# Erosion Resistant Hydrophobic Coatings for Passive Ice Protection of Aircraft
## 2 Materials and Methods
### Methods
The erosion rates were calculated by dividing the mass loss of the coatings with the mass of solid particles consumed. The maximum particle loads were estimated by dividing the amount of sand required to fail the coating sample (e.g., penetration) with the eroded area (i.e., approximately a circle of ca. 10 mm diameter). The water droplet erosion (WDE) resistance of the coated samples were evaluated according to ASTM Standard G73 using a testing rig that was equipped with a working chamber, a vacuum system, a compressed air driven turbine and a water droplet generating system. During testing, two test coated coupons having mass differences of less than 0.1 g were mounted on the opposite ends of a rotating disc, with one as the comparative control and the other as the test sample. The disc was rotated at a specific speed while water droplets were formed in the test chamber on a path of the test coupons. A 30-50 mbar vacuum was maintained during the test to avoid temperature rise caused by friction between the rotating disc and air. In this study, the disc was rotated at a rate of 7000 rpm, corresponding to a water droplet impingement velocity of 175 m/s. The impingement angle was 90\({}^{\circ}\). The average size of water droplets produced using a 400 \(\upmu\)m shower head was about 463 \(\upmu\)m. The test coupon underwent about 42,000 individual water droplet impingements during each minute of testing. For the test coupon preparation, AA2024 specimens (0.32\({}^{\prime\prime}\)\(\times\) 0.97\({}^{\prime\prime}\)\(\times\) 0.12\({}^{\prime\prime}\)) were first treated using Akzo Nobel's Metaflex(r) SP 1050 pretreatment, followed by priming using Aerodur HS 2118 primer (ca. 30 \(\upmu\)m thick). The coatings of this study were applied by dispensing ca. 0.10 g of the coating solutions on the primed surface, followed by drying and curing at ambient conditions for 7 days. The coating layers had an average thickness of ca. 0.35 mm. Static and dynamic water contact angles (CAs) were measured on an Automated Goniometer (Model 290, Rame-hart Instrument Co., Succasunna, NJ, USA). For static contact angles, a water droplet of 4 \(\upmu\)L was dispensed on the coating surface using an automated dispenser. The contact angles were analyzed by the Dropimage Advanced software. The measurement of dynamic contact angles, i.e., the advancing and receding angles, was performed using the volume addition and subtraction method, with each volume step being 2 \(\upmu\)L and a delay time of 0.5 s. The maximum contact angle during volume addition was taken as the advancing angle, while the contact angle where the drop edges started to slide during volume subtraction was recorded as the receding angle. Ice adhesion of the coatings was examined using the push-rig schematically shown in Figure 1[26]. The set-up comprises a \"pusher\" shaft connected to a load cell, which, in turn, is connected to a threaded rod that provides lateral movement of the assembly into the test sample. The threaded rod was manually turned to drive the pusher against the ice block enclosed by a PTFE lined aluminum mold. Output from the load cell was recorded and the maximum force required to detach the ice block from the coating surface is used to calculate IAS. The aluminum mold has a dimension of 3\({}^{\prime\prime}\)\(\times\) 3\({}^{\prime\prime}\)\(\times\) 5/16\({}^{\prime\prime}\), the PTFE has a thickness of 3/8\({}^{\prime\prime}\) and the cavity for ice block preparation is 1\({}^{\prime\prime}\)\(\times\) 1\({}^{\prime\prime}\)\(\times\) 5/16\({}^{\prime\prime}\). To prepare the test sample, the PTFE lined aluminum mold was clamped tightly to the coating surface, with a rubber cushion (1/16\" thick) sandwiched between the two. De-ionized water was filled into the cavity and the assembly was then kept in a cold room at \(-\)14 \({}^{\circ}\)C to freeze the water overnight. Figure 1 also shows the PTFE-lined aluminum mold and the preparation of the ice cube on the coating surface.