Understanding the Formation Mechanism of Plasma-sprayed Ni and Ni20Cr Splats through Experimental and Numerical Study
In the deposition of plasma-sprayed coatings, the molten (or semi-molten) droplets impact on the substrate surface, spread and solidify to form splats with various morphologies. Splats serve as the basic building blocks of the bulk coating where their morphology changes and splat-substrate interactions are critical for microstructure characteristics and properties of the coatings. Such droplet impingement onto the substrate surface with small diameter scales, high temperatures and velocities involves complex phenomena such as fluid dynamics, solidification, bubble entrapment and interfacial heat transfer. In this thesis, the formation of single plasma-sprayed metallic splats, including droplet impact, spreading dynamics and droplet-substrate interactions, is studied experimentally and numerically. Starting with experimental work on the morphology characterisation of Ni splats on stainless steel substrates, splat overspreading and fragmentation were closely correlated with the presence of a large proportion of moisture on the substrate surface. For regularshaped splats (finger splashing or perfect disk), a numerical simulation was developed to investigate droplet spreading behaviour and the solidification process. The influence of the interfacial thermal contact resistance, the liquid-solid contact angle, the droplet surface tension, the droplet thermal conductivity and substrate preheating temperature were investigated. Interfacial interactions between the splat and the substrate, including solidification microstructure, contact quality and edge delamination, were carefully characterised by FIB and TEM microscopes. Combined with the simulation results of energy conversion and fluid dynamics during droplet spreading, solidification was found to be crucial in generating splashing in the absence of substrate surface moisture/adsorbates. The formation of Ni20Cr splats on stainless steel substrates was studied by focusing on the changes in splat morphologies, droplet spreading dynamics and splat-substrate interactions compared to Ni splats. The addition of Cr was helpful for suppressing splat fragmentation, promoting droplet spreading and facilitating the elemental interdiffusion along the interface due to the improved wettability and changes in droplet thermophysical properties. The splat formation mechanism developed on stainless steel substrates was further applied for Ni and Ni20Cr splat formation on chromium substrates. The different thermal properties (especially thermal conductivity) of substrate materials significantly influenced the droplet solidification process, and thus the spreading behaviour and splat morphology changes.
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