Abstract:

Objective Three-dimensional woven angle interlock SiC_f/SiC composites have the advantages of high temperature resistance, low density and long service life, and are an ideal candidate material for thermal aviation terminal components. At present, the research of SiC_f/SiC composites mainly focuses on the first- and second-generation SiC fibers, but few studies on the mechanical properties of the third generation SiC fibers and their three-dimensional woven angle interlocking composites were reported.
Method In order to explore the influence of interfacial layer on fracture toughness of SiC_f/SiC composites, the third generation SiC fibers made in China was selected. Three-dimensional woven angle interlock SiC_f/SiC composites with four interface phases including pyrolytic carbon (PyC), pyrolytic carbon/silicon carbide (PyC/SiC), boron nitride (BN) and boron nitride/silicon carbide (BN/SiC) were prepared by the precursor infiltration pyrolysis processes, chemical vapor deposition process and chemical vapor infiltration process. On this basis, combined with acoustic emission technology, the normal temperature fracture toughness test was carried out, and the microscopic damage mode was evaluated by scanning electron microscopy.
Results All samples showed the characteristic of "pseudo-plastic fracture" (Fig.8). The fracture strengths of P-SiC_f/SiC, P/S-SiC_f/SiC,B-SiC_f/SiC and B/S-SiC_f/SiC are 193.36, 233.97, 89.43 and 218.49 MPa, respectively, and the modulus thereof are 33.86, 33.36, 32.03 GPa and 31.37 GPa, respectively (Fig.9). It was found that the samples with PyC as the main interfacial layer offer good fracture toughness, and the fracture toughness of P-SiC_f/SiC and P/S-SiC_f/SiC are 13.99 and 16.93 MPa·m^1/2, respectively (Fig.10). On the other hand, the B-SiC_f/SiC samples show strong interfacial bonding, with the lowest fracture toughness of 6.47 MPa·m^1/2. However, the fracture toughness of B/S-SiC_f/SiC samples is significantly increased to 15.81 MPa·m^1/2 when the SiC layer is introduced into the interface. The results show that the interfacial layer in the SiC_f/SiC composites has a strong influence on the fracture strength and fracture toughness, but has no great influence on their initial modulus, which mainly depends on the fiber structure and the stiffness of the matrix. In the microscopic damage morphology of SiC_f/SiC composites, the meso-damage of the four samples all involve the matrix fracture, the interface damage, the debonding between fiber and matrix, the fiber fracture and the fiber pulling-out (Fig.11, Fig.12). However, the types of main body damage are obviously different, the sample with composite interface layer produces more AE events before the fiber failure due to the blocking effect of SiC layer on the crack (Fig.13).
Conclusion It can be concluded from the research that the introduction of SiC layer enhances the energy dissipation mechanism of the interface and prevents the crack propagation in the matrix, the cracks in PyC layer and BN layer can be deflected effectively, and the mechanical properties of SiC_f/SiC composite are improved. In addition, acoustic emission (AE) event energy values and numbers of impact can completely describe the real-time damage process of SiC_f/SiC composites. Several problems should be further investigated in the study of the properties of three-dimensional woven angle interlock SiC_f/SiC composites. Firstly, it is difficult to distinguish SiC_f/SiC composites due to their relatively complex microscopic composition and the close density of fiber and matrix. How to monitor the more detailed real-time damage process of materials in the bearing process by means of advanced characterization techniques is a focus of future research. In addition to the experimental testing, it is necessary to develop a high-fidelity numerical simulation method for three-dimensional woven angle interlock SiC_f/SiC composites, establish a more accurate meso-structural model to achieve progressive damage analysis and reveal the failure mechanism.

Key words: SiC_f/SiC composite, three-dimensional woven angle interlock, interface layer, fracture toughness, acoustic emission