Ice Adhesion Strength and Durability of Nanostructured Coatings for Aerospace Applications

Abstract

The aerospace industry has been focused on improving the efficiency of its ice protections systems, also by supporting the systems with icephobic coatings, but lacks suitable candidate materials and processes for manufacturing such surfaces. To be able to protect its next generation aircraft and light rotorcraft without ice protection systems from icing, the industry requires research into durable and effective icephobic coatings. In this thesis, progress toward the implementation of icephobic materials for aerospace applications has been made on two fronts: on new methods for characterizing the performance of icephobic coatings, and on the exploration of a multitude of candidates based on different working principles in terms of durability and ice adhesion strength. The characterization of icephobic performance in this thesis consisted of durability tests and ice adhesion strength measurement. The principle qualitative durability test used was accelerated rain erosion. A step was added to standard test method to evaluate the change in surface functionality after erosion. Ice adhesion strength measurement was performed using a in-situ vibrating cantilever method in an icing wind tunnel. To enhance the vibrating cantilever ice adhesion test method, an uncertainty analysis was performed according to the Guide for the Expression of Uncertainty in Measurement (GUM) for the first time. The parameter that contributed most to this uncertainty was the Young’s modulus of ice. A test method was herein developed for the measurement of the Young’s modulus of ice using the unmodified vibrating cantilever test method. For demonstration, the method was used for measuring the modulus of 4 different ice types produced by the conditions used for ice adhesion strength testing. The measured Young’s moduli were between 5 – 7 GPa, between 60-80% of the literature value of 9 GPa. The superhydrophobic surfaces tested for durability and ice adhesion were prepared on aluminum alloy and stainless-steel alloy substrates. The most durable of these surfaces proved to be least icephobic, as shown by its high ice adhesion strength in comparison to non-superhydrophobic surfaces. Superhydrophobic surfaces prepared on titanium substrates showed improved icephobicity when a nano-scale roughness was present on a micro-scale roughness. This result led to the conclusion that micro-scale roughness provided enhanced durability for a nano-scale roughness which enhanced icephobicity. The icephobicity of these surfaces also relied on a hydrophobic surface chemistry, which is known to degrade on exposure to UV radiation. To address this issue, silicone nanofilament coatings with intrinsic nano-scale roughness, a hydrophobic surface chemistry, and resistance to UV were considered. Silicone nanofilament coatings prepared on polyester fabrics were exposed to a water droplet cloud in a high-speed airflow to test their durability. Water contact angle and roll-off-angle were measured following progressively aggressive exposure (increasing airspeed). The coating was superhydrophobic until exposure to water droplet clouds in an airstream at a speed of 120 m/s. A coated fabric was also exposed to icing conditions, whereby it remained hydrophobic for 3 icing/de-icing cycles, and locally superhydrophobic in the leading-edge region. This result showed that the coating was durable enough for the exterior of light aircraft, and for controlled ice adhesion tests. The silicone nanofilament coating was then grown on aluminum alloy and titanium alloy, resulting in nanostructured superhydrophobic coatings. As a novel characterisation of this coating, ice adhesion strength measurements were performed, resulting in a 50-70% reduction in ice adhesion strength than the untreated surface on aluminum substrates, and a reduction between 70-80% compared to the untreated surface on titanium substrates. The infusion of lubricant into the nanoporous coating on titanium resulted in the 80% reduction in ice adhesion strength and was consistent for 4 icing/de-icing cycles in each of the 4 icing conditions tested. Silicone nanofilament coatings are therefore suitable for aircraft applications and provide a durable, easy ice-release functionality.