Characterization of Polyurethane at Multiple Scales for Erosion Mechanisms Under Sand Particle Impact
Thin polyurethane films have been widely used as erosion-resistant coatings on helicopter rotor blades. Published research has mainly focused on empirical studies that relate the mechanical properties such as rebound resilience and hardness of polyurethane to solid particle erosion resistance. However polyurethane possesses phase mixing at multiple scales and thus sand particle erosion resistance depends also on the micro structure and the phase mixing. Hence, it is very important to carry out detailed and systematic investigations to understand the step-by-step mechanism of erosion and how it relates to the polyurethane micro, meso, and macrostructure. Thermal transitions of the pristine films have been studied through Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) yielding micro-scale information such as glass transition temperatures of the hard and soft segments and melting temperature of the soft segment. The next stage of our study involved sand particle erosion tests carried out at 500 mph, at an impact angle of 30 degrees. Test specimens were exposed to two different sand media at different mass loadings ranging from 0.1 to 20 g/cm^2. The tools of characterization used on the pristine polyurethane are once again used on the eroded specimens, with the goal to compare pre- and post- erosion results. The comparison of FTIR results on pre-eroded and eroded films reveal the removal of macromolecular bonds corresponding to soft segments in the micro scale. The reduction of the crystalline portion of the soft segment observed from DSC results supports the FTIR findings. Scanning electron microscopy (SEM) images of the eroded specimens are used to correlate the sequence of the damage due to erosion. The observations revealed that after initial ductile deformation of the soft segments on the surface, brittle cracks are formed on the hard segments. The increased exposure to sand particles leads to formation of fragments containing mainly soft segments with cracks in the hard segments propagating in a brittle manner. As exposure increases, cracks intersect and material on the surface gets removed which mainly contains the soft segments as revealed by the FTIR and DSC results.