Highly Electrically Conducting Glass-Graphene Nanoplatelets Hybrid Coatings

Abstract

Hybrid coatings consisting of a heat resistant Y2O3-Al2O3-SiO2 (YAS) glass containing 2.3 wt % of graphene nanoplatelets (GNPs) were developed by flame spraying homogeneous ceramic powders-GNP granules. Around 40% of the GNPs survived the high spraying temperatures and were distributed along the splat-interfaces, forming a percolated network. These YAS-GNP coatings are potentially interesting in thermal protection systems and electromagnetic interference shields for aerospace applications; therefore silicon carbide (SiC) materials at the forefront of those applications were employed as substrates. Whereas the YAS coatings are nonconductive, the YAS-GNP coatings showed in-plane electrical conductivity (∼10(2) S·m(-1)) for which a low percolation limit (below 3.6 vol %) is inferred. Indentation tests revealed the formation of a highly damaged indentation zone showing multiple shear displacements between adjacent splats probably favored by the graphene sheets location. The indentation radial cracks typically found in brittle glass coatings are not detected in the hybrid coatings that are also more compliant.

Superior Performance of Ablative Glass Coatings Containing Graphene Nanosheets

“…This anisotropic behavior is similar to that observed in bulk ceramic materials having oriented GNPs lying in the direction perpendicular to the heat flux, being the reduction for the cross‐plane thermal conductivity mainly related to the intrinsically lower conductivity of the graphite C‐axis. However, it differs from the thermal performance previously reported for the G glass coating containing 3.6 vol% GNPs; in that case, an augment of κ by 30% was measured for both orientations because GNPs were also connected in the direction perpendicular to the coating surface . For the present coatings, the reduced amount of GNPs intensifies the thermal anisotropy of the hybrid GC/GNP coating (the α in‐plane /α thickness ratio rises from 3.7 to 6.4), which foresees good performance for thermal shield purposes, as they would enhance the in‐plane release of heat avoiding hot spots while hindering heat propagation toward the substrate in the through‐thickness direction.…”

“…On the other hand, the GC/GNPs coating [Fig. (b)] presents similar amount of pores and also some elongated features at the splat boundaries that, although resembling pores, do correspond to the nanoplatelets as it has been demonstrated in a previous work of the present authors . As the DTA of the GC ground coating [Fig.…”

“…The major concerns regarding aerospace coatings include reducing weight, decreasing vulnerability to orbital debris, while keeping minimal thermal conductivity and high emissivity to ensure the maximum rejection of incoming convective heat through radiative heat transfer . In that context, we see hybrid graphene/glass–ceramic coatings as very promising materials because of the exceptional properties of carbon allotropes as well as the heat resistance and self‐healing capability of glass–ceramics . The feasibility of the flame spraying method for coating SiC, C f /SiC, and C f /C substrates using glass (G) and glass–ceramic (GC) compositions of the Y 2 O 3 –Al 2 O 3 –SiO 2 (YAS) system has been proved previously by some of the present authors .…”

“…This powder mixture was conditioned for obtaining spherical‐shaped particles of ~30 μm following the procedure described elsewhere . Similar size granules for the GC composition with 10 vol% (6.5 wt%) of graphene nanoplatelets (N008‐100‐P‐10, Angstron Materials Inc., Dayton, OH), labeled as GC/GNPs, were also prepared …”

“…These coatings were continuous, homogeneous, dense, and presented good adhesion to SiC, C f /SiC, and C f /C substrates, particularly for SiC‐based materials . Furthermore, the inclusion of graphene nanoplatelets (GNPs) as filler for the YAS glass produced anisotropic hybrid coatings with aligned GNPs, exhibiting enhanced damage tolerance and also added electrical and thermal functionalities . Preliminary ablation tests performed for those YAS glass coatings showed endured ablation resistance at 900°C, but at higher temperatures the complete removal of the coating occurred .…”