Solid particle erosion mechanisms of hard protective coatings

In order to assess the material loss mechanisms of monolithic coatings subjected to solid particle erosion (SPE) using angular alumina particles with relatively low velocities (< 100 m/s), we studied the erosion behavior of several hard coatings deposited by pulsed DC magnetron sputtering. We first validated a new methodology for the measurement of volume loss and optimized the testing conditions to obtain a measured erosion rate (ER) free from experimental artifacts. We then correlated the measured ERs to the mechanical properties, measured by depth sensing indentation, and found that the ER was strongly dependent on the target hardness (Ht) of the materials (ER ∝ Ht− 6.8 ± 0.5). In order to understand the material loss mechanisms, we studied three coating systems in greater detail with the help of fracture characterization and a morphological study of the eroded surfaces. It was found that fracture toughness was not a good predictor of ER and that the material removal was the result of ductile indentation and cutting. Finally, in an effort to understand the role of particle fracture, we measured the particle size distributions of the powders before and after erosion testing and found that particle breakup was proportional to the target hardness but not sufficiently large to explain the large drop in ERs with increasing Ht.