Understanding how titanium nanoparticles deform and coat surfaces when deposited at supersonic velocities

Cold spraying is a coating deposition method where solid powders undergo plastic deformation and adhere to the target surface, due to the very high velocities they are usually accelerated to. As such, deposition of metallic nanoparticles by cold spray method, has gained considerable attention from the research community. While the conventional kinetic spraying of microparticles is a well-recognized process, the concept of nanoparticle deposition in cold spray has remained a challenge and needs more investigation. In response to this shortcoming; recently, some experimental works have been reported in critical engineering fields including metallic, ceramic and metal matrix composite (MMC) coatings. Such studies have revealed that; in most cases, the particles being deposited need to attain a critical velocity that is required for optimal coating. Regrettably, the estimation of local temperature rises and shear instabilities by experimental means is very difficult with current technology. Recent developments in the computing world have facilitated computer-based simulations. To this end, researchers have carried out computer simulations using various techniques; where, so far, molecular dynamics and FEM simulations stand out. Owing to the complexity of these approaches, there is not much simulation work to understand and interpret the experimental results and to identify basic physical processes involved in the formation of the nanostructured coatings in Cold spraying films.

Considering the importance of deposited particle size in Cold spraying process, it is imperative to explore these aspects by molecular dynamics simulations. On this account, researchers from the Faculty of Engineering at The University of Sydney: Mr. Hesamodin Jami (PhD candidate) and Professor Ahmad Jabbarzadeh proposed to study the deposition of titanium particles on a titanium surface using sophisticated models for both particle and substrate. Simply put, they indulged in a molecular dynamics simulation of titanium nanoparticles deposition on a titanium substrate. Their work is currently published in the research journal, Applied Surface Science.

The researchers studied the local stress, temperature and particle deformation behavior in detail during the impact. Ultra-high strain rates in the order of 10¹⁰ s⁻¹, and very high temperatures lead to ultrafast deformation behavior. The researchers conducted the simulations for three different particle velocities, which were achievable in real experimental settings. In their approach, realistic potentials were used to allow for the deformation of the substrate and nano-particle upon impact.

The authors showed that there was a direct relationship between the deposition velocity and temperature. In fact, for a small particle of 2nm size, the maximum temperature was seen to reach to around 842 K at the highest impact velocity of 700 m/s. However, for the 20nm particle temperature locally and globally rose further up to 1369K. The two scientists were also able to show that at higher velocities (> 500 m/s) deformation of the particle in the elastic regime was followed by strong plastic behavior; an observation they attributed to the very high temperatures.

In summary, the Jami-Jabbarzadeh study derived the relationship between particle velocity, local stress and the local temperature in a bid to understand the processes that lead to deformation of particles and the underlying substrate. Overall, the results of molecular dynamics simulations indicated distinct behaviors for the impact of titanium nanoparticles: the maximum calculated temperature for particle and substrate (locally and globally), were lower than the melting point of titanium. In an interview with Advances in Engineering, Professor Ahmad Jabbarzadeh highlighted that the fast strain rates accompanied by high temperatures and complex size effects play a dominant role in the deformation process of nano-meter size titanium particles. Jabbarzadeh’s research group currently studies deposition of ceramic particles and the effect of particle shape and is working with researchers in Germany to optimize the deposition process to create better coating methods for biomedical applications.