Creep Performance and Damage Mechanism of Al₂O₃/Al₂O₃ Ceramic Matrix Composites

With the rapid development of aero-engine and aerospace technology, there is increasing demand for heat resistance materials. At present, defense and civil aircrafts are flying at supersonic speed, while the service temperature of the hot-end components even reaches 1600 ℃. Therefore, higher temperature resistance requirements are proposed for aviation engines and their surrounding materials. Currently, most of the engines are nickel-based high-temperature alloys with high temperature resistance, light weight and anti-deformation capability. Because the working temperature of the alloy materials is already close to their temperature limit, it is impossible to further increase the operating temperature. Comparatively, ceramic matrix composites have various advantages, such as high bearing capacity, high erosion resistance, low density and high reliability, which are ideal alternative materials for high temperature hot end components. In this study, higher oxidation resistance and lower cost oxide fiber oxide composites were studied

A high-temperature creep machine was used to test creep performance of the composite materials made of Al₂O₃/Al₂O₃ ceramic matrix containing Nextel 720 fibers. The test was conducted at three temperatures, i.e., 1000 ℃, 1100 ℃, and 1200 ℃. After the temperature was stabilized, a constant mechanical load was applied at 100 MPa and 120 MPa. After the creep test, the fractured morphology of the creep samples was observed using a scanning electron microscope (SEM), to identify the creep damage mechanism of the Al₂O₃/Al₂O₃ ceramic matrix composite materials. Modulus and hardness of the fibers were measured by using nano-indentation, while the microstructure was observed by using transmission electron microscope (TEM).

The creep rate at 1200 ℃/120 MPa is much greater than that at 1000 ℃/100 MPa. The creep activation energies for the Al₂O₃/Al₂O₃ composite at 120 MPa and 100 MPa are 798.9 kJ·mol⁻¹ and 766.0 kJ·mol⁻¹ respectively. A large number of cracks are observed on surface of the matrix, while the matrix is quite porous. The presence of pores and cracks reduced strength of the matrix. HRTEM and Fourier transform filtering images of selected areas were analyzed at 1200 ℃/100 MPa. The phase isα-Al₂O₃, with a rhombohedral structure. The Al₂O₃ grain size gradually increased with increasing temperature. The rise in temperature caused softening of the matrix, resulting in decrease in hardness.

During the initial stage of creep, the fiber and matrix together shared the load, but the matrix was cracked before the fiber. As a result, the fiber bridged the cracks of the matrix and bore the load at the later stage of creep. The Al₂O₃/Al₂O₃ ceramic matrix composites experienced significantly different creep behaviors at temperatures of below 1100 ℃ and 1200 ℃. At the same time, the creep sample showed fiber pull-out, with no obvious matrix cracks, suggesting that the matrix and fiber were relatively strong and the cracking of the matrix was suppressed. In this case, the matrix and fiber shared the load, leading to a long creep life and slow deformation rate. However, at 1200 ℃/100 MPa and 1200 ℃/120 MPa, hardness of the matrix decreased. Then, the matrix started cracking. As the cracks of the matrix reached certain scale, the fiber would bridge the cracks and bear the load. Therefore, the Al₂O₃/Al₂O₃ ceramic matrix composite has higher creep resistance at 1200 ℃ than at 1100 ℃.

In addition, since the grain size inside the fiber is dependent on temperature, the creep resistance of the composite is also related to temperature. When the creep temperature is below 1100 ℃, due to the stable microstructure of the fiber, the composite has excellent creep resistance. However, when the creep temperature reaches 1200 ℃, the performance of the fiber decreases, due to the grain growths inside the fiber, leading to significant reduction in the creep resistance. In addition, Temperature affects the creep resistance of composite materials by influencing the grain size inside the fibers. When the creep temperature is lower than 1100 ℃, due to the stable microstructure of the fibers in the composite material, the composite material exhibits excellent creep resistance. However, when the creep temperature reaches 1200 ℃, the performance of the fibers deteriorates, and the grains in the fibers bond together, resulting in a significant decrease in the creep resistance of the Al₂O₃/Al₂O₃ ceramic matrix composite material.