Brierley, Jack
(2025)
Design and evaluation of icephobic coatings for efficient thermal de-icing.
PhD thesis, University of Nottingham.
Abstract
Ice accretion on engineering structures is a significant safety concern, and the current in-use mitigation strategy of electro-thermal de-icing systems is energy-intensive. Icephobic coatings have been identified as a potential zero-energy solution, passively de-icing the surface or altering the accretion formation. Numerous approaches are being studied, and progress is being made towards this goal. However, coating durability remains a significant hurdle.
This work is primarily motivated by the extreme use case of ice accretion on a helicopter blade. The leading edge, where ice typically forms, experiences abrasive wear through sea spray and particulate erosion so severe that damage to the titanium erosion shield can be a life-limiting issue. Icephobic coatings may never have a sufficient lifespan to be applied as a stand-alone passive solution to solve this particular problem.
We consider a hybrid approach, with the coating considered a sacrificial barrier, extending the life of the erosion shield with the additional benefit of icephobic properties, leading to a reduction in the electro-thermal energy required to de-ice the leading edge. The goal of this coating would be to minimise cumulative energy costs by reducing the frequency and severity of icing incidents, the energy cost per de-icing cycle, or both.
While the need for a hybrid solution may only be suitable for niche or low-volume applications, many others where a passive solution would be appropriate will have that passive solution retroactively applied. Testing such coatings alongside an electro-thermal de-icing system would not be necessary to improve function. Still, it would provide confidence in early candidates. If the coating failed to passively de-ice due to unexpected performance outcomes or incurred damage inhibiting its capabilities, the active system would remain effective as a secondary safety system. In comparison, a hybrid coating system would be an electro-thermal de-icing system that is enhanced by an icephobic coating and not replaced by one.
This thesis addresses the new considerations required of a hybrid icephobic coating deployed alongside an electro-thermal de-icing system. We show that altering a coating's properties to optimise the performance of an electro-thermal de-icing system produces detrimental changes to its performance in passive icephobic metrics. An example of such a modification would be the addition of fillers to increase the thermal conductivity for use as part of an active system, but which also increases the frequency and growth rate of ice accretions and also increases the Young's modulus of the elastomer matrix material, increasing the ice adhesion strength. Every factor that can be manipulated to improve one performance metric is often a detriment to another, meaning any potential success would be application-specific, with carefully delineated optimisation requirements.
Polydimethylsiloxane (PDMS) was chosen as a matrix material following accepted best practices from icephobic literature. It was tested through a range of thicknesses and with the addition of filler material. A static electro-thermal de-icing experiment was produced that was capable of characterising the energy cost associated with a coating's surface properties and bulk materials properties. The energy required to de-ice the surface increases linearly with coating thickness. PDMS containing 8.3 wt% of SiC submicron fibres reduced the per-thickness energy cost by 59% compared to PDMS alone. Dynamic electro-thermal de-icing was tested by attaching an iced sample atop an electro-thermal de-icing system at the end of a rotating arm. Spinning the arm at a fixed speed applied shear stress to the surface-ice interface subject to electro-thermal de-icing.
When the rotational speed was sufficiently high, airflow over the rotor increased the heat lost to the environment, significantly increasing the energy cost to de-ice. The linear relationship between coating thickness and energy cost was lost, with some samples failing to de-ice with sufficiently thick coatings. This highlights a key safety concern if an early passive coating fails.
Adding thermally conductive fillers to a coating will also change the accretion process as the transfer of latent heat of freezing will be higher for a more thermally conductive coating. Altering the coating could therefore alter the accretion formation process for a given environment. If a coating changes the accretion structure to one that has not been optimised to de-ice, then an iterative design would be necessary.
A cloud was simulated with a nebuliser in a freezing environment to form ice on the leading edge of spinning turbine blades. Properties of the cloud, flow conditions relative to the blades' motion and environmental temperature were successfully used to analyse a wide range of ice accretion structures and to understand how they form. This experiment has shown that it could provide a basis for modelling techniques to predict ice accretions and show how implementing a coating could alter the accretion. A second icing experiment forming accretions on a spinning disc shows promise as a preliminary icing test for candidate coatings. The results can inform how the coating will alter the accretions formed in the blade icing experiment.
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