Ice Adhesion Strength Mitigation via Low Surface Roughness Erosion Resistant Coatings
Open Access
- Author:
- Schneeberger, Grant M
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 06, 2023
- Committee Members:
- Puneet Singla, Major Field Member
Edward Smith, Major Field Member
Amy Pritchett, Program Head/Chair
Jose Palacios, Chair & Dissertation Advisor
Stephen Lynch, Outside Unit & Field Member
Douglas Wolfe, Dissertation Co-Advisor - Keywords:
- ice adhesion
ice protective coatings
ice protection
aircraft icing
anti-ice coating
rotorcraft icing
helicopter icing
erosion resistant coatings
icing - Abstract:
- Ice accumulation on the aerodynamic surfaces of wings or blades is a problem encountered by fixed wing and rotorcraft vehicles. Unlike fixed wing vehicles, helicopters are required to operate in environments such as deserts and at low altitudes such that sand/dust particles are ingested by the rotor. Ingestion of sand/dust coupled with large impact velocities (up to 650 mph at the rotor tip) results in erosion of the leading edge of the rotor blades. As such, low power cost effective de-icing solutions such as pneumatic boots are not feasible. Presently, only electro-thermal systems have been certified for use on rotorcraft. Electro-thermal systems require significant power to operate and the slip rings required to transmit that power from the fixed frame to the rotating rotor are not robust. In many cases, the wires for the electro-thermal systems are cut and as much of the system as possible is removed from the vehicle to reduce maintenance requirements. Ice protective coatings could solve this problem as they could assist electro-thermal systems by reducing the power required or they could replace them all together. Erosion resistance and ice protection are typically seen as competing parameters, this PhD work presents a novel method for developing a coating which is both erosion resistant and ice protective. Two new facilities were developed to aid in the experimental work conducted in this thesis. The first is a novel rain erosion testing facility with the capability to switch between rain erosion and ice adhesion strength testing. Pennsylvania State University’s new rain erosion testing facility can generate a rain cloud with a droplet median volumetric diameter (MVD) of 1.4 mm, rainfall rate of 2 inches of water per hour and impact velocities of up to 650 mph. The facility is capable of testing small test coupons or larger blades with a maximum rotor diameter of 3.5 m. The second facility constructed was a dedicated ice adhesion testing rig for the rapid screening of ice protective surfaces. The facility features a 0.65 m rotor capable of testing 1 inch diameter flat disk surfaces. A slip ring allows icing load to be quantified via full bridge strain gauges and allows the temperature in the rotor plane to be monitored using thermistors. The spray system uses a NASA standard icing nozzle such that droplet sizes of 20-50 microns can be achieved. Liquid water content was measured using airfoils and cylinders for 4 different droplet sizes and was found to vary linearly from 0.9 g/m3 at 20 microns to 0.6 g/m3 at 50 microns confirming Appendix C icing conditions are possible in the facility. To determine how to best develop the coating, the effects due to homogeneous surface roughness and controlled surface morphology due to laser ablating were explored. Homogeneous surface roughness experiments were conducted on 304 stainless steel 1 inch diameter flat surfaces and showed that ice adhesion strength decreased linearly from 68.9 kPa to 13.8 kPa as surface roughness decreased from 80 nm to 10 nm. When compared with rougher surfaces, an asymptotic trend was seen around 10 nm. This showed that the target value for an ice protective surface should be about 13.8 kPa at a temperature of -8◦C and the surface should have a roughness of approximately 10 nm. Controlled surface morphology due to laser ablating was explored using 304 stainless steel 1 inch diameter flat surfaces polished to a surface roughness of 12 nm. Ablations of the same depth and width were made into each of the surfaces at different spacing resulting in different amounts of clamping area. Ice adhesion strength testing was conducted and at -8◦C it was found that as the controlled clamping area decreased from 202.39 mm2 to 111.8 mm2 , the ice adhesion strength decreased linearly from 125.5 kPa to 88.9 kPa. The results of both studies show that surface morphology is a driving factor in determining the adhesion strength of a surface. Finally, a novel erosion resistant ice protective coating was created based on the results of the surface morphology on ice adhesion strength study. Erosion resistance was achieved by starting with an erosion resistant coating and ice protection was achieved by reducing homogeneous surface roughness as well as minimizing macro scale surface morphologies such as scratches, pin holes, and peaks. Cathodic arc physical vapor deposition was used to deposit titanium aluminum nitride (TiAlN) coatings on 304 stainless steel and was vibratory polished for several days to achieve the ice protective surface finish. The ice protective surface was found to have a surface roughness of 12.6 nm and the ice adhesion strength was found to decrease linearly from 21.4 kPa to 9.7 kPa as temperature increased from -16 to -8◦C. Additionally, it was observed that when plotted against rougher TiAlN surfaces the dependence between ice adhesion strength and temperature decreased as the surface got smoother. The ice protective TiAlN surface (Sa=12.6 nm) showed only an 11.7 kPa change for an 8◦C temperature change (-8 to -16◦ ) as opposed to the roughest (Sa=1690 nm) TiAlN surface which showed a change of 448.5 kPa for the same temperature. The TiAlN coating was subjected to fatigue testing and rain erosion to demonstrate robustness. A cantilevered beam vibration test was setup for fatigue. Titanium strips coated with TiAlN were subjected to 1500 µϵ for 1 million cycles to simulate blade flexure during level flight and at 3000 µϵ 500 thousand cycles to simulate blade flexure during maneuvers. For both cases, there were no visual signs of fatigue or damage to the coatings. Rain erosion testing was conducted using Pennsylvania State University’s novel rain erosion facility. Rain testing was conducted at a droplet size of 1.4 mm, rainfall rate of 2 inches of water per hour, and impact velocity of 154.7 m/s (70% span location on a Blackhawk blade). Two total hours of rain erosion were conducted. After 2 hours the icing performance was found to decrease slightly, the ice adhesion strength was found to be 32.4 kPa at -16◦C and decreased linearly to 11.0 kPa at -8◦C.