Cold Spraying of Cu-Al-Bronze for Cavitation Protection in Marine Environments
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S. Krebs, F. Ga¨rtner, and T. Klassen (Submitted June 9, 2014; in revised form September 15, 2014) Traveling at high speeds, ships have to face the problem of rudder cavitation-erosion. At present, the problem is countered by fluid dynamically optimized rudders, synthetic, and weld-cladded coatings on steel basis. Nevertheless, docking and repair is required after certain intervals. Bulk Cu-Al-bronzes are in use at ships propellers to withstand corrosion and cavitation. Deposited as coatings with bulk-like properties, such bronzes could also enhance rudder life times. The present study investigates the coating formation by cold spraying CuAl10Fe5Ni5 bronze powders. By calculations of the impact conditions, the range of optimum spray parameters was preselected in terms of the coating quality parameter g on steel substrates with different temperatures. As-atomized and annealed powders were compared to optimize cavitation resistance of the coatings. Results provide insights about the interplay between the mechanical properties of powder and substrate for coating formation. Single particle impact morphologies visualize the deformation behavior. Coating performance was assessed by analyzing microstructures, bond strength, and cavitation resistance. These first results demonstrate that cold-sprayed bronze coatings have a high potential for ensuring a good performances in rudder protection. With further optimization, such coatings could evolve towards a competitive alternative to existing anti-cavitation procedures.
Keywords
bronze, cavitation, cold spraying
1. Introduction Ship rudders are exposed to corrosion-erosion, erosion by dispersed sediments, and cavitation-erosion (Ref 1). Figure 1 illustrates the types of damage caused by cavitation-erosion on a typical ship rudder. Due to pressure oscillation at the ships rudder, vapor bubbles arise and collapse as pressure increases. Bubble collapses are associated with the formation of water jets locally causing loads of up to 1 GPa (Ref 2). As a consequence, the exposed material deforms and microstructural defects are generated. The cyclic loading and unloading leads to small cracks that grow and finally result in local removal of surface material. This affects the surface roughness and finally results in considerable material losses and deep cavities. Cavitation occurs in liquid flows at high speeds when the static pressure drops below the thermal saturation vapor pressure of the fluid. Under such conditions, cavitation bubbles arise, but implode as soon as the static pressure exceeds the saturation pressure again. The sudden conThis article is an invited paper selected from presentations at the 2014 International Thermal Spray Conference, held May 21-23, 2014, in Barcelona, Spain, and has been expanded from the original presentation. S. Krebs, F. Ga¨rtner, and T. Klassen, University of the Federal Armed Forces, Helmut-Schmidt-University, Hamburg, Germany. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
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