Nickel-based coatings and structures for hydrophobic/icephobic applications

Wang, Jie (2020) Nickel-based coatings and structures for hydrophobic/icephobic applications. PhD thesis, University of Nottingham.

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Ice accretion often causes severe hazards and results in performance degradation to outdoor facilities and infrastructures ranging from aviation systems to energy constructions and so on, leading to huge economic loss or even catastrophic failures. Ice protection systems, typically in the form of icephobic coatings, have been receiving intensive attention in recent years due to the increasing requirement in relevant application areas. However, the design and creation of icephobic coatings for practical applications in severer environmental conditions (e.g. rain erosion) remains a big challenge, especially the limited service life and poor mechanical durability of icephobic coatings in long-time service.

This study aims to develop effective hydrophobic/icephobic coatings and structures with high durability because high durability could ensure the service life in long-time service. To start, the feasibility of metallic based coatings for hydrophobic/icephobic applications was investigated based on Ni-Cu-P ternary coatings due to its high mechanical properties and good anti-corrosion resistance. The time-dependent wettability phenomenon has been verified. The effect of time-dependent wettability and surface gaseous adsorption on the coating icephobicity was also studied for the first time. A new construction of durable icephobic layer structures using porous Ni foam skeletons and polydimethylsiloxane (PDMS) fillings was proposed, which demonstrated good mechanical durability and icephobic performance.

First, Ni-Cu-P ternary coatings with hierarchical cauli-flower structures were prepared using an electrodeposition method on 304 stainless steel (SS) substrates and the optimised preparation parameters were also investigated. The adsorption of airborne hydrocarbon molecules and decrease of lattice oxygen groups would alter the surface free energy and introduce more hydrophobic groups onto the coating surface, resulting in the changes of wetting behaviour from hydrophilic state to hydrophobic state, and causing the observed time-dependent surface wettability.

To better investigate the time-dependent wettability phenomenon, a comparison study using different surfaces and coatings including electroless deposited Ni-Cu-P coatings, 304 SS, CeO2 and PDMS polymeric coatings has been carried out and help with the understanding of the mechanism. The results showed that the main reason for the time-dependent wettability among the studied systems could be similar to that of Ni-Cu-P coatings, which is ascribed to the adsorption of airborne hydrocarbons and the change of lattice oxygen on the coating surface. The stable hydrophobicity of PDMS layer that does not show any time-dependent wettability also verified the significant role of the surface gaseous adsorption.

The effect of time-dependent wettability on icing behaviour has also been studied using Ni-Cu-P ternary coatings. The results showed that the icing time of water droplets on the Ni-Cu-P coatings in hydrophilic state was decreased significantly, and the ice adhesion strength suffered a sharp increase. The ice removal efficiency with electrothermal heating also deteriorated in the hydrophilic state, accompanied by a significantly increased energy input and longer detachment duration and higher detachment temperature. Due to the spontaneous surface adsorption of gaseous species (mainly hydrocarbon groups) during the ageing process in ambient air, the coating could recover back to the hydrophobic state, and the increased icephobicity is also regained. The adsorbed hydrocarbon species would promote the formation of air pockets between the ice-coating interfaces, which could effectively reduce the interfacial contact of the formed ice with the coating. When the adsorbed hydrocarbon species were removed by plasma cleaning, water droplets tended to have more direct contact with the coatings prior to icing, leading to the formation of interlocked ice, which significantly increased the ice adhesion on the surface. In-situ water condensation and ice formation process observed by environmental scanning electron microscopy (ESEM) confirmed the role of air pockets and the presence of interlocked ice. The variation of surface icephobicity could also be attributed to the changes in surface energy due to the surface adsorption. The results indicated that the surface gaseous adsorption in air played an important role in determining the surface icing behaviour and the icephobicity of the materials.

A new design concept of combining robust porous metallic skeletons and icephobic filling was then proposed to construct a two-phase layer structure with good anti-icing and de-icing performance, as well as excellent mechanical durability. Nickel/PDMS layers with small surface cavities were prepared using porous Ni foam skeletons (phase I) impregnated with PDMS (phase II) as the filling material by a two-step method (vacuum impregnation and spinning coating). The surface topographies, wettability and icing process were studied. Excellent icephobicity and mechanical durability of the two-phase Ni foam/PDMS icephobic layers have been verified. Under an external force, micro-cracks could be easily initiated at the Ni foam/PDMS interface region due to the small surface cavities and elastic modulus difference between Ni and PDMS, which would promote the fracture at the ice/solid interface and thus significantly lower the ice adhesion strength. Electro-thermal heating tests also confirmed the good thermal transfer efficiency of Ni foam/PDMS layers. The surface morphology and icephobicity remained unchanged after the water-sand erosion test, showing the good mechanical durability. ESEM was also used to study the in-situ icing process on the samples. The icing and de-icing cycle supported the low ice adhesion of the two-phase layer structures. By combining the advantages of mechanical durability of porous Ni skeleton and icephobicity of PDMS matrix, the Ni foam/PDMS layer demonstrates great potential for ice protection with long-term service time.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Hou, Xianghui
Hussain, Tanvir
Xu, Fang
Scotchford, Colin
Keywords: Ice protection systems, Nickel-based coatings, Icephobic, Hydrophobic
Subjects: T Technology > TS Manufactures
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 63810
Depositing User: Wang, Jie
Date Deposited: 07 Jan 2021 15:14
Last Modified: 07 Jan 2021 15:15

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