Deposition of graphene as a coating material over large-scale areas is an intense topic of research because of complexities involved in the existing deposition techniques. Higher defects and compromised properties restricted in realizing the full potential of graphene coating. This work aims to deposit graphene coatings by adopting a traditional technique, that is, plasma spraying, which has inherent merits of extremely high cooling rate (∼106 K/s) and low plasma exposure time (∼0.1–10 μs). Graphene nanoplatelets (GNPs) were spray-dried into spherical agglomerates (∼60 μm dia.) and coatings were deposited over a wide range of surfaces. Continuous monitoring of temperature and velocity of in-flight GNPs was done using a diagnostic sensor. Deposition of GNP coatings was the result of striking of quasi-2D melted GNPs with higher velocity (∼197 m/s) toward the substrate. Postcharacterizations confirmed that GNPs did not collapse even after being exposed to harsh environments in plasma. Instead, high temperatures proved to be beneficial in purifying the commercial GNPs. The coatings were transparent even in the short-wavelength infrared region and remained electrically conductive. A proof-of-concept was established by carrying out preliminary corrosion and antifriction tests. Outstanding reduction of ∼3.5 times in corrosion rate and 3 times in coefficient of friction was observed in GNP-deposited coating. It is envisaged that graphene coating by plasma spraying can bring a revolution in commercial sectors.
In this study a simple, direct, one-step, scalable technique for instant tuning of all the different states of wetting characteristics using atmospheric plasma spray (APS) technique was used. We observed that, just by changing the process parameters in the APS technique, the wetting characteristics of an intrinsically hydrophilic aluminum metallic surface can be tuned to superhydrophilic (contact angle (CA): 0°), hydrophilic (CA: 19.6°), hydrophobic (CA: 97.6°), and superhydrophobic (CA: 156.5°) surfaces. Also, tuned superhydrophobic surface showed an excellent self-cleaning property. Further, we demonstrated that these surfaces retain their superhydrophobic nature even after exposure at elevated temperatures (up to 773 K) and on application of mechanical abrasion. Manipulation in different wetting behavior was possible mainly due to the presence of varying degrees of smooth surface as well as micropillars, which incorporated the multiscale roughness to the surface. “Re-entrant”-like microstructures such as mushroom, cauliflower, and cornet microstructures were observed in the case of tuned superhydrophobic surface, which is well-known for achieving the excellent water repellency over the hydrophilic surface.
Lanthanum zirconate (La2Zr2O7), an emerging candidate for TBC system, faces stringent concern owing to poor fracture toughness. We report an exceptionally high fracture toughness (5.3 ± 0.4 MPa m0.5) value for a La2Zr2O7 coating prepared by atmospheric plasma spray technique reinforced by carbon nanotubes (2 wt %); the enhancement in fracture toughness was noted to be more than 300% compared to all previous works. Several factors, viz. increased density, uniform distribution of CNTs in La2Zr2O7 matrix, stabilization of pyrochlore La2Zr2O7 phase and various associated toughening mechanism offered by CNTs (e.g., CNT pull out, CNT bridging and crack arresting etc.) was attributed for this surprising enhancement. In addition, splat sandwiching and nanomechanical interlocking between CNT and La2Zr2O7 was found to be unique toughening mechanism. Apart from it, thermal shock resistance of LZC-SD coating performed at 1300 °C, showed the 145 numbers of cycle before the partial delamination of the coating. While, it was only 87 numbers of cycle for LZ-SD coating. The density, hardness, elastic modulus values also depicted a suitable increase. The introduction of CNTs in the La2Zr2O7 matrix did not alter/affect any of the physical properties (phase purity, compositional integrity etc.) of the pristine La2Zr2O7 system.
In this study in-situ fabrication of oxide-free titanium nitride (TiN) coating by conventional plasma spraying technique. Complete oxide free TiN coating without using low pressure or vacuum environment was achieved by using the N2 shroud with plasma spraying. X-Ray diffraction and Transmission electron microscope confirmed the absence of any traces of oxide in the coating. Coating showed exceptionally higher mechanical properties (H: ∼18 GPa; E: ∼317 GPa). Outstanding reduction of ∼15 times in wear rate and ∼4 times in corrosion rate was observed compared to bare substrate (i.e. Ti-6Al-4V).
In this study, article communicates the successful fabrication of plasma sprayed graphene nanoplatelets (GNPs) reinforced ceria (CeO2) composite coating. Later, various characterizations confirm the presence of survived GNPs into CeO2 matrix and its transformation to few layered graphene during plasma spraying. The reinforcement of GNPs led a significant enhancement in corrosion resistance and mechanical performance. Addition of 5 wt% GNPs in CeO2 displayed an exceptional reduction in corrosion rate about ∼15 times in 3.5 wt% NaCl solution, while superior improvement of 50% in hardness, 98% in elastic modulus and 185% in fracture toughness during indentation has been observed. The GNPs acted as promising corrosion inhibitor and prevented infiltration of electrolytic salts (NaCl) through the coating. Apart from these, entanglement of few layer GNPs with CeO2 matrix and GNP crack bridging between two crack banks enhanced the mechanical performance of the matrix. As a whole, GNPs reinforcement owed the combined improvement in corrosion resistance as well as mechanical performance, which would make it a better option for advanced technological applications.
In this study, graphene nanoplatelets (GNPs: 1–2 wt. %) reinforced TiN coating were successfully fabricated over titanium alloy using a reactive shroud plasma spraying technique. All coatings were completely oxide free, while the addition of GNPs suppressed the non-stoichiometric TiN0.3 phase. Improvement of 19%, 18% and 300% in hardness, elastic modulus and fracture toughness was achieved by mere addition of 2 wt. % GNP. The addition of GNP in TiN also reduced the wear volume loss and the wear rate of the coatings for the entire range of temperature (293–873 K). Moreover, GNPs also manifested the coefficient of friction (COF) of the coating. Post wear characterization revealed that the presence of GNP throughout the wear track even at 873 K. The multi-layer structure of GNPs assisted in long term lubricity to the surface and increased the wear resistance of the coating.