- Impact-induced adhesion of microparticles is at the essence of cold spray coatings and additive manufacturing via kinetic deposition. Despite extensive research in the past decade, the key gaps in understanding impact-induced adhesion have not been fully addressed. The proposed mechanisms have not been directly proven yet, and the predictive theories are still of empirical nature.
- In two recently published papers, we, in collaboration with the Schuh group (DMSE, MIT), for the first time, observed in-situ impact bonding of metallic microparticles on metallic substrates.
- The critical velocity for adhesion was accurately measured for 4 metals (with matching particle-substrate pairs): Al, Ni, Cu, and Zn.
- We demonstrated that adhesion is a pressure-governed mechanism and is related to the formation of a solid jet at the periphery of the particle.
- Finally, we also found that, under some conditions, melting of the substrate upon impact hinders particle adhesion. We showed that if the particle bounces away before the substrate solidifies adhesion cannot occur.
FIG. 1. In-situ observation of the bonding moment in microparticle impact. Multi-frame sequences with 5 ns exposure times showing 45-μm Al particle impacts on Al substrate at (a) 605m/s and (b) 805 m/s, respectively below and above critical velocity. The micro-projectiles arrive from the top of the field of view. Material jetting is indicated with white arrows.
- Our in-situ observations are in line with the post-mortem observations of material jets in bonded particles and emphasize the importance of jetting and plastic ejection of material to obtain conditions for bonding. Large plastic deformation, caused by such jetting, can provide fresh metallic surfaces and facilitate pristine atomic contact between particle and substrate leading to bonding in cold spray as it does in a similar manner for explosive welding
- In contrast to Al particle impacts on Al substrates, we never observed a single Al particle adhering to Zn even at very high impact velocities, close to 1400 m/s (see Fig. 2). Although these velocities are far beyond the critical adhesion velocity for both of the two constituent metals, there is apparently no critical adhesion velocity, at least over the studied range, for the mismatched Al-Zn pair.
FIG. 2. Multiframe sequences with 5 ns exposure times showing 15-μm Al particle impacts on a Zn substrate (top) and Al substrate (bottom) at 940 and 950 m/s, respectively, impact velocity. The microprojectile arrives from the top of the field of view. It rebounds after impacting on Zn but adheres to Al.
- SEM observations suggest that at high impact velocity, the Zn substrate underwent melting and resolidification (see. Fig. 3) and we proposed that the anomalous lack of adhesion in this case is caused by the emergence of melting, which hinders impact-induced adhesion.
FIG. 3. SEM observations of Al microparticle supersonic impact-induced indentations on a Zn substrate at 930 (a), 1195 (b), and 1368 m/s (c) velocities.
- If an impacting particle resides on the top of a molten surface layer of a substrate for a long enough time, it should eventually fuse to the substrate thanks to chemical mixing and the molten layer resolidifying. In a supersonic impact, however, the residence time of the particle on the substrate is limited and if the time needed for solidification is longer than the residence time of the particle, it will rebound with no mechanical resistance from the adjacent unsolidified liquid.
- We estimated that for Al impacting Zn, the resolidification time is orders of magnitude longer than the time that the particle resides on the substrate, explaining the lack of adhesion.
- This mechanistic finding should prove useful for a broader understanding of impact-induced adhesion and particularly for the design of impact-based additive manufacturing processes.
For more details, read the papers: