and thermal stress in the top coat. Most of the AE events due to the generation of vertical cracks were detected during rapid heating of the surface of the top coat by the heat flux from the torch. It showed that the vertical cracks were induced due to the tensile stress caused by the temperature difference in the top coat from the rapid heating. Furthermore, the estimated critical thermal stress for vertical cracking from the monitoring results was consistent with a previously reported strength of YSZ coating deposited by thermal spray.

APS Deposition of MnCo2O4 on Commercial Alloys K41X used as Solid Oxide Fuel Cell Interconnect: The Importance of Post Heat-treatment for Densification of the Protective Layer

The continuous contamination of the cathode with chromium released from metallic interconnects speeds up the degradation of the performance of the cell. For this reason Solid Oxide Fuel Cell (SOFC) interconnects are coated with a dense protective layer. This layer plays the role of a diffusion barrier for the chromium evaporation at operating conditions. The deposition of a dense, gas tight and stable MnCo2O4 spinel layer on ferritic alloy using the atmospheric plasma spraying (APS) process is studied. The first experimental results show that the plasma spray process combined with an appropriate post thermal-treatment is suitable to form a dense spinel protective layer.