Surface oxide layer strengthening and fracture during flattening of powder particles

Surface oxide layer fracture and the subsequent exposure of clean metallic surfaces are critical in various solid-state processes for powder consolidation and additive manufacturing. We resolve this process in-situ by deforming individual spherical powder particles inside a scanning electron microscope. We reveal three fracture modalities, i.e., meridian, radial, and circumferential cracking that sequentially activate with particle flattening. Real time measurements of load and displacement upon particle flattening also reveal a significant strengthening effect by surface oxide. We attribute the strengthening to two mechanisms: the composite strengthening and the strain gradient strengthening.