Elastic properties of hard cobalt boride composite nanoparticles

This paper reports on the determination of elastic and hardness properties of Co-B composite nanoparticles (CNP). Co boride materials is usually known for their functional properties (hydrogen catalysis, magnetism, corrosion, biomedics), but nanoscale dimensions also bring significant mechanical properties. In situ compression tests of 70-150 nm core-shell silica-coated Co2B CNP (Composite nanoparticles) were performed for the first time with a nanoindenter in the load range 30-300 μN. The CNP modulus is comparable with the bulk material (ECNP = 159-166 GPa), but the hardness is as much as 5 times higher (∼4.5 ± 1.0 GPA). Both modulus and hardness (to a lesser extent) are found to increase with the applied pressure. The paper first addresses the limitations of ordinary contact analysis intended for single-phase NP, and then presents a hybrid Oliver-Pharr strategy suitable for CNP, where numerical modeling overcomes issues related to anisotropy and heterogenety of the composite nanostructure that hinder the direct application of basic contact models. An alternative regression-based approach for estimating modulus and hardness is also considered for comparison. The importance of the model selection for the contact area A for accurate modulus and hardness results is emphasized. Besides typical Hertzian, geometrical and cylindrical area models, a new one is formulated from a "rigid-sphere" approximation, which turned out to perform best and consistently in this study, on a par with the cylindrical model. Finally, evidence of the magnetic nature of CNP and, unexpectedly, reverse plasticity is provided. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.