A thermal spray coating is formed by the successive deposition of a stream of molten droplets through the processes of impacting, flattening, and rapid solidification and cooling. The behavior of droplet following the impacting is the most fundamental aspect involved in thermal spraying process, which determines the adhesion, cohesion, and the structure of subsequent coating. Therefore, it has become one of the most concerned subjects to be clarified during last decade. The direct observation of the splatting behavior of the thermal spray droplet is much difficult because of much limited time period in several microseconds during which the splatting is completed. Therefore, the experimental examination of splat morphology is an important approach to understand the involved phenomena.

In the present investigation, the variations of the morphology of splats were systematically investigated under different spray conditions (subsequently droplet conditions), surface state of substrate, and the combination of the droplet/substrate materials). The results revealed that the morphology of splat is significantly influenced by the factors including surface state and the combination of droplet/substrate, surface roughness of substrate. The morphology of the splats is complicatedly varied owing to the extensive splashing when droplet impacts even on a flat substrate surface in ambient atmosphere. The variations of splat morphology basically depend on the splatting behavior and splashing pattern occurred during splatting. When the flat substrate surface is preheated to over 200oC degree, the splashing will be suppressed and the regular disk type splat is formed despite the combinations of droplet/substrate materials in the case that no substrate melting occurs by molten droplet. The evaporated gas induced splashing mechanism and the surface melting induced splashing mechanism have been proposed to explain the splashing related phenomena. An ideal surface model is proposed for the examination of the effect of the surface roughness on splatting behavior.

Prof. Li Chang Jiu graduated with B.Eng. from Xi'an Jiatong University and proceeded to obtain M.Eng and Ph.D. from Osaka University. He is presently Full Professor at Xi'an Jiaotong University. He has been active in thermal spray research for the past 15 years. Prof. Li is presently in NTU as Tan Chin Tuan Fellow. His area of interest includes Process-Microstructure-Property Relationship in thermal spray coatings.

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For further clarification, please contact Assoc Prof Michael Khor, School of Mechanical & Production Engineering at Tel: 790 5526.