Effects of combustion and salt-fog exposure on fatigue behavior of two ceramic matrix composites and a superalloy

Tension–tension fatigue behavior of two ceramic matrix composites (CMCs), SiC/SiC (Nicalon™/SiC) & oxide/oxide (Nextel™720/AS), and a superalloy (Rene 41) was characterized under combustion environment as well as under alternate salt-fog exposure for 4 h and combustion fatigue for 6 h. The run-out limit was 25 h (90,000 cycles) of fatigue. Test temperatures were 900, 1050, and 1050 °C for Rene 41, SiC/SiC, and oxide/oxide CMCs, respectively, and the applied maximum fatigue stress was 62 MPa. All three materials survived up to 25 h under combustion fatigue without any exposure to salt. Rene 41 and Nicalon™/SiC survived up to 25 h (90,000 cycles) under the alternate exposure of salt-fog and combustion fatigue. However, exposure to salt-fog exposure (5 wt%) had very detrimental effect on the combustion fatigue resistance of Nextel™720/AS. The reduction in salt concentration to 0.5 wt% had less detrimental effect. These tests were supplemented with more tests, where applied stress level, test temperature, and salt concentration were changed. Out of these three gas turbine engine materials, Nicalon™/SiC exhibited better fatigue performance in terms of applied temperature and stress level under combustion condition as well as when exposed to alternate salt-fog and combustion.