La₂Si₂O₇ is a candidate environmental barrier coating (EBC) material for silicon carbide ceramic matrix composites in next-generation gas turbine engines, yet its degradation behavior under the simultaneous attack of molten CMAS and water vapor remains insufficiently understood. This study systematically examines atmospheric plasma-sprayed La₂Si₂O₇ coatings exposed to four environments (air, CMAS alone, H₂O/O₂ alone, and combined CMAS + H₂O/O₂) at 1400 °C for 8 h. Under dry air, the coating recrystallizes to La₂Si₂O₇ with negligible corrosion. CMAS attack triggers a dissolution–reprecipitation mechanism forming needle-like CaLa₄(SiO₄)₃O apatite within a denser reaction zone, which partially impedes further infiltration. Water vapor accelerated the growth of the thermally grown oxide at the bond coat interface. The combined CMAS + H₂O/O₂ environment produced a pronounced synergistic acceleration: water vapor reduced CMAS melt viscosity, enabling deeper CMAS penetration, while concurrent silica volatilization disrupted the protective apatite barrier, generating extensive porosity and through-coating cracking. These findings reveal that the inherent CMAS resistance of La₂Si₂O₇ via apatite formation is critically compromised by simultaneous water vapor, highlighting a key challenge for its application in realistic engine environments.

Phenolic resin is widely used as a binder in high-temperature industries; however, its pyrolysis generally yields isotropic glassy carbon, which strongly influences its high-temperature oxidation behavior. In this work, cupric tartrate was introduced as a catalyst precursor to investigate its effects on the thermal decomposition behavior, microstructural evolution, and oxidation behavior of the phenolic resin pyrolytic carbon. Upon heating, cupric tartrate decomposed at 250–320 °C into nanoscale Cu/Cu₂O composites, which were then converted into metallic Cu nanoparticles through reduction by gaseous products generated during the pyrolysis of phenolic resin. The in situ formed Cu nanoparticles were associated with the growth of tapered carbon nanofibers (CNFs), reaching maximum lengths of 30–50 μm at 700 °C. Based on the observed microstructural features and established literature reports, a dissolution–precipitation pathway is proposed to rationalize the formation of these CNFs. The presence of Cu-catalyzed CNFs correlates with enhanced structural ordering of the pyrolytic carbon, as reflected by reduced I_D/I_G ratios, and with an increased apparent oxidation activation energy in the selected fitting region (from 103.73 to 137.45 kJ/mol). Overall, this work demonstrates a facile strategy in which cupric tartrate serves as an effective catalyst precursor that generates Cu nanoparticles in situ; these nanoparticles then catalyze CNF growth from phenolic resin, enabling the construction of low-dimensional carbon nanostructures.