JIAO Chunrong, SUN Shijie, WANG Hongjie, YANG Jinhua, JIAO Jian

(AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China)

Extended abstract:

[Background and purposes] SiC_f/SiC composites, featuring high temperature resistance, low density and long service life, are one of the most promising thermal structural materials for hot-end components of aero-engines. The L-shaped structure of SiC_f/SiC composites is a common form of core turbine structural parts in aero-engines, functioning as connection and load-bearing components. L-shaped structure is usually located in the connection and transition areas of turbine parts, which is a stress concentration point. Therefore, it is a key point in the design of SiC_f/SiC composite components for aero-engines. Therefore, the study on the load-bearing characteristics and failure mechanisms of the L-shaped structure is of considerable significance. In this paper, L-shaped structure of SiC_f/SiC composites was prepared by using Prepreg-MI process. The static strength, at room temperature, 800 ℃ and 1200 ℃, was tested, while the failure mechanism was analyzed. The results can be used as basis and guidance for the structural optimization of SiC_f/SiC composite turbine guide vanes.

[Methods] The reinforcement phase of the L-shaped structure was the second-generation continuous SiC fibers, with a BN/SiC interface layer on the surface of SiC fibers via CVI. The SiC fibers were impregnated to be prepreg unidirectional tapes, which were laid in an L-shaped mold according to the [0/90] lay-up pattern. After hot pressing, carbonization and molten silicon infiltration, the L-shaped structure of SiC_f/SiC composites was obtained. The internal quality was inspected by using CT. Three groups of static stress tests were carried out on an electronic universal testing machine at room temperature, 800 ℃ and 1200 ℃. A vertical load was applied to the rear end of the specimen at a loading rate of 0.1 mm·min⁻¹. The test load was increased from 0 until the L-shaped specimen failed, while the performance data, such as maximum load and fracture displacement, were obtained from the load-displacement curve. A stereomicroscope and a scanning electron microscope were used to observe crack propagation path, macroscopic fracture morphology and microscopic fracture morphology at the fracture position of the L-shaped structure specimens. An X-ray diffractometer was used for micro-area composition analysis of different regions of the fracture.

[Results] Internal structure of the L-shaped component was dense and uniform, with no obvious defects, such as delamination, pores or density inhomogeneity in the R-region. The load-displacement relationships at room temperature and 800 ℃ were basically similar. Before reaching the maximum load, the load-displacement curve exhibited a good linear relationship. After reaching the maximum load, internal damage of the specimen gradually accumulated, matrix cracks occurred which prevented load transfer and fibers began to be pulled out, leading to a rapid decrease and an obvious nonlinearity in the load-displacement curve. Stereomicroscopic observations indicated that, at room temperature, 800 ℃ and 1200 ℃, the initial cracks of the specimens appeared at the corner near the mounting hole, originated from the outer side of the corner on one end face and propagated to the other side, and deflected away from the mounting hole in the later stage of propagation. The origin position was relatively flat, while the fracture surface gradually became rough from the outer side to the inner side and along the crack propagation direction to the other end face, where fiber pull-out in the composite could be observed. At room temperature, 800 ℃ and 1200 ℃, cracks in all L-shaped specimens were originated from the end face outside the corner of the L-shaped structure. Fracture at this location mainly occurred under shear force, while the cracks propagated toward the other end face and along the thickness direction of the specimen. The stress at the crack tip region gradually transitioned from shear to tensile force perpendicular to the fracture surface, resulting in more sufficient fiber/matrix interface debonding and pull-out at room temperature and 800 ℃. At room temperature, the area near the crack was characterized by an overall flat fracture surface with short fiber pulled-out, while clear propagation ridges were visible on the fiber surface. At 800 ℃, the overall morphology of the crack origin was relatively rough, with obvious fiber debonding and pulled-out; most of the pulled-out fibers had no visible BN coating, but propagation ridges were still present on the fiber surface. At 1200 ℃, the fiber fracture surface was smooth and flat, the initial fracture position showed complete oxidation morphology covered by glassy oxides, and the final fracture region exhibited characteristics of shear overload instantaneous fracture. For the specimen tested at room temperature, the element contents at different positions were basically equivalent. Besides Si and C, the content of O was very low, while the content of B was relatively high (B mainly came from the coating on the fiber surface and O was an impurity introduced during the manufacturing process). For the specimen tested at 800 ℃, the oxygen content on the fracture surface was between 2.00 wt.% and 5.00 wt.%, showing slight oxidation, while the B content at the initial fracture region decreased slightly, indicating that the coating on the SiC fiber surface had started to be oxidized. For the specimen tested at 1200 ℃, the EDS results of different fracture positions showed that the oxygen contents were 46.48 wt.%, 43.69 wt.% and 8.31 wt.%. The initial fracture region and the propagated fracture region experienced severe overall oxidation during the test, while the instantaneous fracture position showed slight oxidation. No B element was detected at the initial fracture region and the propagated fracture region, indicating that the coating on the SiC fiber surface had completely disappeared and lost its protective effect for the fibers.

[Conclusions] The maximum failure load of the L-shaped SiC_f/SiC composite structure exceeds 250 N at room temperature, 800 ℃ and 1200 ℃. There is no significant differences in the maximum failure load with increasing temperature. At room temperature, 800 ℃ and 1200 ℃, cracks in the L-shaped SiC_f/SiC composite structure all originated from the end face outside the corner of the L-shaped specimen, propagate toward the other end face and along the thickness direction of the specimen, and finally fracture and fail at the corner near the mounting hole. At room temperature and 800 ℃, the fracture surface of the L-shaped SiC_f/SiC composite structure is overall flat, with obvious fiber debonding and pulled-out at the fiber/matrix interface and clear propagation ridges visible on the fiber surface. At 1200 ℃, the fracture surfaces of the matrix and fibers are smooth and flat without crack propagation characteristics, showing a complete oxidation morphology. The SiC_f/SiC composite and its fiber surface coating undergo slight oxidation at 800 ℃, while both the SiC_f/SiC composite and the fiber surface coating suffer severe oxidation at 1200 ℃. This indicates that, without other protective measures, the SiC_f/SiC composite is difficult to be used for a long time in a high-temperature oxygen environment at 1200 ℃.

Key words: SiC_f/SiC composites; L-shaped structure; load-bearing characteristics