Evaluation of Ice-Adhesion Strength on Erosion-Resistant Materials

Abstract

Ice-adhesion properties were evaluated for coating systems based on titanium nitride applied via cathodic-arc physical vapor deposition developed for rotorcraft erosion caps. The ice-adhesion strength of titanium nitride and titanium aluminum nitride was determined experimentally and compared to the ice-adhesion strength of uncoated metallic materials currently used on rotor-blade leading-edge caps: stainless steel 430, Inconel 625, and titanium grade 2. Environmental and material parameters were investigated to identify which were most influential on impact ice-adhesion strength. The effects of median volumetric diameter of the cloud droplets, liquid water content of the cloud, ambient temperature, surface roughness, and material grain direction were tested on stainless steel 430. Tests revealed that surface roughness and temperature have the greatest effect on ice-adhesion strength. There was an increase in adhesion strength of 670% from to and 250% increase from 0.61 to . An increase in water-droplet size from 20 to decreased the ice-adhesion strength by 65%. The adhesion strength increased 15% when shear forces were applied 90 deg with respect to the grain direction as compared to a 0 deg loading configuration. Although within the Federal Aviation Regulation Part 25 and Part 29 Appendix C icing envelope for liquid water content, an increase from 0.5 to had a 7% reduction in ice-adhesion strength. A test matrix to evaluate ice-adhesion strength of erosion-resistant materials was developed, investigating the effects of temperature and coating surface roughness. An empirical extrapolation method to predict ice-adhesion strength with varying temperature is presented and validated on metallic materials. To compare the different materials, the adhesion strength was divided by the temperature to eliminate temperature variation in the data. This temperature-adjusted adhesion strength averaged over the tested conditions for the titanium-nitride-based coatings was 32% higher than the uncoated metallic materials. The titanium nitride had the highest average temperature-adjusted adhesion strength of , and titanium grade 2 has the lowest with over all of the test conditions. Titanium aluminum nitride is an ideal coating because it has low ice-adhesion strength at low surface roughness values and will erode much slower than the other coatings.