Deposit formation dynamics and microstructure development during thermal spraying.

摘要

In this investigation, experimental approach was integrated with theoretical to study the physics of splat formation process and deposit microstructure development in thermal spraying.;Splat morphology is significantly affected by droplet/substrate interaction although droplet fragmenting tendency increases fundamentally with droplet Reynolds number and Weber number. Improved droplet/substrate contact facilitates contiguous splat formation, whereas localized substrate melting/deformation promotes splat fragmentation as exemplified by molybdenum splat break-up due to steel substrate melting. Condensates and adsorbates on substrate surface lead to large degree of splat fragmentation at low temperature, while they are reduced/eliminated at temperatures above 200 C. The cleaning of substrate surface associated with substrate heating is the dominant factor for the splat morphology change with substrate temperature.;Solidification can significantly affect splat dimensions through the arresting of droplet spreading as in the case of molybdenum droplet on steel or molybdenum substrate where Solidification time scale is significantly shorter than the droplet spreading time scale. In systems such as molybdenum on glass substrate or zirconia on steel substrate, splat dimension is basically determined by the viscous dissipation of droplet kinetic energy.;Significant substrate deformation is assisted by the softening/melting of the substrate. In the case of molybdenum droplets on a stainless steel substrate, the normalized crater volume is proportional to droplet kinetic energy and heat input carried by the droplet. Alloying enhances the splat/substrate contact.;Droplets form less fragmented splats on preheated, roughened substrate. Subsequently arriving droplet can form basically contiguous splat on top of them. Equiaxed pores and inter-laminar pores are produced as a result of trapped air underneath impinging droplet and splat curling respectively. Both types of porosity decreases with increase of deposition temperature and dramatically improved splat/splat adhesion, as well as enhanced properties is obtained. Deposit integrity, cohesion and properties are improved with increasing of particle kinetic energy and thermal energy. Particle kinetic energy is more responsible for the deposit densification, and thermal energy is responsible for improved splat/splat adhesion and coalescence of adjacent splats. High kinetic energy combined with high thermal energy is crucial to achieve low porosity, integrity and enhanced cohesion.