Abstract
The hypersonic environment is one of the harshest materials can experience. Leading edge materials are exposed to temperatures ranging from 840°C at Mach 5 to above 5500°C at Mach 10 while also interacting with dissociated oxygen, dissociated nitrogen, and plasma. These highly oxidizing environments create a unique challenge for material survivability. Materials used for leading edges are required to withstand high temperatures and severe oxidation conditions while maintaining their mechanical strength for the duration of flight. Carbon/carbon (C/C) composites are commonly used in hypersonic applications for their ability to retain their strength at extreme temperatures, but their poor oxidation performance typically requires them to be coated for protection. Silicon carbide (SiC) is often used as a coating material with C/C composites due to the protective oxide scale it forms. While carbon and SiC oxidation have been widely studied in molecular oxygen (MO) environments, very few prior studies have examined the impact of atomic oxygen (AO) on C/C and SiC oxidation. One of the primary challenges to understanding material behavior in the hypersonic environment is the limited test capability. State-of-the-art test facilities like arc jets can create hypersonic-representative flows, but these facilities cost upwards of $150K/day to use and have years-long wait times. Therefore, cost and schedule often prohibit conducting thorough arc jet testing. Additionally, arc jets do not allow for the isolation of variables (pressure, temperature, velocity, gaseous dissociation) which makes determining the primary failure mechanisms of a material a challenge. New test capabilities are necessary to investigate the effects of atomic oxygen on a material at high temperatures. To help address this gap, a novel Microplasma Resistive Heating System (MRHS) has been developed at the University of Virginia to test materials at temperatures up to 2400°C while simultaneously exposing them to molecular or dissociated oxygen under otherwise identical conditions. This system allows for the isolation of oxidation effects on a material. Investigation of the microplasma yielded that the oxygen dissociation percentage can be tailored to be representative of up to Mach 10 flight regimes. When the microplasma energizes air, an environment of dissociated oxygen similar to that of Mach 7-10 is created. After determining the hypersonic-representative conditions, the MRHS was utilized to assess the oxidation of SiC-coated C/C in a dissociated oxygen environment. The material was tested at 950°C and 1300°C for two, six, and ten minutes and at 1800°C for two minutes. The SiC actively oxidized aggressively at 1800°C limiting the tests times to two minutes. Scanning electron microscopy (SEM) was used to assess the resulting oxidation morphologies. Macroscopic facets formed during the 1800°C tests that have not been described in prior oxidation literature. Localized regions of both passive and active oxidation occurred almost instantaneously at all temperatures particularly near and along cracks. The relative abundance of oxidation followed thermal gradients through the material with the highest amount of oxidation concentrated in the center of the samples where the temperature was highest. Compared to MO literature, oxidation occurred four orders of magnitude faster in AO due to the increased reactivity and enhanced diffusivity of AO. After environmental exposure, the composite was then tested in tension at room temperature to determine the residual strength. The results of the tensile testing revealed that oxidation under these test conditions did not have a significant impact on mechanical strength. This is inconsistent with what has been found in literature. The discrepancy in results can be attributed to the short times that the material was exposed to extreme temperatures and dissociated oxygen along with how thin the test specimens were.
