SiC_f/SiC ceramic matrix composites can replace high-temperature alloys in manufacturing high-temperature structural parts of aerospace engines due to their excellent properties. In a gas environment, SiC_f/SiC composite materials have a challenge from corrosion by water vapor and oxygen. The specific process of SiC_f/SiC corrosion by water vapor-oxygen coupling is not yet clarified. Therefore, investigating its corrosion behavior is of extremely important significance in guiding the protection of SiC_f/SiC from corrosion and ensuring the application of high-temperature components.

In this paper, the water-oxygen corrosion behavior of SiC_f/SiC was simulated via hydrothermal corrosion experiment in water environment and water-oxygen corrosion experiment in water-oxygen coupling environment, respectively.

In the hydrothermal corrosion experiment, SiC_f/SiC was placed in deionized water in a closed container and kept in an oven at 200 ℃ for 2 450 h. The pressure inside the container was approximately 2 MPa. After the corrosion was completed, the aqueous solution was taken out and dried in an oven at 35 ℃, and the dried powder was characterized. In the water-oxygen corrosion experiment, SiC_f/SiC was placed in a closed high-temperature furnace tube, and water vapor-oxygen mixed gas (A volume ratio of 90%∶10%) was injected into the furnace tube. The pressure inside the furnace tube was normal pressure, the temperature was maintained at 1 300 ℃ for 50-200 h.

The crystal structure of water-oxygen corrosion products was analyzed by X-ray diffraction (XRD, Smart Lab., Japan). The morphology and composition of SiC_f/SiC and its corrosion products were determined by scanning electron microscopy (SEM, Zeiss Co., Germany). The Si content in hydrothermal corrosion products dissolved in aqueous solution was measured by inductively coupled plasma (ICP, Leeman Prodigy 7, USA). The electronic states of Si and O in the hydrothermal corrosion products were characterized by X-ray photoelectron spectroscopy (ESCALAB 250Xi, XPS). The thermal stability of hydrothermal corrosion products was analyzed by thermal gravimetric analysis-differential scanning calorimetry (TGA-DSC, Netzsch Co., Germany).

Based on the observation of the microstructure, the connected pores of SiC_f/SiC composite materials are the entry and exit channels for corrosion media/products. Corrosion products mainly exist in three types of areas, i.e., pores remaining during the precursor impregnation process, lamellar holes produced in the gaps between fiber cloths, and suture holes in the z direction.

In a hydrothermal environment at 200 ℃, the corrosion of SiC_f/SiC in water is a volume expansion process, the matrix becomes loose, and the pores are filled with corrosion products. After hydrothermal corrosion for 2 450 h, the Si content in the milky white colloid measured is 1 045.4 mg/L. According to the EDS analysis, the O/Si atomic number ratio is 2.9, and the chemical composition is close to H₂SiO₃ (SiO₂·H₂O). The comprehensive analysis by XRD (PDF 38-0448) and XPS (O 1s has two chemical states, namely Si—O and Si—O—H) indicates that the milky white product is H₂SiO₃.

In the water-oxygen coupled corrosion environment at 1 300 ℃, the fibers and interface layers of the SiC_f/SiC composite material are corroded, and cracks are formed. After corrosion for 200 h, the thickness of the sample expands from 4.85 mm to 6.66 mm, which is increased by 37%. The internal structure of the sample becomes loose, the corrosion intermediate product is H₂SiO₃, and the final product is cristobalite SiO₂.

This article studied the corrosion process of SiC_f/SiC composite materials in hydrothermal (oxygen-free) and water-oxygen coupling environments. The connected pores of SiC_f/SiC composite materials were channels for corrosive media/products, which could not be avoided. In an oxygen-free hydrothermal environment at 200 ℃, the corrosion of SiC_f/SiC by water was a volume expansion process, forming a milky white product H₂SiO₃. In a water-oxygen coupled corrosion environment at 1 300 ℃, the SiC_f/SiC composite material expanded in volume and formed cracks. The fibers and interface layers were corroded and the structure became loose. The corrosion product was cristobalite SiO₂.