On the determination of hardness and elastic modulus in BaFe 2 As 2 lamellar-like material
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Lincoln B.L.G. Pinheiro Instituto Federal de São Paulo, 13.565-905, São Carlos, SP, Brazil
Fábio T. Dias Department of Physics, Universidade Federal de Pelotas, 96.010-900, Pelotas, RS, Brazil
Alexandre Mikowski Universidade Federal de Santa Catarina, 89.218-035, Joinville, SC, Brazil
Sergey L. Bud’ko, Alex Thaler, and Paul C. Canfield Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011 (Received 14 December 2015; accepted 6 April 2016)
The mechanical behavior of superconductor lamellar-like BaFe2As2 single crystals was investigated at nanoscale by instrumented indentation. The unique responses of the ab- and a(b)ccrystallographic planes were discussed based on their influence in hardness (H) and elastic modulus (E). The results allowed two main conclusions. (i) The choice of testing parameters strongly affected the scaling of mechanical properties on the lamellar surfaces. Lamellar cracking was the leading mechanism of deformation, featuring a brittle-like behavior and affecting considerably H and E. However, the plastic deformation history allowed different elastic–plastic responses on the ab-plane owing to the compaction of the material. Threshold loads for cracking depended on both loading rate and penetration velocity, pointing out to time-dependent plastic deformation mechanisms. (ii) Proper estimates were achieved for H in multiple loading tests [3.4 GPa for ab- and ;1 GPa for a(b)c-planes], and for E under loads less than 3 mN (;55 GPa for both planes).
I. INTRODUCTION
Several materials of technological importance have in common a lamellar or lamellar-like structure, where the atomic structure confers them high physical and mechanical anisotropy. Some important examples are the high toughness nacre structure-based materials,1 graphite,2 hierarchical biological materials,3 REBaCuO hightemperature superconductors4 and the recently discovered iron-arsenic based superconductors.5 In addition, natureinspired composites employ intercalated brittle and soft lamellae (e.g., SiC and graphite) to locally modulate the elastic properties, at micron and submicron scales, originating materials with superior fracture strength.6 Lamellar and lamellar-like materials subjected to normal loading deform plastically due to the dislocation mobility as well as fracture. The gathering of dislocations can originate microcracks, which eventually cause delamination of basal planes (in lamellar structures) or
Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected]; [email protected] DOI: 10.1557/jmr.2016.166 J. Mater. Res., 2016
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cleavage planes (in lamellar-like materials).2,7 Bending forces can fracture adjacent layers perpendicularly to the cleavage plane. As a result, indentation tests using pyramidal indenters disclose pop-ins (rapid incursions) in the loading curves, as observed by several authors.2,8–11 Thus, careless analyses based strictly on the ready applica
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