[en] An optimized nanoindentation pillar splitting technique is used for the fracture toughness measurement of spinel Li_xMn_2O_4 cathode material under different states of charge (SoC), along with the high-speed nanoindentation results for nanomechanical property mapping. High-speed nanoindentation enables for a robust and efficient evaluation of elastic modulus and hardness as a function of the SoC on strongly heterogeneous materials. The fracture toughness decreases linearly upon de-lithiation, with an overall reduction of 53% from 0% to 100% SoC. Decrease in fracture toughness is associated with the volume change, increase of defect density and stresses related to diffusion of lithium upon de-lithiation.

[en] Highlights: • Nanomechanical properties of thermal barrier coatings were measured by high-speed nanoindentation and pillar splitting • During thermal cycling, delamination damage progresses slowly as long as the bond coat forms a dense, tough oxide scale • The fracture toughness of the thermally grown oxide scale drops when its thickness exceeds a 5-μm threshold • Severe delamination damage starts from the embrittled thermally grown oxide and propagates across the zirconia top coat • Such propagation is favoured by a simultaneous drop in top coat strength, due to accumulated microstructural alterations -- Abstract: This paper studies how the nano-mechanical properties of thermal barrier coatings (TBCs) vary during thermal cycling, as a way to shed new light on their failure mechanisms. In particular, high-throughput nanoindentation revealed the evolution of hardness and elastic modulus distributions of plasma-sprayed yttria-stabilized zirconia (YSZ) top layers. The evolution of fracture toughness of the YSZ layers and the thermally grown oxide (TGO) formed onto the vacuum plasma-sprayed NiCoCrAlY bond coat were investigated by nanoindentation micro-pillar splitting. The TGO fracture toughness increases up to ≈2.5–3.5 MPa√m at the early stages of thermal cycling, followed by a rapid decrease to ≈2.0 MPa√m after a critical TGO thickness of ≈5 μm is reached. Consequently, interface damage is initially limited to short cracks within the YSZ material. As TGO thickness exceeds the critical threshold, multiple cracks originate within the TGO and join through the YSZ to form long delamination cracks. Joining is favoured by a simultaneous loss in YSZ strength, testified by a decrease in the nanomechanical properties (hardness, elastic modulus) of both high- and low-porosity top coats. This is due to microstructural changes occurring because of the continuous interplay between sintering and thermal shock cracking in the YSZ layers.