Processing and evaluation of UHTC loaded composites

Conference Dates

September 17-20, 2017


The ceramics composite research team at the Air Force Research Laboratory (AFRL) has been investigating the impact of adding UHTC fillers to traditional SiC/SiC and C/SiC matrices. This talk will highlight these efforts and their corresponding results. A more thorough review of a SiC/SiC-UHTC composite will be presented to discuss processing development and the evaluation regiment. Ceramic matrix composites with BN/SiC coated Hi Nicalon STM SiC fibers and matrices derived from a combination of polymer-derived SiC ceramic and powders of SiC and HfB2 were prepared. Flat plates and leading edges were processed using a wet layup of slurry infiltrated 8HS fabric. The processing was designed to result in a graded microstructure where the bulk of the sample was SiC/SiC and the outer layers to be exposed to the high heat flux environment were SiC/SiC-HfB2. In addition, a HfB2-rich coating was applied using a novel processing method amenable to preceramic polymer infiltration and pyrolysis (PIP) processing. The samples were tested for oxidation resistance using two methods: (1) arcjet and (2) laser heating. Arcjet testing was performed at the Italian Aerospace Research Centre (CIRA) using a mixed Air/Ar plasma in the Ghibli facility. Surface temperatures were set between 1500 and 2200oC by controlling the current. After high temperature exposure, an adherent oxide scale formed on the surface of the composite that limited further damage to the interior of the CMC. A rapid heating occurred near 1750oC that may be related to active oxidation. To further elucidate oxidation mechanisms, samples were also tested in the Laser Hardened Materials Evaluation Laboratory (LHMEL) at Wright Patterson AFB. The samples were subjected to gas flows using mixtures of N2 and air to control the environment while the sample was heated using a laser. As seen in Figure 1, damage was shown to be intensified by exposure to a flowing oxygen deficient atmosphere. The oxidation behavior of both the arcjet and laser heated samples were compared using SEM/EDS analysis of cross sections. Results will be discussed to elucidate oxidation mechanisms of a refractory loaded SiC/SiC in different environments.

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