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7407 |
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Friday, May 06, 2022, Hall G1 1:40 PM Cold Gas Spraying IV |
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Adhesion of cold spray coatings: Effect of substrate roughness and hardness |
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Alessia Bruera* / University of Modena and Reggio Emilia , Italy Luca Lusvarghi / University of Modena and Reggio Emilia, Italy Giovanni Bolelli/ University of Modena and Reggio Emilia, Italy Pietro Puddu/ University of Modena and Reggio Emilia, Italy Thomas Klassen/ Helmut-Schmidt-Universität, Germany Frank Gärtner/ Helmut-Schmidt-Universität, Germany Stephan Theimer/ Helmut-Schmidt-Universität, Germany Maria Villa-Vidaller/ Helmut-Schmidt-Universität, Germany Alexander List/ Helmut-Schmidt-Universität, Germany |
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When depositing a soft metal coating (e.g. pure Cu) onto a harder material (e.g. stainless steel) by cold spraying, attainable coating adhesion depends on both the hardness and roughness of the substrate. To decouple their respective influences, the present work focuses on a model system of copper cold sprayed onto steel surfaces with different hardness and roughness. AISI 304 stainless steel plates were either polished (Rz = 0.3 ?m) or grit-blasted to different roughness values (Rz = 16, 34, 51?m), and then employed as substrates either in “as blasted” condition or in a softened, vacuum-annealed state. SEM analyses and nano-indentation tests showed that grit-blasting causes severe plastic deformation of the surface, where any visible austenite grain structure is lost up to a depth of some tens of microns. The corresponding microstructures are work-hardened, reaching a hardness of about 400 HV. Vacuum annealing induced recrystallization and restored the original, lower hardness of AISI 304. By performing cavitation tests on single-particle deposition experiments, it was found that cold sprayed Cu particles adhere much worse onto grit-blasted substrates, which are simultaneously roughened and hardened, than they do onto polished surfaces. In contrast, the situation is reversed for roughened and annealed (soft) substrates, showing an increase in adhesion of cold-sprayed Cu particles with increasing roughness up to a maximum at Rz of about 34 ?m. These trends are confirmed by bond strength testing of complete Cu coatings. The underlying mechanisms are discussed under viewpoints of particle deformation and needed substrate surface topographies to trap areas of shear instabilities of the cold spray powder to ensure bonding. |