research briefs Extreme testing: Mechanical testing of UHTCs at ultra-high temperatures “Failure is not an option” applies to ultra-high-temperature ceramics and ceramic composites for extreme environments. UHTC materials are expected to see service temperatures upward of 1,500°C for applications that include hypersonic aircraft, scramjet engines, rocket propulsion systems, atmospheric reentry, and next-generation gas turbine engines. Although failure may not be an option, it is always a possibility, and engineers developing these materials are challenged with testing them at temperatures high enough to generate meaningful data, especially with regard to mechanical properties. Data are critical to validating and tweaking the models used to predict the materials’ performance and, ultimately, their safety. Designing experiments that can test UHTC materials under load and at very high temperatures is itself an engineering challenge. Issues include designing furnaces that can reach test temperatures, fixture materials, accurate temperature measurement, controlling atmospheres, and more. A group at Berkeley Lab published a new paper in Nature Materials on its use of in-situ X-ray computed microtomography (CT scanning) of UHTC silicon carbide-fiber–silicon carbide-matrix composites that are subject to tensile loads at temperatures up to 1,750°C. This technique produces 3D images of microcracks in solid objects with a resolution of about 1 micrometer. The technique itself is nondestructive, so the observed damage comes only from the effects of load and temperature. The group turned to in-situ CT scanning as a way to better understand the risk of failure in extreme service conditions. In a press release, corresponding author of the study and ACerS Fellow Rob Ritchie says, “Complexity in composition brings complexity in safe use. For ceramic composites in ultra-hightemperature applications, especially where corrosive species in the environment must be kept out of the material, relatively small cracks, on the order of a single micron, can be unacceptable.” Key to evaluating failure risk is understanding the mechanisms of crack formation and growth. As the authors say in the paper, “Measurements made at high temperature are the only faithful source of the details of failure.” They explain in the paper, “Exactly how microcracks are restrained by such a tailored microstructure becomes the central question for the materials scientist, who seeks to find the optimal composition or architecture, and the design engineer, who must predict the failure envelope.” The group tested two composite configurations: a single-tow SiC-fiber–SiCmatrix composite and a textile-type carbon-fiber–SiC matrix composite. Samples were tested at 1,750°C at tensile loads starting at 10 newtons and ranging until failure (in one case, 127 newtons). The paper reports that the 3D images “reveal a wealth of information” on the interior failure mechanisms of the two composite configurations they tested, including the locations of the failure of individual fibers, the load at failure, the extent to which fibers relaxed after breaking, the opening displacement of matrix cracks, and the 3D surface morphology of surface matrix cracks. Although nobody disputes that 1,750°C is a very high temperature, there are some materials that are expected to be used at even higher temperatures. For example, refractory metal borides, such as ZrB2 and HfB2, are candidates for the leading edges of hypersonic vehicles and service temperatures of more than 2,000°C are expected. Collecting realistic mechanical test property data at these temperatures is not an off-the-shelf capability. The Berkeley team’s paper is “Real-time quantitative imaging of failure events in materials under load at temperatures above 1,600°C,” by H.A. Bale, A. Haboub, A.A. Mac- Dowell, J.R. Nasiatka, D.Y. Parkinson, B.N. Cox, D.B. Marshall, and R.O. Ritchie, Nature Materials (doi:10.1038/ NMAT3497). n American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 17 (Credit: Ritchie; LBNL.) CT scans show the formation of microcracks in ceramic composites under applied tensile loads at 1,750°C. They were obtained at Berkeley Lab’s Advanced Light Source using a unique mechanical testing rig.
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