Della Gatta, Roberta (2022) Cold spray deposition of metallic coatings on polymers: analysis of process feasibility and coating properties. [Tesi di dottorato]

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Abstract

Nowadays, the consumption of polymeric materials and polymeric matrix composite materials is increasing because of the many advantages of polymeric materials compared to other materials, such as metals or ceramics. They are widely used in aerospace and military due to their low density, high specific strength and rigidity, and other noteworthy properties such as ease of formability and machining. Although these materials have various benefits, some more applications are still precluded because bulk polymers cannot guarantee appropriate electrical conductivity, erosion resistance, and high operating temperature. To give these materials such properties, their metallization with an additive manufacturing technique appears to be a promising solution to wide their applications. Among different techniques, the Cold Spray one can be considered the most prominent. The Cold Gas Dynamic Spray generally referred Cold Spray (CS), is a relatively new spray technology that falls under the larger family of thermal spray processes. The cold spray uses less thermal and more kinetic energy, so the powder particles remain in a solid-state upon impact with the substrate. During CS depositions, upon particles' impact with the target surface, a conversion of kinetic energy into plastic deformation occurs, and the solid particles deform and bond together for the coating formation and growth. Therefore, it can cope with the production of different kinds of coatings, pure metals, alloys, composites, nanostructured materials as well as amorphous materials, on a wide variety of substrates. Although this technique is widely used to create coatings on metals, its implementation on polymers and FRPs is relatively new. The experimental evidence of cold spray deposition of metal particles on polymers has been proved, but all the involved phenomena are not clear. To date, despite the rich excess of approaches dealing with which is the best strategy to achieve adhesion, the wide range of materials and parameters make it difficult to identify which is the best strategy to follow, and the phenomena that govern the adhesion are not clear. It is known that PMCs metallization’s quality through cold spray depends on substrate typology and stratification, which in turn, depend on PMCs manufacturing processes. Anyway, the manufacturing of the substrate is usually not considered, and the influence of the fibre is not well understood. Therefore, the customization of PMCs to be used as substrates for the cold spray deposition is crucial, the target surface should be designed and manufactured to fit better the cold spray requirements. Also, there is no model in the literature that aims to thoroughly examine and explain the phenomena that lead to the adhesion of metallic particles on composite substrates. On this basis, this thesis aims to give a complete knowledge of the Cold Spray process, trying to understand why certain phenomena occur and answer the main unsolved questions about the subject matter: (i) Is it possible to achieve deposition on all the substrates? (ii) What are the main phenomena occurring because of the impact of a particle on a composite substrate? (iii) How can the stratification of the composite and the fiber typology contribute to the effectiveness of CS depositions? (iv) Which is the role of the reinforcement when spraying on composites? (iv) is it possible to model the process? To answer all these questions, a complete study of the process is required starting from an interpretation of the phenomena occurring to the manufacturing of the substrate and the optimization of process parameters. The aim of the doctoral project is therefore to resolve the open questions about this type of process. A comprehensive approach has been proposed by considering three main research flows to be developed at the same time: to develop a numerical model that makes us able to understand the physical mechanisms that regulate adhesion between polymer and metals; provide guides for the choice of process parameters closely related to the choice of materials so that we can also achieve adequate growth of the coating; customized realization of the substrate so that we can identify the best strategy for a panel to be coated. For these two-last purposes, in the experimental set-up, both the layup sequence and stratification of the substrates and the manufacturing technology and the cold spray process parameters were varied to study the manufacturing process in its wholeness. The three flows are closely linked: since the mechanisms are not clear, the feasibility of the process for different substrate/powder pairs cannot be predicted and experimental work is necessary. To achieve these goals, this thesis aims: to find the best process and strategies for the manufacturing of the substrate, optimize process parameters and provide an interpretation of the phenomena occurring during cold spraying though the development of an interpretative model able to clarify and discuss what are the involved phenomena. In the first chapter of this work, a wide literature overview of the cold spray process and its application to composite materials will be shown, highlighting all the gaps presented in the literature. In the second chapter, the manufacturing of the substrates used for the experimental campaign and the cold spray process parameters used for the deposition will be widely discussed for each combination. Three different types of substrates are analyzed: thermoplastic substrates, thermosetting substrates and 3D-printed substrates. In the third- and fourth-chapter analysis of the feasibility of the cold spray process and analysis of the influence of process parameters, substrate material and manufacturing will be extensively discussed. In the fifth chapter, a preliminary development of an analytic model has been proposed to analyze and clarify which phenomena occur as a result of the impact of a particle on a composite substrate that leads to the coating formation.

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