A Comparison of Microscale Techniques for Determining Fracture Toughness of LiMn₂O₄ Particles

Accurate estimation of fracture behavior of commercial LiMn₂O₄ particles is of great importance to predict the performance and lifetime of a battery. The present study compares two different microscale techniques to quantify the fracture toughness of LiMn₂O₄ particles embedded in an epoxy matrix. The first technique uses focused ion beam (FIB) milled micro pillars that are subsequently tested using the nanoindentation technique. The pillar geometry, critical load at pillar failure, and cohesive FEM simulations are then used to compute the fracture toughness. The second technique relies on the use of atomic force microscopy (AFM) to measure the crack opening displacement (COD) and subsequent application of Irwin's near field theory to measure the mode-I crack tip toughness of the material. Results show pillar splitting method provides a fracture toughness value of ~0.24 MPa.m^1/2, while COD measurements give a crack tip toughness of ~0.81 MPa.m^1/2. The comparison of fracture toughness values with the estimated value on the reference LiMn₂O₄ wafer reveals that micro pillar technique provides measurements that are more reliable than the COD method. The difference is associated with ease of experimental setup, calculation simplicity, and little or no influence of external factors as associated with the COD measurements.