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

Significance 

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.

About the author

Ahmad Jabbarzadeh is an Associate Professor in Applied Mechanics at the School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney. He received his PhD (1998) and Masters (1994) degrees, both in Mechanical Engineering, from the University of Sydney. He also has an Associate Diploma in Higher Education Studies from the University of Sydney.

He has held academic and research positions at the University of Sydney for the past 22 years. Associate Professor Ahmad Jabbarzadeh is an internationally recognized molecular rheologist (the science of flow and deformation of materials) and tribologist (the science of friction and lubrication) whose research mainly deals with studying the properties of complex materials by advanced computational techniques at the micro and nanoscales. His primary areas of specialty and interest are surface science, molecular tribology, molecular rheology, polymer processing, and high-performance computational nanotechnology. Ahmad studies the properties of complex materials by advanced computational techniques at the micro and nano scales aiming to understand material properties, flow and phenomena at nano-scale. A/Prof. Jabbarzadeh has significant experience in molecular simulations of simple and complex fluids in bulk and complex geometries. His contributions include the development of a domain decomposition message-passing parallel algorithm for molecular dynamics simulation of liquids in nano-scale geometries, and use of molecular dynamics to investigate the ultrathin-film nanorheology and nanotribology. His most recent activities related to surface science and tribology include nanoscale studies of the surface coating by aerosol and cold spray, friction anisotropy in molecularly thin films and frictional properties of self-assembled monolayers. He also teaches Tribology and Computational Nanotechnology methods to Masters Students at the University of Sydney. He works in other areas related to surface science such as nano-scale wetting phenomena, and liquid-solid interaction phenomenon, that includes nano-scale lubrication and also phase transition in polymeric nanocomposites.

His previous contributions have explained why confined films of linear alkanes (main constituents of mineral oil) less than a few molecular diameters thick exhibit extremely high viscosity. In these studies, the structural origins of transition to rigidity, and the role of confining surfaces roughness, in-plane order and relative orientation have been unravelled. These works have been reported in Physical Review Letters, Tribology International, and Soft Matter.

About the author

Mr Hesamodin Jami

He carried out this research using molecular dynamics simulation as part of his PhD work in mechanical engineering at the University of Sydney. His main interests are research in Manufacturing Engineering, Mechanical Engineering and Materials Engineering. He is particularly interested in using vacuum cold sprayed to create better implants for biomedical applications.

Reference

Hesamodin Jami, Ahmad Jabbarzadeh. Unravelling ultrafast deformation mechanisms in surface deposition of titanium nanoparticles. Applied Surface Science, volume 489 (2019) page 446–461.

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