Thermo-Chemical-Mechanical Performance of Silica- and Graphene-Reinforced Epoxy Coatings for Geothermal Proppants
Abstract
Polymer coatings are widely used to improve proppant durability; however, their stability under thermo-hydro-mechanical-chemical conditions relevant to enhanced geothermal systems remains insufficiently constrained. This study evaluates epoxy nanocomposite coatings incorporating silica (SiO2), graphene nanoplatelets (GNP), and a SiO2–GNP hybrid using a multi-scale characterization framework. Two-stage curing (24 h at room temperature followed by 24 h at 80 °C) increased rebound hardness across most formulations, with 0.5 wt% GNP improving hardness from ~478 HLD to ~556 HLD (~16% increase) relative to direct curing. Under constant-load scratch testing, GNP systems exhibited increasing hardness with loading, reaching 0.2086 GPa at 2 wt% GNP (~43% higher than neat at 0.1456 GPa), accompanied by a reduction in scratch width from ~589.8 µm (neat) to ~494.1 µm (~16% decrease). Thermogravimetric analysis showed that nanoparticle incorporation increased degradation onset temperatures, with 1 wt% GNP exhibiting the largest increase (~114% relative to neat), while high-temperature residue increased from 7.53% (neat) to 26.27% for 1 wt% SiO2 (~249% increase). Hydrothermal testing at 220 °C for 72 h showed that neat epoxy exhibited the highest fluid uptake (~16.6% mass gain), whereas 1 wt% GNP reduced uptake to ~4.5% (~73% reduction relative to neat). Post-exposure scratch testing showed that neat epoxy experienced ~7–8× increases in friction, penetration, and energy, while 1 wt% GNP limited these increases to ~1.6×. These results demonstrate that nanoparticle incorporation improves coating performance in a formulation-dependent manner, with GNP systems providing the most consistent improvements in hydrothermal resistance and mechanical performance retention under geothermal conditions. Keywords: Enhanced geothermal systems (EGS); Epoxy nanocomposites; Graphene nanoplatelets (GNP); Hydrothermal stability; Proppant coating.s