A combustion facility which includes uniaxial mechanical loading was implemented that enables environmental conditions more akin to jet engine environments compared to conventional static environment tests. Two types of woven SiC/SiC ceramic matrix composites (CMCs), melt-infiltrated (MI) and chemical vapor infiltrated (CVI), were subjected to fatigue loading in the combustion facility and under isothermal furnace conditions. Some CVI test coupons were coated with a multilayer environmental barrier coating (EBC) of mullite + ytterbium monosilicate using slurry infiltration process to demonstrate the performance with a coating. Combustion conditions were applied using a high velocity oxy fuel gun on the front side of the specimen and mechanical loading was applied using a horizontal hydraulic MTS machine. All the specimens considered were subjected to tension-tension fatigue loading at 100 MPa, stress ratio of 0.1 and specimen front-side surface temperature of 1200 ± 20 °C. Nondestructive evaluation (NDE) methods, such as electrical resistance (ER), was used as an in-situ health monitoring technique. Similar fatigue tests were performed in an isothermal furnace for comparison. A much lower fatigue life was observed for the uncoated specimens tested under combustion conditions in comparison to isothermal furnace condition. This difference in fatigue life was attributed to damage associated with added thermal stress due to the thermal gradient and higher rate of oxidative embrittlement due to the presence of high velocity combustion gases in the combustion environment. EBC coating increased the fatigue life in combustion environment. However, EBC coated specimens experienced spallation in the high-velocity flame due to the presence of micro cracks in the coating surface. Fracture surfaces of the failed specimens were investigated under the scanning electron microscope (SEM) to determine the extent of oxidation and damage.

Fatigue behavior of woven melt infiltrated (MI) SiC/SiC ceramic matrix composites (CMCs) was investigated under a tension–tension fatigue condition in a combustion environment. A special experimental facility is designed to subject the CMCs under simultaneous mechanical and combustion conditions which is more representative of some conditions experienced by the hot section components of a jet engine. The MI SiC/SiC CMCs considered in this study consists of a SiC matrix densified with liquid Si infiltration, BN interphase, and reinforced with two different fibers, namely, Hi–Nicalon type S and Tyranno SA fibers. A high velocity oxygen fuel (HVOF) gun is used to create the representative combustion condition and a horizontal hydraulic MTS machine to apply the mechanical loading. Several fatigue tests were conducted at different stress levels with a stress ratio of 0.1, frequency of 1 Hz, and the specimen surface temperature at 1200 °C. Similar tests were conducted in an isothermal furnace condition at 1200 °C for comparison. Electrical resistance (ER) was used to monitor the tests. A reduction in the fatigue life was observed for the two MI systems under combustion conditions in comparison to the isothermal furnace condition at the same applied stress level. This is attributed to the presence of harsh combustion environment present in the burner rig. ER showed some promising results in monitoring the temperature and detecting damage in the specimen. Runout condition was set as 24 H (86400 cycles) in burner rig and 100 H (360000 cycles) in furnace environment. Specimens that achieved the runout condition were subsequently tested under monotonic tension testing at room temperature after cooldown to evaluate the residual properties. Residual strength results showed a significant strength reduction in both the furnace and burner rig environments. Post-test microscopy was conducted on the fracture surfaces of the failed specimens to understand the oxidation behavior and damage mechanisms.

Electrical resistance (ER) has become a technique of interest for monitoring SiC-based ceramic composites. The typical constituents of SiC fiber-reinforced SiC matrix composites, SiC, Si, and/or C, are semiconducive to some degree resulting in the fact that when damage occurs in the form of matrix cracking or fiber breakage, the resistance increases. For aero engine applications, SiC fiber reinforced SiC, sometimes Si-containing, matrix with a boron nitride (BN) interphase are often the main constituents. The resistivity of Si and SiC is highly temperature dependent. For high temperature tests, electrical lead attachment must be in a cold region which results in strong temperature effects on baseline measurements of resistance. This can be instructive as to test conditions; however, there is interest in focusing the resistance measurement in the hot section where damage monitoring is desired. The resistivity of C has a milder temperature dependence than that of Si or SiC. In addition, if the C is penetrated by damage, it would result in rapid oxidation of the C, presumably resulting in a change in resistance. One approach considered here is to insert carbon “rods” in the form of CVD SiC monofilaments with a C core to try and better sense change in resistance as it pertains to matrix crack growth in an elevated temperature test condition. The monofilaments were strategically placed in two nonoxide composite systems to understand the sensitivity of ER in damage detection at room temperature as well as elevated temperatures. Two material systems were considered for this study. The first composite system consisted of a Hi-Nicalon woven fibers, a BN interphase and a matrix processed via polymer infiltration and pyrolysis (PIP) which had SCS-6 monofilaments providing the C core. The second composite system was a melt-infiltrated (MI) prepreg laminate which contained Hi-Nicalon Type S fibers with BN interphases with SCS-ultramonofilaments providing the C core. The two composite matrix systems represent two extremes in resistance, the PIP matrix being orders of magnitude higher in resistance than the Si-containing prepreg MI matrix. Single notch tension–tension fatigue tests were performed at 815 °C to stimulate crack growth. Acoustic emission (AE) was used along with ER to monitor the damage initiation and progression during the test. Post-test microscopy was performed on the fracture specimen to understand the oxidation kinetics and carbon recession length in the monofilaments.