**4. Conclusions**

The microstructural changes observed in the IF-WS2 particulates as a consequence of treatments with ultrasonic horn, shock tube and military rounds could be categorized in two distinct fracture modes.

The most commonly observed was the phase transformation of the 3D cage-like structures to the 2D layered polymorphs, which involved a crack forming at the particulate surface leading to the structure collapse and subsequent agglomeration of the plate-like sheets, to produce larger particle sizes. Such mechanism dominated the samples' microstructure for experiments performed using ultrasonic waves and the ones exposed to military rounds. Incipient cracks were identified in shock wave postmortem samples.

A less common secondary mechanism of particle breakage was also identified; the delamination of IF-WS2. However, the later was mainly observed as an incipient process, with only very small sections of the shells separating from the IF main body and the survival of the IF hollow cage characteristics.

The steps required for particle failure by the first mechanism included: (i) an initial crack formed in the surface of the IF particles; (ii) crack propagation towards the core, leading to a complete fracture of the walls; (iii) structure collapse and disappearance of the hollow cores; (iv) sheet re-arrangement; and (v) new bonds being formed, which resulted in an overall agglomeration. We encountered evidence that the IF-WS2 structure collapse initiated at the edges of the polyhedral particles, which acted as stress concentrators, demonstrating that general fracture mechanics concepts can be applied to materials at the nanoscale. Based on those findings and in agreement with previous reports, defect-free perfectly spherical IF-WS2 surfaces are expected to present improved shock absorbing performance than that observed for polyhedral shape IFs.

We found that surface area measurements could be correlated to the extent of particle damage when agglomeration could be recognized as the main effect of the pressure load.

We demonstrated that more irregular particles (faceted) tend to fail at defect sites that act as stress concentrators independently of how energy is delivered: shock being applied in fractions of a second, or over long periods of time, as an isotropic or non-isotropic event, as a single occurrence or by cyclic treatment.

#### **Supplementary Information**

**Figure S1.** TEM image of IF-WS2 nanoparticles (left) along EDS spectra (right) showing that in some locations WO3 can be identified.
