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4875 |
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Wednesday, May 11, 2016, Room 3E 11:20 AM Cold Gas Spraying III |
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Influences on coating quality in cold spraying |
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Frank Gärtner* / Helmut Schmidt University of the Federal Armed Forces, Germany Maria Villa-Vidaller / , Sebastian Krebs/ , Henning Gutzmann/ , Hamid Assadi/ , Thomas Klassen/ , |
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Within the last two decades, cold spraying has developed from a laboratory deposition technique to a reliable process for applications that demand a high coating purity and the preservation of unique feedstock properties. For metals, the process is well understood, and forecasting coating performance can already be performed by commercially available software. Exceeding a material specific critical velocity, successful bonding occurs by the formation of adiabatic shear instabilities, means local thermal softening at particle interfaces over-compensating strain and strain rate hardening. The present contribution is aimed to summarize and to explain the different influences on coating quality attainable by cold spraying. The most obvious influence on coating performance is given by the choice of spray nozzle, process gas type, process gas pressure and temperature, all directly determining individual particle impact conditions. Whether particles do bond or not, then depends on materials properties, means mainly their strength and their softening behavior. In addition, impact conditions and critical conditions for bonding vary for different particle sizes and particle preheating conditions. Since bonding by adiabatic shear instabilities depends on the material strength and on the rate of deformation, the deceleration of impacting particles and related effects by substrate strength and temperature on coating adhesion and cohesion also have to be considered. The observed dependencies result in a better understanding of cold spraying. Exploring the several effects for a wide range of materials is in progress, and will enhance the development of new applications. |