Advanced Helicopter Rotor Blade Coatings

The erosion degradation of helicopter rotor blades and their subsequent repair and replacement has become one of the largest logistics and maintenance burdens for the U.S. Armed Forces when deployed in a desert environment.

Helicopter rotor blades experience severe sand erosion when operating in desert climates. Typically, the rotor blade is protected from erosion using a leading-edge metallic erosion strip consisting of nickel on the outboard portion of the blade and titanium on the inboard portion of the blade. Although both nickel and titanium are hard metals, their hard structures are significantly lower than that of sand, which is primarily made of quartz. This hardness differential results in the excessive erosion and degradation of rotor blades. An equally important problem with titanium protection is that a corona or halo is generated around the rotor blades at night from the sand impacting the leading edge and causing titanium to spark and oxidize, thus providing enemy forces with a visible target.

Rotor blades are also subject to rain erosion. The impact pressure from rain creates both high tensile and shear stresses at the raindrop impact sites. Repeated raindrop impact can cause stresses that create cracks in the leading edge. Prior sand erosion exacerbates raindrop damage by generating cracks that become the weak points under rain erosion. Metal erosion protection has better erosion resistance to rain than sand.

Hard and strong bulk materials such as monolithic ceramics showed sharply reduced erosion rates compared to metals in both sand and rain erosion tests. In order for a high hardness protection system scheme to be effective, the substrate must have reasonable stiffness relative to the protection system, otherwise it cracks under impact. It is also difficult to achieve compatibility between the large strain from a flexible substrate and the small strain from a stiff system.