Issue 8

R. Ghelichi et alii, Frattura ed Integrità Strutturale, 8 (2009) 30-44; DOI: 10.3221/IGF-ESIS.08.03 30 Coating by the Cold Spray Process: a state of the art Ramin Ghelichi, Mario Guagliano Dipartimento di Meccanica, Politecnico di Milano, Via La Masa 1, 20156 Milan, Italy R IASSUNTO . Si descrive il processo di cold-spray per l’ottenimento di rivestimenti di superfici metalliche e non metalliche. La memoria, dopo una parte introduttiva relativa alle applicazioni del processo, intende discutere criticamente le attuali conoscenze al riguardo, principalmente indirizzate all’influenza dei parametri di processo sull’efficienza di deposizione (DE). Si focalizza l’attenzione su tutti i parametri di interesse ed è indirizzata, in particolare, all’analisi dell’influenza della rugosità della superficie del substrato sul’efficienza di deposizione. Vengono poi analizzati i principali effetti del cold-spray sulle proprietà del substrato. Infine, sulla base delle attuali conoscenze si tracciano e sottolineano alcuni possibili sviluppi di ricerca in questo settore. A BSTRACT . A brief description of cold spray coating process is presented. This paper intends to review some the previous works which are mostly about the influences of the cold spray parameters, mostly the surface of the substrate, on the deposition efficiency (DE). Almost all the important parameters, with more focus on the roughness of the substrate, on increasing the DE are briefly studied; this review also includes a description of application of cold spray and of some important effect of this method on substrate properties. On this basis, some possible development in this field of research are drawn and discussed. K EYWORDS : Cold spray coating, Surface treatments, Thermal coating I NTRODUCTION he ever increasing demand to manufacture weight efficient structures that are damage tolerant and can operate at elevated temperatures has fueled the development of novel alloy compositions and radically different processing approaches over the last decades. In recent years, thermal spray technologies have evolved from fairly crude processes that were relatively difficult to control into increasingly precise tools for which the process is tailored to take into account the properties of both the deposited material and the required coatings. The limitation some coating types(for instance HVOF process, Plasma spray systems, thermal spray technology of coating, etc) seem to be overcome for some materials by the newest thermal spray process-cold spray. Cold spray is an all- solid-state process, thus making it suitable for the deposition of oxygen-sensitive materials such as aluminum, copper, or titanium or for temperature sensitive materials such as nanostructured and amorphous powders. Furthermore, cold spray is known to present other characteristics that offer unique advantages compared to existing spray technology: - The coatings can exhibit wrought-like microstructures with near theoretical density values; - The spray trace is small (typically 1-25 mm 2 ) and well defined allowing for precise control on the area of deposition; - The coatings can be produced with compressive stresses, thus ultra thick (5-50 µ m) coatings can be built-up without adhesion failure ; - Coatings can be deposited on temperature-sensitive materials such as glass or polymers[1]. Cold gas-dynamic spray (or simply cold spray) is a process of applying coatings by exposing a metallic or dielectric substrate to a high velocity (300–1200 m/s) jet of small (1–50 µm) particles accelerated by a supersonic jet of compressed gas at a temperature that is always lower than the melting point of the material, resulting in coating formation from T