Surface oxide and impact ratio dictate interfacial adhesion of cold spray bulk metallic glass single splats

In this paper, we conducted a joint experimental and computational study on the impact and adhesion of cold spray (CS) bulk metallic glass (BMG) single splats on aluminum and steel substrates, using molecular dynamics (MD) simulations to qualitatively fill in gaps and provide mechanistic insight into the experimental data. From scanning electron microscopy, it was discovered that both substrates consisted of single splats and empty craters, with BMG‑aluminum having significant splat penetration depths and BMG-steel having moderate splat penetration depths. Transmission electron microscopy and energy-dispersive X-ray spectroscopy demonstrated microstructural gradients and three distinct phases across all splats; for the aluminum, the microstructure consisted of elongated Cr-rich grains in a Fe-rich and (W, Si, Mo)-rich matrix, while for the steel, the microstructure included larger Cr-rich grains in a Fe-rich matrix with (W, Si, Mo)-rich layers. Nanoscale scratch testing showed that values for coefficient of friction (0.24 ± 0.02 for aluminum and 0.37 ± 0.04 for steel) were within the range for unlubricated metal contacts, while values for adhesion strength σ_a (35.4 ± 3.3 MPa for aluminum and 262.3 ± 29.6 MPa for steel) were highly dependent on substrate. MD revealed that the large σ_a for steel was likely due to small effective oxide thickness t_ox via splat fracture and plastic deformation, as this translated to large attractive interactions and removal force. Finally, the experimental and MD results were interpreted in terms of impact ratio R = (ρC_o²Y_d)_p/(ρC_o²Y_d)_s, where ρ, C_o, and Y_d are the density, velocity, and dynamic yield strength of the particle (p) and substrate (s). This correlation pointed to a significant increase in σ_a in going from mechanical bonding on aluminum to metallurgical bonding on steel (i.e., σ_a increased as R → 1).