Surface chemistry modification for elimination of hydrophilic Al4C3 in B4C/Al composites

Highlights: • The hygroscopic and deleterious Al₄C₃ phase was successfully eliminated by a post-processing approach. • A hydrophobic and impermeable fluoride coating multiphase layer is deposited on the composites. • The stability of fluoride phases and the integrity of the composites are confirmed. • Optimal conditions to maximize the amount of AlF₃ deposited were established. The presence of deleterious Al₄C₃ phase has been for long a major challenge in the manufacture of Al matrix composites reinforced with carbide phases. In this contribution, the hydrophilic Al₄C₃ phase has been successfully eliminated from the surface of B₄C/Al composites prepared by pressureless infiltration, using a post-processing approach by the hybrid system chemical vapor deposition (HYSY-CVD) route. B₄C/Al composites containing intentionally formed Al₄C₃ were first prepared by pressureless infiltration and subsequently treated by HYSY-CVD, varying temperature (400 and 500 °C), time (30 and 60 min) and amount of solid precursor Na₂SiF₆ (10 and 20 g). In conformity with reaction thermodynamic predictions using the HSC 6.1® and FactSage® programs and databases, analysis by XRD and SEM confirms the superficial elimination of Al₄C₃ and the seedless formation of passivating AlF₃ and Na₅Al₃F₁₄. Optimal conditions to maximize the amount of AlF₃ deposited were established by ANOVA and the effectiveness of the fluoride phases was evaluated in hardness and corrosion tests, confirming the stability of fluoride phases and the integrity of the composites. Microstructure characterization and hardness tests of specimens after three years substantiate the efficacy of AlF₃ and Na₅Al₃F₁₄ phases. A hydrophobic and impermeable fluoride coating multiphase layer is thus obtained.