B. T. Golesich, Kuchera Defense Systems, Windber, PA; T. Eden, The Pennsylvania State University, University Park, PA; D. L. Ballard, Air Force Research Laboratory, Wright-Patterson AFB, OH; J. E. Barnes, Lockheed Martin Aeronautics, Fort Worth, TX; M. Sharma, Bucknell University, Lewisburg, PA; K. M. Anderson, Kuchera Engineering, Windber, PA
Cold Spray technology is an emerging and alternative coating method to thermal spray coating applications that is gaining acceptance as a method of applying a corrosion resistant coating for aerospace aluminum alloys. In the Cold Spray process, ductile or composite ductile/hard particles are accelerated to supersonic velocity by expanding a low-molecular weight gas through a converging-diverging nozzle. Upon impact with a substrate, the particles deform and create a mechanical bond. Unlike traditional Thermal Spray processes where the particles are molten or semi-molten, lower temperatures are used and the particles remain in a solid state during the deposition process. Another benefit of the Cold Spray process compared to Thermal Spray is the reduced oxide in the coating. For some applications, to achieve acceptable adhesion strength and coating build-up, proper surface preparation is required. The purpose of this preparation is to remove the surface oxide layer inherent to the substrate, increase the surface roughness for improved mechanical bonding, and remove any debris or oils that could affect the coating performance. For this study, the focus was to determine the effect that different surface preparation techniques have on coating quality and adhesion for a commercially pure aluminum coating on AA2024-T3 substrates. The primary techniques involved dry grit blasting with SiC, Al2O3, and glass bead mediums. Different nozzle trajectories were examined in an attempt to minimize the amount of imbedded grit to the substrate. Chemical de-oxidation was also evaluated as a non-aggressive preparation technique. Unprepared substrates were used as the baseline for comparison. Once the substrates were prepared and coated with the Cold Spray process, the coating microstructure and the substrate/coating interface were examined using optical microscopy. Adhesion testing was also conducted following the ASTM International Pull and Bend test procedures. Results of this study will be presented.
Summary: For Cold Spray coating applications, surface preparation is an essential step necessary to achieve acceptable coating characteristics. This preparation can be as simple as an alcohol wipe and as aggressive as a grit blasting operation. Kuchera Defense Systems has been involved in studying the effects of surface preparation on the adhesion properties for various Cold Spray coating applications. Under an AFRL Phase II SBIR, a direct task was to examine different surface preparation procedures applied to AA2024-T3 substrates with a commercially pure aluminum coating. The outcome was to determine the least abrasive surface preparation technique that yielded the best coating properties based on a combination of bond strength, interface characteristics, and rate of deposition. The bond strength, coating density and interface structure were evaluated by researchers at Kuchera Defense Systems and the Pennsylvania State University’s Applied Research Laboratory (ARL/PSU). Also assisting with the program were researchers at the Army Research Laboratories (ARL/Army), Lockheed Martin Advanced Development Programs (LM/ADP), and the Bucknell Mechanical Engineering Department (BU/ME).
