The effect of temperature on the erosion of polyurethane coatings for wind turbine leading edge protection

Wind turbine leading-edge erosion can degrade the aerodynamic properties of blades and reduce their efficiency. Previous theoretical work has suggested that low temperatures might affect the erosion performance of leading-edges and protective leading-edge coatings. Solid particle erosion is caused by dust, sand and hailstones impacting the leading edges. For polymer coatings, temperature is a particular concern; the low temperatures can cause a transition from ductile to brittle failures. Polyurethane (PU) coatings were eroded at two temperatures: ambient (25°C) and cold (-30°C). An adapted solid-air erosion facility was used to accelerate sub-angular sand particles of 96.2 μm mean size to a velocity of 68±8 m/s. Low volumetric sand concentrations of 1.3×10^-4 % were studied at two impingement angles of 45 and 90 degrees. The results showed that cold temperatures influenced the erosion rate and erosion mechanism of the coatings, with the erosion rate at the cold temperature increasing significantly. The erosion classification values and the shape of the wear scar suggested plastic erosion behaviour of the PU at cold temperatures, as opposed to the more erosion-resistant elastic behaviour. A temperature-controlled nanoindentation study demonstrated that the ratio of hardness to modulus reduced and the plasticity index increased with a reduction in temperature, implying the PU coatings had an increased propensity to plastically deform during cold erosion. This supports the erosion performance seen in experiments; however, the cold erosion surfaces developed more pits than the ambient case. Cross-section analysis of the eroded coatings showed accumulation of damage subsurface with evidence of delamination at the weakest interfaces in the layered coating systems, across all temperatures.