# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 2 Materials and Methods
### Methods  
For property studies, coating and free-standing thin film samples were prepared by casting the coating solutions onto a fiberglass substrate (FR-4, 2 \(\times\) 2 \(\times\) 1/4 in) and into an aluminum mold (4.5 \(\times\) 4.5 in) pre-treated with X-9032/G401 Nix Stix(r) release agent, respectively, followed by drying and curing at ambient conditions. Through controlling the amount of coating solutions cast, all the coatings on FR-4 and free-standing thin films have comparable thicknesses of ca. 0.35 mm. Differential scanning calorimetry (DSC) was performed on DSC Q2000 (TA Instruments, New Castle, DE, USA) in nitrogen at specific heating and cooling profiles. FT-IR spectroscopy was recorded on Nicolet 6700 (Thermo Fisher Scientific, Waltham, MA, USA) equipped with Smart iTR(tm) attenuated total reflectance (ATR) sampling accessory. Test samples were measured in the form of thin films that were pressed against a diamond plate with single bounce mode. The complex viscosity (\(\eta\)) of the coatings were measured by a DHR-2 rheometer (TA Instrument Co.) at 25 \({}^{\circ}\)C using a cone-plate geometry (40 mm, 0.9786\({}^{\circ}\)). The strain was set to be 5.85% and the angular frequency was set to be 10 rad/s. The X-ray photoelectron spectroscopy (XPS) data were collected using a Kratos AXIS Ultra DLD spectrometer with a monochromated Al K-alpha beam (1486.6 eV) under high vacuum (5 \(\times\) 10\({}^{-9}\) Torr). The binding energy scale was charge corrected by shifting the main peak of the C 1s spectrum to 284.8 eV. The electron collection lens aperture was set to sample a 700 \(\mu\)m \(\times\) 300 \(\mu\)m spot, the largest possible, and at least two spots were measured for each sample to ensure the compositional information was representative of the average for the surface. The charge neutralizer was used in all measurements because the samples are non-conductive. The data were analyzed using CasaXPS (version 2.3.17 PR1.1). Tensile properties of thin films were measured on Instron model 5565 tensile tester equipped with pneumatic grips according to standard ASTM D412. Dumbbell-shaped film coupons were die cut using a DIN-53504-S3A type cutting die (ODC tooling & molds). All samples were conditioned at 23 \(\pm\) 2 \({}^{\circ}\)C and 50 \(\pm\) 5%RH for at least 24 h before testing. Due to the fact that slippage at the grip areas occurred during testing, benchmarks of 10 \(\pm\) 1 mm distance (LO) in the middle of the dumbbell-shaped samples were drawn and followed during testing to obtain true elongation at break. The rate of grip separation was 500 mm/min. After rupture, the distance (LF) between the benchmarks was measured within ca. 1 min for the calculation of tensile set. The Shore A hardness of the thin films was measured using a Rex DD-4 digital durometer with an OS-1 operating stand. Solid particle erosion tests were performed according to ASTM Standard G76-04 using angular alumina particles of ca. 50 \(\upmu\)m (AccuBRADE 50, part No. : AP106, S.S. White Technologies, Petersburg, FL, USA) as the erodent. During testing, the alumina particles were fed into a compressed air carrier stream from a pressurized vibrator-controlled hopper, which was allowed to pass through a silicon carbide nozzle with an inner diameter of 1.14 mm and directed towards the test sample at a preset impingement angle of 30\({}^{\circ}\) with respect to the test sample surface. The impingement speed of the ejected alumina particles was 150 m/s, which was controlled through adjusting the pressure of compressed air. The particle flux was regulated at 3-5 g/min by changing the vibrating amplitude of the hopper. The stand-off distance was maintained constant at 50 \(\pm\) 1 mm. After each 4 min of testing, the test sample was removed from the erosion rig and measured for its weight using an analytical balance with an accuracy of \(\pm\) 0.01 mg. At the same time, the weight of the consumed erodent was measured. Then the sample was returned to the test rig and erosion testing was resumed.