Carbon–Aluminum Composite with a Barrier Coating on Carbon Fibers

Barrier SiO₂ coatings on the surface of carbon fibers are deposited by the dip coating method from sol–gel solutions based on tertraethoxysilane Si(C₂H₅O)₄. The average thickness of the barrier SiO₂ coating on carbon fibers is 127 ± 30 nm. Carbon–aluminum composites are prepared by the shell molding process, being a variety of the liquid-phase infiltration method. The components of the composite are placed in a steel hermetic shell. After shell evacuating and heating to aluminum melting, the fibers are infiltrated with the melt under an external pressure followed by cooling. Composites with the SiO₂ coating on the carbon fibers and without coating are prepared. After taking the composite from the metal shell, the structure, phase composition, and mechanical properties of the samples are studied. The study of the composite structure shows that the interfiber space is filled with an aluminum melt without porosity and macroscopic defects. The study of the phase composition of the composite reinforced with uncoated carbon fibers shows peaks of aluminum carbide at the angles 2θ = 41°, 67°, and 74°. The Rietveld quantitative analysis reveals that the amount of aluminum carbide in the composite is 12.0 ± 1.3%. The deposition of the barrier SiO₂ coating on carbon fibers leads to a decrease in the intensity of the main peaks of aluminum carbide, while the amount of Al₄C₃ decreases by 4 times (to 3.0%). An analysis of the fracture surface of the samples after mechanical tests reveals that the fracture surface of the composite reinforced with uncoated carbon fibers is almost planar. No protrusions and no relief are observed on the fracture surface. The fracture surface of the composite reinforced with coated carbon fibers has a relief, and separately sticking out fibers are observed. Mechanical three-point bending tests of the samples show that the barrier coating on the fibers increases the strength to 520 ± 50 MPa, and the strength of the uncoated composite is 350 ± 8 MPa. The barrier SiO₂ coating deposited on the carbon fiber surface prevents the formation of aluminum carbide and fiber degradation in the carbon–aluminum composite.