Fracture behavior of metakaolin-based geopolymer reinforced with carbon nanofibers

We investigate the fracture response of metakaolin-based geopolymer reinforced with 0.1 wt%, 0.2 wt%, and 0.5 wt% carbon nanofibers. We measure the elastoplastic response using microindentation tests. We note an increase in indentation modulus of 5%, 13%, and 21%, and an increase in indentation hardness of 9%, 18%, and 25%, respectively. We measure the fracture energy using cutting-edge microscopic fracture tests. In our tests, a sphero-conical diamond indenter pushes across the specimen's surface under a prescribed vertical force. We analyze the recorded penetration depth and horizontal force using nonlinear fracture mechanics and extract the fracture parameters. We find that carbon nanofibers enhance fracture resistance. The fracture toughness increases by, respectively, 38%, 40%, and 45%; meanwhile, the fracture energy increases by, respectively, 83%, 72%, and 74%. We find that carbon nanofibers lead to a densification of the microstructure. Moreover, we observe crack-bridging mechanisms in geopolymer nanocomposites. This study is important to pave the way for novel enhanced-performance and multifunctional structural materials. © 2020 The Author. International Journal of Ceramic Engineering & Science published by Wiley Periodicals, Inc. on behalf of American Ceramic Society.