Abstract
Successive impingement of droplets after refining in supersonic plasma jet generally yields a submicron-sized lamellar coating with excellent comprehensive properties. Nevertheless, physical insight into the flattening and rapid solidification with crystallization behavior of supersonic impingement of refined droplets is difficult to understand. In this research, the content of refinement droplets reached 90% and displayed the multi-scale distribution of equiaxed grains. The boundary migration of equiaxed grains and anisotropic coalescence was found in the dynamic temperature gradient. Furthermore, an optimized model was established in order to accurately reproduce the multi-physical coupling process of supersonic impingement of single or two refined droplets, which was based on the numerical calculation of nonlinear equations (including the Mass and momentum, energy balance, Cahn–Hilliard, phase-field and orientational field equations). The size distribution and growth orientation of columnar grains within single or two flattened droplets were in good agreement with the experimental results. Epitaxial growth of columnar grains was found in the two-flattened droplet interface during the extremely rapid cooling stage. This optimized model could be an effective method in predicting the flattening and solidification with crystallization behavior of droplets during plasma spraying.
Highlights
The widely used industrial manufacturing processes usually involve the growth of single crystal, metal additive manufacturing, laser-assisted thin film and thermal spray coating [1,2].Generally, the thermal spray process can be realized by rapidly quenching techniques
The crystallographic orientation can be studied by in situ heating high-resolution transmission electron microscopy (HRTEM) technology [11,13], the dynamic growth behavior of multi-scale grains of ceramic droplets is still scarcely observed by some experimental methods
The content of refined droplets was about 90%
Summary
The widely used industrial manufacturing processes usually involve the growth of single crystal, metal additive manufacturing, laser-assisted thin film and thermal spray coating [1,2]. The thermal sprayed coating is associated with the rapid solidification and crystallization behavior of flattened droplets [5,6,7]. The rapid solidification rate resulted in the formation of fine-lamellar-structured coating with multi-scale crystallographic texture, which exhibited excellent comprehensive performances (e.g., bonding strength, heat temperature resistance, anti-oxidation and thermal shock resistance) [15,16,17]. The crystallographic orientation can be studied by in situ heating high-resolution transmission electron microscopy (HRTEM) technology [11,13], the dynamic growth behavior of multi-scale grains of ceramic droplets is still scarcely observed by some experimental methods. Physical insight into the growth and formation process of multi-scale grains within the fine-lamellar structure at extremely rapid solidification is great of importance and is necessary. The relationship between the solidification rate and crystallographic orientation of grains is elaborated in order to clarify the underlying growth mechanism of equiaxed or columnar grains of flattened supersonic droplets at an extremely rapid solidification rate
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